Distribution System Optimization for Water Quality

By Brian T. Bisson, P.E.This paper is based on a collaborative effort of the Ohio

EPA and the Technology Committee and Distribution Committee of the Ohio Section of AWWA.

Paper Outline Technology Committee Synopsis Water Quality Monitoring Biofilm Control and Assessment Distribution System Piping and Storage Flushing Programs Hydraulics and Water Quality Monitoring

Technology Committee Committee Make-up Guidance Documents and White Papers Committee’s work has informed and saved capital

funds for water utilities (AWWAJ Article) Ohio EPA and Water Utility Industry working together

– Practical approach for all factions “Special purpose” samples collected for the purpose of

distribution system optimization are not required to be reported.

Water Quality Monitoring Baseline Monitoring Program

Planning (goals, resources, end user) Design (parameters, frequency, equipment, sites) Implementation (fix quality issues as they are

confirmed to be problems) Data collection (grab and on-line)

Common parameters: pH, chlorine residual, HPC, DBPs, pressure, temp, taste and odor, and also ammonia, nitrite, and nitrate for chloraminated systems

Water Quality Monitoring, continued Baseline Monitoring

Monitoring Parameter Indication Disinfectant residual Decrease = main break, biofilm, cross-connection, etc

Increase = chlorinator problems, change in valving Turbidity main break, cross connection, fire flow, flushing, flow

reversal, O & M, security breach, post precip, pump trip pH, Conductivity, Alk Corrosion control issues, cross connections, treatment issues,

breach, new cement mortar lining VOCs presence = probable cross connections Trace Metals Corrosion control problems, cross connection TOC Increase = biofilm sloughing off, cross connection

Decrease = biofilm consumption, DBP formation Water Source Conductivity, other parameters: DBPs, chlorine, fluoride,

chloride, nitrate, sulfate, sodium, potassium, hardness, magnesium, and calcium

Water Quality Monitoring, continued Baseline Monitoring, continued

Monitoring Parameter Indication Tracer Studies Fluoride can be used as an aid in determining water

age Leak investigations Chlorine, fluoride, hardness, alkalinity, pH, conductivity,

DBPs can all be used to determine if a leak is drinking water of from groundwater intrusion.

Biofilm Control and Assessment Definition: A diverse association of

microorganisms and their byproducts existing together. Biofilms are typically very sparse. Typically, the levels and composition of biofilms are not known until the biofilm begins causing problems.

Biofilm Control and Assessment Biofilm Problems

The type of organisms in a biofilm are usually not a health concern,

Not all biofilm organisms benign. Unclear the extent that pathogens can grow in biofilms, but clear that biofilms can shelter these organisms.

Biofilm growth can produce taste and odor, e.g. fungi and musty t & o, iron reducing bacteria can release sulfur compounds, and decay of dead biomass.

Biomass can produce acid and promote the formation of tubercules.

Biofilm Control and Assessment Factors that influence the growth

Seed – Good disinfection helps prevent introduction Food – High TOC has been shown to support growth Disinfectant – maintaining residual key Type of disinfectant – Chloramines Vs. free chlorine Hydraulics – Low flow tends to favor Temperature – High temp favors development/diversity Pipe condition and material – Corroded pipes favor

Biofilm Control and Assessment Biofilm control

Decrease nutrients: enhanced coagulation, activated carbon, source water protection

Nitrogen may be the limiting factor. For systems using chloramines careful control of ammonia is important. Suggest a goal of < 0.1 mg/L leaving the plant.

Corrosion control: Iron has chlorine demand. Adequate disinfectant concentration: Couple with corrosion

control and flushing program. Chloramines more sustainable. Flushing: Remove debris and bring fresh water. Only temporary

Storage Facilities In general poor mixing and turnover increases

water age, reduces disinfection residual, increases microbial counts, increases DBPs, and nitrification (in chloraminated systems)

Volume turnover: OEPA recommends 25%/day Using water quality data to evaluate tank mixing:

residuals, DBPs, bacteria counts, temperature data

Examples of poorly mixed tanks:

Temperature Free Chlorine TTHM HAA5Tank No. Top Bottom Top Bottom Top Bottom Top

Bottom 1 80 79 0.8 2.0 75 54 42 20 2 78 78 0.2 1.8 78 58 35 41 3 81 78 0.0 1.9 74 56 12 44 4 81 80 0.0 1.7 66 69 25 51 5 81 78 0.0 1.9 74 50 22 47

Storage Facilities Inlet Momentum

Storage Facilities Inlet Location

Storage Facilities Poorly Mixed Tank

Storage Facilities Well-Mixed Tank

Storage Facilities Other Tank Mixing Comments

Computational Fluid Modeling (CFD) Qualitative visual image (AWWARF CFD package)

Avoid Baffling Increased water age, less residual, greater DBPs

Excess Storage Historically built for hydraulics Oversized or hydraulically submerged Distribution system analysis to ensure storage needs met

Distribution Piping Pipe looping: dead zones; cul-de-sacs Managing valves Blow-offs

Photo courtesy of Hydro-Guard International.

Distribution PipingPipe Material Cast Iron

Ductile Iron (cement mortar-lined)

Potential Water Quality Impacts May exert higher disinfection demand Loss of disinfection residual Increased DBP’s from higher chlorine dose to

overcome higher demand Color (red water) Taste and odor Increased microbial activity Nitrification for chloraminated systems Lack of quality control may lead to increased metals

concentration, e.g. barium, cadmium, chromium, or aluminum

Distribution PipingPipe Material Asbestos-Cement Prestressed

Concrete Cylinder Lead Copper

Galvanized Plastic (HDPE)

Potential Water Quality Impacts Inc asbestos, barium, cadmium, chromium, or Aluminum Leaching of calcium in non-stable waters Lack of quality control may lead to increased metals Increased tap lead under certain conditions Increased tap copper under certain conditions Microbial-influenced corrosion under certain conditions Pitting corrosion may result in home plumbing failures Increased zinc, iron, lead, copper, cadmium, and others Possible leaching of VOC’s from surrounding soils

Distribution Piping System Expansion Alternatives

Smaller Planning Horizons – Build-out Vs. 5 to 10 yrs Dual storage tanks Smaller mains – Capital and O & M impacts

Flushing Program Objectives:

To remove impurities: Accumulated, new and repaired mains, complaints, hazardous

To reduce: bacteria concentrations, chemical contamination, To increase chlorine residual To eliminate taste and odors To remove discolored water To reduce turbidity To remove accumulated sediment To respond to customer complaints To maintain the life of the mains

Flushing Program Data Collection and Monitoring

Complaint code Pressure in surrounding mains >20 psi Records of color, clarity, turbidity, DO, pH, and temp Chlorine residual at start, middle, and end of flushing Visual clarity and time to clear Lab results for samples collected Location and time of maintenance work

Flushing Program Flushing Process

Flushing plan: from source toward periphery Flush one short section at a time to maintain > 20 psi Consider flushing at night Flushing velocities: Min of 2.5 fps; Goal for 8-inch and

smaller mains of 5 to 7 fps Do not try to flush large dia mains supplied by a small

dia main Notify all customers (hospitals and laundries)

Flushing Program Unidirectional Flushing

Proper Planning - AWWARF Report: “Development of Distribution System Water Quality Optimization Plans”

Identify Target area (within one pressure zone) Gather Data (water source, infrastructure, critical

customers) Program layout: single clean water source; < 1,000’; possible

exclusion of segments > 4 fps; do not extend past: change in pipe size, large unclosed branch, intersection connecting unflushed segments; delineate and sequence additional segments

Hydraulics Hydraulic “Surges” or “Water Hammer” or “Transients”

Use of high speed pressure monitors to identify transients

Disrupt pipe scales, biofilms, and sediments leading to taste and odor, color, or other customer complaints

Negative pressures can create backflow Eliminating water hammer: pump soft starts, VFD’s,

controlled closing of valves, pres-reducing valves, air-release valves, and other system controls

Water Quality Modeling EPANET: http://www.epa.gov/nrmrl/wswrd/dw/epanet.html

Modeling Basics:

Demand node for customers

Connection to other system

Water Quality Modeling Types of models

Skeletonized Vs. All Pipe Skeletonized: transmission and some distribution mains Skeletonized for master planning and fire flow testing

Steady-state Vs. Extended Period Simulation (EPS) EPS essential for water quality analysis

Hydraulic Vs. Water Quality Need a calibrated hydraulic model for an accurate water

quality model

Water Quality Modeling Model Applications

Master Planning – quantity and quality Regulations – Residuals and DBPs: predictions of non-

conservative parameters requires extensive model calibration and validation

Security – sensor location, contaminant tracing, contaminant containment

Customer complaints – flushing plans

Conclusions Distribution system impacts water quality Distribution system regulations increasing Evaluate impacts and make physical or operational

improvements to minimize degradation. Get to know your system Baseline data to determine if conditions are unusual Be proactive – don’t wait for an “event” to investigate

water quality issues.

Questions ?