arlington wpcp solids master plan

ARLINGTON WPCP SOLIDS MASTER PLANFINAL REPORTMarch 2018

5400 Glenwood Avenue, Suite 400

Raleigh, North Carolina 27612

tel: 919-325-3500

March 19, 2018

Mary StrawnProject OfficerArlington County, Virginia3402 S. Glebe Rd.Arlington, VA 22202

Subject: Solids Master Plan for the Arlington County Water Pollution Control PlantFinal Report

Dear Ms. Strawn:

CDM Smith is pleased to submit seven (7) hardcopies plus one (1) electronic copy of the Final Report for the Solids Master Plan. The report incorporates comments received from Arlington County staff on the draft report.

The CDM Smith project team appreciates the opportunity to assist Arlington County in this important project over the last three years. We want to thank you, Mr. Tom Broderick, County leadership and staff for the valuable input and engagement throughout the planning process. Your commitment and collaboration on this process with all stakeholders is essential in making this project a success.

We look forward to the opportunity to further assist the County in the implementation of the solids management plan. If you have any questions, or require additional information, please do not hesitate to contact me.

Sincerely,

K. Richard Tsang, PhD, P.E., BCEEProject ManagerCDM Smith Inc.

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Table of Contents

Executive Summary

Section 1 – Project Background and Introduction1.1 Background ...............................................................................................................................................................1-11.2 Program Goals and Objectives ..........................................................................................................................1-31.3 Summary of Previous Master Planning Efforts .........................................................................................1-41.4 Project Methodology and Team Members...................................................................................................1-4

Section 2 – Review of Existing Facilities2.1 Overview of Plant Process ..................................................................................................................................2-12.2 Solids Handling and Treatment Processes ..................................................................................................2-3

2.2.1 Gravity Thickeners................................................................................................................................2-32.2.2 Dissolved Air Flotation Thickness..................................................................................................2-42.2.3 Sludge Storage Tanks...........................................................................................................................2-52.2.4 Dewatering ...............................................................................................................................................2-62.2.5 Lime Stabilization and Truck Loading..........................................................................................2-7

2.3 Odor Control .............................................................................................................................................................2-82.4 Bio Building...............................................................................................................................................................2-9

Section 3 – Solids Production and Basis of Planning3.1 Objectives...................................................................................................................................................................3-13.2 Historical Flows and Loadings..........................................................................................................................3-1

3.2.1 Historical Solids Production .............................................................................................................3-53.2.2 Solids Mass Balance ..............................................................................................................................3-6

3.3 Future Biosolids Production..............................................................................................................................3-83.3.1 Future Wastewater Flows..................................................................................................................3-83.3.2 Summary and Conclusions ................................................................................................................3-93.3.3 Biosolids Production .........................................................................................................................3-11

3.4 References...............................................................................................................................................................3-11

Section 4 – Evaluation Methodology and Criteria4.1 Screening Criteria...................................................................................................................................................4-14.2 Evaluation Criteria.................................................................................................................................................4-3

4.2.1 Evaluation Criteria Development ...................................................................................................4-34.2.2 Criteria Prioritization...........................................................................................................................4-54.2.3 Performance Measure Development.............................................................................................4-8

4.3 Community Outreach and Communications Plan..................................................................................4-124.3.1 Community Outreach ........................................................................................................................4-124.3.2 Communications Plan .......................................................................................................................4-14

4.4 Summary .................................................................................................................................................................4-15

Table of Contents Arlington County WPCP Solids Master Plan – Final Report

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Section 5 – Technology Identification and Screening5.1 Organization of Technology Review ..............................................................................................................5-15.2 Non-Digestion Stabilization ...............................................................................................................................5-3

5.2.1 Composting...............................................................................................................................................5-35.2.2 Chemical Treatment .............................................................................................................................5-7

5.2.2.1 Alkaline Stabilization ......................................................................................................5-75.2.2.2 BCR Environmental – CleanB (Class B), Neutralizer (Class A)......................5-85.2.2.3 Lystek.....................................................................................................................................5-9

5.2.3 Screening Exercise and Results ....................................................................................................5-105.3 Non-Digestion Stabilization ............................................................................................................................5-12

5.3.1 Aerobic Digestion................................................................................................................................5-125.3.2 Autothermal Thermophilic Aerobic Digestion.......................................................................5-145.3.3 Anaerobic Digestion...........................................................................................................................5-16

5.3.3.1 Mesophilic Anaerobic Digestion ..............................................................................5-165.3.3.2 Thermophilic Digestion...............................................................................................5-185.3.3.3 Enhanced Digestion.......................................................................................................5-20

5.3.3.3.1 Acid/Gas Digestion...............................................................................5-205.3.3.3.2 Temperature Phased Digestion ......................................................5-22

5.3.3.4 Anaerobic/Aerobic Digestion ...................................................................................5-235.3.3.5 Enzymatic Hydrolysis/ Anaerobic Digestion .....................................................5-245.3.3.6 High Solids Digestion....................................................................................................5-25

5.3.4 Screening Exercise and Results ....................................................................................................5-255.4 Digestion Process Enhancements Technology Screening..................................................................5-26

5.4.1 Thermal Hydrolysis Pretreatment ..............................................................................................5-265.4.1.1 CamiTHP™ by Cambi.....................................................................................................5-275.4.1.2 Exelys ™ by Krüger ........................................................................................................5-295.4.1.3 Biotheyls by Krüger.......................................................................................................5-305.4.1.4 Comparison.......................................................................................................................5-31

5.4.2 Thermal Hydrolysis – Post-Digestion ........................................................................................5-325.4.3 Thermochemical Hydrolysis ..........................................................................................................5-325.4.4 Cavitation................................................................................................................................................5-345.4.5 Screening Exercise and Results ....................................................................................................5-34

5.5 Drying Technology Screening ........................................................................................................................5-355.6 Thermal Processes Technology Screening ...............................................................................................5-38

5.6.1 Incineration ...........................................................................................................................................5-395.6.1.1 Multiple Hearth Furnace.............................................................................................5-395.6.1.2 Fluidized Bed Incineration.........................................................................................5-395.6.1.3 Alternatives for Incineration for Arlington County.........................................5-42

5.6.2 Gasification ............................................................................................................................................5-445.6.3 Pyrolysis..................................................................................................................................................5-445.6.4 Supercritical Water Oxidation.......................................................................................................5-445.6.5 Catalytic Hydrothermal Gasification ..........................................................................................5-445.6.6 Wet Air Oxidation and VERTAD....................................................................................................5-445.6.7 Anuvia ......................................................................................................................................................5-455.6.8 Results of Screening Evaluation for Thermal Processes....................................................5-45


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Section 5 – Technology Identification and Screening (continued)5.7 Solids Thickening Technology Screening .................................................................................................5-46

5.7.1 Gravity Thickening .............................................................................................................................5-465.7.2 Dissolved Air Flotation Thickening.............................................................................................5-475.7.3 Centrifuge Thickening.......................................................................................................................5-485.7.4 Gravity Belt Thickening....................................................................................................................5-485.7.5 Rotary Drum Thickening .................................................................................................................5-495.7.6 Rotary Screw Thickening.................................................................................................................5-495.7.7 Summary of Screening Process.....................................................................................................5-50

5.8 Solids Dewatering Technology Screening................................................................................................5-515.8.1 Centrifuge Dewatering......................................................................................................................5-515.8.2 BFP Dewatering ...................................................................................................................................5-525.8.3 Screw Press Dewatering ..................................................................................................................5-525.8.4 Rotary/Fan Press Dewatering.......................................................................................................5-535.8.5 Bucher Press Dewatering ................................................................................................................5-535.8.6 Summary of Screening Process.....................................................................................................5-54

5.9 Summary of Technologies Considered for Further Evaluation .......................................................5-55

Section 6 – Overview of Cost Development6.1 Background ...............................................................................................................................................................6-16.2 Cost Analysis Parameters ...................................................................................................................................6-26.3 Operating Costs .......................................................................................................................................................6-36.4 Capital Costs .............................................................................................................................................................6-56.5 Cost Development for Long-Term Capital Needs .....................................................................................6-6

Section 7 – Current and Emerging Biosolids Regulations7.1 Introduction..............................................................................................................................................................7-17.2 Existing Biosolids Regulations..........................................................................................................................7-1

7.2.1 Federal Regulations for Biosolids Use and Disposal ..............................................................7-17.2.1.1 Land Application Requirements (40 CFR 503)....................................................7-27.2.1.2 Management Practices and Product End Use .......................................................7-67.2.1.3 Sewage Sludge Incinerator (SSI) Rule Under 40 CFR Part 60

and Part 62 ........................................................................................................................7-67.2.2 Virginia Department of Environmental Quality (VDEQ) – 9VAC35-32..........................7-7

7.2.2.1 General Requirements ..........................................................................................................7-77.2.2.2 Management Practices and Product End Use .............................................................7-87.2.2.3 Implementation of Recent VDEQ Regulatory Changes (September 2013)....7-97.2.2.4 Nutrient Management Requirements..........................................................................7-12

7.3 Review of Pending Regulations Applicable to Biosolids Recovery and Reuse..........................7-127.3.1 Federal Updates to 40 CFR Part 503...........................................................................................7-13

7.3.1.1 Land Application and Risk Assessment Updates....................................................7-137.3.1.2 Biosolids Core Risk Assessment Model Screening Tool (BCRAM

Screening Tool) ...............................................................................................................7-137.3.2 Mid-Atlantic Regional Regulatory Perspective and TMDLs .............................................7-13

7.3.2.1 Nutrient Management Initiatives ..................................................................................7-13


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Section 7 – Current and Emerging Biosolids Regulations (continued)

7.4 Air Pollutant Permitting....................................................................................................................................7-157.4.1 Major New Source Review Permits.............................................................................................7-16

7.4.1.1 Prevention of Significant Deterioration (PSD) Permit.........................................7-167.4.1.2 Nonattainment Permit .......................................................................................................7-16

7.4.2 Minor New Source Review Permits ............................................................................................7-167.4.3 Federal Title V Operating Permits ...............................................................................................7-177.4.4 State Major Permits............................................................................................................................7-177.4.5 State Operating Permits ...................................................................................................................7-177.4.6 Article 7 Permits for Major Sources of Hazardous Air Pollutants .................................7-17

7.5 Anticipated Near-Term Biosolids Regulatory Changes.......................................................................7-177.5.1 Federal 40 CFR Part 503 ..................................................................................................................7-17

7.5.1.1 TNSS Pollutant Risk Assessment...................................................................................7-177.5.1.2 Biosolids Core Risk Assessment Model Screening tool (BCRAM

Screening Tool) ...............................................................................................................7-177.5.2 Federal Renewable Fuel Standard Program ...........................................................................7-187.5.3 Virginia Department of Environmental Quality – 9VAC25-32 ........................................7-18

7.5.3.1 Implementation of Storage Requirements................................................................7-187.5.3.2 Need for Focus on New and Innovative Recovery and Reuse Options .........7-19

7.5.4 Regulatory Considerations for Biosolids-Derived Products ............................................7-197.6 Long-Term Biosolids Regulatory Changes and Strategies.................................................................7-20

7.6.1 Pharmaceuticals and Personal Care Products (PPCPs) and Emerging Pollutants Update......................................................................................................................................................7-20

7.6.2 Emerging Pathogens Update..........................................................................................................7-207.6.3 Virginia and Other Mid-Atlantic States......................................................................................7-207.6.4 Greenhouse Gas Emissions .............................................................................................................7-21

7.7 Summary .................................................................................................................................................................7-22

Section 8 – Opportunities for Using Biosolids in the Mid-Atlantic Region8.1 Introduction..............................................................................................................................................................8-18.2 Types of Products...................................................................................................................................................8-28.3 Potential Products and Uses ..............................................................................................................................8-3

8.3.1 Biosolids Products.................................................................................................................................8-38.3.1.1 Class B Dewatered Cake.................................................................................................8-68.3.1.2 Class A Dewatered Cake.................................................................................................8-68.3.1.3 Class A Heat-Dried Pellet...............................................................................................8-88.3.1.4 Class A Heat-Dried Non-Pellet.....................................................................................8-98.3.1.5 Nutrient Enhanced Pellet............................................................................................8-108.3.1.6 Class A Compost..............................................................................................................8-108.3.1.7 Soil Blend ...........................................................................................................................8-118.3.1.8 Ash ........................................................................................................................................8-12

8.3.2 Energy Products ..................................................................................................................................8-138.3.2.1 Electricity and Heat Recovery from Biogas or Biosolids-Derived

Material ..............................................................................................................................8-138.3.2.2 Steam ...................................................................................................................................8-14


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Section 8 – Opportunities for Using Biosolids in the Mid-Atlantic Region (continued)8.3.2.3 Conversion to Biomethane.........................................................................................8-14

8.3.2.3.1 Injection into Natural Gas Pipeline................................................8-158.3.2.3.2 Vehicle Fuel from Biogas ...................................................................8-15

8.3.3 Nutrient Products (non-biosolids derived) .............................................................................8-168.3.3.1 Phosphorus Fertilizer Product .................................................................................8-16

8.4 Regional Perspective ..........................................................................................................................................8-178.4.1 Mid-Atlantic Overview......................................................................................................................8-17

8.5 Opportunities for Product Use within Arlington County ...................................................................8-198.5.1 Biosolids and Nutrient Products ..................................................................................................8-208.5.2 Energy Opportunities........................................................................................................................8-21

8.5.2.1 Electrical Power..............................................................................................................8-238.5.2.2 Biogas ..................................................................................................................................8-24

8.6 Product Associated Risks .................................................................................................................................8-248.7 Summary .................................................................................................................................................................8-258.8 References...............................................................................................................................................................8-28

Section 9 – Process Evaluations9.1 Introduction..............................................................................................................................................................9-19.2 Process Train Development...............................................................................................................................9-4

9.2.1 Thickening ................................................................................................................................................9-49.2.2 Dewatering ...............................................................................................................................................9-99.2.3 Stabilization...........................................................................................................................................9-12

9.2.3.1 Digestion Pretreatment – Thermal Hydrolysis .................................................9-139.2.3.2 Digestion ............................................................................................................................9-13

9.2.3.2.1 Autothermal Thermophilic Aerobic Digestion (ATAD)........9-139.2.3.2.2 Mesophilic Anaerobic Digestion .....................................................9-149.2.3.2.3 Temperature Phased Anaerobic Digestion (TPAD) ...............9-149.2.3.2.4 Thermal Hydrolysis Pretreatment and Mesophilic

Anaerobic Digestion.............................................................................9-149.2.3.2.5 WAS-only Thermal Hydrolysis Pretreatment and

Mesophilic Anaerobic Digestion .....................................................9-159.2.3.3 Non-Digestion..................................................................................................................9-159.2.3.4 Comparison of Digestion Stabilization Technologies.....................................9-15

9.2.4 Thermal Drying....................................................................................................................................9-169.2.4.1 Digested Product ............................................................................................................9-179.2.4.2 Raw Product .....................................................................................................................9-179.2.4.3 Cost Comparison of Drying Alternatives..............................................................9-17

9.3 Summary of Evaluations...................................................................................................................................9-189.3.1 Preferred Process Alternatives .....................................................................................................9-18

9.3.1.1 Thickening.........................................................................................................................9-199.3.1.2 Dewatering........................................................................................................................9-209.3.1.3 Stabilization......................................................................................................................9-209.3.1.4 Thermal Drying...............................................................................................................9-21

9.3.2 Preferred Process Alternatives .....................................................................................................9-219.3.3 Justification for Elimination of Alternatives............................................................................9-22


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Section 10 – Overview of Selected Biosolids Management Strategies10.1 Introduction ...........................................................................................................................................................10-110.2 Alternative 1: Lime Stabilization with Improvements ........................................................................10-1

10.2.1 Description and Process Flow .......................................................................................................10-110.2.2 Process by Process Description and Sizing..............................................................................10-2

10.2.2.1 Gravity Thickeners............................................................................................................10-210.2.2.2 Gravity Belt Thickeners...............................................................................................10-310.2.2.3 Solids Holding Tanks ....................................................................................................10-310.2.2.4 Solids Screening..............................................................................................................10-310.2.2.5 Dewatering........................................................................................................................10-410.2.2.6 Lime Feed and Truck Loading ..................................................................................10-510.2.2.7 Odor Control.....................................................................................................................10-510.2.2.8 Sidestreams ......................................................................................................................10-5

10.2.3 Product Uses .........................................................................................................................................10-510.2.4 Conceptual Layout of Facilities .....................................................................................................10-510.2.5 Capital and Life Cycle Costs ............................................................................................................10-7

10.2.5.1 Capital Costs .....................................................................................................................10-710.2.5.2 Annual Costs.....................................................................................................................10-7

10.2.6 Energy Balance.....................................................................................................................................10-910.3 Alternative 2: Mesophilic Anaerobic Digestion ......................................................................................10-9

10.3.1 Description and Process Flow .......................................................................................................10-910.3.2 Process-by-Process Description and Sizing..........................................................................10-11

10.3.2.1 Solids Holding Tanks..................................................................................................10-1110.3.2.2 Co-Thickening ...............................................................................................................10-1110.3.2.3 Solids Screening ...........................................................................................................10-1110.3.2.4 Anaerobic Digestion ...................................................................................................10-1110.3.2.5 Holding Tanks ...............................................................................................................10-1210.3.2.6 Dewatering.....................................................................................................................10-1210.3.2.7 Cake Storage and Truck Loading ..........................................................................10-1310.3.2.8 Odor Control ..................................................................................................................10-1310.3.2.9 Sidestreams....................................................................................................................10-13

10.3.3 Product Users.....................................................................................................................................10-1310.3.4 Conceptual Layout of Facilities ..................................................................................................10-1410.3.5 Capital and Life Cycle Costs .........................................................................................................10-14

10.3.5.1 Capital Costs ..................................................................................................................10-1410.3.5.2 Annual Costs ..................................................................................................................10-16

10.3.6 Energy Balance..................................................................................................................................10-1810.4 Alternative 3: Thermal Hydrolysis Pretreatment with Mesophilic Anaerobic

Digestion ...............................................................................................................................................................10-2210.4.1 Description and Process Flow ....................................................................................................10-22


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Section 10 – Overview of Selected Biosolids Management Strategies (continued)10.4.2 Process-by-Process Description and Sizing..........................................................................10-22

10.4.2.1 Solids Holding Tanks..................................................................................................10-2210.4.2.2 Solids Screening ...........................................................................................................10-2310.4.2.3 Pre-Dewatering with Cake Hopper......................................................................10-2310.4.2.4 Thermal Hydrolysis Pretreatment Process......................................................10-2310.4.2.5 Anaerobic Digestion ...................................................................................................10-2310.4.2.6 Holding Tanks ...............................................................................................................10-2410.4.2.7 Post-Dewatering ..........................................................................................................10-2410.4.2.8 Cake Storage and Truck Loading ..........................................................................10-2510.4.2.9 Odor Control ..................................................................................................................10-2510.4.2.10 Sidestreams....................................................................................................................10-25

10.4.3 Product Uses.......................................................................................................................................10-2510.4.4 Conceptual Layout of Facilities ..................................................................................................10-2510.4.5 Capital and Life Cycle Costs .........................................................................................................10-27


10.4.6 Energy Balance..................................................................................................................................10-3010.5 Alternative 4: Anaerobic Digestion with Thermal Drying...............................................................10-34

10.5.1 Description and Process Flow ....................................................................................................10-3410.5.2 Process-by-Process Description and Sizing..........................................................................10-34

10.5.2.1 Solids Holding Tanks..................................................................................................10-3410.5.2.2 Co-Thickening ...............................................................................................................10-3510.5.2.3 Solids Screening ...........................................................................................................10-3510.5.2.4 Anaerobic Digestion ...................................................................................................10-3510.5.2.5 Holding Tanks ...............................................................................................................10-3510.5.2.6 Dewatering.....................................................................................................................10-3510.5.2.7 Thermal Drying ............................................................................................................10-3510.5.2.8 Product Storage and Truck Loading....................................................................10-3610.5.2.9 Odor Control ..................................................................................................................10-3610.5.2.10 Sidestreams ....................................................................................................................10-36

10.5.3 Product Uses.......................................................................................................................................10-3610.5.4 Conceptual Layout of Facilities ..................................................................................................10-3610.5.5 Capital and Life Cycle Costs .........................................................................................................10-36


10.5.6 Energy Balance..................................................................................................................................10-41

Section 11 – Evaluation of Alternatives11.1 Economic Criteria Evaluations.......................................................................................................................11-111.2 Operational Criteria Evaluations ..................................................................................................................11-611.3 Environmental Criteria Evaluations .........................................................................................................11-1011.4 Social Criteria Evaluation ..............................................................................................................................11-1511.5 Summary of Scoring Evaluation .................................................................................................................11-1711.6 Selection of Preferred Alternative .............................................................................................................11-18


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Section 12 – Overview of Preferred Alternative 12.1 Introduction ...........................................................................................................................................................12-112.2 Summary of Improvements.............................................................................................................................12-112.3 Conceptual Layout of Facilities......................................................................................................................12-312.4 Biogas Utilization.................................................................................................................................................12-612.5 Sidestream Treatment.......................................................................................................................................12-712.6 Capital Costs of Recommended Improvements......................................................................................12-7

Section 13 – Implementation Plan 13.1 Implementation Process...................................................................................................................................13-1

13.1.1 Project Planning...................................................................................................................................13-113.1.2 Engineering Design ............................................................................................................................13-213.1.3 Execution ................................................................................................................................................13-2

13.2 Design and Construction Delivery Options ..............................................................................................13-313.2.1 Design/Bid/Build................................................................................................................................13-313.2.2 Alternative Delivery...........................................................................................................................13-3

13.3 Implementation Recommendations ............................................................................................................13-413.4 Implementation Schedule ................................................................................................................................13-5


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List of FiguresFigure ES-1 Evaluation Criteria Weighting...................................................................................................ES-5Figure ES-2 Map of Solids Management Practice at Mid-Atlantic Wastewater Treatment

Facilities ...........................................................................................................................................ES-7Figure ES-3 Results of Alternative Scoring ...................................................................................................ES-8Figure ES-4 THP with Mesophilic Anaerobic Digestion Process Flow ...........................................ES-10

Figure 1-1 Arlington County WPCP Contributing Areas .........................................................................1-2Figure 1-2 Existing Site Plan ...............................................................................................................................1-3

Figure 2-1 Process Flow Diagram for Arlington County WPCP ...........................................................2-2Figure 2-2 Gravity Thickener and Fiberglass Cover .................................................................................2-4Figure 2-3 Dissolved Air Flotation Thickener .............................................................................................2-5Figure 2-4 Solids Storage Tank ..........................................................................................................................2-6Figure 2-5 Dewatering Centrifuge ....................................................................................................................2-7Figure 2-6 Lime Silo ................................................................................................................................................2-8Figure 2-7 Dewatering Building Chemical Scrubber ................................................................................2-9Figure 2-8 Biological Solids Processing Building .......................................................................................2-9

Figure 3-1 Flow to WPCP, January 2013 through September 2015 ..................................................3-4Figure 3-2 BOD Influent Loading at WPCP, January 2013 through September 2015................3-4Figure 3-3 TSS Influent Loading at WPCP, January 2014 through September 2015..................3-4Figure 3-4 Arlington County WPCP Liquids Flow Schematic................................................................3-7Figure 3-5 Arlington County WPCP Solids Flow Schematic...................................................................3-7Figure 3-6 Projected WPCP Flows through 2040 (mgd)......................................................................3-10Figure 3-7 Projected WPCP Influent Loading through 2040 (mgd)................................................3-11

Figure 4-1 Application of Screening and Evaluation Criteria for Solids Plan Development ...4-3Figure 4-2 Evaluation Criteria Development Process..............................................................................4-3Figure 4-3 Paired Metric Comparison Matrix ..............................................................................................4-6Figure 4-4 Evaluation Criteria Weighting .....................................................................................................4-8

Figure 5-1 Simplified Schematic of CambiTHP ™ Process ...................................................................5-28Figure 5-2 Rendering of Cambi Installation...............................................................................................5-28Figure 5-3 Exelys-LD ™ Thermal Hydrolysis Configuration ...............................................................5-29Figure 5-4 Exelys-DLD ™ Thermal Hydrolysis Configuration ............................................................5-30Figure 5-5 Schematic for Typical FBIS .........................................................................................................5-40

Figure 8-1 Belt Dryer Non-Pellet Product from an Andritz System...................................................8-9Figure 8-2 DC Water Class A Product Logo................................................................................................8-12Figure 8-3 GE Jenbacher CHP Engine Generator .....................................................................................8-14Figure 8-4 Map of Solids Management Practice at Mid-Atlantic Wastewater Treatment

Facilities ..............................................................................................................................................8-17Figure 8-5 Biosolids Generated in Mid-Atlantic 2010...........................................................................8-18


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List of Figures (continued)Figure 8-6 Biosolids Generated in Mid-Atlantic 2016...........................................................................8-18Figure 8-7 Class A Distribution 2016............................................................................................................8-19Figure 8-8 Freestate Compost Facility in Prince William County ....................................................8-21

Figure 9-1 Alternatives Evaluation Process .................................................................................................9-1Figure 9-2 Current Solids Train .........................................................................................................................9-2

Figure 10-1 Lime Stabilization Process Flow...............................................................................................10-1Figure 10-2 Solids Screens...................................................................................................................................10-4Figure 10-3 Alternative 1: Lime Stabilization with Improvements ...................................................10-6Figure 10-4 Energy Balance for Lime Stabilization ................................................................................10-10Figure 10-5 Mesophilic Anaerobic Digestion Process Flow ..................................................................10-9Figure 10-6 Alternative 2 – Mesophilic Anaerobic Digestion ............................................................10-15Figure 10-7a Energy Balance for Mesophilic Digestion – Biogas Used On-Site for

Process Purposes ......................................................................................................................10-19Figure 10-7b Energy Balance for Mesophilic Digestion –Biogas Use Off-Site, Process

Heating Needs by Natural Gas.............................................................................................10-20Figure 10-7c Energy Balance for Mesophilic Digestion – Biogas Used On-Site, Process

Heating by CHP Heat Recovery with Supplemental Natural Gas .........................10-21Figure 10-8 THP with Mesophilic Anaerobic Digestion Process Flow...........................................10-22Figure 10-9 Alternative 3: Thermal Hydrolysis Pretreatment with Digestion...........................10-26Figure 10-10a Energy Balance for THP plus Mesophilic Digestion – Baseline with

Biogas Used On-Site for Process Purposes ....................................................................10-31Figure 10-10b Energy Balance for THP plus Mesophilic Digestion – Biogas Use Off-Site,

Process Heating Needs by Natural Gas............................................................................10-32Figure 10-10c Energy Balance for THP plus Mesophilic Digestion – Biogas Used On-Site,

Process Heating by CHP Heat Recovery with Supplemental Natural Gas ........10-33Figure 10-11 Anaerobic Digestion with Thermal Drying Process Flow...........................................10-34Figure 10-12 Alternative 4: Digestion with Thermal Drying ................................................................10-37Figure 10-13a Energy Balance for Mesophilic Digestion plus Thermal Drying – Baseline

With Biogas Used On-Site for Process Purposes .........................................................10-42Figure 10-13b Energy Balance for Mesophilic Digestion plus Thermal Drying – Biogas

Use Off-Site, Process Heating Needs by Natural Gas .................................................10-43Figure 10-13c Energy Balance for Mesophilic Digestion plus Thermal Drying – Biogas

Used On-Site, Process Heating by CHP Heat Recovery with Supplemental Natural Gas ..................................................................................................................................10-44


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List of Figures (continued)Figure 11-1 Results of Economic Criteria Evaluation ..............................................................................11-4Figure 11-2 Sensitivity Analysis Results (no CNG)....................................................................................11-5Figure 11-3 Sensitivity Analysis Results (with CNG) ...............................................................................11-5Figure 11-4 Results of Operational Criteria Evaluation .......................................................................11-10Figure 11-5 Estimated Energy Intensity per Dry Ton of Solids in Year 2021.............................11-11Figure 11-6 Estimated Carbon Intensity per Dry Ton of Solids in Year 2021 ............................11-12Figure 11-7 Results of Environmental Criteria Evaluation.................................................................11-15Figure 11-8 Results of Social Criteria Evaluation....................................................................................11-17Figure 11-9 Results of Alternative Scoring ................................................................................................11-18

Figure 12-1 THP with Mesophilic Anaerobic Digestion Process Flow..............................................12-1Figure 12-2 Preferred Alternative: Thermal Hydrolysis Pretreatment with Digestion............12-4Figure 12-3 3-D Rendering of Facility Improvements, Looking North.............................................12-3Figure 12-4 3-D Rendering of Facility Improvements, Looking East ................................................12-5Figure 12-5 3-D Rendering of Facility Improvements, Looking Southwest at THP Skid..........12-5Figure 12-6 3-D Rendering of Facility Improvements, Looking Southeast at THP Skid ...........12-6


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List of Tables

Table ES-1 Screening Criteria Use to Identify Preferred Technologies ..........................................ES-4Table ES-2 Capital Costs for THP with Anaerobic Digestions ..........................................................ES-11

Table 3-1 WPCP Average Influent Flows and Mass Loadings.............................................................3-2Table 3-2 WPCP Maximum Influent Flows and Mass Loadings .........................................................3-2Table 3-3 WPCP Influent Peaking Factors...................................................................................................3-3Table 3-4 WPCP Historical Solids Production Data.................................................................................3-5Table 3-5 WPCP Annual Average Solids Loads, Normalized to Influent Flows...........................3-5Table 3-6 WPCP Liquid Process Flows at 22.5 mgd................................................................................3-8Table 3-7 WPCP Solids Process Flows at 22.5 mgd.................................................................................3-8Table 3-8 Estimated Arlington County Base Sanitary Flow Increase from 2010 (mgd).........3-9Table 3-9 WPCP Projected Solids Production .........................................................................................3-12

Table 4-1 Screening Criteria..............................................................................................................................4-2Table 4-2 Evaluation Criteria ............................................................................................................................4-4Table 4-3 Evaluation Criteria Weighting .....................................................................................................4-7Table 4-4 Preliminary Performance Measures .........................................................................................4-9Table 4-5 Solids Master Plan Stakeholder Meeting Dates and Topics..........................................4-13

Table 5-1 Technology Identification ..............................................................................................................5-2Table 5-2 Summary of Composting Processes ..........................................................................................5-5Table 5-3 Screening Exercise for Non-Digestion Stabilization Technologies ...........................5-11Table 5-4 Screening Results for Non-Digestion Stabilization Technologies..............................5-11Table 5-5 Typical Performance of an Aerobic Digester ......................................................................5-13Table 5-6 Design Parameters for ATAD Systems (Stensel and Coleman, 2000)......................5-15Table 5-7 Screening Exercise for Digestion Stabilization Technologies......................................5-25Table 5-8 Screening Results for Digestion Stabilization Technologies ........................................5-26Table 5-9 Digestion Process Enhancements Comparison Matrix Screening .............................5-35Table 5-10 Digestion Process Enhancements Screening Evaluation Results ..............................5-35Table 5-11 Summary of Drying Processes...................................................................................................5-36Table 5-12 Dryer Technology Comparison Matrix – Screening .........................................................5-38Table 5-13 Dryer Technology Screening Evaluation Results..............................................................5-38Table 5-14 Thermal Process Technology Comparison Matrix Screening......................................5-45Table 5-15 Thermal Process Technology Screening Evaluation Results .......................................5-45Table 5-16 Process Options for Thickening Arlington County WPCP .............................................5-46Table 5-17 Thickener Technology Comparison Matrix – Screening................................................5-50Table 5-18 Thickener Technology Screening Results ............................................................................5-51Table 5-19 Dewatering Technology Comparison Matrix – Screening.............................................5-54Table 5-20 Dewatering Technology Screening Results .........................................................................5-54Table 5-21 Technologies to Be Considered Moving Ahead..................................................................5-55

Table 6-1 Unit Costs Used to Develop Annual Operating Costs .........................................................6-4Table 6-2 Cost Estimating Classification Matrix per AACE ..................................................................6-5


xiii

List of Tables (continued)Table 7-1 40 CFR §503.13 Pollutant Limits ................................................................................................7-2Table 7-2 40 CFR §503.33 - Summary of VAR Requirements.............................................................7-5

Table 8-1a Summary of Products: Biosolids Products .............................................................................8-4Table 8-1b Summary of Products: Energy and Non-Biosolids Derived Products.........................8-5Table 8-2 Typical Biosolids Characteristics................................................................................................8-6Table 8-3 Products of Interest for Use Within Arlington County ...................................................8-20Table 8-4 Baseline and Target Net GHG for Government and Public Schools –

2007 through 2050.....................................................................................................................8-22Table 8-5 Market Outlet Associated Risks or Uncertainties .............................................................8-26

Table 9-1 Solids Peaking Factors.....................................................................................................................9-1Table 9-2 Basis of Planning................................................................................................................................9-2Table 9-3 Mass Balance Assumptions ...........................................................................................................9-3Table 9-4 Thickening Process Requirements ............................................................................................9-4Table 9-5 Thickening Equipment Options...................................................................................................9-4Table 9-6 DAF Design Criteria ..........................................................................................................................9-5Table 9-7 Centrifuge Thickener Design Criteria .......................................................................................9-6Table 9-8 Gravity Belt Thickener Design Criteria ....................................................................................9-6Table 9-9 Rotary Drum Thickener Design Criteria..................................................................................9-6Table 9-10 Comparison of WAS Thickening Alternatives .......................................................................9-7Table 9-11 Comparison of Co-Thickening Alternatives ...........................................................................9-7Table 9-12 Comparison of Thickening Capital Costs.................................................................................9-8Table 9-13 Comparison of WAS Thickening Life Cycle Costs ................................................................9-8Table 9-14 Comparison of Co-Thickening Life Cycle Costs ....................................................................9-9Table 9-15 Dewatering Process Requirements ...........................................................................................9-9Table 9-16 Centrifuge Dewatering Design Criteria .................................................................................9-10Table 9-17 2-meter Belt Filter Press Dewatering Design Criteria ....................................................9-10Table 9-18 Screw Press Dewatering Design Criteria..............................................................................9-10Table 9-19 Comparison of Dewatering Performance.............................................................................9-11Table 9-20 Comparison of Pre-Dewatering Capital Costs ....................................................................9-11Table 9-21 Comparison of Pre-Dewatering Life Cycle Costs...............................................................9-11Table 9-22 Comparison of Final Dewatering Capital Costs..................................................................9-12Table 9-23 Comparison of Final Dewatering Life Cycle Costs ............................................................9-12Table 9-24 Digestion Process Requirements .............................................................................................9-12Table 9-25 Digester Tank Sizing Criteria and Dimensions ..................................................................9-13Table 9-26 Comparison of Digestion Capital Costs..................................................................................9-15Table 9-27 Comparison of Digestion Life Cycle Costs ............................................................................9-16Table 9-28 Thermal Drying Requirements .................................................................................................9-16Table 9-29 Thermal Drying Equipment........................................................................................................9-16Table 9-30 Summary of Operating Information for Drying Technologies ....................................9-17Table 9-31 Comparison of Thermal Dryer Capital Costs ......................................................................9-18


xiv

List of Tables (continued)Table 9-32 Comparison of Thermal Drying Life Cycle Costs...............................................................9-18Table 9-33 Processes Included in Each Alternative ................................................................................9-19Table 9-34 Preferred Dewatering Technologies by Solids Type .......................................................9-20Table 9-35 Preferred Stabilization Technologies.....................................................................................9-21

Table 10-1 Gravity Thickener Sizing..............................................................................................................10-2Table 10-2 Gravity Belt Thickener Sizing ....................................................................................................10-3Table 10-3 Dewatering Centrifuge Sizing....................................................................................................10-5Table 10-4 Capital Costs for Alternative 1: Lime Stabilization with Improvements ................10-7Table 10-5 Operating and Maintenance Labor Requirements for Alternative 1........................10-7Table 10-6 Summary of Alternative 1 Annual Costs for Power, Chemical, Labor,

Fuel, and Hauling .........................................................................................................................10-8Table 10-7 Summary of Costs for Alternative 1: Lime Stabilization with Improvements......10-9Table 10-8 RDT Sizing (Co-Thickening) ....................................................................................................10-11Table 10-9 Sizing for Class B Anaerobic Digestion ...............................................................................10-12Table 10-10 Sizing for Dewatering with Class B Anaerobic Digestion ...........................................10-13Table 10-11 Capital Costs for Alternative 2: Mesophilic Anaerobic Digestion............................10-14Table 10-12 Operating and Maintenance Labor Requirements for Alternative 2 .....................10-16Table 10-13a Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel,

And Hauling – Biogas Used for Process Purposes ......................................................10-17Table 10-13b Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel,

And Hauling – Biogas Used Off-Site as Renewable Fuel...........................................10-17Table 10-13c Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel,

And Hauling – Biogas Used On-Site in CHP System....................................................10-18Table 10-14 Summary of Costs for Alternative 2: Mesophilic Anaerobic Digestion.................10-18Table 10-15 Pre-Dewatering Sizing ...............................................................................................................10-23Table 10-16 Sizing for Post-THP Anaerobic Digestion ..........................................................................10-24Table 10-17 Sizing for Post-Dewatering with THP and Anaerobic Digestion .............................10-24Table 10-18 Capital Costs for Alternative 3: THP with Anaerobic Digestion...............................10-27Table 10-19 Operating and Maintenance Labor Requirements for Alternative 3 .....................10-28Table 10-20a Summary of Alternative 3 Annual Costs for Power, Chemical, Labor, Fuel,

And Hauling – Biogas Used for Process Purposes ......................................................10-29Table 10-20b Summary of Alternative 3 Annual Costs for Power, Chemical, Labor, Fuel,


And Hauling – Biogas Used On-Site in CHP System....................................................10-30Table 10-21 Summary of Costs for Alternative 3: THP with Anaerobic Digestion....................10-30Table 10-22 Thermal Dryer Sizing Criteria.................................................................................................10-35Table 10-23 Capital Costs for Alternative 4: Anaerobic Digestion with Thermal Drying.......10-38Table 10-24 Operating and Maintenance Labor Requirements for Alternative 4 .....................10-38Table 10-25a Summary of Alternative 3 Annual Costs for Power, Chemical, Labor, Fuel

And Hauling – Biogas Used for Process Purposes ......................................................10-39


xv

List of Tables (continued)Table 10-25b Summary of Alternative 3 Annual Costs for Power, Chemical, Labor, Fuel


And Hauling – Biogas Used On-Site in CHP System....................................................10-40Table 10-26 Summary of Costs for Alternative 4: Anaerobic Digestion with Thermal

Drying................................................................................................................................................10-41

Table 11-1 Summary of Economic Criteria for Alternatives ...............................................................11-2Table 11-2 Economic Criteria Evaluation....................................................................................................11-3Table 11-3 Summary of Operational Criteria for Alternatives ...........................................................11-6Table 11-4 Operational Criteria Evaluation................................................................................................11-9Table 11-5 Summary of Environmental Criteria for Alternatives..................................................11-13Table 11-6 Environmental Criteria Evaluation ......................................................................................11-14Table 11-7 Summary of Social Criteria for Alternatives.....................................................................11-16Table 11-8 Social Criteria Evaluation .........................................................................................................11-16Table 11-9 Results of Scoring Evaluation .................................................................................................11-17

Table 12-1 Capital Costs for Preferred Alternative: THP with Anaerobic Digestion ....................12-8Table 12-2 Capital Costs with Biogas Utilization and Sidestream Treatment .................................12-8

Table 13-1 Implementation Schedule for Solids Master Plan with Capital Expenditures..........13-5


xvi

AppendicesAppendix A Permits

Appendix B Summary of Previous Master Planning

Appendix C Process Control System Evaluation Memo

Appendix D Disinfection Alternatives Evaluation Memo

Appendix E Communication Plan

Appendix F Individual Process Comparisons

Appendix G Capital Cost Details

Appendix H Operation and Maintenance Cost Details

Appendix I Summary of Biosolids Use in Mid-Atlantic Region

Appendix J Major Vendor Proposals

Appendix K Air Emissions Modeling Memo


xvii

AcronymsAACE American Association of Cost EngineersAADF Average Annual Daily FlowACCF Arlington County Civic FederationACE Arlingtonians for a Clean EnvironmentADMM Average Day Maximum MonthAIRE Arlington Initiative to Rethink EnergyAlexRenew Alexandria Renew EnterprisesASP Aerated Static PileATAD Autothermal Aerobic DigestionBCRAM Screening Tool Biosolids Core Risk Assessment Model Screening ToolBio Building Biological Solids Processing BuildingCEP Community Energy PlanCNG Compressed Natural GasCO Carbon MonoxideCO2e Carbon Dioxide Equivalent COEP County Operations Energy PlanDAFT Dissolved Air Flotation ThickenersDC Water District of Columbia Water and Sewer AuthorityDTPD Dry Tons Per DayDWB Dewatering BuildingE2C2 Environment & Energy Conservation CommissionEDCs Endocrine Disrupting CompoundsEPA U.S. Environmental Protection AgencyEQ Exceptional QualityFAAC Fiscal Affairs Advisory CommissionFBC Fluidized Bed CombustorFBI Fluidized Bed IncineratorFBIS Fluidized Bed Incineration SystemFIV Fertility Index ValueFTE Full-Time EquivalentGHG Greenhouse Gasgpm gallons per minuteHAP Hazardous Air Pollutantslb/MG pounds per million gallonsMAD Mesophilic Anaerobic DigestionMDE Maryland Department of the Environment(MACT) Maximum Achievable Control Technologymgd million gallons per dayMHF Multiple-Hearth FurnacesMo MolybdenumMOPO Maintenance Of Plant Operations


xviii

MSW Municipal Solid WasteN NitrogenNAAQS National Ambient Air Quality StandardsNACWA National Association of Clean Water AgenciesNMP Nutrient Management PlanNOx Nitrogen OxidesNPW Net Present WorthO&M Operation and MaintenanceORP Oxidation-Reduction PotentialOPCC Opinion of Probable Construction CostP PhosphorusPCS Process Control SystemPEC Pathogen Equivalency CommitteePFRP Process to Further Reduce PathogensPMC Pairwise Metric ComparisonPMT Phosphorus Management Tool POTWs Publicly Owned Treatment WorksPPCPs Pharmaceuticals & Personal Care ProductsPSD Prevention of Significant DeteriorationPSRP Process to Significantly Reduce PathogensPTB Preliminary Treatment BuildingPTE Potential To Emit QBL Quadruple Bottom LineRECs Renewable Energy CertificatesRFS Renewable Fuel StandardRIN Renewable Identification NumbersRPS Renewable Portfolio Standardscfm standard cubic feet per minuteSNDR Simultaneous Nitrification-Denitrification RateSO2 Sulfur DioxideSRT Solids Retention TimeSSIs Sewage Sludge IncineratorsTHP Thermal Hydraulic PretreatmentTMDLs Total Maximum Daily Loadingstpy tons per yearVAR Vector Attraction ReductionVDACS Virginia Department of Agriculture and Consumer ServicesVDEQ Virginia Department of Environmental QualityVFD Variable Frequency DriveVOC Volatile Organic CompoundsVPA Virginia Pollution AbatementVPDES Virginia Pollutant Discharge Elimination SystemVS Volatile Solids

ES-1

Executive Summary

The Arlington County Water Pollution Control Plant (WPCP) treats incoming wastewater flows for

residents, businesses, and government agencies in the Arlington County sewer service area to

protect public health and the environment. The residuals produced as a byproduct of the

treatment process are currently stabilized using lime to produce Class B biosolids that are

beneficially used through application on agricultural land. The previous Master Plan project

(MP01) focused on implementing state-of-the-art technology for the liquid treatment processes.

This Solids Master Plan focuses on planning for the future of solids processing at the facility.

The purpose of the Solids Master Plan is to provide a roadmap for Arlington County with long-

terms goals and strategies to achieve those goals identified. The plan recognizes the potential for

a regional solids management solution that might include partnership with other utilities.

Potential partners including DC Water, Fairfax County, and others were contacted for interest;

however, at the time of this report, no regional opportunity has emerged. The recommendations

in this Solids Master Plan allow the County to proceed with the planning and implementation of a

solids management strategy while continuing to explore potential regional opportunities that may

arise.

ES.1 Solids Master Planning Background and Goals The Arlington County WPCP is an advanced wastewater treatment plant located on South Glebe

Road in Arlington, Virginia with capacity to treat up to 40 million gallons per day (mgd). The

facility provides wastewater treatment for a service area that includes most of Arlington County

plus areas of Falls Church, Alexandria, and Fairfax County. The area is densely populated with a

mix of residential, institutional, and commercial customers. The service area includes over

220,000 residents plus landmarks including Reagan National Airport and the Pentagon.

The WPCP discharges treated wastewater effluent into Four Mile Run, part of the lower Potomac

River sub-basin in the Chesapeake Bay watershed, under Virginia Pollutant Discharge Elimination

System (VPDES) Permit No. VA0025143. The WPCP uses a combination of physical, chemical, and

biological processes to treat wastewater to a high standard. Solids removed from the treatment

processes receive additional treatment before being hauled off-site by trucks. Solids are thickened

and dewatered prior to lime stabilization. Approximately 36,000 wet tons of lime-stabilized

biosolids are hauled annually by truck from the site for beneficial use as Class B biosolids in bulk

land application, which equates to about 30 dry tons per day.

Several goals for this Solids Master Plan were identified at the outset of the project. They include:

� Replacing failing and end of life equipment

� Mitigating the risk of potential future regulatory changes to the current practice of

recycling Class B biosolids through application to agricultural land

� Providing a solution that reduces the energy and greenhouse gas footprint of the WPCP

� Achieving additional County-wide sustainability goals

� Developing a solids management strategy that offers long-term reliability

� Establishing an implementation plan compatible with County CIP funding

Executive Summary •

ES-2

ES.2 Project Team and Communications The master planning project team was comprised of representatives from multiple stakeholder

groups including:

� Arlington County

• Department of Environmental Services representatives from Management, Financial,

Communications, and Energy teams

• WPCP engineering and Bureau Chief

• WPCP operations staff

• WPCP maintenance staff

� CDM Smith consulting core team

� Technical advisory committee consisting of recognized industry experts

� Multiple subject matter experts

� External stakeholders including civic associations, financial commissions, and

environmental groups

The team members met regularly over the project period to advance the project from initial goal-

setting and technology screening, to developing and evaluating solids management alternatives,

and ultimately to identifying a preferred alternative and developing an implementation plan.

The project team recognized the specific and direct impacts this project would have on WPCP

customers, the community, and in particular, WPCP’s closest neighbors. The team developed a

communication plan early on and has been conducting community outreach since the project

began in the fall of 2015. The purpose of this outreach was to ensure early, frequent and two-way

communication with key stakeholders and residents throughout the multiple phases of the

project. The communication plan and outreach facilitated an information exchange that allowed

the project team to inform stakeholders of progress and recommendations of the Solids Master

Plan. Additionally, the County received feedback from stakeholders that was valuable to the

project.

WPCP hosted a series of workshops and presented information about the plan at several

community meetings over a two-year period. Workshop attendance included representatives

from the closest neighborhoods (Aurora Highlands, Arlington Ridge, Crystal City, and Long

Branch), as well as members from the Arlington County Civic Federation, Neighborhood

Conservation Advisory Commission, the Fiscal Advisory Affairs Commission, Arlingtonians for A

Clean Environment and the Energy and Environment Conservation Commission.

At each workshop stakeholders received a presentation about progress on the project and were

able to provide input at key decision points. Meeting summaries, presentations and questions and

answers were posted to a public project website (https://projects.arlingtonva.us/projects/water-

pollution-control-plant-solids-master-plan/). Stakeholders’ input provided along the way

included participation in developing weightings for the evaluation criteria used in the

alternatives analysis. Stakeholder participation resulted in an increased emphasis on social and


ES-3

environmental impacts of the project; the importance of exploring regional solutions; and the

need for an early evaluation of potential air emissions resulting from the technologies

recommended for implementation. These priorities have been addressed as part of this study.

ES.3 Project Need and Basis of Planning Much of the existing solids handling infrastructure at the WPCP is over 25 years old with some

facilities over 40 years old. The equipment requires frequent attention from maintenance staff. A

condition assessment was completed for the solids equipment early in the Master Plan project.

The assessment reviewed both the criticality and condition of the equipment. The assessment

identified numerous process equipment that was approaching or past its useful life. Frequent

maintenance was noted for the equipment associated with the lime stabilization and truck loading

process. Considerable improvements to the solids processes are needed.

The basis of planning for the master plan was determined by estimating future wastewater flows,

influent loadings, and solids production for the WPCP. The projections, presented in Section 3 of

the report, were based on conclusions from previous studies and adjusted based on recent plant

operating records. On average, the solids projections reflect an increase of approximately 1.5%

per year from current levels through year 2040.

� Facility improvements and capital costs described in the report are based on the WPCP’s

existing permitted capacity of 40 mgd.

� Annual costs and 20-year life cycle costs were developed using annual projections of solids

for years 2021 – 2040.

ES.4 Evaluation Methodology The evaluation methodology used in the master planning process includes multiple levels of

evaluation to select a preferred solids management alternative from a list that initially included

nearly 70 different potential technologies.

An initial review of technologies considered is presented in Section 5 of the report. The project

team completed a screening exercise to identify preferred technologies that were appropriate for

Arlington County based on a set of five criteria (Table ES-1).

Following an initial technology screening, the project team combined the preferred technologies

into twelve (12) potential solids management alternative process trains. Ten of the process trains

focused on processes that could be constructed at the WPCP. The remaining two process trains

were developed around off-site solids management solutions. The potential for regional

partnerships and/or third-party agreements were explored for these off-site management

solutions.


ES-4

Table ES-1. Screening Criteria Used to Identify Preferred Technologies

Criterion Comparative Basis Preferred

Development Status Technical Development Level defined by WE&RF

Conventional/established technologies

Typical Application Scale Typical solids production at wastewater treatment plants using the technology

10 - 100 dry tons per day

Site Requirements Relative amount of land required, or off-site solution

Technologies that fit on site with minimal demolition; or technologies where solids would be processed elsewhere

Relative Costs Compare to current solids management costs

Comparable to (or lower than) current biosolids management costs

Permitability History of permitting technology Technology has been permitted and/or no difficulty is anticipated

Due to the uncertainty associated with developing a regional partnership, the project team put

this effort on its own parallel track and focused the master plan evaluation on solutions that

Arlington County could construct and maintain long-term ownership. Arlington County will

continue to consider any options for a regional solution; however, at this time, there are no

feasible facilities or partners in place or anticipated in the near future.

Section 9 of the report presents the evaluation of the individual technologies and processes. Two

project workshops were conducted to review the preliminary process evaluations with a focus on

economics, space requirements, energy requirements, and end products. The outcome of the

workshops resulted in the project team identifying four alternatives for detailed evaluations.

The four alternatives identified will result in either a Class A or a Class B biosolids product. Both

classes of biosolids products are suitable for beneficial use in land application. Key differences

between Class A and Class B biosolids are summarized below:

� Class A biosolids are treated for pathogen reduction to a level where pathogens cannot be

detected. Class A biosolids can be distributed and applied with fewer restrictions than

Class B biosolids. The result is that Class A biosolids can be distributed to additional

markets outside of bulk agricultural land application.

� Class B biosolids are not treated to the same level for pathogen reduction as Class A

biosolids. Regulators have developed management measures such as buffer requirements

and access restrictions for Class B application sites to protect public health.

The four alternatives are:

� Alternative 1: Lime Stabilization. Lime stabilization represents the current process

employed at the WPCP and is used as a baseline for comparison of other alternatives. The

current process produces Class B biosolids.

� Alternative 2: Mesophilic Anaerobic Digestion. Anaerobic digestion was identified as a

preferred process for stabilization of WPCP solids at the screening level and preliminary

process evaluations. The process includes thickening solids ahead of digestion, anaerobic

digestion, and dewatering of the final product. The process will produce Class B biosolids

and biogas that can be captured and utilized as fuel. Phosphorus recovery is also potential

for all digestion options.


ES-5

� Alternative 3: Thermal Hydrolysis Pretreatment + Anaerobic Digestion. THP

combined with anaerobic digestion will produce Class A biosolids and biogas that can be

captured and utilized as fuel. The process involves pre-dewatering of solids, thermal

hydrolysis pretreatment, anaerobic digestion, and dewatering of the final product.

� Alternative 4: Anaerobic Digestion + Drying. Similar to Alternative 2, the mesophilic

digestion process of this alternative will produce a biosolid that can be dewatered. The

dewatered material can then be thermally dried to produce a Class A product with

significant volume reduction compared to other alternatives.

The detailed evaluation of the four alternatives was based on nineteen (19) criteria that were

developed by Arlington County at the beginning of the project. The criteria were weighted based

on input from numerous project stakeholders, including Arlington County staff and the external

stakeholder group.

The evaluation criteria were distributed between four categories: economic, operational,

environmental, and social. These categories and criteria support a “Quadruple Bottom Line”

approach to the evaluation. Figure ES-1 is a representation of the criteria in each category with

weightings.

Figure ES.1. Evaluation Criteria Weighting


ES-6

ES.5 Alternative Evaluations A detailed engineering evaluation of the alternatives was completed. Sections 10 and 11 of the

Master Plan report provide additional information on the development of the alternatives and the

results of the evaluation.

ES.5.1 Economic Criteria Evaluation

The project team considered multiple economic criteria as part of the analysis. Capital cost,

annual cost of operations & maintenance, and 20-year life cycle costs were developed for each

alternative using vendor information, utilities rates, County contracts, and solids quantity

projections. Annual costs were adjusted for inflation as part of the cost development. A sensitivity

analysis, comparing the impacts of cost increases or decreases, was also completed.

Alternative 2 (Mesophilic Anaerobic Digestion) received the highest score and was viewed most

favorably under the economic criteria. Other digestion based alternatives (Alternative 3 and

Alternative 4) also scored favorably compared to the existing lime stabilization process. The

factors influencing the more favorable scores include:

� Moderate capital cost and reduced annual O&M cost resulting in comparable life cycle costs

� Reduced quantity of product hauling associated with digestion based alternatives

� Reduced life cycle cost sensitivity to variables such as changes in hauling costs or energy

costs, resulting in reduced risk

ES.5.2 Operational Criteria Evaluation

Operational criteria evaluations focused on the impact to the operations of the WPCP. The criteria

considered the number of hours projected to operate and maintain equipment, the reliability of

the process and equipment to meet performance goals, the impact on other treatment plant

processes, and operator safety.

Alternative 1 (Lime Stabilization) and Alternative 2 (Mesophilic Anaerobic Digestion) scored the

highest (most favorable) in this category. In general, the scores are more favorable for these

alternatives due to the following:

� Operability of systems that show a lower staffing requirement to operate and maintain (i.e.,

fewer processes and equipment)

� Processes that are more established with a longer operating history and more installations

� Processes that require smaller footprints based on preliminary layouts

� Alternative 1 (Lime Stabilization) scored slightly higher than Alternative 2 (Mesophilic

Anaerobic Digestion) when considering the impact of the solids process on plant

operations. Digestion-based alternatives will produce a dewatering sidestream with

increased nutrient loading when compared to the lime stabilization process. The potential

impacts of returning the nutrient-rich sidestream to the liquid process will need to be

addressed.


ES-7

The trend in the mid-Atlantic region for treatment facilities similar in size (and larger) than

Arlington County’s WPCP has been to move towards processes capable of producing a Class A

biosolids product. Many of the facilities similar in size to Arlington County are either currently

producing a Class A biosolids or have plans to produce a Class A product in the future. Figure ES-

2 presents a map of most Mid-Atlantic wastewater treatment facilities located in urban areas. For

clarity, the map does not include all biosolids sources in each state; however, the size of

treatment plants presented covers 2 mgd to 370 mgd. The map indicates the type of biosolids

product or management process relied upon to manage solids produced at the facility.

Figure ES-2. Map of Solids Management Practice at Mid-Atlantic Wastewater

Treatment Facilities

Land application programs have successfully been used to manage biosolids for many years. Since

Class B biosolids are not treated to the same level for pathogen reduction as Class A biosolids,

regulators have developed management measures such as buffer requirements and access

restrictions for Class B application sites to protect public health. Class A biosolids are treated to a

higher level and can be distributed and applied without the same restrictions. The result is that

Class A biosolids can be distributed to additional markets outside of bulk agricultural land

application.


ES-8

The added flexibility of processes that produce Class A biosolids was reflected in the evaluation of

this criterion. While the criteria rankings reflect a low weighting for ‘flexibility,’ the project team

recognized the value of Class A biosolids in increasing operational flexibility, even noting the

County’s long-term objective was to move towards a process that could produce Class A biosolids.

ES.5.3 Environmental Criteria Evaluation

For the environmental criteria, Alternative 3 (THP with Anaerobic Digestion) scored the highest

(most favorable) of the alternatives considered. Other digestion based alternatives (Alternative 2

and Alternative 4) also scored favorably compared to lime stabilization.

Biogas production as a resource to be recovered favored all options with anaerobic digestion.

Multiple biogas utilization opportunities were identified, including on-site combustion in a

combined heat and power (CHP) system, local use of a cleaned biogas as CNG fuel, and pipeline

injection of biomethane produced from cleaning of the biogas. Utilization of biogas locally aligns

with Arlington County’s Community Energy Plan to reduce carbon footprint along with

generating and utilizing local, renewable energy.

As the project team considered the process that would produce a Class A biosolid, the reduced

energy intensity and carbon footprint required to achieve Class A were key differentiators of the

THP alternative.

ES.5.4 Social Criteria Evaluation

Alternative 3 (THP with Anaerobic Digestion) received the highest score in the social criteria

evaluation. Alternative 4 (Anaerobic Digestion with Drying) also received a high score. The

acceptability of Class A biosolids suitable for distribution with fewer restrictions was a key

differentiator for the two highest scoring alternatives. Concerns with thermal dryer (Alternative

4) potential emissions, in particular odors, led to a preference for THP.

Concerns with process and product odors, lower acceptability of a Class B product, and higher

traffic associated with product hauling all contributed to Alternative 1 (Lime Stabilization)

receiving the lowest score in the social criteria evaluation.

ES.5.5 Results of Evaluation

The individual scoring results for each criterion are presented in Section 11. Figure ES-3

presents the overall results of the scoring evaluation graphically. The ranking of the end results

indicates that Alternative 2, Mesophilic Anaerobic Digestion, scored the best against the

evaluation criteria. Alternative 3, THP pretreatment followed by anaerobic digestion, was the

second ranked alternative based on the scoring. Alternative 4 (Anaerobic Digestion + Drying)

ranked third and Alternative 1 (Lime Stabilization) ranked last.


ES-9

Figure ES-3. Results of Alternative Scoring

ES.6 Recommended Alternative The project team reviewed the final rankings and considered these rankings in determining the

recommended alternative. The Quadruple Bottom Line analysis was not the definitive means of

determining the preferred alternative, but did help the team clarify their thinking about what was

important and helped move the discussion forward as described below.

� The team agreed that all three digestion-based alternatives were preferred over lime

stabilization.

� The team also noted master planning goals included mitigating regulatory and social

acceptability risks by moving towards a Class A biosolids program. While there is no

foreseeable regulatory risk on Class B land application, implementation of a process that

produces Class A biosolids product will provide the County with added flexibility in seeking

product outlets. Improving the level of treatment can also address potential public concerns

related to product uses.

As the project team considered additional factors, such as the County’s Community Energy Plan,

Alternative 3 (THP followed by anaerobic digestion) was preferred over Alternative 4 (anaerobic

digestion and drying). The Community Energy Plan identified goals for reducing the County’s

carbon footprint and developing/ utilizing renewable energy. Each of these goals aligns with the

THP and anaerobic digestion processes.


ES-10

THP provides Arlington County an opportunity to recover multiple resources suitable for use in

the local area. These include a Class A biosolids product, biogas, and potentially recovered

phosphorus.

� The Class A biosolids product is able to be distributed to the public as well as to other

County departments and commercial entities. The biosolids will likely require additional

processing if local distribution is desired. Additional processing could include blending

with soil or bulking agent to create a soil amendment and developing a distribution center.

� Biogas can be used to generate steam for the THP process, heat process buildings, generate

electrical power with heat recovery, cleaned and converted to compressed natural gas

(CNG) for local use, or cleaned and injected into the natural gas grid. Initial review of biogas

utilization opportunities has identified the Arlington Rapid Transit (ART) bus fleet as a

potential CNG customer. The bus fleet has been converted to CNG with a fueling station

located across the street from the WPCP.

Implementation of THP with anaerobic digestion aligns with the goals Arlington County

established at the beginning of this project. Additionally, the recommendation to implement THP

with Anaerobic Digestion aligns the WPCP Solids Master Plan with the County’s Community

Energy Plan.

A critical element of the recommended plan involves the beneficial use of biogas generated as

part of the anaerobic digestion process. The potential impacts of various biogas uses on site to the

air quality in the WPCP vicinity is a subject of concern to civic and neighborhood groups. An

emission study was conducted to evaluate the potential contributions of air pollutants under

various biogas use scenarios. The study concluded that with proper use of control technologies,

biogas use on or off site will not significantly impact the air quality in the vicinity of the WPCP,

and is fully protective of the health of sensitive populations bordering the facility site. Detailed

evaluation results are presented in Appendix K.

While off-site options were considered early on in the project, the decision-making process in the

master plan yielded top-ranked alternatives that Arlington could construct, own, and operate.

The potential for regional partnerships remains an option for the County. For example, both DC

Water and Fairfax County have existing solids treatment infrastructure, and both indicated early

in the project that they might have available capacity to take some or all of the County’s residuals.

However, subsequent conversations indicated challenges to a long-term partnership such as

capacity limitations and logistics of implementation. Opportunities for regional partnerships will

be revisited during the next phase of the project.


ES-11

ES.7 Community Input Multiple meetings were held over the course of the project with the external stakeholder group.

The recommended alternative was presented at the June 22, 2017 public meeting. During that

meeting, external stakeholders provided feedback on the project. The general consensus was the

group appreciated the sound engineering, clear presentations, and patience County staff showed

through the master planning process. Most agreed that they enjoyed participating in thoughtful

discussions. They also liked the concept of energy recovery from the biogas, that the process could

be safely managed, and that the end result was a Class A product. They reported appreciating the

opportunity to see the master planning process evolve, and one noted, “It was beneficial that the

County set up the [stakeholder] group at such an early point in the process to get feedback so

early on.” Many of the stakeholders agreed that the recommended technology includes several

potential environmental and social benefits like providing a safe, nutrient positive soil

amendment product to the community, minimizing truck traffic into and out of the plant, and

generating a sustainable energy source for use by the plant, County and/or the community.

ES.8 Implementation of Master Plan Recommendations Figure ES-4 presents a simplified process flow for THP with anaerobic digestion. The two sources

of solids (primary sludge and secondary sludge) are blended, screened, and pre-dewatered ahead

of the THP reactors. Hydrolyzed solids from the THP reactors are digested and dewatered. The

dewatered solids can be land applied in bulk as a Class A biosolid or distributed to other markets

such as soil blenders.

Figure ES-4. THP with Mesophilic Anaerobic Digestion Process Flow

Table ES-2 provides a summary of the capital cost associated with the implementation of THP

with anaerobic digestion at the WPCP. The costs presented are planning level estimates which

reflect the engineer’s opinion of full implementation costs including project management, design,

construction, and construction management. The FY17-26 Capital Improvement Program also

included a sufficient contingency to account for changes in these costs over time.


ES-12

Table ES-2. Capital Costs for THP with Anaerobic Digestion

Process Capital Cost

Primary Solids Holding $ 700,000

WAS Holding $ 700,000

Blended Solids Holding $ 700,000

Screening $ 1,800,000

Pre-Dewatering $ 10,100,000

THP $ 17,900,000

Mesophilic Digesters and Building $25,600,000

Digested Solids Holding $ 3,400,000

Post-Dewatering $ 25,600,000

Biological Solids Processing Building Demolition $ 3,500,000

Allowance for Odor Control System Improvements $ 5,000,000

Alternative Capital Cost $ 95,000,000

Biogas Utilization $ 10,200,000

Planning level costs representing an accuracy of -30%/+50% Costs are

presented in 2017 dollars

Implementation of the recommendations from the Solids Master Plan is anticipated to occur over

the next several years with completion targeted for 2027. The next phase in the process is

engaging a Program Manager to serve as the owner’s representative through the process. Section

13 presents a proposed implementation schedule that begins with Arlington County procuring a

program manager in FY 2019. The program manager will prepare a Facility Plan that advances

the project to a 15-20% design level. The Facility Plan should confirm assumptions used in the

development of the Master Plan and determine whether a regional solution has become available

and is worth considering. A more detailed biogas utilization study and a nutrient sidestream

treatment analysis, including phosphorus recovery opportunities, are recommended components

of the Facility Plan. Arlington should revisit the assumptions of the Solids Master Plan in the

economic evaluation of biogas utilization as more information becomes available. Additionally,

the Facility Plan should develop preliminary process flow schematics and identify pipe routing

and other design details. Finally, the opinion of probable construction cost should be updated

with the Facility Plan.

Implementation of thermal hydrolysis pretreatment with anaerobic digestion, as recommended

in this Solids Master Plan, meets the goals and objectives established by Arlington County. The

recommendations provide a roadmap for the County to implement a long-term solids

management strategy that aligns with County energy goals, reduces greenhouse gas emissions,

and reduces the County's risk associated with the continued land application of Class B lime

stabilized biosolids.

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Section 1Project Background and Introduction

Arlington County WPCP treats incoming wastewater flows for the residents, businesses, and government agencies in the Arlington County sewer service area to protect public health and the environment. The plant produces alkaline-stabilized Class B biosolids that are beneficially reused as fertilizer. Recent facility upgrades have focused on improving the liquid treatment stream as part of the program to restore the Chesapeake Bay. This master planning project is primarily focused on planning to improve treatment of solids generated at the facility.

1.1 BackgroundThe Arlington County Water Pollution Control Plant (WPCP) is a 40 million gallons per day (mgd) advanced wastewater treatment plant (WWTP) located on South Glebe Road in Arlington, Virginia. The facility operates under various water quality and air quality permits which are provided in Appendix A. Facility discharge is regulated by Virginia Pollutant Discharge Elimination System (VPDES) Permit No. VA0025143, issued by the Virginia Department of Environmental Quality (VDEQ). The facility discharges treated effluent into Four Mile Run, part of the Potomac River subbasin in the Chesapeake Bay watershed.

Areas served by the WPCP include most of Arlington County, as well as areas of Falls Church, Alexandria, and Fairfax County (see Figure 1-1). The area is densely populated urban/suburban with a mix of residential, institutional, and commercial customers. Local population growth has been steady over the past 30 years and is expected to continue over the next 20+ years; however, the growth rates are expected to diminish over that time. In 2015, the estimated population of Arlington County was about 220,000 people and an estimated employment number of nearly 210,000. The average daily flow to the WPCP in 2015 was approximately 24 mgd.

The WPCP uses a combination of physical, chemical, and biological processes to treat wastewater to a high standard for discharge (see Figure 1-2). Incoming flow is treated by screening and grit removal before primary clarification. Primary effluent is sent to the secondary process, including the aeration basins and secondary clarifiers. To limit primary effluent flow to the secondary treatment facilities, part of the primary effluent is sometimes diverted to on-site flow equalization tanks for storage. Secondary effluent is treated by deep bed filtration (with denitrification), chlorination, dechlorination, and re-aeration before discharge. Iron salts are added at multiple locations in the process, primarily to help remove phosphorus (P) from the wastewater.

Solids removed in the primary and secondary clarifiers are treated with additional processes before being hauled off-site. Primary solids and waste activated solids are thickened, dewatered, and stabilized by adding lime to the dewatered cake. In 2015, approximately 30,000 wet tons of lime-stabilized biosolids were hauled from the site for beneficial use as Class B biosolids in bulk land application. Biosolids are currently hauled to land application sites in central and southern Virginia.

_̂Arlington CountyWPCP

£¤29

£¤1

£¤50

§̈¦395

§̈¦66

1 inch = 5,000 feet0 5,000 10,000 15,0002,500

Feet±Figure 1-1

Arlington County WPCPContributing Areas

LegendContributing Area

AlexandriaArlington CountyFairfax CountyFalls Church

Section 1 Project Background and Introduction

1-3

Figure 1-2. Existing Site Plan

The lime stabilization process was constructed in the mid-1990s and requires significant attention from WPCP operations and maintenance (O&M) staff. To address concerns with the current process sustainability and to plan for the future, Arlington County began developing this Solids Master Plan.

1.2 Program Goals and ObjectivesThe Solids Master Plan will modernize plant facilities (solids treatment) while creating opportunities for process sustainability and resource recovery.

The main plan drivers are to:

Replace aging infrastructure by identifying immediate and long-term needs.

Weigh the possibility of regulatory changes with other criteria to determine treatment options.

Make better use of valuable resources.


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Perform cost and risk analyses.

Select a long-term solids treatment solution and prepare capital funding program.

1.3 Summary of Previous Master Planning EffortsThe Arlington County WPCP Master Plan 2001 Update was developed to outline continuing capital improvements at the WPCP, specifically geared toward capacity increases and performance improvements. The Master Plan 2001 Update project team evaluated regulatory impact in the liquid and solids treatment areas, address aging infrastructure, and address capacity issues. A summary of the 2001 Master Plan is provided in Appendix B. The following issues were identified and addressed in the 2001 plan:

Redundancy and capacity of the BNR process – 1998 regulation changes required a BNR process that resulted in a plant de-rating to 30 MGD capacity. The Master Plan 2001 Update developed options to increase the capacity to 40 MGD.

More stringent effluent limits.

Biosolids management to ensure a means of ultimate disposal or reuse.

The final recommendations for solids improvements from the Master Plan 2001 Update are quoted below:

Continue to upgrade the existing lime stabilization system and its operation to reduce odor and improve the product for beneficial use. The County should recognize, however, that lime stabilization for beneficial use land application programs may not provide long-term acceptability.

Provide County budget for implementation of anaerobic digestion processing, as a minimum, within a decade. Anaerobic digestion options rated highly and provide increased flexibility for solids management. The County should be prepared to move ahead rapidly with anaerobic digestion if the situation warrants.

Provide County budget for processing beyond anaerobic digestion (e.g. heat drying) so that capital is available to implement biosolids processing to this extent, if necessary.

Continue to evaluate and assess biosolids processing technologies to determine if more cost-effective and environmentally friendly methods are available than digestion and heat drying.

Due to budgetary limitations, the improvement projects recommended in the 2001 Master Plan were ultimately not implemented. This was used as a driver for the current master planning project, as previous studies have also supported anaerobic digestion.

1.4 Project Methodology and Team MembersThe current master planning project focused primarily on solids management at the Arlington WPCP. The project team consisted of operations, maintenance, and management staff from the WPCP; CDM Smith consulting core team; a technical advisory committee; multiple subject matter experts, and external stakeholders. The team members met regularly over the approximately 18


1-5

month project period to advance the project from initial technology screening, to developing and evaluating solids management alternatives, and ultimately to identifying a preferred alternative and developing an implementation plan.

The consulting and advisory team members include:

CDM Smith represented by K. Richard Tsang, PhD, PE, BCEE and Jonathan Treadway, PE, BCEE

Rosegate Construction Solutions represented by Patti Psaris, PE

LA Stone, LLC represented by Lori Stone, PE

Environmental Finance Group represented by Scott Harder

Technical Advisory Committee Members:

John T. Novak, PhD, PE

Roger Tim Haug, PhD, PE, BCEE

Richard Kuchenrither, PhD, PE

Jan A. Oleszkiewicz, PhD, Peng, CEng

Water Environment Research Foundation (WERF) Collaborative Research represented by Lauren Fillmore, MS

Section 4 of this report describes the project evaluation methodology. The project was advanced through a series of six workshops mixed with process specific technical memorandum, multiple project team meetings, and site visits to review installations with equipment similar to the technologies being considered. The six workshops are identified as:

Workshop 1: Kick-off and Project Quality Management

Workshop 2: Develop Screening and Evaluation Criteria

Workshop 3: Technology Identification and Screening

Workshop 4: Process-Specific Technology Evaluation

Workshop 5: Review Alternative Process Trains

Workshop 6: Detailed Evaluation of Selected Alternatives

Attendance at each workshop included Arlington County staff representing the Department of Environmental Services, including the WPCP; consulting team members; members of the technical advisory committee; and representation by WERF. Workshop handouts and discussion were documented with meeting notes distributed to all attendees.

The master planning process also included an extensive outreach to solicit input into the planning process from stakeholders representing civic and community organizations in Arlington County.

Stakeholders represented include:

Four Civic Associations: Arlington Ridge, Aurora Hills, Crystal City, and Long Branch Creek


1-6

Arlington County Civic Federation (ACCF)

Fiscal Affairs Advisory Commission (FAAC)

Arlingtonians for a Clean Environment (ACE)

Environment & Energy Conservation Commission (E2C2)

A public relations consultant (SaViPR represented by Samatha Villegas, APR) was contracted to assist Arlington County and its communications team with the outreach program. The consultant developed a communications plan that provided guidance for the team. Six quarterly stakeholder meetings were conducted to update the group and to obtain feedback. The meetings allowed the stakeholders an opportunity to ask questions and provide feedback on goals, criteria, and desired outcomes for the project. A Solids Master Plan Project website was created that was accessible to the public through the County’s website.

In addition to the primary objective to prepare a solids master plan for the WPCP, the project included evaluating the disinfection system alternatives, process control system (PCS) improvement opportunities; and evaluating improvements for five specific process areas identified as immediate needs:

influent screening

primary scum collection

scum concentrator

gravity thickening

Motor Control Center -1 (MCC-1)

Separate technical memorandum presenting the results of the evaluations in the area noted were prepared and are included as Appendices.

A final component to the Master Planning project is the evaluation of the plant’s PCS and an evaluation of long-term alternatives for disinfection. The technical memorandum summarizing these efforts is provided as Appendix C and Appendix D, respectively.

2-1

Section 2Review of Existing Facilities

2.1 Overview of Plant ProcessFigure 2-1 presents a simplified process flow schematic for the WPCP. Preliminary treatment of influent flow is provided using bar screens and vortex-type grit separators. Excess water is removed from the screenings in the preliminary treatment process. The dewatered grit and screenings are collected in bins and hauled off-site for disposal by incineration.

Primary treatment is provided using eight rectangular clarifiers. Clarifier overflow is either pumped to equalization tanks or sent directly for secondary treatment. Scum from the primary clarifiers would normally be sent to the scum concentrator. The out of service scum concentrator is being repaired to bring it back into service. Secondary and tertiary treatment includes step feed aeration basins, secondary clarifiers, and denitrification filters. Ferric salts and sodium hypochlorite are added during the secondary and tertiary treatment processes, respectively.

The solids handling system at the Arlington County WPCP includes the following major treatment, storage, and conveyance units:

Gravity Thickeners

Dissolved Air Flotation Thickeners (DAFT)

Sludge Storage Tanks

Dewatering Centrifuges and Polymer System

Cake Conveyance and Storage

Lime Storage and Blending

Truck Loading

Odor Control System

Primary solids are thickened using two gravity thickeners, which are described in Section 2.2.1. Thickened solids from the gravity thickeners are blended with thickened waste activated sludge (WAS) from the DAFTs and sent to two blended sludge storage tanks. Overflow from the gravity thickeners and underflow from the DAFTs are sent back to the plant headworks. The sludge transfer pumps move solids from the blended sludge storage tanks to the sludge wet well. Centrifuge feed pumps move solids from the sludge wet well to dewatering centrifuges, which are housed in the dewatering building (DWB). Dewatered cake is sent to the lime stabilization process, while centrate is returned to the plant headworks. Major components of the solids handling system are described in the following sections. Condition assessments of solids processing equipment were conducted in 2016 in order to determine remaining useful life and rehabilitation needs.

PrimarySettling

TankClimber Bar

Screens

Potomac and Four Mile RunInterceptors

Four Mile RunRelief Pump

Station

EqualizationTanks Aeration

TanksSecondary

Clarifier

Blowers

Flow Equalization Pumping Station

Return Activated Sludge To

Aeration Tanks

AWT/PAF Plant Effluent Water Pump Station

Outfall to Four Mile Run

Water for Plant Use (PEW)

Wet Weather Facility

Optional

Optio

nal

Backwash Effluent Tanks

Overflow to Headworks

Gravity Thickeners

Blended Sludge Storage

Sludge Wet Well

Centrifuges

Cake

Centrate to Headworks

To Land Application

Lime Stabilization

Lime Silo

From Primary Settling Tanks

Dissolved Air Floatation Thickeners (DAF)

Underflow to Headworks

Screening, Grit and Scum Boxes

To Solid Waste Incinerator

Scum to Headworks

Waste Activated Sludge to Flotation Thickeners

Vortex Grit Collectors

PRELIMINARY TREATMENT BUILDING

ScumConcentrator

Sludge toGravity Thickeners

Equalization Tanks Drain to Four Mile Run Interceptor / PEC

A

C

B

B

C

A

Waste Activated Sludge from Wet Well

Chlorine ContactChambers

FILTRATION FACILITY

Figure 2-1Process Flow Diagram for Arlington County WPCP

Section 2 Review of Existing Facilities

2-3

2.2 Solids Handling and Treatment Processes2.2.1 Gravity ThickenersGravity thickening facilities at the Arlington County WPCP are used to thicken primary solids before blending with thickened WAS and storage in two 65-foot diameter solids storage tanks. The two tanks have a 23-foot side wall depth. The tanks have dome fiberglass covers with ventilated headspace for treatment by the South odor control system, which also serves the solids storage tanks.

Primary solids are pumped to the thickener at approximately 1.5 percent solids. Gravity thickener solids are removed at 3.5 – 4 percent solids. Elutriation water – up to 1,000 gallons per minute gpm per thickener – can be added to freshen the solids. Gravity thickener overflow is returned to the main liquids process at the plant headworks.

A condition assessment of the gravity thickeners was completed in 2016. Corrosion damage was observed on the inlet baffling and structural steel of the drive unit for Gravity Thickener #2. Staff indicated that the rakes on both gravity thickeners are damaged due to the corrosion caused by ferric chloride. The drive and collector units need rehabilitation including localized reinforcement, reanchoring, and recoating of metal surfaces.

The drive unit currently used for the gravity thickeners causes problems for the maintenance staff. The unit fails unpredictably and causes operational issues thickening the primary solids, impacting the centrifuge dewatering of all facility solids that are combined with the primary solids.

During the inspection, Gravity Thickener #2 sludge pumps were not in an operable condition in that Arlington maintenance staff had the ball check valves removed, but staff indicated that the pumps are operable, if needed. The County is in the process of replacing these pumps. Gravity Thickener #1 Sludge Pump 2 has damage to the middle piston, which allows sludge to leak past the piston packing and flow onto the floor. The lighting in the gravity thickener gallery is very limited and should be increased.

In addition, Gravity Thickener #2 has some slight structural damage, with concrete missing on the outside of the tank in various locations. Although the covers to the gravity thickeners are in good condition, the fiberglass is thin in some places.


2-4

Figure 2-2. Gravity Thickener and Fiberglass Cover

2.2.2 Dissolved Air Flotation ThickenersThe DAF system consists of two steel tank Eimco DAF units, two air compressors, two saturation tanks, dry polymer feed system, batch tanks, and aging tanks. Typically, one unit is in service and one unit is in standby. The DAF units receive waste activated sludge (WAS) at 1.0 – 1.2 percent solids, and produce thickened solids at approximately 2.4 percent solids concentration. The thickened solids are pumped using progressive cavity pumps and blended with gravity thickened solids. Blended sludge is stored in the sludge storage tanks (Section 2.2.3). DAF unit underflow is returned to the plant headworks.

Each DAF tank is approximately 50 feet long and 12 feet wide, with a side wall depth of 12 feet. The tanks bottoms have previously been epoxied to repair cracks. The chain and flight collector drives and saturation pumps operate on a timer, but do not have any other automatic controls. The ultrasonic transmitters by the beaches have been abandoned and are not energized. The DAF collector drive units have been recently modified to allow for speed control and to prevent over speeding, which damages the DAF collectors. Both DAFs have corrosion on their drive shafts and sprockets.

The dry polymer system for the DAF is mostly a manual operation. There are some hard wired interlocks to prevent damage to the rotor and stator on the polymer pumps. The polymer pumps and their Variable Frequency Drives (VFDs) have been replaced recently. Polymer can be added through a siphon, but at times operators have added polymer with a 5-gallon bucket. Staff report that the polymer system is problematic, with the polymer tank mixers often failing. The staff have not been able to determine the cause of the polymer mixer failures.


2-5

Base plates on the saturation pumps have significant rust from equipment age. The staff report that the saturation pumps cavitate due to the piping arrangement and the low suction head on the pump from water being saturated with air. Various pressure gauges on the DAFs are inoperable.

CDM Smith noted that #2 Air Compressor has short cycles, which causes excessive wear on the equipment. The compressor ran for 20 seconds and then remained off for approximately 5 minutes.

Figure 2-3. DAF Thickener2.2.3 Sludge Storage TanksTwo sludge storage tanks are installed and in operation. Both tanks are approximately 53 feet in diameter, with a side wall depth of 25.5 feet, not including the sloped bottom. The tanks receive solids from the gravity thickeners and DAFTs. The current operation is to fill one and drain the other tank. Two centrifugal transfer pumps were designed to transfer the solids from the storage tanks. WPCP staff indicate that the original pumps are operable, but have reduced flow capacity. Staff report the transfer pumps have never been able to meet design flow of 600 gpm. A third temporary disc pump was installed and has been tested to pump thickened solids to the centrifuge wet well. The disc pump is less efficient than the transfer pumps at the design point, requiring higher power.

The centrifuge wet well level is controlled by a position modulating valve in the DWB and manual valves in the feed and return lines of the solids storage tanks. The sludge transfer pumps run continuously; the modulating valve on the inlet to the centrifuge wet well will open or close based on the wet well level. Solids are returned to the sludge storage tank. For approximately 5.5 days each week, solids are directed to the centrifuge wet well. During centrifuge operations, less flow is returned to the sludge storage tanks, resulting in less mixing.

The odor control for the sludge storage tanks occurs by drawing air off with one of the two blowers on each tank. The blowers discharge into the suction header of the gravity thickener odor control fans.


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Figure 2-4. Solids Storage Tank

2.2.4 DewateringA four-story DWB houses the plant’s dewatering facilities, which include a polymer system, sludge feed pumps, and dewatering centrifuges. The DWB also contains storage areas for caustic, sulfuric acid, and lime, as well as two truck bays for loading of lime-stabilized cake. The L-shaped building is approximately 120 feet by 140 feet.

The centrifuge feed pumps are variable speed progressing cavity pumps capable of pumping 50 to 250 gpm. Three of the four pumps are dedicated to each of the three centrifuges. The fourth centrifuge feed pump is a common spare to the three dedicated feed pumps. All of the centrifuge feed pumps have evidence of lubricant leaking from the coupling and gearbox. Staff report the pumps have wear consistent with their age.

The DWB has three Andritz centrifuges to dewater thickened solids. They were installed in 1996 and are capable of dewatering a feed of 150 gpm to over 25% solids. The plant normally runs two centrifuges at a time, with a spare centrifuge available to dewater solids and maintain normal treatment plant operations. The centrifuges were recently rebuilt in-house. Each centrifuge discharges cake into one or two of the bin distribution screws, which move the cake into one of the four cake bins for storage.

The centrate is returned to the plant headworks. Staff report that the centrate lines are being affected by scum, oil, and grease, causing air gap clogging. Since the scum concentrator has been removed from service, the centrate lines are cleaned quarterly, and staff has had to add cleanouts in the centrate line to help clean the centrate lines.


2-7

Figure 2-5. Dewatering Centrifuge

2.2.5 Lime Stabilization and Truck LoadingDewatered cake is stored in four 1,500 cubic-foot bins, allowing dewatering and truck loading of lime-stabilized product to occur on different schedules. A complex series of conveyors and cake diverters is used to distribute dewatered cake to the bins.

The lime system contains a single silo used to store quicklime on-site used to stabilize the cake to meet Class B biosolids requirements. Currently the plant adds approximately 0.25 dry pound of lime for every dry pound of dewatered solids. The lime silo, which has a total volume of approximately 200 cubic yards, is normally filled using the transport truck and the truck’s air compressor to push the lime up and into the silo. An unloading control panel associated with the lime silo allows the plant’s air compressors to move the lime into the silo, but it is not currently used; its operational status is unknown.

A bag filter on top of the silo is used to capture the lime dust during filling operations. Lime from the silo is then fed through a line with compressed air to one of the two day tanks for use. From the day tank, the lime is added to the plow blenders where lime and solids are mixed together. The lime stabilization process is a labor-intensive, requiring the attention of multiple staff to monitor the system for lime dosing, mixing, and truck loading. Two bays and two primary loading conveyors are used for truck loading. The truck loading process is messy and requires multiple operators. The process is also a significant source of odor, including ammonia release from lime addition and organic reduced nitrogen (N) species (mercaptans, methylamines, etc.). Odor control systems are discussed in more detail in Section 2.3.


2-8

Figure 2-6. Lime Silo

2.3 Odor ControlThe Arlington County WPCP has three scrubber systems for solids handling odor control: the North scrubber, the South scrubber and the DWB scrubber. The South chemical scrubber services the Four Mile Run Pump Station, Preliminary Treatment Building (PTB), DAFTs, gravity thickeners, and solids storage tanks. Air is conveyed to the South chemical scrubber from these odor sources through a 54-inch diameter concrete process pipe that crosses South Glebe Road and was repurposed as a foul air duct. CDM Smith completed a Foul Air Study for the PTB and DWB in April 2017, which summarized the observations of a November 2016 site visit focusing on the odor control systems.

Two odor control fans are used for each gravity thickener. The individual fans draw suction on their respective gravity thickener and discharge the foul air to the North odor control scrubber for treatment. The fans for the solids storage tanks also discharge into the suction line of each pair of the gravity thickener odor control fans. The make-up air for the gravity thickeners is provided by a louver in each of the gravity thickeners’ respective domes, allowing fresh air to be pulled in. The staff report that the louvers have failed and are no longer able to operate as designed. At the PTB, only one of the two foul air fans that draw air from the building is functioning. The motorized dampers on are also stuck in the open position and cannot be closed by hand. While both exhaust fans in the DAFT building are operational, the tempered air supply system is not operational, resulting in significant negative pressure in the room.

The DWB chemical scrubber services the sludge processing areas of the building and the truck bays. The primary purpose of the DWB scrubber is to remove ammonia and reduced sulfur compounds. The DWB contains four odor control fans. Three exhaust fans draw suction off of their individual spaces and discharge to a common header, where an odor control system fan draws suction and discharges the combined air to the scrubber for treatment. Sodium hypochlorite is added to the scrubber to oxidize compounds in the foul air before discharge to the


2-9

environment. The plant has also added the ability to add sulfuric acid to help treat the ammonia in the foul air stream.

Figure 2-7. Dewatering Building Chemical Scrubber

2.4 Bio BuildingThe biological solids processing building (Bio Building) is a multi-story building located immediately south of the DWB. Although it has been generally abandoned, the building still serves as a bulk sodium hypochlorite and polymer storage facility and has active electrical panels. Some critical piping runs through the building, including the 8-inch sludge line between the solids storage tanks and the dewatering process.

Figure 2-8. Biological Solids Processing Building

3-1

Section 3

Solids Production and Basis of Planning

3.1 Objectives This section presents the results of CDM Smith’s review of historical biosolids production data for

the WPCP, as well as the projected flows and biosolids production through the 2040 planning

year. The purpose is to estimate the flows and biosolids loading at a level of accuracy needed for

master planning level projections and decision making. The projected solids loadings will form

the basis to define the future treatment needs for the Master Plan.

3.2 Historical Flows and Loadings Using historical operating data provided by WPCP staff, the following approach was used to

assess current solids production:

� Daily WPCP influent wastewater flows and quality data from January 1, 2013, through

December 31, 2015, were analyzed to evaluate influent mass loadings and peaking factors

for biochemical oxygen demand and total suspended solids. Influent and effluent flows and

concentrations provided for specific stages were used to determine mass loadings and

solids production from different processes within the plant.

� To determine the impact of recycle streams on influent concentrations of BOD and TSS,

Arlington County adjusted the recycle location to be downstream of the influent sampler

for approximately 2 months in 2016. This showed recycle streams, which include gravity

thickener overflow, DAF subnatant flow, and centrate flow, did not impact the overall

average; however, the short-term increases in TSS concentration that had periodically been

observed in the historical data were not seen during the trial period suggesting that

historical spikes in TSS concentration in the influent were likely associated with recycle

streams. CDM Smith applied a statistical analysis to the influent data and excluded outlier

data above the 95% confidence range to adjust for the spikes in influent TSS expected to be

associated with recycle streams.

� Maximum 7-day, 14-day, and 30-day flows and influent loadings were calculated as the

maximum of 7-day, 14-day, and 30-day moving average loads, respectively. The maximum

30-day load is also referred to as the average day maximum month (ADMM) and is typically

used to size most solids handling processes; sizing certain processes can be influenced by

the maximum 7-day and 14-day loads. The maximum 7-day, 14-day, and 30-day peaking

factors were calculated by dividing the maximum 7-day, 14-day, and 30-day load by the

average daily load for each year of the historical data period.

� Primary solids and WAS quantities were calculated from available historical data. Primary

solids were calculated using the difference in flow and concentration between primary

clarifier influent and effluent. WAS generation was calculated as the metered WAS flow

multiplied by the WAS solids concentration. Unit mass of solids production in pounds per

million gallons (lb/MG, normalized to WPCP effluent flows) were calculated as an annual

Section 3 • Solids Production and Basis of Planning

3-2

average. Influent flow was typically 3-5 mgd higher than effluent flow. The impact of

recycle flows, filter backwash in particular, is considered the reason. Effluent flow data was

used as a surrogate for influent flow as it is more representative of raw influent flow to the

WPCP.

� Maximum 7-day, 14-day, and 30-day moving average solids production were calculated and

peaking factors were developed by comparing the annual average against the relevant

maximum solids production.

� Based on the historical data, a mass balance for the current WPCP solids process was

developed, identifying the average flows and solids throughput for each treatment process

stage.

Influent flows in mgd were assumed to be equal to the outfall flow as provided by WPCP. Average

influent flows, concentrations, and mass loadings for BOD and TSS are summarized in Table 3-1.

7-day, 14-day, and 30-day maximum flows and loads are presented in Table 3-2.

Table 3-1. WPCP Average Influent Flows and Mass Loadings

Annual Average

Year Flow (mgd) BOD (mg/L) BOD (lb/day) TSS (mg/L) TSS (lb/day)

2013 20.8 329 57,100 248 43,000

2014 23.0 320 61,400 228 43,800

2015 23.4 309 60,200 233 45,500

Average 22.4 319 59,500 237 44,100

Wastewater flows to WPCP from 2013 through 2015 ranged from 20.8 to 23.4 mgd average

annual daily flow (AADF). Average influent TSS loads increased slightly over the same time

period, with annual averages increasing from 43,000 lb/day to 45,500 lb/day. BOD loads have

also increased over the period from 57,100 lb/day to 60,200 lb/day.

Table 3-2. WPCP Maximum Influent Flows and Mass Loadings

Year

7-Day Maximum

Flow (mgd) BOD (lb/day) TSS (lb/day)

2013 29.1 90,500 77,300

2014 34.4 101,600 87,000

2015 30.1 100,800 79,900

Average 31.2 97,600 81,400

Year

14-Day Maximum


2013 25.1 81,200 63,400

2014 29.6 86,900 80,200

2015 29.4 92,300 73,900

Average 28.0 86,800 72,500


3-3

Table 3-2. WPCP Maximum Influent Flows and Mass Loadings (continued)

Year

30-Day Maximum


2013 23.6 68,700 52,700

2014 27.8 79,200 72,300

2015 27.8 83,900 67,100

Average 26.4 77,300 64,000

Table 3-3 presents the 7-day maximum, 14-day maximum, and 30-day maximum peaking factors

for influent flow, BOD loading, and TSS loading. The peaking factors fall within the range

presented in guidance documents and industry standards.

Table 3-3. WPCP Influent Peaking Factors

Flow

Year 7-Day Peaking Factor 14-Day Peaking Factor 30-Day Peaking Factor

2013 1.40 1.21 1.14

2014 1.50 1.28 1.21

2015 1.29 1.26 1.19

Average 1.40 1.25 1.18

BOD


2013 1.58 1.42 1.20

2014 1.65 1.41 1.29

2015 1.67 1.53 1.39

Average 1.63 1.45 1.29

TSS


2013 1.80 1.47 1.22

2014 1.99 1.83 1.65

2015 1.76 1.62 1.49

Average 1.85 1.64 1.45

Figures 3-1, 3-2, and 3-3 show the influent wastewater flow, BOD mass loading, and TSS mass

loading, respectively, at WPCP over the past 3 years. In general, flows are highest in the spring

and early summer. Influent BOD and TSS loadings are highest in the summer months.


3-4

15.0

20.0

25.0

30.0

35.0

40.0F

low

(m

gd

)

-

40,000

80,000

120,000

160,000

200,000

BO

D (

lb/

da

y)

-

40,000

80,000

120,000

160,000

200,000

TS

S (

lb/

da

y)

Figure 3-2. BOD Influent Loading at WPCP, January 2013 through September 2015

Figure 3-3. TSS Influent Loading at WPCP, January 2013 through September 2015

Figure 3-1. Flow to WPCP, January 2013 through September 2015


3-5

3.2.1 Historical Solids Production

Arlington County staff provided historical data for critical parameters from daily operations

summaries at WPCP. These data formed the basis of the solids mass balance. When parameters

required for this analysis were not available from daily records, they were estimated based on the

available data and the anticipated performance of the treatment equipment.

The results of the historical solids production data analysis appear below in Tables 3-4 and 3-5.

Table 3-4. WPCP Historical Solids Production Data

Table 3-5. WPCP Annual Average Solids Loads, Normalized to Influent Flows

Year

Primary TSS Removal

(%) Primary Solids

(lb/MG)

WAS

(lb/MG) Combined Solids2

(lb/MG)

2013 72% 1,482 746 2,228

2014 66% 1,260 839 2,099

2015 75% 1,452 878 2,330

Average 71% 1,398 821 2,219

1. P.F. = Peaking Factor

2. Combined solids based on annual average data and is not necessarily the sum of primary solids and WAS.

Year

Primary Solids (lb/day)

Annual Avg. Day

7-Day 7-Day 14-Day 14-Day 30-Day 30-Day

Max P.F.1 Max P.F. Max P.F.

2013 30,800 60,600 2.0 50,300 1.6 41,000 1.3

2014 29,000 73,900 2.5 66,900 2.3 57,200 2.0

2015 33,900 62,700 1.8 62,700 1.8 52,400 1.5

Average 31,200 65,700 2.1 60,000 1.9 50,200 1.6

Year

WAS (lb/day)

Annual Avg. Day


Max P.F. Max P.F. Max P.F.

2013 15,500 28,300 1.8 26,200 1.7 25,000 1.6

2014 19,300 47,800 2.5 44,800 2.3 39,600 2.1

2015 20,500 33,200 1.6 29,300 1.4 26,600 1.3

Average 18,800 36,400 2.0 33,400 1.8 30,400 1.7

Year

Total Solids Production (lb/day)

Annual Avg. Day


Max P.F. Max P.F. Max P.F.

2013 46,300 73,800 1.6 64,700 1.4 62,400 1.3

2014 48,900 83,100 1.7 67,300 1.4 64,100 1.3

2015 54,300 82,600 1.5 78,100 1.5 70,100 1.3

Average 49,800 79,800 1.6 70,000 1.4 65,500 1.3


3-6

As Table 3-5 shows, the average primary solids production over three-year period was

1,398 lb/MG (normalized to effluent flows). The calculated WAS production over the same period

was 821 lb/MG (also normalized to effluent flows). The combined solids production calculated

over the time period was 2,219 lb/MG (0.26 kg/m3), which is consistent with typical combined

solids production at municipal WWTPs based on CDM Smith’s experience with facilities of similar

size and treatment process. It is also consistent with the benchmark of 0.25 kg/m3 cited by WEF

Manual of Practice No. 8 for domestic WWTPs (Water Environment Federation (WEF), 2010).

3.2.2 Solids Mass Balance

The historical operations data from Arlington County was used to construct a mass balance for

the WPCP solids treatment process. When solids loads or other parameters required for this

analysis were not available from daily records, they were estimated based on assumptions

regarding the performance of the treatment equipment (e.g., solids capture in the DAFT). The

mass balance was calibrated to correlate to typical quantities of dewatered biosolids hauled from

the WPCP.

The following assumptions were made to create this mass balance:

� Primary influent flow is calculated as the sum of four flows: measured influent flow

(includes raw influent + filter backwash), gravity thickener overflow, DAF subnatant flow,

and centrate flow. The latter three are recycled flows, which are typically pumped back to

the headworks.

� Primary solids concentration is 1.5 percent, based on the average solids loading and flow of

primary solids to the gravity thickeners in the data provided.

� To balance flows in and out of the gravity thickeners, an average of 65 gpm of treated water

was assumed to be added as either elutriation water or surface spray.

� Because DAF thickener subnatant and thickened solids concentrations were not available,

the mass balance around the DAF thickener was calculated assuming a solids capture rate

of 95 percent.

� The DAF subnatant flow is calculated as the difference between the WAS flow and

thickened WAS flow.

� Combined solids are calculated as the sum of gravity thickener underflow and thickened

WAS flow. The amount of combined solids does not change while inside the solids blending

tank.

� The mass balance around the dewatering centrifuges was calculated assuming that

95 percent of solids entering the centrifuges become dewatered cake.

� Lime addition, in pounds per day as CaO, was assumed to be 20 percent of the total

dewatered cake in dry weight (i.e., 500 lb lime per dry ton of dewatered cake).

Figures 3-4 and 3-5 present the liquids and solids flows at WPCP, respectively, with each

relevant process flow labeled as a letter, A through O. Tables 3-6 and 3-7 present the average


3-7

liquids and solids process flows, respectively, based on the 2013 to 2015 plant data. Note that the

units of measurement for the solids process are different from those used for the liquid process.

Figure 3-4. Arlington County WPCP Liquids Flow Schematic

Figure 3-5. Arlington County WPCP Solids Flow Schematic


3-8

Table 3-6. WPCP Liquid Process Flows at 22.5 mgd

Step A B C D E F G

Flow (mgd) 22.5 23.0 23.0 26.1 22.5 0.251 0.200

TSS (mg/L or % Solids)1 237 257 107 9.1 1.3 1.5% 913

Solids Load (lb/day) 44,430 49,350 20,470 1,990 250 30,400 1,520

1. Values are in mg/L unless designated as a percentage (%).

Table 3-7. WPCP Solids Process Flows at 22.5 mgd

Step H I J K L M N O

Flow (gpd1) 88,000 207,000 90,000 117,000 178,000 - 185,000 -

TSS (mg/L or % Solids)2 4.0% 1.1% 2.4% 965 3.2% - 1,596 30%

Solids Load (lb/day) 28,880 18,880 17,940 940 46,820 12,170 2,460 60,860

1. gpd = gallons per day.

2. Values are in mg/L unless designated as a percentage (%).

3.3 Future Biosolids Production 3.3.1 Future Wastewater Flows

To calculate future wastewater flows for the WPCP, projected increases in population and

employment were used to estimate the base sanitary flow increases using rates of 100 gallons per

capita daily for residential development and 35 gallons per employee daily for non-residential

flows. These population increases were based on the Arlington County Department of Community

Planning, Housing and Development – Planning Division, which provided estimates of

employment and population for census blocks.

The estimated sanitary flow increases, using 2010 as the base year, were presented in planning

work completed by Arlington County, such as the Sanitary Sewer Model Update and Study of

Crystal City Development (January 31, 2014). The flow increases, calculated based on residential

and employment projections at the census block level, are presented in Table 3-8. The current

split of base sanitary flows is approximately 60 percent residential and 40 percent non-

residential. The following assumptions were used to develop estimated flow increases.

� The population and employment projections do not include other jurisdictions contributing

flow to WPCP. However, the projections do include areas of Arlington County (North

Arlington) where the wastewater flows are conveyed to District of Columbia Water and

Sewer Authority (DC Water). The projection methodology assumes that flows from areas

served by the WPCP in the City of Falls Church, City of Alexandria, Fairfax County, and

Arlington County will exhibit growth rates similar to those experienced across Arlington

County. The age and development density of both areas is roughly equal, and the project

team agreed to work with similar growth rates for both. Potential high density projects in

areas outside of Arlington County’s jurisdiction could impact this assumption; however,

Arlington County could negotiate acceptable flows from the potential development areas if

allocated flows are exceeded.


3-9

� Employment numbers for the Pentagon and National Airport are inherent in the base 2010

flows. Employment projections provided by Arlington County from 2008 did not indicate

projected increases for either of these locations.

The Arlington County Water Master Plan Update (May 12, 2014) assumed that base sanitary flow

accounted for approximately 80% of the total flow seen annually at the WPCP when the base

sanitary flow equaled the billed water amount. The additional 20% of the annual flow resulted

from dry weather and wet weather inflow and infiltration, particularly groundwater infiltration.

The flow factors (100 gpd/resident and 35 gpd/employee) already include allowance for inflow

and infiltration. As a result, the flow increases calculated in Table 3-8 were applied without any

adjustment. The table presents the actual and projected plant flows in 5- year increments from

Year 2015 to Year 2040.

Table 3-8. Estimated Arlington County Base Sanitary Flow Increase from 2010 (mgd)

Flow Type 2015 2020 2025 2030 2035 2040

Residential (100 gpd/capita) 1.24 2.32 2.91 3.50 3.73 3.97

Non-residential (35 gpd/employee) 0.85 1.51 2.07 2.29 2.64 2.75

Total Base Sanitary Flow Increase from 2010 (mgd) 2.09 3.82 4.97 5.79 6.37 6.72

Average Annual Plant Flow (mgd) 24.1

(actual) 29.8 31.0 31.8 32.4 32.7

2010 average annual flow = 26.0 mgd

Blocks with employment decrease were assumed to have zero growth

Arlington County’s efforts to reduce the amount of inflow and infiltration may impact the actual

plant flow. Efforts such as rehabilitating the sanitary sewer system and redevelopment requiring

disconnecting foundation drains from the sanitary sewer could reduce the per capita flow factors

used. Installing water saving fixtures will reflect both a reduction in the billed water amount and

the flow factors used. Per capita flow factors have decreased in recent years as a result of these

efforts.

Solids production rates have been determined using 2013 – 2015 operating data and flow

records. Assuming the same level of inflow and infiltration looking ahead, the influent

concentration (TSS and BOD) and solids production rates normalized to unit volume of flow are

assumed to be the same. The mass loading, calculated by multiplying the flow times the

concentration, is expected to increase with flow. If inflow and infiltration are significantly

reduced or the presence of water saving fixtures increases, this would increase the influent TSS

and BOD concentrations and solids production rates (in lb/MG). However, the mass loadings

(computed as flows times concentration) are expected to remain similar and a function of the

number of residents and employees.

3.3.2 Summary and Conclusions

Figure 3-6 presents wastewater flow projections starting in 2020, at 5-year increments. Despite

a slight downward trend in flows between 2002 and 2015, wastewater flows are projected to

increase between 2020 and 2040. The projections show an increase in wastewater flow from 29.8

mgd in Year 2020 to 32.7 mgd in Year 2040. This is an average increase of approximately 0.5%


3-10

per year, although the rate of increase diminishes with time. The VDPES permitted capacity of

the WPCP is 40 mgd, and it is recommended this capacity be used to develop improvements

associated with the solids management facilities.

The flow projections developed for this report are based on conclusions from previous studies

and are adjusted based on recent plant operating records. Key projection assumptions include:

� Development and growth rates within Arlington County are similar to neighboring

jurisdictions.

� Employment numbers for the Pentagon and National Airport will remain relatively

constant.

� Per capita loadings of BOD and TSS will not be significantly impacted by efforts to reduce

inflow and infiltration or increases in water saving plumbing fixtures.

Figure 3-7 presents influent BOD and TSS loading projections starting in 2020, at 5-year

increments. The projections show an increase in BOD loading from 83,000 lb/day in 2020 to

91,100 lb/day in 2040, and an increase in TSS loading from 63,400 lb/day in 2020 to 69,500

lb/day in 2040. It should be noted that efforts by Arlington County aimed at conserving water

through low-flow plumbing fixtures and other measures, coupled with collection system

improvements aimed at reducing inflow and infiltration, have the effect of concentrating influent

loadings to the WPCP. The result is that influent loadings of BOD and TSS may be increasing while

flow to the treatment plant remains steady or decreases. When planning for future solids

facilities, the project team considered influent loadings to be a more viable method of solids

projections than wastewater flow projections.

29.831.0 31.8 32.4 32.7

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

2000 2005 2010 2015 2020 2025 2030 2035 2040 2045

Infl

ue

nt

(mg

d)

Wastewater Flow Projections

Recorded Plant Flow Projected Flow

Figure 3-6. Projected WPCP Flows through 2040 (mgd)


3-11

3.3.3 Biosolids Production

To determine planning criteria for the WPCP, the design solids treatment capacity will be

described in terms of the ADMM flow that the solids processes are expected to treat. Table 3-9

summarizes the projected wastewater flows and solids production, calculated in the mass

balance, at the plant through 2040 and at the design capacity of 40 mgd. The combined solids

production at WPCP is 2,219 lb/MG on an annual average basis and a maximum month solids

production of 2,885 lb/MG. Design capacity values are used for sizing and costing capital

improvements, while projected solids production values through 2040 are used for estimating

operating and maintenance costs.

3.4 References Arlington County Forecast Round 8.2: Population and Employment Forecasts. Arlington, Virginia:

Arlington County Department of Community Planning, Housing and Development, April 2014.

Arlington County Water Master Plan. Arlington, Virginia: Arlington County Department of

Environmental Services, May 2014.

Design of Municipal Wastewater Treatment Plants: WEF Manual of Practice No. 8, ASCE Manuals

and Reports on Engineering Practice No. 76, Fifth Edition. Water Environment Federation:

McGraw-Hill Professional, 2010.

Sanitary Sewer Model Update and Study of Crystal City Development, Technical Memorandum,

January 31, 2014

83,00086,400 88,600 90,300 91,100

63,400 65,900 67,600 68,900 69,500

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

2010 2015 2020 2025 2030 2035 2040 2045

Infl

ue

nt

Lo

ad

ing

(lb

/d

ay

)Influent Loading Projections

BOD BOD (Projected) TSS TSS (Projected)

Figure 3-7. Projected WPCP Influent Loading through 2040 (mgd)


3-12

Table 3-9. WPCP Projected Solids Production

Year

Projected Annual

Average Flow (mgd)

Primary Solids (lb/day)

Annual Avg. Day

Max

7-Day

Max

14-Day Max

30-Day

2020 29.8 41,700 88,400 80,400 67,300

2025 31.0 43,400 92,000 83,600 70,000

2030 31.8 44,500 94,400 85,800 71,800

2035 32.4 45,300 96,200 87,400 73,200

2040 32.7 45,700 97,100 88,200 73,900

Design 40.0 55,900 118,800 107,900 90,400

1. Total solids production at the maximum 7-day, 14-day, and 30-day condition is less than the sum of the maximum 7-day, 14-day, and 30-

day primary solids production and maximum 7-day, 14-day, and 30-day WAS production since peaking factors for each grouping are

different due to differences in timing between the maximum conditions.

Year

Projected Annual

Average Flow (mgd)

WAS (lb/day)

Annual Avg. Day

Max

7-Day

Max

14-Day Max

30-Day

2020 29.8 24,500 48,400 44,400 40,500

2025 31.0 25,500 50,300 46,200 42,100

2030 31.8 26,200 51,600 47,400 43,200

2035 32.4 26,700 52,600 48,300 44,000

2040 32.7 26,900 53,100 48,700 44,400

Design 40.0 32,900 65,000 59,600 54,300

Year

Projected Annual

Average Flow (mgd)

Total Solids Production

(lb/day)

Annual Avg. Day

Max

7-Day

Max

14-Day Max

30-Day1

2020 29.8 66,200 106,200 92,900 87,200

2025 31.0 68,900 110,500 96,600 90,700

2030 31.8 70,700 113,400 99,100 93,000

2035 32.4 72,000 115,500 101,000 94,800

2040 32.7 72,600 116,600 101,900 95,700

Design 40.0 88,800 142,600 124,600 117,100

4-1

Section 4Evaluation Methodology and Criteria

This section describes developing the decision-making framework for the Solids Master Plan, specifically focusing on criteria to be applied to evaluate long-term solids management alternatives. These criteria were mutually developed by project stakeholders in a workshop setting. Performance measures developed after the workshop that were used for scoring and ranking purposes are also described.

Two separate sets of criteria were developed to guide project decision making; each set was used for different purposes:

Screening Criteria: These criteria, typically considered fatal flaw criteria, are applied to the wide range of available solids processing technologies for the sole purpose of screening technologies that are not suitable at the Arlington County WPCP.

Evaluation Criteria: These criteria are more numerous and more specific. They are applied to assess the relative merits of long-term alternatives developed from technologies that remain through the screening process. These criteria support a “Quadruple Bottom Line (QBL)” (e.g., economic, social, environmental, and operational elements) approach to evaluate alternatives.

Applying these criteria in the context of the overall project is shown in Figure 4-1.

Developing these criteria sets as well as prioritizing them and associated performance measures are described below.

4.1 Screening CriteriaTable 4-1 presents screening criteria developed for the project, as well as a rubric and associated score, for each criterion.

When the scoring system was discussed during Workshop 2, it was determined that a score of 0 for any criterion would not be considered a fatal flaw and a basis for elimination; instead, all technologies would be scored. At the end of the screening process, those technologies receiving the lowest scores were dropped from further evaluation. Technologies with a score of 8 or above were kept; however, some technologies with lower scores were kept based on owner or County interest and Technical Advisory Committee input.

Section 4 Evaluation Methodology and Criteria

4-2

Table 4-1. Screening Criteria

CriterionComparative

Basis Description ScoreResearch and Development Status

Technologies in the early development stage and/or bench-scale tested or proof of concept pilot scale. 0

Technologies have been successfully tested at a demonstration scale or a sufficient scale that can establish the basis of the first generation of full-scale facilities in a relevant environment.

0Emerging Technology Demonstrations or First Generation Technologies

Technologies have been successfully applied at 2 to 4 full scale facilities in an operational wastewater environment. 1

Production & Implementation

Technologies have been qualified through testing and implemented under full operational conditions and have some degree of initial use, but are not considered established in the wastewater sector.

1

Conventional Technologies considered established and have been typically used at treatment facilities or have been available and widely implemented for more than 5 years.

2

Development Status (based on WE&RF Technical Development Level)

Adaptive Use

Some wastewater treatment processes have been established for years, but their use has not been static. In some cases, an established technology may have been modified or adapted resulting in an emerging technology. In other cases, a process that was developed to achieve one treatment objective is now being modified to serve in different ways or to achieve additional treatment objectives. During the operation of treatment systems using these established technologies, engineers, and operators have altered and improved their efficiency and performance.

1

Small Primarily for facilities treating less than 5 dry tons per day (DTPD) of biosolids 0Typical Application

ScaleMedium Primarily for facilities treating from 10 to 100 DTPD of biosolids 2

Large Requires purchase of adjacent or off-site property to adequately fit technology and ancillary systems to the site 0

Moderate

Requires no additional land purchase but may require demolition or relocation of existing facilities (functional or non-functional) to accommodate technology footprint and ancillary systems

1

MinimalEasily fits on the existing plant site with minimal footprint and minimal demolition or relocation required to accommodate technology and ancillary systems

2

Site Requirements (on-site, except where noted)

Offsite Solids processed elsewhere 2High Higher than current stabilization/end use costs 1

Relative CostsModerate Comparable to or lower than current stabilization/end use costs 2Unlikely History indicates that permit could not likely be obtained 0Uncertain Process is new and/or has varying permitting history 1PermitabilityEasy Process has been permitted in the past and/or no permitting

difficulties are expected 2


4-3

4.2 Evaluation CriteriaEvaluation criteria developed for the Arlington County WPCP Solids Master Plan have been organized into four categories in accordance with a QBL approach: economic, environmental, social, and operational. Allocating the evaluation criteria into these categories helped identify alternatives that best meet Arlington County’s overarching goals.

Criteria development was a multi-step process (Figure 4-2). The project team first developed preliminary criteria, which were finalized and ranked through a pairwise metric comparison (PMC) during a preliminary workshop and subsequent stakeholders meeting. Pairwise comparison worksheets were completed by Arlington County DES and WPCP staff as well as representatives for the external stakeholders group. As a final step in the process, the project team assigned performance measures to each criterion. These steps are further described in the following sections.

4.2.1 Evaluation Criteria DevelopmentEvaluation criteria were used to assess the relative merits of solids management alternatives that remained after the screening process. The project team discussed and finalized evaluation criteria and definitions during Workshop 2. The consensus for criteria and definitions from the workshop are presented in Table 4-2 and generally reflect criteria included in the Scope of Services for the project.

Figure 4-1. Application of Screening and Evaluation Criteria for Solids Plan Development

Figure 4-2. Evaluation Criteria Development Process


4-4

Note that the criterion presentation in the table reflects an allocation into QBL categories. The allocation, initially presented to the County along with preliminary evaluation criteria, was finalized during a project workshop involving the project core team and technical advisory committee.

Table 4-2. Evaluation Criteria

Criterion Description

Economic

Capital CostAssesses upfront project costs including design, construction, construction oversight, capitalized internal labor expenses and other soft costs including legal and administrative expenses.

Total Annual CostReflects annual expenses to operate and maintain the facilities delivered by the alternative, as well as offsite costs for solids end use/disposal and potential revenues/offsets for byproduct (solids, biogas) use.

Life Cycle Cost

Reflects total project cost over a 20-year planning period, considering upfront capital costs, annual operating costs, annual maintenance costs, potential revenues or offsets from resource recovery, periodic refurbishments, periodic replacements, and a salvage value measured at the end of the twentieth year.

Financial Options/ Risk Offset

Addresses financing options like private public partnerships, PPA, and ESCO. Could also include innovative financing or ownership and lease options, where they exist and can offset the risk either financially by the County or operationally through vendor maintenance/performance agreements.

End Use Management and Control

Measures the reliability of the end use outlet (e.g., how likely is it that the final use outlet will be available during the project life), as well as the ability to control/mitigate and respond to off-site problems (e.g., "bad load" applied, odor incident in field, hauling problems off-site).

Operational

Flexibility

Addresses flexibility with respect to implementation (construction phasing), expansion potential, and diversification potential for outlets. Greater product flexibility means there are multiple different final use outlets (based on availability and cost) that can be accessed with little or no impact on equipment or operations.

Operability and Safety

Focuses on a broad range of O&M issues, including complexity, potential training needs, and safety. Considers complex systems will generally be more susceptible to downtime than less complex systems. Complexity also impacts the skill level required for O&M staff, and most likely require more training. Considers the level of operational safety provided by the system.

Proven System/ Technology

Measures the relative experience of candidate systems, focusing on operating history and performance.

Reliability

Reliability is the ability to effectively monitor, assess, predict, and generally understand the working of an alternative and its assets to successfully deploy a cost-effective and optimal maintenance strategy. Reliable functioning allows routine and predictive maintenance to be identified, dominant failure modes to be ascertained, and consequences of failure to be estimated with a degree of confidence.

ConstructabilityMeasures both the ability to construct an alternative and reflects both unforeseen site conditions as well as physical limitations that might be presented with respect to site constraints and/or existing facilities.


4-5

Criterion Description

MOPO/ Impacts on Plant Processes and Facilities

Measures the potential impacts of an alternative on other plant processes and facilities. Key considerations include: maintenance of plant operations (MOPO) during construction (e.g., minimization of plant outages), the impact of constructed facilities on plant processes and the potential need for additional processes to address those impacts (i.e., side-stream treatment, PEW treatment required), and ease of integration with other plant processes.

Environmental

Resource Recovery Potential

Addresses the level of product resource recovery that can be achieved with an alternative, with a focus on energy, nutrients, and organics.

Energy Intensity Considers the relative energy requirements (energy intensity) that the system might require.

Carbon FootprintAssesses the magnitude of air emissions impacts, including greenhouse gas (GHG) emissions. For example, what is the estimate of GHG emissions for the anticipated process and final use associated with that process? Also includes carbon sequestration.

Regulatory Permits

Deals with the difficulty and timeframe needed to permit the technology with respect to air quality, process and selected final product use.

Gas and Product Quality

Addresses the expected energy and biosolids product quality characteristics and the ability to produce them reliably and consistently to meet the intended product use (e.g., digester gas, CHP, steam, etc. and various biosolids products).

Social

Odor Generation Potential/Reduction

Considers both process and product odor. For processing, it reflects the odor potential associated with the process exhausts at the plant site and how easily they can be mitigated. With respect to products, assesses the comparative product odor compared to other biosolids products.

AcceptabilityAssesses community impacts and acceptability associated with both the process and final use option. Encompasses noise, visual impacts, odor concerns, and other concerns that might be voiced by communities around the WPCP and at product end use locations.

HaulingReflects the potential for neighbor impacts and complaints due to truck traffic at the plant site and product end use locations (land application sites), and potential mitigation needs to address related concerns.

4.2.2 Criteria PrioritizationWhile each criterion listed in Table 4-2 is important to Arlington County and project stakeholders, the relative importance of each criterion differs. To determine the extent of this difference (so that it could be reflected in project evaluations), a PMC exercise was conducted with the stakeholders, which required the completing a matrix worksheet (Figure 4-3).

Over twenty participants completed the PMC worksheet. Participants included staff representing management, operations, and maintenance with the Arlington County WPCP and other participants/stakeholders from DES, as well as external stakeholders representing community and civic associations participating in the project.


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Figure 4-3. Paired Metric Comparison Matrix

For the exercise, each participant was given a worksheet to compare each criterion against each other, comparing one pair of criteria at a time. Participants were asked to record which criterion of the pair was more important to them, and then assign a factor (from 1 to 3) indicating how much more important it was using the following guidelines:

1 = slightly more important

2 = moderately more important

3 = significantly more important

As each participant completed the worksheet, their scores were compiled, providing the relative priority, or weight for each evaluation criterion. These criteria weights were then determined using the following equation:

Criterion weighting = PMC Score of Criterion / Highest PMC Score of an Individual Criterion * 100%

Based on this methodology, the highest ranked criterion received a weighting of 100 percent. The results of the paired metric comparison are summarized in Table 4-3. The results in Table 4-3 are summarized graphically by criterion and by category in Figure 4-4.


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Table 4-3. Evaluation Criteria Weighting

Evaluation Criteria Criteria Weighting (%)

Economic

Capital Cost 51%

Annual O&M Cost 42%

Life Cycle Cost 68%

Financial Options/ Risk Offsets 20%

End Use Management & Control 32%

Operational

Flexibility 38%

Operability and Safety 91%

Proven System/Technology 43%

Reliability 78%

Constructability 64%

Impacts on Plant Processes 64%

Environmental

Resource Recovery Potential 39%

Energy Intensity 38%

Carbon Footprint 38%

Regulatory Permits 100%

Gas and Product Quality 29%

Social

Odor Generation Potential/Reduction 75%

Acceptability 66%

Hauling 16%

Capital cost received a weighting factor of 51%, which is near the average of all criteria. Life cycle cost was the economic criteria with the highest significance, having a weighting factor of 68%. In general, operational criteria were deemed especially important to the County, as reflected in the large number of high-scoring criteria. Input from external stakeholders reflected an additional emphasis on social and environmental criteria. Further discussion of the evaluation criteria and overall assessment results are provided in Section 11.


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Figure 4-4. Evaluation Criteria Weighting

4.2.3 Performance Measure DevelopmentPerformance measures provide a quantitative basis on which to compare alternatives with respect to evaluation criteria. These measures were established for each evaluation criterion (for some criteria, multiple performance measures were defined) (Table 4-4). As shown in the table, ratings between 1 and 5 are assigned to each performance measure (the basis for these ratings is also shown in the table), with an emphasis on the relative performance of alternatives when compared to each other. Where multiple performance measures are established for a criterion, scores for those measures will be averaged to reach a composite score for the criterion. In some cases, there was overlap between the various performance measures. For example, hauling, odor generation, and product quality are interdependent; participants discussed these issues and rated the alternatives accordingly.


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Table 4-4. Preliminary Performance MeasuresRating Basis

1 to 5 (Worst to Best)

Evaluation Criteria Performance Measure 1 3 5

Economic

Capital Cost

Estimated total project cost considered as a Class IV estimate (-30%/+50%) including demolition of existing facilities.

Significantly higher capital cost as compared with the median cost of competing alternatives. Highly variable results when considering the range of capital costs and other risk factors.

Capital costs and risk exposure near to the median of competing alternatives.

Significantly lower capital cost as compared with the median cost of competing alternatives. Relatively stable results when considering the range of capital costs and other risk factors.

Annual O&M Cost

Estimated change in annual operating expense resulting from the alternative, considered in its first year of operation and splitting out operations labor, maintenance labor, materials, allowances for renewal and replacement, energy, and chemicals.

Significantly large change in operating cost as compared with the median cost of competing alternatives. Highly variable results when considering the range of operating costs and other risk factors.

Change in operating costs and risk exposure near to the median of competing alternatives.

Significantly lower change in operating cost as compared with the median cost of competing alternatives. Relatively stable results when considering the range of operating costs and other risk factors.

Life Cycle Cost

Calculated 20-year life cycle cost, expressed both as NPV and levelized cost per dry ton. This measure should be considered within a -30%/+50% range of probable capital cost estimates and various operating cost risks.

Significantly higher life cycle cost as compared with the median cost of competing alternatives. Highly variable results when considering various risk factors.

Life cycle costs and risk exposure near to the median of competing alternatives.

Significantly lower life cycle cost as compared with the median cost of competing alternatives. Relatively stable results when considering various risk factors.

Financial Options/ Risk Offsets

Likelihood of securing grant funding, allocating significant financial risks to other parties, or otherwise offsetting risks through project revenues or other credits.

Significantly lower likelihood of reduced life cycle cost as compared with the median of competing alternatives.

A likelihood of reduced life cycle cost near to the median of competing alternatives.

Significantly higher likelihood of reduced life cycle cost as compared with the median of competing alternatives. Specific opportunities have been identified.

End Use Management & Control

Long-term Outlet Availability

Outlet is non-existent or future of outlet is uncertain

Outlet access will require significant effort to develop/maintain

Outlet is proven, accessible, with no identified significant threats


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Rating Basis1 to 5 (Worst to Best)


Ability to Control/Mitigate Offsite Problems

Significant likelihood of risk/failure associated with final use outlet with limited ability for Arlington County to mitigate (offsite odor event, truck spill or accident, contractors, etc.)

Little likelihood of risk/failure associated with final use outlet with reasonable ability for Arlington County to mitigate (offsite odor event, truck spill or accident, contractors, etc.)

No risk/failure associated with final use outlet with complete Arlington County control

Operational

Construction Phasing Phasing not possible due to equipment issues

Phasing possible, but may negatively impact ops/costs

Phasing to support capacity additions possible

Expansion Potential

No room to expand beyond Master Plan capacity

Can be expanded (through additional facilities or process changes)

Can be expanded to add addition treatment train

Flexibility

Diversification of Product

Product suitable for single outlet only

Multiple product outlets but majority to single outlet

Multiple products and/or multiple outlets for same product with little dependence on any one outlet

Operability for Existing Plant Staff

Technically complex and difficult to operate, requires skilled/trade professionals

Moderately difficult to operate, can use existing staff with training

Little complexity with equipment familiar to operators

Number of new operators/maintenance staff required to run and maintain new facilities

Addition of new skill sets for maintenance and operations - adding more than 10% to existing staff

Addition of new skills set for maintenance and operations - adding less than 10% to existing staff

No new skill set required for maintenance and operations - less than 5% new staff required

Operability and Safety

Plant Safety

Safety issues need to be mitigated with specialty equipment

Safety issues would require that new safety training be incorporated into plant safety training

No new safety issues beyond what is currently experienced at plant

Proven System/Technology

Operating History/ Performance

No full-scale installations of system technologies

Full scale installations with short duration (1-3 years) of operating and performance data

System technologies are well-proven and commonly used

Reliability

Ability to monitor asset and deploy maintenance strategy

Components difficult to access for repair, failure modes not understood, specialized or off-site maintenance routinely required

Limited access to components for maintenance, uncertainties regarding failure modes, specialized maintenance periodically required

Documented and predictable maintenance schedule, failure modes well understood, routine maintenance procedures


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ConstructabilityPhysical limitations for on-site construction

Cannot be constructed on site

Will be difficult to construct on existing site

Few spacing or access requirements

Number and duration of major plant outages required to construct

Frequent major outages, with mitigation process/equipment (temp facilities) required.

One or two major outages planned throughout construction - with minor mitigation facilities required

Facilities construction requires no major plant outages

Impacts on Plant Processes Impact of

constructed facilities on plant processes/ease of integration

Ancillary processes required to mitigate impact of alternative on existing plant processes

Operational or minor modifications to existing plant processes needed to accommodate new facilities

Minimal ancillary processes or modifications needed to accommodate alternative

Environmental


Ability to recover all resources in solids

No recovery of energy, nutrients or organics

Recovery of nutrients/organics or energy only

Recovery of nutrients, organics and energy

Energy IntensityAmount of energy required

Requires a significant quantity of purchased energy

Requires a moderate quantity of purchased energy

Requires little to no purchased energy

Carbon Footprint

Estimated change in direct and indirect GHG emissions as a result of implementing the alternative, measured against baseline plant emissions.

Significantly higher change in GHG emissions as compared with the median of competing alternatives.

Change in GHG emissions near to the median of competing alternatives.

Significantly lower change in GHG emissions as compared with the median of competing alternatives.

Regulatory Permits

Ease of permitting (air, process, product)

Current regulations present significant permitting issues and/or alternative has significant air emissions

Current regulations and expected future changes in fed/state regulations may be difficult to address

Supports anticipated federal/state mandates for process and final use requirements; reduction in total air emissions

Gas and Product Quality

Ability of gas and biosolids product quality to meet use requirements

Gas and biosolids product must be significantly treated with additional processing to high quality for intended end-use

Gas and biosolids product requires some additional processing to improve quality to ensure minimal maintenance and consistent end-use

Gas and product quality are sufficient for required end-use

SocialImpact of alternative on process odor

Known significant process odor concerns or potential mitigation difficulty

Odor concerns typical for solids treatment or odors amenable to treatment

Known minimal odor production, such that odor treatment may not be necessary

Odor Generation Potential/Reduction

Impact of alternative on product odor

Product known to be odorous

Product odor not acceptable to some

Minimal product odor, likely


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markets, but typical for agricultural applications

acceptable to high end/high value markets

Acceptability

Community impacts and acceptability with respect to process and product use.

Known negative impact on public image and conflicts with political/regional goals, may incur environmental justice complaints

No impact on public image, and/or is neutral with respect to political/regional goals

Provides significant improvement in public image and support to political/regional goals

Hauling

Impacts due to traffic and corresponding mitigation needs

Highest truck traffic volume

Moderate truck traffic volume

Lowest truck traffic volume

4.3 Community Outreach and Communications PlanIn recognition of the specific and direct impacts a project of this nature can have on its neighbors and County residents, the WPCB team has conducted community outreach since the master plan study began in the fall of 2015 and created a Communications Plan to identify steps to maintain that outreach with the community. The outreach was to provide a format for early, frequent, and two-way communication with key stakeholders and residents throughout the planning, design, and construction phases of the project, with the goal of exchanging ideas and knowledge, as well as garnering recommendations and asking support for the technologies selected. The purpose of the Communications Plan was to provide a guiding document of strategies and tactics for keeping the community informed.

4.3.1 Community OutreachA key component of the community outreach was a series of face-to-face meetings hosted and attended by staff, with the facility’s most critical external stakeholders, which included representatives from the closest neighborhoods including:

Aurora Highlands Civic Association

Arlington Ridge Civic Association

Crystal City Civic Association

Long Branch Civic Association

The meetings also included representatives from key County organizations, such as:

Arlington County Civic Federation

Fiscal Advisory Affairs Commission

Arlingtonians for a Clean Environment

Energy and Environment Conservation Commission


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Neighborhood Conservation Advisory Committee

Table 4-5 lists each meeting hosted, the date, and the topics covered.

Table 4-5. Solids Master Plan Stakeholder Meeting Dates and TopicsDate Meeting Type Topic

December 15, 2015Stakeholder Kickoff Meeting Introduce the Master Plan project & team

Outline purpose of the stakeholder group Overview of treatment and equipment

March 16,2016

Stakeholder Meeting Review of capacity and solids loading Describe plan to address immediate needs Provide regulatory review of biosolids Explain evaluation process tool; obtain

stakeholders to participation

April 18, 2016Staff Presented to ACE Provide project overview: Master Plan process,

schedule, and goals

June 7, 2016Staff presented to internal Operations teams (5x)

Provide project overview: Master Plan process, schedule, and goals

June 7, 2016Staff presented to CivFed Provide project overview: Master Plan process,

schedule, and goals

July 13, 2016

Stakeholder Meeting Provide status of project, share results of paired comparison analysis

Review the financial planning and budget process for project

October 27, 2016

Stakeholder Meeting Review purpose of stakeholder meetings Provide update on Master Plan study Review financial planning and budgeting process

for project Review project next steps

April 4, 2017 Staff presented to the Arlington Civic Federation

Project Update – Solution Recommendation

April 17, 2017Staff presented to ACE Project Update – Solution Recommendation

April 24,2017Staff presented to E2C2 Project Update – Solution Recommendation

May 10, 2017 Staff presented to Aurora Highlands Civic Association

Project Update – Solution Recommendation

May 18, 2017Staff presented to FAAC Project Update – Solution Recommendation and

Budget ImpactsMay 25, 2017 Optional Stakeholder Meeting Review and respond to questions

June 22, 2017

Stakeholder Meeting Review the Master Plan process Review technology selected, why Review financial/budgeting process Provide overview of emissions study Provide update on regional solution

November 18, 2017 Tour of Plant Provide tour of liquids and solids treatment trains


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A project website was created and posted in the first quarter of 2016, at https://projects.arlingtonva.us/projects/water-pollution-control-plant-solids-master-plan/. The site serves as the main hub and repository of documents and information related to the master plan process, including fact sheets, meeting presentations, meeting summaries, questions and answers, and supplemental information. A fact sheet providing an overview of the master plan process was created for residents and others to download. Other fact sheets are planned to be developed as the project progresses.

The stakeholder group has provided critical input to the solids master plan process throughout the study period, including, for example, participation in the weighted ranking of project evaluation criteria. The citizen’s input made an impact on the final results by shifting more weight to the social and environmental aspects of the criteria. Other influences the stakeholders have had are in the creation and execution of an emissions study, as well as a push to explore a more regional solution.

It is intended that stakeholder meetings will continue on an annual or semi-annual basis and that staff regularly attend civic group meetings to keep stakeholders apprised of the project as it continues through design and construction phases.

4.3.2 Communications PlanA comprehensive Communications Plan (Appendix E) was developed to provide a roadmap of suggested strategies and tactics tailored to the different audiences affected by the project. The Plan includes the following sections: Background Research

Meetings

Communications Audit

Content Analysis

Opportunities and Challenges

Goals

Target Audiences

Measurable Objectives

Strategies and Tactics (By Phase and Audience)

Key Message Platform

Evaluation

The Plan is meant to be a dynamic document, managed by WPCB staff, designed to promote frequent and timely communication and engagement with internal and external stakeholders. It should be evaluated annually by staff to ensure continued relevance. The Plan also includes recommendations for outreach through the design and construction phases. These recommendations include, but are not limited to, hosting annual stakeholder meetings, offering

https://projects.arlingtonva.us/projects/water-pollution-control-plant-solids-master-plan/


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periodic facility tours, attending civic and other group meetings, posting outdoor signage, and continued communications through the County’s established digital channels.

4.4 SummaryDefining a basis for decision making was a critical first step in the master planning process to assure that Arlington County priorities were reflected in all project/alternative evaluations, as well as the final selected alternative/recommendation. For the Solids Master Plan, decision making was founded on two sets of criteria: one to screen the universe of solids processing technologies and another to evaluate the relative merits of alternatives derived from screened technologies. The criteria were collaboratively developed by the Project Team, Arlington County staff, and the TAC.

Criteria presented in this section supported subsequent decision-making and evaluating alternatives as follows:

Screening criteria were applied to select preferred technologies, with preferred technologies forming the basis for process train alternatives.

Evaluation criteria, and their associated performance measures, were applied to evaluate and rank each alternative. Alternatives were first assessed to each performance measure. Each raw score was prioritized according to the weights developed through the PMC exercise; weighted criterion scores were summed to determine the total score for each alternative.

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Section 5 Technology Identification and Screening

As part of the master planning process, the project team identified an exhaustive list of potential biosolids treatment and handling technologies to be considered. Members of the core team consulted with WERF and with members of the technical advisory committee in the identification process. The screening process reduced the number of technologies considered from over 70 to 20. The screening process included an initial pre-screening to remove technologies the core team agreed were not applicable to Arlington County, as well as a formal screening exercise based on ratings in five criteria. This section provides an overview of the technologies considered and presents the results of the screening process.

5.1 Organization of Technology Review Technologies were divided into major categories focused on the function of the process. For example, biosolids stabilization technologies are organized into categories for thermal processes, digestion processes, and non-digestion processes. Non-stabilization solids handling technologies, including thickening and dewatering, are also included in the review.

Table 5-1 presents the list of technologies considered and the organization for the review. The table includes technologies that were identified but were pre-screened and not considered in the formal screening process. The pre-screened technologies were not considered for various reasons described in this section.

The table presents general information on the process, as well as information to score the technology against five screening criteria. The criteria were developed in the initial stages of the master planning process and are described in Table 4-1 of this report. The criteria are:

Development Status (based on WERF Technical Development Level)

Typical Application Scale

Site Requirements (on-site, except where noted)

Relative Costs

Permitability

The technology review for each grouping concludes with a summary of the scoring and a decision on whether a technology will be considered further in the master planning process. Technologies that were identified by the core team but removed through pre-screening are not described herein.

Section 5 • Technology Identification and Screening

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Table 5-1. Technology Identification

Technologies Considered for Initial Screening

Technologies Identified but Eliminated Through Pre-Screening

Non-Digestion Stabilization

Composting

Covered Aerated Static Pile (ASP)

Agitated Bin

Tunnel/Vessel Chemical Treatment

Alkaline Stabilization – Class A

BCR – CleanB and Neutralizer

Lystek

Composting

Windrow

Open-Air ASP

Enclosed ASP

Digestion Stabilization

Aerobic Digestion

Conventional

Autothermal Thermophilic Aerobic Digestion (ATAD)

Anaerobic Digestion

Single Phase • Mesophilic • Thermophilic

Multi-Phase • Temperature Phased • Acid/Gas

Anaerobic/ Aerobic Digestion Enzymatic Hydrolysis/ Anaerobic Digestion High Solids Digestion

None

Digestion Pretreatment

Thermal Hydrolysis

Pre-Digestion

Post-Digestion Cavitation Thermochemical Hydrolysis (Pondus, Lystek)

Ozone Microwave Kady Mill Electrical Disintegration

OpenCel

BioCrack Crown Ultrasound Pasteurization Microsludge Cannibal


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Technologies Considered for Initial Screening

Technologies Identified but Eliminated Through Pre-Screening

Drying Solar Dryer Direct Thermal Dryer

Rotary Drum Dryer

Belt Dryer

Microwave Dryer

Fluidized Bed Dryer Indirect Thermal Dryer

Paddle Dryer

Disc Dryer

Thin-Film Dryer Tray Dryer

Sand Drying Beds Vacuum Assisted Drying Beds

Thermal Processes Incineration Wet Air Oxidation

VERTAD (deep shaft) Anuvia Gasification Pyrolysis Liquid Sludge Thermal Oxidation

Supercritical Water Oxidation

Hydrothermal Catalytic Gasification

Vitrification

Thickening Gravity Thickener DAF Thickener Gravity Belt Thickener Rotary Drum Thickener Screw Thickener Centrifuge

Membrane Thickener Disc Thickener

Dewatering Centrifuge Belt Filter Press Screw/ Volute Press Rotary/ Fan Press Bucher Press

Recessed Chamber Filter Press Vacuum Filtration Electro-dewatering

5.2 Non-Digestion Stabilization Technology Screening 5.2.1 Composting Composting is an aerobic biological process where microorganisms break down organic matter and their heat of respiration increases the temperature of the composting mass. The compost process temperature can be high enough to kill disease organisms and meet Class A pathogen criteria. In addition to time and temperature, the process is dependent on other operational parameters, such as oxygen availability, moisture content, and biodegradable VS content.


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Amendment, in the form of wood chips or other woody material, is added to the biosolids to increase porosity of the material and maintain aerobic conditions during the composting process. Aerobic conditions are maintained through mechanical turning or mechanical aeration. The mixture is composted for a sufficient period of time to allow substantial decomposition of organic material, typically 21 to 30 days. After the active composting phase, compost is frequently allowed to cure in storage piles, ensuring that the final product has minimal odors. Curing durations vary, but can be as long as 30 to 60 days. Depending on the amendment, the product is screened to remove large material/amendment and then used in landscaping or as a soil amendment.

There are several forms of composting. ASP is the most common composting technology in North America, and includes composting, curing, screening, and storage. Almost all municipal biosolids ASP facilities are covered/enclosed, and large volumes of air must be treated for odor control. The enclosed ASP method is non-proprietary and has independent process components, allowing more flexibility and process control when dealing with operational difficulties and variations in feedstocks. However, larger volumes of building and process air must be collected and treated.

In-vessel technology, such as a tunnel reactor system, is used more often at new composting installations to reduce footprint and odor concerns. Older generations with in-vessel technology (vertical plug flow and circular systems) are not commercially available today.

When selecting a compost technology, a detailed evaluation is needed for the following: land area requirements; materials handling equipment (e.g., front-end loaders, hoppers, mixers, conveyors, etc.); odor control (for building and process air); process control; fire prevention and control; and technology operational experience. Uncovered/unenclosed windrow-type composting is not considered suitable for Arlington County biosolids and is not described herein.

One of the major advantages of composting is its adaptability to a variety of feedstocks. Composting has been applied successfully to municipal solids, yard waste, animal manures, and a variety of agricultural materials. Sawdust, wood chips, leaves, agricultural wastes, and even recycled composting itself have been used as amendment in the compost mix. The ability to segregate and specify amendment quality and feedstocks is important for Arlington County product branding and quality control purposes. If biosolids are added to the compost mix, regardless of the amendment, the final product must meet federal and state requirements for biosolids (e.g., Federal Part 503 and VDEQ regulations).

Fire prevention and control is a major safety issue at compost facilities. Excessive material drying creates the possibility of the compost catching fire from careless equipment maintenance sparks (in the case of ASP or agitated bed systems) or from spontaneous combustion in the mass. The ability to access the fire, control aeration (moisture), contain, and extinguish is critical – open process technologies, such as enclosed ASP and agitated bed systems, are the least likely to be disrupted in the event of a fire.

A comparison of costs for operating biosolids composting facilities is difficult as there are many variables to consider; most operational costs are site-specific and related to the type of solids/feedstocks being composted. The most significant cost factors are land cost, amendment type and costs, degree of mechanization and materials handling, and odor control and treatment.


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For third-party contracting, costs depend on the volume of biosolids, availability of existing capacity, duration of contract, and terms for distribution and marketing.

A comparison of various composting processes is summarized in Table 5-2 below.

Table 5-2. Summary of Composting Processes

Process Enclosed/

Configuration Amendment

Type Reactor & Equipment

Aeration & Odor Control

Procurement

Conventional ASP

Enclosed/ composting (21-30 days), curing in piles (30+days), amendment screening and recycle

Typically woodchips

Build piles with loaders

Conveyors, hoppers, mixers

Positive aeration

High rate ventilation for building air

Multi-stage scrubbing and/or biofilter for process air

Non-proprietary

Agitated Bin Enclosed/tunnels with wall separation

Tunnel configuration conducive to different amendment types and feedstocks

Loaders

Machine mixing/aeration

Positive aeration

Curtains separate process and work areas

Ammonia odor control required

Proprietary

In-vessel Enclosed contained reactors; 20 days composting

Exterior (covered) curing and storage

Typically, sawdust; configuration and aeration very sensitive to amendment type

Mixers

Loaders to feed reactors

Discharge conveyors

Less non-process air than other configurations

Positive and negative

aeration floors

Chemical scrubbing for process air

Proprietary

Due to plant site limitations, composting would be an off-site option for Arlington County. For the third-party composting option, the County would commit to pay the third party an agreed rate per mass (typically wet ton) of solids received. Contract terms and conditions would have to be specified, including responsibilities for the final product quality, distribution, and alternative management in the event the solids could not be delivered or processed at the compost facility.


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The ability to segregate Arlington County biosolids from other feedstocks is a concern and the County’s product branding potential is limited.

A composting facility located in Waverly, VA, is within hauling distance from the plant. The current capacity would need to be determined in order to ensure the receipt and processing of the current and future volumes of biosolids from the County. Depending on the quantities and duration of the contract, the company would consider expansion of the facility to handle the additional volume.

Advantages Composting produces a marketable, familiar, high-quality Class A biosolids product. Since it requires woody material as amendment, green waste that is already being source-separated in Arlington can be used in the compost process, thereby reducing costs for amendment purchase and providing a use for the community’s green waste. As a contract option, Arlington County could negotiate a contract to haul and treat a portion, or the full production, of solids to an off-site facility.

Drawbacks Composting can have high capital and operating costs. It is a labor intensive process and requires a large footprint. Some in-vessel systems have narrow specifications for the amendment quality; dust and fines can plug the reactor aeration systems. Odor potential from the composting process is significant; thus proper odor control design is critical. As a contracted solution, terms and conditions would need to be negotiated and developed. Operational control (source segregation) and product quality oversight depends on the contractor. Product branding potential for Arlington County would be limited. Additional storage at the plant site, including on-site and cake storage, may be required depending on the trucking and receiving schedule at the third-party composting facility.

Applicability to Arlington County Development status: Established, mature

Typical application scale: Amenable for small and large-scale applications

Site requirements: The process cannot fit on the site and would need to be performed at an off-site, third-party location.

Relative cost: If the compost site/operation is owned by a contractor, final use O&M costs would be expected to increase compared to the current program.

Permitability: Proven permit process and product familiarity.

Product Use: Difficult to brand for Arlington County due to other feedstocks; limited operational oversight and control at contractor facility.

Community: Slight reduction in the amount of current truck traffic. Limited return of an “Arlington” branded product to the community for use.


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5.2.2 Chemical Treatment 5.2.2.1 Alkaline Stabilization Alkaline stabilization is a process by which an alkaline chemical is added to raise the pH of a liquid sludge feed or dewatered cake. With sufficient contact time and adequate mixing, a pH of 12 or higher inactivates pathogens and microorganisms. Although both large and small treatment plants have used lime stabilization as their primary stabilization process, it is more common for small facilities. Typical system design criteria include the following:

Feed sources and characteristics (quantity, type, quality, percent solids)

Contact time, pH, and temperature

Alkaline chemical types and doses

Energy requirements

Storage requirements

Alkaline stabilization, using lime, can meet Class A and/ or Class B pathogen standards. The difference to meet Class A is the increased lime dose and process monitoring to achieve Class A. Typically, the lime dose is more than 2X greater than current doses at the Arlington County WPCP.

Other processes can achieve Class A at lower lime doses, but heat is typically added, such as the RDP EnVessel process. Howard County, MD, which uses this technology, is not benefitting from their Class A lime stabilization product since material is still handled as Class B.

Bioset offers a Class A process at a potentially lower lime chemical dosage. Sulfamic acid can be added to aid in production of heat, but acid is not always used. Bioset was pilot tested as a back-up option to incineration at Prince William County Service Authority (PWCSA).

In the past, Virginia Department of Health (VDH)/VDEQ has considered eliminating lime stabilization as a Class A process alternative.

Advantages Established process and established demand for biosolids product

Low capital cost – Cost is dependent on solids characteristics and quantities. However, the incremental cost for meeting Class A requirements is the lowest among stabilization alternatives.

Systems adjust easily to changing flows – The amount of lime added to the slurry or dewatered cake can be easily regulated and adjusted to account for flow variations.

Small land area required relative to other stabilization processes

Drawbacks Greater mass & volume of solids handling – The process does not result in solids

destruction. Rather, the mass is increased because of added lime.

High O&M costs associated with transportation for usage or disposal


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Dust production associated with lime

Process and product odor is a concern

Applicability to Arlington County Development status: Established, mature

Typical application scale: Amenable for small and large-scale applications

Site requirements: Lime stabilization is currently used at Arlington County WPCP to produce Class B biosolids.

Relative cost: Low capital cost, but high O&M costs. Enhancements to achieve Class A biosolids will increase capital and O&M costs.

Permitability: Proven permit process and product familiarity. In the past, VDEQ has considered eliminating lime stabilization as a Class A process alternative.

Product Use: Product use for a Class A lime-stabilized biosolid is similar to Class B biosolids. The conversion to Class A is not expected to significantly affect the value of the biosolids product.

Community: Likely some increase in the amount of current truck traffic due to additional lime demand for a Class A process. Likely increase in odor emissions.

5.2.2.2 BCR Environmental - CleanB (Class B), Neutralizer (Class A) BCR Environmental offers two chlorine dioxide-based solids ‘disinfection’ processes aimed for utilities seeking alternatives to digestion and lime stabilization for biosolids stabilization. CleanB is permitted as a process to produce Class B biosolids and Neutralizer is permitted for Class A biosolids production.

The process works by blending sulfuric acid and sodium chlorite in an on-site generator to create chlorine dioxide. Chlorine dioxide is injected into the solids as a disinfectant. The CleanB process can be set as a continuous flow process. The Neutralizer process also relies on chlorine dioxide, but operates in a batch mode. In the second stage of the Neutralizer process, sulfuric acid is used to lower the pH of the solids ahead of sodium nitrate addition. Solids are held for a specified period of time (approximately six hours) and caustic is added to adjust the pH back to a minimum of six.

The Neutralizer process is permitted to meet Class A pathogen requirements and vector attraction reduction (VAR) requirements for land application. The CleanB process will meet Class B pathogen reduction (demonstrated by fecal coliform testing), but vector attraction compliance is achieved through incorporation into the soil at the application site (Option 10).

BCR process is typically used at smaller facilities, but it is scalable. The process is commonly used on WAS and not primary solids. Many of the chemicals required for the process are already used at the WPCP, but in limited quantities. The BCR process (CleanB or Neutralizer) will require a significant increase in chemical handling, including on-site generation of a strong oxidizer.

This technology is generally considered too costly (ongoing chemical cost) for facilities with sizes similar to Arlington.


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Advantages Low capital cost – Cost is dependent on solids characteristics and quantities.

Systems adjust easily to changing flows – The amount of chemical can be easily regulated and adjusted to account for flow variations.

Small land area required relative to other stabilization processes

Drawbacks Significant chemical handling requirements, including on-site generation of strong oxidizer

High O&M costs associated with chemical consumption of the process

Typically applied at smaller facilities that only produce WAS solids

Applicability to Arlington County Development status: Established, however, application on primary solids is unknown

Typical application scale: Typically applied as small facilities

Site requirements: Small site requirements, primarily chemical storage tanks

Relative cost: Low capital cost, but high O&M costs. Ongoing chemical costs. Class A process will increase capital and O&M costs.

Permitability: Generally accepted for Class B pathogen reduction, but vector attraction is met at the application site through incorporation. The project team is not aware of any Class A (Neutralizer) installations in VA.

Product Use: Product use for Class A and Class B biosolids is similar to the current product use. The conversion to Class A is not expected to significantly affect the value of the biosolids product.

Community: Neutral or likely slight increase in the amount of truck traffic due to chemical deliveries.

5.2.2.3 Lystek Lystek is a technology process vendor offering several proprietary systems for biosolids hydrolysis and stabilization. The core process is a combination of chemical treatment (alkali to raise pH), heat (low pressure steam), and mechanical mixing (high shear). The process is completed in batches in a reactor vessel. Process controls for the batch reactor ensure time and temperature requirements for Class A pathogen reduction are met.

Holding tanks on the feed and discharge side of the reactor allow for ‘semi’ continuous operation of the other processes, including solids thickening, dewatering, and product hauling. The process requires pre-thickening or pre-dewatering of solids with the concentrated solids stored in bins. Published literature on the process indicates that solids concentrations up to 17 percent solids can be fed into the reactor. The product leaving the reactor has a lower viscosity and will remain flowable, even at high solids concentrations.


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Within the U.S., the process is recognized for meeting Class A pathogen requirements, but permitting uncertainties exist. Vector attraction requirements are achieved by incorporating the material at the site. Class A EQ cannot be achieved without altering the process.

Advantages Moderate capital cost – chemical dosing, reactor vessels, steam generator, and pre-

dewatering/ thickening

System is batch and adjustments in operating times can be made to adjust to changing solids production. The amount of chemical can be easily regulated and adjusted to account for flow variations.

Drawbacks Limited installation and permitting uncertainty in the U.S.

Higher degree of operating complexity than other chemical stabilization systems

High O&M costs associated with chemical and steam consumption

Applicability to Arlington County Development status: Developing, technology is proven, but few installations

Typical application scale: Amenable for small applications, uncertainty for application at larger scale

Site requirements: Less space required compared to digestion

Relative cost: Moderate capital cost with high O&M costs

Permitability: Process can meet Class A pathogen requirements, but permitting process is unproven in the U.S. Does not meet VAR requirements by process and relies on incorporation into the soil at the time of application to remain compliant.

Product Use: Class A liquid product. Dewatering can be employed, but few facilities currently dewater.

Community: Likely similar to the amount of current truck traffic, may see a slight decrease if final product dewaterability improves.

5.2.3 Screening Exercise and Results Scoring of the technologies against the screening criteria was reviewed. In general, the highest scores (8 or higher) in each grouping were selected as technologies to be considered moving forward. Exceptions are noted in the comments section of Table 5-3 and Table 5-4.


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Table 5-3. Screening Exercise for Non-Digestion Stabilization Technologies

Table 5-4. Screening Results for Non-Digestion Stabilization Technologies

Technology Development Application

Scale Site

Requirements Relative

Costs Permit-ability Total

Com

post

ing Covered ASP 2 2 2 (offsite) 1 2 9

Agitated Bin 2 2 2 (offsite) 1 2 9

Tunnel/ Vessel

2 1 2 (offsite) 1 2 8

Chem

ical

Class A Alkaline Stabilization

2 2 1 1 2 8

BCR- CleanB 1 0 2 1 2 6

BCR – Neutralizer

1 0 2 1 2 6

Lystek 2 2 1 1 1 7

Technology Score

(Max = 10) Comments Consider For Further

Evaluation?

Composting – Covered ASP

9 Off-site Yes

Composting – Agitated Bin

9 Off-site Yes

Composting – Tunnel/ Vessel

8 Off-site; typically smaller application scale compared to other composting processes; will be considered moving ahead with other composting options

Yes

Alkaline Stabilization (Class A)

8 No perceived benefits from Class A lime stabilization compared to existing Class B lime process.

No

BCR – Class A and Class B

6 Lack of installations in medium- to large-sized facilities

No

Lystek 7 Uncertainty with permitting the product No


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5.3 Digestion Stabilization Technology Screening 5.3.1 Aerobic Digestion Aerobic digestion is a suspended-growth biological treatment process in which aerobic microorganisms consume oxygen and biodegradable organic matter to produce a biologically stable product. Concurrent objectives of the aerobic digestion process include reduction of solids mass and volume, reduction of pathogenic organisms, and conditioning for further processing. The aerobic digestion process has been used for many years, and has been successfully used to treat WAS, mixtures of WAS and primary solids, waste sludge from extended aeration plants, and waste sludge from membrane bioreactors. Although it is most commonly used in plants with design capacities of less than 5-mgd, it has been used in plants with capacities exceeding 50-mgd.

There are numerous factors affecting the design of aerobic digestion systems, including desired reduction in VS, feed quantity and characteristics, operating temperature, oxygen transfer and mixing, and retention time requirements. The process is capable of obtaining 35- to 50-percent reductions in VS levels. A 38-percent VS reduction (VSR) is required by the 40 CFR Part 503 regulations in order to attain vector-attraction reduction requirements. However, other parameters, such as the residual rate of oxygen demand, pathogen levels, or oxidation-reduction potential (ORP) may be more indicative of stabilization. In terms of feed quantity and characteristics, the influent solids concentration is an important factor in aerobic digester design and operation. One of the techniques used to optimize aerobic digestion is pre-thickening. The main advantages of this technique are increased solids retention time (SRT), smaller digester volume requirements, and increased levels of VS destruction.

The operating temperature in an aerobic digester is a critical parameter, as it significantly affects the rate of VSR. Typical operating temperatures range from 10 to 40° C. 20 to 45° C is known as the mesophilic zone of bacterial action. Within the mesophilic temperature zone, the rate of biological processes generally increases with temperature. At temperatures lower than 10° C (the cryophilic zone), the process is less effective. Because most aerobic digester systems use open tanks, operating temperature is typically a function of ambient weather conditions.

Because aerobic digestion is a biological process, it requires oxygen to convert organic matter to cellular material. At the same time, adequate mixing is required to ensure contact and interaction between oxygen, microorganisms, and their food supply (biodegradable organic matter). Typically, the aeration system provides both the oxygen and mixing. For the treatment of secondary biological sludge, air flow requirements typically range from 30 to 40 standard cubic feet per minute (scfm) per 1,000 square feet. For primary solids, air requirements may range from 45 to 70 scfm per 1,000 square feet. Mixing requirements range from 0.5 to 4.0 horsepower per 1,000 cubic feet, which typically corresponds to airflow rates between 20 and 40 scfm per 1,000 cubic feet. Actual requirements will vary depending on the tank geometry and mixing mechanism. In cases where the mixing requirement exceeds the oxygen transfer requirement, supplemental mechanical mixing can be considered before overdesigning the aeration system.

The SRT is another significant factor in the operation of aerobic digesters. The SRT typically governs the required volume of an aerobic digester system. Overall, SRTs typically range from 10 to 40 days. Generally, increased SRT results in increased VSR. However, higher retention times also reduce the dewaterability of the digested solids. If dewaterability is an important


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consideration, SRT should be kept on the lower end of this range. It should be noted that the 40 CFR Part 503 regulations require an SRT of 40 days at 20° C to meet pathogen reduction requirements.

Parameters for typical performance of an aerobic digester are presented in Table 5-5.

Table 5-5. Typical Performance of an Aerobic Digester.

Parameter Value

Dissolved Oxygen ≥ 1 mg/L

SRT 40 – 60 days

VSS Reduction 30 – 50%

Aerobic digesters are typically constructed of reinforced concrete or steel. A minimum of two tanks are installed to allow flexibility for maintenance or repair. Aeration systems may consist of air or pure oxygen transfer equipment, including conventional mechanical aerators, coarse-bubble diffusers, fine-bubble diffusers, or jet aerators. Blowers supply the air to these systems.

Advantages Aerobic digestion systems have a number of advantages compared to anaerobic digestion:

More simple operational control

Safer operation with no potential for gas explosion – No production of methane gas, as in anaerobic digestion

Lower BOD5 and ammonia concentrations in supernatant

Less prone to upsets and less susceptible to toxicity

Drawbacks Reduced efficiency of the process during cold weather – Temperature variability

throughout the year causes variability in operating performance

Inability to produce a useful byproduct, such as methane

High power costs associated with aeration and mixing

Addition of primary solids increases oxygen demand

Larger footprint

Applicability to Arlington Development status: Established, mature

Typical application scale: Amenable for small and medium-sized applications


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Site requirements: Application of aerobic digestion at WPCP would be difficult due to the large footprint required. Lower ambient temperatures during the winter months may affect the performance of an aerobic digester system, requiring long detention times in the process.

Relative cost: Moderate capital cost, with high O&M costs primarily associated with power for aeration and mixing.

Permitability: Proven PSRP for a Class B biosolid. VAR compliance is sometimes difficult to demonstrate when digesting only WAS.

Product Use: Land application as a Class B biosolid. Process cannot meet Class A by itself and lack of lime in the product may be perceived as a negative.

Community: A decrease in the amount of current truck traffic due to removal of lime addition and destruction of solids in the digestion process.

5.3.2 Autothermal Thermophilic Aerobic Digestion ATAD is a Class A biosolids process that represents a variation of conventional aerobic digestion. The ATAD process achieves thermophilic operating temperatures (40 to 80° C) without any supplemental heat besides that supplied by mixing, hence the term autothermal. Feed solids are typically thickened to provide a digester feed concentration of four percent solids or greater. In addition, the reactors are insulated to conserve the heat produced during the breakdown of organic matter by thermophilic bacteria. The high temperatures of the ATAD process provide a number of benefits:

Similar VSRs to conventional aerobic digestion (35- to 50-percent) can be achieved with a shorter HRT. At typical design SRTs of 12 to 14 days, VSRs between 50- and 70-percent may be achieved.

Excess heat generated by the process can be used to heat the building or to preheat the ATAD influent feed.

Pathogens are significantly reduced via pasteurization.

The ATAD process requires an integrated approach to solids handling, including proper feed characteristics, a proper environment for the autothermal reaction, and post-process solids cooling and storage. Thickening is a key aspect of many ATAD systems, as influent to the ATAD reactor must typically have a minimum of three percent solids, with a recommended range of four- to six-percent. Feeds with solids concentrations less than three percent may contain too much water to achieve autothermal conditions. On the other hand, concentrations greater than six percent are more difficult to mix and aerate. It is also recommended that the feed contains a minimum of 40 g/L of COD, and a minimum VS content of 25 g/L.


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In terms of basin configuration, two or more enclosed, insulated reactors are typically used. Each reactor is equipped with mixing, aeration, and foam control equipment. Although both continuous and batch loading have been used, withdrawal and feeding of the solids to the reactors is performed on a batch basis when compliance with pathogen regulations of Class A biosolids is desired. In this case, pumps are designed to withdraw and feed the daily amount of solids in one hour or less. The temperature in the second stage is typically maintained at a minimum of 55 °C. Design parameters for ATAD systems are summarized in Table 5-6.

Table 5-6. Design Parameters for ATAD Systems (Stensel and Coleman, 2000).

Parameters Range Typical

Number of reactors 2 to 3 2

Pre-thickened Solids 4 to 6% 4%

Reactors in series Yes

Total HRT in reactors 4 to 30 days 6 to 8 days

Temperature – Stage 1 35 to 60° C 40° C

Temperature – Stage 2 50 to 70° C 55° C

Proper aeration is crucial to the performance of ATAD systems, as both adequate oxygen transfer and mixing are needed to ensure complete stabilization. Several aeration and mixing devices have been used, including diffused air, aspirating aerators, and jet aeration. First generation ATAD systems commonly used aspirating aerators, which limited their flexibility in terms of oxygen transfer capacity. However, there is a trend among second generation ATAD systems to employ floor-mounted jet aeration systems. The primary benefits of jet aeration are higher oxygen transfer efficiencies and the ability to mix and aerate irrespective of tank depth. Jet aerators also provide flexibility in that the motor can be controlled by VFD to regulate the aeration and mixing intensity. Aerator motor speed can be tied to ORP monitoring in the tank to ensure that the process stays aerobic, and that energy is not wasted due to over-aerating. The ability to maintain the aerobic nature of the process via variable oxygen supply has provided the added benefit of reducing odors caused by periods of anaerobic activity within the reactors.

Advantages Decrease in solids retention times (SRT) – A smaller volume is required to achieve a given

VSR, compared to conventional aerobic digestion. An SRT of approximately five to six days is needed to achieve a VSR of 35- to 50-percent, whereas with conventional aerobic digestion, SRTs of 10 or more days are typically required to achieve the same reduction. An SRT of 10 to 12 days is typically used in designing ATAD systems.

Greater reduction of bacteria and viruses compared with mesophilic anaerobic digestion (MAD) (Metcalf and Eddy, 2002) – When ATAD reactors are well-mixed and maintained at 55° C and above, they can reduce pathogenic viruses, bacteria, and other parasites to below detectable levels, and can meet the pathogen reduction requirements for Class A biosolids.


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Reduction in solids volume compared to lime stabilization – The Middletown (OH) WWTP, a 26-mgd facility, saw a 63-percent reduction in solids volume when they converted from lime stabilization to an ATAD process. The facility produces 17 DTPD.

Drawbacks Biosolids produced by ATAD may have poor dewatering characteristics. However,

dewatering issues are lessened with improved sludge conditioning.

Although odor problems have been much improved from first generation to second generation systems, objectionable odors may still form if aerobic conditions are not maintained.

Relatively high capital and energy costs

Foam control is required to ensure effective oxygen transfer – If left uncontrolled, a thick, brown foam layer can form, which reduces oxygen transfer efficiency.


Typical application scale: Amenable for small and medium-sized applications. Arlington is on the upper end of the typical application scale.

Site requirements: Application of ATAD at WPCP would be difficult due to the large footprint required for the digester reactors and the denitrification process.

Relative cost: Moderate capital cost with high O&M costs, primarily energy.

Permitability: Proven permit process for Class A biosolids.

Product Use: Product use for digested Class A biosolids is similar to Class B biosolids. The conversion to Class A using ATAD is not expected to significantly affect the value of the biosolids product.

Community: A decrease in the amount of current truck traffic due to removal of lime addition and destruction of solids in the digestion process.

5.3.3 Anaerobic Digestion 5.3.3.1 Mesophilic Anaerobic Digestion MAD is a widely used stabilization method that uses anaerobic microbes to perform a series of biochemical transformations. These transformations break down complex organic compounds in wastewater solids into methane and carbon dioxide.

The key components of a MAD system include a closed and insulated reaction tank, a mixing system, a heating system, and gas handling equipment. The microbe-rich environment inside the digester is deprived of dissolved oxygen and nitrate in order to facilitate the conversion of VS to digester gas and water. Although they are more costly to construct, egg-shaped digesters are advantageous because they minimize scum formation and facilitate grit removal. A less-expensive cylindrical design with a sloped floor can also perform well.


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A mesophilic digester is designed to operate within a temperature range of 90 to 100 degrees F. This temperature is typically maintained by an automated digester heating system, which uses a hot water loop and a heat exchanger to transfer heat to the solids. A mixing system keeps the contents of the digester tank well-mixed, preventing accumulation of solids at the bottom of the tank and ensuring a uniform temperature throughout the tank. This mixing system may be a gas mixing system, which uses biogas to create an upflow pattern in the tank, or a mechanical or pumped mixing system, which recirculates solids via propeller mixers or through the use of centrifugal solids handling pumps and specially designed mixing nozzles located on the tank floor. A recent development in digester mixing is the linear motion mixer that may offer a reduced energy requirement compared to other mixing technologies.

According to 40 CFR Part 503, wastewater solids are considered a Class B biosolid with respect to pathogens if it meets the required minimum retention time of 15 days at 35° C (95° F). VAR requirements are fulfilled when VSR is at least 38 percent. A properly designed and operated anaerobic digestion system will meet these criteria and will produce biosolids suitable for land application. MAD can also be a core process in an enhanced process train to produce a Class A biosolids product.

A useful byproduct of MAD, digester gas, is captured from the headspace of the tank. Also known as biogas, the digester gas typically consists of approximately 65 percent methane and 35 percent carbon dioxide and has a heating value of about 600 BTU/cubic foot. Energy available in the digester gas can be recovered and used to power a variety of processes, including the digester solids heating or thermal drying systems. The gas can also be fed to a combined heat and power (CHP) system to generate useful electricity and waste heat. Before it can be used, however, the gas must first be treated to remove moisture and chemical compounds, such as hydrogen sulfide and siloxanes. Additional facilities for storage of the treated biogas can require a large amount of space.

Advantages Well-established stabilization method with a wealth of operational experience

Requires less energy than aerobic processes such as ATAD

Biogas can be beneficially used for heating, power generation, or alternative fuel

Reduces volume of biosolids to be disposed

Drawbacks Capable of meeting Class B pathogen reduction requirements; additional processes are

required to meet Class A pathogen reduction requirements

More complex operations compared to aerobic digestion and lime stabilization

Requires control of corrosive and explosive digester gases and odors

Requires more careful attention to process, such as precise control of SRT, a consistent digester feed, attention to mixing and scum/foaming potential

Can be sensitive to temperature changes

Can have a large footprint depending on the tank configuration


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Relatively high capital cost (size) and reduced O&M cost (mixing, heating)


Typical application scale: Amenable for medium and large applications. Arlington is within the typical application scale.

Site requirements: Application of MAD at Arlington County WPCP requires a large footprint. Utilizing tall cylindrical or egg-shaped digesters will be required.

Relative cost: High capital cost with lower O&M costs.

Permitability: Proven permit process for a Class B biosolid.

Product Use: Land application as a Class B biosolid. Process cannot meet Class A by itself.

Community: A decrease in the amount of current truck traffic due to removal of lime addition and destruction of solids in the digestion process. Enclosed digesters provide odor containment, but anaerobic process requires odor control system.

5.3.3.2 Thermophilic Digestion Thermophilic digestion is a type of anaerobic digestion that operates at a higher temperature (49 to 57 degrees C) compared to MAD (32 to 38 degrees C). This higher operating temperature increases the metabolic rates and growth rates of the microbial population, allowing the VS loading rate to be increased and the SRT to be lower. Compared to mesophilic digesters of the same size, thermophilic digesters achieve a higher VSR and produce more biogas. Thermophilically-digested biosolids also tend to have better dewatering characteristics than mesophilically-digested solids. On the other hand, its higher operating temperature requires a higher input of heat, thus consuming more energy than MAD. Operating costs for thermophilic digesters can therefore be higher than those of MAD systems. In some applications, the reduced cost to dewater the digested solids and the potential to recover waste heat may offset the higher O&M costs.

Thermophilic digestion produces solids that contain fewer pathogens, but two provisions of the Part 503 rule can prevent thermophilic digestion systems from receiving Class A pathogen reduction status. First the rule groups non-batch, non-phased MAD, and thermophilic digestion in the same category as PSRPs, or Class B processes. Consequently, a continuous-flow thermophilic digester does not receive credit for pathogen reduction. Secondly, Part 503 requires any PFRP to precede, or operate concurrently with, the VAR process (MAD or thermophilic digestion) in order to mitigate concerns regarding pathogen regrowth.

Of the six alternatives for achieving Class A pathogen reduction, operators of thermophilic digestion systems typically use one of the following three alternatives to demonstrate Class A pathogen reduction.

Option 1: The digester is operated partially or entirely in batch mode to meet Alternative 1 – Time and Temperature. Alternative 1 is the most conservative in terms of time and temperature and requires the least amount of proof of its effectiveness.


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Option 2: Operators of thermophilic digesters can also apply to EPA for approval as a Class A system under Alternative 3 – Documented Virus and Helminth Ova Destruction. Approval under Alternative 3 is contingent upon the operator’s implementation of a testing program that regularly checks the raw and treated biosolids for helminths and enteric viruses.

Option 3: Lastly, operators may also apply for approval under Alternative 6 – Treatment with Processes that are equivalent to a PFRP. Data from pilot-scale studies could be used to demonstrate to EPA that certain alternative combinations of batch time and temperature can achieve Class A treatment.

Advantages Established technology

Higher VSR and more biogas

Lower SRT than MAD

Capable of meeting Class A pathogen reduction requirements – when approved

Higher VS loading rates due to higher operating temperature

Digested biosolids are more easily dewatered

Drawbacks Not as widely implemented as MAD

Consumes more energy for heating compared to MAD

Energy balance in colder climates may not be favorable compared to conventional MAD

Heat recovery from thermophilic digester effluent is challenging, inefficient

Pathogen regrowth is a concern

Requires control of digester gas and odors

Process and product odor is a concern; odors peak at 10 to 15 days after production

Class A pathogen reduction must be demonstrated by the operator before EPA will approve it; testing programs may require additional O&M cost

Large footprint

More sensitive to temperature changes than MAD

Moderate O&M costs due to higher heat demand and mixing needs



Site requirements: Application of thermophilic digestion may require a reduced footprint compared to mesophilic digestion processes. Utilizing tall cylindrical or egg-shaped digesters is likely required.


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Relative cost: High capital cost with moderate O&M costs.

Permitability: Proven permit process for a Class B biosolid. Class A can be achieved, but site-specific operating program and testing will be required ahead of approval.

Product Use: Land application as Class A or B biosolid.

Community: A decrease in the amount of truck traffic due to removal of lime addition and destruction of solids in the digestion process.

5.3.3.3 Enhanced Digestion Numerous variations on the anaerobic digestion process, from standard rate to two-stage digestion, have been developed over decades of practice. These variations have been developed in attempts to increase solids destruction, increase pathogen reduction, or reduce the time required to meet stabilization goals. Two variations, acid/gas digestion and two-stage digestion, are discussed below.

5.3.3.3.1 Acid/ Gas Digestion Acid/gas anaerobic digestion, also known as two-phase digestion, provides individual reactors to separate the acid formation (acidogenesis) reaction from the methane generation (methanogenesis) reaction. The use of separate reactors enables the operator to optimize the loading rate and detention time for each reactor. This separation allows for higher VSR and biogas production compared to a single-phase system. In addition to producing more biogas, the biogas also has a higher methane content. Two-phase anaerobic digestion systems also offer increased pathogen reduction and minimal foaming.

The first reactor is the acid-phase digester, a low-pH environment (pH 5.5 to 6.2), which receives the raw sludge. It is usually operated at mesophilic temperatures. The higher loading rate and short SRT of the acid-phase digester is an unfavorable environment for methane formers, thus making it favorable for acidogenic organisms. In this digester, accelerated hydrolysis of suspended organic matter takes place, and low-molecular-weight fatty acids are formed.

After a short SRT of one to two days in the acid-phase digester, the solids flow into the larger methane-phase digester, which is operated more like a conventional mesophilic or thermophilic digester and favors methane formers. The short-chain fatty acids present in the influent to the methane-phase digester are converted into biogas by the methanogens. This digester is designed to provide an SRT of 10 to 15 days. Higher loading rates to the methane-phase digester are possible because of the conditions that promote hydrolysis of the suspended organic matter in the acid-phase digester.

WEF MOP-8 reports mixed results from implementation of two-phase digestion. VSR was reportedly significantly improved at a facility in DuPage County, IL, but was not significantly increased at other facilities, including Denver, CO. There are more than ten two-phase digestion installations in North America.


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Students and faculty at Virginia Tech examined two-phase digestion at the Hampton Roads Sanitary District. The study found that acid-phase digesters operating at pH 5 did not appear to provide a significant benefit over conventional MAD, and while a lower SRT was possible, it was unlikely that the design parameters for the two-phase system would consider a lower combined acid-phase and methane-phase SRT compared to MAD. Furthermore, the need for separate acid-phase and methane-phase digesters would result in a larger footprint for the system and a higher construction cost.


Can operate at higher loading rates compared to MAD

Lower SRTs are possible

Minimal foaming

Higher VSR and biogas production, in some cases

Drawbacks Class A biosolids are not a guarantee

Unlikely to yield a benefit compared to conventional MAD

Larger footprint and higher construction cost

More complex compared to MAD

Requires chemical addition and heating to achieve acidogenic conditions in the acid-phase digester


Applicability to Arlington Development status: Established, but not as mature as single phase digestion


Site requirements: Application of multi-phase digestion can be achieved at the site. Utilizing tall cylindrical digesters is likely required.


Permitability: Proven permit process for a Class B biosolid. Class A can be achieved, but site specific operating program and testing will be required ahead of approval.

Product Use: Land application as a Class A or B biosolid.



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5.3.3.3.2 Temperature Phased Digestion In a temperature phased digestion system, two digester tanks are arranged in series. Among the many two-stage variations, one that is considered for Arlington County is temperature-phased anaerobic digestion (TPAD, or thermophilic/mesophilic).

The first reactor in a TPAD system is operated at thermophilic temperatures, and the second reactor is operated at mesophilic temperatures. Some TPAD installations use more than one mesophilic reactor. The residence times in each reactor are 4 to 10 days for the thermophilic stage and 6 to 12 days for the mesophilic stage; these times vary by installation. TPAD offers a higher thermophilic digestion rate, better performance during shock loads, less foam, and higher VS destruction compared to MAD. TPAD-digested biosolids reportedly dewater more easily compared to conventional MAD-digested biosolids, and the process is capable of meeting Class A pathogen reduction.

Similar to a single-stage thermophilic digestion system, the need to maintain a higher temperature in the thermophilic reactor results in higher energy costs. Heat recovery equipment can take advantage of the heat in the thermophilic solids by using it to heat the cold feed solids, heat buildings, or other uses. The transition from the thermophilic digester to the mesophilic digester requires cooling of the solids in sludge-to-water heat exchangers, which require cooling water pumps and a source of cooling water. In freezing weather, it may be possible for some facilities to discharge heat to the atmosphere rather than through the use of a cooling system.

As of 2007, more than 20 TPAD plants have been built in North America. Installations typically meet Class A requirements by providing batch processing under Formula H of Part 503 Alternative 1.


Provides higher VS destruction and enhanced pathogen reduction than MAD alone – can achieve Class A biosolids

Allows lower SRTs and digester volumes

Reduced foaming

Performs better during shock loads

Drawbacks Similar to drawbacks of single-stage thermophilic digestion

Not as common as conventional MAD


Temperature control is critical; low pH can result if controls are inadequate

Class A is not guaranteed with the process


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Applicability to Arlington Development status: Established, but not as mature as single-phase digestion


Site requirements: Application of TPAD can be achieved at the site. Utilizing tall cylindrical digesters is likely required.


Permitability: Class A can be achieved, but site-specific operating program and testing will be required ahead of approval.

Product Use: Land application as a Class A or B biosolid.


5.3.3.4 Anaerobic/ Aerobic Digestion Some benefits, including reduced ammonia recycle and improved dewaterability, have been observed when sequencing anaerobic digestion followed by aerobic digestion. One demonstration in Spokane, WA, indicated nitrification with partial denitrification resulting in 90-percent removal of ammonia at less than 10-day SRT in the aerobic digester. Improved dewatering and lower odors have also been observed.

Applicability to Arlington Development status: Innovative. Individual processes are established, benefits of aerobic

digestion stage are still being researched

Typical application scale: Amenable for medium and large applications. Arlington is expected to fall within the typical application scale.

Site requirements: Site constraints will result in challenges to layout the process tanks

Relative cost: High capital cost with high O&M costs. Benefit of process is reduced N loadings in post-digestion sidestreams

Permitability: The extent of pathogen reduction will be a function of the anaerobic digester process. The addition of aerobic digestion is not expected to impact the ability to permit the project.

Product Use: Land application as a Class B biosolid.



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5.3.3.5 Enzymatic Hydrolysis/ Anaerobic Digestion Also referred to as enzymic hydrolysis or biological hydrolysis, enzymatic hydrolysis involves the expansion of the acid phase of the digestion system into as many as six or more reactors in series, all operated at or slightly above mesophilic temperatures. The presence of multiple reactors allows a particular reaction to occur in each tank, and it also allows for a system that behaves more like a plug-flow reactor than a complete-mix batch reactor. Enzymes can either be added or produced naturally by microbes in the digestion process. The use of enzymes improves hydrolysis prior to acidogenesis and methanogenesis, which are the limiting steps in digestion. It can also remove unfavorable cations that are present after most conventional treatments.

Enzymatic hydrolysis can offer increased biogas production and VS destruction while making the solids easier to dewater. The resulting product can meet Class B pathogen reduction requirements, but this technology has only been implemented overseas. Enzymatic hydrolysis is difficult to scale to a large WWTP.

Advantages Increased biogas production, VS destruction

Enhanced enzymatic hydrolysis offers improved pathogen reduction

Can meet Class B pathogen reduction requirements

Small footprint

Low initial investment

Disadvantages No U.S. installations

Extra capital cost and high O&M costs associated with maintaining proper conditions in the digesters

Additional reactors mean added complexity


Applicability to Arlington Development status: Developing, no U.S. installations

Typical application scale: Amenable for small and medium applications. Arlington is within, but near the upper end, of the potential application scale.

Site requirements: The series of tanks required for the process may be challenging with the site constraints; however, less volume is expected to be required compared to mesophilic digestion. Utilizing tall cylindrical digesters is likely required.

Relative cost: High capital cost with high O&M costs.

Permitability: Mesophilic digestion is a proven process for a Class B biosolid; however, the addition of commercial enzymes to the process has not been permitted in the U.S.




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5.3.3.6 High Solids Digestion One consideration to reduce required digestion volume is to digest at high solids concentration. The higher solids concentration is more difficult to mix and requires pre-dewatering. Feed concentrations can be up to 20-percent solids. At this concentration, the reactor cannot be mixed, and operates as plug flow. However, a product meeting Class B pathogen reduction can be produced.

Applicability to Arlington Development status: Process is still developing with limited installations

Typical application scale: Amenable for medium and large applications. Arlington is expected to fall within the typical application scale.

Site requirements: Pre-dewatering is required, but the objective is to reduce the overall volume of digesters required to address site constraints.

Relative cost: High capital cost with moderate to high O&M costs.

Permitability: Digestion is likely to produce a Class B biosolid.



5.3.4 Screening Exercise and Results Scoring of the technologies against the screening criteria was reviewed. In general, the highest scores (eight or higher) in each grouping were selected as technologies to be considered moving ahead. Exceptions are noted in the comments section of Table 5-7 and Table 5-8.

Table 5-7. Screening Exercise for Digestion Stabilization Technologies

Technology Development

Application

Scale Site


Costs Permit-ability TOTAL

Aerobic Digestion 2 0 0 1 2 5

ATAD 2 2 1 1 2 8

Anaerobic Digestion

(single phase)

2 2 1 1 2 8

Anaerobic Digestion

(multi-phase)

1 2 1 1 2 7

Anaerobic/ Aerobic 1 0 1 1 2 5

Enzymatic Hydrolysis/

Anaerobic Digestion

1 2 1 1 2 7

High Solids Digestion 0 2 1 1 2 6


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Table 5-8. Screening Results for Digestion Stabilization Technologies

Technology

Score

(Max = 10) Comments

Consider For Further

Evaluation?

Aerobic Digestion 5 Does not meet screening threshold No

ATAD 8 Yes

Mesophilic Digestion 8 Anaerobic digestion (single or multi-phase) will be considered for further evaluation.

Yes

Anaerobic Digestion (multi-phase)

7 Anaerobic digestion (single or multi-phase) will be considered for further evaluation.

Yes

Anaerobic/ Aerobic Digestion

5 While technology did not score highly, it may be considered moving forward as an emerging technology, particularly for sidestream treatment benefits.

Yes

Enzymatic Hydrolysis/ Anaerobic Digestion

7 No

High Solids Digestion 6 No

5.4 Digestion Process Enhancements Technology Screening Mechanical, thermal, chemical, and/or biological treatment of solids prior to digestion with the goal of enhancing to the digestion process and reducing digestion volume requirements; digestion improvements vary, depending on WAS quantity and characteristics.

Many technologies have been developed and marketed, but a large number are no longer marketed. Alexandria uses pasteurization ahead of digestion to produce a Class A product; however, the vendor is no longer marketing the process. This section considers technologies viewed by the project team as either established or as developing with the potential for implementation in the future.

5.4.1 Thermal Hydrolysis Pretreatment Thermal hydrolysis pretreatment (THP) is a biosolids pre-treatment option that applies pressure and temperature to residuals prior to digestion. The THP system conditions solids by fracturing cellular material and long-chain fatty acids, thus making the solids more amenable to downstream digestion and dewatering processes.

THP is best suited for medium to large size facilities. Digestion after THP yields high VS destruction (up to 65 percent) and allows for increased loading to the digesters, as the drastically altered rheology of the solids makes them easier to pump and mix. This increase in loading results in smaller digester capacity requirements and increased biogas production.


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Prior to being fed to the THP system, the solids must be dewatered; depending on the system, the pre-dewatered cake solids content must be between 16 and 22 percent solids. Once injected into the THP, solids are treated for about 30 minutes at 166° C and up to 160 psi. These treatment conditions exceed those required by 40 CFR Part 503 for producing Class A biosolids. Coupled with anaerobic digestion followed by downstream mechanical dewatering (also called post-dewatering), the hydrolyzed, digested biosolids can produce a cake that typically exceeds 30 percent solids. The final dewatered or dried product exhibits excellent properties for soil blending and land application with low odor. It should be noted that a setup in which THP receives WAS only will result in a Class B end product. Utilities considering this option may also consider pasteurizing the primary solids to meet Class A pathogen requirements.

While THP has not been widely adopted in the U.S., the number of THP facilities is growing. The world’s largest THP installation is operational at the DC Water Blue Plains Advanced WWTP in Washington, D.C. This facility is the first U.S. THP installation, and several other facilities are under design or construction in the U.S.

The major suppliers of THP systems for wastewater solids are Cambi Group AS, based in Asker, Norway; and I. Krüger, Inc., part of Veolia Water Technologies, located in Cary, NC. Cambi is regarded as the market leader. Of the more than 50 worldwide installations of THP, Cambi holds the majority. Krüger/Veolia has one operating Exelys™ installation and seven operating BioThelys installations. Other suppliers are introducing thermal hydrolysis processes as the technology is seeing increasing interest worldwide.

Because these competitors’ systems differ significantly from one another, each is described in greater detail below.

5.4.1.1 CambiTHP™ by Cambi CambiTHP™ is a batch process consisting of three basic steps: solids heating in the pulper/pre-heater tank; heating, pressurization, and thermal hydrolysis in the reactor; and pressure release to the flash tank. Pre-dewatered cake at 16- to 18-percent solids is continuously fed from cake storage bins to the pulper, whose contents are mixed with circulation pumps and preheated with steam. Cake is then transferred to batch reactors, where steam is added to increase the temperature and pressure. The batch reactors are raised to a temperature of approximately 330 degrees F and 140 to 190 psi. After a prescribed amount of time has elapsed, a pressure discharge opens and allows steam to travel to the pulper. The remaining pressure is used to transfer the solids slurry through the blowdown valve to the flash tank. Excess flash steam from the flash tank is conveyed to the pulper to pre-heat the incoming cake.

The hydrolyzed solids are continuously removed from the flash tank by digester feed pumps, which convey the material to a hydrolyzed sludge booster/circulation system that increases the pressure and keeps the solids constantly moving to prevent settling. Between the flash tank and the digesters, the hydrolyzed solids are diluted with water (potable water or disinfected plant effluent) from 13- to 15-percent down to 8- to 12-percent solids. This dilution helps to reduce the ammonia concentration in the digesters, and it eases digester mixing. The dilution water also cools the solids, protecting the digester feed pump components from heat damage.


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Finally, the hydrolyzed solids are cooled by mixing with recycled digested solids and then routed to the digester. Heat exchangers may be added upstream of each digester to cool the solids to the proper temperature for high-rate digestion. A schematic of this process is shown in Figure 5-1. An example equipment installation is illustrated in Figure 5-2.

Figure 5-1. Simplified Schematic of Cambi THP Process

Figure 5-2. Rendering of Cambi Installation

Hydrolysed Solids to Digestion

Dewatered Solids

Steam

Recycled Steam

FLASH

TANK PULPER REACTOR


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5.4.1.2 Exelys™ by Krüger In contrast to the CambiTHP™ system, Krüger’s Exelys™ is a continuous plug flow process that requires a higher solids feed. Dewatered solids (up to 22 percent solids) from a storage silo is conveyed to the Exelys™ system via a progressive cavity pump. Steam is injected continuously and begins to heat the solids up to the level at which hydrolysis can occur. The heated solids pass through a self-cleaning static mixer before entering the reactor. The reactor operates within a temperature range of 285 to 330° F and a pressure range of 130 to 220 psi. After exiting the reactor, the solids enter a heat exchanger system, where excess thermal energy can be recovered and exported from the system.

Krüger offers two Exelys™ configurations, the most common of which is the Exelys-LD™. In the LD configuration, thermal lysis (L) is followed by digestion (D), as shown in the schematic in Figure 5-3. The process can also be configured into a digestion-lysis-digestion configuration, called Exelys-DLD™ (shown in Figure 5-4). In this configuration, solids are digested and dewatered before they are fed to the thermal hydrolysis reactor, thus reducing the solids loading on the reactor. Next, the dewatered, hydrolyzed solids are cooled and diluted before being sent to a second digester.

The Exelys-DLD™ configuration offers a number of advantages that are not available with conventional thermal hydrolysis followed by digestion. Approximately 20- to 30-percent of the total solids entering the first digester is converted to biogas. Since the first digestion step reduces the mass and volume of sludge, the Exelys-DLD™ system can be approximately two-thirds of the size required in an Exelys-LD™ configuration under the same conditions.

Figure 5-3. Exelys-LDTM Thermal Hydrolysis Configuration (image courtesy Krüger)


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5.4.1.3 Biotheyls™ by Krüger Krüger also markets a batch process called BiothelysTM, which is similar to CambiTHP™. The process utilizes three distinct stages: a pre-heating tank; batch thermal hydrolysis reactors; and a buffer tank. In the pre-heating tank, contents are heated to a temperature of 176 to 212 degrees F using flash steam recycled from the buffer tank. Pre-heated cake is then pumped through a dynamic mixer where steam is injected. At this point the sludge structure and viscosity are changed.

The material is then introduced into the batch reactors. The batch reactors operate in three distinct phases of fill, react, and drain. In nearly all instances one tank is in the fill phase, one tank in the drain phase, and the remaining one or two tanks are in the reaction phase. For the BioThelysTM process, no additional steam is injected into the batch reactors. The reactors operate at a temperature of 310-329 degrees F and a pressure of 101 psi. The reaction phase is a minimum of 20 minutes. During the drain phase, depressurization occurs in the buffer tank. Flash steam recovered in the buffer tank is recycled to the pre-heat tank. Depending on the design feed solids concentration, steam can be injected at the pre-heat tank by either: 1) direct injection using lances and recirculation pumps, or 2) injection ahead of the pre-heat tank using a dynamic mixer.

Temperatures in the buffer tank are typically 212 to 248 degrees F and cooling is required ahead of introduction to the anaerobic digesters. Cooling occurs through the introduction of water that dilutes the material to approximately 10 – 11 percent solids and pumping the diluted material through a dedicated heat exchanger.

Figure 5-4. Exelys-DLDTM Thermal Hydrolysis Configuration (image courtesy Krüger)


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5.4.1.4 Comparison While Exelys™ and CambiTHP™ both rely on thermal hydrolysis for solids conditioning, there are a few key differences in their designs. Because the Exelys™ system does not recycle steam like the CambiTHP™ system, the Exelys™ system requires more steam. The CambiTHP™ process produces biosolids that meet Class A requirements of the Part 503 Rule, while Exelys™ does not. Though the Exelys™ process meets the Class A time and temperature requirements, there is potential for the system to short circuit since it is not a batch process. The BioThelysTM process also operates in batch mode and can meet the Class A requirements.

Advantages Reduces digester capacity requirements (and capital costs for construction of digesters and

supporting equipment) through increased loading to the digesters

Enhances biogas production and VSR in digesters, further reducing volume of biosolids to be disposed

Eliminates the need for digester heating equipment

Produces solids that are less odorous, easier to dewater and dispose

Can achieve Class A pathogen reduction (see above)

When coupled with AD, has low O&M costs; potentially energy-neutral

Drawbacks Requires additional solids screening and pre-dewatering upstream of THP

High temperature, high pressure processes and boilers may require specialized O&M staff

Depending on the amount of biogas available, additional fuel input may be needed to produce steam

High capital cost

Applicability to Arlington Development status: Established – Cambi has multiple installations worldwide, including

the world’s largest at DC Water’s Blue Plains WWTP. Other processes are still developing in the U.S. and international market.


Site requirements: Application of THP can be achieved at the site. Utilizing tall cylindrical digesters is required.


Permitability: Coupled with anaerobic digestion, THP is a proven process for Class A biosolids.

Product Use: Land application or distribution and marketing as Class A biosolids.


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Community: A decrease in the amount of truck traffic due to removal of lime addition, destruction of solids in the digestion process, and improved dewaterability of the product.

5.4.2 Thermal Hydrolysis – Post-Digestion Thermal hydrolysis following digestion is currently in development as a process aimed to increase the dewaterability of the digestion solids. The process is capable of meeting Class A pathogen requirements; however, the full requirements for land application as a Class A biosolids may not be satisfied. This is because the 40 CFR 503 rules require pathogen reduction to be achieved before or at the same time as VAR is achieved.

Currently only one technology provider is marketing a post-digestion thermal hydrolysis process. Cambi’s SolidStream™ is aimed to improve the dewaterability of the digested solids. Increases from 20- to 25-percent dewatered cake solids to over 40-percent dewatered cake solids can be achieved.

Applicability to Arlington Development status: Process is still in development.


Site requirements: Application of post-digestion THP will be challenging given the site constraints.


Permitability: Process currently has no impact on pathogen class. Under EPA requirements, Class A pathogen reduction must be achieved before or at the same time as VAR.

Product Use: Land application or distribution as a soil amendment if Class A compliance can be demonstrated.

Community: A decrease in the amount of truck traffic due to removal of lime addition, destruction of solids in the digestion process, and improved dewaterability of the product.

5.4.3 Thermochemical Hydrolysis In a chemical hydrolysis system, chemicals are added to hydrolyze cell walls and solubilize their contents, increasing VS destruction and biogas production. The chemicals used for hydrolysis can vary but are usually acids, bases, or oxidizers. Thermochemical hydrolysis systems require downstream anaerobic digestion in order to meet Class B pathogen reduction requirements.

The Pondus Thermo-Chemical Hydrolysis Process (TCHP), by cnp-Technology Water and Biosolids Corporation, continuously hydrolyzes thickened solids through the combination of heat and application of caustic soda (sodium hydroxide) in the hydrolysis reactor. Class A pathogen reduction is not achieved in the hydrolysis step alone; instead, the Pondus system relies on digestion and a final thermal drying step to produce a Class A dried product. The thermally/chemically hydrolyzed WAS and thickened primary solids are fed to a primary digester, followed by a single secondary digester. Waste heat from CHP is used for drying.


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cnp-Technology Water and Biosolids Corporation, located in Kenosha, WI, has a single installation, also located in Kenosha. This facility is currently in startup, and there are two to three installations overseas.

Lystek, a Canadian firm known for its alkaline stabilization system, is also developing a similar low-temperature thermochemical hydrolysis system for digestion pretreatment. The system combines low-pressure steam, high-shear mixing, and a pH of 9.5 (adjusted through alkali addition) at ambient temperatures to achieve hydrolysis. The process is simple to operate and has a small footprint, and it can be designed to provide pre- or post-digestion treatment.

Advantages Achieves Class B pathogen reduction with downstream anaerobic digestion

Can achieve Class A pathogen reduction if digestion and drying are included

Enhanced biogas production and VS destruction

cnp-Pondus system allows beneficial use of waste heat for drying process

Drawbacks Handling and storage of chemicals presents a safety risk

Added operational complexity

Added O&M cost for chemicals

cnp-Pondus is typically WAS-only; requires separate thickening of primary and WAS, if there are primary solids

Depends upon digestion and potentially drying to achieve Class A pathogen reduction

Applicability to Arlington Development status: Developing, technology is established, but limited application with

municipal wastewater solids


Site requirements: Application can be achieved at the site. Utilizing tall cylindrical digesters is likely required.

Relative cost: High capital cost (although slightly lower than THP) with moderate O&M costs.

Permitability: Class A can be achieved if all solids (not just WAS) are pre-treated.

Product Use: Land application as a Class A or Class B biosolid; distribution as a soil amendment if Class A is achieved.

Community: A decrease in the amount of truck traffic due to removal of lime addition, destruction of solids in the digestion process, improved dewaterability of the final product, and potential drying.


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5.4.4 Cavitation Cavitation occurs when a liquid is subjected to rapid pressure changes. These changes result in the formation of cavities, or voids of low pressure, that implode when they are subsequently subjected to higher pressure. This implosion generates a shock wave. While uncontrolled, undesired cavitation can cause significant wear in pumps, controlled cavitation can be useful in breaking up the cell walls in wastewater sludges to release their cellular materials. This increases biogas production and VS destruction in a downstream digestion process.

Two of the more common methods of sludge cavitation are ultrasound and hydrodynamic cavitation. These systems have been marketed in various forms over the years. In their current form, cavitation systems remain in various stages of development. Cavitation system vendors offer equipment packages that are touted as energy- and space-efficient, but they have experienced difficulties with scaling of the technology.

Advantages Achieves Class B pathogen reduction with downstream anaerobic digestion

Enhanced biogas production and VS destruction

Drawbacks Not widely used

Currently experiencing issues moving beyond lab scale to pilot or full scale.

Applicability to Arlington Development status: Developing

Typical application scale: Small scale at present, increasing scale seems to increase cost significantly

Site requirements: Can be located at WPCP.

Relative cost: True cost is unknown, but expected high capital cost with high O&M costs.

Permitability: Land application requirements are met with downstream anaerobic digesters. Cavitation does not impact permitability of the product.



5.4.5 Screening Exercise and Results Scoring of the technologies against the screening criteria was reviewed. In general, the highest scores (eight or higher) in each grouping were selected as technologies to be considered moving ahead. Exceptions are noted in the comments section of Table 5-9 and Table 5-10.


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Table 5-9. Digestion Process Enhancements Comparison Matrix Screening

Technology Development Application

Scale Site


Costs Permit-ability TOTAL

Thermal Hydrolysis – Pre Digestion

2 2 1 1 2 8

Thermal Hydrolysis – Post Digestion

0 2 1 1 1 5

Cavitation 0 0 1 1 2 4 Thermochemical Hydrolysis

0 0 1 1 2 4

Table 5-10. Digestion Process Enhancements Screening Evaluation Results

Technology Score

(Max = 10) Comments Consider For Further Evaluation?

Thermal Hydrolysis – Pre-Digestion 8 Yes

Thermal Hydrolysis – Post-Digestion 5 No

Cavitation 4 No

Thermochemical Hydrolysis 4 No

5.5 Drying Technology Screening Drying is the process of applying heat to evaporate water from biosolids, and offers the benefits of increased solids concentration and volume reduction. Generally, three types of drying technologies are considered for biosolids applications: solar dryers, direct thermal dryers, and indirect thermal dryers. Solar drying uses air heated by radiative solar energy. Typically, solar drying processes result in a total solids content between 70- and 80-percent.

Direct thermal drying processes use direct contact between the biosolids and the drying air. Examples include rotary drum dryers, belt dryers, microwave dryers, and fluid bed dryers. Because these technologies use large amounts of air in direct contact with the biosolids, large air pollution control (APC) systems are typically required. In some cases, a regenerative thermal oxidizer (RTO) may be recommended for Volatile Organic Compounds (VOC) reduction and hazardous air pollutant (HAP) elimination.

Conversely, indirect thermal dryers do not use direct contact between the material and heated air. Rather, the dryer shell is heated and the biosolids material is dried via conduction and radiation. Examples of indirect thermal drying technologies include paddle/disc dryers, thin-film dryers, and tray dryers.

Thermal drying typically involves the use of a supplemental energy source to produce heat, such as fuel or biogas. In some cases, the dryer product itself can be used as a furnace fuel, which is used to produce the heat for drying. Typically, thermal drying processes are capable of producing 90-percent total solids or greater.


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It is recommended that drying be coupled with an upstream digestion process for a number of reasons. First, the biogas produced in the digestion process can be used as a fuel supplement in the drying process. Second, digestion would result in a reduction in feed quantity to the dryer. Lastly, pairing anaerobic digestion with drying results in an improved product quality, particularly in terms of reduced odor potential.

Overall, drying significantly reduces the amount of product hauling, but requires a substantial capital investment and ongoing energy costs to operate the dryer. Without modifications to the Arlington County WPCP site, finding the appropriate space for a dryer may prove challenging, depending on the type of dryer. In a review of multiple dryer configurations, the project team found that the technologies most applicable to facilities of similar size to Arlington County are drum dryers, belt dryers, paddle/disc dryers, and tray dryers. Tray dryers were of particular interest to County staff due to their potential footprint advantages over other dryer technologies, however the project team was not aware of any vendors currently marketing tray dryers.

While the decision on type of dryer can be deferred, it should be noted that different types of dryers will produce different quality products. It is important to understand the biosolids product market when selecting the type of dryer. The value of dried product has ranged from $0 to $40 per ton. A full biosolids market assessment of the Mid-Atlantic region is presented in Section 8. It is worth noting that several other utilities in the region have drying facilities, including Leesburg, Baltimore, Hagerstown, and the Upper Occoquan Service Authority (UOSA); however, the excess drying capacity is limited at these facilities. Additionally, it is unknown if these facilities would be willing to accept undigested solids. Opportunities to haul and use drying capacity at existing facilities may be considered as part of a regional solution.

A comparison of various drying processes is summarized in Table 5-11 below.

Table 5-11. Summary of Drying Processes

Technology Biosolids Product Odor Potential Cost Solar Dryer Class B Cake High, Control Required Low O&M, High Capital

Drum Dryer Class A Pellet High, Control Required High Capital, Potentially High O&M

Belt Dryer Class A Non-Pellet (if not coupled with a pelletizer)

High, Control Required High Capital, Potentially High O&M

Microwave Dryer Class A Non-Pellet High, Control Required High Capital, Potentially High O&M

Fluid Bed Dryer Class A Non-Pellet High, Control Required High Capital, Potentially High O&M

Paddle/Disc Dryer Class A Pellet Control Required, Reduced Air Volume

High Capital, Potentially High O&M

Thin-Film Dryer Class A Non-Pellet Control Required, Reduced Air Volume


Tray Dryer Class A Non-Pellet Control Required, Reduced Air Volume



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Advantages Thermal drying (all technologies listed except solar drying) produces a marketable, high-quality Class A biosolids product. Pellet products are typically more marketable, since both bulk and retail markets are well-established. However, all Class A dried products can be used for land application or fertilizer blending. Dried product can also be used as a furnace fuel. As opposed to dewatering, which is capable of producing a product that is approximately 20- to 40-percent solids, thermal drying typically results in a product with a solids content of 90-percent or greater. Since drying significantly reduces the volume of biosolids, it greatly reduces the amount of product hauling required.

Drawbacks Drying can have high capital and operating costs. In the case of solar drying, a Class A product is typically not produced, and a relatively large footprint is required. Direct thermal drying technologies require significant amounts of carrier gas and require large APC systems to treat the exhaust gas. While smaller than solar drying systems, thermal drying technologies would still present footprint challenges at Arlington County WPCP due to space constraints. Both direct and indirect thermal drying processes require an external heating medium, which may be steam, warm water, thermal oil, or electricity.

Applicability to Arlington County Development status: Some technologies are established and mature (solar dryer; rotary

drum dryer; paddle/disc dryer, belt dryer). Others have limited use or are innovative (microwave dryer; fluid bed dryer; thin film dryer; tray dryer).

Typical application scale: Amenable for medium and large-scale applications.

Site requirements: Finding the appropriate space for a dryer may prove challenging without modifications to the existing site.

Relative cost: Addition of a drying process would require a substantial capital investment and ongoing energy costs to operate the dryer.

Permitability: Thermal drying technologies capable of producing Class A product; have proven permit process and product familiarity. Less certainty about product quality and permitting of solar drying process.

Product Use: May be used for land application and fertilizer blending. Dried product may also be used as a furnace fuel.

Community: Reduction in the amount of current truck traffic. Production of an “Arlington” branded product to the community for use.

The comparison matrix used to screen drying technologies is presented in Table 5-12. The results of the screening evaluation are presented in Table 5-13. Ultimately, the four technologies considered for further evaluation were the drum dryer, belt dryer, paddle/disc dryer, and tray dryer.


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Table 5-12. Dryer Technology Comparison Matrix - Screening

Dryer Technology

Development Application

Scale Site


Costs Permit-

ability TOTAL

Solar 2 0 0 1 1 4

Drum 2 2 1 1 2 8

Belt 2 2 1 1 2 8

Microwave 0 0 1 1 2 4

Fluid-Bed 1 0 1 1 2 5

Paddle/Disc 2 2 1 1 2 8

Thin-Film 1 0 1 1 2 5

Tray 1 2 1 1 2 7

Table 5-13. Dryer Technology Screening Evaluation Results

Dryer Technology

Score

(Max = 10) Comments

Consider For Further Evaluation?

Solar 4 Does not meet screening threshold No

Drum 8 Yes

Belt 8 Yes

Microwave 4 Does not meet screening threshold No

Fluid-Bed 5 Does not meet screening threshold No

Paddle/Disc 8 Yes

Thin-Film 5 Does not meet screening threshold No

Tray 7

Will consider due to potential advantages with equipment footprint compared to other drying technologies; however, uncertainty exists about the manufacturer’s marketing of the equipment.

Yes

5.6 Thermal Processes Technology Screening Thermal conversion systems are designed to expose the solids to high temperatures in order to achieve pathogen reduction requirements, convert the material into useful byproducts, reduce the volume of solids to be disposed, or some combination of all three objectives. The exposure to high temperature and pressure evaporates a large fraction of the water in the material, reducing its volume and disposal cost. Thermal oxidation processes, such as incineration, also combust (burn) the solids to further reduce their volume. Gasification and pyrolysis processes can convert the solids into beneficial products, such as syngas and biochar.


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Because VS in the sludge can be combusted to produce useful energy, digestion is not a component of the process train at facilities that use thermal processes. As the equipment is sized based on the evaporative capacity, dewatering or drying of the feed solids is an essential step prior to the thermal process in order to keep the size and cost of the equipment to a minimum.

5.6.1 Incineration Incineration, a thermal oxidation process, exposes solids to high temperatures inside a closed reactor in the presence of excess air. The evaporation of water and complete burning of combustible matter in the solids produces carbon dioxide, water vapor, sulfur dioxide (SO2), and inert ash. Incineration is considered the most mature of the thermal processes, with many others in the demonstration and development stages.

The two major types of incinerators in North America are multiple-hearth furnaces (MHF) and fluidized bed incinerators (FBI or FBIS). Because of Arlington County’s familiarity with MHFs, the following sections are focused on a discussion of FBISs, as well as FBIS’ advantages and disadvantages compared to MHFs and other disposal technologies.

5.6.1.1 Multiple Hearth Furnace A MHF consists of several stacked, horizontal refractory shelves, or hearths, enclosed within a vertical, refractory-lined carbon steel cylindrical vessel. The furnace has zones for drying, combustion, and cooling. In the drying zone, the temperature increases from about 800 °F to 1,400 °F; in the combustion zone, the combustible material in the solids burns at 1,400 °F to 1,700 °F. In the lowest hearths of the furnace, the incoming combustion air cools the ash to between 200 °F and 400 °F before the ash drops out of the bottom of the furnace.

At the center of the hearth is a common, cast-iron rotating shaft with attached rabble arms. These rabble arms, which may be designed to carry cooling air, have teeth that sweep and plow the solids on each hearth over and over again, exposing fresh surfaces to the hot air that enters at the bottom of the hearth and flows upward through holes in the hearths. Solids are introduced at the top of the furnace; the arrangement of the hearths and rabble arms causes the movement of the solids to alternate between outward and inward as they drop from one hearth to the next.

Part 503 regulations are favorable to MHF upgrades; in order to meet Part 503 emissions requirements and improve efficiency, MHF suppliers have introduced various improvements, including afterburners, upgraded burner designs, dual-fuel burners, and VFDs. MHFs have generally been reliable and easy to operate. However, WEF reports that since 1988, more than 50 FBISs have been installed at WWTPs in North America. Out of these installations, 18 have replaced existing MHFs. Aging infrastructure and the ability to upgrade MHFs in a cost-effective manner must be considered when evaluating the long-term viability of MHFs.

5.6.1.2 Fluidized Bed Incineration Fluidized bed incineration combusts solids in a high-temperature reactor containing a bed of fluidized sand. The fluidized sand is a hot, turbulent suspension of sand and combustion gases that provides a large surface area for combustion. FBIS are preferred over MHFs because of their ability to achieve better flue gas emissions at lower O&M costs.


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The incineration process reduces the quantity of solids to be disposed by about 95-percent. The feed solids must be dewatered to between 20- and 35-percent solids in order for them to combust properly in the reactor.

Figure 5-5 presents a process schematic for a typical FBIS. The principal component of an FBIS is the fluidized bed combustor (FBC), also known as the fluidized bed reactor. The FBC is a cylindrical, refractory-lined, carbon steel vessel containing three sections: the windbox, the bed, and the freeboard. The windbox is a chamber at the bottom of the vessel which supports a refractory arch that is perforated with tuyeres (air distribution nozzles). A bed of silica or olivine sand lies on top of the refractory arch; in its unfluidized state, the bed is about three- to four-feet deep. The bed is the primary combustion zone. The freeboard is a large open space above the bed which acts as an afterburner.

Figure 5-5. Schematic for Typical FBIS

The bed is initially preheated to combustion temperatures by firing the preheat burner and also by injecting auxiliary fuel directly into the bed. Fluidizing air (which also serves as combustion air) is blown into the windbox and up into the bed to fluidize the sand. Once the bed temperature is raised to 1,250° F, the biosolids can be injected into the bed. In the fluidized bed, evaporation of water in the biosolids and incineration of combustible matter occurs concurrently and instantaneously. Completion of the combustion reactions occurs in the freeboard. A long gas residence time of at least seven seconds and high temperatures (greater than 1,400° F) in the freeboard ensure complete combustion of organic matter and low emissions of carbon monoxide (CO). Also, the reduced gas velocity in the freeboard allows the sand to disengage from the gas stream and fall back into the bed.

The sand in the bed of the FBC will eventually break down into very fine particles and will be carried out with the flue gases. A reduction in the pressure drop across the sand bed will be the operator’s indicator that periodic sand addition is required. Replacement sand is pneumatically conveyed from a nearby sand storage silo to the FBC.


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Water, as required, is fed through bed water guns into the incinerator. Water addition is required when high BTU wastes and/or high-percent solids are being burned in the incinerator and cause freeboard temperatures to rise above the maximum permitted temperature limit. The FBC is also equipped with high-pressure roof sprays, which are used to add water to the flue gas and reduce excessively high flue-gas temperatures. Flue gas temperatures greater than 1,650° F can damage the downstream combustion air heat exchanger.

Natural gas is typically used to fire the preheat burner. Auxiliary fuels, such as fuel oil or processed fats, oils, and grease (FOG), or natural gas can be fed into the bed through fuel injectors. The FBIS control system will automatically add auxiliary fuel when the combustion temperature falls below a predetermined set point temperature. Depending on the heating value and moisture content of the feed solids, its combustion can become a self-sustaining reaction, also known as autogenous combustion, where no auxiliary fuel is needed.

The hot flue gas from the FBC enters the combustion air preheater where the thermal energy in the flue gas is used to preheat the incoming combustion air, and thereby minimizes auxiliary fuel use. The combustion air preheater is a large vertical flue-gas-thru-the-tube heat exchanger which reduces the flue gas temperature from a minimum temperature of 1,400° F to approximately 1,050° F, while heating the combustion air from 180° F to 1,200° F. If there are beneficial uses for the heat of the flue gas, the gas may next proceed to waste heat recovery equipment, such as a boiler or a thermal oil heat exchanger.

Following the heat recovery system, the flue gas enters the secondary heat exchanger, which is used to raise the temperature of the cooled flue gas after it exits the wet scrubbing system. The secondary heat exchanger is a flue-gas-thru-the-tube heat exchanger, similar to the combustion air preheater but considerably smaller in size. The secondary heat exchanger raises the temperature of the scrubber exhaust from 110° F to 160° F, which is required by the downstream carbon absorber.

From the secondary heat exchanger, the flue gas is conveyed through the APC train consisting of a venturi scrubber, a tray scrubber, a wet electrostatic precipitator (WESP), and a carbon absorber. In the venturi scrubber, the flue gas is forced through a narrow constriction (the throat) which is flooded with water. A water column pressure drop of approximately 20- to 25-inches is maintained across the throat, atomizing the water into fine droplets that entrain and capture most of the particulate matter in the flue gas. The particulates and scrubber water exit the bottom of the unit.

The flue gas then enters the tray scrubber, which contains two sections: a subcooling section and an acid gas removal section. In the subcooling section, a large flow of plant water is used to subcool the flue gas to 110° F. Additional particulate matter is removed in the subcooler, and the large volume of water vapor in the flue gas is condensed, thus minimizing the vapor plume from the exhaust stack. In the acid gas removal section, additional plant water is used to remove SO2, hydrogen chloride (HCl), and other acid gases. Typically, the plant water has sufficient alkalinity to control the acid gases. If needed, additional alkalinity in the form of caustic soda (NaOH) is supplied from a nearby storage tank and chemical metering pump system.


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Following the tray scrubber, the scrubbed flue gas enters the WESP, which removes sub-micron particulate matter including metals and salts. The cleansed flue gas then proceeds through a demister, which removes fine water droplets from the flue gas, and then travels through the secondary heat exchanger to be reheated to 155° F. The flue gas then enters the ID fan, which provides the static pressure necessary to convey the flue gas through the waste heat recovery and APC systems.

The ID fan discharges the flue gas to the carbon absorber, which removes mercury. The carbon absorber is a large, cylindrical vessel containing three layers of media: a prefilter of zeolite for fine dust removal and two layers of customized activated carbon. The carbon layers are actually a mixture of sulfur-impregnated activated carbon and an inert adsorbent. The mercury is adsorbed as mercuric sulfide and then converted to mercuric sulfate in a solid phase. Since these chemical reactions are exothermic, the inert adsorbent is required to dissipate the heat of the chemical reactions. After approximately two years, the carbon layers will become loaded with mercuric sulfate and will have to be disposed of as hazardous waste. Following the carbon adsorber, the cleansed flue gas is discharged through an exhaust stack to the atmosphere.

A continuous emission monitoring system (CEMS) is provided for analysis of oxygen (O2), CO, and any other pollutants required to be monitored. The system extracts a gas sample from the stack, transports the sample back to the CEMS cabinet for conditioning, and then sends the sample through gas analyzers.

The scrubber water from the venturi scrubber is sent to an ash thickener, where the particulate matter (ash) will settle out of solution. Dewatering of the ash slurry can be accomplished through gravity drainage of ash storage basins or through the use of dewatering equipment, such as rotary drum filters, which can dewater the ash slurry to a 30- to 40-percent solids cake. Depending on the concentrations of constituents in the ash, the material may be suitable for landfill disposal, or it may be subject to special waste or hazardous waste regulations.

5.6.1.3 Alternatives for Incineration for Arlington County Fairfax County operates four MHFs at the Noman M. Cole Jr. Pollution Control Plant (NMCPCP), located approximately 15 miles from the Arlington County WPCP. Solids cake pumps are piped such that the dewatered solids can be pumped to any one of the four MHFs. Two MHFs are located in Building K1 and two are located in Building K2. Incinerators P1 and P2 in Building K1 are 18-ft-diameter, seven-hearth furnaces; Incinerators P3 and P4 in Building K2 are larger 25-ft-diameter, 6-hearth units. The County typically operates P3 or P4 and one dewatering train, keeping the other incinerator in standby mode.

The MHFs have a rated capacity, based on average solids concentration, of 60 DTPD, but the current solids throughput is approximately 40 DTPD. Fairfax County estimates that 15- to 20-DTPD capacity is available. Both Arlington County and Fairfax County have expressed an interest in taking advantage of this unused capacity at the NMCPCP by sending Arlington County’s solids to the facility. Arlington County would pay a tipping fee to Fairfax County to accept the solids.


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Because NMCPCP does not have facilities for receiving dewatered cake, Arlington County’s liquid sludges would be brought to the plant via tanker truck. Once received, the solids would be lime conditioned (to reduce clinkers in the incinerators), dewatered, and incinerated. Energy recovery equipment is not yet installed at NMCPCP but is being considered for Incinerators P3 and P4. Fairfax County’s payback scenario improves with additional solids processing. Once the energy recovery system is online, the value of the energy produced from the incineration of Arlington’s solids could be made a part of the overall contract and tipping fee structure.

Additional dewatering and incineration capacity is available at NMCPCP, but to utilize this capacity would require operation of additional dewatering and incineration trains and additional O&M cost. Therefore, if Fairfax County were to handle more than 20 DTPD of Arlington County’s solids, they would do so at higher cost. While contracting with Fairfax County may not be a viable long-term strategy, it may have value as an intermediate option for project phasing purposes.

Advantages & Disadvantages of Sending Solids to Fairfax County Advantages Utilizes capacity of an existing facility.

Only requires the solids to be thickened; reduces O&M costs associated with further treatment of the solids beyond thickening.

Solids would be transported in closed tanker trucks to minimize odor.

Could be a valuable intermediate treatment strategy while improvements at Arlington County WPCP are constructed.

Disadvantages NMCPCP cannot accept Arlington County WPCP’s entire solids output.

Increases truck traffic within and outside Arlington County WPCP due to hauling of liquid sludges with a lower solids content (i.e., thickened solids).

Not a long-term treatment strategy.

Applicability to Arlington County Development status: Established technology, although the number of manufacturers is

limited

Typical application scale: Amenable for medium and large-scale applications.

Site requirements: Not considered acceptable as an on-site solution. May be implemented under agreement with Fairfax County at the NMCPCP

Relative cost: Limited capital, but high O&M costs associated with hauling and fees paid to Fairfax County. Not considered a viable long-term solution

Permitability: Significant uncertainty related to the air quality permits

Product Use: No identified product use for the ash

Community: Significant increase in amount of truck traffic if hauling thickened (not dewatered) solids from the WPCP


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5.6.2 Gasification Gasification involves the application of high temperatures to organic solids in an oxygen starved environment. Products from the gasifier include syngas and ash. Gasification of biosolids has several drawbacks, including the ‘wet’ condition of dewatered biosolids requiring significant quantity of water evaporation ahead of the gasifier. Drying would be accomplished in a thermal dryer that is a part of the gasification process train. Multiple vendors have experience in biomass gasification; however, experience with biosolids is limited and generally has not proven sustainable. The Sanford, FL project operated for a short period of time but has been shut down.

Gasification may be promising as a future technology for biosolids, but limited experience at present.

5.6.3 Pyrolysis Pyrolysis is a thermal process in which biosolids are converted into biochar, along with pyrolysis gas and bio-oil, which can be used as energy sources. Biochar’s resistance to biological and physical degradation when incorporated into soils makes it a valuable soil amendment. The pyrolysis process involves the thermochemical decomposition of organic material by heating in the absence of oxygen or any other reagents. Possible concerns with this process include handling of the oil, char, and gas char products and the possibility of combustion once contacted with oxygen. Pyrolysis has limited operational experience and is considered a developing technology when applied to biosolids.

5.6.4 Supercritical Water Oxidation Supercritical water oxidation utilizes high temperature, high pressure reactor vessel to convert biosolids to inert gases (CO2 and N2), water, and inert material. The inputs are thickened solids, heat, and oxygen. The theory behind the technology is to raise the temperature and pressure of the solids above the critical point for water (374° C and 22.1 MPs). The technology is emerging and has demonstrated a high operating and maintenance cost at small scale.

5.6.5 Catalytic Hydrothermal Gasification Catalytic hydrothermal gasification involves the conversion of biomass into a synthetic gas product similar to other gasification processes. The hydrothermal processing utilizes temperatures above 300° C and pressures above 2,300 psia to maintain the water in the biomass in a liquid phase. The technology is still under development and is not ready for application at full-scale.

5.6.6 Wet Air Oxidation and VERTAD Wet air oxidation is another form of treatment using heated water, air, and pressure. The Zimpro process is considered a form of wet oxidation. VERTAD is another form of wet oxidation that uses a deep-shaft to generate pressure and heat needed for the reactions. The shaft is a 10-foot diameter shaft extending more than 300-feet below ground. Thickened solids are pumped through a pipeline to the bottom of the shaft. Stabilized solids are removed from the lower portion of the shaft for additional handling (thickening or dewatering). Limited installations of the process have been identified with the majority serving as a biological process for BOD removal.


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5.6.7 Anuvia Anuvia is a fertilizer manufacturer that produces fertilizer using biosolids as an organic component of a modified ammonium sulfate process. Anuvia produces Class A product, but the bigger market is manufacturing fertilizer for commercial uses. Currently, Anuvia constructs the facility and contracts a tipping fee. This would be implemented as an off-site solution for Arlington County.

5.6.8 Results of Screening Evaluation for Thermal Processes The technologies were scored using the screening criteria listed in Table 5-14. In general, the highest scores (eight or higher) in each grouping were selected as technologies to be considered moving ahead. Exceptions are noted in the comments section of the table.

Table 5-14. Thermal Process Technology Comparison Matrix Screening

Technology Development Application Scale

Site Requirements

Relative Costs

Permit-ability

TOTAL

Incineration 2 2 1 1 0 6 Gasification 0 2 2 1 1 6 Pyrolysis 0 2 2 1 0 5 Supercritical Water Oxidation 0 2 0 1 0 3

Hydrothermal Catalytic Gasification 0 2 1 1 0 4

Wet Air Oxidation 0 0 2 1 1 4 Anuvia 0 2 2 (offsite) 2 1 7

Table 5-15. Thermal Process Technology Screening Evaluation Results

Technology Score

(Max = 10) Comments


Incineration 6 Potential near-term off-site/regional solution; previously existed at site, but not considered permittable on-site.

Yes

Gasification 6 Limited demonstration with biosolids, especially at full scale facilities. No

Pyrolysis 5 Limited biosolids experience. No Supercritical Water Oxidation 3 No

Hydrothermal Catalytic Gasification 4 No

Wet Air Oxidation 4 Limited information on technology and installations available. Additional research being completed.

No

Anuvia 7 Off-site solution with contracted/merchant arrangement for implementation. Yes


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5.7 Solids Thickening Technology Screening Numerous processes and technologies exist for thickening of primary solids and WAS generated at municipal WWTPs. Table 5-16 lists several of these processes.

Table 5-16. Process Options for Thickening Arlington County WPCP

Process Description Typical Thickened Solids

Typical Solids Capture

Efficiency

Gravity Thickening

Similar to clarifier or settling tank; currently used for thickening primary solids.

3% to 5% (2% to 3% when thickening

WAS) > 85%

DAF Thickening Existing WAS thickening process; air bubbles injected into the flow attach to floc, causing it to rise; solids are skimmed from the surface.

3% to 5% >90%

Centrifuge Thickening

Rotating bowl and scroll use centrifugal force to separate solids and liquids; can be used with or without polymer.

5% to 8% > 90%

Gravity Belt Thickening (GBT)

Water is drained from flocculated solids on a fabric mesh belt; belt wash cleans belt before return to the influent end of equipment.

4% to 8% > 95%

Rotary Drum Thickening (RDT)

Rotating drum conveys flocculated feed through basket; basket allows drainage of water with flocculated solids being retained.

4% to 8% > 95%

Rotary Screw Thickening (RST)

Rotating screw conveys flocculated feed through and inclined basket; mesh on basket allows drainage of water with flocculated solids being retained.

4% to 8% > 95%

The following sections review the evaluation and comparison of these technologies.

5.7.1 Gravity Thickening Gravity thickeners are used in Arlington County WPCP’s existing primary solids thickening process. Each of the two tanks is 65 feet in diameter and has a side wall depth of 23 feet. The tanks have dome fiberglass covers and the headspace is ventilated to an odor control system. Primary solids are pumped to the thickeners at 1.5 to 2 percent solids. Gravity thickener solids are removed at 3.5 to 4 percent solids. Overflow from the gravity thickeners is returned to the main liquids process at the plant headworks.

Gravity thickeners are used across all sizes of treatment facilities. They also require the lowest amount of operator attention compared to the other thickening processes considered. If uncovered, gravity thickeners can be a potential source of odor. Dilution water can be used to control odors by “freshening” the primary solids. The existing gravity thickeners at Arlington County WPCP have fiberglass covers to capture foul air, which is sent to the odor control system.


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Applicability to Arlington Development status: Technology is established and mature.

Typical application scale: Amenable for small, medium, and large applications.

Site requirements: Existing gravity thickeners on-site for primary solids thickening. Large footprint compared to other thickening processes.

Relative cost: Relatively low capital cost to upgrade gravity thickener equipment, compared to other thickening options. Relatively low O&M costs.

5.7.2 Dissolved Air Flotation Thickening DAFT are used for Arlington County WPCP’s existing WAS thickening process. The units utilize air compressors, recycle flow, and a retention tank to introduce air to the process. Floating solids are skimmed from the surface and collected in a trough at the end of the tank. Subnatant is removed through effluent piping at the opposite end of the tank and is returned to the head of the plant. The technology has been in use since the 1960s, and advancements over the past 50 years have kept the technology viable for thickening biological solids. DAF thickening is suitable for thickening WAS and primary scum; however, primary scum can impact the performance of the unit with respect to solids concentration and efficiency of solids capture.

DAFT use air bubbles to buoy solids particles to the surface, where they are skimmed from the flotation tank. The basic equipment necessary for a DAF includes a flotation tank with surface skimmer, air saturation tank, recycle pump, air compressor, and associated piping and valves. The flotation tanks can be circular or rectangular. The tanks at Arlington County WPCP are rectangular with a chain-and-flight float collector mechanism, as well as lower scraper mechanisms to remove solids that settle to the bottom of the tank.

While it is not necessary for DAF operation, polymer is often used to enhance thickening performance. The polymer serves to increase solids capture during peaks in solids loading and to increase the solids concentration of the captured material. The existing DAF units at the Arlington County WPCP were designed and installed with an associated polymer system, but the polymer system has been reported by staff as problematic, as discussed in Section 2.

DAF thickening is applicable across all plant sizes, although DAF thickening units have a large footprint compared to GBTs, screw thickeners, and RDTs. They also have higher power costs compared to GBTs, screw thickeners, and RDTs. Although it is an established process for thickening WAS, DAF is not typically used to thicken primary solids. Due to the higher amount of heavier material, primary solids are more likely to settle to the bottom of the tank. As such, DAFT also have limited applications in co-thickening.

Applicability to Arlington Development status: Technology is established and mature.

Typical application scale: Applicable across all plant sizes.

Site requirements: DAF thickening currently used on-site for WAS thickening. Larger footprint compared to GBTs, screw thickeners, and RDTs.


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Relative cost: Capital cost to rehab two existing units and construct one new unit is lower than all other WAS thickening options.

5.7.3 Centrifuge Thickening The centrifuge thickening process involves the rapid rotation of a cylindrical bowl to separate solids from the feed liquid. Centrifuges are currently used on-site for dewatering. Centrifuges are a proven thickening technology, and are applicable for medium to large facilities. They may not be suitable for primary solids due to the presence of abrasive material. The units require a small amount of floor space relative to their capacity, making them an attractive technology for plants with space constraints. However, centrifuge thickeners have a higher capital cost compared to RDTs and GBTs, as well as higher power consumption.

Applicability to Arlington Development status: Established and proven technology.

Typical application scale: Amenable for medium and large applications. Arlington falls within the typical application scale.

Site requirements: Construction of a new thickening building to maintain operations.

Relative cost: Highest capital cost and O&M cost compared to all other WAS thickening options.

5.7.4 Gravity Belt Thickening Gravity belt thickening (GBT) arose from belt presses for solids dewatering, where thickening occurs in the gravity drainage portion of the press. The technology has since become a common method for thickening of wastewater solids. GBTs operate on the principle of coagulation and flocculation of solids in dilute solids. The solids are conditioned with polymer to coagulate and concentrate the solids. The conditioned solids are then fed to a distribution box, which evenly applies it to a slow-moving, porous fabric belt. Separation of solids from the liquid occurs through gravity drainage and capillary suction forces imparted by the fabric’s interstitial voids. As the conditioned solids are conveyed along the belt, plow blades create furrows that allows water to release from the solids and pass through the belt. Solids are scraped and deposited into a hopper, while the drained water is captured and typically returned to the liquid stream for treatment.

GBTs are often manufactured in standard equipment sizes and the capacity is a function of the belt width. Multiple GBTs may be used throughout as required for redundancy. GBTs are not particularly suitable for co-thickening WAS and primary solids because the units are typically open to the atmosphere, resulting in uncontrollable odors and corrosive emissions. GBTs can be furnished with enclosures to contain foul air and an exhaust to an outside area or a foul air treatment system. The enclosures provide access to the belt and rollers through hinged and/or bolted access panels.

Applicability to Arlington Development status: Established technology.

Typical application scale: Amenable for medium and large applications. Arlington falls within the typical application scale.


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Site requirements: Construction of a new thickening building to maintain operations. Small space requirement compared to DAF and gravity thickening.

Relative cost: Comparable in cost to other WAS thickening processes.

5.7.5 Rotary Drum Thickening A rotary drum thickener (RDT) is a rotating cylindrical drum screen that can be used to thicken WAS only, or to co-thicken WAS and primary solids. A RDT functions similar to a GBT unit by achieving solid-liquid separation through coagulation and flocculation of solids and drainage of free water through porous media. The porous media typically consists of a drum with wedge wire, perforated plate, or woven stainless steel mesh screen.

The RDT consists of an internally fed drum with flights that assist in transporting the thickened solids to the discharge end of the drum. The drum rotates on trunnion wheels and is driven by a variable speed drive through a gearbox. Solids are polymer conditioned and mixed in a flocculation tank prior to entering the drum. The conditioned solids are fed directly to the interior of the drum via overflow piping from the flocculation tank. As the drum rotates, free water passes through the drum perforations into a collection trough. Thickened solids are retained inside the drum and conveyed by internal flights to a discharge chute at the end of the drum. A continuous fixed-spray bar extends along the entire length of the drum for cleaning and reducing blinding of the screen.

RDT is a relatively low energy process and uses about 50 percent of the washwater of a similar capacity GBT. The RDT process can be operated continuously with minimal operator attention. RDT units are more compact and require less floor space than many other thickener technologies. In addition, their closed construction provides good odor containment compared to other thickening technologies.

Applicability to Arlington Development status: Established technology.

Typical application scale: Applicable to all sizes of plants. Multiple units required as capacity requirements increase.

Site requirements: Construction of a new thickening building to maintain operations. Small space requirement compared to other thickening technologies.

Relative cost: Similar capital and operating costs to GBTs.

5.7.6 Rotary Screw Thickening A rotary screw thickener (RST) consists of a drum screen with an internal rotating screw that can be used to thicken WAS only, or to co-thicken WAS and primary solids. An RST functions much like an RDT unit, achieving solid-liquid separation by coagulation and flocculation of solids and drainage of free water through porous media. The porous media typically consists of a drum with stainless steel wedge wires. Unlike an RDT, an RST’s drum screen is stationary and utilizes an internal screw (independent of the screen) to convey the thickened solids through the screen. Depending on the manufacturer, the unit may be oriented in horizontal configuration or inclined.


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The RST consists of an internal-feed drum basket with a screw that transports the thickened solids out of the screen. The screw rotates on a shaft and is driven by a variable speed drive. Solids are polymer conditioned and mixed in a flocculation tank prior to thickening. The conditioned solids overflow through a pipe and feeds directly to the interior of the basket. An internal rotating screw auger transports solids through the basket. Free water passes through the basket perforations into a collection trough, leaving thickened solids inside the basket that are discharged by the internal screw at the opposite end. High pressure backwash is used to clean and prevent blinding of the screen.

RST units can run continuously without operator attention; however, operator attention at least once per shift is recommended to monitor performance. The RST units are compact and completely encapsulated, which provides odor containment.

Applicability to Arlington Development status: Established technology, but limited installations in the U.S.

Typical application scale: Applicable to all plant sizes. Multiple units required as capacity requirements increase.

Site requirements: Construction of a new thickening building to maintain operations. More compact than other thickeners.

Relative cost: Comparable capital and operating costs to GBTs and RDTs.

Lower capacity units compared to other thickening technologies.

5.7.7 Summary of Screening Process The comparison matrix used to screen thickening technologies is presented in Table 5-17. The results of the screening evaluation are presented in Table 5-18. In addition to the existing technologies, gravity belt thickeners, rotary drum thickeners, and centrifuge thickeners are considered for further evaluation. The selection of the thickening process will consider the fit of the process with long-term strategies.

Table 5-17. Thickener Technology Comparison Matrix - Screening

Technology Development

Status Application

Scale Site


Costs Permit- ability

TOTAL

Gravity Thickener 2 2 2 2 2 10

DAF Thickener 2 2 2 2 2 10

Gravity Belt Thickener

2 2 2 2 2 10

Rotary Drum Thickener

2 2 2 2 2 10

Rotary Screw Thickener

2 0 2 2 2 8

Centrifuge Thickener

2 2 2 1 2 9


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Table 5-18. Thickener Technology Screening Results

Technology

Score

(Max = 10) Comments


Gravity Thickening

10 Yes

DAF Thickening 10 Yes

Gravity Belt Thickening

10 Yes

Rotary Drum Thickening

10 Yes

Screw Thickening

8 Lower capacity units compared to other processes. Other thickening technologies are suitable and more often seen in municipal wastewater treatment.

No

Centrifuge Thickening

9 Yes

5.8 Solids Dewatering Technology Screening Numerous processes and technologies exist for the dewatering solids generated at municipal WWTPs. Dewatering offers the advantages of reduced solids volume, which can result in savings on storage and transportation. The technologies to be considered for Arlington County are the centrifuge, belt filter press (BFP), screw press, rotary/fan press, and the Bucher press. Each technology is discussed further in the following sections.

5.8.1 Centrifuge Dewatering The centrifuge dewatering process involves the rapid rotation of a cylindrical bowl to separate solids from the feed liquid. Centrifuges are currently used on-site for dewatering. They have been used in wastewater treatment since the 1930s. Centrifuges operate as continuous feed units which separate solids using a scroll conveyor in the middle of the unit. The conical-shaped bowl lifts solids out of the liquid, allowing them to dry before being discharged. The liquid stream is discharged over a weir on the opposite end of the unit.

Centrifuge dewatering units require a small amount of floor space relative to their capacity, making them an attractive technology for plants with space constraints. They also require minimal operator attention when operations are stable. The process typically involves an in-line polymer injection, which can be used to maintain the percent solids recovery at higher design loadings. However, dewatering centrifuges have a higher power consumption compared to other dewatering technologies. The units are relatively noisy and have greater vibration than other technologies. Performance is also difficult to monitor due to the enclosed nature of the unit.

Applicability to Arlington Development status: Widely used technology.


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Typical application scale: Applicable for medium and large size plants.

Site requirements: Small amount of floor space required relative to capacity. If existing DWB is used for pre-dewatering, construction of a new DWB is required.

Relative cost: Comparable capital costs to BFPs, but higher operating costs due to energy requirements.

5.8.2 BFP Dewatering BFPs dewater by applying pressure to the solids to squeeze out the water. Biosolids which have been conditioned with polymer are sandwiched between two tensioned belts and are passed over and under rollers of various diameters. The pressure increases as the porous belts pass over rollers with decreased diameter. All belt filter dewatering processes incorporate a polymer conditioning zone, gravity drainage zones, low-pressure squeezing zones, and high-pressure squeezing zones. The main components of a BFP include dewatering belts, rollers and bearings, a belt tracking and tensioning system, and a belt washing system.

Because BFPs are an open process, they have a high odor potential. Enclosed units are available for odor control. Maintenance on BFPs is relatively simple, with belt replacement being the major maintenance cost. The units can be started and stopped quickly compared to dewatering centrifuges. However, if not automated, the belt washing process can be time-consuming. BFPs have moderate energy requirements compared to other dewatering technologies. BFPs have a relatively large footprint compared to other dewatering technologies. In addition to the press itself, additional equipment is required for residuals conditioning, including tanks, mixers, and chemical feed equipment.

Applicability to Arlington Development status: Widely used technology with proven track record.

Typical application scale: Used in facilities of all sizes.

Site requirements: Relatively large footprint compared to other dewatering technologies. If existing DWB is used for pre-dewatering, construction of a new DWB is required.

Relative cost: Comparable capital cost to centrifuges, but lower operating costs.

5.8.3 Screw Press Dewatering Screw press dewatering involves a rotating helical screw inside a cylinder. As solids move with the screw along the cylinder, increasing pressure is applied and water is removed via the perforated drum or wedge wire basket. They are manufactured in both horizontal and inclined orientations. The assembly usually includes a solids feed pump, a floc tank with polymer injection, a filtrate drain, and a cake drop. Screw press units are typically designed for unattended operation except during periods of start-up and shutdown.

Screw presses are a relatively new technology with growing interest. The technology originated from the pulp and paper industry. The screw press is considered to be a relatively simple, low maintenance system compared to the other dewatering options evaluated. It can be easily started up and shut down. The enclosed system provides good odor containment. It also has relatively


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low energy requirements. However, screw presses have lower throughput than other dewatering technologies, which means more units are typically required. This results in a larger overall footprint. In addition, the cake solids are expected to be lower than that from a centrifuge.

Applicability to Arlington Development status: Relatively new technology. Strong focus on west coast.

Typical application scale: Small to medium sized facilities.

Site requirements: Moderate footprint due to lower throughput per unit. If existing DWB is used for pre-dewatering, construction of a new DWB is required.

Relative cost: Higher capital cost compared to centrifuge and BFP, but lower power consumption and maintenance requirements.

5.8.4 Rotary/Fan Press Dewatering A rotary press, or “fan” press, involves two parallel wedge wire filter screens rotating at speed to push feed solids through the unit. Solids are rotated between the two parallel revolving filter elements. Filtrate flows through these elements and collects at the bottom of the press. The friction of the plates and backpressure at the outlet produces the dewatered cake. The rotary press is a relatively simple process that is best suited for raw sludges and solids that are more fibrous.

Since rotary press units are enclosed, they have low odor potential and low noise levels. They are easy to startup and shut down, and have low energy requirements compared to other dewatering technologies. The units have a small footprint, but also have limited throughput. The system can be easily expanded to accommodate additional rotary presses.

Applicability to Arlington Development status: Relatively new technology.

Typical application scale: Used in small- to medium-sized facilities.

Site requirements: Small footprint, but relatively small throughput per unit. If existing DWB is used for pre-dewatering, construction of a new DWB is required.

Relative cost: Capital costs can be high. Lower power consumption and maintenance requirements compared to centrifuge and BFP.

5.8.5 Bucher Press Dewatering The Bucher Press is a patented technology originating from the fruit pressing industry that uses a hydraulically driven cylinder-piston system to dewater biosolids. Bucher Presses have been in operation for approximately 45 years, with over 2,000 installations worldwide.

The complete pressing cycle consists of filling, pressing, loosening, and emptying phases. In the filling phase, the press space is filled using a pump. In the pressing phase, the press piston is moved forward, squeezing the solids and forcing the liquid through several drainage elements made of porous cloth material. The cake is retained inside the cylindrical shell. In the loosening phase, the press piston is pulled back. The filling, pressing, and loosening phases are repeated


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until a sufficient quantity of dewatered cake has developed in the press space. Then the press space is opened and the fragmented cake is discharged by the press piston.

Applicability to Arlington Development status: Widely used technology, but limited solids dewatering applications.

Typical application scale: Applicable for medium and large size plants.

Site requirements: Relatively large footprint required. If existing DWB is used for pre-dewatering, construction of a new DWB is required.

Relative cost: High capital cost compared to other technologies. Moderate power consumption, but relatively low operating costs due to automated process.

5.8.6 Summary of Screening Process The comparison matrix used to screen dewatering technologies is presented in Table 5-19. The results of the screening evaluation are presented in Table 5-20. In addition to centrifuges, BFPs are considered for further evaluation. The selection of the dewatering process will consider the fit of the process with long-term strategies.

Table 5-19. Dewatering Technology Comparison Matrix - Screening

Technology Development Status

Application Scale

Site Requirements

Relative Costs

Permit-

ability TOTAL

Centrifuge 2 2 2 2 2 10

BFP 2 2 1 2 2 9

Screw Press 2 0 2 2 2 8

Rotary/Fan Press

2 0 2 2 2 8

Bucher Press 1 2 1 1 2 7

Table 5-20. Dewatering Technology Screening Results

Technology Score

(Max = 10) Comments

Consider For Further

Evaluation?

Centrifuge 10 Yes

BFP 9 Yes

Screw/ Volute Press

8 Lower capacity units compared to other processes. Selected dewatering technologies are suitable and more often seen in municipal wastewater treatment.

No

Rotary/ Fan Press

8 Lower capacity units compared to other processes. Selected dewatering technologies are suitable and more often seen in municipal wastewater treatment.

No

Bucher Press 7 No


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5.9 Summary of Technologies Considered for Further Evaluation Table 5-21 provides a summary of the technologies to be considered in the process train development for biosolids management at the Arlington County WPCP.

Table 5-21. Technologies to Be Considered Moving Ahead

Processing Function Selected Technologies

Non-Digestion Stabilization Composting (off-site)

Digestion Stabilization ATAD Anaerobic Digestion (single and multi-phase)

Digestion Pre-Treatment Thermal Hydrolysis – Pre-Digestion

Thermal Drying Drum Dryer Belt Dryer Paddle/Disc Dryer Tray Dryer

Thermal Processes Incineration (off-site) Anuvia (off-site)

Thickening Gravity Thickening DAF Thickening Gravity Belt Thickening Rotary Drum Thickening Centrifuge Thickening

Dewatering Centrifuge Dewatering BFP Dewatering

Moving ahead, the off-site processes (composting, incineration, and Anuvia) will be considered in one category as opposed to separate processes. The focus of the master plan will not be on the off-site alternatives; however, Arlington County should continue to monitor these opportunities and weigh the benefits and drawbacks of third party biosolids management at an off-site facility. The benefits are expected to include reduced capital investment by Arlington County; however, the drawback is expected to be a higher annual cost for services provided by the third-party merchant. Other drawbacks include limited cost control by Arlington County as agreements expire, and the potential for loss of an outlet in the event the third party cannot accept the solids due to numerous reasons. Reasons may include facility closure or loss of capacity.

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Section 6Overview of Cost Development

Capital, operating, and life cycle costs give an economic and financial basis to compare competing biosolids management alternatives. This cost development overview documents the sources and methods used, degree of accuracy, and collaborative process undertaken to identify, screen, and rank competing alternatives. Comparative costs for each biosolids management alternative can be found in Section 10 of this report. Capital cost development is presented in Appendix G. Operating and life cycle cost development, presented at 20-year net present value, is presented in Appendix H.

6.1 BackgroundThrough the master planning process, a series of workshops were conducted with the project team and Arlington County staff. At each workshop, the level of capital and operating cost information was refined as the number of alternatives considered was reduced.

At the screening level, relative cost was identified as a key criterion to compare competing biosolids management and technology alternatives against the current process. Relative cost was measured on a life cycle basis, comparing the cost of each competing alternative against current county biosolids stabilization and end use costs. For this purpose, a baseline was defined projecting current stabilization operating and capital costs through a 20-year planning analysis period. The 20-year period aligns with typical funding opportunities for projects of similar size and scope, including low-interest loan programs such as state revolving funds. It is also generally consistent with the expected life of major process equipment. Annual periods are assumed to represent fiscal years in accordance with the county’s financial auditing and reporting practices. Initial screening for short- and long-term improvement technologies was completed by eliminating technologies clearly unsuitable and selecting technologies with favorable screening-level scores. For purposes of the initial screening, qualitative measures of capital and operating costs were considered.

Following the initial screening, the short list of technologies was further refined to a list of preferred technologies. The preferred technologies were assembled into process train alternatives and the alternatives were evaluated considering process sizing and cost information together with other evaluation criteria (economic, operational, environmental and social). To achieve this, conceptual level costs were developed using a combination of vendor proposals, preliminary quantity take-offs, and recent costs of similar projects adjusted using cost indices. Operating costs were developed using unit costs provided by county staff and/or the project team as described in Section 6.2 below. Capital costs were developed according to the methods described in Section 6.3 below. Life cycle costs expressed as net present worth (NPW) were then developed using methods and criteria described in Section 6.4 below.

Section 6 Overview of Cost Development

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The project team selected a baseline and three alternative process train configurations for detailed cost analysis and evaluation using additional scoring criteria. Capital cost, operating cost, and life-cycle cost were presented for each competing process alternative compared to the lime stabilization baseline. Life-cycle costs expressed over the 20-year planning period along with their sensitivity to a 50 percent increase in capital cost, a 30 percent decrease in capital cost, a 100 percent increase in hauling cost, and a 100 percent increase in energy cost. The sensitivity analysis also included a 100 percent increase in produced energy value as well as zero energy value (no credit for power or biogas produced). Costs were also compared and contrasted using financial terms, annualizing the capital cost and adding that to the first year operating cost to derive a total annual cost that could also be expressed on a unit cost per dry ton and wet ton basis.

6.2 Cost Analysis ParametersThe following life cycle cost analysis parameters were used to analyze competing immediate needs as well as short- and long-term solids processing capital improvements as part of this Master Plan project.

Analysis PeriodThe analysis period extends from 2021 through 2040. For comparison of the alternatives, construction was assumed to occur in 2020 with operations extending from 2021 through 2040. Depending on the timing and implementation pace, the construction period could be considerably longer, with operations beginning in a piecemeal fashion depending on decommissioning of existing facilities, testing, and other factors affecting construction and implementation. Such details are beyond the needs of this master plan level alternatives analysis.

Six Life Cycle Cost ElementsCapital Cost: design, construction, professional services during construction, and other project soft costs including administration and legal expenses. Includes the cost to remove existing equipment and the salvage value of any equipment removed to implement the alternative. Capital costs were developed and presented as year 2016 costs for comparison purposes.

Operations Labor Cost: direct costs based on current fiscal year average hourly labor rates and escalated to the first year of the operating period and then adjusted annually thereafter.

Maintenance Labor Cost: direct costs based on current fiscal year average hourly labor rates and escalated to the first year of the operating period and then adjusted annually thereafter.

Energy Costs: direct electric power and natural gas. Energy costs were escalated to the first year of the operating period and then adjusted annually thereafter.


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Chemical Costs: direct expenses for various chemicals used in the normal course of project operations. Chemical costs were escalated to the first year of the operating period and then adjusted annually thereafter.

Hauling and Disposal Costs: direct expenses for hauling and disposal based on current contract. Costs were escalated to the first year of the operating period and then adjusted annually thereafter.

Discount RateA discount rate was determined to reflect the cost of financing the project, including upfront costs and long-term interest rates. A discount rate of 5 percent was selected based on the expected future average bond rate for an AAA-rated municipal enterprise fund. The discount rate, a proxy for the long-term financing cost, necessarily takes an inflationary perspective. Costs are first escalated and then discounted to a Year 2016 basis for comparison purposes. This rate was approved by WPCP’s Utility Fund Manager.

Inflation Factors5 percent applied to labor costs, compounded annually beginning in year 1 based on the current US implicit price deflator.

3 percent applied to energy and chemical costs, compounded annually beginning in year 1 based on the current US implicit price deflator.

Salvage ValueSalvage values are often included in a life cycle analysis. They are used to estimate the remaining, undepreciated value of facilities at the end of an analysis period, thus crediting capital facilities that are longer-lived. For purpose of this master plan cost comparison, all assets are assumed to have a 20-year service life to align with the analysis period and will have no residual, salvage value in 2040 at the end of the analysis period.

6.3 Operating CostsAnnual operating costs for 2021 through 2040 were developed for baseline and selected process alternatives. These annual operating costs are itemized and reported separately for each process area (e.g. pre-dewatering, dewatering, hauling and disposal). The unit costs used to develop these costs are given in Table 6-1. Initial year 2016 unit costs were adjusted for inflation to year 2021, the first year of the operating period and adjusted annually.


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Table 6-1. Unit Costs Used to Develop Annual Operating Costs

Operating Cost Parameter Unit2016 Unit

CostAnnual

Inflation2021 Unit

Cost

Electric Power kWh $0.060 3.0% $0.070

Operating Labor Hour $45.75 5.0% $58.39

Maintenance Labor Hour $45.75 5.0% $58.39

Plant Effluent Water Gallon $0.00 3.0% $0.00

Potable Water Gallon $0.005 3.0% $0.0058

Polymer (Dewatering) Pound $1.46 3.0% $1.69

Polymer (Thickening) Pound $1.38 3.0% $1.60

Natural Gas Therm $1.00 3.0% $1.160

Biosolids Hauling and Disposal1 Wet Ton $49.50 3.0% $53.90

Dryer Pellet Disposal2 Wet Ton $0.00 3.0% $0.00

Liquid Solids Hauling Gallon $0.030 3.0% $0.035

Liquid Solids Disposal Dry Ton $260.00 3.0% $301.41

Composting Facility Tipping Fee Wet Ton $65.00 3.0% $75.35

Lime Wet Ton $170.00 3.0% $197.08

Hauling to Compost Facility Wet Ton $25.00 3.0% $28.981. All Class B biosolids land applied in Virginia are subject to a $7.50 per dry ton fee to cover VDEQ and local government testing and

monitoring programs. These fees are paid directly by the biosolids product generator. Monthly and annual reporting requirements also apply to Class A/EQ products; however, the VDEQ fee of $7.50 per dry ton is not assessed. While production of Class A versus Class B biosolids has the potential to affect land application costs, it is assumed for the purposes of this report that hauling and disposal costs are similar for Class A and Class B products, based on the existing hauling and disposal contract.

2. Class A only. Assumed to be revenue neutral.

Operations and Maintenance ElementsThe following O&M cost elements were included in developing operating costs, life cycle costs and financial costs.

Operations Labor Cost: direct costs based on current fiscal year average hourly labor rates. Burdened operations labor costs were used, but indirect labor was not included.

Maintenance Labor Cost: direct, burdened costs based on current fiscal year average hourly labor rates.

Energy Costs: direct electric power, natural gas, diesel fuel, and other purchased energy and combustion fuels. Purchased power and energy used for onsite, stationary combustion should be presented as separate line items. Unit costs were multiplied by annual quantity estimates to derive total annual energy costs.


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Chemical Costs: direct expenses for various chemicals used in the normal course of project operations. Unit costs were multiplied by annual quantity estimates to derive total annual chemical costs.

Hauling and Disposal: direct expenses for hauling and disposal of final products and waste materials. Unit costs were multiplied by annual quantity estimates to derive total annual hauling and disposal costs.

6.4 Capital CostsCapital costs were developed for baseline (lime stabilization) and competing process alternatives. Detailed capital costs for baseline and alternative process trains are presented in Appendix G.

The American Association of Cost Engineers (AACE) recommends five levels of accuracy for construction cost estimating, which are shown in Table 6-2. The level of cost estimation depends on the project stage and scope; as expected, accuracy improves as the project moves through planning and design. Conceptual and study estimates have a wide range of accuracy because many design features and details are undetermined at these stages.

Table 6-2. Cost Estimating Classification Matrix per AACE

Primary Characteristic Secondary Characteristic

Estimate Class

Maturity Level of Project Definition

DeliverablesExpressed as % of

complete definition

End UsageTypical Purpose of

Estimate

MethodologyTypical estimating

method

Expected Accuracy Range

Typical variation in low and high ranges

Class 5 0% to 2% Concept screening

Capacity factored, parametric models,

judgment, or analogy

L: -20% to -50%H:+30% to +100%

Class 4 1% to 15% Study or feasibility

Equipment factored or parametric models

L: -15% to -30%H: +20% to +50%

Class 3 10% to 40%Budget

authorization or control

Semi-detailed unit costs with assembly

level line items

L: -10% to -20%H: +10% to +30%

Class 2 30% to 75% Control or bid/tender

Detailed unit cost with forced detailed take-

off

L: -5% to -15%H: +5% to +20%

Class 1 65% to 100% Check estimate or bid/tender

Detailed unit cost with detailed take-off

L: -3% to -10%H: +3% to +15%

Construction cost estimates prepared for baseline and the other biosolids management alternatives are accurate to the Class 4 (study or feasibility) level. The estimate was prepared using previous estimates for similar projects, historical data from comparable work, and equipment costs obtained from vendors.


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Costs include indirect costs for permits, taxes, bonds, insurance, contractor general conditions, overhead and profit, and construction contingency. In addition, a 35 percent owner’s implementation fee was added for engineering design & construction services, owner’s administration, and legal fees.

6.5 Cost Development for Long-Term Capital NeedsLife cycle cost analysis also supported developing, prioritizing and selecting among competing alternatives to manage the dewatering, pre-treatment, stabilization, processing and final disposal and/or beneficial reuse of biosolids including digestate and side-stream treatment, heat recovery, and energy generation. The analysis was conducted using planning level engineering information to generate Class 4 cost estimates with a degree of accuracy to within -30% to +50%.

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Section 7Current and Emerging Biosolids Regulations

7.1 IntroductionReviewing existing regulations, as well as anticipated changes and emerging concerns, is essential for all strategic planning efforts. Considering ongoing nutrient management requirements in the Mid-Atlantic region, this regulatory review is important for wastewater and biosolids programs. Additionally, air and water quality requirements may affect the selection, cost, and implementation of solids management options and approaches.

This section summarizes current federal and Virginia state regulations as well as trends observed in the Mid-Atlantic region (consisting of Delaware, District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia). Recent regulatory changes are also described, their impacts explained, and anticipated near and long-term regulatory changes are evaluated. With continued regulatory pressures associated with nutrient management planning and Total Maximum Daily Loadings (TMDLs) to the Chesapeake Bay, biosolids land application programs are coming under greater scrutiny. Diversification of biosolids outlets along with new product development derived from nutrient and energy recovery are changing the landscape of solids management. This section addresses the potential opportunities and the necessary pathways for Arlington County to move forward within the regulatory framework.

7.2 Existing Biosolids Regulations 7.2.1 Federal Regulations for Biosolids Use and DisposalFederal regulations governing the use and disposal of municipal wastewater solids include: 40 CFR 503 Standards for the Use or Disposal of Sewage Sludge, 40 CFR 257 Criteria for Classification of Solid Waste Disposal Facilities and Practices, and 40 CFR 258.2 Criteria for Municipal Solid Waste (MSW) Landfills. This section focuses mainly on the 40 CFR 503 standards. Although solids placed in a MSW landfill are not subject to the 503 regulations, they must meet the requirements of both 40 CFR 257 and 40 CFR 258.2.

The 40 CFR 503 regulations were promulgated in 1993 and set forth standards for the following use and disposal options:

Beneficial use through land application, distribution, and marketing

Disposal at dedicated sites or in sludge-only landfills

Incineration in sludge-only incinerators

The 503 regulations include standards that apply to wastewater sludge and septage generators, processors, end-users, and disposers. The 503 regulations set two important standards. The first is a comprehensive risk assessment of pollutant pathways. These risk assessments set limits for heavy metal concentrations related to all biosolids practices. The second governs biosolids use

Section 7 Current and Emerging Biosolids Regulations

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and disposal. Management practices are designed to limit exposure and ensure biosolids are used in a way that protects human health and the environment.

7.2.1.1 Land Application Requirements (40 CFR 503) The 503 regulations specify requirements in the following three categories for solids applied to land:

Pollutant limits

Pathogen Reduction requirements

VAR requirements

Pollutant Limits To date, the 503 regulations have established pollutant limits for nine metals (Table 7-1).

Table 7-1. 40 CFR §503.13 Pollutant Limits

Pollutant

Pollutant Ceiling Concentration

(mg/kg) (1)(2)

Cumulative Pollutant Loading

Rate(kg/hectare) (2)

Pollutant Concentration(mg/kg) (1)(3)(4)

Annual Pollutant Loading Rate

(kg/hectare/yr) (4)

Arsenic 75 41 41 2.0

Cadmium 85 39 39 1.9

Copper 4,300 1,500 1,500 75

Lead 840 300 300 15

Mercury 57 17 17 0.85

Molybdenum (Mo)

75 -- -- --

Nickel 420 420 420 20

Selenium 100 100 100 5.0

Zinc 7,500 2,800 2,800 14mg = milligrams(1) Dry Weight basis (3) For sludge applied to a lawn or home garden kg = kilograms(2) For sludge applied to land (4) For sludge sold or given away in a bag

Pathogen Reduction The 503 regulations clearly specify two classifications depending upon the quality of biosolids and the level of pathogen reduction achieved, Class A or Class B. Class A pathogen reduction requirements reflect a Process to Further Reduce Pathogens (PFRP) standard, while Class B requirements reflect a process to significantly reduce pathogens (PSRP) standard. Class A pathogen treatment reduces biosolids pathogen levels to minimal detection, while Class B biosolids may have higher pathogen levels that must be managed in accordance with specific practices that ultimately provide the same level of protection as Class A treatment. Typically, Class A treated biosolids can be distributed in a bag or container and used on lawns and home


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gardens, similar to commercial fertilizers; Class B biosolids may only be applied to sites with buffer zones, limited public access, and harvesting restrictions.

Listed below is a summary of the U.S. Environmental Protection Agency (EPA) Class A and Class B pathogen treatment options (§503.32):

Class A Class A material must have either

density of fecal coliforms of less than 1,000 MPN per gram of total solids (dry weight basis), or

density of Salmonella sp. Bacteria less than 3 MPN per 4 grams of total solids (dry weight basis)

In addition to the above microbiological requirements, it must meet one of the following process requirements:

Alternative 1: Time and temperature

Alternative 2: pH, time and temperature

Alternative 3: One-time demonstration correlating pathogen levels (enteric viruses, helminth ova) and operating parameters

Alternative 4: Concentration of enteric viruses and helminth ova

Alternative 5: PFRP

Alternative 6: Equivalent to PFRP

Class B Alternative 1: Density of Fecal Coliform (Geometric mean of 7 samples < 2,000,000 MPN/g or

CFU/g total solids (dry weight basis)

Alternative 2: Process to Significantly Reduce Pathogens (PSRP)

Alternative 3: Equivalent to PSRP

Appendix B to Part 503 (Pathogen Treatment Processes) describes the recognized Class A (PFRP) and Class B (PSRP) treatment options. Listed below are the approved PFRP and PSRP technologies:

Class A - PFRP 1. Composting - Using the within-vessel composting method or the static aerated pile

composting method, sewage sludge temperature is maintained at 55° C or higher for 3 days.

Using the windrow composting method, sewage sludge temperature is maintained at 55° or higher for at least 15 days. When compost is maintained at 55° or higher, the windrow will be turned at least five times.


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2. Heat drying—Sewage sludge is dried by direct or indirect contact with hot gases to reduce the moisture content of the sewage sludge to 10% or lower. Either the temperature of the sewage sludge particles exceeds 80° C or the wet bulb temperature of the gas in contact with the sewage sludge as the sewage sludge leaves the dryer exceeds 80° C.

3. Heat treatment—Liquid sewage sludge is heated to a temperature of 180° C or higher for 30 minutes.

4. Thermophilic aerobic digestion—Liquid sewage sludge is agitated with air or oxygen to maintain aerobic conditions and the mean cell residence time of the sewage sludge is 10 days at 55 - 60° C.

5. Beta ray irradiation—Sewage sludge is irradiated with beta rays from an accelerator at dosages of at least 1.0 megarad at room temperature (ca. 20° C).

6. Gamma ray irradiation—Sewage sludge is irradiated with gamma rays from certain isotopes, such as cobalt-60 and cesium-137, at dosages of at least 1.0 megarad at room temperature (ca. 20°C).

7. Pasteurization— Sewage sludge temperature is maintained at 70° C or higher for 30 minutes or longer.

Class B - PSRP 1. Aerobic digestion—Sewage sludge is agitated with air or oxygen to maintain aerobic

conditions for a specific mean cell residence time at a specific temperature. Values for the mean cell residence time and temperature are between 40 days at 20° Celsius (C) and 60 days at 15° C.

2. Air drying—Sewage sludge is dried on sand beds or on paved or unpaved basins. The sewage sludge dries for at least three months. During two of the three months, the ambient average daily temperature is above 0° C.

3. Anaerobic digestion—Sewage sludge is treated anaerobically for a specific mean cell residence time at a specific temperature. Values for the mean cell residence time and temperature are between 15 days at 35 to 55° C and 60 days at 20° C.

4. Composting— Using the within-vessel, static aerated pile, or windrow composting methods, the temperature of the sewage sludge is raised to 40° C or higher and remains at 40° C or higher for 5 days. For 4 hours during the 5 days, the compost pile temperature exceeds 55° C.

5. Lime stabilization—Sufficient lime is added to the sewage sludge to raise the sewage sludge pH to 12 after 2 hours of contact.

In 1985, EPA created the Pathogen Equivalency Committee (PEC), a federally sponsored technical group that provides technical assistance and recommendations on process equivalencies for pathogen reduction in sewage sludge to government and industry.

The PEC reviews and makes recommendations to relevant federal and/or state permitting authorities on the merits of applications proposing new innovative or alternative sewage sludge pathogen reduction processes are equivalent to the processes currently listed in the 40 CFR Part 503, Subpart D, §503.32.


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The PEC process may be necessary for some enhanced digestion processes and/or newer pathogen reduction technologies. The EPA website (http://www.epa.gov/biosolids/pathogen-equivalency-committee-documents) provides guidance from the PEC to demonstrate the effectiveness of innovative and alternative sewage sludge pathogen disinfection processes to receive a recommendation of PSRP or PFRP equivalency.

Vector Attraction Reduction (VAR) All biosolids products must meet one of the ten VAR options (Table 7-2) when used for land application or options 1 – 8 when applied to a lawn or home garden.

The biosolids industry has coined the term Exceptional Quality (EQ) for biosolids that meet the most stringent requirements for all three parameters (pathogen reduction, VAR, and pollutant limits). EQ biosolids meet a Class A - PFRP pathogen reduction process, options 1-8 of the VAR requirements, and metal limits under EPA §503.13 Table 3 Pollutant Concentrations. EQ biosolids are exempt from additional management practices and may be used freely as soil amendments and/or fertilizers as allowed by the local regulatory agency.

Biosolids that meet pathogen reduction through a Class B - PSRP must be land applied in accordance with specific management practices as listed below, and applied at agronomic rates equivalent to the N need of the crop to be grown.

Table 7-2. 40 CFR §503.33 - Summary of VAR Requirements

VAR Option Requirement

1. Volatile Solids (VS) Reduction > 38% VS reduction during solids treatment

2. Anaerobic Bench-Scale Test < 17% VS loss, after 40 days at 30°C to 37°C

3. Aerobic Bench-Scale Test < 15% VS reduction, after 30 days at 20°C

4. Specific Oxygen Uptake Rate (SOUR) SOUR at 20°C is ≤ 1.5 mg oxygen/hr/g total solids

5. Aerobic process > 14 days at > 40°C with an average > 45°C

6. pH adjustment pH > 12 at 25°C and remain at pH > 12 for 2 hours and pH > 11.5 for additional 22 hours

7. Drying without Primary Solids > 75% Total Solids prior to mixing

8. Drying with Primary Solids > 90% Total Solids prior to mixing

9. Soil injection No significant amount of sludge on the land surface within 1 hour after injection. Class A must be injected within 8 hours after pathogen treatment process.

10. Soil Incorporation Incorporation into the soil within 6 hours after application. Class A must be incorporated into the soil within 8 hours after pathogen treatment process.

http://www.epa.gov/biosolids/pathogen-equivalency-committee-documents

http://www.epa.gov/biosolids/pathogen-equivalency-committee-documents


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7.2.1.2 Management Practices and Product End UseFor Class B (PSRP) biosolids to be land applied and achieve the same level of protection as Class A (PFRP) biosolids, the following management practices (§503.14) must occur:

1. Sludge shall not be applied to the land if it is likely to adversely affect a threatened or endangered species.

2. Sludge shall not be applied to agricultural land, forest, a public contact site, or a reclamation site that is flooded, frozen, or snow-covered so that the bulk sewage sludge enters a wetland or other waters of the United States.

3. Sludge shall not be applied to agricultural land, forest, or a reclamation site that is 10 meters or less from waters of the United States.

4. Sludge shall be applied to agricultural land, forest, a public contact site, or a reclamation site at an application rate that is equal to or less than the agronomic rate, unless, in the case of a reclamation site, otherwise specified by the permitting authority.

5. A label shall be affixed to the bag or container in which sludge is sold or given away for application to the land, or an information sheet shall be provided to the person who receives the sludge. The label or information sheet shall contain the following information:

Name and address of preparer of the sewage sludge,

a statement that application of the sludge must occur in accordance with label, and

the annual whole sludge application rate for the sludge that does not cause any of the annual pollutant loading rates in Table 4 of §503.13 to be exceeded.

7.2.1.3 Sewage Sludge Incinerator (SSI) Rule Under 40 CFR Part 60 and Part 62SSIs and their air emissions have been regulated under the Clean Water Act 40 CFR Part 503 regulations since 1993. In April 2010, EPA's Office of Air & Radiation proposed new air emissions regulations for SSIs under the Clean Air Act. After public comment, EPA responses, and amendments, the new regulations became final on March 21, 2011.

On March 21, 2011, EPA published the Identification of Non-Hazardous Secondary Materials that are Solid Waste. Under this rule, biosolids incinerated by SSIs at Publicly Owned Treatment Works (POTWs) were designated as non-hazardous solid waste. Consequently, biosolid incineration no longer fell under the domestic sewage exclusion provision of the Clean Water Act; instead, Section 129 of the Clean Air Act was applied. Under Section 129, air emissions requirements for SSIs were established based on maximum achievable control technology (MACT) standards. Compliance with these regulations (MACT regulations) is required by March 2016.

In response to the new rule, the National Association of Clean Water Agencies (NACWA) and others sued EPA. In 2013, the court required EPA to review some of the rule's background information and calculations. In 2015, the court denied NACWA's petition to overturn EPA's determination that incinerated sewage sludge is a solid waste. In 2015, EPA proposed moving forward with rule implementation. At the same time, EPA plans to review the data and calculations on which the regulation was based, as required by the court.


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Under the SSI rule, EPA defined all wastewater sludge combusted as solid waste unless they meet the following criteria to be classified as a renewable fuel:

Sludge is managed as a valuable commodity, and

Sludge has meaningful heating value and is used as a fuel in a combustion unit that recovers energy, and

Sludge contains contaminants at levels comparable to or lower than those in traditional fuels the combustion unit is designed to burn.

The MACT regulations requires compliance with 10 pollutants, and the emission limits are applicable at all times, including normal operation and periods of startup and shutdown.

The new SSI standards do not apply to dried biosolids being used as an alternative fuel in a cement kiln, as the facilities fall under existing regulations. According to NACWA, “the SSI rule only applies to SSIs and if the cement kiln burns biosolids that have not been excluded from the solid waste regulations, the appropriate standards for the kiln burning solid waste would apply.” However, the petition process to remove biosolids as solid waste has been successfully performed. Petitioners have been able to demonstrate that the biosolids are processed sufficiently to meet the three fuel conditions listed above, but the contaminant levels remain the biggest concern. In most cases the petitioners were able to show low enough contaminant levels in the biosolids to be comparable to coal and qualify as a fuel. In 2012, the City of Detroit, Michigan received EPA approval that their biosolids were deemed a non-solid waste and a renewable fuel.

7.2.2 Virginia Department of Environmental Quality (VDEQ) – 9VAC35-32In Virginia, biosolids regulations are enforced through the VDEQ. The VDEQ regulations mimic the 503 regulations as it relates to pollutant limits, pathogen reduction, and VAR requirements. However, Virginia has a more comprehensive set of regulations that requires permitting all biosolids activities and management practices. All biosolids activities are enforced through a Virginia Pollution Abatement (VPA) permit.

Under the VPDES, POTWs and other treatment works treating domestic sewage must report sewage sludge activities and production annually as part of their VPDES permit.

7.2.2.1 General RequirementsThe VDEQ follows 503 regulations for both Class A and Class B pathogen reduction requirements, as well as VAR and pollutant concentration limits, but has additional requirements. Class A biosolids must be registered with the Virginia Department of Agriculture and Consumer Services (VDACS). These biosolids may be distributed and marketed for sale or distribution after VDEQ approval. Facilities that produce and land apply or dispose of Class B biosolids must obtain a VPA permit from the VDEQ. Class B biosolids are also subject to operational standards and management practices including land-applier training and extensive notification systems.


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7.2.2.2 Management Practices and Product End UseThe Virginia biosolids rules include detailed operational and management standards, along with monitoring, recordkeeping, and reporting requirements. VPA site-specific permits are issued by political jurisdictions (county or city) and are typically held by contracted land appliers. Only a few biosolids generators hold their own VPA permits for land application.

The VPA biosolids permitting process includes pre-approval of individual WWTP. Each WWTP must supply and certify detailed information regarding the processing and treatment of each biosolids type produced. Once approved, the biosolids from those WWTPs can be permitted for land application within the state. The WWTPs are required to submit biosolids quality data to VDEQ to remain on the approved biosolids source list.

Land ApplicationClass B biosolids may be land applied after VPA permit issuance once all operational and management standards are achieved. VDEQ requires a Virginia Certified Land Applier oversee all Class B land application operations and ensure that all applications are performed in accordance with a Nutrient Management Plan (NMP) prepared by a Certified Nutrient Management Planner. Biosolids must also be applied in accordance with the permittee’s approved Biosolids Management Plan. This includes an Odor Management Control Plan for each product. Most biosolids applied in Virginia are managed through contracting arrangements with certified land appliers who hold VPA permits.

Before beginning land application activities, VDEQ and local government notification and site signage requirements must be completed. Site-specific field parameters must be documented, such as soil pH, P and potassium levels, depth to groundwater and bedrock determined, restrictions on food crop growth, animal grazing and public access, as well as setbacks and slopes flagged out from the proposed application area.

VDEQ requires monthly and annual reporting for all biosolids activities. These reports are completed by the permit holder. All Class B biosolids land applied in Virginia are subject to a $7.50 per dry ton fee to cover VDEQ and local government testing and monitoring programs. These fees are paid directly by the biosolids product generator.

Product MarketingVDEQ allows for the distribution and marketing of Class A/EQ biosolids under a general distribution VPA permit. EQ products must be registered with VDACS and marketed and distributed throughout the state in accordance with an approved label. EQ biosolids must be used in accordance with an approved NMP unless the biosolids are greater than 90% total solids or the intended use is for purposes other than land application.

If EQ biosolids are mixed with inert materials before marketing and distribution, additional VDEQ approvals/permits are required. Mixed or blended EQ products intended for agricultural purposes require proper testing and/or research to assess product suitability and is approved by VDEQ on a case-by-case basis.

Monthly and annual reporting requirements also apply to Class A/EQ products; however, the VDEQ fee of $7.50 per dry ton is not assessed. The reporting requirement for Class A/EQ


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products are fulfilled by the permit holder. To date, most Class A/EQ distribution and marketing permits in Virginia are held by certified land appliers that will contract with utilities.

Landfill DisposalAlthough the VDEQ biosolids regulations require monitoring, recordkeeping, and reporting of all biosolids landfilled in the state, they do not regulate the activity. Biosolids placed in a MSW landfill must comply with the criteria in the Virginia Solid Waste Management Regulation, 9VAC-20-80. Only non-hazardous wastes can be accepted at MSW landfills.

Analytical testing of the biosolids to ensure acceptability and confirmation of non-hazardous status includes a complete Toxicity Characteristic Leaching Procedure, reactivity, corrosivity, ignitability, and pass a paint filter test.

With increased recycling, landfills are looking to replace trash inputs. Landfill pricing for biosolids in the Virginia area ranges from approximately $30 - $50 per wet ton. Landfills are limited by trash to biosolids ratios, stabilization requirements, and odor management requirements. Therefore, landfills have been known to turn away biosolids due to low trash inputs and odor management concerns. Landfill operators are also cognizant of the fact that inclement weather and winter impact land application activities and have been known to increase their tip fees to cover market demand during those timeframes.

7.2.2.3 Implementation of Recent VDEQ Regulatory Changes (September 2013)The Virginia Governor signed new VDEQ biosolids regulations in June 2013, which became effective September 1, 2013. The regulatory change was a major revision of the state’s biosolids program, requiring resubmittal of all previously permitted sites in accordance with the new VPA permit requirements. The new regulations not only included significant changes to the permitting structure and fees, but also to the nutrient management planning and storage requirements for both routine and on-site storage. All storage facilities in the state were required to comply with the new regulations by winter 2014. Most regulatory changes were related to Class B land application criteria, and the requirements fall upon the individual permit holders, which in Virginia is typically a land applier that will contract with utilities. Although these additional constraints are being placed on the permit holder, the impacts will also be felt by WWTPs, as fees may go up and land and storage availability may decrease.

Permitting Changes and FeesUnder the new (2013) VDEQ regulations, all previously permitted sites were required to be re-permitted under the new VPA permit structure, which included new consent forms, updated maps, and samples. If sites were not re-submitted and deemed administratively complete, the sites would no longer be approved for biosolids land application. Virginia’s land application permits are typically held by contracted land appliers, most of them hold 25 - 30 VPA permits encompassing 200,000+ acres. The new regulations also instituted significant permitting fees. A VPA Municipal Sludge Operation (biosolids may also include water treatment plant residuals) initial fee is $5,000 with an annual maintenance fee of $1,000, as well as a modification fee of $1,000 for to add sites to the permit. This change alone was a significant burden on Virginia land appliers from both a labor and cost perspective.


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Land Application NotificationsThe notification process was significantly changed under the new VDEQ regulations. Before applying biosolids to newly permitted lands, the permit holder must first notify the local government and then wait 100 days. At least 30 days before land application can occur, a NMP must be submitted. VDEQ and the local government must be notified within 14 days of any biosolids land application activity. Within 5 days of land application, the site must be posted with a sign, local government and VDEQ must be notified, and VDEQ must be notified 1 day before operations.

Within 15 days of completing the land application process, the land applier must supply the farmer, VDEQ, and VDCR with a copy of the NMP. The land application sign must remain posted for a minimum of 30 days after completing the land application activity. These changes have increased both field labor and administrative requirements on the Virginia land appliers.

Storage RequirementsUnder the new (2013) VDEQ storage regulations, two storage types can be integrated into a Biosolids Management Plan:

1. On-site storage, or

2. Routine storage.

Both storage types require VDEQ permitting and are located off-site from the WWTP. Contracted land appliers in Virginia hold storage permits. These storage requirements do not apply to Class A/EQ biosolids that have an approved VDEQ Distribution and Marketing Permit.

On-site storage is a short-term option that allows for up to 45 days storage, limited to the amount of biosolids specified in the NMP for the fields managed by the farmer/operator in which the on-site storage is located. On-site storage requires the location to be approved by VDEQ and constructing a surface with sufficient strength to support operational equipment along with the maximum permeability of 10-7 cm/sec. The on-site storage pad must use best management practices to prevent contact of biosolids with storm-water run on and runoff. All on-site storage pads must either have a cover or collect and manage the storm-water that comes in contact with the biosolids.

Routine storage is a constructed facility that allows for long-term storage of biosolids. Biosolids may be stored for up to one year in routine storage facilities and may be taken off-site for use at any permitted land application site. Biosolids routine storage facilities not only require VDEQ permitting, but also require local zoning approval and conditional use permits. These facilities are designed to hold large volumes of biosolids. Examples are lagoons or large-scale enclosed structures. At this point, there are only a handful of permitted routine storage facilities in Virginia.


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Routine storage facilities designed to hold dewatered biosolids must have a roof to prevent contact with precipitation. The facility’s floor surface must also be engineered with a maximum permeability of 10-7 cm/sec and sufficient strength to support operational equipment. Existing facilities permitted as routine storage for liquid biosolids may also be used to store dewatered biosolids, but the supernatant must be managed as liquid biosolids and requires maintaining freeboard within the facility. This allows for the continued use of previously permitted storage lagoons.

The VDEQ storage regulations have significantly increased the cost of both on-site and routine storage. The requirement for an impermeable surface of 10-7 cm/sec ultimately means a concrete floor must be constructed and either a roof/cover or a storm-water collection system installed and monitored. These new requirements have added tens of thousands of dollars in cost to on-site storage and hundreds of thousands of dollars in cost to routine dewatered biosolids storage. The increased costs of routine storage incurred by contracted land appliers are passed on to biosolids generators through more expensive contracts.

Since implementing the new VDEQ regulations, no new routine storage facilities have been permitted, and many of the previously approved on-site storage pads have been eliminated.

Class A/EQ Distribution and MarketingAlthough not much changed in the new (2013) VDEQ regulations regarding Class A/EQ biosolids distribution and marketing, the VDEQ has been very conservative in approving general distribution for the newer Class A technologies that have been introduced into the state over the last few years. Historically the VDEQ has approved general distribution of Class A/EQ products such as heat-dried pellets, compost, and RDP-processed (heat/lime treatment) biosolids. Class A/EQ products are typically drier than dewatered Class B biosolids. Many times the Class A/EQ product will also have a different appearance and handling requirement than most Class B biosolids.

With the emergence of new Class A technologies and their resultant products, such as pre-pasteurization (Alexandria Renew Enterprises (AlexRenew), VA) and thermal hydrolysis (DC Water), the VDEQ has not yet granted general distribution and marketing of these products because they resemble and handle like Class B dewatered biosolids. These products are being evaluated on a site-specific basis. VDEQ approved the distribution and marketing of Alexandria biosolids after they were blended with mulch fines and cured. The general distribution and marketing of Alexandria’s blended product required a separate VPA treatment and processing permit after the product was tested, and a greenhouse study was performed to assess product suitability. The only permitted site approved for treatment and processing of the Alexandria biosolids never came to fruition, therefore AlexRenew Class A/EQ dewatered biosolids are currently land applied on permitted Class B sites. DC Water is currently working through the VDEQ Class A/EQ general distribution and market permitting process, but to date is still land applying their Class A/EQ Cambi dewatered biosolids on Class B permitted sites. It is unclear at this time if VDEQ will allow for general distribution and marketing of Class A/EQ biosolids that look and handle like Class B biosolids.


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All biosolids in Virginia must be land applied in accordance with a NMP written by a certified nutrient management planner. The Virginia Department of Conservation and Recreation (VDCR) enforces the Nutrient Management Standards and Criteria that were last revised in July 2014. The latest revision included additional biosolids management, similar to the Maryland NMP program where the NMP’s become enforceable documents.

7.2.2.4 Nutrient Management RequirementsPhosphorus LimitsUnder the revised Nutrient Management Standards, organic sources cannot be applied to soils with a 65% P saturation level. Soil P 65% saturation levels were established for each EPA region. For fields that do not exceed the maximum P saturation levels, either the Environmental Threshold or the Virginia P Index must be used to determine organic nutrient source P applications for fields contained in NMPs. In either instance, the new P nutrient management requirements tend to limit using land application sites that have used biosolids over the years. These new P limitations reduce biosolids application rates and, in some instances, remove entire farms from the biosolids program. More land will be required to manage the same volume of biosolids.

The addition of enhanced nutrient treatment at WWTPs only compounds the nutrient management issues associated with land application, as more nutrients end up in the biosolids. Some biosolids have increased 1 - 2% in N and/or P due to enhanced nutrient removal and digestion.

Nutrient Application TimingThe Nutrient Management Standards also require reduced and limited biosolids applications on environmentally sensitive sites in winter. The new regulations define an environmentally sensitive site to mean any field particularly susceptible to nutrient loss to groundwater or surface water. One criteria for these sites is soils identified with light, sandy textures and a high potential for leaching. These are the sandy, light textured soils that have historically been land applied in fall and winter, as they tend to dry quicker and allow equipment access quickly after inclement weather.

The VDEQ regulations also require a crop to be planted within 60 days of applying biosolids. This tends to be problematic during the month of January, unless you are applying to an existing crop, as most spring crops are not planted until March/April.

7.3 Review of Pending Regulations Applicable to Biosolids Recovery and Reuse With nutrient management planning and TMDLs to the Chesapeake Bay, biosolids land application programs are facing increasing regulatory pressures. Since 2012, numerous influences and changes were made to many of the Mid-Atlantic state biosolids programs. Details of those changes and implications are discussed in this section.


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7.3.1 Federal Updates to 40 CFR Part 5037.3.1.1 Land Application and Risk Assessment UpdatesUnder the Clean Water Act, Section 405(d)(2)(C), EPA has statutory requirements to review the 40 CFR Part 503 standards not less than every 2 years under Biennial Review summaries to regulate new pollutants where sufficient data exist.

EPA plans to address risk assessment for the Targeted National Sewage Sludge Survey Pollutants, starting with the ten Phase I pollutants (i.e., barium, beryllium, manganese, Mo, silver, pyrene, 4-Chloroaniline, fluoranthene, nitrate and nitrite). These peer reviewed risk assessments are being addressed and EPA is revising the draft report. Comment responses and a revised report were circulated for internal EPA review in October 2015. After the internal review, EPA will publish the risk assessment for public comment later this year. There will be a 60-day comment period after which EPA will respond to comments and make decisions as to what additional numeric standards are needed, if any. There is no definitive timeframe identified, but EPA has identified Mo and the other Phase I chemicals as priorities for completion.

Any potential changes to the numerical standards do not appear to be of significant impact to most WWTPs.

7.3.1.2 Biosolids Core Risk Assessment Model Screening Tool (BCRAM Screening Tool)EPA is also developing the BCRAM Screening Tool, an easy-to-use multimedia, multi-pathway, multi-receptor screening level deterministic model that estimates high-end human and ecological risk based on potential exposures associated with land application of biosolids. The BCRAM Tool can also calculate the allowable concentration of a constituent in biosolids to manage risk.

Screening level risk assessment results can be used to identify pollutants, pathways, and receptors of greatest interest and inform decisions about the need to perform more refined modeling or to address data gaps or uncertainties. This model has been peer reviewed, and later this year EPA will begin refining and developing the model based on comments received.

After completing the Phase 1 TNSSS risk assessment for the original ten pollutants, EPA will begin to screen (using the BCRAM Screening Tool) the balance of the 135 TNSSS pollutants, where sufficient data exist.

7.3.2 Mid-Atlantic Regional Regulatory Perspective and TMDLsEach state in the Mid-Atlantic region is focusing on achieving individual state TMDL limits. As part of that initiative, each have outlined different approaches to meet their specific needs as it relates to wastewater, agriculture, urban systems, and trading/offsets. However, no matter what approach is used, each jurisdiction is further restricting limits on wastewater discharges and land applied nutrients. These additional restrictions are and will continue to impact costs for WWTPs. A more detailed explanation of specific impacts is listed below.

7.3.2.1 Nutrient Management InitiativesStates in the Mid-Atlantic region have also increased regulatory initiatives associated with all agricultural practices. In particular, the Chesapeake Bay TMDLs are driving more detailed and


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restrictive NMPs. Some of the increased regulatory initiatives include reduced fall and winter nutrient applications. In some instances, the updated nutrient management regulations have completely eliminated applying any nutrients in the winter. Increased P monitoring and P-based NMPs are required in most Mid-Atlantic states. Additional administrative requirements are also becoming more burdensome.

Continued Limits on Fall and Winter Nutrient ApplicationsThe Mid-Atlantic states have all implemented some form of nutrient management planning that restricts fall and winter nutrient applications. Pennsylvania has placed the least restrictions on nutrient applications in the fall and winter, still allowing for temporary field stockpiling of biosolids. Delaware has also implemented a very limited nutrient application window from mid-December through February. Maryland has the most restrictive nutrient management regulations, including very limited fall nutrient applications and the winter ban on all nutrient applications in Maryland, which became effective November 2016. No nutrients can be applied in Maryland from November through February.

The current Maryland nutrient management regulations allow for the field stockpiling of dewatered manures and residuals over the fall and winter months, but the Maryland Department of the Environment (MDE) regulations do not allow for field stockpiling of Class B biosolids. Only Class A/EQ biosolids may be field stockpiled in Maryland, but they can only be stockpiled for up to 90 days. The winter ban on nutrient applications is longer than 90 days.

Increased restrictions on fall and winter nutrient applications, along with limited storage options are creating a precarious position for many biosolids operations. In the Mid-Atlantic region, most biosolids land appliers hold multiple state permits to manage their biosolids programs year-round.

Increased Phosphorus Monitoring and Nutrient Management PlanningThe increased focus on the environmental impacts of P from land applied manures has broadened to include all residuals land applied. Biosolids are similar to manures in that the levels of N and P are unbalanced with respect to crop needs. Applying biosolids at an agronomic rate for N crop needs typically over-applies the amount of P needed. Years of continued fertilization at N-based rates with animal manures and/or biosolids have increased soil P levels. Since runoff and soil erosion are the primary pathways for P to enter surface waters, higher soil P levels can make water bodies more susceptible to P overloads and eutrophication.

With the deadline approaching to meet the next phase of the Chesapeake Bay TMDLs, most Mid-Atlantic States have put additional restrictions on nutrient applications, in particular more focus on P loadings and a move toward P-based NMPs. Most states are using a risk based tool known as a P-Site Index to rank the potential for P loss from individual fields. Most Mid-Atlantic State’s P-Site Index allows for the return to N-based application rates if the P Index places the field into a low category for P loss.

Maryland has recently taken P-based NMPs even one step further. They adopted a new P risk tool called the Phosphorus Management Tool (PMT), which became effective June 8, 2015. The new P regulations do provide for a multi-year process for farmers to transition from the old P-Site Index to the new PMT. The PMT no longer allows for a field to return to a N based application, even if it


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falls into the low risk category. Once a field reaches the threshold soil P level requiring the use of the PMT, it can no longer receive a N based application until the soil P level is reduced. The new Maryland PMT regulations also placed an immediate ban on fields with a Fertility Index Value (FIV) of 500 from receiving any additional P regardless of the outcome of the PMT program. These new P requirements will remove many farms from the biosolids program.

State Level Focus on meeting TMDLsOn December 29, 2010, EPA established the Chesapeake Bay TMDL. The TMDLs are being implemented using an accountability framework that guides restoration efforts using four elements:

Watershed Implementation Plans (WIPs)

2-year milestones

EPA’s tracking and assessment of restoration progress

specific federal actions if the Bay jurisdictions do not meet their commitments

Chesapeake Bay jurisdictions include Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia, and the District of Columbia.

The WIP goals and milestones outline steps each jurisdiction is taking to have all pollution control measures in place by 2025 to fully restore the Chesapeake Bay. By 2017, practices should be in place to meet a 60% reduction of N, P, and sediment compared to 2009.

EPA evaluates the milestone that each jurisdiction makes every 2 years and conducts biennial progress reviews that each jurisdiction has made toward achieving their target TMDLs. As of May 2015, EPA identified Virginia and Maryland as being on target for meeting their TMDL goals for agriculture and wastewater. EPA has identified specific concerns with Delaware’s wastewater strategy and may take additional federal actions, as well as noticed a potential downgrade in their agriculture strategy. The recent review of Pennsylvania’s progress shows that EPA has identified substantial concerns with their agriculture strategy, but their wastewater strategies are on target. The back-slide in Pennsylvania’s agricultural strategies may be attributed to Pennsylvania being one of the few states in the Mid-Atlantic region that have not implemented P-based NMPs, nor have they limited fall or winter applications.

7.4 Air Pollutant Permitting Modifications or improvements to at the Arlington County WPCP will be subject to the air pollution regulations from EPA and the VDEQ. All sources of air pollutants, as classified by the National Ambient Air Quality Standards (NAAQS), at the Arlington County WPCP are subject to permitting requirements. The type of permit required depends on the potential to emit (PTE) pollutants, the existing air quality on the county, and if the existing source is already a major source of air pollutants. The Arlington County WPCP is not a major source of air pollution. EPA has determined that Arlington County is in a Marginal Nonattainment Area for the 2008 8-Hour Ozone NAAQS (40 CFR 81.322). EPA and VDEQ are concerned about the ozone precursors: Nitrogen Oxides (NOx) and VOCs. All air permits are administered through VDEQ.


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7.4.1 Major New Source Review Permits7.4.1.1 Prevention of Significant Deterioration (PSD) PermitArlington County would be required to obtain a PSD permit if plant modifications cause it to become a major emissions source. In addition, these permits are only issued in areas that are in attainment of the NAAQS. As previously noted, Arlington County is not in attainment; however, this status should be reevaluated at the time of permitting. For PSD permitting, a major source is considered to be facilities with the PTE:

From any single criteria pollutant over 250 tons per year (tpy), or

From any single criteria pollutant over 100 tpy and is a source category listed in 9VAC5-80-1615, or

Over 100,000 metric tons of carbon dioxide equivalent (CO2e).

Source categories in 9VAC5-80-1615 include fossil fuel boilers with a heat input totaling more than 250 million BTU per hour.

7.4.1.2 Nonattainment PermitAs long as Arlington County continues to be in nonattainment for the 2008 8-Hour Ozone NAAQS, the WPCP may be subject to Nonattainment Permitting. This would be applicable if plant modifications result in it becoming a major source of NOx or VOC emissions. In addition, these permits are only issued in areas that are in nonattainment of the NAAQS. Similar to PSD permitting, a major source is considered to be facilities with the PTE:

From any single criteria pollutant over 250 tpy, or

From any single criteria pollutant over 100 tpy and is a source category listed in 9VAC5-80-1615.

The major source levels may be lower for NOx or VOC at the discretion of VDEQ. This type of permit would require the plant to achieve the Lowest Achievable Emission Rate and to obtain air emissions offsets as necessary.

7.4.2 Minor New Source Review PermitsMinor New Source Review permits are required for facilities that are below the major source emissions thresholds and above the exemption thresholds in 9 VAC 5-80-1105 C or D. One exemption threshold is for gaseous fuel burning external combustion units with a maximum heat input of less than 50 million BTU per hour. Arlington County may opt to use a minor new source review permit to limit the uncontrolled emissions rate as part of the enforceable permit conditions to avoid becoming a major source of emissions. To qualify for this type of permit, the facility would have a PTE:

From any single criteria pollutant under 100 tpy, or

From HAPs, more than the exemption level by less than 10 tpy or 25 tpy of any combination of air pollutants.

This permit must be obtained before construction.


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7.4.3 Federal Title V Operating PermitsA Title V Air Operating Permit must be obtained within 12 months of beginning operation for the new source of major air pollution. A Title V major source of air pollution in Arlington County would be one that has the PTE of 100 tpy of NOx, VOC, SO2, CO, or particulate matter with an equivalent aerodynamic diameter of 10 micrometers or less (PM10). Again, VDEQ would be particularly interested in the NOx and VOC emissions given the County’s nonattainment status.

7.4.4 State Major PermitsA State Major Permit is for facilities that have the PTE between 100 and 250 tpy of a criteria pollutant, but are not listed as one of the major source categories in 9VAC5-80-16151. These facilities require a Title V permit, but are below the thresholds for PSD permit. Arlington County may opt to use the state major permit to limit the uncontrolled emissions rate as part of the enforceable permit conditions to avoid becoming a PSD major sources of emissions. This permit must be obtained before construction.

7.4.5 State Operating PermitsTo avoid New Source Review permitting (Section 3.2.5.1) or Title V permitted (Section 3.2.5.3), facilities in Virginia may opt for a State Operating Permit that limits the facility’s emission below major source permitting thresholds. In this case, the permit would be referred to as a synthetic minor permit. These types of permits are also used to combine multiple air permits at one facility or as a way to allow for emissions trading. These permits are issued at the request of the facility owner or at the discretion of VDEQ, such as to cap emissions at a station source.

7.4.6 Article 7 Permits for Major Sources of Hazardous Air PollutantsIn addition to permits for criteria pollutants, the Arlington County WPCP may be subject to an air permit for HAP through Article 7 permitting. This type of permit is a preconstruction permit for sources that have the PTE 10 tpy of a single HAP or 25 tpy of a combination of HAPs.

7.5 Anticipated Near-Term Biosolids Regulatory Changes 7.5.1 Federal 40 CFR Part 503 7.5.1.1 TNSS Pollutant Risk AssessmentAs previously mentioned, EPA plans to address the risk assessment for the TNSSS Pollutants, starting with the ten Phase I pollutants (i.e., barium, beryllium, manganese, Mo, silver, pyrene, 4-Chloroaniline, fluoranthene, nitrate and nitrite). They intend to publish the risk assessment for public comment later this year. It is likely that a new Mo pollutant concentration limit may be promulgated in the upcoming year. The proposed change does not appear to be of significant impact the most WWTPs.

7.5.1.2 Biosolids Core Risk Assessment Model Screening Tool (BCRAM Screening Tool)The BCRAM screening tool is an easy-to-use multimedia, multi-pathway, multi-receptor screening-level deterministic model that estimates high-end human and ecological risk based on potential exposures associated with land application of biosolids. The BCRAM tool can also


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calculate the allowable concentration of a constituent in biosolids that will result in a management level risk.

Screening level risk assessment results can be used to identify pollutants, pathways, and receptors of greatest interest and inform decisions about the need to perform more refined modeling or to address data gaps or uncertainties. This model has been peer reviewed, and later this year EPA will begin refining the developing model based on comments received.

As EPA continues to develop the BCRAM screening tool, they will begin to use the BCRAM screening tool to screen the balance of the 135 TNSSS pollutants, where sufficient data exist.

7.5.2 Federal Renewable Fuel Standard Program EPA has started to move toward some resource recovery initiatives as they developed the Renewable Fuel Standard (RFS) program. The RFS program was authorized under the Energy Policy Act of 2005 and expanded under the Energy Independence and Security Act of 2007. The RFS program was created to reduce GHG emissions and expand the nation’s renewable fuels sector while reducing reliance on imported oil. This was a mechanism to ensure that transportation fuels begin to contain a minimum amount of renewable fuel.

The requirements to meet the RFS standards have evolved over the years and in July 2014 EPA included fuels derived from digester biogas at municipal WWTPs. EPA also announced that the following fuel pathways meet the lifecycle GHG reduction requirements for cellulosic biofuels established under the RFS program:

Compressed natural gas (CNG) produced from biogas landfills, municipal WWTP digesters, agricultural digesters and separated MSW digesters

Liquefied natural gas produced from biogas landfills, municipal WWTP digesters, agricultural digesters, and separated MSW digesters

Electricity used to power electric vehicles produced from biogas landfills, municipal WWTP digesters, agricultural digesters, and separated MSW digesters

EPA noted that including these fuels in the RFS program will help achieve program goals and may provide credits (Renewable Identification Numbers or RINs) to biofuel producers. Each gallon of renewable fuel equals one RIN, which can be bought and sold as a commodity. Renewable energy along with volume reduction technologies are two areas that the biosolids industry would benefit from investigating further.

7.5.3 Virginia Department of Environmental Quality (VDEQ) – 9VAC25-32 7.5.3.1 Implementation of Storage RequirementsThe VDEQ and permitted land appliers have had 1 year under their belt with the new storage requirements. As previously noted, no new routine storage facilities have been permitted and many of the previously approved on-site storage pads have been eliminated. Total biosolids storage capacity in Virginia has been reduced due to these new requirements.

With substantial increases in storage costs for both on-site and routine storage from the requirement for an impermeable surface of 10-7 cm/sec and a roof or a storm-water collection


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system, land appliers are scrambling to ensure adequate storage capacity. Due to the hundreds of thousands of dollars associated with even a small routine storage facility, it is very unlikely that any additional routine storage facilities will be permitted and built unless a local municipality is funding the project. Such a facility also requires local zoning approval and a conditional use permit. This task alone may preclude the sighting of any new routine storage facilities.

Land application contractors appear to be leaning toward more on-site storage pads with impermeable surfaces with covers and/or storm-water collection systems due to the extremely high costs of a routine storage facility. However, constructing these smaller on-site pads with storm-water management requirements has added several tens of thousands of dollars in cost. The other concerns with on-site pads are that local government approval is also required, and the biosolids stored on the pad can only be used for the farm in which the pad is located. These on-site storage pads tend to have limited life spans, as after several years of biosolids applications, the soil P levels and/or pH levels may not allow for additional applications.

7.5.3.2 Need for Focus on New and Innovative Recovery and Reuse OptionsVirginia as well as the surrounding Mid-Atlantic States would benefit by working together to develop additional sustainable solutions and implement regulatory strategies to achieve such goals. There is no single solution, and the solution may require multiple smaller scale programs. Listed below are a few areas that may be evaluated further as the industry begins to diversify and supplement traditional Class B land application programs:

Energy Recovery

Renewable Fuels & Fuel Alternatives

Class A - Soil Blending (move toward non-agricultural outlets)

Class A – Fertilizer Manufacturing

VDEQ and Virginia generators would also benefit by implementing strategies that allow for WWTPs to embrace new technologies, allow for both energy and nutrient recovery, as well as re-evaluate its Class A/EQ distribution and marketing permit process and approval to provide incentives for municipalities to move toward a more sustainable and product- based program.

7.5.4 Regulatory Considerations for Biosolids-Derived ProductsVDEQ is already addressing permitting for a traditional biosolids-derived product - (manufactured soil), establishing requirements on a case-by-case basis within a biosolids regulation context; as technologies evolve, however, new biosolids products further push the boundaries of our understanding of how these materials should be regulated. Two products that fall into this category are harvested struvite and Anuvia fertilizer.

For harvested struvite, EPA has not defined a permitting path, but had been leaning toward regulating these products under the 503 regulation; producers argue, however, that the products are not biosolids and that falling under the 503s would hinder market acceptance. Discussions with the agency on this issue (led by NACWA) continue, and it is expected that the pressure to define a regulatory path for these products will increase as more struvite recovery facilities come online. NACWA is planning additional discussions to reach consensus on this issue.


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The EPA has also been silent on the distribution of Anuvia fertilizer. However, Anuvia expects to distribute its product freely and without specific notification of biosolids content, based upon its EQ status. While some states have labeling or other requirements for EQ materials, Florida (where the first Anuvia facility is located) does not. Thus, the product can be distributed freely and without tacit acknowledgement that wastewater residuals are a component.

7.6 Long-Term Biosolids Regulatory Changes and Strategies 7.6.1 Pharmaceuticals and Personal Care Products (PPCPs) and Emerging Pollutants UpdateTrace organics, also known as microconstituents, compounds of emerging concern and emerging pollutants are terms that the WEF and others have adopted to describe natural and manmade substances that have been detected at small but measurable quantities in the environment and in biosolids. The 2002 National Research Council (NRC) report identified the following two types of trace organics as high priority topics for future biosolids research due to their preference to adhere to solids rather than pass through the wastewater treatment process.

Endocrine Disrupting Compounds (EDCs) are chemicals that may have an adverse impact on hormonal systems in humans and wildlife. EDCs include flame retardants, surfactants, and estrogens. EDCs have gained particular attention in recent years due to researchers noting hormonal effects in fish and wildlife populations.

Pharmaceuticals & Personal Care products (PPCPs) have also been raised as compounds of concern. PPCPs include prescription and over-the-counter-drugs (such as antibiotics and steroids) as well as fragrances, lotions, and cosmetics.

Both types of trace organics have been detected in biosolids. Due to the complexity regarding compound-specific impacts, bioassays are a potential approach to assess the effects of trace organics.

7.6.2 Emerging Pathogens UpdateSince the promulgation of the EPA 503 regulations, a few pathogens have emerged as being potentially public health concerns. These include a variety of bacteria (E.coliO157:H7, Listeria, Clostridium perfringens, Campylobacter), parasites such as Cryptosporidium and viruses (Hepatitis A, enteric viruses and adenoviruses). Exposure routes of concern include ingesting pathogens through groundwater and inhaling bioaerosols. Recent laboratory and field studies investigating the fate and transport of these emerging pathogens have found that transport through soil and air is minimal. Although the recent studies show minimal impacts, EPA recommends continued verification of these results through additional research and modeling.

7.6.3 Virginia and Other Mid-Atlantic States The Mid-Atlantic state level biosolids and wastewater regulations do not currently address nutrient recovery or energy recovery and reuse at WWTPs. Updated regulations are needed to address new technologies as well as allow new market development and new practice advancement to alleviate the continued constraints placed on land application programs due to increased nutrient management requirements. With the advancement of digestion and drying


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technologies, as well as biogas and biofuel production, it would be beneficial for regulatory programs to begin to accommodate the technology movement toward energy recovery and reuse.

In 2007, the Virginia General Assembly adopted a statewide Renewable Portfolio Standard (RPS) which set forth a phased series of renewable energy goals through 2024. Renewable Energy was defined to include (among other sources) energy derived from biomass. Compliance with these RPS goals requires that such renewable energy be generated within the Commonwealth and purchased by a participating electric utility under a power purchase agreement. Such power purchase agreements then transfer renewable “attributes” to the purchaser. As such, electric power utilities can trade renewable energy represented by these attributes as renewable energy certificates (RECs). The Virginia RPS goals are an average of 4 percent of total electrical energy sold in 2015, increasing to 7 percent from 2017 to 2021 and to 12 percent by years 2023 and 2024.

The Virginia RPS may allow Arlington County to develop a revenue stream of RECs that can be sold to compliance buyers. These revenue streams may improve the economics of biogas power generation and other energy generation strategies associated with managing biosolids. Current REC pricing in Mid-Atlantic markets ranges from $15 to $20 per MWh.

7.6.4 Greenhouse Gas Emissions The Arlington County WPCP generates GHG emissions both directly and indirectly. Direct emissions include those generated directly by the nitrification/denitrification processes, building heating processes, and stationary sources of combustion of fossil fuels. Indirect emissions are those generated through purchased electricity from grid-connected sources. Additional indirect emissions are those generated through mobile sources of fossil fuel combustion and other sources related to the upstream supply chain to the plant (e.g., chemical manufacture, construction, transport).

Several emerging reporting protocols and regulatory requirements define GHG emissions to include carbon dioxide, methane, and nitrous oxide. As a single measure, these three gases are aggregated using standard global warming potential factors to define a single GHG emissions rate expressed in equivalent tons of carbon dioxide (CO2e). Oftentimes, fluorinated gases including hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and N trifluoride (synthetic greenhouse gases that are emitted from a variety of industrial processes) are also regulated. These fluorinated gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases.

GHG emissions are of emerging regulatory concern and may affect the long-term viability of biosolids management practices. Biosolids processing often avoids methane generation that would otherwise result from landfill or other static forms of disposal. This benefit is often weighed against the GHG emissions generated through using fossil fuels, truck transport, and other activities undertaken in the course of processing biosolids. EPA has published guidance to POTWs as to the proper procedures for calculating a carbon footprint, vernacular for tons of CO2e emitted annually.


7-22

In 2008, EPA issued the Mandatory Reporting of Greenhouse Gases Rule (74 FR 56260) that requires reporting GHG data and other relevant information from large sources and suppliers in the United States. In general, the Rule is referred to as 40 CFR Part 98 (Part 98). Implementation of Part 98 is referred to as the Greenhouse Gas Reporting Program. Facilities that emit 25,000 metric tons or more annually of GHGs must submit annual reports to EPA. Municipal WWTPs are currently exempt from these reporting requirements.

In 2010, EPA set GHG emissions thresholds to define when permits under the New Source Review PSD and Title V Operating Permit programs are required for new and existing industrial facilities. This final rule tailors the requirements of these Clean Air Act permitting programs to limit covered facilities to the nation's largest GHG emitters: power plants, refineries, and cement production facilities. Municipal WWTPs are also currently exempt from these requirements.

Prospectively, GHG emissions are expected to remain a continuing concern, and regulatory changes should be monitored. Further, market mechanisms through which regulated sources of GHG emissions may purchase offset credits for qualifying activities and projects may provide Arlington County an opportunity to generate a revenue stream to offset the costs of biogas use, power generation, and other biosolids management activities.

7.7 Summary With no substantial changes expected in the 503 regulations, continued modifications to the recently adopted state level regulations are expected. With TMDL goal requirements of 60% reduction by 2017 and complete restoration of the Chesapeake Bay by 2025, it is likely that additional nutrient reduction strategies will be evaluated.

Increased constraints on agricultural land application practices due to increased nutrient management requirements will continue to stress local land-based options and increase costs for biosolids land appliers. The Mid-Atlantic States and their respective regulatory agencies would benefit by working together to develop additional sustainable solutions and implement regulatory programs to achieve these goals. There is no single solution, and the solution may require multiple smaller scale solutions. Diversification and supplementing traditional Class B land application programs may be a start. Include Class A solutions, energy recovery and reuse, nutrient recovery, additional storage capacity and/or disposal options and identify non-agricultural outlets. Some solutions may even divert biosolids from beneficial use options.

As WWTPs look to diversify and supplement traditional Class B land application programs, the following areas/outlets have potential merit in an overall diversification plan:

Energy Recovery

Renewable Fuels & Fuel Alternatives

Class A - Soil Blending (move toward non-agricultural outlets)

Class A – Fertilizer Manufacturing


7-23

As previously mentioned, the Mid-Atlantic state level biosolids and wastewater regulations do not address nutrient recovery or energy recovery and reuse at WWTPs. The region would benefit from developing new regulatory programs that allow WWTPs to embrace new technologies, allow for energy and nutrient recovery, as well as accommodate Class A/EQ distribution and marketing to provide incentives for facilities to move toward a more sustainable and product-based program.

An industry focus on developing new markets outside the agricultural arena to alleviate the continued constraints placed on land application programs due to increased nutrient management requirements would be beneficial. The continued advancement of digestion and drying technologies that reduce biosolids volumes, along with biogas and biofuel production, will also provide industry movement toward energy recovery and reuse options. As nutrient reduction strategies continue to be enforced in the Mid-Atlantic region, the biosolids industry’s need to move toward volume reduction and energy recovery will become more apparent.

Solutions should be identified soon, as many options require years to develop, permit, and implement.

8-1

Section 8

Opportunities for Using Biosolids in the Mid-

Atlantic Region

8.1 Introduction This section addresses the various types of biosolids that are typically produced at wastewater

treatment plants similar in size and process to the WPCP, and summarizes the product-market fit

for biosolids and related products that could be generated at the WPCP. This information will be

used in conjunction with a rigorous screening and evaluation process to allow Arlington County

to focus on the most promising systems that will both produce high-level products and meet

other predetermined criteria and external drivers for solids processing.

Products could be produced through the solids handling and treatment technologies that are

evaluated in this Master Plan, including: biosolids, energy, and non-biosolids derived products.

Their characteristics, market potential, need for a third-party for distribution, and use/familiarity

in the region are presented.

Identified products that will be included in the overall evaluation process as alternatives are

examined and compared. Evaluation criteria specifically account for product impacts on

wastewater treatment, costs, reuse and resource recovery potential, and product acceptability.

The WPCP currently produces a Class B lime-stabilized biosolids product that is hauled off-site

and land applied by contractors. Refer to Section 3 for current and projected solids quantities.

The biosolids are produced by adding lime to a dewatered blend of primary sludge, WAS, and

primary scum at the WPCP. Lime is added to the dewatered material to meet the requirements of

the Virginia Pollution Discharge Elimination System Permit. The lime stabilization process is

monitored to document compliance with regulatory requirements for Class B pathogen reduction

as a Process to Significantly Reduce Pathogens (PSRP) and requirements for VAR. Refer to Section

7 for regulatory requirements for this and other processing technologies.

Product hauling is typically accomplished using trucks with open-top trailers with water tight

gates and sides. They are covered with secured tarps before leaving the DWB loading bay. Each

truck can transport up to wet 20 tons per load. Typically, the contractor hauls 30 to 35 loads

weekly from the plant.

For the Arlington County Solids Master Plan, the Class B lime stabilized cake product is

considered the baseline product.

Section 8 • Opportunities for Using Biosolids in the Mid-Atlantic Region

8-2

8.2 Types of Products According to the USEPA (April 2012), more than 16,500 publicly owned wastewater treatment

works in the United States daily treat over 40 billion gallons of wastewater, generating over 8

million dry tons of biosolids annually. Wastewater treatment facilities should generally be viewed

as water resource recovery facilities (WRRFs) that produce clean water and can recover energy

and generate nutrients. Many utilities strive to manage resources to generate value for the utility

and its customers, improve environmental quality at the least cost to the community, and

contribute to the local economy.

Solids treatment presents opportunities to recover and produce valuable products for use. These

products can be grouped into nutrients (biosolids and non-biosolids), energy, and water

products. Solids are the primary, waste-activated matter removed from wastewater. After

treatment, biosolids can be used as a fertilizer for agricultural, horticultural, or reclamation

purposes.

From an energy production perspective, there are two primary pathways for energy recovery:

thermal conversion and biodegradation. The potential to recover energy and resources from

wastewater biosolids is dictated by numerous drivers, such as the quality of and markets for the

biosolids and energy products, as well as regulatory and public perceptions that influence the

choice of recovery options. The characteristics of the end products of thermal conversion and

biodegradation are significantly different.

Combustion, a thermal conversion process, oxidizes the organic matter in the biosolids, leaving

behind only the inert (ash) fraction. The digestion process (a form of biodegradation) at

wastewater treatment plants produces biogas, an energy source that is comprised of methane

and carbon dioxide. Biogas can be used for process heating and can provide other benefits when

coupled with combined heat power (CHP) systems, such as displacing fossil-fuels normally

purchased for facility needs, increasing power reliability for the plant, and providing renewable

fuel for green power programs. While biogas is a very broad term that can include biomass-

derived gas, or gas from the anaerobic digestion of animal or food waste, the term biogas in this

report refers to the gas derived from the wastewater anaerobic digestion process. Anaerobic

digestion consumes most of the readily biodegradable organics in the biosolids, but leaves a

larger mass/volume of biosolids for use or disposal than thermal conversion (Stone, et al., 2010).

For this reason, the product market assessment addresses both potential biosolids and energy

products.

In addition to biosolids-derived products, nutrient resources can be recovered (and marketed)

from side streams or other elements of the treatment process. In this document, non-biosolids

products include nutrient products devoid of significant organic matter resulting from extractive

nutrient recovery processes. These products include a variety of N, P, and iron compounds;

however, products like P-enriched biosolids, potentially resulting from struvite harvesting, are

excluded from this category. It is not currently clear whether these non-biosolids derived

products come under the same regulatory reviews and restrictions as biosolids products for end-

use and marketing.


8-3

8.3 Potential Products and Uses Potential biosolids and energy products are summarized in Tables 8-1a and 8-1b. While various

combinations of Arlington County and third-party/offsite production and handling contractors

are possible options for most products, the most likely or common scenario is listed for each.

Table 8-1a provides information for biosolids products; Table 8-1b includes energy and non-

biosolids derived products.

8.3.1 Biosolids Products

The biosolids products listed in Table 8-1a are assessed below. A brief product description, a

discussion on the stabilization process or treatment required to produce the product, comparison

of relative product value, and probable market outlets are included.

While each biosolids beneficial reuse product is different, all share a relatively high organic

content. The organic matter in biosolids is often cited as the basis for their positive impact on soil

tilth or fertility like other soil amendment products such as mulch, top soils, and leaf compost.

Biosolids products offer the following specific benefits:

� Improved soil structure: Biosolids can greatly enhance the physical structure of soil,

reducing its erosion potential.

� Improved drought resistance: Increased organic matter provided through biosolids can

increase water retention, improving drought resistance, and promoting more efficient

water use.

� Increased Cation Exchange Capacity (CEC): An increased CEC improves a plant’s ability

to more effectively use nutrients, reducing nutrient loss by leaching.

� Enhanced soil biota: The activity of soil organisms is essential in productive soils and for

healthy plants. Their activity is largely based on the presence of organic matter, which can

be provided through biosolids applications.

� Slow release N: The N in biosolids is predominantly organic N, and must be converted to

inorganic N by soil microbes to become available to plants. This process is generally slow,

and consequently the N in biosolids is referred to as slow release.

The characteristics of biosolids will define, to a large extent, appropriate uses for these products.

General physical and chemical characteristics of typical dewatered solids (cake) and other

products are summarized in Table 8-2.


8-4

Table 8-1a. Summary of Products: Biosolids Products

Biosolids Products

Product Cake Cake Heat Dried Pellet Heat Dried Non-

Pellet

Nutrient Enhanced

Pellet

Compost Soil Blend

Class B A A A A A A

Outlet � Lime-stabilized:

Bulk land

application

(primarily

agricultural)

(CURRENT

PROGRAM)

� Land reclamation

� Digested: Bulk

land application

� Bulk land

application


� Feedstock for

blended

products

�

� Bulk land

application

� Wholesale to

agricultural

market

� Retail sale

� Fuel

� Feedstock for

blended product

� Bulk land

application

� Wholesale to

agricultural

market

� Retail sale

� Fuel

� Feedstock for

blended product

� Bulk land

application

� Wholesale to

agricultural

market

� Retail sale

� Fuel

� Feedstock for

blended product

� Bulk land

application

� Wholesale to

agricultural

market

� Retail sale


� Feedstock for

blended product

(higher value

horticultural

blend)

� Bulk land

application

� Wholesale to

agriculture

� Retail sale


� Urban

Reclamation

Market

Potential

Currently well-

established;

reduced market

expected from

increased

regulatory

restrictions and

competition for

sites

Established and

growing market in

the mid-Atlantic

region;

appearance-wise it

is difficult to

differentiate from

Class B Cake

Both bulk and retail

markets are well-

established; effort

required to enter

regional & national

market; good

branding potential

Established

wholesale market;

effort required to

enter regional

market; fuel

opportunities exist

Established

wholesale and retail

markets; effort

required to enter

regional market;

excellent branding

potential

Established

wholesale

markets; effort

required to enter

regional market;

limited branding

potential

Established

wholesale market

and growing retail

market; good

branding potential

Possible

Production

/Handling

Options

� Production by

Arlington

� Management by

3rd party

� Production by

Arlington

� Management by

3rd party

� Production by

Arlington

� Management by

3rd party

� Production by

Arlington

� Management by

3rd party

� Production by

Arlington

� Enhancement by

3rd party

� Management by

3rd party

� Production by

Arlington

� Composting by

3rd party

� Management by

3rd party

� Production by

Arlington

� Blending by 3rd

party

� Management by

3rd party


8-5

Table 8-1b. Summary of Products: Energy and Non-Biosolids Derived Products

Energy and Non-Biosolids Derived Products

Product Ash Electricity Steam Biogas as Renewable

Fuel

Recovered P Fertilizer

Outlet � Landfill

� Cement additive

� Soil additive

� To Grid

� On-site

� On-site use

� Clean and compress as

CNG for Utility Fleet or

buses

� Clean and Injection

into Natural Gas

pipeline

� P fertilizer

Market Potential Limited market, may be

difficult to develop

Market complex; electrical

power can be utilized on-

site

Limited off-site market;

mostly for on-site use

Emerging for utility fleet

vehicle; embryonic for

commercial sale

Currently processed

and marketed without

biosolids regulatory

restrictions (this may

be changing);

established retail

market; proprietary

product

Possible

Production

/Handling

Options

� Production by 3rd party

� Management by 3rd

party

� Production by Arlington

� Management by Arlington

or 3rd party

� Production by

Arlington

� Management by

Arlington

� Production by

Arlington

� Management by

Arlington or 3rd party

� Production by 3rd

party

� Management by 3rd

party


8-6

Table 8-2. Typical Biosolids Characteristics

8.3.1.1 Class B Dewatered Cake

Class B dewatered cake is the primary biosolids product generated in the Mid-Atlantic region.

The Class B dewatered cake products meet the federally mandated Process to Significantly

Reduce Pathogens (PSRP), and they typically consist of lime-stabilized or digested (aerobically or

anaerobically) biosolids. Most Class B biosolids are dewatered with a belt filter press or

centrifuge. The average percent total solids for dewatered Class B cake ranges from 20 to 27

percent for digested product up to 31 to 43 percent for lime-stabilized product.

Nearly all Class B dewatered cake biosolids in the Mid-Atlantic are beneficially used for bulk land

application on both agricultural and land reclamation sites. Some Class B dewatered cake

products are disposed in a landfill. Although most is used in agriculture, typical Class B

dewatered cake products are not well-balanced fertilizer replacements. Class B dewatered cake

has nearly as much N as P and almost zero potassium (K). Applying Class B cake at an agronomic

rate for N crop needs typically over applies the amount of P needed and supplies no K. Years of

continued fertilization at N based rates with both Class B cake and/or animal manures have

shown to increase soil P levels. Most Mid-Atlantic States have put additional restrictions on

nutrient applications, more focus on P loadings, and a move toward P-based NMPs.

8.3.1.2 Class A Dewatered Cake

Only a few generators in the Mid-Atlantic region produce a Class A dewatered cake product. DC

Water’s new Class A dewatered cake product significantly increased the volume of Class A cake

being used in the region.

Parameters

Class B Lime

Stabilized Cake

(Arlington

County)

Class B

Digested

Cake

Class A

Compost

Class

A

THP

Cake

Class A

Heat-

Dried

Class A Nutrient

Enhanced Pellet

Total Solids (%) 31 - 43 20-27 58 30-40 92-95 95

Volatile Solids (%

of total solids) 58 - 74 60 60 50-60 67 25

Bulk Density

(lb/cf)

63 (est.) 55 45 40-45 33-46 35

Organic Content

(%)

60 (est.) 60-75 50-75 50-60 75-80 18-20

Nitrogen (%) 3.1 - 3.7 3-5 2-3 4-6 4-6 15-17

Total P (%) 1.2 - 1.6 2-3 1-2 6-7 2-3 1-2

Potassium (%) 0.1 - 0.2 0-0.05 0-0.5 0.1 0-0.05 0


8-7

The Class A dewatered cake products have gone through a PFRP, which is a treatment process

that is able to consistently reduce pathogens to below detectable levels at the time the sludge is

used or disposed. However, the Class A dewatered cake products look and handle just like Class B

dewatered cake products unless they are further conditioned or blended. The Class A treatment

process typically consists of enhanced digestion and/or pasteurization processes without further

processing such as heat drying. The average percent total solids for dewatered Class A cake can

range from 30 to 40 percent (depending on the treatment processes used).

Most Class A dewatered cake products are being beneficially used for bulk land application on

both agricultural and reclamation sites; these are the same outlets as Class B dewatered cake. In

some Mid-Atlantic States, distribution and marketing of Class A dewatered cake products do not

require individual site permits.

The Virginia Department of Environmental Quality (VDEQ) has not yet granted the general

distribution of Class A dewatered cake products. All general distribution of Class A dewatered

cake products in Virginia that are less than 40 percent total solids requires the Class A dewatered

cake biosolids be blended with other inert materials. A separate permit for blending must be

issued before general distribution will be granted. As a result, only a very small percentage of

Class A dewatered cake products are being blended with other materials and being used outside

of the typical agricultural markets, such as for landscaping or soil blending.

A treatment process that is relatively new to the US, the thermal hydrolysis process (THP),

creates a Class A cake that has unique quantities and marketing potential. Thermal hydrolysis is a

pretreatment technology that uses high pressure and temperature to condition the solids before

digestion, making the solids more easily biodegradable during digestion. It is a Class A process

that creates a Class A dewatered cake that could be used for land application (in the traditional

sense of material spread on agricultural fields) or blended to create a manufactured soil.

The general impacts of THP on solids processing are significant and can enhance product uses

and acceptability. Process impacts include:

� Improved dewaterability: hydrolyzed solids release water more easily, and increases in

cake solids content of up to 10 percent total solids over mesophilically digested cake are

typical. THP conditioned biosolids can be dewatered with belt presses to 30 percent total

solids or better.

� Increased biogas production: THP improves solids degradability in digesters, typically

increasing volatile solids reduction (VSR) 10 to15 percent compared to VSR achieved by

mesophilic digestion alone, at comparable SRTs.

� Reduced product odor: – both research and experience indicate the odor from THP

hydrolyzed and digested solids are significantly lower than mesophilically digested and

lime stabilized biosolids.


8-8

� Class A biosolids: generally, the process time and temperatures in a THP system can meet

40 CFR 503 Class A pathogen reduction criteria, with the VAR requirements for beneficial

use achieved in the mesophilic digestion that follows THP. If metals are low, and VAR

requirements are met in the digesters, THP biosolids would also be EQ materials and could

be distributed with minimal requirements.

8.3.1.3 Class A Heat-Dried Pellet

The nutrient content of biosolids is generally low compared to chemical fertilizers, and they are

used primarily as soil conditioners, reflecting the soil benefits noted previously. Along with Class

A THP Cake, Class A Heat-Dried Pellets are an exception— the N content of these materials can

range from 4 to 6 percent.

Class A Heat-Dried product typically meets EQ standards and has solids contents between 92 and

95 percent. As with other biosolids products, heat-dried products must meet applicable quality

standards with respect to trace metals and synthetic organics to be sold for agricultural use. A

dense, uniform, and pelletized product, it provides the benefit of slow N release fertilization

(compared to mineral-based fertilizers) and a supply of both secondary nutrients and

micronutrients. For this reason, heat-dried pellets have been referred to as a multivitamin for the

soil.

This product is most commonly used as a fertilizer on agricultural lands and turf. Agricultural

applications include a variety of uses, such as cropland, pastures, and rangeland. Turf-based uses

include fertilization for golf courses, athletic fields, sod farms, and lawns.

A key benefit to all users is that heat-dried biosolids can be spread with conventional fertilizer

spreading equipment. Heat-dried biosolids are often sold as a component, or filler, in mineral

fertilizer blends. For fertilizer blenders and distributors, they offer the advantages of a low-cost

additive that adds micronutrients, slow-release N, and organics. Biosolids used in fertilizer blends

should have the same size and bulk density as the fertilizers they are blended with to ensure that

the blend remains homogenous.

Uses for bulk and bagged product are similar and include fertilization for agriculture, parks and

recreation, golf courses, and other lawns (including residential). Regardless of whether they are

sold in bulk or bagged, a key characteristic of heat dried pelletized biosolids is their

transportability. Heat-dried pellets can generally be transported much greater distances than

other biosolids because of their dry and light form and because of their higher value from higher

N content than other biosolids products.


8-9

8.3.1.4 Class A Heat-Dried Non-Pellet

The Class A Heat-Dried Non-Pellets

(produced by processes such as belt dryers)

also typically meet requirements for a Class

A product with EQ standards and solids

contents between 92 and 95 percent. They

differ from the pellet product by varying

widely in shape and uniformity. The

material produced is mostly irregular,

varying in both shapes (from granules to

fragmented sticks) and size (from dust particles

up to approximately 6 millimeters). Product size

and density will vary by manufacturer due to the

different methods of preprocessing and feeding the cake to the dryer, with material from one

belt-dried facility compared to Cheetos in appearance. Figure 8-1 shows product from one

supplier’s system.

Factors that affect product marketability vary by market sector and region. Some of the most

common factors include nutrient content, physical characteristics, product availability, and

delivery method and application. For dried product, particle size is a key component. Fertilizer

blenders prefer uniform shaped product within a narrow particle size distribution, while

consumers in other markets may prefer the irregular dirt like appearance of this non-pellet, less

uniform product.

Due to the variance in appearance, heat-dried non-pellets cannot access the same market

diversity as pellet form. While they can be used in agricultural applications – such as cropland,

pastures and rangeland – the dust and lack of uniform size mean that they cannot typically be

used by fertilizer blenders and may not generally be as desirable to demanding markets such as

golf courses.

The finished product is well suited for use as a fuel or a biofuel. Both heat-dried products typically

have the heat value of a low-grade coal and have been shown to be a good substitute for coal in

cement production. The non-pellet product is somewhat more desirable as a coal replacement

because it is typically crushed before entering the burner.

The cement industry co-fires dried biosolids in locations across the country. At the Lehigh

Cement Plant in Union Bridge, Maryland, several municipal heat-dried products are delivered to

the plant for use as an alternative fuel. Lehigh burns the heat-dried products as a supplemental

fuel source to coal that usually fires the cement kiln. The pellets are crushed before use, whereas

the non-pelletized, dustier products can go directly into the process. Approximately 14,000

metric tons are combusted annually, with plans to increase capacity to 36,000 metric tons

annually. This represents approximately 3- 5% of its average daily fuel use and is reported to

have no adverse impacts to cement quality (Maestri, 2009).

Figure 8-1. Belt dryer non-pellet product

from an Andritz system


8-10

8.3.1.5 Nutrient Enhanced Pellet

The last 10 years have seen a great deal of interest in value-added biosolids products, which are

typically Class A, EQ biosolids suitable for marketing and retail. One such value-added product is

a proprietary material marketed under the name of Anuvia, which converts digested or

undigested dewatered biosolids into a high-value ammonium sulfate granular fertilizer. The

resulting product is a dry (greater than 99 percent), hard granule of 2 to 3 millimeters in size,

having a N:P:K nutrient content of approximately 16:2:0, along with significant sulfur

(16 percent), iron (1 percent), and organic content (18-20 percent).

Anuvia has operated the first commercial plant in Zellwood, Florida, that has optimized the basic

organic ammonium sulfate fertilizer process by separating acidification and ammoniation steps to

enhance process control, chemical consumption, and manufacturing economics. The acidification

and ammoniation reactions not only destroy all microorganisms, but also hydrolyzes

macromolecules, such as proteins, into smaller peptides and amino acids. This fertilizer liquid mix

is then sprayed into a granulator, and the resulting granules are then conveyed to a dryer,

followed by product sizing, cooling, and coating processes that use standard granular fertilizer

manufacturing equipment.

The commercial-grade, granular, slow-release fertilizer provides beneficial crop responses in

yield and quality and can be sold into the turf and agricultural markets. Even though it

represents only 16 percent of the product weight, the biosolids will qualify the fertilizer as an

enhanced efficiency fertilizer, which is in increasing demand. In addition, the product’s relatively

high N:P ratio means that it can be applied at a higher rate without exceeding allowances for P

application in agricultural settings.

Due to the proprietary nature of this process, site constraints at the WPCP and the manufacturing

nature of nutrient enhancement, processes such as Anuvia would be an off-site option for

Arlington County. Such a third-party option for Arlington would mean the County would commit

to pay an agreed rate per wet ton of biosolids sent to an Anuvia facility. Contract terms and

conditions would have to be specified, including responsibilities for the final product quality,

distribution, and alternative management if the biosolids could not be delivered or processed at

the nutrient enhancement facility.

8.3.1.6 Class A Compost

Composting produces a marketable, familiar, high-quality Class A biosolids product. Since it

requires woody material as amendment, community green waste can be used in the compost

process, reducing costs for amendment purchase and providing a use for the community’s green

waste. As a stand-alone product, compost is often used as topdressing by homeowners or turf

managers or mixed directly into planting beds to improve poor soils. The compost provides

nutrients and organic matter to the soil as it degrades. In some places, biosolids compost can be

purchased in bulk or bagged form by users; alternately, the compost can be spread by

landscaping and maintenance services.


8-11

It is not uncommon, however, to see compost mixed with other materials such as sand (to make a

topdressing) and soil (manufactured topsoil). Compost can also be used for mulching and at

schools, parks, or distributed through a network of licensed commercial vendors for retail. The

amount of material handled by vendors can vary widely.

Composting can also be used for agriculture. While this market is new to some areas, compost has

been used on a large scale in California to improve soils, and has played a key role in the state’s

initiative on carbon farming where processes promote carbon sequestration.

A key advantage of composting is its adaptability to a variety of feedstocks. Composting has been

applied successfully to municipal solids, yard waste, animal manures, and a variety of agricultural

materials. Sawdust, wood chips, leaves, agricultural wastes, and even recycled compost itself have

been used for this purpose. The ability to segregate and specify amendment quality and

feedstocks is important for Arlington County product branding and quality control purposes. If

biosolids are added to the compost mix, regardless of the amendment, the final product must

meet federal and state requirements for biosolids.

Due to plant site limitations, composting would be an off-site option for Arlington County. As a

contract option, Arlington County could negotiate a contract to haul and treat a portion, or the full

production of solids to an off-site facility. For the third-party composting option, the County

would commit to pay the third party an agreed rate per wet ton of biosolids received. Contract

terms and conditions would have to be specified, including responsibilities for the final product

quality, distribution, and alternative management if the biosolids could not be delivered or

processed at the compost facility. The ability to segregate Arlington County biosolids from other

feedstocks is a concern; the County’s product branding potential is limited.

The McGill Regional Composting Facility in Sussex County, Virginia near the town of Waverly, is

within hauling distance from the plant. It opened in 2008 and serves the coastal Mid-Atlantic

region, including biosolids from DC Water and Hampton Roads Sanitation District (HRSD).

Multiple feedstocks, including yard waste, industrial wastes (i.e., Smithfield Farms), and biosolids,

are composted in bays with biofilters and aerated outdoor curing. The current capacity would

need to be determined to ensure the receipt and processing of the current and future volumes of

biosolids from the County. Depending on the quantities and contract duration, the company

would consider facility expansion to handle additional volume. Additional plant site storage,

including on-site and cake storage, may be required depending on the trucking and receiving

schedule at the third-party composting facility.

8.3.1.7 Soil Blend

Due to increased nutrient management regulations in the past few years that have started to

impact the timing and amount of biosolids applied to agricultural land, many municipalities have

begun to discuss alternative uses of Class A biosolids products, such as blending for soil

manufacturing. Soil blending is being contemplated as an alternative to historical agricultural

land application programs. As with other Class A marketable products, soil blends with Class A

biosolids are removed from the agricultural program and their inherent nutrient management

restrictions to become products associated with landscaping or construction industries. This

allows for flexibility and sustainability to historical agricultural programs when nutrient

management regulations continue to become more restrictive. Even though soil blending


8-12

alternatives have great merit for opening new product markets for Class A biosolids, it has yet to

become a viable option in the Mid-Atlantic States. The regulatory framework needed to

accommodate the blending of Class A biosolids with inert soils and woody materials has not been

fully accepted or implemented. Most states have been very cautious to approve general

distribution of Class A dewatered cake products as they look and handle like Class B dewatered

cake. Most municipalities and historical contract land appliers do not have the space or

equipment required to blend Class A biosolids into manufactured soils, therefore a partnership

with soil blending operators are necessary. This partnership requires the movement of Class A

dewatered biosolids off-site with regulatory general distribution approval. Without regulatory

general distribution approval, Class A products cannot be taken off-site and blended into soil

products. As a result, the current Mid-Atlantic regulatory framework limits the development of

soil blending and therefore limits this expanded market potential which falls outside of the

typical agricultural product uses.

Regulatory agencies have only been willing to give general distribution and marketing approval

to heat dried and composted Class A products, as they look and handle differently from Class A

dewatered cake. In addition, using Class A heat dried pellets for soil blending has proven

somewhat problematic due to the density and size of the pellets as they tend to fall out of a

blended soil product.

With the recent installation of the thermal hydrolysis process

(THP) at the DC Water Treatment Facility, a new Class A

dewatered cake has been introduced into the Mid-Atlantic

region. This process produces a unique Class A dewatered cake

that is generated from the thermal hydrolysis pretreatment

technology described earlier in this section. Even with the

addition of a new Class A product that is a great candidate for

the soil blending program, the local regulatory agencies are still

very cautious about giving the necessary general distribution

approvals needed to accommodate off-site soil blending operations. DC Water has been working

with Virginia Tech to develop specific soil blends and mixes that contain saw dust and/or sand

that could be distributed to landscapers and the construction industry. The DC Water’s Class A

biosolids product is marketed as BLOOM, Figure 8-2. This product is similar to the City of

Tacoma Washington’s TAGRO mix that has been sold to local communities for years. TAGRO has

been sold in both bulk and bags at many local hardware and garden shops.

8.3.1.8 Ash

An incineration process by-product, ash is an inert, sterile material that can be landfilled or

beneficially used. While ash is suitable for use as an ingredient for cement production or as a soil

enhancement, the most common practice is landfill disposal. Some inert materials in ash may be

recovered, such as P. The major advantage of incineration and other thermal conversion

processes is the overall reduction in solids. Of the incoming solids, approximately 30 percent of

the solids fraction by weight remains as ash after incineration. This is a 70 percent reduction in

solids to be used or disposed.

Figure 8-2. DC Water

Class A Product Logo


8-13

Fairfax County operates four MHFs at the Noman M. Cole Jr. Pollution Control Plant (NMCPCP),

located approximately 15 miles from the Arlington County WPCP. Solids cake pumps are piped so

that the dewatered solids can be pumped to any one of the four MHFs. The MHFs have a rated

capacity based on average solids concentration of approximately 60 DTPD, but the current solids

throughput is approximately 40 DTPD. In the past, both Arlington County and Fairfax County

have expressed interest in taking advantage of this unused capacity at the NMCPCP by sending

Arlington County’s solids to the facility. Arlington County would pay a tipping fee to Fairfax

County to accept the sludge.

Because NMCPCP does not have facilities to receive dewatered sludge cake, Arlington County’s

thickened solids would be brought to the plant via tanker truck. Once received, the solids would

be lime conditioned (to reduce clinkers in the incinerators), dewatered, and incinerated. Energy

recovery equipment is not yet installed at NMCPCP, but is being planned. Once the energy

recovery system is online, the value of the energy produced from incinerating Arlington’s solids

could be included in the overall contract and tipping fee structure.

The Arlington County WPCP currently produces more biosolids than the NMCPCP could accept in

its entirety. While contracting with Fairfax County may not be a viable long-term strategy, it may

have value as a shorter-term option.

8.3.2 Energy Products

Energy consumed at wastewater treatment facilities includes electricity for equipment, heat for

processes and building heating, and petroleum-based fuels for trucks and other mobile

equipment. Many utilities are looking at renewable energy options to reduce energy costs and

their overall carbon footprint. Renewable energy options at wastewater facilities cover a broad

range of sources and technologies. Some of these are discussed further in the following sections.

8.3.2.1 Electricity and Heat Recovery from Biogas or Biosolids-Derived Material

Renewable energy sources derived from biological processes, such as biogas, are referred to as

bioenergy sources. Using biogas from anaerobic digestion is one of the most readily available,

proven, and cost-effective energy sources at wastewater facilities. Biogas is comprised of

approximately 60 percent methane and can be easily combusted for heat and/or power.

According to the WEF Biogas Survey, 85 percent of WRRFs with anaerobic digestion beneficially

use their biogas (Beecher, et al. 2013). Heat recovery is the most common use of biogas, with

most facilities using biogas in boilers or recovering heat to heat digesters and/or buildings.

Cogeneration is the utilization of normally wasted heat energy produced by an industrial process,

especially to generate electricity. In wastewater treatment, cogeneration most often applies to

using excess biogas to generate electric power for plant use or sale. Power generation from

biogas is particularly attractive in areas with high electricity rates.


8-14

Systems that incorporate recovering waste heat from power generation for plant use are referred

to as combined heat and power (CHP)

systems. CHP technology has improved

significantly in recent years with overall

energy efficiencies as high as 80 percent.

Many CHP technologies are available, from

internal combustion engines or widely used

micro-turbines, to fuel cells with minimal,

clean emissions. Biogas cleaning to remove

moisture and siloxanes is an important design

consideration depending on the CHP

technology. Figure 8-3 shows an 850-kW

engine generator before installation.

CHP systems at wastewater treatment and solids management facilities vary in complexity from

simple systems designed to consume only biogas, to complex systems that consume both biogas

and other types of fuel. The more complex systems can often provide full standby power and

produce excess electricity to sell back to the utility. In addition, several technologies are available

for power generation; internal combustion engine generators and gas turbines are most

commonly used.

In addition to generating power, heat can be recovered from an engine to help heat the anaerobic

digestion process, which frees up more biogas for alternative uses.

Production may also be boosted through digesting high strength organic wastes (co-digested

sludge and fats, oil, and grease (FOG)). Due to grease’s high biodegradability, adding it to the

anaerobic digestion process could boost biogas production, with minimal impact on overall

biosolids production. Disadvantages of accepting grease include increased truck traffic at the

plant and operational issues associated with conveying grease into the digesters. Private hauler

preprocessing reduces these impacts, but grease characteristics make it a nuisance material for

conveyance and requires closer operator attention. In addition, the grease could increase foam

production, warranting closer operator attention to feeding patterns and overall operation.

8.3.2.2 Steam

Steam is necessary to operate several thermal hydrolysis processes (e.g., Cambi, Exelys).

Typically, biogas from MAD is processed using CHP facilities to generate electricity and most

steam necessary for THP. In the Cambi reactors, approximately one ton of steam is injected per

one dry ton of sludge. Little off-site market exists for any additional steam not used by the THP.

The quantities and potential demand for additional steam beyond on-site use could be evaluated

further, but off-site demand would need to be almost adjacent to the plant site to be practical.

8.3.2.3 Conversion to Biomethane

In addition to providing electricity, biogas can be further cleaned and converted to a biomethane

such as a CNG product that could be used to power vehicles. The biomethane process takes biogas

that has been treated to remove water, H2S, and siloxanes, and further processes the biogas to

remove CO2. The biomethane can then be used the same as natural gas in HVAC systems, in

Figure 8-3. GE Jenbacher CHP Engine Generator


8-15

natural gas distribution lines, or compressed for use as a vehicle fuel. CNG use is a high value use

in that it directly displaces diesel or gasoline cost for vehicle engines. It should be noted however,

that vehicles may already need CNG engines in order for this option to be feasible.

Several different processes can process biogas into a biomethane product. These processes

include water wash scrubbing, pressure swing adsorption (PSA), and molecular sieves

(membranes) for CO2 exclusion. Each of these systems has advantages and disadvantages. In all

systems, a side stream gas will be produced that must be treated before discharge. Required

treatment will depend on the system selected. Treatment methods can include biofiltration,

chemical scrubbing, or thermal oxidation. A small liquid side stream may also be produced.

Currently, only thermal oxidation (ground flare) is considered for treating side stream gas in the

air emissions study being performed by CDM Smith. The water blowdown could contain CO2 and

H2S as well as other contaminants removed from the biogas. This side stream could be processed

along with the other plant side streams. Some makeup water would be required for this process,

as well. PSA has proven to be an effective process for producing biomethane from biogas. The

waste gas from the PSA process is typically oxidized using a gas burner. Emissions from the

burner should be considered when reviewing air permitting requirements for the process.

8.3.2.3.1 Injection into Natural Gas Pipeline

Injection of biomethane into the natural gas pipeline has been explored throughout the country.

The primary advantage for pipeline injection is the limited on-site requirement for biomethane

storage. The primary disadvantage for pipeline injection is the tight specification for meeting

pipeline quality gas and the extent of treatment that may be required. Historically, pipeline

injection was not deemed economical due to the high capital investment in infrastructure

required and cost to clean the gas.

8.3.2.3.2 Vehicle Fuel from Biogas

Using biogas to create CNG to power fleet vehicles, such as buses, trash trucks, or biosolids

hauling trucks, has been widely used in Europe for more than a decade, but has not been as

common of a practice in the U.S. Reasons for this include the complexity of the cleaning processes,

relatively high cost of cleaning, the low cost of natural gas, and the need for a vehicle fleet with a

constant fuel demand. Purity requirements for vehicular fuel are lower than those for pipeline

injection. Often, the biggest barrier to CNG conversion is the lack of a widespread infrastructure

for gas filling stations and the cost of vehicle conversion for CNG use. As scrubbing technologies

improve and become less costly—and using CNG becomes more widespread—we anticipate that

biomethane production will become more common in the U.S., especially at larger plants where

the economy of scale make this a more cost-effective process. Unit costs for CNG are based on the

gasoline gallon equivalent (GGE), with approximately 127 standard cubic feet of biogas

equivalent to 1 gallon of gasoline.

Some examples of municipalities creating biomethane include King County, Washington, and

Janesville, Wisconsin. King County has been cleaning its biogas using a CO2 wash system and

injecting the biomethane into a natural gas distribution system for more than a decade. Janesville

implemented a biogas cleaning and CNG facility to fuel city vehicles in early 2012. Most recently,

St. Petersburg, Florida, has decided to sell vehicle fuel derived from WWTP biogas.


8-16

Selling vehicle fuel is expected to provide an annual revenue stream, a portion of which is due to

trading Renewable Identification Numbers (RINs), a compliance measure for the EPA’s

Renewable Fuel Standard (RFS) Program. Covered parties meet their obligations under the RFS

by surrendering renewable fuel credits to EPA equal to the number of gallons in their annual

obligation. These credits, known as RINs, are generated when a batch of biofuel is produced, and

separated from the fuel by obligated parties. RINs work much like Renewable Energy Credits

(RECs) in the generation and trading of renewable electricity. RINs can be traded between

parties, bought as attached RINS to fuel purchased, or bought unattached on the open market.

Note that energy equivalence assignments in the RIN coding is relative to a gallon of ethanol, not

gasoline. Each gallon of pure liquid ethanol has about 0.077 MMBTU, which is thus the energy

value of one RIN.

8.3.3 Nutrient Products (non – biosolids derived)

8.3.3.1 Phosphorus Fertilizer Product

A challenge at WRRFs is managing effluent nutrients. Due to regulatory limits, treatment

processes strip the soluble nutrients and convert them into insoluble solids, concentrating these

constituents into the biosolids product.

One solution to this challenge can be taking the waste product high in nutrients and recovering

nutrients through precipitation and harvesting using processes such as those designed by Ostara,

Multi-form Harvest, or AirPrex. In some cases, up to 85 percent of the P and 40 percent of the

ammonia load from a municipal WWTP can be removed from sludge dewatering liquid and

marketed as a commercial fertilizer.

With nutrient management process, P and ammonia are recovered and struvite controlled to

form extremely pure, crystalline struvite pellets. These pellets start out as microscopic crystals

that grow until the desired size, approximately 0.9 to 3.0 millimeters (mm). These pellets are

bagged and distributed as a slow-release, commercial fertilizer to a variety of customers locally

and regionally.

The pellets are usually sold, marketed, and distributed by a third-party, often the process

manufacturer. A locality study could be conducted to determine market viability in the local area,

but distribution is typically not restricted to the local area.

If the customer and the process manufacturer agree, the manufacturer would provide the

equipment to the customer with the understanding that the processed pellets would be returned

to the manufacturer, who would be responsible for final product marketing.

In addition to providing a beneficial reuse product, this option helps in other areas of the process

by helping to meet effluent discharge and biosolids land application limits and prevents struvite

formation. There are some recent concerns that US EPA intends to regulate the non-biosolids

struvite pellets as a biosolids material, thus falling under Part 503 federal regulations.

This nutrient management approach has been used in the region, with three Ostara reactors

installed at Nansemond WWTP in Suffolk, Virginia, in 2010; two were installed (retrofit) at York

WWTP in York, Pennsylvania, in 2010. Though not required, the feasibility of this approach is

improved if the liquid treatment process includes biological phosphorus removal.


8-17

8.4 Regional Perspective 8.4.1 Mid-Atlantic Overview

A summary of WWTPs in the Mid-Atlantic region is presented in Appendix I. The Appendix is

intended to summarize what WWTPs in the region are currently doing with biosolids. It

specifically lists each plant’s design flow capacity, annual tons generated, biosolids treatment

process and end use markets, along with any energy or non-biosolids products generated from

their treatment process.

The end use patterns for biosolids in the mid-Atlantic region has been heavily skewed toward

land application of Class B biosolids. In 2010, over 76 percent of biosolids in the region was land

applied Class B product. According to Robert Crockett of the Virginia Biosolids Council, biosolids

were applied to approximately 52,000 acres of agricultural land, cultivated in pasture, row crops,

or silviculture in Virginia in 2014. When comparing the end use patterns of Class A products in

2010 (Virginia Biosolids Council) to those presented in this Appendix (2016), we see a shift from

a mix of Class B land application at 76 percent and Class A products at 12 percent to

approximately Class B land application at 43 percent and Class A products totaling 40 percent.

The trend in the mid-Atlantic region for treatment facilities similar in size (and larger) than

Arlington County’s WPCP has been to move towards processes capable of producing a Class A

biosolids product. Figure 8-4 presents a map of major Mid-Atlantic wastewater treatment

facilities located in urban areas. The map indicates the type of biosolids product or management

process relied upon to manage solids produced at the facility. Many of the facilities similar in

size to Arlington County are either currently producing a Class A biosolids or have plans to

produce a Class A product in the future.

Figure 8-4. Map of Solids Management Practice at Mid-Atlantic Wastewater Treatment Facilities


8-18

The shift to increased production and distribution of Class A biosolids products is significant. It is

largely due to the change in biosolids treatment at DC Water and the City of Philadelphia to

produce Class A products. DC Water and Philadelphia accounted for approximately 590,000 wet

tons of Class B land application in 2010, versus the 210,000 wet tons of Class A product

production in 2016. DC Water is currently land-applying in Virginia, whereas Philadelphia is

producing heat-dried pellets for marketing and distribution in Pennsylvania, Maryland, and

Virginia. The Class A technology shift has also significantly reduced the volume of biosolids

produced in the region. This shift in end use patterns is shown on Figure 8-5 for 2010 and

Figure 8-6 for 2016, below.

Most Class A materials are still being used for agricultural land application purposes (Figure 8-

7). The movement toward non-agricultural markets such as soil blending and landscaping has

been limited. Most Mid-Atlantic state regulatory agencies have not approved the general

distribution of Class A dewatered cake and have also required additional permitting when other

Class A products such as pellets or compost are blended with other inert materials, making the

processing and ultimate marketing of these blended products restrictive and cumbersome.

Class A ,

151,000 WT

(11.8%)

Landfill/Incineration,

154,900 WT

(12.1%)

Class B Land App,

974,100 WT

(76.1%)

Figure 8-5. Biosolids Generated in the Mid-Atlantic 2010

(Based on 1.28 M WT)

Class A ,

360,000 WT

(39.8%)

Landfill/Incineration,

155,600 WT (17.2%)

Class B Land App,

389,000 WT

(43.0%)

Figure 8-6. Biosolids Generated in Mid-Atlantic 2016

(Based on 904,600 WT)


8-19

AlexRenew installed a pasteurization process upstream of existing anaerobic digestion to

generate a Class A cake (dewatered, digested) in 2005. The switch from Class B to Class A cake

was driven by the AlexRenew’s concerns with possible regulatory changes that could limit Class B

land application programs. AlexRenew’s long-term goal is to minimize bulk land application by

generating a product that is suitable for distribution and marketing. A second area utility, DC

Water, recently implemented a Class A program consisting of thermal hydrolysis and anaerobic

digestion. The previous DC Water program relied on several outlets for its lime-stabilized cake,

including bulk land application and, within the last ten years, composting. DC Water is pursuing

soil blending to develop a marketable biosolids product with its Class A cake. In May 2016, DC

Water launched its new Class A biosolids soil conditioner BLOOM.

The shift to more Class A technology continues to grow as the Hampton Roads Sanitation

District’s Atlantic Plant and the Washington Suburban Sanitary Commission’s treatment plants

are scheduled to convert to Class A technologies. This would bring the percent Class A product

produced in the Mid-Atlantic to 49 percent of the total biosolids produced (by 2022). It will be

important for biosolids professionals and state agencies to work together to assure a safe and

easily distributed use of Class A biosolids products across many markets.

8.5 Opportunities for Product Use within Arlington County The following County Government agencies have expressed an interest in the Solids Master Plan

that is being conducted by the WPCP with respect to potential product development and reuse:

� Department of Parks and Recreation

� Solid Waste Bureau

� Arlington Initiative to Rethink Energy (AIRE)

� Transit Bureau

Pellets,

111,600 WT

(31%)

Compost ,

39,600 WT (11%)

Cake Land App,

208,800 WT

(58%)

Figure 8-7. Class A Distribution 2016

(Based on 360,000 WT)


8-20

Table 8-3 summarizes what products might have potential for use within the County as well as at

the treatment facility. Because Arlington County is primarily urban, there is no opportunity to use

products best suited for large-scale agriculture, such as biosolids cake. The primary opportunity

for use within the County is in the local markets catering to urban gardens, homes, urban soil

amendments, and other soil improvement projects. The county maintains approximately 1,800

acres of parklands, open space, and recreational facilities that may require occasional top

dressing or more aggressive use of soil amendments or blended soil products.

Table 8-3. Products of Interest for Use within Arlington County

Classification Product

Potential

County

Interest Comments

Biosolids

Class B Cake No Too difficult to handle, limited to large scale treatment

and/or application sites

Class A Cake No Too difficult to handle, limited to large scale treatment

and/or application sites

Class A Heat

Dried Pellet Yes

Local distribution and marketing – citizen use for urban

gardens/home

Class A Heat

Dried Non-Pellet No

Potential use as fuel at Resource Recovery Plant in

Alexandria, Covanta

Class A Heat

Dried Enhanced

Pellet

Yes Local distribution and marketing – citizen use for urban

gardens/home

Class A Compost Yes

Use on county lands and urban markets – citizen use for

urban gardens/home; County currently provides small

amounts of leaf compost to citizens

Soil Blend

Product

(Class A)

Yes

Use on county lands and urban markets – citizen use for

urban gardens/home; County currently produces small

amounts of soil blend materials (not with biosolids)

Energy

Electricity Yes On-site use

Steam Yes On-site use

Biogas Yes On-site or use as CNG vehicle fuel

Non-Biosolids

Nutrients

Recovered P

Fertilizer Yes

Fertilizer for County owned properties and/or local

distribution and marketing

8.5.1 Biosolids and Nutrient Products

Organic soil amendment product use within the county is currently limited. The Department of

Parks and Recreation indicates that landscaping and forestry divisions within the county do use

some soil amendments for their urban areas and projects. These amendments can sometimes be

obtained from the Earth Products Yards that are managed and maintained by the Solid Waste

Bureau. Fertilizer use is very low as the county has moved away from planting annuals to using

perennial plants that do not require fertilizers. When fertilizers are used, there is also a

reluctance to use chemical fertilizers due to potential environmental impact. The Parks and

Recreation Department expressed interest in working with an organic product if one was

produced at the plant.


8-21

The Solid Waste Bureau manages leaf collection and composting within the county. Leaves are

stockpiled at several sites, but space is at a premium. Residents may come and pick up the leaf

compost if they choose; however, most leaf compost is used internally by the county. The leaf

composting operation is practiced seasonally, and soils are mixed into the leaf clippings to create

recycled soils. Earth Products Yards blends the products, sometimes customized for the County’s

use. The soils are largely spoils from County construction projects and are of poor quality.

Blending these soils with amendments can greatly improve the product quality making them very

attractive for use by the county and local community. Arlington County has an Urban Agriculture

Task Force that outlined nine priority recommendations to the County in 2013. One of which was

a request for the County to “initiate municipal composting system to ensure an effective and

ecologically appropriate disposal, reuse, and recycling system for yard waste and other organic

materials”.

Because of the limited space and lack of storage within the County, much of the collected material

is taken to composting facilities in neighboring jurisdictions including Loudoun County, Prince

William County, and Maryland. The Solid Waste Bureau is looking into collecting food waste and

is considering using the new food waste

digestion and compost facility being developed in

Prince William County (Freestate Farms LLC,

Figure 8-8). Arlington County currently collects

approximately 35,000 tons of waste annually, 15

to 18 tons is organics, food, and yard waste. Solid

waste is currently taken to the Covanta operated

Resource Recovery Plant (waste to energy) in

Alexandria, which has a current tipping fee of

approximately $43 per ton of waste. Compost

facilities are currently taking the county yard

waste for a tipping fee of about $32 per ton. This

is a significant savings for the County, with the added benefit of producing an environmentally

sound product. The down side of this strategy is that once the yard waste and other organics are

removed from the solid waste stream and taken out of the County for processing, the compost

product does not come back to the county.

8.5.2 Energy Opportunities

Accessing and using the energy available in biosolids products or derived from biosolids

treatment can provide Arlington County opportunities to reduce the energy footprint of the

WPCP. The following paragraphs describe the current energy reduction programs actively

underway at the County including priorities, goals, energy reduction targets, and potential

opportunities for product reuse.

Arlington Initiative to Reduce Emissions (AIRE) Program started with a County Board initiative in

2007 and was defined by an aggressive 5-year program and a dedicated funding stream achieved

by the addition of a utility tax. The program was renamed in 2013 the “Arlington Initiative to

Rethink Energy” (AIRE) and program goals were established for 2020 and 2050.

Figure 8-8. Freestate Compost Facility

in Prince William County


8-22

The County Operations Energy Plan complements the Community Energy Plan (CEP) (adopted

2013) and is an element of the County’s Comprehensive Plan. The long-term goal is to reduce

GHG emissions by more than 70 percent from 2007 baseline to 3 metric tons of CO2e per capita

per year by 2050. Also included is a goal to reduce emission from County government activities

by 76 percent by 2050 with an interim target of 25 percent reduction by 2020. Baseline emissions

in 2007 and targets for 2020 through 2050 are presented in Table 8-4.

The priority to improve energy productivity is as follows:

� Increase the energy efficiency of buildings, vehicles, and infrastructure

� Use waste or excess heat

� Use renewable sources of energy

� Use distributed energy generation close to points of energy use

The County facilities including Arlington Public Schools total nearly 900 billion BTU annually and

represent approximately 3 to 4 percent of the entire county (residents, businesses, and

government) emissions. For the county-owned facilities, buildings, and activities the COEP

roadmap set the following objectives:

Table 8-4. Baseline and Target Net GHG for Government and Public Schools – 2007 through 2050

Year GHG Emissions; CO2e metric tons GHG Emissions; % Change from 2007

2007 Baseline 93,575 -

2020 Target 70,181 25 percent reduction




The WWTP is the single largest energy user in county operations, consuming 13 percent of all

energy used by the county and the school system.

GHG emissions per unit of energy are determined by the choice of fuel for an energy service. At

the plant, efficiency in energy use is measured as energy per 1,000 gallons treated. For the 2012

emissions goal, energy reduction at the WWTP was achieved due to water volume reduction to

the plant by 11 percent and electricity became 7 percent cleaner for a lower GHG emission rate.

The 2020 GHG Emissions Goals (from 2012 levels) is expected to be reduced three ways:

1. no change in building area

2. reduced energy per 1000 gallons by 10 percent

3. decreased emission rate from electricity by 15 percent

The COEP includes the following objectives and strategies that specifically call out the WWTP.

However, currently the trucks used to haul biosolids are not included in the energy footprint of

the plant as they are accounted for in the overall Community inventory.


8-23

� Objective 1: Increase facility and infrastructure energy productivity through continuous

improvements in efficiency:

• Strategy 4 – Develop or re-examine energy management plans for large, energy-

intensive and special-purpose county facilities

• Strategy 6 – Participate in and achieve the goal of the DOE Better Buildings Challenge

� Objective 3: – Make use of waste heat and process optimization for efficiency and resource

recovery:

• Strategy 2 – Consider combined heat and power and district energy

• Strategy 3 – Maximize efficiency and opportunities for resource recovery at the WWTP

� Objective 5: Be a leader in early adoption and promotion of innovative technology

• Strategy 1 – Pilot and deploy new and innovative technologies

� Objective 6: – Pursue sustainable funding strategies

• Strategy 1 – Fund clean energy and resource efficiency projects in the Capital

Improvement Plan

Reducing emissions and energy saving priorities are a major county focus and plans specifically

address the energy usage at the wastewater treatment facility. Reuse opportunities for steam,

heat, and gas, all byproducts of various biosolids treatment alternatives being evaluated in the

Solids Master Plan, will have an impact on these goals and opportunities.

8.5.2.1 Electrical Power

As discussed in Section 5, digester gas can be recovered and used to power a variety of processes,

including digester solids heating or thermal drying systems. However, the gas can also be fed to a

CHP system to generate useful electricity and waste heat. In some cases, CHP may be able to

create a net positive energy balance on electrical power, as the CHP system can produce more

power than required of the solids handling equipment. The additional power would be available

to offset WPCP electrical demands or available to sell to the electrical utility.

Renewable Energy Credits (RECs) are currently being purchased by the plant (approximately

500 megawatts monthly). This cost is embedded in the power charged by Dominion Power, who

is required to sell these credits as part of their Clean Energy Program. This rate is set once every

3 years. In evaluating the energy saving potential available for some of the biosolids treatment

alternatives, the potential for selling RECs may also become part of the financial analysis.

In addition, the plant is considering participation in the Department of Energy’s Better Plants

Challenge that sets voluntary goals for a given metric of energy consumption to be reduced by a

set amount. This program would provide additional incentive for the plant’s participation in

overall energy reduction.


8-24

8.5.2.2 Biogas

Biogas used for vehicle fuel is another opportunity available to the WPCP. Currently there is a

new Arlington Regional Transit Maintenance and Fueling Facility directly across from the plant

on South Eads Street. The facility will be used for fueling, maintenance and washing the existing

ART CNG buses. The existing ART fleet has approximately 65 buses, with plans to expand to 85

buses. The ART buses currently fuel at the County Trade Center Site or they use the Washington

Metropolitan Area Transit Authority (WMATA) CNG fueling station in Crystal City. WMATA also

uses CNG buses and currently houses some of their buses directly across from the plant on South

Eads Street, immediately adjacent to the new county ART bus facility. Converting the biogas to an

acceptable fuel for the CNG buses would require additional equipment for cleaning and storing

the biogas. The County Transit Bureau has expressed interest in exploring this opportunity.

An opportunity for revenue exists when converting biogas into vehicle fuel in the recently

released (July 2014) Renewable Fuel Standards (RFS) Pathways II and Technical Amendments to

the RFS Standards (Pathways-II Rule). The RFS Program was created under the Energy Policy Act

of 2005. The recent change in the Rule allows all municipal wastewater digester gas to be

considered “cellulosic” when converted to renewable fuels (for natural gas fueled vehicles – or

electric vehicles if the electricity is derived from digester gas). The classification of digester gas as

a cellulosic fuel enables the sale of biofuel RINs as cellulosic which have traded at a premium over

the previously allocated RINs for these same fuels. St. Petersburg Florida has used RINs to

significantly improve the return on investment for their proposed biogas fueling facility. In other

cases, high-strength organic waste such as deicing fluids have been used to increase biogas

production though a co-digestion process. However, co-digestion may impact the classification

and subsequent RINs value of biogas derived from non-cellulosic sources.

8.6 Product Associated Risks As with any product, there is risk or uncertainty in its development, marketing, and distribution.

However, with products produced as a byproduct of wastewater treatment, that risk or

uncertainty can be magnified by people’s perceptions, regulations, contractual obligations, and

production. Changes in inputs to the treatment plant (external to plant control) and changes to

the water stream treatment can impact the products produced.

Looking at just at the market outlets, some of the more significant risks or uncertainties can be

identified. Table 8-5 summarizes risks or uncertainties associated with the products evaluated in

the Solids Master Plan.

Some uncertainties can be mitigated through careful planning and access to multiple product

outlets; however, many may not be under the control of the treatment facility or the county.

The most visible and active risk to biosolids distribution is the change taking place with respect to

land application. States are adopting more stringent land application regulations, which has led to

the loss of land availability for application. Increasing costs of permitting and managing these

sites, along with the uncertainty of public acceptance, threaten the continued production of Class

B solids whose only beneficial product outlet is land application.


8-25

Given that a Class A product does not rely solely on land application but can be handled to

increase the distribution outlets for this material, moving to Class A would create more options

and opportunities to mitigate product risks.

Energy products provide another type of opportunity that draws on environmental soundness

and sustainability. When biogas is derived from treating plant solids, the gas creates products

that can mitigate energy dependency, provide benefits to the plant and the County, and can

potentially provide a revenue stream that continues beyond the study life of any of the solids

management alternatives. Energy recovery from biogas is also evaluated as part of the economic

and financial analysis presented in this report.

8.7 Summary We offer the following observations in evaluating long-term solids treatment alternatives for the

Arlington WPCP:

1. Class B Biosolids Product. The most likely outlet for Class B biosolids products (cake) will

remain as bulk application to agricultural sites using a third-party contractor to haul, store

apply, track, and comply with all the required regulations governing this practice. It is likely

that the program costs for this end use will increase as regulations for land application

tighten in the mid-Atlantic states and restrict the quantity of biosolids that can be applied to a

given site as well as dictate when applications can take place (i.e., seasonal restrictions).

2. Class A Biosolids Product. Depending on the product produced and its quantity, both

government agencies and citizens appear to be interested in an organic biosolids product that

could be managed by a third-party distributor and be used locally with minimal restrictions.

Class A biosolids products include cake, heat dried pellets or non-pellets, nutrient enhanced

pellets, compost, and soil blend materials. Currently much of the Class A dewatered cake

material produced in the Mid-Atlantic region is being land applied to agricultural lands, but

this may change as more and more major wastewater treatment facilities move to produce a

Class A product. Outlets for soil blends, compost, and organic fertilizers used in urban

settings and gardens have gained significant support from communities and the agricultural

industry. Implementing a Class A marketing program to generate interest and distribute the

product is a long-term prospect and will require effort and investment whether this comes

from the plant itself or through a third-party contractor.


8-26

Table 8-5. Market Outlet Associated Risks or Uncertainties

Market Outlet Risks or Uncertainties

Bulk Agricultural Land Application for Class B

Cake

Potential Future Class A Requirement

P Regulations

State or Local Bans/Restrictions

Potential Public Opposition

Contractor Fee Stability

Contractor Inability to Perform

Odor Event On-Site or Off-Site

Bulk Agricultural Land Application for Class A

Products

P Regulations

State or Local Bans/Restrictions

Potential Public Opposition



Distribution and Marketing of Class A Products State and/or Local Permitting

Product Licensing

Odor Potential

Additional Processing Requirements

Additional Packaging Requirements

Product Marketing and Acceptance



Energy Products: Biogas for Vehicle Fuel Gas Treatment and Storage Requirements

Vehicle Engine Specifications

Product Revenue

Federal Incentives, RINs, and Reuse Fuel Market Stability

Energy Products: Biogas for CHP Gas Treatment and Storage Requirements

Emissions Impact

On-sited usage vs power to grid



Non-Biosolids Product: P Fertilizer Federal and State Regulations moving toward regulating as a

Biosolids Product

Off-Site Treatment and Processing Hauling Unstabilized Solids (interim solution only)

Third Party Agreement Duration

Third Party Fee Stability

Third Party Inability to Perform


8-27

3. Class A versus Class B Biosolids. The drivers and benefits of moving from a Class B biosolids

product (dewatered, lime stabilized cake) to a Class A biosolids product need to be clearly

defined and articulated. Some of the larger treatment plant facilities in the Mid-Atlantic region

have already shifted toward Class A products.

Class B land application remains a viable option for many treatment facilities in the Mid-

Atlantic. In areas with either regulatory or practical restrictions on Class B bulk land

application, Class A digestion processes may maximize land availability. In addition, some

Class A processes have significant benefits in terms of solids reduction and biogas generation.

However, an emerging trend for producing a Class A product for distribution and marketing is

driven by the fact that this may allow the utility to get out of the land application business

altogether. If Arlington County changes from a Class B to a Class A product, the above defined

reasons would help to justify any potential increased investment required to produce a Class

A product. This decision is best made with local community, county, and industry support.

4. Energy Products. The ability to generate energy from biosolids is well documented.

Renewable energy options are a recommended focus of any biosolids treatment analysis and

should include reuse of energy to supply heat, mixing, steam, fuel, or other energy

requirements of the on-site treatment process. Excess energy production may also be

available. As a result, using renewable energy is an integral part of the alternatives analysis

and evaluation. Developing renewable energy options that can contribute to the major

energy goals of the County’s Community Energy Plan supports the entire county. Options

mentioned in this Master Plan include using biogas to supplement the natural gas vehicle bus

fueling facility currently being constructed adjacent to the treatment plant site as well as CHP

on site to convert the biogas to electricity for use at the treatment plant. A preliminary

economic analysis of these alternatives is presented in Section 11 of this Master Plan.

An initial look at the GHG impact of using digester gas to offset electricity use at the plant, as

conducted by the county, shows biogas use in 2040 to produce electricity could reduce the

plant’s GHG emissions by approximately 40 percent as compared to the current lime

stabilization process. This substantially reduces the plant’s carbon footprint and would

impact the county’s overall operations for GHG emissions reduction.

5. Non-Biosolids Derived Products. The technical viability and economics of producing a non-

biosolids product are evaluated as part of the alternatives analysis (Section 11). Options exist

to recover P that have proven to be cost effective for several treatment facilities throughout

the United States. However, a third-party vendor is typically contracted to produce and

distribute this product. Interest from the vendor and economic feasibility for the county

would be required. The P fertilizer product could be used by the County Parks and Recreation

Department or by county citizens if made available to them by the vendor. A disadvantage to

pursing this opportunity is the current movement to treat these products as biosolids derived

products, which would constrain product distribution as a regulated biosolids product.


8-28

6. Off-Site Opportunities. Several off-site opportunities exist in the region and have been

mentioned in this Master Plan. They include the McGill Regional Composting Facility in

Sussex County Virginia, Lehigh Cement Plant in Union Bridge Maryland, the Covanta Resource

Recovery Plant in Alexandria, Virginia, and other local government solids treatment and end-

use processors such as the Fairfax County Noman M. Cole Jr. Pollution Control Plant. In

addition, vendors may have interest in capitalizing on capturing recycled materials (such as

P) or energy may approach the county with project proposals and financing options. All of

these options require negotiation, procurement, and contract development.

7. County Opportunities. Other considerations to be included in the alternatives analysis

beyond on-site use of products derived from the treatment of solids at the WWTP include

developing potential partnerships within the county government structure and other county

entities. Considerable interest exists within the county for an organic soil amendment or

fertilizer that could be used by the county and by its citizens. Blending biosolids with other

County wastes, such as leaf or food waste, means that federal and state biosolids

distribution/quality regulations will apply to the final material regardless of the

proportionate amount of biosolids included. Biogas-derived fuels appear to warrant further

investigation working with the County Transit Bureau, and an opportunity exists to work

with Reagan National Airport to explore potential partnerships for energy needs or the

processing of de-icing liquids in digestion facilities (high in organics to augment biogas

production). In addition, analysis of the Solids Master Plan’s impact on the County’s energy

goals will play a significant role in the decision process.

8.8 References Beecher, N., Qi, Y., Stone, L., (2013) Biogas Production and Use at Wastewater Treatment Plants in

the United States; Water Environment Federation: Alexandria, Virginia.

Maestri, T. (2009), The Use of Biosolids as a Renewable Fuel, Chesapeake Water Environment Association Conference, Hunt Valley, Maryland.

Stone, L., Kuchenrither, R., Quintanilla, A., Torres, E., Groome, M., Pfeifer, T., Dominak, R., Taylor, D. (2010) Renewable Energy Resources: Banking on Biosolids; National Association of Clean Water Agencies, Washington, D.C.

Planning Division of the Department of Community Planning, Housing and Development, (2016),

Urban Design + Research, Profile 2016, Arlington, Virginia.

9-1

Section 9Process Evaluations

9.1 IntroductionThis section presents the evaluation results of four solids treatment processes being considered at the Arlington County WPCP: thickening, dewatering, stabilization, and thermal drying. Taking into account the design criteria for each technology, equipment options at each part of the process train are compared on the basis of capital cost, life cycle cost, and footprint. This evaluation intends to aid in developing process train alternatives that will be further evaluated and compared (Figure 9-1).

Figure 9-1. Alternatives Evaluation Process

Table 9-1 presents the 30-day, 14-day, and 7-day solids peaking factors for primary solids, WAS, and combined solids (Section 3). The peaking factors were used to determine the maximum month solids production values used in the solids projections through 2040 (Table 9-2). The basis for solids planning, which is used to determine the design solids loads that each process must handle, assumes an annual average flow of 40 mgd.

Table 9-1. Solids Peaking Factors

Flow Condition Primary WAS Combined

Max 30-Day Average 1.6 1.6 1.3



Section 9 Process Evaluations

9-2

Table 9-2. Basis of Planning

Primary Solids(lbs/day)

WAS(lbs/day)

Total Raw Solids Production(lbs/day)

Year

Projected Annual

Average Flow

(mgd)

Ann. Avg. Day

Max. 30-Day

Max. 14-Day

Max. 7-Day

Ann. Avg. Day

Max. 30-Day

Max. 14-Day

Max. 7-Day

Ann. Avg. Day

Max. 30-Day

Max. 14-Day

Max. 7-Day

2015 28.1 38,000 62,500 75,000 82,300 23,100 37,700 41,300 44,900 61,100 80,900 86,300 98,600

2020 29.8 40,300 66,300 79,500 87,300 24,500 40,000 43,800 47,600 64,800 85,800 91,500 104,600

2025 31.0 41,900 69,000 82,700 90,800 25,500 41,600 45,600 49,500 67,400 89,300 95,200 108,800

2030 31.8 43,000 70,800 84,800 93,100 26,200 42,700 46,800 50,800 69,200 91,600 97,700 111,600

2035 32.4 43,800 72,100 86,400 94,900 26,700 43,500 47,700 51,800 70,500 93,300 99,500 113,700

2040 32.7 44,200 72,800 87,200 95,800 26,900 43,900 48,100 52,300 71,100 94,200 100,400 114,800

Basis for Solids

Planning40.0 54,100 89,100 106,700 117,200 32,900 53,700 58,800 64,000 87,000 115,200 122,800 140,400

The current solids process train was presented in Section 2 and is shown in Figure 9-2. Primary solids are sent to the gravity thickeners, and WAS is sent to the DAFT. Thickened solids from both the gravity thickeners and DAF units is then sent to a solids blending tank, where it is held before moving to centrifuge dewatering. Lime is added to the dewatered solids to produce Class B biosolids that can be land applied off-site.

The mass balance assumptions used to determine the performance requirements for each process are presented in Table 9-3. This includes solids concentrations, capture rates, and volatile solids removal rates for the various digestion processes being considered.

Figure 9-2. Current Solids Train


9-3

Table 9-3. Mass Balance AssumptionsProcess Parameter Value Units

Primary Solids Production 1,352 lb/MGPrimary

Primary Solids Concentration 1.5 %

WAS Production 823 lb/MGWAS

WAS Solids Concentration 1.1 %

Primary Thickening Capture Rate 95 %Primary Thickening

Thickened Primary Solids Concentration 4.0 %

WAS Thickening Capture Rate 95 %WAS Thickening

Thickened WAS Solids Concentration 4.0 %

Blended Thickened Solids Concentration1 4.0 %Combined Thickened Solids

Blended Thickened Solids Concentration2 6.5 %

Digester Feed Volatile Solids Content 75 %

Digester VS Removal 50 %Mesophilic Digestion

Digester Gas Production 15 cf/lb VS removed


Digester VS Removal 55 %Temperature Phased Anerobic Digestion (TPAD)


Solids Screening Capture Rate 98.5 %

Screenings Dryness 45.0 %

Pre-Dewatering Capture 95.0 %

Pre-Dewatered Solids Concentration 17.0 %

THS Solids Concentration 10 %


Digester VS Removal 60 %

THP


Post Dewatering Solids Capture 95.0 %

Dewatered Cake Solids Content (No Stabilization) 28 %

Dewatered Cake Solids Content (AD and TPAD) 25 %

Dewatered Cake Solids Content (THP) 30 %

Lime Demand (for No Digestion) 0.25 lb/lb solids

Post Dewatering

Biosolids Product Solids Content After Drying 92 %1. For process trains that do not include digestion.2. For process trains that include digestion. This is the high end of allowable solids concentration to maintain adequate mixing in the

digesters.


9-4

9.2 Process Train DevelopmentFour main processes are being considered as potential components of the process train: thickening, dewatering, stabilization, and thermal drying. In this section, equipment options are evaluated based on the process requirements at the design condition. Capital and life cycle costs for each option are also compared.

9.2.1 ThickeningBecause primary solids and WAS are currently sent to different thickening processes, three types of thickening are considered: primary solids thickening, WAS thickening, and co-thickening. The process requirements for each of these is presented in Table 9-4.

Table 9-4. Thickening Process Requirements

Primary Solids WAS Co-Thickening

Annual Average Production @ 40 mgd 54,100 lb/day 32,900 lb/day 87,000 lb/day

7-Day Production Peaking Factor 2.2 1.9 1.6

Design Condition (mass loading) 117,200 lb/day 64,000 lb/day 140,400 lb/day

Feed Solids Concentration 1.5% 1.1% 1.4%

Thickened Solids Concentration Varies; 4-8% Varies; 3-8% Varies; 4-8%

Desired Solids Capture Efficiency >95% >95% >95%

Design Operating Schedule 24/7 24/7 24/7

Table 9-5 presents the five thickening equipment options and indicates which options are considered for each type of thickening. Given the current solids treatment train at the plant, gravity thickening is only considered for primary solids, and DAF thickening is only considered for WAS thickening. Centrifugal thickening, gravity belt thickening (GBT), and rotary drum thickening (RDT) are considered for both WAS thickening and co-thickening.Table 9-5. Thickening Equipment Options

ProcessConsider for

Primary SolidsConsider for

WASConsider for

Co-Thickening

Gravity Thickening

Dissolved Air Flotation (DAF) Thickening

Centrifugal Thickening

Gravity Belt Thickening (GBT)

Rotary Drum Thickening (RDT)


9-5

The sizing of gravity thickeners can be based on two criteria: the solids loading rate (in lb/day/ft2) or the hydraulic loading rate (in gpd/ft2). Typically, solids loading rates range between 20 and 30 lb/day/ft2, and hydraulic loading rates are between 400 and 800 gpd/ft2. The two existing gravity thickening tanks are each 65 feet in diameter. Given that the maximum 7-day solids loading for these tanks at the design condition is 22 lb/day/ft2, the existing facilities are adequate for primary solids thickening. Non-potable water is added to the gravity thickeners to aid in managing the hydraulic detention, solids thickening performance, and as elutriation water. The increase in hydraulic loading can be as much as 1,000 gpm per thickener.

Typical design criteria for DAF thickening is presented in Table 9-6. Currently, each DAF is rated for approximately 24,000 lb/day WAS. Given the design condition of 64,000 lb/day of total WAS, the solids loading for the two existing DAF units would be 2.7 lb/hr/ft2, which is above the manufacturer recommended 2 lb/hr/ft2. Accordingly, a third DAF unit would be required for the design condition. It is assumed that the tank dimensions of the new unit would match those of the two existing units.

Table 9-6. DAF Design Criteria

Parameter Value

Hydraulic Loading 0.8 gpm/ft2 (basis for existing DAF units)

Solids Loading 2 lb/hr/ft2 (manufacturer recommended)

Operating Schedule 24 hr/day × 7 day/week

Surface Area Required 1,230 ft2 (based on manufacturer recommended solids loading)

Tank Dimensions 41.6 ft L × 12 ft W × 12 ft D (matches existing)

Design criteria for centrifuge thickening, gravity belt thickening, and rotary drum thickening are presented in Tables 9-7, 9-8, and 9-9, respectively. In each case, criteria for both WAS thickening and co-thickening are presented, which includes the hydraulic loading, solids loading, operating schedule, and number of units required. The solids loading design condition for the Master Plan is 64,000 lb/day of WAS and 140,400 lb/day of combined solids. The hydraulic loading design conditions for WAS thickening is 767,000 gpd (530 gpm) at 1 percent solids and the design for co-thickening is 1,202,000 gpd (835 gpm) at 1.4 percent solids.


9-6

Table 9-7. Centrifuge Thickener Design Criteria

Parameter WAS Thickening Co-Thickening

Hydraulic Loading 550 gpm 550 gpm

Solids Loading 2500 lb/hr 2500 lb/hr

Operating Schedule24 hr/day × 7 day/week(for 7 day max condition)

24 hr/day × 7 day/week(for 7 day max condition)

Number of Units Required 2 (1 duty, 1 standby) 3 (2 duty, 1 standby)

Table 9-8. Gravity Belt Thickener Design Criteria


Hydraulic Loading 200 gpm/m 200 gpm/m

Solids Loading 750 lb/hr/m 1500 lb/hr/m

Operating Schedule 24 hr/day × 7 day/week (max week)

24 hr/day × 7 day/week (max week)

Number and Size of Units RequiredTwo 3-m GBTs (1 duty, 1 standby); or Three 2-m GBTs (2 duty, 1 standby)

Three 3-m GBTs (2 duty, 1 standby); or Four 2-m GBTs (3 duty, 1 standby)

Table 9-9. Rotary Drum Thickener Design Criteria


Hydraulic Loading 350 gpm 350 gpm

Solids Loading 1500 lb/hr 2500 lb/hr

Operating Schedule24 hr/day × 7 day/week(max week)

24 hr/day × 7 day/week(max week)

Number of Units Required 3 (2 duty, 1 standby) 4 (3 duty, 1 standby)

Table 9-10 compares operating parameters for the WAS thickening alternatives. As shown, the major differences between alternatives are the power requirements and equipment operating times. For example, the power requirement for centrifuges is significantly higher than for any other alternative, while the operating time for the DAF units would be much higher than for any other alternative. A similar comparison of operating parameters for co-thickening alternatives is presented in Table 9-11.


9-7

Table 9-10. Comparison of WAS Thickening Alternatives

DAF Centrifuge 2-m GBT 3-m GBT RDT

Operating & Maintenance Labor (hr/week) 20 20 20 20 20

Typical Polymer Dose (lb/DT) 4 - 10 4 - 10 6 - 12 6 - 12 6 – 12

Assumed Polymer Dose (lb/DT) 6 6 10 10 10

Equipment Power, kW (HP) 30 kW (40 HP) 149 kW (200

HP)9.3 kW (12.5

HP)11.2 kW (15

HP)7.5 kW (10

HP)

Equipment Operating Time Year 2021, First Year of Planning Period (hr/week)

2 units @ 86 hr/week each

(Avg)2 units @ 164 hr/week each (Max 7-Day)

1 unit @ 69 hr/week (Avg)1 unit @ 131

hr/week (Max 7-Day)

1 unit @ 86 hr/week

(Avg)1 unit @ 164

hr/week (Max 7-Day)

1 unit @ 58 hr/week

(Avg)1 unit @ 110

hr/week (Max 7-Day)

1 unit @ 99 hr/week (Avg)2 units @ 94

hr/week each (Max 7-Day)

Table 9-11. Comparison of Co-Thickening Alternatives

Centrifuge 2-m GBT 3-m GBT RDT

Operating & Maintenance Labor (hr/week) 20 20 20 20

Typical Polymer Dose (lb/DT) 4 - 10 6 - 12 6 - 12 6 – 12

Assumed Polymer Dose (lb/DT) 6 10 10 10

Equipment Power, kW (HP) 149 KW (200 HP) 9.3 kW (12.5 HP) 11.2 kW (15 HP) 7.5 kW (10 HP)

Equipment Operating Time Year 2021, First Year of Planning Period (hr/week)

2 units @ 92 hr/week each (Avg)

2 units @ 147 hr/week each (Max

7-Day)

1 unit @ 152 hr/week (Avg)2 units @ 61

hr/week each (Max 7-Day)

1 unit @ 102 hr/week (Avg)1 unit @ 163

hr/week (Max 7-Day)

2 units @ 92 hr/week each

(Avg)2 units @ 147 hr/week each (Max 7-Day)

Estimates of thickening capital costs are presented in Table 9-12. This table presents the total estimated cost of equipment as well as the total capital cost of each alternative. A more detailed breakdown of capital costs for the thickening alternatives is provided in Appendix G.


9-8

Comparisons of life cycle costs for WAS thickening and co-thickening alternatives are presented in Tables 9-13 and 9-14, respectively. These comparisons take into account the net present value (NPV) of O&M to estimate the NPW, or total life cycle cost, for each alternative. As shown, rehabilitating the existing DAF units and constructing the new unit has the lowest life cycle cost for WAS thickening alternatives, while WAS thickening with centrifuges has the highest. For co-thickening options, the 3-meter GBT option has the lowest life cycle cost, which centrifuge co-thickening would be most costly.

Table 9-12. Comparison of Thickening Capital Costs

DAF Centrifuge 2-m GBT 3-m GBT RDT

Equipment

$1.2M(includes rehab of two existing plus

one new)

2 @ $1.7M each

3 @ $500,000

each

2 @ $600,000

each

3 @ $500,000

eachWAS Only Thickening2

Capital Cost $4.0M1,2 $8.3M1,2 $6.5M1,2 $6.0M1,2 $6.5M1,2

Equipment N/A 4 @ $1.7M each

4 @ $500,000

each

3 @ $600,000

each

5 @ $500,000

eachCo-Thickening

Capital Cost N/A $15.3M $9.7M $9.4M $10.6M

1. Capital cost does not reflect approximately $1.4M for GT improvements.2. Capital cost reflects cost of new building, but does not reflect cost of demolition of existing DAF.

Table 9-13. Comparison of WAS Thickening Life Cycle Costs

WAS Only ThickeningCost Presented as 2016 $, million DAF Centrifuge 2-m GBT 3-m GBT RDT

Capital Cost $ 4.0 $ 8.3 6.5 $ 6.0 $ 6.5

1st Year of O&M $ 0.10 $ 0.11 $ 0.11 $ 0.11 $ 0.11

20-Year NPV of O&M $1.8 $2.1 $2.0 $2.0 $2.0

Total NPW $ 5.8 $ 10.4 $ 8.7 $ 8.0 $ 8.7


9-9

Table 9-14. Comparison of Co-Thickening Life Cycle Costs

Co-ThickeningCost Presented as 2016 $, million

Centrifuge 2-m GBT 3-m GBT RDT

Capital Cost $ 15.3 $ 9.7 $ 9.4 $ 10.6

1st Year of O&M $ 0.21 $ 0.20 $ 0.20 $ 0.21

NPV of O&M $ 4.5 $ 3.7 $ 4.2 $ 4.2

Total NPW $ 19.8 $ 13.4 $ 13.6 $ 14.8

9.2.2 DewateringDewatering involves reducing the volume of solids and converting it from a liquid to a solid product. In most cases, dewatering is performed after digestion (if a digestion step is present) and before drying. However, in the case of THP or WAS-only THP plus digestion, a pre-dewatering step is required. This section will evaluate both pre-dewatering and dewatering equipment options. Dewatering process requirements for raw and digested solids are presented in Table 9-15.

Table 9-15. Dewatering Process Requirements

Raw Digested

Annual Average Solids Throughput @ 40 mgd (lb/day) 83,000 – 87,000 1 45,000 – 52,000 2

30 Day Production Peaking Factor 1.3 1.3

Design Condition (mass loading) (lb/day) 107,000 – 113,000 1 59,000 – 72,000 2

Design Operating Schedule – Final Dewatering (hr/week) 70 70

Design Operating Schedule – Pre-dewatering (hr/week) 168 N/A

1. Assumes 87,000 lb/day solids production with a 95% capture efficiency in a thickening process.2. Digested biosolids mass ranges as a function of volatile solids reduction in the digesters. THP plus digestion is assumed to provide the

largest VSR (60%). Class B mesophilic digestion VSR is assumed to be less (50%).

The three equipment options being considered for dewatering are centrifuges, belt filter presses, and screw presses. The design criteria for centrifuge dewatering of digested solids is provided in Table 9-16. It should be noted that although the THP and WAS-only THP pre-dewatering steps require three and two centrifuges, respectively, they would also require four post-dewatering centrifuges as presented in the Digested Biosolids Scenarios column.


9-10

Table 9-16. Centrifuge Dewatering Design Criteria

Parameter Raw SolidsTHP

Pre-DewateringWAS-only THP

Pre-DewateringDigested Biosolids

Scenarios

Solids Loading (lb/hr) 3,800 2,900 2,200 3,800

Requirements (lb/day) 107,000 113,000 52,000 72,000

Operating Schedule (hr/week) 70 168 168 70

Number of Units Required

4 (3 duty, 1 standby)

3 (2 duty, 1 standby)

2 (1 duty, 1 standby) 3 (2 duty, 1 standby)

Similar design criteria for belt filter press and screw press dewatering are provided in Tables 9-17and 9-18, respectively.

Table 9-17. 2-meter Belt Filter Press Dewatering Design Criteria

Parameter Digested Biosolids

Solids Loading (lb/hr) 1,500

Requirements (lb/day) 72,000

Operating Schedule (hr/week) 70

Number of Units Required 5 (4 duty, 1 standby)

Table 9-18. Screw Press Dewatering Design Criteria

Parameter Digested Biosolids


Requirements (lb/day) 72,000

Operating Schedule (hr/week) 70

Number of Units Required 8 (6 duty, 2 standby)

Table 9-19 presents the anticipated performance of the dewatering technologies with respect to cake solids produced for multiple types of solids. The values shown in the table are typical values based on performance of similar solids at other treatment plants.


9-11

Table 9-19. Comparison of Dewatering Performance

Final Dewatering THP Pre-DewateringAnticipated % Cake Solids

Centrifuge Belt Filter Press Screw Press Centrifuge

No Stabilization 28% 25% 25% N/A

Anaerobic Digestion & TPAD 25% 20% 20% N/A

ATAD 25% 22% 22% N/A

THP with MAD & WAS-only THP with MAD

30% 30% 30% 17%

The analysis of dewatering technologies for digested biosolids indicates a similar lifecycle cost between the technologies. For pre-dewatering ahead of THP, the same technologies could be considered. Arlington County has both experience and an existing DWB that is set up for centrifuges. Possible reuse of this building suggests that pre-dewatering be performed by centrifuges. As such, only centrifuges are being considered for pre-dewatering.

For all technologies and alternatives, the dewatering process is assumed to have a 95-percent solids capture efficiency. It is also assumed that 18 pounds of dry polymer are used for every dry ton of solids entering the dewatering process. A comparison of pre-dewatering capital costs and life cycle costs are provided in Tables 9-20 and 9-21, respectively. Similar comparisons for post-dewatering capital and life cycle costs are presented in Tables 9-22 and 9-23.

Table 9-20. Comparison of Pre-Dewatering Capital Costs

THP WAS-Only THP

Centrifuge Centrifuge

Dewatering Equipment 3 units 2 units

Reuse Existing Building? Yes Yes

Capital Cost (presented in year 2016, $) $10.1 M $9.5M

Table 9-21. Comparison of Pre-Dewatering Life Cycle Costs

THP WAS-Only THPCost Presented as 2016 $, million

Centrifuge Centrifuge

Capital Cost $ 10.1 $ 9.5

1st Year of O&M $ 0.36 $ 0.28

NPV of O&M $ 6.6 $ 5.2

Total NPW $ 16.7 $ 14.7


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Table 9-22. Comparison of Final Dewatering Capital Costs

Raw Solids Digested Biosolids THP/ Digested Biosolids

Centrifuge Centrifuge Centrifuge Belt Filter Press Screw Press

Dewatering Equipment 4 units 3 units 3 units 5 units 8 units

Reuse Existing Building? Yes Yes No No No

Capital Cost $12.9M $9.5 M $25.6M $24.7M $37.4M

Table 9-23. Comparison of Final Dewatering Life Cycle Costs

Raw Solids

Digested Biosolids THP/Digested Biosolids

Cost Presented as 2016 $, million

Centrifuge Centrifuge Centrifuge Belt Filter Press

Screw Press

Capital Cost $ 12.9 $ 9.5 $ 25.6 $ 24.7 $ 37.4

1st Year of O&M $ 0.54 $ 0.36 $ 0.37 $ 0.37 $ 0.35

NPV of O&M $ 9.8 $ 6.6 $ 7.9 $ 6.8 $ 6.9

Total NPW $ 22.7 $ 16.1 $ 33.5 $ 31.5 $ 44.3

9.2.3 StabilizationStabilization processes are designed to reduce pathogens and odors in the thickened solids. The design parameters for the digestion process are presented in Table 9-24. The values shown in the table assume 95% solids capture efficiency in the solids thickening or pre-dewatering process. This section compares the capital and operating costs of the different stabilization options considered.

Table 9-24. Digestion Process Requirements

Combined Thickened Primary + Thickened WAS

Annual Average Production @ 40 mgd 83,000 lb/day

14 Day Production Peaking Factor 1.4

Design Condition (mass loading) 116,000 lb/day

Digester Feed Volatile Solids Content 75%


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9.2.3.1 Digestion Pretreatment – Thermal HydrolysisOne of the stabilization options under consideration is digestion pretreatment with thermal hydrolysis (THP). This process uses steam injection under high temperature and pressure to pretreat solids prior to digestion. A pre-dewatering step is required before the THP process, which can be fed with either combined solids or WAS only. Based on the digestion process requirements listed in Table 9-24 above, the vendor equipment for the THP process would consist of one pulper (where the dewatered solids are mixed and heated), four reactors, one flash tank, and various feed pumps and air compressors. This equipment is provided by the manufacturer as one package. A heat exchanger for sludge cooling is also part of the THP system and can be supplied as a part of the vendor package or as a separate system.

9.2.3.2 DigestionFollowing the screening results for digestion stabilization technologies presented in Section 5.3.4, four main digestion options are considered: ATAD, MAD without pretreatment, MAD following THP, and temperature phased anaerobic digestion (TPAD). The equipment needs of each option, as well as a comparison of the digestion alternatives, are provided in the following sections. A summary of digester tank sizing and criteria for each digestion option is shown in Table 9-25. The following sections provide additional details on the digester tanks and associated equipment.

Table 9-25. Digester Tank Sizing Criteria and Dimensions

ATAD Meso AD TPAD THP/ AD WAS THP/ AD

Solids Loading (lb/day) 116,000 116,000 116,000 116,000 116,000

Feed Solids Concentration (%) 6% 6.5% 6.5% 10% 8%

Hydraulic Loading (gal/day) 232,000 214,000 214,000 139,000 174,000

Solids Retention Time (days)12 (ATAD)5 (SNDR)

183 (Thermo)12 (Meso)

15 17

Digester Volume Required (MG)2.8 (ATAD)1.2 (SNDR)

3.90.6 (Thermo)

2.6 (Meso)2.1 3.0

Number of Digester Tanks3 (ATAD)1 (SNDR)

32 (Thermo)

2 (Meso)2 2

9.2.3.2.1 Autothermal Thermophilic Aerobic Digestion (ATAD)The ATAD process tanks would consist of three thermophilic reactors and one mesophilic reactor. Each thermophilic reactor would be 100 feet long and 55 feet wide, with a side water depth of 24 feet (30-foot sidewall height). These tanks would be covered and would have two jet aerators per tank. Each jet aerator would have a 25-horsepower blower and a 50-horsepower pump. The mesophilic reactor, also known as the storage nitrification denitrification reactor (SNDR), would be 170 feet long and 40 feet wide, with a side water depth of 24 feet (30-foot sidewall height). This tank would also be covered and would have two jet aerators with each aerator having a 50-


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horsepower blower and a 60-horsepower pump. For comparison purposes, cast in place concrete tanks with allowances for a deep pile foundation are used.

The ATAD process will produce a Class A biosolids that can be dewatered and hauled off-site for bulk land application or other end uses.

9.2.3.2.2 Mesophilic Anaerobic DigestionMAD would require three 1.3 MG tanks. For comparison purposes, cast in place concrete tanks with allowances for a deep pile foundation are assumed. Each tank would be 65 feet in diameter with a 60-foot sidewall height. The tanks would have conical floors and fixed steel covers. Each tank would have five 15 horsepower draft tube mixers, each 36 inches in diameter. A separate digester building would house hot water boilers, digester heat exchangers, recirculation pumps, and temperature control loops.

MAD alone will produce a Class B biosolids that can be dewatered and hauled off-site for bulk land application. Additional treatment will be required to produce a Class A biosolid product.

9.2.3.2.3 Temperature Phased Anaerobic Digestion (TPAD)Temperature phased anaerobic digestion (TPAD) would consist of two thermophilic tanks and two mesophilic tanks. For comparison purposes, cast in place concrete tanks with allowances for a deep pile foundation are assumed. The thermophilic tanks would be 0.3 MG each, with a 40-foot diameter and 40-foot sidewall. The mesophilic tanks would be approximately 1.3 MG each, with a 65-foot diameter and 60-foot sidewall. All tanks would have a conical floor, fixed steel cover, and draft tube mixing. A digester building would also be required, which would contain hot water boilers, heat exchangers, recirculation pumps, transfer pumps, and temperature control loops.

TPAD process is capable of producing a Class A biosolids product. Facility design and operating procedures must be developed with the objective of meeting Class A pathogen reduction requirements.

9.2.3.2.4 Thermal Hydrolysis Pretreatment and Mesophilic Anaerobic DigestionUtilizing THP in combination with MAD would be accomplished using the thermal hydrolysis vendor package to deliver diluted and partially cooled hydrolyzed solids to the digester tanks.

The anaerobic digestion process that follows would consist of two mesophilic tanks of approximately 1.1 MG each (60-foot diameter and 60-foot sidewall). It is assumed that tank walls would be cast-in-place concrete with deep pile foundations. The tank would have a conical floor and a fixed steel cover. Alternate materials can be considered as the project advances. Each tank would also have five draft tube mixers, each 15 horsepower and 36 inches in diameter. A digester building would house heat exchangers, transfer pumps and steam boilers for the thermal hydrolysis process.

THP with anaerobic digestion will produce a Class A biosolids that can be dewatered and hauled off-site for bulk land application or other end uses.


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9.2.3.2.5 WAS-only Thermal Hydrolysis Pretreatment and Mesophilic Anaerobic DigestionThermal hydrolysis pretreatment of WAS would be accomplished using a smaller thermal hydrolysis vendor package. Thickened primary solids would blend with the hydrolyzed WAS to produce a feedstock to the digester tanks.

MAD would require two 1.5 MG tanks, each 65 feet in diameter with a 65-foot sidewall height. The tanks would have conical floors and fixed steel covers. Each tank would also have five draft tube mixers, each 15 horsepower and 36 inches in diameter. A digester building would house heat exchangers, transfer pumps and steam boilers for the thermal hydrolysis process.

WAS-only THP with anaerobic digestion will produce a Class B biosolids that can be dewatered and hauled off-site for bulk land application or other end uses. Process configurations, including pasteurization of primary solids, have been explored to produce a Class A biosolids.

9.2.3.3 Non-DigestionCurrently, the Arlington County WPCP stabilizes dewatered cake with lime for pathogen and vector attraction reduction. The Class B cake product is then hauled off-site for land application.

Other non-digestion stabilization options including composting, incineration, and Anuvia fertilizer production would be accomplished off-site and by a third-party vendor.

9.2.3.4 Comparison of Digestion Stabilization TechnologiesTable 9-26 presents a comparison of digestion stabilization capital costs.

Table 9-26. Comparison of Digestion Capital Costs

ATAD Meso AD TPAD THP/ AD WAS THP/ AD

Pre-Dewatering -- -- -- $10.1 M $9.5 M

Digestion (including THP as appropriate) $40.4 M $35.8 M $48.6 M $42.4 M $44.3 M

Allowance for Deep Pile Tank Foundations $3.5 M $2.7 M $2.4 M $1.1 M $1.4 M

Total Capital Cost $43.9 M $38.5 M $51.0 M $53.6 M $55.2 M

Table 9-27 compares the life cycle cost for the different digestion options. Overall, the THP and WAS-only THP options have considerably higher life cycle costs compared to the other digestion alternatives. It should be noted these costs do not reflect the costs of final dewatering or disposal. MAD has the lowest life cycle cost of the digestion options.


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Table 9-27. Comparison of Digestion Life Cycle Costs

Cost Presented as 2016 $, million ATAD Meso AD TPAD THP/AD WAS THP/AD

Capital Cost $ 43.9 $ 38.5 $ 51.0 $ 53.6 $ 55.2

1st Year of Operations $ 0.21 $ 0.16 $ 0.12 $ 0.56 $ 0.45

NPV of O&M $ 3.6 $ 2.8 $ 2.2 $ 10.3 $ 8.5

Total NPW $ 47.5 $ 41.3 $ 53.2 $ 63.9 $ 63.7

9.2.4 Thermal DryingThe thermal drying design requirements are outlined in Table 9-28. It is assumed that the dryers operate on a 24 hour per day, 7 day per week schedule at the 30 day peak design condition. The drying equipment assumed for each dryer type is listed in Table 9-29. Table 9-30 presents a summary of the operating information for each dryer type that is used in developing the life cycle costs.

Table 9-28. Thermal Drying Requirements

Raw ATAD or Meso-Digestion

THP (or WAS-only THP) + Digestion

Dewatered Solids (annual average) 78,500 lb/day 49,100 lb/day 43,200 lb/day

30 Day Production Peaking Factor 1.3 1.3 1.3

Dewatered Solids (30 Day Peak, Design Condition) 107,000 lb/day 63,800 lb/day 56,100 lb/day

Dewatered Solids Concentration 28% 25% 30%

Design Operating Schedule 24 hr/day × 7 day per week

24 hr/day × 7 day per week

24 hr/day × 7 day per week

Water Evaporation Rate 11,000 lb/hr 7,700 lb/hr 5,200 lb/hr

Table 9-29. Thermal Drying Equipment

Process Raw Digested (THP or no THP)

Drum Dryer Andritz DDS-60 Andritz DDS-40

Belt Dryer 2 × Andritz BDS-30 Andritz BDS-40

Belt Dryer 2 × Veolia/ Kruger Bio-Con Veolia/ Kruger Bio-Con

Paddle Dryer 2 × Andritz GDP-14W190 Andritz GDP-17W240


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9.2.4.1 Digested ProductIf drying a digested product, the maintenance labor, estimated fuel consumption, and equipment operating times are the same for drum, paddle, and belt dryers. The difference between these three drying options is in the power consumption. At 283 kW, the paddle dryer has the least power consumption, followed by the drum dryer (343 kW), and belt dryer (402 kW).

9.2.4.2 Raw ProductDrying a raw product would require more maintenance labor for the paddle and belt dryer, based on the O&M recommendations provided by equipment manufacturers. Again, the paddle dryer would have the lowest power consumption (417 kW), followed by the drum dryer (447 kW) and belt dryer (685 kW).

Table 9-30. Summary of Operating Information for Drying Technologies

Digested Product Drum Paddle Belt

Operating Labor (hr/week)1 40 40 40

Maintenance Labor (hr/week) 4 4 4

Estimated Fuel Consumption (BTU/lb water evaporated) 1,700 1,700 1,700

Equipment Power, kW (HP) 343 kW (460 HP) 283 kW (380 HP) 402 kW (540 HP)

Raw Product Drum Paddle Belt

Operating Labor (hr/week) 40 40 40

Maintenance Labor (hr/week) 4 8 8

Estimated Fuel Consumption (BTU/lb water evaporated) 1,700 1,700 1,700

Equipment Power, kW (HP) 447 kW (600 HP) 417 kW (560 HP) 685 kW (920 HP)

1. Operating labor is specific to dryer operation only. Dryers can run unattended for periods of time.

9.2.4.3 Cost Comparison of Drying AlternativesA capital cost comparison of different dryer options is presented in Table 9-31. The total capital costs presented include the cost of a new dryer building, which varies in size depending on the drying technology. The life cycle costs of each option are presented in Table 9-32.


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Table 9-31. Comparison of Thermal Dryer Capital Costs

Dewatered Raw Solids Dewatered Digested Solids

Drum Belt Paddle Drum Belt Paddle

Dryer Equipment Cost $8.8 M 2 @ $5.5 M

each2 @ $3.5 M

each $7.5 M $6.5 M $4.3 M

RTO Equipment Cost1

2 @ $0.7 M each -- -- $0.7 M -- --

Capital Cost $ 45.7 M $ 73.4 M $ 59.2 M $ 35.8 M $ 36.5 M $ 28.5 M

1. RTO expected to be required for drum dryer to manage odors in emissions.

Table 9-32. Comparison of Thermal Drying Life Cycle Costs

Dewatered Raw Solids Dewatered Digested SolidsCost Presented as 2016 $, million Drum Belt Paddle Drum Belt Paddle

Capital Cost $ 45.7 $ 73.4 $ 59.2 $ 35.8 $ 36.5 $ 28.5

1st Year of Operations $ 1.2 $ 1.3 $1.2 $0.8 $ 0.8 $ 0.8

NPV of O&M $ 22.2 $ 23.6 $22.2 $ 14.3 $ 14.6 $ 14.0

Total NPW $ 67.9 $ 97.0 $ 81.4 $ 50.1 $ 51.1 $ 42.5

9.3 Summary of Evaluations9.3.1 Preferred Process AlternativesThis section outlines the preferred technologies that were selected for each process step as a result of the process alternatives evaluation. The evaluation considered the capital costs, O&M costs, 20 year NPW of each technology. The evaluation also considers the types and quality of the products (biosolids, biogas, etc.) produced with each process.

The project team completed an exercise to consolidate the selected individual technologies or processes into process trains that could be considered for a more detailed evaluation. Table 9-33 presents the alternative process trains developed using a marker to indicate which processes are included with the process trains. The following sections summarize the results of the individual process evaluations and the methodology used to select the top process train alternative strategies for a more detailed evaluation.


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Table 9-33. Processes Included in Each Alternative

No. Alternative

Prim

ary

Solid

s Th

icke

ning

WAS

Th

icke

ning

Blen

ding

/Ho

ldin

gSo

lids

Scre

enin

g

Pre-

Dew

ater

ing

Ther

mal

Hy

drol

ysis

ATAD

Anae

robi

c Di

gest

ion

Fina

l De

wat

erin

g

Lim

e Ad

ditio

n

Ther

mal

Dr

ying

Off

-site

Pr

oces

sing

1 Lime Stabilization (Baseline) X X X X X

2 Anaerobic Digestion (Mesophilic, Class B) X X X X X

3

Thermal Hydrolysis Pretreatment (THP) + Anaerobic Digestion

X X X X X X

4 Anaerobic Digestion + Drying X X X X X X

5Anaerobic Digestion (Class A, TPAD or similar)

X X X X X

6Autothermal Thermophilic Aerobic Digestion

X X X X X

7 ATAD + Drying X X X X X X

8 THP + Anaerobic Digestion + Drying X X X X X X X

9THP (WAS only) + Anaerobic Digestion + Drying

X X X X X X

10 Thermal Drying X X X X X

11 Offsite (haul cake) X X X X X

12 Offsite (haul liquid) X X X X

9.3.1.1 ThickeningBased on the design solids loading at 40 mgd of annual average flow, additional capacity would be needed for WAS thickening at the planning condition. While the DAF rehabilitation option would allow the county to reuse the existing DAF building and expand upon it to accommodate the new unit, the other WAS thickening options would require constructing a new thickening building to provide capacity and to maintain operations of the DAF units during construction.


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Co-thickening provides the opportunity to consolidate thickening processes, making space on the site available for other purposes. Odor control will be a part of the final implementation of any co-thickening option. Overall, centrifuge thickening has the highest life cycle cost in the current evaluation for both WAS thickening and co-thickening. GBT and RDT options have lower NPVs, and are comparable to each other.

9.3.1.2 DewateringBased on the solids projections, the current DWB has sufficient capacity for centrifuge dewatering at the planning condition, for either raw or digested solids. For options that include THP, the County should consider using the DWB to house the pre-dewatering equipment needed. If one of the two THP options is selected, a new dewatering facility would be required for final dewatering. Similarly, belt press or screw press dewatering options would require constructing a new dewatering facility, due to the equipment footprint. Table 9-34 presents the preferred dewatering technologies for WAS, combined solids, and digested solids.

Table 9-34. Preferred Dewatering Technologies by Solids Type

WAS Combined Raw Solids Digested Solids

Centrifuge

Belt Filter Press Not Preferred Not Preferred

Screw Press Not Preferred Not Preferred

9.3.1.3 StabilizationOverall, the net present values for the THP and WAS-only THP plus digestion options are significantly higher compared to the mesophilic AD and TPAD options. It should be noted, however, that the increased solids reduction and improved product dewaterability achieved with THP would result in a significantly lower product disposal cost. WAS-only THP offers little advantage from a present worth standpoint over THP for all solids and presents operating challenges associated with separate handling of WAS and primary solids ahead of the digesters. Table 9-35 presents the preferred stabilization technologies to be considered further.

All digestion options require more footprint than what is currently identified on-site. The demolition of some existing facilities is required. Structures that are considered for demolition include the sludge holding tanks and the Bio-Building.

ATAD has both a higher life cycle cost and does not produce biogas compared to conventional mesophilic anaerobic digestion and TPAD, making either conventional AD or TPAD a more attractive option. Note that biogas use is not being considered in this analysis. Adding this consideration would likely make some of the anaerobic digestion options even more appealing on a life-cycle cost basis.


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Table 9-35. Preferred Stabilization Technologies

Preferred? Justification

ATAD N Higher operating cost than MAD due to energy input for blowers and mixing pumps. Largest tank volume. Process does not produce biogas.

MAD Y Lowest life cycle cost of all digestion technologies. Digestion + dewatering results in reduced hauling requirements compared to lime stabilization.

TPAD NSlightly more complex system to operate due to requirement to heat and cool biosolids, plus batch holding operation. Has some challenges with pathogen reduction and does not always produce Class A product.

THP + MAD YHigher digester VS removal. Can consider pre-dewatering blended primary solids and WAS without the intermediate step of co-thickening, which reduces complexity of operation.

WAS-Only THP + MAD N

Separating digester pretreatment of WAS and primary solids results in a more complex operation. Concerns with odor potential and dewatering impacts if primary solids are not hydrolyzed.

Lime Stabilization Y This is the WPCP’s current stabilization strategy (baseline condition).

9.3.1.4 Thermal DryingIn the case of thermal drying, the quality of the dried product and the surrounding market for biosolids often drives the technology selection. As shown in Table 9-32, the dryers rank fairly similarly in terms of capital, O&M, and life cycle costs, especially when drying digested solids. There are a number of drawbacks to direct drying raw sludge, including product and process odors, influence of scum and grease on the dryer process; and quality of the final dried product. Digestion before drying results in both reduced dryer throughput, reduced fuel requirements, potential to off-set a portion of the fuel requirement with biogas, and an improved dried product quality with respect to consistency and product odor. Moving forward, only thermal drying of digested solids will be considered. Alternative evaluations will be used to determine if thermal drying as a process is advantageous, and will not necessarily focus on the type of dryer.

9.3.2 Preferred Process AlternativesOf the twelve alternatives listed in Table 9-33, the top four were selected following Workshop 4 (March 25, 2016) and Workshop 5 (June 22, 2016).

The alternatives selected for detailed evaluation include:

Alternative 1: Lime Stabilization. Lime stabilization represents the current process employed at the WPCP and is used as a baseline for comparison of other alternatives. The current process produces a Class B biosolids. The age and condition of the existing equipment will require significant rehabilitation or replacement over the planning period.

Alternative 2: MAD. Anaerobic digestion is identified as a preferred process for stabilization of WPCP solids at the screening level and preliminary process evaluations.


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The process will produce a Class B biosolids and biogas that can be captured and utilized as a fuel. The process reduces the quantity of biosolids produced at the WPCP compared to the baseline.

Alternative 3: THP + Anaerobic Digestion. THP combined with anaerobic digestion will produce a Class A biosolids and a biogas that can be captured and utilized as a fuel. THP results in a reduced footprint for the digester tanks, although space for the THP reactors must be provided. The process will significantly reduce the quantity of biosolids produced at the WPCP compared to the baseline.

Alternative 4: Anaerobic Digestion + Drying. Similar to Alternative 2, the mesophilic digestion process of this alternative will produce a Class B biosolids that can be dewatered. The dewatered material can then be thermally dried to produce a Class A product with significant volume reduction compared to other alternatives. The biogas produced with anaerobic digestion can be captured and used a fuel. Supplemental fuel (natural gas) will be required to dry all of the biosolids.

9.3.3 Justification for Elimination of AlternativesThe eliminated alternatives and justifications for elimination are the following:

Alternative 5: Temperature Phased Anaerobic Digestion (TPAD). TPAD does not offer a guaranteed Class A biosolids product. However, the potential for a Class A material does exist and the process offers a potential cost savings compared to other Class A processes, such as THP + Digestion + Drying.

Alternative 6: ATAD. ATAD does not provide product benefits over anaerobic digestion and has higher operating costs and energy input requirements.

Alternative 7: ATAD + Drying. ATAD does not provide product benefits over anaerobic digestion and has higher operating costs and energy input requirements. The addition of the thermal dryer increases the energy input of the alternative.

Alternative 8: THP + Anaerobic Digestion + Drying. The cost for this option (both capital and life cycle) was not comparable to other Class A processes. This option offers the highest level of redundancy for Class A; however, there is little justification for the many levels of redundancy. Thermal drying can be considered as a potential phasing option in the future if the market drives Arlington County to consider a heat dried product.

Alternative 9: THP (WAS Only) + Anaerobic Digestion + Drying. WAS-only THP has several operational challenges (blending feed to digesters, etc.) and does not produce a Class A product.

Alternative 10: Thermal Drying of Raw Solids. Concerns with dryer performance, dryer safety, product quality, and odor were noted. Moving ahead, only considering drying of a digested product.

Alternative 11: Off-Site Processing (Haul Cake). While Arlington County maintains the option to evaluate proposals from third-party vendors, no viable proposals have been received during the preparation of this master plan.


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Alternative 12: Off-Site Processing (Haul Liquid). The number of trucks required to haul liquid is not likely to be acceptable to the community. The identified haul location can only accept about half of the WPCP’s solids production without modifying operations. The county is concerned with the risk of hauling large quantities of unstabilized material.

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Section 10Overview of Selected Biosolids Management Alternatives

10.1 IntroductionThis section presents an overview of each of the four short-listed biosolids management alternatives. The overview includes process-by-process descriptions and sizing, end product uses, a conceptual layout of facilities, capital and life cycle costs, and energy balances for each alternative. This overview provides the basis for completing a detailed evaluation in which the alternatives are scored on established criteria for the project, including economic, operational, environmental, and social criteria.

10.2 Alternative 1: Lime Stabilization with Improvements10.2.1 Description and Process FlowAlternative 1 assumes that WPCP continues with lime stabilization of dewatered cake and makes several capital improvements to the biosolids treatment process. A process schematic of this alternative is presented in Figure 10-1.

Figure 10-1. Lime Stabilization Process Flow

Section 10 Overview of Selected Biosolids Management Alternatives

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10.2.2 Process-by-Process Description and Sizing10.2.2.1 Gravity ThickenersThe gravity thickeners receive solids from the primary clarifiers and thicken them to approximately 4.0 percent solids. It is assumed that 95 percent of primary solids are captured in the thickening process, while this is higher than ‘typical’ gravity thickener performance, it is consistent with the historical data. Thickened solids are sent to the solids holding tank. Supernatant from the gravity thickening process is sent to the plant headworks.

Typically, solids loading rates for gravity thickeners range between 20 and 30 lb/day/ft2. The two existing gravity thickening tanks are each 65 feet in diameter. Given that the maximum 7-day solids loading for these tanks at the design condition is 22 lb/day/ft2, the existing facilities are adequate for primary solids thickening. However, it should be noted that the current mode of operation is to have one thickener in service and one out of service, whereas both units are needed in service for the design condition. A summary of gravity thickener sizing criteria is presented in Table 10-1.

Table 10-1. Gravity Thickener Sizing

Design Factor Value

Average Solids Produced (lb/day) 54,100

Max 7-Day Average Solids Produced 119,000

Expected Solids Concentration 4.0%

Equipment Size 65 ft dia.

Surface Area Available (ft2) 5,409

Surface Area Per Unit (ft) 3,318

Operating Schedule (hr/wk) 168

Required Capacity

Solids (lb/hr) 4,960

Hydraulic (gpm) 660

Solids Loading 20-30 lb/day/ft2

Hydraulic Loading 400-800 gpd/ft2

Duty Units Required 2

Standby Units 0

WPCP currently uses primary clarifier effluent to achieve necessary hydraulic overflow rates on the gravity thickeners and maintain thickened solids quality. It is expected that WPCP will continue this process.

While the gravity thickeners are adequately sized for the design condition, replacement of the internal mechanisms were recommended following a recently completed condition assessment. As such, new mechanisms and new covers are included in capital costs.


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10.2.2.2 Gravity Belt ThickenersThe gravity belt thickeners receive WAS from the secondary clarifiers. These units will replace the DAF unit and will be housed in the existing DAF building. Similar to the gravity thickeners, it is assumed that the gravity belt units thicken solids to approximately 4.0 percent and capture approximately 95 percent of solids entering the unit. For cost purposes, polymer is assumed to be added at a dose of 10 lb/DT to promote flocculation. Thickened solids are sent to the solids holding tank, while filtrate is sent to the plant headworks.

Sizing of the gravity belt thickeners for WAS thickening assumes use of the 3-meter units at a solids loading rate of 750 lb/hr/m. Two 3-meter units are required, one duty and one standby, assuming a 24 hr/day, 7 day/week operating schedule.

Table 10-2 presents the sizing criteria for gravity belt thickeners used for WAS thickening.

Table 10-2. Gravity Belt Thickener Sizing

Design Factor Value


Max 7-Day Average Solids Produced 63,000

Expected Solids Concentration 4.0%

Equipment Size 3-m GBT


Required Capacity


Hydraulic (gpm) 477

Solids Loading 750 lb/hr/m

Hydraulic Loading 200 gpm/m

Duty Units Required 1

Standby Units 1

10.2.2.3 Solids Holding TanksThickened solids are sent from both primary and secondary thickening to holding tanks where the blended solids are stored before dewatering. The two tanks are each approximately 60 feet in diameter with a sludge depth of 25 feet. Each tank provides 2 days of hydraulic retention time at the design condition of 40 mgd. In this alternative, it is assumed that the existing solids holding tanks will continue to be used.

10.2.2.4 Solids ScreeningArlington County’s existing headworks screens are bar screens with 1/2-inch openings. While solids screening options were not evaluated, planning level costs for solids screens are provided for all options to improve the quality of the biosolids produced. Blended thickened solids are sent through a screening process before dewatering to separate coarse material. The screens have a


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recommended opening size of 5 mm. Assuming intermittent operation at a design feed rate of 630 gpm, three screening units are recommended: two duty and one standby.

The proposed screens have a maximum hydraulic loading rate of 400 gpm per unit. The internally fed drum-style screens would be located inside a closed structure, with screenings falling into dumpsters inside the structure. The dumpsters would also be equipped with foul air capture and treatment. Figure 10-2 shows a set of typical installed solids screens.

Figure 10-2. Solids Screens

10.2.2.5 DewateringAfter the thickened solids are blended and stored, they will be sent to the centrifuges for dewatering. Polymer is added to the feed immediately before entering the centrifuge. It is assumed that the dewatering process captures 95 percent of influent solids, and produces cake at a solids content of approximately 28 percent from an influent feed of 4.0 percent solids. For the 30-day maximum design condition, four units (3 duty/ 1 standby) are needed assuming an operating schedule of 70 hours per week and a per-unit loading rate of 3,800 lb/hour. Table 10-3 presents the centrifuge sizing criteria.

The capital cost of dewatering presented in Section 10.2.5 includes replacing the existing centrifuges due to age. The new centrifuges would be housed in the existing DWB.

Section 10 • Overview of Selected Biosolids Management Alternatives

10-5

Table 10-3. Dewatering Centrifuge Sizing

Design Factor Value

Required Capacity (lb/day, 30-day maximum design) 107,000

Unit Throughput (lb/hr) 3,800



Feed Solids Concentration 4.0%

Number of Units Required 3 duty/ 1 standby

10.2.2.6 Lime Feed and Truck Loading

Dewatered solids are stored and conveyed to the lime stabilization process, where lime is added

and mixed with the cake to achieve compliance with pathogen and vector attraction reduction

requirements. Lime demand has historically been near 0.25 lb lime/lb of dry solids. This

alternative includes lime mixing and product conveyor improvements.

10.2.2.7 Odor Control

Chemical scrubbers are used to treat foul air removed from multiple process areas at the WPCP.

The scrubbers are rated to treat specific air volumes. The volumes of treated space for existing

and new processes were compared to determine future treatment requirements against the

existing system capacity. When considering structure demolition or facility abandonment, the

volume of the proposed facilities is nearly identical to the volume of the existing facilities. Capital

costs for odor control presented in Section 10.2.5 include an allowance for the replacement of the

existing chemical scrubbers, fans, and ductwork.

10.2.2.8 Sidestreams

Sidestreams in this alternative include gravity thickener supernatant, gravity belt thickener

filtrate, and dewatering centrifuge centrate. These sidestreams are returned to the plant

headworks for further treatment. Pollutant and nutrient loadings of the sidestreams is expected

to be similar to the current sidestreams and no additional sidestream treatment is included.

10.2.3 Product Uses

Similar to the current operation, this alternative produces lime-stabilized Class B biosolids that

can be land applied. Currently, approximately 40,000 wet tons of biosolids are produced per year.

Using this product involves bulk land application through a third party contractor. While

producing a Class B biosolid remains a viable option, program costs are expected to escalate due

to modifications to nutrient management plan requirements and regulatory restrictions.

10.2.4 Conceptual Layout of Facilities

A conceptual facilities layout for this alternative (Figure 10-3) is similar to the current layout,

except for demolition of the biological solids processing building and adding holding tanks for

WAS and blended solids.

BSP

GBT

GTGT

DW

Figure 10-3

Alternative 1:Lime Stabilizationwith Improvements

Blended Sludge HoldingBiological Solids ProcessingDewatering BuildingGravity Belt ThickenerGravity Thickener

Facility Key

BSHBSPDWGBTGT

Arlington CountyVirginia

Legend

Existing

Existing (With Modifications)

New

Demolition

BSH

BSH


10-7

10.2.5 Capital and Life Cycle Costs10.2.5.1 Capital CostsA summary of the capital costs for Alternative 1 is provided in Table 10-4. The basis for these capital costs is provided in Appendix G.

Table 10-4. Capital Costs for Alternative 1: Lime Stabilization with Improvements




GBT Thickening $ 6,000,000

Gravity Thickening $ 1,500,000


Final Dewatering $ 12,900,000

Lime System Improvements $ 8,000,000

Bio-Building Demolition $ 3,500,000


Baseline Capital Cost $ 40,100,000

10.2.5.2 Annual CostsAnnual costs for Alternative 1 are summarized in the subsections below.

Operating Staff RequirementsO&M needs for this alternative will require nearly four full-time equivalents (FTEs), with efforts divided among thickening, dewatering, lime addition, and truck loading. FTEs are intended to provide an indication of the level of effort required to operate and maintain the equipment. FTEs do not reflect the number of staff or new hires associated with the process as it does not account for balancing workloads, shared resources with other processes, supervision, and adjustments for time off such as holidays and vacation days. The estimated hours for each of these processes and the total annual O&M costs for the years 2021 and 2040 are presented in Table 10-5. For presentation purposes, the costs in the table are presented for the year in which they are incurred with no adjustment to current dollars. Adjustments to a present worth (in Year 2016 dollars) for alternative comparisons is presented later in this section.

Table 10-5. Operating and Maintenance Labor Requirements for Alternative 1

Year Thickening (hr/yr)

Dewatering (hr/yr)

Lime and Hauling (hr/yr)

Total Labor (hr/yr) FTEs Total Labor

Cost

2021 1,040 2,398 4,145 7,583 3.6 $ 440,000

2040 1,040 2,573 4,328 7,942 3.8 $ 1,200,000


10-8

Hauling RequirementsHauling lime-treated solids is expected to range from 40 trucks/ week in 2021 to 43 trucks/ week in 2040. This assumes each truck is filled to a maximum capacity of 20 tons. Assuming a land application unit cost of $57 per ton, the annual hauling and disposal cost for 2021 is approximately $2,400,000 (Year 2021 dollars).

Power RequirementsAdditional power costs are calculated by estimating the energy requirements for thickening and dewatering. In 2021, thickening is expected to require approximately 640 kWh per week, whereas dewatering is expected to require approximately 18,800 kWh per week, assuming the centrifuge and feed pumps require 220 HP per train for nearly 40 hours per week. Overall, the annual power cost for 2021 is estimated at $70,000 (Year 2021 dollars).

Chemical RequirementsAdding thickening polymer, dewatering polymer, and lime contribute to the plant’s chemical requirements under this alternative. It is assumed that polymer is added to the gravity belt thickeners at a rate of 10 lb/DT and added to the dewatering centrifuges at a rate of 25 lb/DT. Lime is added at a rate of 0.25 lb/lb of dry solids. Overall, these chemicals are expected to cost the plant $1,000,000 in 2021 (Year 2021 dollars).

Heating/Natural Gas RequirementsThere are no additional heating or natural gas requirements with Alternative 1. Additionally, no biogas is produced with this alternative.

20-Year Net Present WorthA summary of hauling, power, chemical, and heating requirements is presented in Table 10-6. Costs are presented in the year in which they are incurred.

Table 10-6. Summary of Alternative 1 Annual Costs for Power, Chemical, Labor, Fuel, and Hauling

Power Polymer Lime Labor Natural Gas HaulingYear

kWh/yr lb/day lb/day hr/yr therm/d lb/dayTotal

2021 1,000,000 900 14,700 7,583 0 227,000 --

2040 1,100,000 980 16,000 7,942 0 247,000 --

$/yr $/yr $/yr $/yr $/yr S/yr

2021 $70,000 $1,000,000 $440,000 0 $2,400,000 $3,910,000

2040 $130,000 $1,600,000 $1,200,000 0 $4,500,000 $7,430,000

The expected NPW for Alternative 1 is shown in Table 10-7. This option has the lowest capital cost of the four alternatives, but also has the highest expected annual costs due to the greater amount of hauling needed. Overall, the 20-year NPW is approximately $89 million. Details of the present worth analysis are provided in Appendix H.


10-9

Table 10-7. Summary of Costs for Alternative 1: Lime Stabilization with Improvements

Capital Costs

Total Capital Costs (see Table 10-1, Year 2016 dollars) $ 40,100,000

Annual Costs

Total Annual Cost (Year 2021) $ 3,900,000

NPV Annual Cost (Year 2016 dollars) $ 54,000,000

Total NPW ($ 2016) $ 94,100,000

10.2.6 Energy BalanceFigure 10-4 presents an energy balance for Alternative 1 using Year 2021 solids projections and mass balance. Overall, the alternative does not product or consume significant energy at the WPCP. The most significant ‘net’ energy in the solids balance is the fuel input for hauling cake.

10.3 Alternative 2: Mesophilic Anaerobic Digestion10.3.1 Description and Process FlowAlternative 2 assumes that WPCP implements a MAD process before dewatering to produce a Class B biosolids. A process schematic of this alternative is presented in Figure 10-5. In this alternative, primary solids and WAS are blended in a solids holding tank and co-thickened by a rotary drum thickener (RDT) rather than having separate thickening processes. Separate thickening can be implemented; however, given the limited space on site, co-thickening consolidates the process preserving space.

Biogas produced can be used for: a) on-site process purposes, such as digester heating; b) cleaned and distributed off-site as a renewable fuel (biomethane or CNG); or c) as a fuel for a combined heat and power process.

Figure 10-5. MAD Process Flow

Gravity Thickener

SidestreamTreatment & Pumping

Recycle toLiquid Stream

Dewatering

LimeStabilization

Hauling

Gravity BeltThickener

Lime Production

Cake

149

375245

20

13

8

09

146

1

0.3

2

PrimarySludge

WAS

258

157

375

LEGENDCOD (MMBtu/d)Fuel (MMBtu/d)

Heat (MMBtu/d)Electrical (MMBtu/d)

Figure 10-4Energy Balance for Lime Stabilization

* Based on 2021 flow condition (30 mgd)

ENERGY FORM NET ENERGY YIELD (MMBtu/d)PowerBiogasNatural GasHaulingTotal

-1200

-146-158


10-11

10.3.2 Process-by-Process Description and Sizing10.3.2.1 Solids Holding TanksThree 33-foot square (with a side water depth of 25 feet) solids holding tanks are used in Alternative 2. Primary solids and WAS are sent to separate holding tanks, then combined in a blended holding tank before co-thickening.

10.3.2.2 Co-ThickeningUnder this alternative, primary solids and WAS are thickened by a single co-thickening process. The process would be housed in a separate co-thickening building next to the solids holding tanks. For planning purposes, it is assumed that RDT are used. Based on the hydraulic loading rate of 800 gpm and solids loading of 5,900 lb/hr, three duty units and one standby unit, each rated for 350 gpm, are required. A summary of the sizing criteria is presented in Table 10-8.The thickening equipment, including feed pumps, polymer systems, and thickened solids pumps will be located in a new thickening building having approximate dimensions of 60 feet × 130 feet.

Table 10-8. RDT Sizing (Co-Thickening)

Design Factor Value


Max 7-Day Average Solids Produced (lb/day) 140,400



Required Capacity


Hydraulic (gpm) 800

Equipment Throughput


Hydraulic (gpm) 350


10.3.2.3 Solids ScreeningThe screening process for Alternative 2 would be similar to the process for Alternative 1. The main difference is that screening would be ahead of the thickening process in this alternative. Assuming continuous operation of the thickeners at a design feed rate of 800 gpm at 1.5 percent solids, three screening units are recommended: two duty and one standby.


10-12

10.3.2.4 Anaerobic DigestionThe mesophilic digesters receive co-thickened solids at approximately 6.5 percent solids concentration. This process produces biogas, which is planned to be beneficially used as a fuel. The mesophilic digestion volume is based on a desired SRT of 18 days at the maximum 14-day design loading. It is assumed that the digester feed has a volatile solids content of 75 percent and that the digesters remove 50 percent of volatile solids. Digester sizing criteria are presented in Table 10-9.

Table 10-9. Sizing for Class B Anaerobic Digestion

Design Factor Value

Maximum 30-Day Solids Loading (lb/day) 115,700

Desired SRT at Maximum 30-Day Solids Loading (days) 18

Feed Solids Concentration (%) 6.5%

Required Digester Volume (MG) 3.9

Number of Tanks 3

Diameter per tank (ft) 65

Minimum sidewall height of each tank (ft) 60

For planning, the digester tanks are assumed to be constructed of cast-in-place concrete having fixed steel covers. The tanks would be constructed with a slightly sloping bottom. Access to the tanks will be provided through manways on the side and at the top.

A new digester building will be required for equipment such as heat exchangers, hot water boilers and pumping system, solids recirculation pumps, and transfer pumps. The anticipated dimensions for the building are 50 feet X 120 feet.

The current Bio Building would be demolished to allow space for the digester and solids holding tanks.

10.3.2.5 Holding TanksDigested solids will be stored in two new 50-foot diameter concrete tanks before dewatering. This allows for 4 days of retention time at the maximum 30-day design flow. These tanks would be located adjacent to the digester building. Covers and gas collection would be provided for these tanks. For this planning level evaluation, membrane type covers suitable for biogas storage are included with these tanks.

10.3.2.6 DewateringThe centrifuge dewatering process will remain in the existing DWB located at the northern end of the plant site. The dewatered cake from digested solids is expected to have a slightly lower solids content compared to solids that have not been stabilized (25 percent as opposed to 28 percent). The dewatering process would require two duty units. Sizing criteria for the dewatering process are provided in Table 10-10.


10-13

Table 10-10. Sizing for Dewatering with Class B Anaerobic Digestion

Design Factor Value

Required Capacity at Maximum 30-Day Design (lb/day) 67,000

Solids Throughput Per Centrifuge (lb/hr) 3,800





10.3.2.7 Cake Storage and Truck LoadingDewatered cake is stored on-site before being loaded onto trucks for Class B land application. The truck loading process and associated annual costs are discussed further in Section 10.3.5.2.

10.3.2.8 Odor ControlThe odor control requirements for this alternative are expected to be similar to the current process. The volume of the proposed co-thickening building is similar to the volume of the existing DAF building. In addition, the volume of the proposed holding tanks are similar to that of the existing gravity thickeners. An allowance for replacement and rehabilitation of the existing DWB HVAC ductwork, fans, and chemical scrubber is included in the capital cost for this option.

10.3.2.9 SidestreamsSidestreams in this alternative include co-thickening filtrate and dewatering centrifuge centrate. Each of these sidestreams would typically be returned to the plant headworks for further treatment. Two sidestream treatment processes were considered: the DEMON process, which would treat dewatering centrate for nitrogen removal, and the OSTARA process, which would treat both co-thickening filtrate and dewatering centrate for P recovery.

The capital costs associated with adding each sidestream processes are presented in Section 10.3.5.

10.3.3 Product UsesAlternative 2 produces a Class B biosolid that can be land applied. Under current flow conditions, this alternative would produce approximately 65 wet tons of biosolids per day. Using this product involves bulk land application through a third party contractor. While producing a Class B biosolid remains a viable option, program costs are expected to escalate due to modifications to nutrient management plan requirements and potential loss of land available within a reasonable hauling distance.

The anaerobic digestion process also produces a biogas that can be capture and utilized on-site or off-site. The analysis considers three alternatives for biogas utilization including: a) on-site for process heating purposes; b) clean and distribute off-site as a renewable fuel (bio-methane to pipeline or CNG fueling station); c) on-site utilization in a CHP system. A more detailed analysis


10-14

of biogas utilization opportunities is recommended if the county elects to implement anaerobic digestion in the future.

10.3.4 Conceptual Layout of FacilitiesA conceptual layout of facilities for this alternative is presented in Figure 10-6. Similar to Alternative 1, the existing biological solids processing building would be demolished, and the existing DWB would be modified to accommodate the new centrifuges. The existing gravity thickeners would also be demolished. New structures include the anaerobic digesters, digester building, solids holding tanks, and co-thickening building.

10.3.5 Capital and Life Cycle Costs10.3.5.1 Capital CostsA summary of the capital costs for Alternative 2 is provided in Table 10-11.

Table 10-11. Capital Costs for Alternative 2: Mesophilic Anaerobic Digestion





Co-Thickening by RDT $ 9,700,000


Mesophilic Digesters (Class B) $ 38,500,000



Bio-Building Demolition $ 3,500,000



Combined Heat and Power (CHP) $ 8,000,000

Compressed Natural Gas (CNG) $ 10,200,000

DEMON $ 6,100,000

OSTARA $ 16,300,000

BSP

DW

BSH

WSH

PSH

CTADAD

DB

AD

WGB

DSH

DAF

Figure 10-6

Alternative 2:MesophilicAnaerobicDigestion


Anaerobic DigesterBlended Sludge HoldingBiological Solids ProcessingCo-Thickening BuildingDissolved Air FlotationDigester BuildingDigested Sludge HoldingDewatering BuildingWaste Gas BurnerPrimary Sludge HoldingWAS Holding

Facility Key

ADBSHBSPCTDAFDBDSHDWWGBPSHWSH

Legend

Existing


New

Demolition

DSH


10-16


Operating Staff RequirementsDespite adding labor associated with the digestion process, O&M needs for this alternative will require less FTEs than the lime stabilization alternative by eliminating labor associated with operating the lime feed and mixing system and reducing the labor associated with the truck loading operation. Operating and maintenance efforts would be divided among thickening, digestion, dewatering, and truck loading. The estimated hours for each process and the total annual O&M costs for the years 2021 and 2040 are presented in Table 10-12.


Year Thickening (hr/yr)

Digestion (hr/yr)

Dewatering (hr/yr)

Hauling (hr/yr)

Total (hr/yr) FTEs Total Labor

Cost

2021 1,040 2,190 2,274 1,341 6,846 3.3 $ 400,000

2040 1,040 2,190 2,439 1,460 7,129 3.4 $ 1,100,000

Hauling RequirementsHauling dewatered cake is expected to range from 26 trucks/week in 2021 to 28 trucks/week in 2040. This assumes each truck is filled to a maximum capacity of 20 tons. Assuming a land application unit cost of $57 per ton, the annual hauling and disposal cost for 2021 is approximately $1,540,000.

Power RequirementsPower costs are calculated by estimating the energy requirements for co-thickening, digestion, and dewatering. In 2021, co-thickening is expected to require approximately 4,000 kWh per week, digestion is expected to require 38,000 kWh per week, and dewatering is expected to require approximately 12,000 kWh per week. Overall, the annual power cost for 2021 is estimated at $195,800, which is greater than the lime stabilization alternative. A significant power cost of this alternative is the power required for digester mixing.

Biogas utilization in a combined heat and power system would offset the increased power demand, resulting in a net reduction in electrical utility demand compared to the current process.

Chemical RequirementsThe plant’s chemical requirements under this alternative include thickening polymer and dewatering polymer. It is assumed that polymer is added to the rotary drum thickeners at a rate of 10 lb/DT, added to the dewatering centrifuges at a rate of 25 lb/DT. Overall, chemicals are expected to cost the plant $490,000 in 2021.

Heating/Natural Gas RequirementsThe analysis considers three alternatives for biogas utilization including: a) on-site for process heating purposes; b) clean and distribute off-site as a renewable fuel (bio-methane to pipeline or CNG fueling station); and c) on-site utilization in a CHP system.


10-17

A summary of hauling, power, chemical, and heating requirements for each of three biogas utilization options is presented in Tables 10-13a, 10-13b, and 10-13c.

Table 10-13a assumes that biogas produced in the digestion process is used on-site for process purposes. The heat balance indicates there is sufficient biogas for process heating. For this table, the remaining biogas is combusted in a waste gas burner. Other tables will consider beneficial use of for all of the biogas produced.

Table 10-13a. Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel, and Hauling – Biogas Used for Process Purposes

Power Polymer Lime Labor Natural Gas Hauling


2021 2,820,000 810 0 6,850 0 147,000 --2040 2,900,000 880 0 7,900 0 160,000 --

$/yr $/yr $/yr $/yr $/yr S/yr2021 $ 200,000 $ 490,000 $ 400,00 $ 0 $ 1,500,000 $ 2,590,0002040 $ 350,000 $ 940,000 $ 1,100,000 $ 0 $ 2,900,000 $ 5,290,000

Table 10-13b assumes that biogas is used off-site, with all process heating needs met by natural gas. Under this scenario, additional capital investment in the biogas cleaning system (noted as CNG costs in Table 10-11) would be required. The table is focused on biosolids processes and does not present the costs to operate the biogas cleaning process or the potential revenues associated with sale of the cleaned gas.

Table 10-13b. Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel, and Hauling – Biogas Used Off-Site as Renewable Fuel



2021 2,820,000 810 0 6,850 580 147,000 --2040 2,900,000 880 0 7,900 620 160,000 --

$/yr $/yr $/yr $/yr $/yr S/yr2021 $ 200,000 $ 490,000 $ 400,00 $ 250,000 $ 1,500,000 $ 2,840,0002040 $ 350,000 $ 940,000 $ 1,100,000 $460,000 $ 2,900,000 $ 5,750,000

Table 10-13c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. Under this scenario, additional capital investment in CHP equipment (noted as HHP costs in Table 10-11) would be required. The table is focused on biosolids processes and does not present the costs to operate and maintain the CHP system or the potential revenues or offset electrical cost associated with the production of power.


10-18

Table 10-13c. Summary of Alternative 2 Annual Costs for Power, Chemical, Labor, Fuel, and Hauling – Biogas Used On-Site in CHP System


kWh/yr lb/day lb/day hr/yr therm/d lb/day

Total

2021 2,820,000 810 0 6,850 0 147,000 --

2040 2,900,000 880 0 7,900 0 160,000 --


2021 $ 200,000 $ 490,000 $ 400,00 $ 0 $ 1,500,000 $ 2,590,000

2040 $ 350,000 $ 940,000 $ 1,100,000 $ 0 $ 2,900,000 $ 5,290,000

20-Year Net Present WorthThe expected NPW for Alternative 2 is shown in Table 10-14. The annual cost used in the net worth presentation is based on biogas utilization option “a”, which considers biogas will be used on-site for process needs. Under this option, no natural gas is purchased to support process needs and a portion of the biogas is available for other utilization. Overall, the 20-year NPW is approximately $114 million.

Table 10-14. Summary of Costs for Alternative 2: Mesophilic Anaerobic Digestion

Capital Costs

Total Capital Costs (see Table 10-11) $ 73,600,000

Annual Costs


NPV Annual Cost (Year 2016 dollars) $ 37,100,000

Net Present Worth ($ 2016) $110,700,000

10.3.6 Energy BalanceFigures 10-7a, 10-7b, and 10-7c present energy balances for Alternative 2 at Year 2021 solids production and mass balance. Figure 10-7a assumes that biogas produced in the digestion process is used on-site for process purposes, with the remainder being available for export. This option represents a scenario where biogas production (148 MMBTU/d) represents a significant positive input to the energy balance. Also indicated on the figure is the energy demand of fuel (95 MMBTU/d) associated with product hauling. This is a significant is decrease compared to the current process configuration (Figure 10-4).

Figure 10-7b assumes that biogas is used off-site, with all process heating needs met by natural gas. The overall ‘net’ energy of this option is similar to option a (Figure 10-7a) with the main difference being more biogas is available for off-site utilization and more natural gas is purchased for process heating requirements.

RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Boiler

Dewatering

258

157

395

21

12

148

49

58

90

234

247

6

2

Biogas Utilization

19


Figure 10-7aEnergy Balance for Mesophilic Digestion • Biogas Used On-Site for Process Purposes



-27+90

0-95-32

Cake

Hauling

95Heat (MMBtu/d)Electrical (MMBtu/d)

RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Boiler

Dewatering

258

157

395

21

12

148

49 58

234

247

6

2

Biogas Utilization

19


Figure 10-7bEnergy Balance for Mesophilic Digestion • Biogas Use Off-Site, Process Heating Needs by Natural Gas



-27+148

-58-95-32

Cake

Hauling

95

Natural Gas


RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Boiler

CHPEngine

Waste Heat

Dewatering

258

157

395

21

12

148

49

58 1

59

56

234

247

6

2

19


Figure 10-7cEnergy Balance for Mesophilic Digestion • Biogas Used On-Site, Process Heating by CHP Heat Recovery with Supplemental Natural Gas



+2900

-95-66

Cake

Hauling

95Heat (MMBtu/d)Electrical (MMBtu/d)


10-22

Figure 10-7c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. The net energy for this option is lower than the net energy for options a and b. This option shows a net positive energy balance on the electrical power as the CHP system can produce more power than required of the solids handling equipment. The power would be available to off-set WPCP electrical demands or available to sell to the electrical utility. There is no biogas available for export under this option.

10.4 Alternative 3: Thermal Hydrolysis Pretreatment with Mesophilic Anaerobic Digestion10.4.1 Description and Process FlowAlternative 3 assumes that WPCP implements a THP step before MAD. A process schematic is presented in Figure 10-8. In this alternative, blended solids go through a centrifuge pre-dewatering process before THP. The final dewatered product will be a Class A biosolids product that can be used in bulk agricultural land application or distributed as a marketable fertilizer or soil amendment.

Similar to other alternatives with anaerobic digestion, three scenarios are considered for biogas utilization – a) process heating needs such as steam generation; b) off-site utilization as a renewable fuel; and c) fuel for a CHP system with recovered heat used for process heating.

Figure 10-8. THP with Mesophilic Anaerobic Digestion Process Flow

10.4.2 Process-by-Process Description and Sizing10.4.2.1 Solids Holding TanksSimilar to the mesophilic digestion alternative, three 33-foot square (with a side water depth of 25 feet) solids holding tanks are used in this alternative. Primary solids and WAS are sent to separate holding tanks, then combined in a blended holding tank before pre-dewatering.


10-23

10.4.2.2 Solids ScreeningThe screening process for Alternative 3 would be identical to the screening for Alternative 2. Assuming continuous operation of the pre-dewatering process at a design feed rate of 800 gpm at 1.5 percent solids, three screening units are recommended: two duty and one standby.

10.4.2.3 Pre-Dewatering with Cake HopperDirect pre-dewatering of the blended primary solids and WAS is presented with this master plan. Multiple centrifuge manufacturers confirmed the ability to dewater a 1.5% solids feed to a 16% cake solids. Based on dewatering units capable of processing 2,900 lb/hr, two duty units and one standby unit will be required. A summary of the pre-dewatering sizing criteria is provided in Table 10-15.

Table 10-15. Pre-Dewatering Sizing

Design Factor ValueAverage Solids Produced (lb/day) 87,000

14-Day Max. Solids Produced 116,000

Influent Solids Concentration 1.5%


Required Capacity


Hydraulic (gpm) 640

Capacity per Centrifuge


Hydraulic (gpm) 390


10.4.2.4 Thermal Hydrolysis Pretreatment ProcessThe THP process uses steam injection under elevated temperature and pressure to pretreat solids before digestion. Based on the digestion process requirements, the vendor equipment for the THP process would consist of one pulper (where the dewatered solids are mixed and heated), three reactors, one flash tank, and various feed pumps and air compressors. The manufacturer provides this equipment as one package.

The thermal hydrolysis equipment layouts presented in this master plan are based on CAMBI’s B-6 reactors.

10.4.2.5 Anaerobic DigestionThe anaerobic digestion process that follows THP would consist of two mesophilic tanks of approximately 1.1 MG each (60-foot diameter and 60-foot sidewall). For planning level purposes, the digesters will be mixed using multiple draft tube mixers for each tank. The cast-in-place concrete tanks would be constructed partially below grade and have a conical floor and a fixed steel cover. Gas collection and safety equipment will be provided for each digester.


10-24

A new digester building would house heat exchangers, steam boilers, transfer pumps, and any recirculation pumps associated with the digestion process.

For Alternative 3, the mesophilic digestion volume is based on a desired SRT of 15 days at the maximum 14-day design condition. It is assumed that the digester feed has a volatile solids content of 75 percent, and that the digesters remove 60 percent of volatile solids. The sizing criteria for the digesters are presented in Table 10-16.

Table 10-16. Sizing for Post-THP Anaerobic Digestion

Design Factor Value

Desired SRT (days) 15

Solids Loading (lb/day) 116,000

Feed Solids Concentration 10%

Total Digestion Volume (MG) 2.1

Number of Tanks 2

Diameter per tank (ft) 60

Minimum sidewall height of each tank (ft) 60

10.4.2.6 Holding TanksDigested solids are stored in two 50-foot diameter concrete tanks located adjacent to the digester building before post-dewatering. This allows for 4 days of retention time at the maximum month design flow. Covers and gas collection would be provided for these tanks. For this planning level evaluation, membrane type covers suitable for biogas storage are included with these tanks.

The current biological solids process building would be demolished to allow space for the digestion and solids holding processes.

10.4.2.7 Post-DewateringThe centrifuge post-dewatering process would be housed in a new three-story solids processing building (150 ft x 65 ft). The dewatered cake from post-THP digested solids is expected to have a slightly higher solids content of 30 percent. The post-dewatering process would require two duty units. Sizing criteria for the post-dewatering process are provided in Table 10-17.

Table 10-17. Sizing for Post-Dewatering with THP and Anaerobic Digestion

Design Factor Value

Required Capacity (lb/day, 30-day maximum design) 59,000

Unit Throughput (lb/hr) 3,800






10-25

10.4.2.8 Cake Storage and Truck LoadingDewatered cake will be stored on-site before being loaded into trucks for Class A land application. The truck loading process and associated annual costs are discussed further in Section 10.4.5.2.

10.4.2.9 Odor ControlThe odor control requirements for this alternative are expected to be similar to the current process. An allowance for replacement and rehabilitation of the existing DWB HVAC ductwork, fans, and chemical scrubber is included in the capital cost for this option.

10.4.2.10 SidestreamsSidestreams in Alternative 3 include pre-dewatering and dewatering centrate. Both of these sidestreams would typically return to the plant headworks for further treatment. Similar to Alternative 2, two sidestream treatment processes were considered: The DEMON process, which would treat dewatering centrate for nitrogen removal, and the OSTARA process, which would treat both pre-dewatering and dewatering centrate for P removal. The expected total load of nitrogen and P in the sidestreams is not anticipated to vary significantly with Alternative 3; however, the dewatering centrate following digestion will have a significantly higher concentration of ammonia. This stream in particular may be well suited for treatment to reduce the nutrient recycle to the plant headworks.

10.4.3 Product UsesWhile Alternatives 1 and 2 produce a Class B product, this Alternative 3 produces a Class A product. The product is more favorable with respect to handling compared to other alternatives, as it dewaters to a drier cake. The THP/ digestion process also generates a product that is typically less odorous than product from conventional anaerobic digestion processes. Under current flow conditions, Alternative 3 would produce approximately 100,000 wet pounds of biosolids daily. Although the product is suitable for distribution into other markets, the cost analysis presented at the planning level assumes product will continue to be bulk land applied using a third-party contractor.

Biogas produced in anaerobic digesters can be beneficially used on-site or off-site. Similar to Alternative 2, the options considered for biogas utilization under this alternative include: a) on-site use for process heating requirements; b) off-site use as a renewable fuel with natural gas supplying fuel for process heating; and c) use in a CHP system with recovered heat used to meet process heating (steam production) requirements.

10.4.4 Conceptual Layout of FacilitiesA conceptual facilities layout for Alternative 3 is presented in Figure 10-9. Similar to Alternatives 1 and 2, the existing biological solids processing building would be demolished. The existing gravity thickeners would also be demolished. The existing DWB would house the new pre-dewatering centrifuges, while the post-dewatering centrifuges will be housed in a new solids processing building (150 ft x 65 ft). Other new structures include the anaerobic digesters, digester building, solids holding tanks, THP equipment pad, and steam generator building.

BSP

BSHWSH

PSH

AD

DB

WGB

SGB

AD

PDW

SP

BDAF

Figure 10-9

Alternative 3:Thermal HydrolysisPretreatment with

Digestion


Anaerobic DigesterBiological Solids ProcessingBlended Sludge HoldingDissolved Air FlotationDigester BuildingDigested Sludge HoldingWaste Gas BurnerPre-Dewatering BuildingPrimary Sludge HoldingSteam Generator BuildingSolids Processing BuildingThermal HydrolysisWAS Holding

Facility Key

ADBSPBSHDAFDBDSHWGBPDWPSHSGBSPBTHPWSH

Legend

Existing


New

Demolition

THP

DSH

DSH


10-27

10.4.5 Capital and Life Cycle Costs10.4.5.1 Capital CostsA summary of the capital costs for Alternative 3 is provided in Table 10-18. The basis for these capital costs is provided in Appendix G.

Table 10-18. Capital Costs for Alternative 3: THP with Anaerobic Digestion







Thermal Hydrolysis $ 17,900,000

Mesophilic Digesters and Building $25,600,000


Post-Dewatering $ 25,600,000






DEMON $ 6,100,000

OSTARA $ 16,300,000


Operating Staff RequirementsAdding labor associated with pre-dewatering and the THP process increases O&M needs for Alternative 3, as approximately six FTE will be required. Efforts would be divided among pre-dewatering, THP, digestion, and post-dewatering. The estimated hours for each process and the total annual O&M costs for 2021 and 2040 are presented in Table 10-19.


10-28


YearPre-

Dewatering (hr/yr)

THP (hr/yr)

Digestion (hr/yr)

Post-Dewatering

(hr/yr)

Hauling (hr/yr)


Cost

2021 5,321 2,496 312 2,979 974 12,082 5.8 $ 820,000

2040 5,756 2,496 312 3,206 1,060 12,830 6.2 $ 2,200,000

Hauling RequirementsDewatered cake hauling is expected to range from approximately 19 trucks/week in 2021 to 21 trucks/week in 2040. This assumes each truck is filled to a maximum capacity of 20 tons. Assuming a land application unit cost of $57 per ton, the annual hauling and disposal cost for 2021 is approximately $1,100,000.

Power RequirementsPower costs are calculated by estimating the energy requirements for pre-dewatering, THP, digestion, and post-dewatering. In 2021, pre-dewatering is expected to require approximately 26,000 kWh per week, THP is expected to require 1,300 kWh per week, digestion is expected to require 15,200 kWh per week, and post-dewatering is expected to require approximately 13,600 kWh per week. Overall, the annual power cost for 2021 is estimated at $203,100, which is similar to the power cost for the mesophilic digestion alternative.

Chemical RequirementsThe plant’s chemical requirements under Alternative 3 include polymer for both pre-dewatering and dewatering. It is assumed that polymer used for pre-dewatering at a rate of 15 lb/DT and used for post-dewatering centrifuges at a rate of 25 lb/DT. Overall, chemicals are expected to cost the plant $561,000 in 2021.

Heating/Natural Gas RequirementsBiogas produced in the anaerobic digesters can provide the fuel needed for process heating requirements of this alternative. The primary fuel requirement is production of steam for the THP reactors. Under options where biogas is treated and used off-site as a renewable fuel, natural gas will be required to product the steam required for thermal hydrolysis.

A summary of hauling, power, chemical, and heating requirements for each of the three biogas utilization options is presented in Tables 10-20a through 10-20c.

Table 10-20a assumes that biogas produced in the digestion process is used on-site for process purposes. The heat balance indicates there is sufficient biogas for process heating. For this table, the remaining biogas is available for other purposes or combustion in a waste gas burner. Other tables will consider beneficial use of for all of the biogas produced.


10-29




2021 2,920,000 910 0 12,082 0 108,000 --

2040 3,100,000 990 0 12,830 0 117,000 --


2021 $ 200,000 $ 560,000 $ 800,000 0 $ 1,200,000 $ 2,760,000

2040 $ 380,000 $ 1,100,000 $ 2,200,000 0 $ 2,100,000 $ 5,780,000





2021 2,920,000 910 0 12,082 970 108,000 --

2040 3,100,000 990 0 12,830 1,060 117,000 --


2021 $ 200,000 $ 560,000 $ 800,000 $ 410,000 $ 1,200,000 $ 3,170,000

2040 $ 380,000 $ 1,100,000 $ 2,200,000 $ 780,000 $ 2,100,000 $ 6,560,000

Table 10-20c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. Under this scenario, additional capital investment in CHP equipment (noted as HHP costs in Table 10-18) would be required. The table is focused on biosolids processes and does not present the costs to operate and maintain the CHP system or the potential revenues or offset electrical cost associated with the production of power.


10-30




2021 2,920,000 910 0 12,082 0 108,000 --

2040 3,100,000 990 0 12,830 0 117,000 --


2021 $ 200,000 $ 490,000 $ 400,00 $ 0 $ 1,500,000 $ 2,590,000

2040 $ 350,000 $ 940,000 $ 1,100,000 $ 0 $ 2,900,000 $ 5,290,000

20-Year Net Present WorthThe expected NPW for Alternative 3 is shown in Table 10-21. The annual cost used in the net worth presentation is based on biogas utilization option “a”, which considers biogas will be used on-site for process needs. Under this option, no natural gas is purchased to support process needs and a portion of the biogas is available for other utilization. Overall, the 20-year NPW is approximately $134 million.

Table 10-21. Summary of Costs for Alternative 3: THP with Anaerobic Digestion

Capital Costs


Annual Costs


NPV Annual Cost $ 39,200,000

Net Present Worth ($ 2016) $ 134,200,000

10.4.6 Energy BalanceFigures 10-10a, 10-10b, and 10-10c present energy balances for Alternative 3 based on the three biogas utilization options using the solids production and Year 2021 mass balance.

Figure 10-10a assumes that biogas produced in the digestion process is used on-site for process purposes, with the remainder being available for export. This option represents a scenario where biogas production (185 MMBTU/d) represents a significant positive input to the energy balance. Also indicated on the figure is the energy demand of fuel (69 MMBTU/d) associated with product hauling. This is a significant is decrease compared to the current process configuration (Figure 10-4).

Pre-Dewatering

Dewatering

ThermalHydrolysis

AnaerobicDigestion

Utilization and/orWaste Gas Burner

Boiler

Cake

11

13

620 214

203

77

98

82

SludgeScreening



PrimarySolids WAS

258

157

409

1

7

389 389


Figure 10-10aEnergy Balance for THP plus Mesophilic Digestion • Baseline with Biogas Used On-Site for Process Purposes



-28+77

0-69-20


Hauling

69

7

Pre-Dewatering

Dewatering

ThermalHydrolysis

AnaerobicDigestion

Utilization and/orWaste Gas Burner

Boiler

Cake

11

13

620 214

203

175

98

82

SludgeScreening



PrimarySolids WAS

258

157

409

1

7

7

389 389


Figure 10-10bEnergy Balance for THP plus Mesophilic Digestion • Biogas Use Off-Site, Process Heating Needs by Natural Gas



-28+175

-98-69-20


Hauling

69

Natural Gas

98

Pre-Dewatering

Dewatering

ThermalHydrolysis

AnaerobicDigestion

Boiler

Cake

11

13

620 214

203

175

70

82

SludgeScreening



PrimarySolids WAS

258

157

409

1

7

7

67

389 389


Figure 10-10cEnergy Balance for THP plus Mesophilic Digestion • Biogas Used On-Site, Process Heating by CHP Heat Recovery with Supplemental Natural Gas



+390

-28-69-58


Hauling

69

Natural Gas

28

CHPEngine


10-34

Figure 10-10b assumes that biogas is used off-site, with all process heating needs met by natural gas. The overall ‘net’ energy of this option is similar to option a (Figure 10-7a) with the main difference being more biogas is available for off-site utilization and more natural gas is purchased for process heating requirements. This option represents the most favorable energy balance of all the alternatives considered.

Figure 10-10c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. The net energy for this option is lower than the net energy for options a and b. This option shows a net positive energy balance on the electrical power as the CHP system can produce more power than required of the solids handling equipment. The power would be available to off-set WPCP electrical demands or available to sell to the electrical utility. There is no biogas available for export under this option.

10.5 Alternative 4: Anaerobic Digestion with Thermal Drying10.5.1 Description and Process FlowAlternative 4 adds a thermal drying step to the processes discussed in Alternative 2. A process schematic is presented in Figure 10-11. The dryer can be fueled by either biogas produced in the digestion process, or by purchased natural gas. It is assumed for this evaluation that all biogas produced by digestion is sent to a waste gas burner. Supplemental natural gas is purchased to fuel the digester boilers and dryer.

Figure 10-11. Anaerobic Digestion with Thermal Drying Process Flow

10.5.2 Process-by-Process Description and Sizing10.5.2.1 Solids Holding TanksSimilar to the mesophilic digestion alternative, three 33-foot square (side water depth of 25 feet) solids holding tanks are used in Alternative 4. Primary solids and WAS are sent to separate holding tanks, then combined in a blended holding tank before pre-dewatering.


10-35

10.5.2.2 Co-ThickeningSimilar to Alternative 2, primary solids and WAS are thickened by a single co-thickening process. The process would be housed in a separate co-thickening building next to the solids holding tanks. See Section 10.3.2.2 for process description and sizing criteria.

10.5.2.3 Solids ScreeningThe screening process for Alternative 4 would be identical to Alternative 2.

10.5.2.4 Anaerobic DigestionThe anaerobic digestion process for Alternative 4 would be identical to Alternative 2. See Section 10.3.2.4 for process description and sizing criteria.

10.5.2.5 Holding TanksDigested solids holding would be identical to Alternative 2. See section 10.3.2.5 for a description of these tanks.

10.5.2.6 DewateringThe centrifuge dewatering process under this alternative would be sized to accommodate a specific capacity of the selected dryer. The centrifuges would be designed to operate on the same schedule at the thermal dryer.

10.5.2.7 Thermal DryingDewatered solids are sent from the centrifuges to the thermal dryer. It is assumed that a drum dryer (Andritz DDS 40) is used for this alternative. Sizing criteria for the dryer are presented in Table 10-22. It is assumed that the dryer operates on a 24-hour per day, 7-day per week schedule to meet maximum 30-day requirements at the design condition.

Due to the significant capital investment required and the space required, it is common for utilities to only install a single dryer train. For the master plan, it is assumed Arlington County will only install a single train under this alternative.

Table 10-22. Thermal Dryer Sizing Criteria

Design Factor Value

Digested Solids (lb/day) 63,800

Dewatered Solids Content (%) 25%

Wet Feed (lb/hr) 10,600

Dried Product Solids Content (%) 92%

Required Evaporation Rate (lb/hr) 7,700

DDS-40 Evaporative Capacity (lb/hr) 8,800


10-36

10.5.2.8 Product Storage and Truck LoadingDried product is stored on-site in two new dryer product silos before being loaded into trucks distribution as a Class A dryer product or fertilizer.

10.5.2.9 Odor ControlCo-thickening, digestion, and dewatering facilities are expected to have odor control requirements similar to the current process. An allowance for replacement and rehabilitation of the existing DWB HVAC ductwork, fans, and chemical scrubber is included in the capital cost for this option.

Odor control requirements are expected to be significant with respect to the operation of the thermal dryer. A regenerative thermal oxidizer (RTO), designed to oxide the treated dryer exhaust is included with this alternative.

10.5.2.10 SidestreamsSidestreams in this alternative are similar to Alternative 2. See Section 10.3.2.9 for a discussion of process sidestreams.

10.5.3 Product UsesIn the case of thermal drying, the quality of the dried product and the surrounding market for biosolids often drives the purchase of the technology. Since the product has a much greater solids content than conventional anaerobically digested solids, the product can be made available as a fertilizer supplement through a distribution and marketing program. It can also be used as a fuel in cement kilns.

10.5.4 Conceptual Layout of FacilitiesA conceptual facilities layout for Alternative 4 is presented in Figure 10-12. It is similar to Alternative 2 except for the dryer building (170-foot x 100-foot x 40-foot high) and dryer product silos (25-foot diameter each).

10.5.5 Capital and Life Cycle Costs10.5.5.1 Capital CostsA summary of the capital costs for Alternative 4 is provided in Table 10-23.


Operating Staff RequirementsAlthough Alternative 4 would require additional labor associated with the drying process, hauling labor would be reduced due to the reduced amount of wet product. O&M needs for this alternative will roughly the same number of full-time as the anaerobic digestion alternative. Efforts would be divided among co-thickening, digestion, dewatering, drying, and hauling. The estimated hours for each process and the total annual O&M costs for 2021 and 2040 are presented in Table 10-24.

DRY

DW

BSH

WSH

PSH

CTAD

AD

DB

AD

WGB

DSH

DSH

DAF

DSDS

Figure 10-12

Alternative 4:Digestion with

Thermal Drying


Anaerobic Digester

Blended Sludge Holding

Co-Thickening Building

Dissolved Air Flotation Building

Digester Building

Dryer Building

Dryer Product Silo

Digested Sludge Holding

Dewatering Building

Waste Gas Burner

Primary Sludge Holding

WAS Holding

Facility Key

ADBSHCTDAFDBDRYDSDSHDWWGBPSHWSH

Legend

Existing


New

Demolition


10-38

Table 10-23. Capital Costs for Alternative 4: Anaerobic Digestion with Thermal Drying





Co-Thickening by RDT $ 9,700,000


Mesophilic Digesters (Class B) $ 38,500,000



Thermal Drying (includes RTO) $ 35,800,000

Product Storage $ 4,500,000






DEMON $ 6,100,000

OSTARA $ 16,300,000


YearCo-

Thickening (hr/yr)

Digestion (hr/yr)

Dewatering (hr/yr)

Drying (hr/yr)

Hauling (hr/yr)


Cost

2021 1,040 2,190 5,120 2,280 610 11,240 5.4 $ 660,000

2040 1,040 2,190 5,540 2,280 660 11,710 5.6 $ 1,700,000

Hauling RequirementsHauling dewatered cake is expected to range from 12 trucks/week in 2021 to 13 trucks/week in 2040. The analysis assumes the dryer product can be hauled and distributed to an end user at no cost to Arlington County. In some instances, the product can be sold to generate a revenue stream. In other instances, the product may be given away with the recipient paying for the hauling costs.


10-39

Power RequirementsProcesses with energy requirements include co-thickening, digestion, dewatering, and drying. In 2021, co-thickening is expected to require approximately 3,890 kWh per week, digestion is expected to require 38,410 kWh per week, dewatering is expected to require approximately 14,850 kWh per week, and drying is expected to require 36,390 kWh per week. Overall, the annual power cost for 2021 is estimated at $340,000, which is greater than the other alternatives considered. This difference is primarily due to the power requirements for drying.

Chemical RequirementsThe plant’s chemical requirements under Alternative 4 include thickening polymer and dewatering polymer. It is assumed that polymer is added to the rotary drum thickeners at a rate of 10 lb/DT, and added to the dewatering centrifuges at a rate of 25 lb/DT. Overall, chemicals are expected to cost the plant $480,000 in 2021.

Heating/Natural Gas RequirementsBiogas produced in the anaerobic digesters can provide the fuel needed for process heating requirements of this alternative. The primary fuel requirement is production of steam for the THP reactors. Under options where biogas is treated and used off-site as a renewable fuel, natural gas will be required to product the steam required for thermal hydrolysis.

A summary of hauling, power, chemical, and heating requirements for each of the three biogas utilization options is presented in Tables 10-25a, 10-25b, and 10-25c.

Table 10-25a assumes that biogas produced in the digestion process is used on-site for process purposes. The heat balance indicates there is sufficient biogas for process heating. For this table, the remaining biogas is available for other purposes or combustion in a waste gas burner. Other tables will consider beneficial use of for all of the biogas produced.




Total

2021 4,880,000 810 0 11,200 480 108,000 --

2040 5,130,000 880 0 11,700 510 117,000 --


2021 $ 330,000 $ 490,000 $ 700,000 $ 200,000 $ 0 $ 1,720,000

2040 $ 630,000 $ 940,000 $ 1,700,000 $380,000 $ 0 $ 3,650,000



10-40




Total

2021 4,880,000 810 0 11,200 2,400 108,000 --

2040 5,130,000 880 0 11,700 2,600 117,000 --


2021 $ 330,000 $ 490,000 $ 700,000 $ 1,000,000 $ 0 $ 2,590,000

2040 $ 630,000 $ 940,000 $ 1,700,000 $ 1,900,000 $ 0 $ 5,330,000

Table 10-25c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. Under this scenario, additional capital investment in CHP equipment (noted as CHP costs in Table 10-18) would be required. The table is focused on biosolids processes and does not present the costs to operate and maintain the CHP system or the potential revenues or offset electrical cost associated with the production of power.




Total

2021 4,880,000 810 0 11,200 1,630 108,000 --

2040 5,130,000 880 0 11,700 1,760 117,000 --


2021 $ 330,000 $ 490,000 $ 700,000 $700,000 $ 0 $ 2,220,000

2040 $ 630,000 $ 940,000 $ 1,700,000 $1,300,000 $ 0 $ 4,570,000

20-Year Net Present WorthThe expected NPW Alternative 4 is shown in Table 10-26. The annual cost used in the net worth presentation is based on biogas utilization option “a”, which considers biogas will be used on-site for process needs. Under this option, the thermal dryer is fueled by a combination of biogas and natural gas to support process needs. Overall, the 20-year NPW for this alternative is the highest of the alternatives evaluated and is approximately $139 million.


10-41

Table 10-26. Summary of Costs for Alternative 4: Anaerobic Digestion with Thermal Drying

Capital Costs


Annual Costs


Present Worth Annual Cost (Year 2016 dollars) $ 24,700,000

NPV (Year 2016 dollars) $ 138,600,000

10.5.6 Energy BalanceFigures 10-13a, 10-13b, and 10-13c present energy balances for Alternative 4 based on the three biogas utilization options using the solids production and Year 2021 mass balance.

Figure 10-13a assumes that biogas produced in the digestion process is used on-site for process purposes, with the remainder being available for export. This option represents a scenario where biogas production (148 MMBTU/d) represents a significant positive input to the energy balance; however, the biogas produced is not sufficient to meet the thermal dryer process requirements. Even with the lowest energy required for final product hauling, the overall net energy for this alternative is the lowest of the alternatives considered, primarily due to the natural gas required for the dryer operation.

Figure 10-13b assumes that biogas is used off-site, with all process heating needs met by natural gas. The overall ‘net’ energy of this option is similar to option a (Figure 10-7a) with the main difference being the biogas is available for off-site utilization and more natural gas is purchased for process heating requirements.

Figure 10-13c assumes that biogas is used on-site, with process heating provided by CHP heat recovery. The net energy for this option is lower than the net energy for options a and b. This option shows a small net positive energy balance on the electrical power as the CHP system can produce more power than required of the solids handling equipment. The natural gas purchased for operation of the thermal dryer is significant in this option, which has a low overall net energy compared to other alternatives considered.

RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Natural Gas

Boiler

Dewatering Direct Dryer

Dried Product

258

157

395

21

12

148

90

58

49

234 234

247 94

2

19

24

6COD (MMBtu/d)Fuel (MMBtu/d)


Figure 10-13aEnergy Balance for Mesophilic Digestion plus Thermal Drying • Baseline with Biogas Used On-Site for Process Purposes



-510

-94-26

-171

LEGEND

Hauling26

RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Natural Gas

Boiler


Dried Product

258

157

395

21

12

5849

234 234

247 184

2

19

24



Figure 10-13bEnergy Balance for Mesophilic Digestion plus Thermal Drying • Biogas Use Off-Site, Process Heating Needs by Natural Gas



-51+148-242

-26-171

LEGEND

Hauling26

BiogasUtilization

148

RDT Co-Thickening

PrimarySolids WAS



AnaerobicDigestion

Natural Gas

Boiler


Dried Product

258

157

395

21

12

49

59

56

234 234

247 184

2

119

24



Figure 10-13cEnergy Balance for Mesophilic Digestion plus Thermal Drying • Biogas Used On-Site, Process Heating by CHP Heat Recovery with Supplemental Natural Gas



+50

-184-26

-205

LEGEND

Hauling26

148

CHPEngine

Waste Heat

11-1

Section 11 Evaluation of Alternatives

Section 10 summarizes four selected biosolids management alternatives and associated process descriptions, process sizing, products and end uses, costs (capital, annual, and life cycle) and energy balances. This section presents the evaluation results of these four alternatives using the evaluation criteria, performance measures scoring guidance, and weighting factors.

This section divides the criteria into the four main categories described in Section 4: economic, operational, environmental, and social. This section ends with an overall assessment of the alternative management strategies and identifies the preferred strategy.

11.1 Economic Criteria Evaluations As described in Section 4, economic criteria represent one of the four elements of the QBL evaluation used as part of the master planning process. Economic criteria include capital cost, annual O&M costs, life cycle cost, financial options/ risk offsets, and end use management and control. The criteria are scored on a scale of 1 – 5 (with 5 being most preferred) and weighted based on the criteria weighting factors described in Section 4.

Capital cost received a weighting factor of 51%, which is near the average of all criteria. Capital cost development is presented in Section 10, with baseline cost summarized in Table 11-1. The capital cost represents a total project cost that includes constructing new facilities and demolishing existing facilities as well as implementation costs (e.g., engineering and design, construction management, and contingencies associated with the planning level estimates).

Operating cost comparisons were based on estimates of labor (operating and maintenance), power, chemicals, gas, transportation, and other costs. Estimates were developed for each year based on inflated unit costs and annual estimates of use. Natural gas costs assume biogas is used to meet on-site heating demands for digester heating and steam production. Supplemental purchased natural gas is required for thermal drying. Operating cost received a weighting factor of 44%, slightly below the average of all criteria.

Life cycle cost is the economic criteria with the highest significance having a weighting factor of 68%. Life cycle costs is presented for a 20-year project duration. Life cycle cost development assumes all capital costs are incurred in the first year (Year 2021), and the annual operating and maintenance costs are discounted to the first year using a discount factor that matches the anticipated project financing rate.

Financial options and risk offsets considers financing options including utility bonds, private public partnerships (PPP), power purchase agreements (PPA), and energy service company (ESCO) arrangements. The criteria also consider potential for innovative financing or ownership and lease options where they exist. These innovative financing options can offset the risk either financially or operationally through vendor maintenance/performance agreements. The estimated financial cost of the annualized capital costs for each alternative were developed

Section 11 • Evaluation of Alternatives

11-2

assuming 100 percent bond financing. The financial options and risk offsets criteria are weighted below the average with a weighting of 20%.

End use management and control considers long-term outlet availability as well as the ability to control and/or mitigate problems off-site from the WPCP. Outlets that either did not exist or had uncertainty about the future were deemed less favorable than proven outlets that may either exist for Arlington County or outlets than can become available with new or enhanced products (biosolids, energy, or other). The weighting for the criterion was below average with a weighting of 32%.

Table 11-1. Summary of Economic Criteria for Alternatives

Alternative 1:

Lime Alternative 2:

MAD Alternative 3:

THP/MAD Alternative 4:

MAD/DRY

Capital Cost (baseline) $ 40 M $ 74 M $ 95 M $ 114 M

Annual O&M Cost, Year 2021

$ 3.1 M $ 2.1 M $2.1 M $1.3 M

20-yr Life Cycle Cost $ 94 M $ 111 M $ 134 M $139 M

Estimated Financial Cost ($/DT)

$ 430/DT $ 490/DT $ 580/DT $600/DT

End Use Management & Control

Class B biosolids; greatest cost sensitivity to contractor hauling cost uncertainty

Reduced quantity of Class B biosolids compared to lime; sensitive contractor hauling cost uncertainty

Significant reduction on quantity of biosolids to be hauled off-site; Class A material but sensitive to contractor hauling cost

Class A biosolids; lowest volume of biosolids product hauling; cost assumes product is revenue neutral, i.e., hauling costs are off-set by revenue of product sale

Table 11-2 presents the raw and weighted scoring for the economic criteria. Raw scoring was collected as the numeric average of participants submitting scoresheets during a project workshop. The raw scores were reviewed by project team members for scoring consistency and criteria definitions. No adjustments were made to the results. Figure 11-1 graphically presents the weighted results. As shown in the graph, Alternative 2, MAD, received the highest score and is viewed most favorably under the economic criteria. The factors influencing the more favorable score include:

Moderate capital cost and lowest annual O&M cost resulted in a low life cycle cost


11-3

Table 11-2. Economic Criteria Evaluation

Alternative Capital Cost Annual O&M Cost Life Cycle Cost Financial Options/

Risk Offsets End Use Management &

Control

Weighted Total

Weighting 51% 44% 68% 20% 32%

Raw Weighted Raw Weighted Raw Weighted Raw Weighted Raw Weighted

Alternative 1: Lime 4.1 2.09 1.4 0.63 2.9 2.00 2.3 0.45 2.1 0.67 5.85

Alternative 2: MAD 3.6 1.84 3.4 1.47 4.4 2.98 3.8 0.75 3.1 1.02 8.06

Alternative 3: THP/MAD 3.0 1.54 3.1 1.38 2.8 1.91 3.4 0.68 3.7 1.20 6.71

Alternative 4: MAD/DRY 2.5 1.29 3.5 1.54 2.1 1.47 2.9 0.58 3.9 1.25 6.12


11-4

Reduced quantity of product hauling reduced life cycle cost sensitivity to variables such as changes in hauling costs or energy costs

Value of Class A product was recognized for Alternatives 3 and 4 with more favorable scores in the End Use Management & Control criterion; however, this criterion was weighted less than cost-focused criteria

Figure 11-1. Results of Economic Criteria Evaluation

As discussed in Section 5, a sensitivity analysis was performed to evaluate the impact of changes in capital, hauling, and energy costs on the net annualized cost of the four alternatives. Scenarios evaluated include a 30 percent decrease in capital costs, a 50 percent increase in capital costs, 100 percent increase in hauling costs, and a 100 percent increase in energy costs. An additional sensitivity analysis was performed on the four alternatives if taking into account the production of CNG. In addition to the scenarios listed above, the analysis with CNG included a 100% increase in the value of produced energy, and a produced energy value of zero. The results of the analyses without CNG and with CNG are presented in Figure 11-2 and Figure 11-3, respectively.

As shown in Figure 11-2, a doubling of hauling costs has the greatest impact on Alternative 1 (Lime) compared to the baseline net annualized cost. A doubling of energy costs has the greatest impact on Alternative 4 (MAD + Drying), due to the high energy demand associated with heat drying. For the sensitivity analysis with CNG, a 50 percent increase in capital cost has a significant impact on Alternative 3 (THP/MAD) and Alternative 4, given the large capital investments associated with THP and heat drying, respectively. A doubling of the produced energy value has


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the greatest impact on Alternative 3, reducing the net annualized cost from approximately $7.8 million to $6.4 million.

Figure 11-2. Sensitivity Analysis Results (no CNG)

Figure 11-3. Sensitivity Analysis Results (with CNG)

$0 $2,000,000 $4,000,000 $6,000,000 $8,000,000 $10,000,000

1-Lime

2-MAD

3-THP/MAD

4-MAD/Dry

Net Annualized Cost

-30% Capital Net Annualized Cost +50% Capital Double Hauling Cost Double Energy Cost

$0 $2,000,000 $4,000,000 $6,000,000 $8,000,000 $10,000,000

1-Lime

2-MAD

3-THP/MAD

4-MAD/Dry

Net Annualized Cost

-30% Capital Net Annualized Cost +50% Capital Double Hauling CostDouble Energy Cost Double Energy Value Zero Energy Value


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11.2 Operational Criteria Evaluations Operational criteria were the second category of criteria used in the evaluation. As a whole, criteria in this category are heavily weighted with a focus on operability, safety, reliability, constructability, and potential impacts of one process on other plant processes. Key parameters from Section 10 related to the operation criteria evaluations are summarized in Table 11-3.

Flexibility considers the potential for operational flexibility allowed by construction phasing, expansion potential of the process based on the space available at the WPCP, and the diversification of biosolids products. Potential opportunities for off-site product use or reuse is considered as well as the potential for increased opportunities with a Class A biosolids product compared to the current Class B project. Flexibility had a weighting factor below the average of all criteria with a weighting of 38%.

Operability and safety considers the staffing requirements and hazards associated with processes and equipment. The basis of comparison is the number of FTEs anticipated for each alternative and not necessarily an exact number of staff or new hires required. Processes and equipment safety was also considered in evaluating this option. Operability and safety is the second highest rated criteria based on the weighting factor of 91%.

Table 11-3. Summary of Operational Criteria for Alternatives

Alternative 1:

Lime Alternative 2:

MAD Alternative 3:


MAD/DRY

Flexibility Class B biosolids, largest quantity of biosolids produced among alternatives; potential to enhance to Class A with increased cost

Class B biosolids, potential to enhance to Class A product with pre-treatment or post-digestion treatment

Class A biosolids cake suitable for bulk land application or distribution as a soil amendment; expansion possible but significant effort required; limited potential for phasing

Class A dryer product suitable for marketing as a soil enhancer or fertilizer; can be phased with Alternative 2 as an initial step

Operability/ Safety

3.6 FTEs; processes familiar to staff; truck loading is labor intensive and requires staff to communicate frequently; lime dust and ammonia gas generation

3.3 FTEs; digestion process is common at municipal wastewater facilities, but is a new process to Arlington staff; biogas is combustible and precautions should be used

5.8 FTEs; pre-dewatering process adds 2 FTEs; high pressure steam boiler does not require a certified operator 24/7 but the process is new to Arlington staff; biogas is combustible and precautions should be used

5.4 FTE; increased dewatering time each week to accompany 24-hour dryer operations; biogas handling precautions required; handling of thermal dryer product requires precautions to prevent combustion


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Alternative 1:

Lime Alternative 2:

MAD Alternative 3:


MAD/DRY

Proven System/ Technology

Current system with numerous installation and many years of performance

Many installations in the United States and worldwide; anaerobic digestion has a long history of performance

Anaerobic digestion has a long history of performance. THP has fewer installations than other alternatives –established history of performance in Europe, U.S. operating history is much shorter

Anaerobic digestion has a long history of performance; thermal drying is accepted in the industry with years of performance records and several drying facilities located in the mid-Atlantic region

Reliability Current system with established maintenance; periodic off-site maintenance of centrifuges

Periodic digester cleaning with frequency depending on many factors but cleanings can be planned; periodic centrifuge maintenance required

Additional equipment to operate and maintain compared to digestion-only alternatives; annual inspections for THP and steam equipment; periodic digester cleaning; periodic centrifuge maintenance required

Periodic digester cleaning; periodic centrifuge maintenance required; significant dryer and air handling components to maintain; dryer downtime requires storage or alternate handling of solids

Constructability Existing system will require temporary solution to facilitate construction of upgrades in the DWB

Limited on-site space will require sequencing of work and demolition of Bio Building to construct facilities

Limited on-site space will require sequencing of work and demolition of Bio Building to construct facilities

Limited on-site space will require sequencing of work and demolition of Bio Building to construct facilities; requires most space at site to implement

Impacts on Plant Processes

Rehabilitation and replacement of equipment will require periodic shutdowns reducing the redundancy available with the existing system; no change to the impacts on plant processes is anticipated

Construction sequencing may require interim processing solutions; increased N loading in the sidestream; additional sidestream treatment can be considered but is not required




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Proven system and/or technology considers the experience and application of the technology and processes required with each alternative. Section 5 of this report describes the range of technologies considered for the master plan and summarizes screening results. The screening exercise described in Section 5 considered the development status of technologies and only established technologies were considered for the final alternatives development. For the alternatives evaluation, proven system and/or technologies were compared based on the operating history and performance of each. The weighting of this criteria is slightly below the average of all criteria with a weighting factor of 43%.

Reliability is the ability to effectively monitor, assess, predict, and generally understand an alternative and its assets and successfully deploy a maintenance strategy. Reliable functioning allows routine and predictive maintenance to be identified, dominant failure modes to be ascertained, and consequences of failure to be estimated with a degree of confidence. This criterion is weighted significantly higher than the average at a factor of 78%.

Constructability measures both the ability to construct an alternative and considers both unforeseen site conditions as well as physical limitations with respect to site constraints and/or existing facilities. Section 5 of this report describes the preliminary screening that was completed and that only technologies that can be located on the WPCP site are considered in the short-list of alternatives. This criterion is weighted higher than average at a factor of 64%

Impacts on plant processes considers the potential impacts of an alternative on other plant processes and facilities. Key considerations include: MOPO during construction (e.g., minimizing plant outages), the impact of constructed facilities on plant processes and the potential need for additional processes to address those impacts (i.e., side-stream treatment, plant water required), and the ease of integration with other plant processes. This criterion is weighted above average at a factor of 64%.

Table 11-4 and Figure 11-4 summarize the evaluation results in this category. As shown, Alternative 1 (Lime Stabilization) and Alternative 2 (MAD) scored the highest (most favorable) in this category. In general, the scores are more favorable for these alternatives due to the following:

Operability and safety of systems that show a lower staffing requirement to operate and maintain (i.e., fewer processes and equipment)

Processes that are more established with a longer operating history and more installations

Processes that require smaller footprints based on preliminary layouts

Flexibility of processes that produce a Class A biosolids product did not significantly influence the final scores

As shown in Table 11-4, Alternative 1 (Lime) scored highest on operability and safety, constructability, and impacts on plant processes. High scores in these categories for Alternative 1 were likely influenced by the fact that this alternative represents the existing process.


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Table 11-4. Operational Criteria Evaluation

Alternative Flexibility Operability and

Safety Proven System/

Technology Reliability Constructability Impacts on Plant

Processes

Weighted Total

Weighting 38% 91% 43% 78% 64% 64%

Raw Weighted Raw Weighted Raw Weighted Raw Weighted Raw Weighted Raw Weighted

Alternative 1: Lime

3.1 1.17 4.0 3.63 4.8 2.06 3.5 2.75 4.2 2.68 3.8 2.41 14.70

Alternative 2: MAD

3.6 1.37 3.8 3.48 4.7 2.03 3.8 2.97 3.4 2.18 3.2 2.04 14.07

Alternative 3: THP/MAD

3.2 1.24 2.8 2.59 3.5 1.52 3.0 2.38 3.3 2.11 3.0 1.95 11.79

Alternative 4: MAD/DRY

4.4 1.66 2.9 2.67 3.8 1.63 2.9 2.24 3.1 2.00 2.9 1.86 12.07


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Figure 11-4. Results of Operational Criteria Evaluation

11.3 Environmental Criteria Evaluations Environmental criteria were the third category evaluated. Criteria in this category generally ranked below the average of the other criteria; however, regulatory permitting was the highest rated of all criteria considered. This category also considers elements related to energy, carbon footprint, the potential to recover resources as energy, nutrients, or organics, and the quality of the biosolids and biogas products.

Resource recovery potential considers the potential products resulting from the treatment train and the ability of the WPCP to recover and use the product value. Products identified for resource recovery include the biosolids product, biogas, and P. Biosolids products include Class B lime stabilized product, Class B digested and dewatered cake, Class A biosolids cake, and Class A thermal dryer product. Biogas is generated with the digestion process and can be used on-site or off-site. The relative amount of biogas is similar for the alternatives with mesophilic digestion and is slightly higher with thermal hydrolysis pretreatment. The relative amount of P that can be recovered is theoretically similar for all alternatives that include anaerobic digestion; however, the recovery potential depends on the liquid stream operation. Resource recovery is weighted below the average of all criteria with a weighting factor of 39%.


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Energy intensity considers the amount of energy required plus the potential energy recovery for the alternative. As shown in Figure 11-5, energy consumption of the processes including purchased electrical power, process heating requirements (noted as purchased natural gas), biosolids hauling energy (based on 200 miles roundtrip from the WPCP to the application site), and the energy credit associated with biogas production. The net energy intensity of the MAD option is close to neutral (-43 MJ/DT) when considering the biogas credit. Adding thermal hydrolysis pretreatment increases biogas production as well as reduced hauling energy and offers the only alternative with a negative energy intensity (-1,300 MJ/DT) based on the process areas considered. Thermal drying requires significant process heating energy and has a net energy of 4,000 MJ/DT based on the factors considered. Lime stabilization has the highest energy requirement, mostly associated with product hauling and does not have any energy credits. The total energy intensity is 4,900 MJ/DT for this alternative. The weighting of the energy intensity is below the average of all criteria with a weighting factor of 38%.

Figure 11-5. Estimated Energy Intensity per Dry Ton of Solids in Year 2021

Carbon footprint is measured by the carbon intensity of GHG emissions as a relative comparison within the solids processing area. The net value of carbon intensity for each alternative is determined by subtracting the biogas credit from the emissions associated with purchased electrical power, process heating requirements (as purchased natural gas), biosolids product hauling based on 200 miles round trip to the application site, and indirect factors associated with producing lime and polymer used in the process. The carbon intensity does not consider the


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product’s fertilizer or carbon sequestration values, which are anticipated to be similar for each considered alternative.

Figure 11-6 illustrates the carbon intensity (presented as mTCO2e/dry ton) for Year 2021. The biogas credit is a significant offset in each alternative with digestion. MAD (net intensity = -0.2 mTCO2e/dry ton) and thermal hydrolysis pretreatment (net intensity = -0.3 mTCO2e/dry ton) are more preferred alternatives under this criteria than lime stabilization and thermal drying. The weighting of the carbon footprint is below the average of all criteria with a weighting factor of 38%.

Figure 11-6. Estimated Carbon Intensity per Dry Ton of Solids in Year 2021

Regulatory permitting was the highest ranked criteria. Each alternative is expected to be permitted with relative ease. Options with digestion will require a review of air permitting requirements and the ultimate permitting effort will depend on the planned use for the biogas. If Arlington County intends to market and distribute Class A dewatered cake or thermally dried material as a fertilizer product, additional registration of the material as a fertilizer will be required. Regulatory permits is the highest rated criteria with a weighting factor of 100%.

Gas and product quality assesses the ability of the biogas and biosolids product to meet the intended use requirements and the level of additional treatment that may be required. While Arlington County has indicated a preference for Class A biosolids, only the alternatives with thermal hydrolysis pretreatment and/ or thermal drying can meet this goal without additional


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treatment; however, a Class A dewatered cake is likely to require additional processing to be distributed into markets outside bulk agricultural land application. Biogas use is anticipated to require additional treatment under each scenario where biogas is produced. Gas and product quality is weighted below the average of all criteria with a weighting factor of 29%.

Table 11-5 is a summary of environmental criteria.

Table 11-5. Summary of Environmental Criteria for Alternatives

Alternative 1:

Lime Alternative 2:

MAD Alternative 3:


MAD/DRY


Class B lime stabilized biosolids

Class B biosolids cake, biogas, potential for P recovery

Class A biosolids cake, biogas, potential for P recovery

Class A biosolids dryer product, biogas, potential for P recovery

Energy Intensity 4,900 MJ/DT 0 MJ/DT -1,300 MJ/DT 4,000 MJ/DT

Carbon Footprint 0.6 mtCO2e/DT -0.2 mtCO2e/DT -0.3 mtCO2e/DT 0.5 mtCO2e/DT

Regulatory Permits Currently permitted with no anticipated modifications required

Established permitting process; air quality permitting is anticipated

Established permitting process; air quality permitting is anticipated

Established permitting process; air quality permitting is anticipated with increased emissions based on dryer fuel and dryer emissions

Gas and Product Quality Class B cake suitable for bulk land application

Class B cake suitable for bulk land application; biogas will require moisture removal and potentially additional cleaning

Class A cake suitable for bulk land application, soil blending, or blending and distribution as a soil amendment; biogas will require moisture removal and potentially additional cleaning

Class A dryer product suitable for soil blending or as a fertilizer, additional processing may be desired depending on dryer type; biogas will require moisture removal and potentially additional cleaning

Table 11-6 presents the raw and weighted score for each alternative. Figure 11-7 is a stacked bar chart showing the weighted scores. As seen in both, Alternative 3 scored the highest (most favorable) of the alternatives considered. Biogas production as a resource to be recovered favored all options with anaerobic digestion. The quality of the final product, a Class A biosolid coupled with the reduced energy intensity and carbon footprint required to achieve Class A (compared to thermal drying) were key differentiators of the thermal hydrolysis alternative.


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Table 11-6. Environmental Criteria Evaluation

Alternative Resource Recovery

Potential Energy Intensity Carbon

Footprint Regulatory

Permits Gas and Product

Quality

Weighted Total

Weighting 39% 38% 38% 100% 29%

Raw Weighted Raw Weighted Raw Weighted Raw Weighted Raw Weighted

Alternative 1:

Lime 1.4 0.55 3.3 1.24 2.9 1.10 4.3 4.29 2.1 0.60 7.78

Alternative 2: MAD 3.2 1.24 3.9 1.48 3.9 1.48 4.0 4.00 3.1 0.89 9.09


4.5 1.73 4.1 1.54 4.4 1.67 4.0 4.00 4.1 1.20 10.14


4.0 1.54 2.9 1.08 2.4 0.89 3.9 3.93 4.6 1.32 8.76


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Figure 11-7. Results of Environmental Criteria Evaluation

11.4 Social Criteria Evaluation Social criteria are the final category evaluated. Of the three criteria in this category, two were weighted significantly above the average weighting and one received the lowest weighting of all considered criteria. This category considers the community impacts of each alternative with respect to odor generation potential and/ or reduction, acceptability of the process and the product, and hauling impacts that are generally associated with increases or decreases with traffic volume. A summary of the criteria considered in scoring the alternatives is presented in Table 11-7.

Odor generation potential and reduction options are an important consideration for any technology or process considered for the WPCP. This criterion received a weighting above the average of all criteria with a weighting factor of 75%. There is zero tolerance for odors at the WPCP site. The facility location is a commercial and residential area of Arlington. The county is committed to integrating the WPCP into the community and recognizes the importance of odor management and the community’s sensitivity to any odors leaving the site. Potential odors from the biosolids product are also considered in the evaluation.

Acceptability is a broadly defined criterion encompassing the expected community response to the alternative. The criterion considers visual aesthetics, odors, and noise – including during the construction phase – as well as potential to reuse the products (biosolids and biogas) within the community. This criterion received a weighting above the average of all criteria with a weighting factor of 66%.


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Hauling represents the community impact with the lowest weighting factor of all criteria with a weighting factor of 16%. Hauling considers the community impacts due to truck traffic and mitigation measures to address these impacts. Visual impacts, hauling emissions, and noise are all potential impacts.

Table 11-7. Summary of Social Criteria for Alternatives

Alternative 1:

Lime Alternative 2: MAD Alternative 3:


MAD/DRY Odor Generation Potential/ Reduction

No changes to process or product odor anticipated; product has some odor

Foul air capture and treatment required, similar to existing facilities; potential for product odor, but generally accepted at application sites

Foul air capture and treatment required, similar to existing facilities; product has minimal odor

Foul air capture and treatment required; dryer emissions will require additional odor management; product has minimal odor, but generally more than THP/MAD product

Acceptability No visual change or changes in emissions sources; product not suitable for use in community

Tanks of significant height required; product not suitable for use in community

Tanks of significant height required; product suitable for distribution locally

Tanks of significant height required; dryer emissions; product is suitable for distribution locally

Hauling 40 trucks/week in Year 2021

26 trucks/week in Year 2021



Table 11-8 presents the raw and weighted score for each alternative. Figure 11-8 is a stacked bar chart showing the weighted scores. As seen in both, Alternative 3 scored the highest (most favorable) of the alternatives considered. The quality of the final product, a Class A biosolid coupled with the reduced energy intensity required to achieve Class A (compared to thermal drying) were key differentiators of this alternative.

Table 11-8. Social Criteria Evaluation

Alternative Odor Generation Potential/

Reduction Acceptability Hauling

Weighted Total Weighting 75% 66% 16%

Raw Weighted Raw Weighted Raw Weighted Alternative 1: Lime

2.0 1.47 2.0 1.32 1.6 0.26 3.04

Alternative 2: MAD 3.3 2.44 2.9 1.89 2.7 0.44 4.77


3.8 2.86 3.2 2.10 3.7 0.59 5.56


3.4 2.56 3.3 2.15 4.5 0.72 5.43


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Figure 11-8. Results of Social Criteria Evaluation

11.5 Summary of Scoring Evaluation The overall results of the scoring evaluation of presented in Table 11-9 and is shown graphically in Figure 11-9. The ranking of the final results indicates that Alternative 2, MAD, scored the best against the evaluation criteria. Alternative 3, THP pretreatment followed by anaerobic digestion, was the second ranked alternative based on the scoring.

Table 11-9. Results of Scoring Evaluation

Alternative Economic Operational Environmental Social TOTAL Rank

Alt 1 Lime 5.9 14.7 7.8 3.0 31.4 4

Alt 2 MAD 8.1 14.1 9.1 4.8 36.0 1

Alt 3 THP/MAD 6.7 11.8 10.1 5.6 34.2 2

Alt 4 MAD/DRY 6.1 12.1 8.8 5.4 32.4 3


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Figure 11-9. Results of Alternative Scoring

11.6 Selection of Preferred Alternative The project team reviewed the final rankings and considered these rankings to determine the preferred alternative. The team agreed that all three digestion-based alternatives were preferred over lime stabilization. The team also noted master planning goals included developing a roadmap for the future and moving toward a Class A biosolids product. With this information, the preferred alternative identified in this master plan is thermal hydrolysis pretreatment followed by MAD.

Thermal hydrolysis pretreatment provides Arlington County an opportunity to produce multiple resources suitable for use in the local area. These include a Class A biosolids product, biogas, and potentially recovered P. The Class A biosolids product will likely require additional processing for distribution locally. Additional processing would include blending with soil or bulking agent to create a soil amendment and developing a distribution center. Biogas can be used to heat process buildings, generate electrical power with heat recovery, cleaned and converted to CNG for local use, or cleaned and injected into the natural gas grid. P recovery is a potential for all digestion options; however, recovery feasibility is also a function of the liquid stream process.

Arlington County places high value in products that can be used locally and recognizes the community would view Class A products more favorably. While the value of a Class A product is considered in many of the individual criteria, the project team feels the value was not fairly represented in the weighting factors. In particular, the weighting factors placed a significant emphasis on the operational criteria. The operational criteria penalized the processes that produced Class A products due to the additional processes and operating effort; however, the


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economic comparison considered additional O&M efforts in determine the annual cost of each alternative.

Comparing the two alternatives that produce a Class A biosolids product, the thermal hydrolysis pretreatment alternative scored nearly equal or more favorable to the thermal dryer alternative in each of the four categories with the key differentiator being the environmental category. Alternatives that have reduced energy intensity and carbon footprint are valued highly by Arlington County and have potential benefits to the county that lie outside the scope of this solids master plan.

As a result of these additional considerations, the thermal hydrolysis pretreatment option with anaerobic digestion is selected as the preferred alternative with this solids master plan.

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Section 12Overview of Preferred Alternative

12.1 IntroductionAs discussed in Section 11, a detailed evaluation for the four shortlisted biosolids management alternative at the Arlington County WPCP was completed. The alternative ultimately chosen as preferred is THP followed by MAD. A simple process schematic is presented in Figure 12-1. In this alternative, blended solids go through a centrifuge pre-dewatering process before THP. Part of the biogas produced by the anaerobic digestion process may be sent to the steam generator, which is used to produce the required heat and pressure for the THP process. Arlington County may also explore other utilization opportunities for biogas included electrical power production in a CHP system or export of cleaned biogas as CNG for vehicle fueling or biomethane for injection into a natural gas pipeline.

Figure 12-1. THP with MAD Process Flow

12.2 Summary of ImprovementsImplementation of thermal hydrolysis and anaerobic digestion at the WPCP will require construction of new facilities and equipment, demolition of facilities, and repurposing of existing facilities.

A summary of the major facility improvements includes:

New solids storage and blending tanks

Three cast in place concrete tanks – 200,00 gallons each

Holding and blending tanks for WAS and primary solids

Covered tanks, ventilated to odor control scrubbers

Section 12 Overview of Preferred Alternative

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Repurposing the existing dewatering building for pre-dewatering

New solids screening – three units rated for 400 gpm each – and screenings dumpsters

Replacement of existing centrifuges with 3 units rated for 2,900 lb/hr; 390 gpm each

Reusing solids cake hoppers to store dewatered cake ahead of THP

Demolition of lime storage, dosing, and mixing equipment

Replacing truck loading conveyors with dewatered cake pumps to feed thermal hydrolysis reactors

Rehabilitation of the existing HVAC and odor control scrubbers

Construction of new thermal hydrolysis process

Vendor process equipment including pulpers, reactor vessels, and flash tanks

Current layouts and costs based on three CAMBITM B-6 reactors

Construction of new anaerobic digesters

Cast in place concrete tanks with slightly sloped bottoms

Two tanks – 1,100,000 gallons each having approximate dimension of 60-ft diameter and 60-ft sidewall height

Digester mixing system – layouts and costs are based on draft tube mixers

Fixed steel covers

New Digester Building

Cooling heat exchanger(s)

Solids recirculation and transfer pumps

System controls

New steam generator building

New waste gas burner

Construction of new digested solids holding tanks

Cast-in-place concrete tanks

Two tanks – 400,000 gallons each having approximate dimension of 50-ft diameter and 28-ft sidewall height

Gas holding membrane covers for bio-gas storage


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Construction of new post-dewatering facility

Multi-story building accommodating drive-thru truck loading lane

New post-dewatering centrifuges, 3 units rated for 3,800 lb/hr each

New centrifuge feed pumps

New polymer storage, dilution, and dosing equipment

New cake storage hopper and truck loading conveyor(s)

Demolition of existing facilities

Bio-Building

DAF Building

Gravity thickeners

Solids holding tanks

12.3 Conceptual Layout of FacilitiesA conceptual layout of facilities for this alternative is presented in Figure 12-2. The existing biological solids processing building and the existing gravity thickeners would be demolished. The existing dewatering building would house the new pre-dewatering centrifuges, while the dewatering centrifuges will be housed in a new solids processing building (150-ft x 65-ft). Other new structures include the anaerobic digesters, digester building, solids holding tanks, THP equipment pad, and a steam generator building. Figures 12-3, 12-4, 12-5, and 12-6 are 3-D renderings of the proposed facilities from various vantage points.

Figure 12-3. 3-D Rendering of Facility Improvements, Looking North


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Figure 12-4. 3-D Rendering of Facility Improvements, Looking East

Figure 12-5. 3-D Rendering of Facility Improvements, Looking Southwest at THP Skid


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Figure 12-6. 3-D Rendering of Facility Improvements, Looking Southeast at THP Skid

12.4 Biogas UtilizationIn addition to the equipment and processes described, the County will continue to evaluate opportunities for biogas utilization. Opportunities identified include utilization of the biogas on-site through a combined heat and power system or cleaning and exporting the gas as a biomethane.

A combined heat and power system would include a combustion engine generator that produces electrical power for use on-site or potential metering the electrical utility. A heat recovery system for the engines and exhaust would allow heat to be captured for potential uses in building or process heating (e.g., steam generation). A biogas cleaning system is recommended for the system. The cleaning system would remove contaminants in the biogas such as hydrogen sulfide, siloxanes, and moisture that could impact engine wear and performance.

The opportunity to purify and export biogas as a biomethane may also be considered. Biomethane production involves increasing the energy content of the gas as well as removing contaminants including carbon dioxide, hydrogen sulfide, and moisture. Multiple technologies exist for producing biomethane. Opportunities to inject the biomethane into the natural gas distribution system or pipeline can be explored. The County may also consider a partnership with the Arlington Rapid Transit’s CNG (compressed natural gas) fueling station located adjacent to the WPCP.

As the County moves ahead with implementation of thermal hydrolysis and anaerobic digestion, biogas utilization opportunities can be explored.


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12.5 Sidestream TreatmentNumerous waste streams are generated from the solids handling and treatment processes at the WPCP. The waste streams are typically collected and recycled to the headworks of the facility where they are combined with influent flow and treated. Thermal hydrolysis and anaerobic digestion will affect the quantity and characteristics of the sidestreams compared to the existing WPCP processes. In particular, nutrient loadings (nitrogen and phosphorus) from the post-dewatering facility are expected to be significantly higher.

Since the WPCP does not operate at the full design and permitted capacity, the increased recycled nutrient loadings may be capable of being treated using the existing facilities. Operating adjustments to aeration and chemical dosing may be required. As influent loadings to the WPCP increase and the facility operates closer to the design capacity, additional mitigation may be required.

Treatment of the sidestreams to reduce the nutrient loadings can be implemented. Biological treatment systems have been used to reduce nitrogen loadings in streams containing high levels of ammonia, which is expected with the post-dewatering centrate. Some biological systems, such as DEMON®, utilize a shortcut nitrogen removal process to reduce energy and carbon requirements compared to more conventional nitrification/denitrification processes. The process relies on aerobic bacteria to partially oxidize ammonia to nitrite (nitritation) and oxidation of the remaining ammonia using nitrite by annamox bacteria.

Arlington County should continue to monitor the performance of the liquid stream treatment process. Construction of a sidestream treatment process may be considered with the final implementation plan.

12.6 Capital Costs of Recommended ImprovementsA summary of the capital costs for this alternative is provided in Table 12-1. The costs presented do not reflect the additional capital required for biogas utilization on-site or export as biomethane. The costs also do not reflect the capital costs for a sidestream treatment process to reduce nitrogen loading in the process recycle stream.


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Table 12-1. Capital Costs for Preferred Alternative: THP with Anaerobic Digestion







Thermal Hydrolysis $ 17,900,000

Mesophilic Digesters and Building $ 25,600,000




Allowance for Odor Control System Improvements $5,000,000


Table 12-2 presents the total project cost including a biogas cleaning system to export biomethane and sidestream treatment process for reducing nitrogen loading in the recycle streams.

Table 12-2. Capital Costs with Biogas Utilization and Sidestream Treatment


Baseline Project Capital Cost $ 95,000,000

Biomethane Cleaning System $ 10,000,000

Sidestream Treatment System $6,000,000


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Section 13Implementation Plan

The WPCP Solids Master Plan is intended to guide Arlington County in selection and implementation of a long-term solids management alternative. This section is intended to provide information on the multiple phases of the project including master planning, preliminary engineering, final design, and construction and commissioning. The recommendations from the Master Plan involve demolition of existing infrastructure as well as construction of new facilities. These improvements must be coordinated and sequenced in a manner that allows the WPCP to maintain operation throughout implementation.

13.1 Implementation ProcessThe recommendations of the WPCP Solids Master Plan described the capital facility improvements necessary to develop thermal hydrolysis pretreatment and anaerobic digestion. Engineering projects of this nature typically have multiple phases as part of the project life-cycle. The phases can generally be assigned to one of three categories: planning, engineering design, and execution.

13.1.1 Project PlanningProject planning involves both concept development and feasibility analysis. This master plan has developed the concept and provides basic direction for the project. Key decisions such as technology selection, project feasibility, and planning level engineering parameters have been identified. Many project planning efforts, including the Solids Master Plan, develop the concept to a 5 -10% level of project definition.

The next step in the process is a Facility Plan that provides additional definition to the concepts identified in the Master Plan. The Facility Plan should review each aspect of the project process-by-process. For Arlington WPCP, a Facility Plan would be used to further define the project scope of work. Additionally, the Facility Plan would confirm the basic assumptions of the Master Plan. Identification of pumping requirements, preliminary pipe routing, and preliminary development of detailed process flow schematics would be completed as part of the Facility Plan.

For Arlington WPCP, a Facility Plan also provides the opportunity to evaluate specific elements identified in the Master Plan but not fully resolved. These include:

Biogas Utilization: Multiple biogas utilization alternatives were identified in the master planning project. Additional evaluation of these alternatives can be completed as part of a Facility Plan or as a separate Biogas Utilization Study. The evaluation should include discussion with potential County and utility stakeholders including Arlington Rapid Transit (ART), Dominion Power (electric), and Washington Gas (natural gas) to solicit interest. The evaluation would also allow refinement of assumptions from the Master Plan and an update on economic projections of capital and operating costs.

Section 13 Implementation Plan

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Recycle Stream Nutrient Loading: Management of sidestreams generated from the solids handling processes was identified as part of the Master Plan. A preliminary review of the liquids stream design conditions, projected influent loadings, and projected nitrogen loadings associated with recycle of these sidestreams did not identify concerns with the liquid stream aeration treatment capacity. However, the review was limited to calculation of aeration requirements based on current performance and projected nitrogen levels. Sidestream treatment may offer an opportunity to reduce the nitrogen loading recycled to the liquids process. The Facility Plan should include a more comprehensive review of sidestream impacts and an evaluation of the benefits of sidestream treatment.

An opinion of probable construction cost will be developed for the Facility Plan. At the completion of the project planning phase, it is expected the project will be at a 15% – 20% design level.

13.1.2 Engineering DesignThe engineering design phase advances the project to a 100% complete design level. The engineering design phase typically includes multiple submittals at design milestones between 30% and 100% complete.

Preliminary design is often the first milestone delivered in the engineering design phase of the project. A design report would be prepared with the project developed to a 30% design level. The preliminary design report is often included with applications for regulatory approval of the project plans.

The preliminary design reports would contain information on the basis of design, description of processes and design criteria, equipment data tables and expected performance, site layouts, process area plans, detailed process flow and control diagrams, control narratives, and materials of construction. The report is used as the basis for design drawings and specifications prepared for the project.

Follow the preliminary design phase, submittals at major design milestones provide additional development of the design. These design submittals provide the opportunity for the design team and the owner to review progress and provide input. An opinion of probable construction cost is often prepared at each design milestone, allowing the scope of the project to remain on budget. At the 100% complete level, the documents are issued for approval to construct and for bidding under the traditional design/bid/build delivery method.

13.1.3 ExecutionThe execution phase of the project includes procurement of equipment and materials as well as construction of the facilities. The execution phase of the project also includes startup and commissioning of the improvements. Finally, the execution phase of the project includes training on operations and maintenance of equipment, performance testing of processes, and development of standard operating procedures.


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13.2 Design and Construction Delivery OptionsVirginia rules allow for multiple project delivery options that could be used for implementation of the improvements recommended in the Master Plan. A brief review of each option is provided in the following sections.

13.2.1 Design/Bid/ BuildTraditionally public utility projects have been delivered using a design/bid/build approach. Under this approach, the owner enters into a contract with an architect and/or engineer to prepare documents for construction. The documents are issued to contractors for pricing and contracts awarded to the lowest priced responsible bidder. The owner then enters into a contract with the lowest priced responsible bidder to construct the improvements.

The design/bid/build process is well understood by owners, engineers, and contractors. Through competitive sealed bidding, the owner receives a known price for the project prior to entering into the construction contract.

The design/bid/build approach requires the owner to have two different contracts for the same project – one for engineering and one for construction. It is also common for a representative of the design engineer to be engaged through the construction phase. Under the design/bid/build approach, the owner assumes the risk for changes during the construction phase. Because the engineer and the construction contractor maintain different contracts with the owner, changes and cost overruns can lead to contentious relationships between the parties looking to protect their specific contractual interests.

13.2.2 Alternative DeliveryHistorically, the design/bid/build approach has been the only project delivery alternative allowed for publicly funded projects; however, Virginia regulations do also allow for alternative delivery.

Design/build project delivery is a method where the owner enters a single contract for the duration of design and construction project. Many forms of design/build teaming arrangements exist, some are contractor led, some are designer led, some are an integrated firm, such as a joint venture. Typically, an owner selects a design/build team based on the team’s qualifications and negotiates a price based on the identified scope of work. Performance based specifications and general process requirements are included as part of the request for qualifications and are used by owners to define the minimum expectations of the project.

Virginia Administrative Code, Title 2.2, Chapter 43.1, § 2.2-4380 defines the requirements for entering into design-build contracts when state public bodies are involved. The procedures allow for a two-step competitive negotiation process. The first step is issuing a Request for Qualification and selecting a list of prequalified firms for the work (typically 2- 5 firms). The second step is issuing a Request for Proposals and evaluating the proposal.

Utilities typically contract with an owner’s representative to assist in issuing the requests and evaluating the qualifications and proposals. The owner’s representative maintains a role throughout the design/build process from developing the initial request for qualifications and


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requests for proposals to reviewing submittals and project progress to ensure compliance with the design/build contract.

Through the design/build project, the design is completed by the contractor with input from the construction team. The owner and owner’s representative will review the design for conformance with the specification and design/build contract. Due to the integrated nature of the design and construction teams, there is a potential to accelerate project delivery by overlapping certain design tasks with construction and to develop innovative design and construction solutions to reduce project costs.

The owner may issue the request for qualifications for design/build contractors between the 10% and 60% design level. At the 10% level, the owner’s requirements are typically more performance based – allowing the design/build team the opportunity to develop more efficient designs. While this may result in some savings, the owner will often have limited contractual ability to have input into design details. At the 60% level, the owner’s requirements may be viewed as overly prescriptive and the opportunities for design/build efficiencies are reduced. However, the owner will have more input into design details. A balance between these extremes is for the owner to develop a preliminary design document (approximately 30% design level) with materials and installation specification that are a part of the design/build contract. Variations of design/build including progressive design/build and construction manager at risk are options that can be considered as well.

The improvements recommended in the Solids Master Plan result is a relatively complex project for delivery. Multiple process areas are impacted and the sequencing of work must allow for maintenance of operations during the construction phase. Site constraints and access also contribute to the complexity of the project. Arlington County may elect to pursue design/ build project delivery with an owner’s representative to take advantage of the innovation, schedule and risk benefits of this delivery method.

13.3 Implementation RecommendationsWhile the opportunity for a regional solution exists, there are no agreements or formal partnerships in place. It is recommended Arlington County continue to advance the recommended Solids Master Plan projects. WPCP staff have identified numerous operating and maintenance challenges of the existing lime stabilization process. The equipment is aging and the process is not a part of the long-term biosolids management program.

The recommended improvements described in Section 12 include a combination of demolition of existing facilities, rehabilitation and repurposing of existing facilities, and construction of new facilities. These improvements can be delivered in multiple phases including:

Phase 1 – Demolition of Bio-Building

Phase 2 – Construction of Remaining Improvements including anaerobic digesters, thermal hydrolysis, new post-dewatering building and truck loading area

Demolition of the Bio-Building will develop available footprint on the project site for construction of the remaining improvements.


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The next step towards implementation of the Solids Master Plan is the completion of a Facility Plan or Preliminary Engineering Report. The Facility Plan will provide additional definition to the project. As a part of the Facility Plan, or as a separate project, a Biogas Utilization Study and a Sidestream Recycle and Treatment Study should be completed.

Arlington County has already commissioned an emissions dispersion modeling study to review the potential impacts of the project and biogas utilization on air quality near the WPCP. The dispersion modeling results can serve as a starting point for air quality permitting of the project which will include steam boilers, waste gas burner, and some form of biogas cleaning and potential on-site utilization.

The project delivery method for either or both phases can continue to be evaluated. The project is similar in the scope of improvements to DC Water’s Blue Plains MPT project that was recently delivered using the design/build delivery method.

13.4 Implementation ScheduleTable 13-1 presents a potential capital program for the implementation of the Solids Master Plan projects using a design/build delivery approach.

Engaging a program manager to act as the County’s representative for the design/build project(s) would be the first step in this process.

Table 13-1 Implementation Schedule for Solids Master Plan with Capital Expenditures

Fiscal Year 2019 2020 2021 2022 2023 2024 2025 2026

Issue RFP for Program Manager/Owner’ Rep X

Prepare Facility Plan $2M

Prepare Phase 1 Design/Build RFQ/RFP $1M

Select Phase 1 Contractor X

Complete Phase 1 Improvements $4M

Prepare Phase 2 Design/Build RFQ/RFP $2M

Select Phase 2 Contractor X

Complete Phase 2 Improvements $34M $34M $33M

Expenditures shown as 2017 Dollars

The next step towards implementation is preparing a Facility Plan. The Facility Plan is intended to further develop the recommendations of the Solids Master Plan. Key elements of the Facility Plan should include:

Confirm basis assumptions of the Solids Master Plan, including the basis of design

Develop concepts of the Solids Master Plan to a 15-20% design level including defining pumping requirements, pipe routing, electrical requirements, and development of process flow schematics


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Complete biogas utilization study that includes discussion with potential utility partners and County stakeholders and confirmation of viability of these options

Complete assessment of recycle stream nutrient loading on WPCP liquids stream and develop options for mitigation of impacts

Update opinion of probable construction cost

The Facility Plan will serve as one of the key documents in developing the scope of the design.

Following completion of the Facility Plan, preparation of the request for qualification (RFQ) and request for proposal (RFP) for design/build contractor selection can begin. The selected design build firm will develop the concept from the Facility Plan to detailed design, construction, and commissioning.

Appendix A Permits

A.1 Arlington County WPCP Discharge Permit

A.2 General Permit for Total Nitrogen and Total Phosphorus Discharges

A.3 Air Permit – Stationary Source Permit to Construct and Operate

Appendix A.1 Arlington County WPCP Discharge Permit

arlington wpcp solids master plan

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