2008 wef manual-alternative sewer systems - with bookmarks

ALTERNATIVESEWER SYSTEMS

Prepared by Alternative Sewer Systems Task Force of theWaterEnvironment Federation

James Kreissl, Chair

Glen R. Behrend, P.E.Daniel G. Burden, Ph.D., P.E.Bruce CorwinGlendon D. Deal, P.E.Douglas K. HammanDavid V. Linahan, P.E.

Pio Lombardo, P.E.,DEEJohn C. MowryRich Naret, P.E.Peter L. Stump, P.E.Thomas M. WalskiMichel Wanna

Under the Direction of the Collection Systems Subcommittee of the TechnicalPractice Committee

2008

Water Environment Federation601 Wythe StreetAlexandria, VA 22314-1994 USAhttp://www.wef.org

http://www.wef.org

ALTERNATIVESEWER SYSTEMS

WEF Manual of Practice No. FD-12Second Edition

Prepared by Alternative Sewer Systems Task Forceof the Water Environment Federation

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About WEF

Formed in 1928, the Water Environment Federation (WEF) is a not-for-profit technicaland educational organization with more than 34,000 individual members and 81 affil-iated Member Associations representing an additional 50,000 water quality profes-sionals throughout the world. WEF and its member associations proudly work toachieve our mission of preserving and enhancing the global water environment.

For information on membership, publications, and conferences, contact

Water Environment Federation601 Wythe StreetAlexandria, VA22314-1994 USA(703) 684-2400http://www.wef.org

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Manuals of Practice of the Water Environment FederationThe WEF Technical Practice Committee (formerly the Committee on Sewage andIndustrial Wastes Practice of the Federation of Sewage and Industrial Wastes Associ-ations) was created by the Federation Board of Control on October 11, 1941. The pri-mary function of the Committee is to originate and produce, through appropriatesubcommittees, special publications dealing with technical aspects of the broad inter-ests of the Federation. These publications are intended to provide background infor-mation through a review of technical practices and detailed procedures that researchand experience have shown to be functional and practical.

Water Environment FederationTechnical Practice CommitteeControl Group

B. G. Jones, ChairJ. A Brown, Vice-Chair

A BabatolaL. W. CassonK.D.ConwayA EksterR. FernandezS. InnerebnerS.S.JeyanayagamR. C. JohnsonE. P. RothsteinA T.SandyAK. UmbleT. O. WilliamsJ. Witherspoon

vi

Contents

Preface xxiList of Figures xxiiiList of Tables xxv

Chapter 1 Overview of WastewaterCollection Systems

Introduction 1History 1Approach 4Commonalities 5Evaluation Issues 7Perspective 11

Overview of Specific Alternative Sewers and Their Applications 11Pressure Sewers 11Vacuum Systems 15Effluent Sewers 21

House Connections 24Interceptor Tanks (Septic Tanks) 24Service Laterals 25

Collector Mains 25

Cleanouis, Manholes, and Vents 25

Lift Stations 25

Evaluation of Alternative Collection Systems and ConventionalCollection Alternatives 27

Population Density 27Growth Inducement 27

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viii Contents

Ground Slopes 28Subsurface Problems 28Discharge to Gravity Sewers .29

References , 31

Chapter 2 Pressure Sewer SystemsIntroduction 35

Background 35History 35

Pressure Sewer Collection System Components 37Isolation Valves 37Cleanouts 37Air-Release Valves 38Pipe Fittings 39Appurtenances 40

Detailed Description of On-Site Pump System Components 42Available Systems 42Detailed Descriptions 42Materials of Construction 44

System Design Considerations .45Hydraulics 45

Design Flows and Their Variability 45

Average Daily Flows .45

Peak Flows from Homes and Required Pumping Rates .46

Design Flows 47Probability Method .48

Rational Method .48

Minimum Flow Velocities in Pipes 49

Applicable Equations 50

Pipelines , 51Mainlines 51

Geometry 51

Contents ix

Pipe Sizing 51Routing " '" 51Trench Section 54

Pipe Materials 54

Appurtenances 55Service Lines 56

Service Line Valves 56

Check Valves 57

Service Line Installation 59

Service Line Pipe Sizing 59

Building Sewers-Building Sewer Appurtenances 60

Pump Systems 61Pump Vault 61

Pump Vault Accessories 64

Vault Cover 64

Pump Discharge Piping 64

Valves 64

Pump Disconnects 65

Pumps/Electrical Service 67Pumps 67

General " .. , '" , , 67Centrifugal Pumps 68

Progressing Cavity Pumps .70

Electric Motors .74

Controls 74

Odors and Corrosion 80Construction Considerations 80Equipment Substitutions 83Scheduling/VVarranties " 83

Operation and Maintenance Considerations 83Operation and Maintenance Manual 83Stafiing~equirements 84

x Contents

Operator Training 85Spare Parts Inventory 86

Maintenance 86Normal 86Grinder-Pump Maintenance 86

Electrical-Related Issues 86Initial Operation 86Ongoing Electrical Problems 87

Pump-Related Issues 87Pump- Vault-Related Issues 88Piping-Related Issues 88

Preventative Maintenance 88Emergency 89Recordkeeping 89Troubleshooting 89

System Costs 90Construction 90

Pipeline Installation 90Grinder-Pump Installation 91

Operation and Maintenance 93User Charges and Assessments 93Systems Management Considerations 94Homeowner Responsibilities 94Sewer Utility Responsibilities 95

References 96

Chapter 3 Vacuum Sewer SystemsIntroduction 101

Overview 101Theory of Operation 102Applicability and Advantages 105

Applicability 105Advantages 105

Manufacturers Active in the United States Market 106

Contents xi

AIRVAC 106Roediger (Roevac) 107Iseki 107

Extent of Use in the United States 109Projects in the United States 109Summary 109

System Plan and Elevation View 110Description of System Components 110

Valve Pits 110Vacuum Mains 110Vacuum Station 114

System Design Considerations 117Hydraulics 117

Design Flows and Velocity 117

Vacuum Transport and Air-Liquid Ratios 117

Sawtooth Profile Design Concept 118Air-Lsquid Ratios 118

Situation to Avoid-Using Vacuum as an Interceptor Sewer 119

Odors and Corrosion 120Static and Friction Losses 121Situations to Avoid 123Effects of Water Conservation 123

Mains and Service Lines 123Definition of Terms 123Mains 124

Geometry and Sizing 124Situation to Avoid-Long Distances with No Connections 125Design Considerations 125Minimizing Static Loss 126Recommended Flowrates 127Routing 128Pipe Materials 129Technology Improvement-a-Ring Gasket Pipe for Vacuum Use 129

Service Laterals (Valve Pit to Vacuum Main) 129

xii Contents

Valve Pit Settings and Building Sewers 130Definition of Terms 130Technology Improvement-Integral Valve Pit 131Valve Pits 132

House-to-Pit-Sharing Ratio 132Fiberglass Settings 132Buffer Tanks 132Situation to Avoid-Overuse of Buffer Tanks 135

Stub-Outs 135Building Sewers 137Backwater Valves 137Air-Intake 138Appurtenances 139

Vacuum Valves 139Vacuum Valve 139Controller/Sensor 140Breathers 141

External Breather 141In-Sump Breather-Recent Systems 142

Appurtenances 143Cycle Counter 143Electronic Air Admission Control 143

Division Valves and Cleanouts 144Division Valves 144Cleanouts 144

Vacuum Station Design 144Discharge Pumps 144

Materials 144Sizing 146

Collection Tank 147Materials 147Sizing 148

Contents xiii

Vacuum Pumps 149Materials 149Sizing .. " 150

Emergency (Standby) Generator 150Station Piping 151Electrical Controls 151

Control Panel 151Motor Control Center 152

Level Controls 153Monitoring System 153Vacuum Gauges 155Vacuum Recorder 155Sump Valve 155Odor Control 155Noise and Heat Considerations 156

Construction Considerations 156Vacuum Main Construction 156

Line Changes 157Grade Control 158Horizontal Directional Drilling 158Vacuum Testing During Construction 159

Valve Pit Installation 160Pit Installation 160

Pit Location 160Pit Orientation 160Pit Installation 160

Connecting the Pit to the Main 162Sump Tightness Test , 163

Vacuum Station Construction 163Final Vacuum Testing and System Startup 163

Vacuum Station Startup 163Collection System Startup 163Vacuum Testing Summary 164

xiv Contents

Construction Inspection 165General 165Duties 165

Vacuum Main Installation 165Valve Pit Installation 166Vacuum Station Construction 166

Record Drawings 166Bidding Issues-Procurement Options 167

Operation and Maintenance Considerations 167Staffing Requirements 167Operator Training 168Key to Successful Operation-The System Approach 168Maintenance 169

Normal and Preventative Maintenance 169Vacuum Station 169Collection System Piping 170Vacuum Valves 171

Emergency Maintenance 172Vacuum Station 172Collection System Piping 172Vacuum Valves 173

Spare Parts Inventory 173Valves and Valve Pits 173Vacuum Station 174Special Tools 175

Operation and Maintenance Manual 175Recordkeeping 176

NormaIMaintenanceRecords 177Prevenattive Maintenance Records 177Emergency Maintenance Records 177Operating Cost Records 178

Contents xv

Evaluation of Operating Systems 178Operating History of Vacuum Sewers " 178Operation and Maintenance Data: 2003 Operator Survey 179

Labor 179Power 181

Mean Time Between Service Calls 182Historical Problems 182Situation to Avoid-Accepting Flow from an ExistingGravity System 184

System Costs 184Constructions Costs 184

General 184Vacuum Mains 185Valve Pits 186Vacuum Stations 186

Operation and Maintenance Costs 187General 187

Operation and Maintenance Information from 1991 U.S. EPA Manual 189Basis of Operation and Maintenance Estimating Charts 189

Labor , .. 189Effort to Operate a System-Actual versus Billable Time 190Power " " '" ., 191Utilities 191Clerical 192Transportation 192Supplies/Maintenance 192Miscellaneous Expenses 192Equipment Reconditioning and Replacement 192

System Management Considerations 193Sewer Authority Responsibilities 193

Customer Connection to System 193Operating Personnel 193Sewer Use Ordinance 194Private versus Public Ownership of Equipment Serving House 195

xvi Contents

Homeowner Responsibilities 195Other Entities 197

Engineer 198Regulatory Agencies 198

Education Process 198Sample Regulations for Vacuum Systems 199

Requirements for Design of Vacuum Collection System 200General Requirements 200Design Requirements for the Vacuum Collection System 201Valve Pit Requirements 202Buffer Tank Requirements 203Vacuum Valve Requirements 204Relation to Water Mains 205Vacuum Collection System Leak Testing 205

Design of Vacuum Stations 205Vacuum Station General Design Requirements 205Vacuum Station Component Sizing 206Vacuum Pump Sizing 207Instrumentation and Controls 208Safety Ventilation of the Vacuum Pumping Station 208Miscellaneous Requirements 208

Design of Wastewater Transmission (Force Main) System 209Customer Connections 209

References " '" 210

Chapter4 Effluent SewersIntroduction 215Effluent Sewer Comparisons 216Myths and Misperceptions 218Description of Effluent Sewers 221Comparison of Septic Tank Effluent Pumping with Septic TankEffluent Gravity System 223

Contents xvii

System Components 224Building Sewers 225Interceptor Tanks 225Service Laterals 227Collector Mains 228Manholes and Cleanouts 228Isolation and Check Valves 229Air-Release Valves and Vents 229Lift Stations 229

Hydraulic Design Considerations 230Design Flow Estimates 232Design Flow Calculation 234Commercial and Industrial Flows 235Infiltration and Inflow 235Flow Velocities 238Hydraulic Equations .238Hydraulic Grade Line Analysis 240

System Components Design 243Building Sewers 243Interceptor Tanks 243

Location 243

Design 244Materials of Construction 246

Service Laterals 246Collector Mains 248

Alignment and Grade 248Hydraulic Design 250Depth. 250Pipe Diameter 251Pipe Material 252Cleanouts 253Air-Release Valves 254

xviii Contents

Odors and Corrosion 255Other Considerations 256

Coordination with Other Utilities 256Power Outages 258Freezing 258Antiflotation Precautions .259Hydrogen Sulfide Control Measures 259

Regulatory Acceptance 260Public Financing 260Construction Considerations .261

Construction Documents 261Lot Facility Plans 262Site Restoration 264Easements 264MainlineConstruction 265

Typical Trench Section 265

Excavation Methods 268

Connections 269

Construction Costs 269Operation and Maintenance Considerations 270

Administration 270Operation and Maintenance Manual 271Staff and Equipment Requirements 272Operator Training 273Spare Parts Inventory 273As-Built Drawings 274Maintenance 274

Interceptor Tanks 274

Collector Mains 276

Obstructions 276

Contents xix

Recordkeeping 277Troubleshooting 277

Odors 277Corrosion 278

Infiltration and Inflow 278

System Costs 279Construction Costs 279Operation and Maintenance Costs 281User Charges and Assessments 282

System Management Considerations 282User Responsibilities 282Responsible Management Entity Responsibilities 282

Example Projects/Case Studies 283Dexter, Oregon, Septic Tank Effluent Gravity System 283Glide, Oregon, Septic Tank Effluent Pumping System 284Lapine, Oregon, Septic Tank Effluent Gravity System 285

Generic Effluent Sewer Design Example 286Basic Steps 286Design Steps 291Critical Sections in Small-Diameter Gravity Sewer Design .294

Step Design Example 295References 298

Index 301

Preface

This second edition of this Manual of Practice was produced under the direction ofJames Kreissl, Chair. The principal authors of this Manual of Practice are as follows:

Chapter 1Chapter 2Chapter 3Chapter 4

James KreisslDavid V. Linahan, PE.Rich Naret, P.E.James Kreissl and Pio Lombardo, P.E., DEE

The purpose of this document is to provide an up-to-date source of informationon alternative collection systems (ACS). It is aimed both at practicing engineers andplanners, with material that can be absorbed by local officials who make decisionsabout infrastructure investments. The state of development of ACS technologies issufficiently advanced to support an overview document of this nature.

The timing for this document is particularly acute in light of the recent recogni-tion by the U.S. Environmental Protection Agency and WEF of the value of decentral-ized technologies and distributed wastewater management in servicing less-urbanareas with affordable wastewater infrastructure. Alternative collection system tech-nologies are an integral part of cluster or neighborhood systems that can affordablyservice new development and older communities that had heretofore been served byseptic systems that have reached the end of their service lives.

One significant change from previous texts on this topic is the combining of smalldiameter gravity sewers or septic tank effluent gravity (STEG) systems with septictank effluent pressure (STEP) collection into a single category of "effluent sewers"because both collect effluent from onsite interceptor (or septic) tanks rather than raw

xxi

xxii Preface

wastewater and these systems typically exhibit some of each technology. In STEP sys-tems, higher elevation structures are often connected with STEG facilities; whereas,with STEG systems, lower elevation structures are connected with STEP facilities.The resulting document should help convey to the engineering, planning, and deci-sion-making readers the inherent value of these systems and the most appropriateconditions for their application as part of the overall infrastructure investment bylocal governments and developers.

Authors' and reviewers' efforts were supported by the following organizations:

AIRVAC, Rochester, IndianaCDM, Sacramento, CaliforniaCTE, Kansas City, MissouriEckler Engineering, Coral Springs, FloridaGeorgia Department of Natural Resources, Atlanta, GeorgiaHazen & Sawyer, P.C, Boca Raton, FloridaKLH Engineers, Pittsburgh, PennsylvaniaLombardo Associates, Chestnut Hill, MassachusettsMacon Water Authority, Macon, GeorgiaYerkes Associates, Inc., West Chester, Pennsylvania

List of Figures

Figure Page

1.1 Grinder-pump system 131.2 AIRVACvacuum sewer system 151.3 Upgrade/level! downgrade transport 171.4 Diagram of a typical vacuum station 181.5 States with vacuum systems 201.6 The STEPsystem 221.7 Schematic of a SDG system. . 232.1 "Converted sewer system" 362.2 Typical service lateral. . 382.3 Cleanout assembly 392.4 Various cleanout configurations. . 402.5 Air valve chamber 412.6 Soilbed for odor absorption 572.7 Typical service lateral 582.8 PVC swing check valve 592.9 Effectivepump curve 602.10 Typical simplex grinder-pump package using slide-away coupling and

guide rails 632.11 Fiberglass cover 642.12 Ball-type check valve 652.13 Slide-away coupling 662.14 Pump guide rail 662.15 Typical centrifugal grinder-pump package with c-channel guide rail system 672.16 Typical centrifugal pump curve 682.17 Head-discharge curve for multiple pumps 692.18 Effectivepump curve 692.19 Rotor and cutaway stator of a progressing cavity-type pump 702.20 Basiccomponents of a progressing cavity-type grinder pump 722.21 A typical head-discharge (H-Q) curve for a progressing-cavity pump 732.22 Single-action float switch "typical" installation 752.23 Wide-angle float switch 752.24 Simplex control panel schematic 77

xxiii

xxiv List of Figures

2.25 Activated cabon filter 812.26 Example lot facility plan , , 823.1 Typicallayout of a vacuum sewer system 1033.2 Similarities between a water system and vacuum system , 1043.3 Earlier design concept-reformer pocket , , 1043.4 Current design concept-sawtooth profile 1043.5 Profile view of typical vacuum sewer line 1113.6 Plan and profile of a typical valve pit , 1123.7 House/pit/main relationship , , , .. , 1133.8 Typical piping diagram-division valve/lift , , 1143.9 Typical vacuum station 1153.10 Vacuum lift capability , 1213.11 Station loss determination 1223.12 Major components of a vacuum system 1243.13 Over-the-top service connection 1303.14 Major components of a valve pit 1313.15 Typicalfiberglass valve pit setting 1333.16 Concrete buffer tank 1343.17 Concrete dual buffer tank 1363.18 Air-intake 1383.19 Early system external breather 1413.20 Recent system in-sump breather 1423.21 Division valve with optional gauge tap 1453.22 NPSHa calculation with typical values 1473.23 Typical elevations of level control probes 1543.24 Typical configuration for gravity connections 1643.25 Vacuum station economy of scale 1883.26 Valvepit installation with home connection 1964.1 Three typical effluent screen devices 2264.2 Typical interceptor/pressurization tank 2274.3 Typical STEPcleanout structure 2284.4 Automatic stainless-steel air-release valve 2304.5 Hydraulic grade line analysis for STEPpipe sizing 2424.6 Lakeshore system 2494.7 Individuallotfacility plan 2634.8 Example centerline easement 2664.9 Simple trench section installation 2674.10 Trenching with a directional drill 2674.11 Example plan and profile for a system analysis plan 2874.12 Effluent drain profile for example design 2924.13 Example pressure sewer design 296

List of Tables

Table Page

1.12.12.23.13.23.33.4

3.53.63.73.83.93.103.113.123.133.143.153.163.173.183.193.203.213.223.233.243.253.263.273.283.29

Quick comparison of collection alternatives 30Maximum probable number of grinder pump cores operating daily 48Pipe size estimation 51Number of operating systems in the United States 108Vacuum sewers: extent of use in the United States 109Recommended maximum design flowrates for valve pits 119Recommended lift height 122Recommended and absolute maximum flowrates for various pipe sizes 127Maximum number of houses served for various pipe sizes . . . .. . 127Discharge pump NPSH calculation nomenclature . 148The A factorfor use in vacuum pump sizing 150Requirements for using HDD to install vacuum sewers 159Valve pit installation tasks 162Vacuum station startup tasks . 164Collection system startup tasks. . . . . . . . . . . . . . . .. . 164Vacuum testing 165Normal vacuum system operation and maintenance tasks and frequencies 170Other preventative maintenance tasks and frequencies 171Time requirements for rebuild tasks 172Spare parts list per every 100 system valves 174Vacuum station spare parts 175Specialty tools and equipment 1752003 operator survey 180Labor: vacuum station 180Labor: vacuum mains 181Labor: vacuum valves 181Power consumption 181MTBSC trend 182Summary of historical problems 183Installed unit costs (4th quarter 2006) for vacuum sewer mains 185Installed unit costs (4th quarter 2006) for valve pits and appurtenances 185Installed unit costs (4th quarter 2006) for vacuum stations 186

xxv

xxvi List of Tables

3.30 General cost comparison of vacuum mains with pressure andgravity sewer mains 187

3.31 Labor estimating factors 1903.32 Vacuum station power consumption estimating factors 1913.33 Typical reconditioning and replacement costs for major equipment 1933.34 Typical rebuilding costs for valves and controllers 1933.35 Normal sequence for connection 1943.36 Potential problems if valve is installed during construction phase 1973.37 Lift height guidelines 2023.38 Vacuum station design nomenclature 2074.1 Comparison of advantages and disadvantages of effluent sewers with

conventional gravity sewers 2174.2 Comparison of typical septic tank materials 2474.3 Comparison of commonly used pipe materials 2274.4 Odor-control methods and applications 2284.5 Approximate construction costs 2704.6 Typical operation and maintenance duties and frequencies 2754.7 Hydraulic computations for SDG sewer design example 293

Chapter 1

Overview of WastevvaterCollection System.s

Introduction 1 Service Laterals 25

History 1 Collector Mains 25

Approach 4 Cleanouis, Manholes, andCommonalities 5 Vents 25

Evaluation Issues 7 Lift Stations 25

Perspective 11 Evaluation of Alternative Collection

Overview of Specific Alternative Systems and Conventional

Sewers and Their Applications 11Collection Alternatives 27

Pressure Sewers 11Population Density 27

Vacuum Systems 15 Growth Inducement 27

Effluent Sewers 21 Ground Slopes 28

House Connections 24 Subsurface Problems 28

Interceptor Tanks Discharge to Gravity Sewers 29

(Septic Tanks) 24 References 31

INTRODUCTION

HISTORY. In the late 1960s, the cost of conventional gravity collection systems insmaller communities was found to dwarf the cost of treatment and disposal. The cap-ital cost of conventional collection systems was averaging almost four times the costof treatment, and operation and maintenance costs followed a similar patternbecause of the greater number of lift stations required per unit length of pipe, owing

1

2 Alternative Sewer Systems

to the increased lengths of pipe needed to service these less densely populated areas.In response to these problems, efforts were initiated by the public and private sectorsto develop low-cost collection systems that could serve the needs of rural communi-ties, which constituted over 80%of the communities in need of wastewater collectionand treatment upgrades, and specific subareas of larger cities.

At that time, there were only two wastewater options available to rural popula-tions (conventional sewers and centralized wastewater treatment or unmanagedindividual home septic systems). The problem was approached from two directions;first, intensified research was initiated to develop and evaluate on-site wastewatersystems that could overcome certain site and soil limitations; second, new waste-water collection approaches that were less costly were also being developed andevaluated. Today, this array of adequately managed low-cost sewers conveyingwastewater to distributed treatment systems and various alternative and advancedon-site systems are known collectively as decentralized wastewater technologies. Thesetechnologies are being applied more frequently in smaller to mid-sized municipali-ties and in new land development projects. They are also being used to service por-tions of what might otherwise be known as a centralized wastewater system. This appli-cation of multiple systems under a single management entity is called distributedwastewater management. In this manual, both types of applications are addressed.

Pressure sewers were an outcome of a study in the late 1960sof separating com-bined sewers by suspending a pressurized sanitary sewer pipe inside of an existingcombined sewer pipe. Vacuum sewers have been around since the late 19th century,but had not been seriously considered for Widespread use until the same post-WorldWar II period. Small-diameter gravity sewers also found 19th century roots in theUnited States, but the principles had been allbut forgotten in the rush to codify urbancivil engineering technology. These systems returned to the United States in the 1970sfrom Australia, where they had been used successfully for more than a decade.

Initial demonstration projects for these technologies were funded by the u.s.Environmental Protection Agency (Ll.S, EPA) (Washington, D.C.) and its predecessoragencies and the Farmers Home Administration (now the Rural Utilities Service ofthe U.S. Department of Agriculture, Washington, D.C.). Because of the promiseshown by these technologies, they were given special status for increased federal costsharing under the innovative and alternative technology provisions of the CleanWater Act of 1977. Thus stimulated, these technologies have mostly flourished insmall communities that were able to secure grants under this program. However,they have also been used to address problematic sections of centrally sewered larger

Overview of Wastewater Collection Systems 3

cities. More than 500 alternative collection systems (ACSs) were installed under theinnovative and alternative technology provisions, and a significant number have alsobeen constructed with state, local, and private funding since that time.

The federal Construction Grants Program was terminated and evolved into theState Revolving Fund Program as the 1990s began. Although the inherent value ofthese systems in small communities and certain sections of larger urban areas wasrecognized by several practitioners, the loss of most available grant funding and theslow rate of technology incorporation to engineering education programs caused aslowing in the rate of ACS applications. However, where communities had the fore-sight to compare the present worth of these technologies with conventional seweragewithout federal grants, the economic advantages were generally obvious. The pri-mary disincentives were the lack of familiarity by engineers with these technologies,public perception of these systems as "temporary substitutes for a real sewer", and apaucity of grant funding for communities willing to adopt these systems. However,in 1997,U.S. EPAissued a report entitled Response to Congress on Use of DecentralizedWastewater Treatment Systems (U.S.EPA, 1997).This report was directed by Congressin response to the same concerns over the high cost of conventional sewerage to solvewastewater problems and the efforts of the rural electric cooperatives and other inter-ested parties to undertake management of rural wastewater systems to better serverural America's needs.

The report clearly stated the value of the decentralized approach in rural andperi-urban areas to make the necessary improvements in wastewater infrastructuremore affordable. It added the emphasis that these technologies must be properlymanaged to provide the level of performance required. The decentralized approachto wastewater management was described to include on-site systems, cluster sys-tems, and mixed solutions under a competent and empowered centralized manage-ment program, similar to what has historically been necessary for conventional sewersystems. More recently, the term distributed wastewater management has been adoptedto better describe the need for centralized management of all wastewater (and water-related) systems.

This report stimulated new federal programs, both within U.S. EPA and throughdirected Congressional funding, to identify, investigate, and minimize the barriers tofull consideration of this approach. The Water Environment Federation(r) (WEF)(Alexandria, VIrginia) has been involved in several of these efforts, and the creationof this document is part of those activities. Some noteworthy examples of barriersinclude state and local codified rules that prohibit the use of any collection tech-


nology other than conventional gravity sewers by local agencies; engineering consul-tants that steer clients away from ACSs through ultraconservative cost estimating,owing to their unfamiliarity with the technology; and federal and state funding pro-grams, whose rules penalize use of any unconventional technology by applicants.

APPROACH. In developing this design manual, several approaches were consid-ered. The existing U.S. EPA design manual, entitled Alternative Wastewater CollectionSystems (U.S.EPA, 1991),was more than 10 years old, but still contained much of therelevant information. However, it needed to be updated. The WEF Manual of Prac-tice FD-12, entitled Alternative Sewer Systems (WPCF,1986),was more than 15 yearsold and had little in the way of performance and management information, given thestatus of technology information at that time.

The course chosen was to use these documents and provide the necessaryupdating of design practice, field performance, and other important managementinformation to create a document that could readily be used by the profession. Sev-eral individuals with extensive practical experience in each of the major technolog-ical categories were contacted to take the lead in drafting the sections on pressure,vacuum, and effluent sewer systems. Several other experts, including those withextensive experience with these low-cost alternative collection systems and thosewho deal with conventional collection only, agreed to provide review comments andassistance in the development of this document.

One other major formatting change is noteworthy. Septic tank effluent pumping(STEP)sewers were moved from the pressure sewer chapter of the existing U.S. EPAand WEF manuals to an "Effluent Sewers" chapter (Chapter 4), with the septic tankeffluent gravity (STEG) or small-diameter gravity (SDG) concept, which had for-merly been discussed separately. Because the two septic tank effluent collectionmethods are commonly combined in most existing systems, this approach wasdeemed to be more appropriate. Thus, the pressure sewer chapter now discussesonly the grinder pump approach.

The chapter authors have extensive personal and professional experience with thesubject systems. The decision was made to concentrate more specifically on contem-porary United States practices. Also, certain details are not fully discussed, owing tothe need for readability and utility. Examples of topics that are not discussed includeflat-grade sewers, which have historically been used almost exclusively in Nebraska,and simplified sewer systems, which are used in Brazil, Ghana, and other countries.


Those seeking additional information on specific topical areas are referred towebsites provided by U'S. EPA and the National Small Flows Clearinghouse in Mor-gantown, West Virginia, and to the references cited in this manual.

COMMONALITIES. Although each alternative collection technology uses dif-ferent motive forces (gravity, pressure, and vacuum) to move wastewater from itssources to its destinations, there are many commonalities in their characteristics. Forexample, all of them use lightweight plastic pipe buried at more shallow depths incomparison with heavier, deeply buried pipes in conventional gravity sewers. Thesepipes come in much longer lengths than typical gravity sewer pipes. The resultingreduction in the number of pipe joints, combined with higher placement in the soilprofile, should minimize or eliminate infiltration. The elimination/minimization ofmanholes and lift stations further enhances infiltration and inflow reduction andreductions in capital and operation and maintenance costs.

Each type of ACS can reduce capital investment from that required by conven-tional systems, in most cases-except for densely populated urban areas. When prop-erly designed and installed in rural and suburban areas, their inherent advantagescan be exploited. They have essentially the same wastewater collection (performance)capabilities as conventional systems for wastewater management, so they can also beused as adjuncts to conventional systems to serve areas that are difficult and expen-sive to serve with conventional systems. This must always be verified through pre-sent worth or life-cycle analyses of each design alternative. Their inherent capabilityto reduce community disruption and duration during construction and to facilitatecluster dispersal, and reuse, with associated reduction in catastrophic failure and riskto public health and water quality, should make their consideration a normal part ofwastewater facilities planning.

Because of their relative newness, all ACS types have suffered from some casesof misdesign, misinstallation, mismanagement, misuse, and misapplication, as haveall new technologies. Because they are still not generally included in mainstreamengineering education, they are still dismissed by engineers who are fearful of newconcepts and operator/managers who have had no management training specific tothese systems. The purpose of this manual is to provide enough information to helpovercome these concerns and facilitate consideration of these technologies to solveexisting wastewater problems and reduce the cost of wastewater management fornew developments.


A common need of all conventional and alternative collection systems is propermanagement. Because the specific needs of each of these technologies are somewhatdifferent from conventional sewers, operation and maintenance staff members mustbe properly trained in the particular operation and maintenance needs of the type ofsystem used. Depending on the design approach and local conditions, the costs asso-ciated with those needs can be higher or lower than for conventional systems. Theoperation and maintenance costs, effects, and reliability of the entire system (collec-tion, treatment, monitoring, and residuals management) should be evaluated in theplanning of wastewater management systems to ensure that the most appropriatesolutions are chosen.

A common advantage of all ACS types is the shallow burial depth, which cansignificantly decrease construction costs and infiltration and inflow compared withconventional collection systems. This characteristic can also increase the potentialfor damage from the ground surface (i.e., subsequent excavation projects). Goodmanagement and design can minimize this problem by inclusion of marking tapeand toning wires in the pipeline trench and surface markers, which direct excava-tors to the operation and maintenance staff for assistance in locating facilities. Localservices are often available that include all buried (often called one call or no dig)utilities and offer information on location and depth of underground infrastructurecomponents (typically in a computer-aided design or geographic informationsystem format). Upon installation completion, production of quality as-built draw-ings and entry of these data into these databases are highly reco:rrunended for all ofthese systems.

Another cost advantage is the lack of regularly spaced (approximately every 100meters or so [every few hundred feet]) and costly manholes that are typically codi-fied as necessary every 90 m (300 ft) or less for conventional sewers. Because ACSsystems allow some horizontal alignment flexibility during construction, far fewermainline access points are required, and those are typically housed in shallow andless-costly structures (i.e., meter boxes). More significant is the lack of lift stationsrequired in conventional sewers. In the conventional engineering approach, all thewastewater is conveyed to a single treatment plant. For example, if several ridges arelocated between sources and the treatment facility, lift stations are required for each,in addition to additional lift stations that may be required, because the requireddepth of excavation becomes too great for long runs between ridges. These facilitiesare expensive to construct and can be the most maintenance-intensive parts of a smallco:rrununityconventional sewer system.


A common concern about ACS systems is the fact that there will be increased on-lot activity by the management staff than would typically be experienced with con-ventional sewers. This requirement is inherent for the ACS alternatives, and it mayrepresent a serious negativity toward their use by the public. All of the ACS alterna-tives are well-served by annual on-lot operation and maintenance visits. Witheffluent sewers, that concern is even greater, owing to the needs of the interceptor orseptic tank and its accoutrements.

Alternative collection systems are sensitive to illegal "clear water" (i.e., down-spout, foundation drains, or sump pump) connections near the served dwellings,because they are not designed to carry this additional flow. Conventional sewers aregenerally designed to handle these extraneous flows, but often that allowance isoverwhelmed during storms or snow melting periods. Careful attention to elimina-tion of these extraneous flows is an important part of ACS design and should be morerigorously applied to conventional systems also. It is desirable to identify and elimi-nate these on-lot problems during the construction stage before connection to thecommunity collection system.

Each of these ACS technologies has critical system components that must belocated near the wastewater source (residence, institution, or commercial facility) andis generally (but not always) sited on private property. Therefore, communities thatare considering these systems must provide considerable public education andinvolvement in the planning process as a vital requirement for success. This approachminimizes the potential for subsequent damage to public relations that can occur ifsuch information were not made clear to the community before implementation.Continuing such efforts after construction can offer additional positive benefits ofmaintaining public support for the project.

During the operational phase of the project, the system staff representative(s)that interact with the serviced community on their property will be considered theembodiment of management and must be trained and capable of relating posi-tively to the public. By arranging for these staff activities at times convenient tothe homeowner and by conducting them in a professional manner, their intrusive-ness can be minimized.

EVALUATION ISSUES. Each section of the manual is concerned with a specificACS technology. Each cites a series of site conditions that favor that technology overconventional gravity sewers. Not surprisingly, that list is somewhat similar for allACS types considering the variety of commonalities discussed above. Only a few site

8 Altemative Sewer Systems

and management conditions clearly favor a given type of ACS over the others. Thereason why one ACS technology has been chosen over the others for the case studiespresented in this manual is rarely, if ever, the result of careful and comprehensiveevaluation of all ACS technologies and subsequent comparison.

Typically, the engineer involved has some familiarity with one type of ACS andattempts to conduct a comparison with conventional sewerage. Depending on howwell that is performed, the availability and rules of financial assistance programs,and the municipality's stated desires, a system is chosen. As incomplete as thatappears to others, it is still well beyond the typical facility plan, which compares onlya few treatment options with conventional collection systems and treatment dis-charge to receiving waters already assumed.

Because of their inherent capital cost-saving potential, ACS systems should beconsidered and evaluated for all municipalities of 10 000 people or less, unseweredareas in metropolitan regions, and new developments. Existing communities withpopulations of 3500 to 10 000 can likely provide a sustainable and effective manage-ment program for any ACS technology, if it is empowered to collect user fees, makeand enforce rules, and ensure proper staff and/or practitioner training. Smallexisting communities with populations under 1000,with no assistance from largergovernmental or private management entities, are probably unable to provide propermanagement for any type of community system. Even the most simple system (i.e., asmall-diameter STEGterminating in a community septic tank and multiple gravitysoil absorption system) requires an organized public or private sector managementprogram. Arrangements with county government, private entities, or other nearbylarger utilities may eliminate this operation and maintenance barrier for thesesmaller communities.

There is no reason that ACS systems should be characterized as "small commu-nity technologies", even though most systems have been located in these settings.The reality is that ACS systems should be considered in suburban and even in cer-tain urban locations that cannot be cost-effectively served by conventional gravitysewers. By their nature, they should be cost-effective in any area where there is sub-stantial distance between sources (larger lot sizes) or topographic, hydrologic, andgeologic conditions that dictate difficult construction (high ground water, shallowbedrock, etc.) or multiple lift stations in a conventional sewer. Thus, one purpose ofthis manual is to dispel the myth of limited application and characterize these tech-nologies as potential options wherever they might be suitable. Indeed, when onelooks at the waterfront areas of most major cities, the potential value of shallow,


water-tight sewers instead of conventional deep collection pipes that are located ingroundwater, one realizes that their potential is clearly greater than their presentrate of application.

Different ACS alternatives have inherent advantages and disadvantages whencombined with other ACS types and existing or newly planned conventionalsewers. The most common combination is the effluent sewer, which combinesSTEG sewers with STEP sewers. Both use septic tank pretreatment and convey it tothe treatment facility.

A vacuum sewer can discharge to either a treatment facility or a conventionalsewer without any negative effects. Grinder-pump pressure systems are commonlyused in combination with conventional gravity sewers to reduce the cost of includinglow-lying or other difficult-to-serve areas into the overall system.

Effluent sewers can discharge to a grinder-pump sewer, but the reverse wouldhave severe repercussions, because the former is not designed to carry settleablesolids and grease. Similarly, any effluent sewer terminated into a vacuum sewershould probably be carefully investigated, to determine the need for some form ofadditional gaseous (i.e., sulfide) emission odor control at the vacuum station and forpotential overloading at the point of entry. All ACS types are theoretically compat-ible with discharge to conventional sewers, but interfacing precautions described inthe manual will need to be evaluated.

Cold-weather conditions have not been dealt with to a great degree in themanual, but all ACS types have been successfully used in Canada. The primary dif-ference lies in insulation of piping and mechanical components from severe wintertemperatures.

One of the most commonly asked questions relates to the treatability of ACSeffluents. Both effluent (STEG and STEP) sewer systems yield an effluent that isessentially septic tank effluent. It is quite biodegradable and weaker in terms of sus-pended and settleable solids and organic matter associated with them (i.e., organicsand fats, oil, and grease). Therefore, it could be characterized as primary effluent, inmany ways. It is generally anaerobic, contains reduced sulfur species, and readilyemits the hydrogen sulfide form upon air stripping (U'S. EPA, 1977;WPCF, 1986).Vacuum sewer conveyance is quite violent in nature, yielding a wastewater similarin strength to conventional raw wastewater that has been aerated (i.e., higher dis-solved oxygen content). However, the gaseous exhaust from the vacuum station mayexhibit an odor potential. With all the ACS options, any site of gaseous release shouldbe carefully designed to ensure minimization of unwanted odors that could affect


nearby residents. Grinder-pump wastewater is generally stronger than conventionalsewer wastewater, in terms of settleable solids and fats, oil, and grease, and organicmatter, owing to the lack of dilution found in the latter from infiltration and inflow.Because these solids (and their associated organics and fats, oil, and grease) are gen-erally amenable to removal by sedimentation, primary sedimentation effluent maybe only slightly more concentrated than the effluent from identical treatment of con-ventional raw wastewater.

Given that most ACS systems have been implemented in smaller communities,many of them have discharged to stabilization ponds, filters, and constructed wet-lands that are more common forms of treatment in rural systems. Facultative pondsystems are sized based on organic loading, so they are sensitive to the organic con-tent of the wastewater. Thus, wastewaters collected by conventional, grinder-pump,and vacuum collection systems will require somewhat larger (approximately 30%)pond sizes than those treating effluent sewers. However, these potential capital costsavings may be offset by the increased costs of periodic interceptor tank pumping,transport, and disposal.

Recirculating media filters (RMFs) are quite popular in smaller systems, owingto their reliability and high quality effluent. Vegetated submerged beds (VSBs)orsubsurface wetlands are also used as pretreatment before soil dispersal. The RMFtreatment systems are generally sized on hydraulic loading rates, so there would beno difference for wastewaters from any of the ACS types, although vacuum, grinder-pump, and conventional sewer effluents will require a sedimentation step beforeapplication to the RMF.The VSBsare sometimes designed on hydraulic loading andsometimes on organic loading, so VSBsizing mayor may not be affected by influentstrength, depending on the VSBdesign criteria used.

Other mechanical treatment systems, such as extended aeration and oxidationditches, have also been used without significant difficulty in some ACS installations.

illmany ACS applications, treated effluent is applied to the soil, rather than dis-charging to surface waters. There are several different subsurface soil absorptionsystem alternatives, but pressure and drip dispersal methods are recommended toattain uniform loading and resting of the soil infiltrative surface. Specific local designcriteria may differ between different types of ACS wastewaters.

Readers are referred to numerous other textbooks for further information onwastewater characteristics versus treatment plant design. Pilot studies may berequired, if treatment of mixed or unique wastewaters from more than one type ofsewer is being considered. Some instances of inappropriate treatment combinations


have been found in the past. For example, anaerobic sewer effluent was applied toinclined screens (used in place of primary clarifiers) for removal of influent solids.Both were perfectly good alternatives, but not with each other. The combination pro-duced such corrosion and odor problems that the screens were quickly removed fromthe treatment sequence. Use of any ACS will require the treatment designer to takecertain precautions to ensure compatibility.

PERSPECTIVE. The timing of this manual is such that experience with the firstand second generations of ACSs has been documented. Problem areas have beenidentified, and solutions generally have been developed. This manual conveys thoseexperiences in a practical manner that permits the engineering profession to designreliable systems that can be managed by competent management programs.

Indeed, there are some unknown or unresolved issues in ACS technology, butthese are rapidly disappearing with time. None are considered serious enough toretard continued and expanded application of these systems. This manual isintended to stimulate consideration of ACS technology and minimize its misusewhere it is an inappropriate solution to a problem.

OVERVIEW OF SPECIFIC ALTERNATIVE SEWERSAND THEIR APPLICATIONSApplications of all ACS technologies were generally stimulated by the Congressionaldirective in Section 104(q) of the Clean Water Act of 1972,which directed U.S. EPAtodevelop wastewater systems that can be applied where implementation of conven-tional practices is impractical, uneconomical, or otherwise infeasible.

PRESSURE SEWERS. Grinder-pump pressure sewer mains consist of a small-diameter pipeline, placed at a shallow depth, generally following the profile of theground, that receives macerated wastewater for conveyance. Typical main diametershave varied upward from a 50-mm (2-in.) minimum. Polyvinyl chloride (PVC) hasbeen the typical piping material in the United States, but high-density polyethylene(HDPE) has also been successfully used. 'Burial depths typically are below the frostline, or a 75-cm (3D-in.)minimum, whichever is greater. Infar northern areas, insu-lated and heat-traced HDPE piping has provided relief from this criterion. Operatingpressures vary with terrain, design, and pressurization equipment capabilities. Somegrinder pumps can generally operate against heads of 30 ill (100 ft) of water. Designs


must accommodate the pump characteristics to attain flushing velocities on a speci-fied frequency, as described in greater detail in Chapter 2, which is devoted to thesesystems. One difficulty that has occurred is related to new development service.Because the grinder-pump main is designed for the ultimate development popula-tion, early periods cannot attain the required flushing velocities at the required fre-quency. This might require the system operation and maintenance staff to provideflushing during the early operational period, to avoid pipeline accumulations thatrequire excessive pumping heads to function.

Installation of a pressure sewer system is far easier and faster than for conventionalsystems. A general sketch of a grinder-pump installation is shown in Figure 1.1.

Typically, each service location uses a small grinder/pressurization device tomacerate and pump wastewater through a small (typically 1.0 to 1.5 in.) service lat-eral to the main .. The grinder pump grinds the solids present in wastewater into aslurry, in the same manner as a kitchen sink garbage grinder, before pumping thatslurry into the lateral under pressure. The lateral connects to the pressure main,which conveys the macerated wastewater to discharge either to a treatment facilityor a larger conventional sewer.

One advantage of grinder-pump systems over effluent sewers is that maceratedsolids are passed through to the sewer, leaving no residuals to be pumped on privateproperty. One disadvantage is that the resulting sewer effluent has higher concentra-tions of organics and solids, which will cause the wastewater to become anaerobicfaster, and it requires additional pretreatment at its terminus.

Grinder pumps that serve individual homes are typically 1.5 kW (2hp), but 0.75-kW (l-hp) units are also used. Some installations use 2.25- to 3.75-kW (3- to 5-hp)motors, but these are generally used when serving several homes or a single largerbuilding source with one pumping unit. The service line leading from the pumpingunit to the main is typically a 25- to 38-mm (1- to l.5-in.) diameter PVC pipe (typi-cally 31 mm [1.25in.j), to ensure necessary velocities in that line to prevent blockages.One or more check valves in the service line prevent backflow, in addition to a redun-dant check valve in the grinder pump unit. Air-release valves (typically with odor-control facilities) are required at major high points throughout the collector main.

If a grinder-pump malfunction occurs, the level of wastewater in the- sumpincreases until a high level alarm is activated. This alarm may trigger a lightmounted inside and/ or on the outside wall of the home, an audible alarm that canbe silenced by the resident, or an alarm at a central location for remotely monitoredsystems. In the first two cases, a resident then notifies the management staff to send


---..::.:l'r'---:::~---:!lrr::=:---_-4.r----,:··tr it'" .'<; to

2

FIGURE 1.1 Grinder-pump system.

1 CONTROL PANEL2 BURIED ELECTRICAL CABLE3 ELECTRICAL JUNCTION BOX4 SEWAGE FLOW FROM HOME5 PLUMBING DISCONNECT6 SHUTOFF VALVE7 SERVICE LINE TO MAIN8 lEVEL SENSORS9 CHECK VALVE

10 GRINDER PUMP

an operator to evaluate the problem and make any necessary repairs. In the lastcase, the staff receives the information directly, and they may attempt to correct itthrough remote controls, if the system is capable of this, or dispatch staff to attendto the offending unit.

The pressure sewer market size is substantial and growing. No comprehensivelist has been kept to document pressure sewer projects, but hundreds of systems areknown to exist throughout the United States. Locations of major grinder-pump pro-jects range from Florida to Alaska and from California to New York. Many of thesesystems serve 50 to 200 homes. Some systems serve over 1000homes each, and a fewsystems even serve over 10 000homes. Inmost cases, these grinder-pump installa-tions were based on local conditions, such as shallow rock, high groundwater, orsome other severe site limitations, which made conventional collection systemsextremely costly.


Some topography does not favor gravity collection. One typical example isaround lakes, where homes are built along the shoreline. The roads servicing thesehomes are often the only practical location for the sewer, and they are generally ups-lope from the homes and their drainage plumbing. Additionally, the grade of thatroad right-of-way may vary substantially, up and down, as its circles the lake.Numerous and costly pumping stations would be necessary if conventional sewerswere used to service such development, when a simple low-cost pressure sewercould easily do the same job for substantially less capital cost and provide less dis-ruption to the service population during construction.

Damage consequential to the installation of deep sewers can also be a factor. Insome cases, rock blasting is required to install conventional sewers. This may causeupheaval of the road, damage to nearby buried utilities and homes, and disruptionto the community. Deeply buried conventional sewers are also more likely to inter-cept and drain groundwater aquifers, resulting in both lowering of water tables(impaired well water supplies) and excessive infiltration that must be processed bythe treatment facility. This groundwater drainage problem has been encounteredmost recently around Boston, Massachusetts, and has further acerbated serious watershortages in the region.

Grinder-pump pressure sewer equipment is often used in conjunction with con-ventional sewer systems. Where low-lying homes or basements are too low to allowgravity flow into a conventional sewer, a grinder pump may easily be used at thathome to discharge to the sewer, thus relieving sewer designers from the need tolower the entire conventional sewer profile or add lift stations to pick up all homes inthe area. The use of pressure sewer components to serve low-lying homes frontinggravity sewers is substantial, but actual numbers are not readily available.

Grinder-pump systems can also be used to discharge raw wastewater to indi-vidual home septic tanks and subsoil drainfields that are located above the buildingsewer elevation, thus allowing the on-site system to be located in the best soils avail-able on the site.

Grinder-pump pressure sewers include mechanical! electrical systems thatmust be properly managed to perform as expected. They have no inherent limita-tions, as long as proper hydraulic principles are followed and operation and main-tenance is provided. There may be some hydraulic limits dictated by headloss andpumping unit size, but these would be found on a case-by-case basis, dependingon local circumstances of use. However, within their hydraulic limitations, they canbe considered capable of serving a community inthe same way that a conventionalsystem can.

Overview of Wastewater Collection Systems

VACUUM SYSTEMS. Vacuum sewer collection systems were patented in theUnited States in 1888.The first commercial applications of such systems were by theLiljendahl Corporation (now known as Electrolux) of Sweden in 1959. Since thattime, several other companies have entered and contributed to this market. Cur-rently, three companies are active in the United States. There are significant differ-ences among the three systems, in terms of design concepts. These are discussed inChapter 3-Vacuum Sewer Systems.

AIRVAC(Rochester, Indiana) presently dominates the United States market andprovides a pneumatically controlled and operated vacuum valve that is used forwastewater systems (Figure 1.2). The AIRVAC system is entirely external to thehouse, with the wastewater flowing by gravity through the building sewer to a com-bined sump/valve pit. The valve starts its cycle when its pneumatic controller sensesthat approximately 38 L (10 gal) has accumulated in the (typically fiberglass-fabri-cated) sump. Itopens for a few (i.e., 3 to 8) seconds, which is enough to evacuate thewastewater in the sump and allow a controlled volume of air (depending on thevalve location) to enter the main and travel toward the vacuum station. Unlikegrinder-pump and STEP collection systems, vacuum valves do not need an on-lotsource of electricity to operate, thus eliminating this major source of service calls.These sump/valve pits can serve several homes. Chapter 3 provides more detail.

VACUUM PUMP

RESERVETANK----~

COLLECTIONTANK

15

VACUUMVALVE/SUMP

SEWAGE PUMP

TO TREATMENT FACILlTIES----,

...., ......... ,,-....tC:t--- LIFT

.....---VACUUMMAIN

FIGURE 1.2 AIRVACvacuum sewer system (courtesy of AIRVAC Rochester, Indiana).


As with a grinder-pump system, there are no on-lot residuals to be collected by avacuum truck, as there are with effluent sewers. Thus, on-lot maintenance time andcost are minimized with these two ACS systems.

The vacuum collection main piping typically consists of 100-and 1S0-mm (4-and6-in.) mains, although recent very large installations include 2S0-mm (10-in.) andlarger mains. Smaller (7S-mm [3-in.]) mains used in early vacuum systems are nolonger recommended. Most systems use PVC pipe, although HDPE has been usedalso. Both solvent-welded and rubber-gasket PVC pipe connections have been used,but accumulated experience indicates that the latter is more reliable. Rubber gasketsmust be certified by their manufacturer as being suitable for vacuum service. Collec-tion main profiles change, depending on installation in uphill, downhill, or level ter-rain (see Figure 1.3).

The pipe profiles recommended by each manufacturer also may differ. There areno manholes in the system; however, access can be gained at each valve pit and at theend of a line, where a cleanout may be installed. Installation of the pipe and fittings fol-lows water distribution system practices. As can be seen in Figure 1.3, grade must becarefully adhered to during construction. Division (isolation) valves are installed onbranches near connections to the main and on the mains near branch connections, toallow for isolation when problem troubleshooting is necessary or when making repairs.

The vacuum station is the heart of the vacuum sewer system. Vacuum stationsvary from manufacturer to manufacturer. Figure 1.4 shows the basic AIRVACvacuum station layout, including the major components.

The equipment used in the vacuum station is slightly more mechanically com-plex than that used in a conventional lift station. Equipment includes a collectiontank, vacuum pumps, wastewater pumps, and pump controls. In addition, an emer-gency generator is standard equipment. During recent hurricanes in the southernUnited States, these systems were shown to suffer little or no downtime, comparedwith the weeks and months of downtime that conventional sewers incurred whilerestoring lift stations.

The collection tank is the equivalent of a wet well in a conventional pumping sta-tion. Some designs have used a reservoir tank between the collection tank andvacuum pumps to minimize the number of on-off cycles of the latter. The wastewaterdischarge pumps are designed with sufficient net positive suction head to overcometank vacuum and conveyance requirements. Level control probes are installed in thecollection tank to regulate the wastewater pumps. Vacuum switches located on thecollection tank regulate the vacuum pumps. A monitoring system alerts the systemoperator should low vacuum or high wastewater level conditions occur.

Overview of WastewaterCollectionSystems 17

~---- 20 FT. MIN. -----110"1

0.2% SLOPE

0.2% SLOPE 1\ VACUUM MAIN

~45·EL80WS

UPGRADE TRANSPORT

1// sal» -1/1 1/1- 1/1 //1 III =!iJ-I/ I /1; - ~ I- r//.. 1// .:::::r+--- 500 FT. TYPICAL --.I

0.2% SLOPE0.2% SLOPE

VACUUM MAIN ...FLOW

LEVEL GRADE TRANSPORT

DOWNGRADE TRANSPORT FLOW·

FIGURE 1.3 Upgrade/level! downgrade transport (1 ft = 0.3 m) (courtesy of AIRVAC,Rochester, Indiana).


VACUUMPUMP

EXHAUST

•

FORCE MAINTO TREATMENT

PLANT-

CONTROLPANEL VACUUM

RESERVOIRTANK

oD ~ no g D

VACUUMg PUMPS (:..;2);.,101'.....,. ....

oo

VACUUMGAUGE

ISOLATIONVALVE

EQUALIZINGLINES (2)

SEWAGECOLLECTION

TANK

FIGURE 1.4 Diagram of a typical vacuum station (courtesy of AIRVAC,Rochester, Indiana).

DISCHARGEPUMPS (2)


Experience has shown that, for vacuum systems to be cost-effective, a min-imum of 75 to 100 customers (houses or equivalents) is generally needed pervacuum station. The average number of customers per station in systems presentlyin operation is approximately 200 to 500, but that average is increasing as largersystems are being implemented that have multiple vacuum stations, each servinghundreds of customers.

Vacuum systems are, to a degree, limited by topography. Their most successfulapplications have been in relatively dense developments with moderate terrainchanges. The vacuum produced by a vacuum station is capable of lifting wastewater4.5 to 6 m (15 to 20 ft), depending on the operating vacuum level of the system. Thisamount of lift is often sufficient to allow the designer to avoid many of the lift sta-tions that would be required in a conventional gravity system.

AIRVAC's first residential system was installed in 1970. Since then, there havebeen more than 200 systems in 27 states constructed and operating in the UnitedStates. Figure 1.5 shows the distribution of operating residential vacuum sewer sys-tems in the United States as of December 2004. There are another 20 to 25 systemspresently in the construction phase, with more being planned and designed. Thereare also numerous worldwide installations using vacuum sewers, with operatingsystems in 23 foreign countries, including many countries in Europe and Asia andalso in Australia, Canada, and Mexico. In addition, several industrial and commer-cial facilities use vacuum systems to collect wastewater.

In terms of applicability, vacuum sewers should be considered in any applicationthat involves more than 75 dwellings. The practical limit of uphill transport histori-cally has been 4.5 to 6 m (15 to 20 ft) of vertical lift, although experimental systemsare being tested that may increase the feasible vertical lift limit.

The vacuum sewer manufacturers universally insist on the need for well-trainedoperators. The operating effort required is not much different from a conventionalgravity system, as the components of the vacuum station are similar to a conven-tional pumping station.

An average-sized vacuum station contains 19-kW (25-hp) vacuum pumps andll-kW (15-hp) wastewater pumps. Considering a typical run-time of 8 hi d for thevacuum pumps and 5 hi d for the wastewater pumps, and assuming the cost of elec-tricity at $0.08/Kwh, the cost of power for the vacuum station is approximately$500/month. A system this size typically serves 250 to 400 customers.

Preventive maintenance of the vacuum station components is also similar to, butsomewhat greater, than that of a conventional lift station. As for the vacuum valves,the small parts that are subjected to wear are very inexpensive. A vacuum valve and


controller can be rebuilt for approximately $50. Rebuilding frequency is reputed tobe 5 to 10 years. An annual visit to each on-lot vacuum valve sump to ensure properoperation is a prudent preventive maintenance plan.

When emergency maintenance is required, it typically is related to a low vacuumcondition. This could be caused by a leak in the piping network (rare) or by a vacuumvalve that does not close. Because virtually all valve failures happen in the open posi-tion, the vacuum continues to try to evacuate the contents of the pit. This will causethe vacuum pumps to run continuously to keep up. A telephone dialer feature avail-able in vacuum stations notifies the operator of this condition. Location (and some-times correction) of the problem can generally be made in less than 1 hour after theoperator arrives at the station.

Like all the ACS alternatives, vacuum sewers have certain design, installation,and management requirements. When these are satisfied, the system will performwith all the reliability of any other collection system. One of the vacuum system sup-pliers also offers design and construction assistance to ensure that the system is prop-erly installed, given the sensitivity of the technology to improper component designand construction. In short, the vacuum system offers the same convenience as anyother type of public sewer system, with reference to the actual discharge from thehome and meeting the needs of the particular locality.

EFFLUENT SEWERS. In this manual, the term effluent sewers is used to describeany sewer, regardless of transport mode, that conveys septic (or "interceptor") tankeffluent, rather than raw wastewater. Thus, it includes gravity and pressure sewersor a combination thereof conveying septic tank effluent. The international literaturecommonly uses the terms small-bore sewers or effluent drains for the gravity option, butthe term septic tank effluent pumped (STEP) sewers is more universal for the pressur-ized parts. This text uses the terms septic tank effluent gravity (STEG) sewers inter-changeably with small-diameter gravity (SDG) sewers, and the term effluent setuers forboth or for a combined system, because almost every installation includes a combi-nation of the two, dictated by local conditions.

The SDG or STEGsewers and the broader category of effluent sewers (STEG +STEP)are rapidly gaining popularity as solutions to wastewater problems in unsew-ered areas because of their low construction and maintenance costs. Unlike conven-tional sewers, primary treatment is provided at each connection, and only the set-tled wastewater is collected. Grit, grease, and other troublesome solids, which mightcause obstructions in the collector mains, are thus separated from the wastewater(see Figures 1.6 and 1.7)before collection. This pretreatment removes the solids that


dictate a required minimum (scouring) velocity in conventional and grinder-pumpsewers and thus eliminates the need to maintain a minimum velocity in the collectorpiping. However, the material that is retained in interceptor or septic tanks (septage)must be periodically removed and transported to a treatment/ dispersal facility.

With conventional sewers, short sections of large-diameter pipes are designedwith straight horizontal alignment between manholes and with sufficient slope tomaintain self-cleansing velocities. With effluent sewers, the collector mains servingthe first hundred or more houses will be smaller in diameter and can be installed atmore shallow depths, with variable or inflective vertical and horizontal gradients,which can minimize existing property damage. Manholes are sparingly (if ever)used, which helps minimize infiltration and inflow and entry of grit. If a manhole isused, it is generally only at major segment junctions. Less intrusive and less expen-sive cleanouts (instead of deep manholes) generally suffice to ensure access to themains if cleaning procedures become necessary. To date, no United States systemshave had occasion to clean an operating system, although a few have had to do soimmediately after construction to remove debris from sloppy installation practices.

-:-;'~~l.. ., ~.

I~~•• - !'.'- -, ··.:i

-~ OTHER COMPONENTSSIMILAR TOGRINDER PUMP

FIGURE 1.6 The STEP system.


EFFLUENT

FIGURE 1.7 Schematic of a SDG system (BOD = biochemical oxygen demand).

The design of the mains is dictated primarily by hydraulics, rather than by solids car-rying capabilities, which dictate the design of conventional gravity sewers.

Designers must be cognizant of infiltration and inflow and ultimate service areagrowth in sizing these systems. Sewer construction costs are reduced (compared withconventional sewers) because STEPor STEGpiping is placed at more shallow depths,follows the topography more closely than conventional sewers, and can easily berouted around most obstacles within their path. All of these features permit signifi-cantly less physical disruption of existing communities and a shorter constructionperiod. However, these advantages may be mitigated by the need for increased on-lot activity by service personnel over that required for conventional systems. As withall ACSs, the responsible management entity must get easements for assurance ofaccess to all on-lot components.

InAustralia, where systems with up to 4000 connections (more than 80 systemsinstalled) have been used for approximately 40 years, STEGsewers have been foundto reduce collection system construction costs by 30 to 65%. Routine maintenancealso proved to be quite low.


In the United States, SDG or STEG sewers were not introduced until the mid-1970s. Some early systems were constructed with a variable grade (i.e., sections ofsewer depressed below the static hydraulic grade line [HGL]), while others had uni-form grades adapted from the Australian guidelines.

By the mid-1980s, over 100 United States effluent sewers had been constructed,and positive experiences with the simpler and less-costly, variable-grade approachmade it the dominant choice in subsequent installations. This variable-grade designis based on the system's capacity to carry the expected peak flows without raising theHGL above the interceptor tank outlet inverts for extended periods of time. By 1990,there were over 600 effluent sewers in the United States.

The STEG systems consist of the following components:

• House connections;

• Interceptor tanks (septic tanks);

• Service laterals;

• Collector mains;

• Cleanouts, manholes, and vents; and

• Lift stations.

These components are described in the following sections.

House Connections. House connections are building sewers that connect the houseto the interceptor tank. Serious hydraulic problems resulted in systems that hadillegal "clear water" connections (roof or foundation drains) to the building sewerahead of the septic tank. Present design practice is to identify and eliminate theseduring the design and construction phases of a project. This practice should be a uni-versal requirement for all collection systems.

Interceptor Tanks (Septic Tanks). These should be buried, water-tight, house-hold-plumbing-vented tanks with effluent-screened outlets to remove any waste-water solids that do not settle or float in the tank. Typical tank design! sizing requiresat least 1 day of retention with maximum allowable sludge and scum accumulation.In practice, 3.78 m3 (1000-gal) tanks are the norm. These tanks also attenuate waste-water flow from each connection, thus reducing the peak to average flow ratio belowwhat is typically used for establishing design flows for conventional gravity sewers.The tanks should be inspected periodically and pumped when necessary. Generally;.


existing septic tanks are not used as interceptor tanks, because they are not water-tight and cannot be inspected and repaired cost-effectively.

Service Laterals. Service laterals connect the interceptor tank to the collector main.Typically, they are 50 to 100 mm (2 to 4 in.) in diameter, but almost always smallerthan the collector main, to take advantage of less-cumbersome saddle connections tothe larger mainlines. In certain instances, they may include a carefully placed checkvalve or other backflow prevention device near the connection to the main, but,owing to the potential for problems related to air capture by the check valves, thisapproach is restricted to low-lying areas, where HGLs may otherwise rise above thetank invert.

Collector Mains. Collector mains are small diameter plastic (typically PVC) pipeswith typical minimum diameters of 75 to 100 mm (3 to 4 in). The mains are typicallyinstalled at a depth sufficient to collect the settled wastewater from most connectionsby gravity. If all sources cannot be accommodated, the remaining low-lying homesare generally served by STEPunits.

Cleanouts, Manholes, and Vents. These provide access to the collector mains forinspection and maintenance. Cleanouts are preferable to manholes, because they aremuch less costly and can be tightly sealed to eliminate most infiltration and grit,which commonly enter conventional sewers through manholes. Vents are necessaryto maintain free-flowing conditions in the mains. Household plumbing vents are suf-ficient, except where depressed sewer sections exist. In such cases, air release valvesor ventilated cleanouts are generally necessary at the high points of the main to pre-vent air-binding.

Lift Stations. These are only necessary where elevation differences do not permitgravity flow. The use of decentralized design principles would generally avoid theneed for lift stations and likely dictate placement of a treatment/dispersal (or reuse)facility rather than a lift station. Designers generally try to avoid lift stations, owingto the demanding operation and maintenance requirements that are inherent withtheir use. Modem small community and suburban approaches consider multiple pas-sive treatment/ dispersal (or reuse) facilities (often referred to as distributed systems)in each natural drainage basin as a better approach than pumping wastewaters outof their natural basin to facilitate the use of a single more complex, treatment facility.

The United States STEP system history traces back into the 1960s. The compo-nents of a STEP system are essentially the same as those listed above for STEG sys-


terns, with certain additions. The pump tank may be part of the septic/interceptortank or a subsequent separate structure. The earliest systems added a separate pres-surization tank after the interceptor tank. In most recent designs, the pump isimmersed in a second compartment of a specially designed interceptor tank, whichis supplied as a package by the manufacturer. Both approaches can work well.Because the existing septic tank is rarely reused in a STEP system, the single pack-aged system is the dominant choice.

The STEPpumping units are simpler than those used for grinder pumps, becausethere is no need for the macerating step. Thus, power requirements are lower, and thedesign is less complex. The types of pump and control designs vary with each sup-plier, but are typically centrifugal pumps. As with other ACS alternatives, an annualvisit to each on-lot facility to check and clean any mechanical components, clean theseptic tank effluent filter, and check sludge and scum accumulations in the tank isrecommended.

Effluent sewers are well-suited for low-density residential and commercial devel-opments, such as small communities and residential fringe developments of largerurban areas. Because of the smaller diameter, longer pipe, with more shallow place-ment, and fewer, if any, manholes and lift stations, they have a distinct cost advan-tage over conventional gravity sewers. However, these collection systems generallyare not well-suited to high-density urban applications because of the need for sub-stantial on-lot components and intrusive activities necessary to maintain them.

Experience has shown that mainline flushing has only been necessary at a fewsystem startups, owing to improper construction techniques that allowed entry offoreign materials. Thus, operation and maintenance requirements have been gener-ally limited to on-lot components.

Experience shows effluent sewers to be relatively trouble-free, for as long as 3decades in the United States. Potential users have resisted them because of themisperception that they are "second-rate systems", and they want "a real sewer".Typical concerns are for odors, the continued need to have periodic andunpleasant maintenance performed on their property, and whether the system canbe expanded to accommodate anticipated community growth. However, whereeffluent sewers have been properly designed, installed, and managed, users havefound them to perform no differently than conventional sewers. However, evenwith good management, the need for on-lot intrusions by operation and mainte-nance personnel is a reality for all ACS systems, and it constitutes the primarytradeoff against the lower capital (and sometimes operation and maintenance)costs inherent in most applications of these technologies.


EVALUATION OF ALTERNATIVE COLLECTIONSYSTEMS AND CONVENTIONAL COLLECTIONALTERNATIVESThe following discussion is a brief list of the various factors that affect the choice ofsewerage alternatives considered in planning a project. Because all wastewater col-lection projects are unique to the area and community served, each of these issuesshould be considered in that process.

POPULATION DENSITY. Because conventional sewers are typically much morecostly to construct on a lineal foot basis, in areas where housing is more sparsebecause of large lot sizes or open spaces between connections, longer reaches of sewerare required, and the cost of providing conventional sewers can be prohibitive. Theseconditions favor ACS alternatives, because they typically prove to be more cost-effec-tive (via present worth or life-cycle analyses) when serving these sparse to medium-density populations. Conversely, in densely-populated areas, such as urban cores,where the required length of sewer between service connections is comparativelyshort and available space for on-lot components is limited, conventional sewers aretypically the system of choice. When some other adverse characteristic exists, such asmassive infiltration along the shore of a water body, the potential for servicing thoseareas with an ACS that discharges to the conventional sewer at some point away fromthe shore may save money and significantly reduce infiltration and inflow.

One recent phenomenon is new housing development that incorporates urban-like housing densities with significant open or common areas. These" conservationstyle" developments are generally best served by ACS,because decentralized waste-water management systems can be built in nearby open spaces for treated waste-water dispersal and! or reuse. This frees such developments from being dependenton costly extensions of existing sewer systems.

GROWTH INDUCEMENT. The minimum diameter for conventional sewers isgenerally 200 mm (8 in.), to accommodate and minimize the need for sewercleaning equipment. Because they are so costly to install, conventional sewers areoften growth-inducing, especially if assessment costs to fronting properties areextremely high. This can induce intensive development of unimproved property toreduce the per-capita costs. By spreading the cost over a greater number of users,the costs per user decrease. A variation on this problem relates to the time cost ofmoney. If anticipated growth exceeds what actually occurs, the excessive investment inunneeded infrastructure can have serious economic repercussions on the community.


Alternative collection system mains can be designed to have less growth-induce-ment capability than conventional sewers. Where land-use plans exist}ACS alterna-tives can be sized to intentionally limit growth to fit land-use plans more specifically.They can also be used to address only the major problem areas of a community, insteadof servicing the entire community-many parts of which require no wastewater man-agement changes. Because many municipalities wish to accommodate some level ofgrowth} alternative sewers can readily be designed to match those needs.

More developers are turning to ACS alternatives to accommodate conservation-style developments. These systems allow them to maximize the number of lots thatthey can sell compared with developments on septic systems that often must meetvery large minimum lot sizes set by state codes or local ordinances. They also havebeen shown to maximize property values over traditional development patterns.

GROUND SLOPES. Where the ground profile of a nonurban area slopes down-ward in the direction of the treatment facility, STEG sewers are likely to be most cost-effective. Insome cases where conventional sewers may become cost-prohibitivelydeep} the use of grinder-pump systems to service low-lying sources may be economi-cally attractive. Effluent sewers, by use of inflective gradients or by use of both STEGand STEP connections, can also be applied to this set of circumstances. Vacuumsewers may also be adaptable to this topographic condition.

In areas with flat terrain, conventional sewers become deep} because of therequirement for minimum slopes to maintain minimum velocities to transport waste-water solids. The typical alternative is to use lift stations to periodically raise theHGL to start the pipe sloping procedure again from a higher location. Both the deepexcavation (especially in areas of rocky formations} high groundwater} and unstablesoils) and the lift stations are expensive-the former in construction cost and thelatter in construction and operation and maintenance cost. Effluent sewers follow theland slope} are less deeply buried, and have fewer slope requirements, owing to theabsence of settleable solids and grease. This results in greatly reduced constructioncosts. Grinder pump and vacuum sewers are also often used in flat areas, as reason-able variations in ground slope are of little concern. In areas where the dischargepoint of the collection system (i.e., treatment facility or interceptor sewer) is at ahigher elevation than the service population} all ACS alternatives} except STEG, arecapable of providing proper wastewater collection.

SUBSURFACE PROBLEMS. Where rock excavation is encountered} the shallowburial depth of alternative sewer mains reduces the amount of rock to be excavated


and, even more significantly, reduces construction costs. The cost of deep excavationsrequired for conventional sewers is exacerbated by rocky soils, high groundwater,and unstable soils. The cost advantages of all ACS approaches will be enhanced, inthese cases, through reduced construction costs and duration of community disrup-tion and also by avoiding the potential for long-term or periodic infiltration that canaccompany the deeper and more vulnerable conventional sewers.

DISCHARGE TO GRAVITY SEWERS. Where homes to be serviced are close toan existing conventional gravity sewer, but where conventional gravity system ser-vice is impractical (i.e., low-lying areas not included in the original design), grinder-pump and STEPsewers may be used effectively.All ACS discharges are amenable tothe conventional biological treatment used in conventional systems.

Effluent sewer discharge to conventional sewers is feasible, as noted above, butthese discharges contain considerable concentrations of sulfides. Thus, the engineermust evaluate the relative flows in the receiving sewer to that discharging from theeffluent sewer and the materials of construction of the receiving sewer in designingthis transition, to avoid potential odor and corrosion problems. This issue is morefully discussed in Chapter 4, which deals with effluent sewer systems.

In seeking information on the performance of existing ACS and communitysatisfaction with them, one can find excellent operating performance and satisfac-tion from well-designed, well-built, and well-managed pressure sewer systems.However, it is also true that one might find poorer performance and satisfactionfrom ones that do not meet contemporary standards of practice. The latter can alsobe valuable if the sources of those problems have been identified. For example,components that were incompatible with the service conditions, misapplieddesign principles, and failure to account for abnormalities of climate, use, andlocal natural features provide a basis of experience that should be incorporated tobetter projects. Inadequate inspection during construction has also been docu-mented to cause serious performance problems. These unfortunate occurrences arebecoming far less frequent as more information becomes available. Because ACSsystems are inherently more sensitive to proper construction practices, they dorequire comprehensive oversight by well-trained inspectors. Users are now usingexperienced engineers who have had demonstrated success with these technolo-gies and inspectors who are trained to ensure the optimum performance of newerACS systems.

A planning-level look at ali collection alternatives is provided in Table 1.1. Spe-cific local conditions may defy the table entries when serious design is undertaken.


oZ

ClI;:l

'"'"-


However, the table offers a quick overview of how the different alternatives comparewith each other.

A frequently held misunderstanding is that ACS alternatives are inherentlymaintenance-intensive. Experience has not supported that opinion. Properlydesigned ACS systems, made easy to maintain by good design and operated by qual-ified personnel, have been relatively easy to maintain. However, they are not tolerantof inadequate operation and maintenance. Just as with a conventional sewer system,proper ACS system management must be provided. Particularly, because of theincreased on-lot facilities of all ACS alternatives, the management program must bewilling to interface closely with the homeowners, and personnel assigned to thesetasks must be knowledgeable and skillfully diplomatic. Any collection system instal-lation can be a source of disruption and inconvenience to the homeowners. The stafftime required for effective public relations is extremely important and is typicallyunderestimated in planning the management program.

REFERENCESUS. Environmental Protection Agency (1991) Alternative Wastewater Collection

Systems, EPA-625/1-91-024; U.S. Environmental Protection Agency, Office ofResearch and Development, Office of Water: Washington, D.C.

US. Environmental Protection Agency (1977) Alternatives for Small WastewaterTreatment Systems, EPA-625/4-77-011, NTIS No. PB-299608; U.S. Environ-mental Protection Agency: Cincinnati, Ohio.

U.S. Environmental Protection Agency (1997) Response to Congress on Use ofDecentralized Wastewater Treatment Systems, EPA-832/R-97-00lb; U.S. Environ-mental Protection Agency, Office of Water: Washington, D.C.

Water Pollution Control Federation (1986) Alternative Sewer Systems, Manual ofPractice No. FD-12; Water Pollution Control Federation: Alexandria, Virginia.

Chapter 3

Vacuum. Sewer Systems

Introduction 101 System Design Considerations 117Overview 101 Hydraulics 117Theory of Operation 102 Deeign Flows and Velocity 117Applicability and Vacuum TransportAdvantages 105 and Air-Liquid Ratios 117

Applicability 105 Sawtooth ProfileAdvantages 105 Design Concept 118

Manufacturers Active in Air-Liquid Ratios 118the United States Market 106 Situation to Avoid-Using

AIRVAC 106 Vacuum as an InterceptorRoediger (Roevac) 107 Sewer 119Iseki 107 Odors and Corrosion 120

Extent of Use in the United Static and Friction Losses 121States 109 Situations to Avoid 123

Projects in the United Effects of WaterStates 109 Conservation 123Summary 109 Mains and Service Lines 123

System Plan and Elevation View 110 Definition of Terms 123Description of System Mains 124Components 110 Geometry and Sizing 124

Valve Pits 110 Situation to Avoid-Vacuum Mains 110 Long Distances with NoVacuum Station 114 Connections 125

(continued)

97


Design Considerations 125 Vacuum Valve 139

Minimizing Static Loss 126 Controller/Sensor 140Recommended Breathers 141Flowrates 127 External Breather 141Routing 128 In-Sump Breather-Pipe Materials 129 Recent Systems 142Technology Appurtenances 143Improvement-O-Ring Cycle Counter 143Gasket Pipe for Electronic Air AdmissionVacuum Use 129

Control 143Service Laterals (Valve Pit Division Valves andto Vacuum Main) 129 Cleanouts 144

Valve Pit Settings and Dioision Valves 144Building Sewers 130

Cleanouts 144Definition of Terms 130

Vacuum Station Design 144Technology Improvement-

Discharge Pumps 144Integral Valve Pit 131Valve Pits 132 Materials 144

House-to- Pit-Sharing Sizing 146Ratio 132 Collection Tank 147Fiberglass Settings 132 Materials 147Buffer Tanks 132 Sizing 148Situation to Avoid- Vacuum Pumps 149Overuse of Buffer Materials 149Tanks 135 Sizing 150

Stub-Outs 135 Emergency (Standby)Building Sewers 137 Generator 150Backwater Valves 137 Statum Piping 151Air-Intake 138 Electrical Controls 151Appurtenances 139 Control Panel 151

Vacuum Valves 139 Motor Control Center 152

Vacuum Sewer Systems

Level Controls 153 Construction Inspection 165Monitoring System 153 General 165Vacuum Gauges 155 Duties 165Vacuum Recorder 155 Vacuum MainSump Valve 155 Installation 165Odor Control 155 Valve Pit Installation 166Noise and Heat Vacuum StationConsiderations 156 Construction 166

Construction Considerations 156 Record Drawmgs 166Vacuum Main Construction 156 Bidding Issues-

Line Changes 157 Procurement Options 167Grade Control 158 Operation and Maintenance

Horizontal DirectionalConsiderations 167

Drilling 158 Staffing Requirements 167Vacuum Testing During Operator Training 168Construction 159 Key to Successful Operation-

Valve Pit Installation 160 The System Approach 168Pit Installation 160 Maintenance 169

Pit Location 160 Normal and Preventative

Pit Orientation 160 Maintenance 169

Pit Installation 160 Vacuum Station 169Connecting the Pit to the Collection System

Main 162 Piping 170Sump Tightness Test 163 Vacuum Valves 171

Vacuum Station Emergency Maintenance 172Construction 163 Vacuum Station 172Final Vacuum Testing and Collection SystemSystem Startup 163 Piping 172

Vacuum Station Startup 163 Vacuum Valves 173Collection System Startup 163 Spare Parts Inventory 173Vacuum Testing Summary 164 Valves and Valve Pits 173

(continued)

99

100 AlternativeSewerSystems

Vacuum Station 174Special Tools 175

Operation and MaintenanceManual 175Recordkeeping 176

Normal MaintenanceRecords 177Preventative MaintenanceRecords 177EmergencyMaintenanceRecords 177

Operating CostRecords 178Evaluation of Operating Systems 178Operating History of VacuumSewers

Operation and MaintenanceData: 2003Operator Survey

LaborPower

Mean Time Between ServiceCalls 182Historical Problems 182Situation to Avoid-AcceptingFlow from an ExistingGravity System 184

System Costs 184Constructions Costs 184

General 184Vacuum Mains 185ValvePits 186Vacuum Stations 186

Operation and MaintenanceCosts

178

187187

Operation and MaintenanceInformation from 1991U.S.EPAManual (U.S.EPA1991a) 189Basis of Operation andMaintenance EstimatingCharts 189

Labor 189Effort to Operatea System-Actual versus BillableTime 190Power 191Utilities 191Clerical 192Transportation 192Supplies/Maintenance 192Miscellaneous Expenses 192Equipment Reconditioningand Replacement

System ManagementConsiderations

Sewer AuthorityResponsibilities

Customer Connectionto SystemOperating PersonnelSewer Use OrdinancePrivate versus PublicOwnership of EquipmentServing House

192

General

179179181

193

193

193193194

195

VacuumSewerSystems 101

Homeowner Responsibilities 195Other Entities 197

Engineer 198Regulatory Agencies 198

Education Process 198Sample Regulations forVacuum Systems 199

Requirements for Design ofVacuum Collection System 200

GeneralRequirements 200Design Requirementsfor theVacuum CollectionSystem 201Valve Pit Requirements 202Buffer TankRequirements 203Vacuum ValveRequirements 204Relation to WaterMains 205Vacuum CollectionSystem Leak

INTRODUCTION

Testing 205

Design of Vacuum Stations 205

Vacuum Station GeneralDesign Requirements 205

Vacuum Station ComponentSizing 206

Vacuum Pump Sizing 207

Instrumentation andControls 208

Safety Ventilation of theVacuum Pumping Station 208

Miscellaneous Requirements 208

Design of WastewaterTransmission (ForceMain)System 209

Customer Connections 209

References 210

OVERVIEW. The use and acceptance of alternative wastewater collection systemshave expanded greatly in the last 30 years. One of these alternatives-vacuumsewers-has been used in Europe for over 100 years, although this technology hasonly been in the United States since the mid-1970s.

Thirty years ago, vacuum sewers were regarded as "new" and only to be used asa system of last resort. Improvements in the technology later led to their acceptanceas "alternative" sewers, but they were still only to be used when significant savingswould result. Now, vacuum sewers have become an acceptable alternative in theproper application and are providing efficient and reliable sewer service to commu-nities all around the world.


The 1991 U.S. Environmental Protection Agency (U.S. EPA) (Washington, D.C.)manual, Alternative Wastewater Collection Systems (U.S. EPA, 1991a), characterizedvacuum sewers as lagging behind other collection types. This was a fair assessment atthat time, but is no longer true, as the vacuum system is now viewed on par with othersystem types. The lessons learned from the early systems resulted in better design andoperation guidelines. Advancements in the technology, which are detailed throughoutthis chapter, and system component improvements have led to more reliable, efficientsystems. Finally, awareness of the technology and its limits has been raised throughthe process of educational seminars, papers, and magazine articles. All of these factorshave led to an increased comfort level with vacuum technology.

THEORY OF OPERATION. Vacuum sewerage is a mechanized system of waste-water transport. Unlike gravity flow, vacuum sewers use differential air pressure tomove the wastewater. A central source of power to operate vacuum pumps is requiredto maintain a vacuum (negative pressure) on the collection system (Figure 3.1). Thesystem requires a normally closed vacuum-gravity interface valve at each entry pointto seal the lines, so that the vacuum can be maintained. These valves, located in valvepits, open when a predetermined amount of wastewater accumulates in collectingsumps. The resulting differential pressure between the atmosphere and vacuumbecomes the driving force that propels the wastewater towards the vacuum station.

A vacuum system is very similar to a water distribution system, except that theflow is in reverse (Figure 3.2) (AIRVAC, 2005a). This relationship would be completeif the vacuum valve was manually opened, like a water faucet is manually opened.

The exact principles of operation of a vacuum sewer system are somewhatempirical, by nature. An early concept centering on liquid plug-flow assumed that awastewater plug completely sealed the pipe bore during static conditions. The move-ment of the plug through the pipe bore was attributed to the pressure differentialbehind and in front of the plug. Pipe friction would cause the plug to disintegrate,thus eliminating the driving force. Therefore, reformer pockets were located in thevacuum sewer to allow the plug to reform by gravity and thus restore the pressuredifferential (Figure 3.3). Inthis concept, the re-establishment of the pressure differen-tial for each disintegrated plug was a major design consideration.

In the current sawtooth profile design concept, the reformer pockets are elimi-nated, so that the wastewater does not completely fill or "seal" the pipe bore. Air flowsabove the liquid, thus maintaining a vacuum condition throughout the length of thepipeline (Figure 3.4). Inthis concept, the liquid is assumed to take the form of a spiral,

Vacuum Sewer Systems 103

FIGURE 3.1 Typicallayout of a vacuum sewer system (courtesy of AIRVAC, Rochester, Indiana).

rotating, hollow cylinder. The momentum of the wastewater and the air carries the pre-viously disintegrated cylinders over the downstream sawtooth lifts. The momentum ofeach subsequent air-liquid slug and its contribution to the progressive movement ofthe liquid component of the previous slugs are the major design considerations.

Both of the above design concepts are approximations and oversimplifications ofa complex, two-phase flow system. The character of the flow within the vacuumsewer varies considerably. The plug-flow concept is probably a reasonable approxi-mation of the flow as it enters the system, whereas the progressive movement con-cept is more likely a better approximation of the flow throughout the vacuum main.

The reformer pocket concept, used in all of the early United States systems, grad-ually gave way to the sawtooth profile concept. Since the early 1980s, virtually allsystems in the United States have been designed using the sawtooth profile.


WATERVALVE

WATER SUPPLY SYSTEMWATER TANK(+) PRESSURE

WATER PUMP

SEWAGE PUMP

SEWAGE COLLECTIONTANK (-) PRESSURE

VACUUM COLLECTION SYSTEM

FIGURE 3.2 Similarities between a water system and vacuum system (courtesy ofAIRVAC,Rochester, Indiana).

FLOW~

FIGURE 3.3 Earlier design concept-reformer pocket (courtesy of AIRVAC,Rochester, Indiana).

FLOW~

SEWAGE AT REST

FIGURE 3.4 Current design concept-sawtooth profile (courtesy of AIRVAC,Rochester, Indiana).


APPLICABILITY AND ADVANTAGES. Applicability. The consulting engi-neer typically drives the community's choice of collection system type during theplanning stages of a wastewater facilities project. This choice is typically based on theresult of a cost-effectiveness analysis. Where the terrain is applicable to a gravitysystem, the engineer may not even consider a vacuum system. However, whilegravity may appear to be less costly in these situations, many small factors consid-ered collectively may result in a vacuum system being the proper choice. Below arethe general conditions that are conducive to the selection of vacuum sewers.

• Unstable soil;

• Flat terrain;• Rolling land, with many small elevation changes;

• High water table;• Restricted construction condition;

• Rock;• New urban development in rural areas;• Existing urban development, where built-out conditions exist; and

• Sensitive ecosystem

Experience has shown that, for vacuum systems to be cost-effective, a min-imum of 75 to 100 customers per vacuum station is generally required. Theaverage number of customers per station in systems presently in operation isapproximately 200 to 500.

Vacuum systems are limited somewhat by topography. The vacuum produced bya vacuum station is generally capable of lifting wastewater 4.5 to 6 m (15 to 20 ft).This amount of liftmay be sufficient to allow the designer to avoid all or many of thelift stations that would be required in a conventional gravity system and the opera-tion and maintenance requirements that they present.

Advantages. The advantage of vacuum collection systems may include substantialreductions in water use, material costs, excavation costs, and treatment expenses. Inshort, there is a potential for overall cost-effectiveness. Specifically, the followingadvantages are evident:

• Small pipe sizes-typically 10,15,20, and 25 em (4, 6, 8, and 10-in)-are used.

• No manholes are necessary.


• Field changes can easily be made, as unforeseen underground obstacles can beavoided by going over, under, or around them.

• Installation of smaller diameter pipes at shallow depths eliminates the needfor wide, deep trenches, reducing excavation costs and potential dewateringcosts.

• High scouring velocities are attained, reducing the risk of blockages andkeeping wastewater aerated and mixed.

• Elimination of the exposure of maintenance personnel to the risk of hydrogensulfide gas hazards.

• The system will not allow major leaks to go unnoticed, resulting in reducedenvironmental damage from exfiltration of wastewater.

• Only one source of power-at the vacuum station-is required. No on-lotpower demand exists at valve pits.

• The elimination of infiltration permits a reduction in size and cost of the treat-mentplant.

• Vacuum stations can be designed to blend with the surroundings more thantraditional lift stations.

• Valve pits are more concealable at the customer's property than are grinder-pump stations.

• A single-source responsibility exists, as one operating entity operates andmaintains the entire system, including the on-lot valve pit and valve.

MANUFACTURERS ACTIVE IN THE UNITED STATES MARKET.AIRVAC. AIRVACis based in Rochester, Indiana, and has been active in the vacuumsewer industry since 1969. In 2005, AIRVACwas purchased by Bilfinger-Berger, ainternational company based in Mannheim, Germany.

AIRVAC's only business is vacuum sewers. Their principal market is theUnited States. They use a combination of their own staff combined with manufac-turer's representatives to market their product in the United States. Their sec-ondary markets include Europe, Asia, and Australia. Outside of the United States,AIRVACis represented by licensees, which are companies licensed to represent andsell AIRVACproducts.


AIRVACmanufactures several sizes of vacuum valves, valve pits, vacuum sta-tion skids, and other system appurtenances. The vacuum valve used for residentialapplication is a full-port 7.S-cm(3-in.)piston-type valve.

AIRVACprovides complete technical assistance, including preliminary layoutand cost-estimating services, design assistance, construction supervision, and systemoperation services. In addition, they have a full-time research and developmentgroup that continues to seek technological advancements. Their design philosophyrevolves around the sawtooth profile. They typically recommend that one valve pitbe shared by two houses.

Roediger (Roevac). The German company, Roediger VHT (Hanau), operates avacuum sewer business under the trade name of Roevac. Roediger's parent companyis Bilfinger-Berger, a German-based international company.

Roevac has been active worldwide in the vacuum sewer industry since the mid-1980s, with their principal market being Europe. Secondary markets include theMiddle East, China, and Australia. Roevac was active in the United States marketfrom 1990until 2004,but withdrew in 2005 to concentrate on other worldwide mar-kets (Note: From 1990 through 2004, Roevac was active in the United States marketthrough their United States affiliate Roediger-Pittsburgh (Pennsylvania). In late 2003,Bilfinger-Berger sold the Roediger-Pittsburgh operation, but continued to market theRoevac product line in the United States, through a license agreement with the newowners of Roediger-Pittsburgh. This arrangement ended in 2004, when Bilfinger-Berger/Roediger VHT ceased all business relations with Roediger-Pittsburgh andbegan marketing their products directly in the United States. Then, in 2005,Bilfinger-Berger purchased AIRVAC and became the parent company of both AIRVAC andRoevac. At the time this manual went to publication, Roevac had withdrawn fromthe United States market to concentrate on other worldwide markets).

Roevac manufactures a vacuum valve and chamber and vacuum station skids.The installation contractor fabricates sumps in the field. The vacuum valve used forresidential application is a nominal6.5-cm (2.5-in.)diaphragm-type valve.

Roevac provides technical assistance, including preliminary layouts and designassistance. Roevac's design philosophy uses a combination of the sawtooth, wave,and reformer pocket profiles. They typically recommend one valve pit per house.

Iseki. The Rediweld Group recently purchased Iseki. Iseki is based in the UnitedKingdom (Daventry) and has been active in the vacuum sewer industry since the late


TABLE 3.1 Number of operating systems in the United States(as of December 31,2006).*

Period AIRVAC Iseki Roevac Vac-Q-Tec EVACactive in the 1969 1992 1995 1969 1969

United States to to to to tomarket 2006 2006 2006 1975 1985

AL 1AK 5 1 1 1AR 4CA 1 2CT 1FL 35 1 6GA 2IN 34IL 1KY 4 1LA 1MA 3MD 19 15MI 2MO 2 2MS 2NC 15NJ 1

NM 14NY 9 1OH 11OR 1 1PA 5 1SC 1 2TN 7TX 6VA 21 3 2WA 13WV 26

--Total 244 5 14 19 5

*Note: Projects are scheduled for design/ construction in Utah and Nevada in the next sev-eral years; other states may be contemplating systems also.


1980s. Their United States activity began in the 1990s. They have no direct presencein the United States, choosing to market via manufacturer's representatives instead.Iseki's primary business is microtunneling. In the vacuum industry, their principalmarket is Europe. Secondary markets include Asia, Australia, and the United States.

Iseki manufactures a vacuum valve. They also provide station skids throughbusiness agreements with subcontractors, who assemble skids for them. The vacuumvalve used for residential application is a nominaI7.5-cm (3-in.) piston-type valve.

Iseki provides technical assistance, including preliminary layout and cost-esti-mating services and design assistance. Iseki's design philosophy revolves aroundthe sawtooth profile. They recommend that one valve pit typically be shared bytwo houses.

EXTENT OF USE IN THE UNITED STATES. Projects in the United States.Table 3.1 provides a summary of the number of operating vacuum systems in theUnited States as of December 31, 2006. A map showing the number of systems in eachstate (as of December 2004) is included in Chapter 1 (Figure 1.5).

Summary. As shown in Table 3.2, almost all systems presently in operation in theUnited States are AIRVAC systems. For this reason, the remainder of this chapterfocuses on that approach. This, inno way, represents any endorsement of that system,but merely reflects the present state of the art invacuum sewerage in the United States.

TABLE 3.2 Vacuum sewers: extent of use in the United States(as of December 31, 2006).

FirstNumber of Number of operatingoperating states with United States Most recent

Manufacturer systems systems* system system

AIRVAC 244 27 1972 2006Iseki 5 3 1999 2002Roevac 14 7 2002 2006EVAC 5 4 1969 1985Vac-Q-Tec 19 3 1969 1975

---Total 287 29

*Somestates have systems by multiple manufacturers; therefore, the total number isnotadditive.


SYSTEM PLAN AND ELEVATION VIEWFigure 3.5 shows a plan and profile view of a typical vacuum sewer line. Figure 3.6shows a plan and profile view of a typical valve pit.

DESCRIPTION OF SYSTEM COMPONENTSA vacuum sewer system consists of three major components-the valve pit, vacuummains, and vacuum station. Figure 3.7 shows the relationship of the three compo-nents to the customer.

VALVE PITS. Valve pits and sumps are needed to accept the wastes from thehouse. These may consist of one unit with two separate chambers. The upperchamber houses the vacuum valve, and the bottom chamber is the sump, into whichthe building sewer is connected. These two chambers are sealed from each other. Thecombination valve pit-sump is typically made of fiberglass and is able to withstandtraffic loads. Buffer tanks are used for large customers or when a pressure-vacuumor gravity-vacuum interface is desired, as would be the case with a hybrid system.

The vacuum valve provides the interface between the vacuum in the collectionpiping and the atmospheric air in the building sewer and sump. The system vacuumin the collection piping is maintained when the valve is closed. With the valve opened,the system vacuum evacuates the contents of the sump. The valve is entirely pneu-matic by design and has a 7.S-cm (3-in.) opening size. Some states have made this aminimum size requirement, as this matches the throat diameter of the standard toilet.

A 10-cm (4-in.) air-intake is installed on the homeowner's building sewer, down-stream of all of the house traps. This air-intake is necessary to provide the volume ofair that will follow the wastewater into the main. This also circumvents the problemof inadequate house venting, which had resulted in trap evacuation in early installa-tions. Some operating entities require the air-intake to be located near a permanentstructure for aesthetic and protection reasons. In some instances, local ordinancesmay stipulate a minimum setback distance from the building structure (Figure 3.7).

VACUUM MAINS. The piping network connects the individual valve pits to thecollection tank at the vacuum station. Schedule 40, standard dimension ratio (SDR)21,or SDR26 polyvinyl chloride (PVC)pipe is used, with SDR21being the most common.Early systems used solvent-welded joints, but most recent systems use O-ring rubbergasketed pipe. Where gasketed pipe is used, the gaskets must be certified for use under

Vacuum Sewer Systems

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AIR INTAKE

VACUUM VALVE VALVE PIT ROADWAY

BUILDING SEWER VACUUMMAIN

SUMP

FIGURE 3.7 House/pit/main relationship (courtesy of AIRVAC).

vacuum conditions. Typical sizes include 7.5-, 10-, 15-, and 20-cm (3-,4-,6-,8-, andIfl-in.) pipe.

Polyvinyl chloride pressure fittings are needed for directional change and for thecrossover connections from the service line to the main line. These fittings may be sol-vent-welded or gasketed. The recent trend is to avoid solvent-welded fittings wherepossible, although there is a cost tradeoff to consider, as the gasketed fittings typicallyare more expensive, but are less labor-intensive than the solvent-welded fittings.

Lifts or vertical profile changes are used to maintain shallow trench depths andfor uphill liquid transport (Figure 3.8). These lifts are made in a sawtooth fashion. Asingle lift consists of two 45-degree fittings connected with a short length of pipe.

Division valves are used to isolate various sections of vacuum mains, therebyallowing operations personnel to troubleshoot maintenance problems in a timelyfashion. Both plug and resilient-wedge gate valves have been used, although mostrecent systems use gate valves. Some designs have included gauge taps installed justdownstream of the division valve. This tap makes it possible for one person to trou-bleshoot without having to check the vacuum at the station. This greatly reducesemergency maintenance expenses, both from a time and manpower standpoint.

Different pipe location identification methods have been used. These includemagnetic trace tape in the top of the trench, metal-toning wires above the pipe duringconstruction, utility-frequency-based electronic markers, and color-coding of thepipe itself (Figure 3.8).


GAGE TAP(OPTIONAL)

DIVISION VALVE

LIFT

---------j:~i~/t;{,I------------------------

FIGURE 3.8 Typical piping diagram-division valve/lift (courtesy of AIRVAC,Rochester, Indiana).

VACUUM STATION. Vacuum stations function as transfer facilities between acentral collection point for all vacuum sewer lines and a pressurized line leadingdirectly or indirectly to a treatment facility. Figure 3.9 shows the major componentsof the vacuum station.

Vacuum pumps are needed to produce the vacuum necessary for liquid-airtransport. They may be either the liquid-ring or sliding-vane type, although mostrecent systems use the sliding-vane type. Efficiency in the normal operating range isoften cited as the reason for this. The optimum operating range is 54 to 68 kPa (16 to20 in. of mercury [Hg]). The vacuum pumps, however, should have the capability ofproviding up to 84 kPa {25in. Hg), as this level is sometimes needed during emer-gency conditions and in the troubleshooting process. Redundancy is required, as thedesign capacity must be met with one pump out of service.

Discharge pumps are required to transfer the liquid that is pulled into the collec-tion tank by the vacuum pumps to its ultimate point of disposal. Dry pit pumps havebeen used extensively, although submersible wastewater pumps located on guiderails within the collection tank may be used as an alternative. The most frequentlyused pump has been the nonclog type. Redundancy is required, with each pump


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capable of providing 100% of the design capacity. The level controls are set for a min-imum of 2 minutes pump running time to prevent excessive pump starting andrelated increased wear. The pumps should have shutoff valves on both the suctionand discharge piping, to allow for removal during maintenance without affecting thevacuum level (Figure 3.9).

Check valves are used on each pump discharge line or on a common manifold afterthe discharge lines are joined to it. Equalizing lines are to be installed on each pump.Their purpose is to equalize the liquid level on both sides of the impeller, so that air isremoved (AIRVAC, 2005a). This ensures that the impeller is filled with liquid, whichallows the discharge pump to start without having to pump against the vacuum in thecollection tank. Because this setup will result in a small part of the discharge flow beingrecirculated to the collection tank, a decreased net pump capacity results.

Discharge pumps are typically located at an elevation significantly below the collec-tion tank, to minimize the net positive suction head (NPSH) requirement. In conjunc-tion with NPSH requirements, pump heads must be increased by 7 m (23 ft), to accountfor the collection tank vacuum. Both vertical and horizontal pumps can be used.

Materials of construction for discharge pumps are commonly cast iron withstainless-steel shafts. Cast aluminum, bronze, and brass should be avoided. Doublemechanical seals, which are adaptable to vacuum service, should be used.

An emergency (or standby) generator is a must. It ensures that on-lot flooding orbackup will be prevented through the continuing operation of the system in the event ofa power outage. Standard generators are available from a variety of manufacturers.

The wastewater is stored in the collection tank until a sufficient volume accumu-lates, at which point, the tank is evacuated. It is a sealed, vacuum-tight vessel made ofcarbon steel; fiberglass, or stainless steel. Fiberglass or stainless-steel tanks are gener-ally more expensive, but do not require the periodic maintenance of a carbon steeltank, which may require painting every 5 to 6 years. A vacuum, produced by thevacuum pumps, is transferred to the collection system through the top part of thistank. The part of the tank below the invert of the incoming vacuum collection linesacts as the wet well. A bolted hatch provides access to the tank, should it be necessary.

Most collection tanks are located at a low elevation relative to most of the com-ponents of the vacuum station. This minimizes the lift required for the wastewater toenter the collection tank, because wastewater must enter at or near the top of thetank, to ensure that the vacuum can be restored upstream. This may result in a deepbasement required in the vacuum station.

Vacuum SewerSystems 117

Vacuum switches located on the collection tank control the vacuum pumps.The typical operating level is 54 to 68 kPa (16 to 20 in. Hg), with a low-level alarmof 47 kPa (14 in. Hg). Seven probes-one for each of the six set points of thepumping cycle and one as a ground-are located inside the collection tank and con-trol the discharge pumps.

The vacuum-system control panel houses all of the motor starters, overloads,control circuitry, and the hours-run meter for each vacuum and wastewater pump.The vacuum chart recorder, collection-tank-level control relays, and fault monitoringequipment are also typically located within the vacuum-system control panel. Fault-monitoring systems include telephone dialers or other telemetry equipment,including radio-based supervisory control and data acquisition (SCADA) systems,digital or fiber-optic-based SCADAsystems, and telephone-based SCADAcommuni-cations systems.

Vacuum gauges, required to allow the operator to monitor the system, are usedon all incoming lines and on the collection tank. These gauges are very important inthe troubleshooting procedures. Chart recorders for both the vacuum and sewerpumps are needed, so that system characteristics can be established and monitored.

It is standard practice in the United States for the vacuum station equipment tobe supplied by the vacuum manufacturer, who preassembles, tests, and then shipsthe equipment to the job site on a skid(s). These skids can then be lifted into thebuilding and connected to the incoming vacuum mains and the outgoing force main.

The vacuum station equipment must be installed in a protective structure. Mate-rials of construction are the choice of the consulting engineer and typically areselected to match the architecture of the surrounding community.

SYSTEM DESIGN CONSIDERATIONSThis section provides a general overview pertaining to the design of the various com-ponents of a vacuum system. The reader is referred to the 2005version of AlRVAC'sDesign Manual for additional and more detailed design information (AIRVAC,2005a).

HYDRAULICS. Design Flows and Velocity. Design flows are maximumflowrates expected to occur once or twice per day and are used to size the vacuumsewer mains and the various vacuum station components. Instantaneous flowratesin excess of design flows can occur under certain situations.


While infiltration and inflow are not inherent in a vacuum system, they can beintroduced in the valve pits via the homeowner's piping and should be taken intoconsideration. Chapter 2 describes various studies of flow versus equivalentdwelling units (EDUs).

Tangential liquid velocities in the typical vacuum sewer are 4.6 to 5.5ml s (15 to18 ftl sec)-obviously well above the minimum required for self-cleaning. The hightransport velocities suggest that the probability of blockages occurring is remote.

Vacuum Transport and Air-Liquid Ratios. The system designer needs an under-standing of the vacuum transport process. With the sawtooth profile design concept(see following section), when no vacuum valves are operating, no wastewater trans-port takes place. All wastewater remaining in the sewers will lie in the low spots, andminimal vacuum loss is experienced throughout the system when this condition exists.

Sawtooth Profile Design Concept. The sawtooth profile design assumes that an openpassage of air between the vacuum station and the interface valves is maintainedthroughout the piping network, providing the maximum differential pressure at theinterface valves, to ensure maximum energy input to the vacuum mains.

When the liquid comes to rest at the base of various lifts, it does not come in con-tact with the crown of the pipe and therefore does not seal the pipe. Should the lift besealed for any reason, liquid is suspended on the downstream side of the lift, and anassociated vacuum loss is incurred.

The AIRVAC design philosophy includes an accounting of all lifts for all flowpaths within a system and the associated potential vacuum loss incurred. The staticloss limit assures that sufficient vacuum will be available for valve operation in theevent of a system upset, such as 100%lift saturation (AIRVAC,2005a).

Air-Liquid Ratios. Vacuum systems are designed to operate on two-phase(air-liquid) flows, with the air being admitted for a time period twice that of theliquid. The open time of the AIRVACvalve is adjustable; hence, various air-to-liquidratios are attainable.

The ability of the vacuum main to quickly recover to the same level of vacuumthat existed before the cycle, commonly referred to as vacuum recovery, is very impor-tant in vacuum sewer design. Vacuum recovery is a function of line length, pipediameter, number of connections, and amount of lift in the system.

The AIRVAC7.5-em (3-in.) valve is rated for 2 Lis (30gpm), assuming sufficientvacuum levels exist at the valve location. This capacity is achieved when the valve


TABLE 3.3 Recommended maximum design flowrates for valve pits.

Pit typeRecommended

peak flow (Us [gpm])Maximum*

peak flow (Us[gpm])

Standard valve pit

Single buffer tankDual buffer tank

Consult manufacturer

0.03 to 0.09 (0.5 to 1.5)

0.2 to 1 (3.1 to 15.0)

1 to 2 (15.1 to 30.0)

>2 (>30.0)

0.2 (3)

2 (30)

3.8 (60)

>3.8 (>60)

*Depending on static and friction loss, the overall amount of peak flow entering the systemthrough buffer tanks, and the exact location of the buffer tank, it may be possible to size aparticular valve pit or buffer tank with the upper limits shown in this column. Consult themanufacturer for guidance.

cycles 3 times/min, with each cycle discharging 38 L (10 gal). The length of one com-plete valve cycle is approximately 6 to 8 seconds, consisting of 2 to 3 seconds for theliquid, followed by 4 to 5 seconds of air. During this time, vacuum levels at the valvelocation temporarily drop, as energy is used to admit the wastewater into the mainline. The valve then "rests" before the next cycle begins, which occurs when another38 L (10 gal) accumulates in the sump. Vacuum recovery occurs during the valve-closed time, as the vacuum level in the main is restored. It is very important to notethat the 2-L/ s (30-gpm) rated capacity of the valve is not to be confused with the rec-ommended design capacity of the valve pit (see Table 3.3).

When vacuum levels decrease, there is a corresponding decrease in the valvecapacity. This is because of the lower pressure differential- which results in less avail-able energy and hence slower evacuation times. A low vacuum condition can occurwhen a larger-than-usual amount of flow enters the main, followed by a relativelysmall amount of air, resulting in a low air-to-liquid ratio. This could occur, forexample, at a buffer tank, where high discharge rates are common and whererepeated firing of the valve over short periods of time may not allow sufficientvacuum recovery. The net result would be progressively decreasing pressure differ-entials and lower evacuation rates. For this reason, a system with nothing but high-flow inputs is not recommended (see following section).

Situation to Avoid-Using Vacuum as an Interceptor Sewer. The idea of col-lecting wastes by gravity and then using a vacuum as an interceptor sewer isintriguing; however, this will not work very well, if at all. An insufficientair-ta-liquid ratio and subsequent waterlogging will result.


Experience has shown that liquid transport in a vacuum system works best whenthere are many small energy inputs (i.e., valve pits) located at various pointsthroughout the system. Conversely, the worst liquid transport characteristics occurwhere there is a single, large flow input located at the extreme end of a line.

The bottom line is that vacuum sewers are intended to be a form of collection, nottransmission (AIRVAC,200Sa).

Odors and Corrosion. There are few odor problems reported with vacuum sewers.The following three contributing factors are responsible for this: (1) the system issealed, (2) air is introduced and not discharged, and (3) detention times are short.

The entire system, from the valve pit setting to the vacuum station, is sealed. Thevalve pit sump containing the wastewater is tested for tightness, both at the factoryand in the field after installation, The piping system contains no air releases. The col-lection tank at the vacuum station, into which all of the sewer mains empty, is avacuum-tight vessel.

There is a large amount of air introduced to the system at each valve pit setting,which aids in the prevention of septic wastewater through the turbulence impartedwithin the collector pipes.

The typical valve cycle volume is approximately 38 L (10 gal). This small volumeresults in frequent valve cycles. Once in the main, the wastewater travels at velocitiesin excess of 4.5 ml s (15 ftl sec). Also, the relative liquid-to-air volume in the main isquite low. These factors result in a short detention time, which also aids in the pre-vention of septic wastewater.

A possible exception to the above discussion on odors can occur when concretebuffer tanks are used. Unlike the fiberglass settings, these tanks are open from thesump to the top of the lid. Operating personnel must be careful of sewer gas buildupin these tanks when performing maintenance, although the volume of wastewaterpresent in the tank may not be large enough to produce dangerous levels ofhydrogen sulfide. Also, these types of tanks typically are used to attenuate largeflows, allowing the wastewater more time to turn septic.

All of the system parts in contact with wastewater are made of corrosion-resis-tant materials (AIRVAC, 200Sa). As such, corrosion has not been a problem invacuum sewers.

The accumulation of grease is a cause for concern in a vacuum station, as itwould be in a conventional lift station. Grease builds up on level controls and on thesides of the vacuum collection tank. Grease traps are typically required in applica-tions such as restaurants to minimize these problems.


Grease has not presented problems in vacuum sewer mains. When the waste-water is evacuated from the sump, the suction generally pulls floatable grease intothe vacuum mains. Because the wastewater moves through the mains at high veloci-ties, there is little opportunity for grease in the sewer to build up in the system to anylevel that could cause a blockage.

Static and Friction Losses. Vacuum systems are designed not to exceed 4 m (13 ft)of static loss or 1.5 m (5 ft) of friction loss. This is a deviation from the early systemdesign, where the sum of the static and friction losses could not exceed 4 m (13 ft).These two losses are now considered independently.

The theory behind the 4-m (13-ft) static loss is simple. Normally, the vacuumpumps are set to operate at 54 to 68 kPa (16 to 20 in. Hg) of vacuum. The minimumvacuum of 54 kPa (16 in. Hg) results in a total available headloss of 5.5 m (18 ft); 1.5m (5 ft) of this headloss is required to operate the vacuum valve, thus leaving 4 m (13ft) available for wastewater transport (Figure 3.10).

Static losses are those incurred by using lifts, or vertical profile changes. Profilechanges are accomplished by using two 45-degree fittings joined by a section of pipe.

VACUUM PUMP

20" HG = 23' H20

30" HG = 34' H20

J__NORMAL OPERATING RANGE

t 16" HG = 18' H20

18' TOTAL AVAILABLE LIFT (AT LOW END)-S' REQUIRED FOR VALVE OPERATION13' AVAILABLE FOR SEWAGE TRANSPORT

FIGURE 3.10 Vacuum lift capability (courtesy of AIRVACrRochester, Indiana)(1 in. Hg = 3.377 kPa; 1 ft = 0.3 m).


TABLE 3.4 Recommended lift height.

Pipe diameter(em [in.])

Lift height(m [ft])

7.5 (3)

10 (4)

15 (6)

20 (8)

25 (10)

0.3 (1.0)

0.3 (1.0)

0.5 (1.5)

0.5 (1.5)

0.6 (2.0)

For efficient use of the energy available, profile changes should be as small as pos-sible. Numerous lifts are recommended rather than one large lift (AIRVAC, 2005a).Table 3.4 shows the recommended lift height for various pipe sizes. Static losses arecalculated by subtracting the pipe diameter from the lift height (Figure 3.11).

Friction losses are only calculated for sewers that are laid on a downward slopebetween 0.20 and 2.0% and are cumulative for each "flow path", from the furthestvalve on the line to the vacuum station. Friction losses in sewers installed at greaterthan 2.0% are ignored. Friction loss charts for SDR 21 PVC pipe and a 2:1 air-liquidratio have been developed by AIRVAC and are contained in their Design Manual(AIRVAC,2005a).

STATIC LOSS = LIFT HEIGHT - PIPE DIA

EX: 1.5 FT LIFT ON A 6" MAIN

STATIC LOSS = 1.5' - 0.5'

=1.0' 1STATIC LOSS

ILIFT HEIGHT

Lr---r=~~ ---'-PIPEDIA

FIGURE 3.11 Station loss determination (courtesy of AIRVAC, Rochester, Indiana)(1 in. = 2.54 em; 1 ft = 0.3 m).


Situations to Avoid. Vacuum sewers are a very reliable, cost-effective alternativewhen designed for the appropriate conditions. However, there are certain situationsthat should be avoided, as they may result in less-than-desirable results. Examples ofthese are the following:

• Using a vacuum as an interceptor sewer,

• Long distances with no connections,

• Overuse of buffer tanks, and

• Accepting flow from existing gravity system.

These situations are discussed in the "Situation to Avoid" sections of this manual.

Effects of Water Conservation. Vacuum sewer systems have been used in areasthat practice water conservation measures. There have been no specific studies doneregarding water conservation and its effect on a vacuum system. However, to date,there have been no negative reports of poor performance resulting from water con-servation from any vacuum system operators.

The general thinking is that a vacuum system can more easily handle a waste-water stream with less liquid than can a gravity system, which requires a certainamount of liquid to carry the solids. The major reason for this is the additional force(i.e., the pressure differential) that assists the natural gravity flow.

The best evidence of this can be seen in the cruise ship industry, where water con-servation goes beyond the low-flush toilets used in residential situations. For manyyears, cruise ships have successfully used internal vacuum systems, with vacuumtoilets that use as little as 0.9 L (1 qt) of water.

MAINS AND SERVICE LINES. Definition of Terms. The major componentsof the vacuum-piping network are defined below and are depicted in Figure 3.12.

• Main line-Larger diameter trunk lines that enter the vacuum station.

• Branch line-Smaller diameter lines that connect to the vacuum main.

• Service lateral-7.5-em (3-in.) vacuum line that connects the valve pit to thevacuum main.

• Valve pit-Point of connection for customer.

• Vacuum station-Heart of the system, where the vacuum is produced andwastewater is collected.


3" SERVICE LATERAL

BRANCH LINE (4" OR 6")

VACUUM MAIN (4", 6", 8", OR 10")

VALVE PIT

FIGURE 3.12 Major components of a vacuum system (courtesy of AIRVAC,Rochester, Indiana) (1 in. = 2.54 em).

Mains. Geometry and Sizing. The geometry of a vacuum sewer system is similar tothat of a water distribution system. Rather than looped, however, it is typicallydesigned in a tree pattern.

The length of vacuum mains is generally governed by two factors; these are staticlift and friction losses, which were previously discussed.

Because of restraints placed on each design by topography and wastewaterflows, it is impossible to give a definite maximum line length (length from vacuumstation to line extremity). Inperfectly flat terrain with no unusual subsurface obsta-cles present, a length of 3000 m (10 000 ft) can easily be achieved. With elevation toovercome, this length would become shorter. With positive elevation toward thevacuum station, this length could be longer. As an example, one operating system hasa line that, from the vacuum station to the line extremity, exceeds 5000 m (16 500 ft)in length.


Situation to Avoid-Long Distances with No Connections. Occasionally, an engi-neer will face a situation where there is a long pipe run with no house connections.'Thisis a situation to avoid, as poor liquid transport characteristics could result.

Movement of liquid within the vacuum main depends on differential pressure.The differential is created when a vacuum valve opens and allows atmospheric air toenter a vacuum main that is under a negative pressure. The only place this canhappen is where a vacuum valve exists.

Although most view the valve pit as simply a connection point for the customer,the designer views this as an Uenergy input" location. The more energy inputs alonga given vacuum main, the better the system operates. Conversely, as fewer energyinputs exist, the transport characteristics become poorer (AIRVAC,2005a).

Design Considerations. The use of horizontal directional drilling (HDD) could resultin significantly shorter lines, as the shallowest slopes that are currently attainablewith HDD are 0.50%rather than the 0.20%normally used. See the Horizontal Direc-tional Drilling section for a more detailed discussion on this subject.

Another important consideration is the location of the valve pits along thevacuum main. Designers should avoid layouts that result in long stretches with noconnections. A lack of valve pits, which act as energy inputs to the system, couldhave a detrimental effect on system hydraulics (see above section).

New developments, where many lots initially may be vacant, require special con-sideration. For these cases, it is suggested that the valve pit, less the valve, beinstalled to each lot at the same time the vacuum mains are installed. Should they beneeded, one or more electronic air admission control (EAAC) valves could beinstalled at strategic pit locations, with a timer (see discussion in the Electronic AirAdmission Control section), to actuate the valve to impart the necessary air andenergy to the piping network.

There are three major items for the designer to consider when laying out avacuum system.

(1) Multiple service zones. By locating the vacuum station centrally, it is pos-sible for multiple vacuum mains to enter the station, which effectivelydivides the service area into zones. 'Thisresults in operational flexibility andservice reliability. With multiple service zones, the operator can respond tosystem problems, such as low station vacuum, by analyzing the collectionsystem on a zone-by-zone basis, to see which zone has the problem. Theproblem zone can then be isolated from the rest of the system, so that


normal service is possible in the unaffected zones, while the problem isidentified and solved.

(2) Minimize pipe sizes. By dividing the service area into zones, the total peakflow to the station is also spread out among the various zones, making itpossible to minimize the pipe sizes.

(3) Minimize static loss. Static loss is generally limited to 4 m (13ft). Factors thatresult in static loss are increased line length, elevation differences, utilityconflicts, and the relationship of the valve pit location to the vacuum main(see following section).

Vacuum sewer design rules have been developed largely as a result of studyingoperating systems. Important design parameters, such as minimum distancebetween lifts, minimum slopes, and slopes between lifts, are contained in AIRVAC's2005 Design Manual (AIRVAC,2005a).

Minimizing Static Loss. Static loss is generally limited to 4 m (13 ft). Factors thatresult in static loss are the following:

• Line length. In perfectly flat ground, lifts are made to keep the vacuum mainat the same invert level. Every lift has an associated loss, depending on the liftheight and pipe diameter. By locating the vacuum station centrally, the lengthof the longest line can be kept to a minimum, resulting in the lowest possiblevacuum loss resulting from lifts.

• Elevation differences. Overcoming elevation differences will also require lifts.While the amount of lift needed to overcome a given elevation differencedepends on the pipe sizes, actual lift height, and so on, in general, the lift lossis similar to the actual elevation overcome.

• Utility conflicts and canal crossings. To cross under utilities and then returnthe pipe to minimum depth requires lift. Crossing over the utility, if possible,will minimize or even totally eliminate lift.

• Relationship of valve pit to main. There are situations where the elevation of ahouse is lower that the road elevation. In this case, the valve pit will have tobe located such that gravity flow from the house is possible. This typically willresult in lift in the service line between the valve pit and the vacuum main.

A key to minimizing vacuum loss is to avoid combinations of the above(AIRVAC,2005a).


TABLE 3.5 Recommended and absolute maximum flowrates for various pipe sizes.


Recommendedmaximum design

flowrate (Us [gpml)

Absolutemaximum

flowrate (Us Igpml)

10 (4)

15 (6)

20 (8)

25 (10)

2.5 (40)

6.62 (105)

13.2 (210)

23.7 (375)

3.5 (55)

9.46 (150)

19.2 (305)

34.4 (545)

Recommended Flowrates. Based on in-house hydraulic testing and an adaptation ofthe Hazen-Williams equation, AIRVAC developed a table showing the recommendedmaximum flowrates that should be used for design purposes and the absolute max-imum flowrate for a given pipe size. Table 3.5 shows these recommendations.

Line size changes are made when the cumulative flow exceeds the maximum rec-ommended design flow for a given line size. As a practical matter, most designerswill make this transition at a logical geographic location, such as a street intersection.

The values in Table 3.5 should be used for planning purposes or as a startingpoint for the detailed design. In the latter case, estimated site-specific flow inputs andthe friction tables should be used in the hydraulic calculations. A correctly sized linewill yield a relatively small friction loss. If the next larger pipe size significantlyreduces friction loss, the line was originally undersized.

The maximum number of houses served by a given line size is shown in Table 3.6,which assumes that the peak design flow for 1 house is 0.03Lis (0.50 gpm).

TABLE 3.6 Maximum number of houses served for vari-ous pipe sizes (based on maximum design flow using apeak design flow of 0.03 Lis [0.50 gpm]/house).


Maximum number ofhomes served

10 (4)

15 (6)

20 (8)

25 (10)

80210

420

750


Routing. An advantage to the use of vacuum sewers is that the small-diameter PVCpipe used is flexible and can be easily routed horizontally around obstacles. The fea-ture allows vacuum sewers to follow a winding path, as necessary.

In most cases, vacuum sewer mains are located outside of and adjacent to theedge of pavement and approximately parallel to the road or street, which reduces theexpenses of pavement repair and traffic control. In areas subject to unusual erosion,the preferred location is often within the paved area. Some municipalities also favorinstallation within the paved area, because subsequent excavation is less likely andmore controlled (via permit application only), and therefore the location is more pro-tected from damage. However, community disruption potential during constructionand maintenance for this approach increases substantially.

With two or more houses sharing one valve pit, overall system construction costscan be significantly reduced, resulting in a major cost advantage. In some circum-stances, however, this approach may require the main line to be located on privateproperty-typically in the backyard. There are two disadvantages to this type ofrouting. First, it requires permanent easements from one of the property owners,which may be difficult to obtain. Second, experience has shown that multiple househookups can be a source of neighborhood friction, unless the pit is located on publicproperty. The designer should carefully consider the tradeoff of reduced costs to thesocial issues before making the final routing decision.

Vacuum sewers are normally buried with a cover of 0.9 m (36 in.). Frost penetra-tion depths typically dictate the depth of burial in colder climates. In the northernUnited States, they are often placed at a depth of 1.2 to 1.5 m (4 to 5 ft). Although linefreezing is a concern with most engineers, it typically is not a problem with vacuummains, because retention time in small-diameter lines is relatively short, and turbu-lence is inherent.

The separation of vacuum sewers from water mains often requires the vacuumsewer to be buried deeper than would be required for other reasons. Horizontal andvertical separation requirements are dictated by state agencies and vary from state tostate. At least one state-Florida-has allowed for smaller separation of vacuum andwater lines, with the idea that, if both the vacuum and main were broken, air and waterwould be pulled into the vacuum sewer rather than raw wastewater leaking out.

Profiles of the mains should always be shown on the plans. Slopes, line sizes andlengths, culvert and utility crossings, inverts, and surface replacements are typicallyshown on the profiles.


Culvert and utility crossings often dictate numerous variations in the depth ofburial of vacuum sewer mains, with many resulting sags and summits in the pipelineprofile. Unlike pressure mains, where air accumulates at a summit, requiring an air-release valve, high points in the profile do not affect vacuum sewers. The sags, how-ever, do add lift to the system. If not designed and constructed properly, sags may trapwastewater at low-flow periods, blocking off the vacuum in the low part of the sewer.

Pipe Materials. Polyvinyl chloride thermoplastic pipe, typically class 200, SDR 21PVC, is typically used for vacuum sewers. In some installations, medium-densitypolyethylene, high-density polyethylene, and acrylonitrile-butadiene-styrene havebeen successfully used. In certain cases, ductile iron pipe has also been used,assuming the joints have been tested and found suitable for vacuum service. Toreduce expansion-and-contraction-induced stresses, flexible elastomeric joint("rubber-ring" joint) pipe is preferred. If solvent-welded joint pipe is used, the pipemanufacturer's recommendations for installation regarding temperature considera-tions should be followed.

Technology Improvement-O-Ring Gasket Pipe for Vacuum Use. Early vacuum sys-tems were constructed using solvent-welded joints. Several factors relating to gluingof pipe, including human error, led to problems with leaking joints.

In the mid-1980s, most designers began using standard O-ring PVC pipe that isequipped with a special double-lipped gasket. The number of main line leaks hasdecreased drastically as a result. Experience has proven this pipe to be economical toinstall, reliable to operate, and easy to repair.

Service Laterals (Valve Pit to Vacuum Main). Vacuum service laterals connectthe vacuum main to the valve pit. Service laterals are typically 7.5 em (3 in.) in diam-eter. As with the vacuum sewer mains, class 200, SDR 21 PVC pipe typically is usedfor the service laterals, with rubber O-ring fittings preferred.

Service laterals should be located distant from potable water lines, to reduce thepossibility of cross-contamination. If possible, they should also be distant from otherburied utilities, to reduce the possibility of damage caused by subsequent excava-tions for maintenance or repair of those utilities.

AIRVAC recommends all connections to the main be made "over the top", byusing a vertical wye fitting and a long radius elbow (Figure 3.13). Because of therestraints placed on the depth of the mainline sewers by the connecting service lat-erals entering "over the top", engineers should consider the minimum ground cover


jVACUUM MAIN

FLOW .. FLOW ..

90° ELL TURNED 45°

3" SERVICE LATERAL

~ FROM VALVE PIT

FIGURE 3.13 Over-the-top service connection (courtesy of AIRVAC,Rochester, Indi-ana) (1 in. = 2.54em).

required on these connections at the design stage. The invert spacing can be reducedby rolling the wye fitting to a 45-degree angle. This method is acceptable, providedthe invert of the connecting pipe is at least 5 em (2 in.) above the crown of the main.

Occasionally- it will be necessary to use lifts in the service line. AIRVAC'sDesignManual (AIRVAC,2005a)provides design parameters for these cases.

VALVEPIT SETTINGS AND BUILDING SEWERS. Definition of Terms.The major components relating to the valve pit are defined below and are depicted inFigure 3.14. This integral valve pit arrangement is an improvement over the earlyside-by-side valve pit arrangement (see following section).

• Pit cone-upper chamber that houses the vacuum valve.

• Sump-lower chamber that accepts wastewater from the building sewer.

• Pit bottom-physical separation between sump and pit cone.

• Vacuum valve-7.5-em (3-in.) interface valve (normally closed).

• Suction line-7.5-cm (3-in.)line used to evacuate the sump .

• Sensor line-5-cm (2-in.) line used to transfer rising sump pressure to valvecontroller.

• Stub-out--short piece of pipe that extends to customer's property.

• Service lateral-7.5 em (3-in.) vacuum line that connects valve pit to thevacuum main.


3" SERVICE LINE(CONNECTS TO FLEXIBLE

CONNECTOR' ~

FLOTATIONCOLLAR

VALVEPIT CONE

FIGURE 3.14 Major components of a valve pit (courtesy of AIRVAC,Rochester, Indi-ana) (1 in. = 2.54 em).

• Flotation collar-fiberglass collar to keep valve pit from floating.

• Valvepit cover-protective traffic-rated lid for pit (not shown in Figure 3.14).

• In-sump breather-device that allows a small amount of atmospheric air fromthe sump to close the valve controller, while protecting it from unwantedwater (not to be confused with the larger amount of air needed for wastewatertransport supplied by the air-intake).

Technology Improvement-Integral Valve Pit. Early (1970s) valve pits had aside-by-side arrangement, with rigid piping connecting the two chambers. Two prob-lems occurred-(l) problems with differential settlement led to pipe breaks and ulti-mately to vacuum leaks, and (2) the glued, rigid connecting piping frequently failed,also causing vacuum leaks.


The current top-bottom arrangement does not include this connecting piping.This eliminated contractor error on two fronts-eompaction of the pits and gluing ofthe connecting pipe fittings (AIRVAC,2005a).

Valve Pits. House-to-Pit-Sharing Ratio. Up to four separate building sewers canbe connected to one sump, each at 90 degrees to one another. However, this is rarelydone, as property line considerations and other factors may render this impractical.By far, the most common valve pit sharing arrangement is for two adjacent houses toshare a single valve pit (AIRVAC,2005a) (refer to the Operation and MaintenanceData: 2003Operator Survey section).

Some have attempted to reduce costs by having additional houses sharing asingle valve pit. Experience has shown that, while this may appear to be viable onpaper, many times, it is not achievable during construction. Also, even if it is, the per-ceived cost savings do not always materialize. Longer runs of gravity laterals arerequired, which results in deeper valve pits needed to accommodate them. Also, theadditional (0.6or 0.9m [2 or 3 £l]) of excavation, of not just the pit, but the gravity lat-erals too, may result in extensive dewatering.

In certain cases, such as the existence of a cul-de-sac or for small lots with shortfront footage, it may be possible to serve three or even four houses with a single valvepit; however, all other design factors must be considered.

Fiberglass Settings. Figure 3.15 shows a premanufactured, fiberglass type of valvepit setting, which is by far the most common. This type of setting is composed of fourmain parts-the bottom chamber (sump), top chamber (valve pit), plate that sepa-rates the two chambers (pit bottom), and lid.

Wastes from the home are transmitted via the building sewer to the stub-out. Thestub-out inlet enters the sump 45 em (18in.) above the bottom. Holes for the buildingsewers are field-cut at the position directed by the engineer. Various valve pitarrangements are available to accommodate varying depths of service (1.5 to 3.0 m[5 to 10 ftj). The shallower arrangements would be used in areas where high ground-water or poor soils exist, and the depth of the building sewers is very shallow.

Buffer Tanks. Buffer tanks are typically used for schools, apartments, nursing homes,and other large-volume users. Their use should be limited to users where service atonly one or two locations is possible (i.e., a commercial user); they should not be usedwhere individual valve pits could otherwise be used (AIRVAC,2005a).


CAST IRON FRAME AND LIDRATED FOR H20 LOADING

SPIRAL WOUND, H20 LOADINGRATED, FIBERGLASS VALVE PIT.27"1.0. AT TOP, 36" ID AT BOnOM

FLEXIBLE CONNECTOR

VACUUM SE ER MAIN(DIRECTION OF FLOW INTO

PAPER)

4" OR 6" GRAVITY SEWERWITH COUPLING

2" SENSOR LINE

30" OR 54" DEEPFIBERGLASS SUMP, 30"'DAT TOP, 16" I D AT BOnOM

3" SUCTION LINE

FIGURE 3.15 Typical fiberglass valve pit setting (courtesy of AIRVAC,Rochester,Indiana) (1.D.= internal diameter; 1 in. = 2.54em).

Buffer tanks are designed with a small operating sump in the lower portion, withadditional emergency storage available in the tank. These types of settings are typi-cally constructed of 1.2-m- (4-ft-) diameter concrete manhole sections, with thebottom section having a prepoured bottom with a 45-cm- (18-in.-) diameter sump(Figure 3.16).

It is very important that all joints and connections be water-tight, to eliminategroundwater infiltration. Equally important is the need for a well-designed pipe sup-port system, because these tanks are open from top to bottom. The support hardwareshould be of stainless steel and Ior plastic.

Considering the discussion in the Vacuum Transport and Air-Liquid Ratios sec-tion, a lower evacuation rate of 1 Lis (15 gpm) is recommended for buffer tanksizing. This lower rate assists proper vacuum recovery. Table 3.3 shows the recom-mended maximum design flows for buffer tanks.


3" VACUUM SERVICELATERAL

CONCRETE MANHOLESECTION WITH STEPS

3" PVC VACUUMSERVICE LATERAL

4" OR 6" GRAVITY INLET

-r '~.

FIGURE 3.16 Concrete buffer tank (courtesy of AIRVAC, Rochester, Indiana)(1 in. = 2.54 em).


A dual buffer tank is similar to a single buffer tank, with the exception that it islarger, to accommodate two vacuum valves (Figure 3.17). These tanks typically usel.5-m- (5-ft-) diameter manhole sections. Dual buffer tanks may also be used, if thesingle buffer tank does not have the capacity for the large flows.

AIRVAC recommends that a specific "not-to-exceed" percentage of the total peakflow be contributed to the vacuum mains from buffer tanks (AIRVAC, 2005a) (see fol-lowing section). This design criterion was established to ensure that an adequate air-to-liquid ratio is maintained in any section of the vacuum mains, thereby preventingthe mains from becoming water-logged, and for the timely recovery of vacuum con-ditions throughout the system.

The amount of flow entering via a buffer tank(s) at a single location is also impor-tant. This is dependent on the location of the buffer tank in the piping network.AIRVAC's Design Manual (AIRVAC, 2005a) provides some general guidelinesregarding this.

Situation to Avoid-Overuse of Buffer Tanks. AIRVAC does not recommend usingbuffer tanks on a widespread basis. Much depends on the location of the buffer tankitself, length of travel to the vacuum station, amount of lift in the system, and numberof other buffer tanks in close proximity. In general, the closer to the station, the lesslift, and the fewer other buffer tanks that exist, the better chance there is of the systemoperating successfully.

Because of the possibility of water-logging, the AIRVAC Design Manual (AIRVAC,2005a) limits the use of buffer tanks as follows: (1) no more than 25% of the total peakflow of the entire system shall enter through buffer tanks, and (2) no more than 50%of the total peak flow of a single vacuum main (i.e., flow path) shall enter throughbuffer tanks (AIRVAC,2005a).

Stub-Outs. Typically, the valve pit setting will include a short piece of pipeextending from the sump to the property line. This short section is called a stub-out(see Figure 3.14). The stub-out is used to minimize the risk of damage to the fiber-glass valve pit during homeowner connection to the system.

Pressure-rated schedule 40 or SDR 21 PVC pipe is recommended for the stub-out,because this type of pipe has an outside diameter that matches the grommet pro-vided in the sump opening. A tight fit at this joint prevents the entrance of infiltra-tion and inflow to the sump.


4" OR 6" GRAVITYINLET (TYP.)

3" VACUUM SERVICE LATERAL

4" OR 6" GRAVITY INLET

18" DIAMETER SUMP

CONCRETE MANHOLESECTION WITH STEPS

3" VACUUM VALVE

18" DIAMETER SUMP

FIGURE 3.17 Concrete dual buffer tank (courtesy of AIRVAC, Rochester, Indiana)(1 In. = 2.54 em).


Building Sewers. The term building sewer refers to the gravity flow pipe extendingfrom the house to the valve pit setting. The building sewer connects to the stub-out.In many cases, state or local authorities regulate the installation of building sewers.Uniform Plumbing Code (IAPMO, 2006) is often referenced.

For residential service, the building sewer should be 10 em (4 in.) and slope con-tinuously downward at a rate of not less than 2% (0.25-in./ft). The line size for com-mercial users will depend on the amount of flow and local code requirements.

Bends should be avoided in building sewers and a cleanout used for each aggre-gate change in direction exceeding 57 degrees. H the building sewer piping networkdoes not have a cleanout within it, one should be placed outside and close to thehome. Some agencies prefer having a cleanout at the dividing line where agencymaintenance begins. Inmost cases, this would be at the end of the 1.8-m (6-ft) stub-out pipe.

Infiltration via leaking building sewers has been common, as has the connectionof roof and yard drains (inflow). A quality inspection during homeowner connectionis advised to determine if these situations exist. If so, steps should be taken to requiretheir elimination before final homeowner connection.

As was the case with the stub-out pipe, pressure-rated schedule 40 or SDR 21PVC pipe is also recommended for the building sewer. Should the air-intake beblocked and the vacuum valve fail in the open position at the same time, the buildingsewer could experience the full system vacuum of 68 kPa (20 in. Hg). Therefore, theexisting building sewer may need to be replaced at most existing houses.

AIRVAC's 2005 Design Manual (AIRVAC, 2005a) contains specific informationregarding recommended pipe types, for both the stub-out pipes and the buildingsewers.

Backwater Valves. Vacuum valves almost always fail in the open position. In thisfailure mode, the homeowner is not affected, as the contents of the sump will con-tinue to be emptied as long as sufficient vacuum levels remain. It is rare that avacuum valve fails in the closed position because of a physical defect or problemswith the internal workings of the valve itself. It is possible, however, that the valvedoes not open because of insufficient vacuum. A broken or leaking vacuum main orsome other system problem that results in a low vacuum condition could cause this.In this case, wastewater backup into the home may occur.

Some entities prefer the use of backwater valves on the building sewer, to pro-vide wastewater backup protection, especially for houses that share a valve pit,when the houses are at different elevations. For the typical "normally closed" -type


backwater valve, positioning is critical. If not installed between the house and theair-intake pipe, the vacuum valve will not operate. The preferred type of backwatervalve is one designed to be in the "normally opened" position. Location is not anissue for this type of valve, and they have proven to have superior performancewhen used in a vacuum system.

Air-Intake. An air-intake, consisting of lO-cm (4-in.)PVC pipe, fittings, and a screen(Figure 3.18),is required for each building sewer. Its purpose is to provide a sufficientamount of air to enter into the vacuum main via the building sewer and valve pit, toact as the driving force behind the liquid that is evacuated from the sump. Becauseplumbing vents allow air out, while the air-intake draws air in, the air-intake is not con-sidered to be a component of the customer's plumbing vent system.

Most plumbing code enforcement entities require the air-intake to be locatedagainst a permanent structure, such as the house or a wall. Because of the variationin local code enforcement agencies, the design engineer and governing utility shouldbe aware of the plumbing code requirements and inform the homeowner beforeinstallation of the required sewer connection.

4ft OR 6" GRAVITY

FLEXIBLE COUPLING l r- 4" OR 6" GRAVITY LINEFROM CUSTOMER

..:.Ro:;:E:..:So:;:ID:..:E:;.N:..:C:.::E:-......._ b:====·=r:c=:I=-~r=~==:a...!T~O~V'!.A~C~U!.!:U~M~V'f..!.AL~V:!:Ec.!:P~IT!...._

SCREEN

4" AIR INTAKE PIPEUSING 90° PVC ELLS

1-------- STRUCTURE OR POST

4"-45° BEND

4"x4"x4" WYE (IF 4" GRAVITY USED)6"x6"x4" WYE (IF 6" GRAVITY USED)

FIGURE 3.18 Air-intake (courtesy of AIRVAC,Rochester, Indiana) (1 in. = 2.54 em).

VacuumSewer Systems 139

The house vent may not be used in place of an air-intake. While it may providethe necessary air, its location could result in the house plumbing traps being evacu-ated during a valve cycle (AIRVAC,2005a). For this reason, the air-intake must belocated downstream of the last house plumbing trap. Also, the use of a check valvewithin the air-intake itself is not recommended, as this also will result in problemswith the traps inside the house.

Appurtenances. The designer should perform buoyancy calculations to see if anti-flotation collars are needed to prevent the valve pit from floating. Until recently, anti-flotation collars consisted of concrete rings. These concrete rings required that care betaken during the valve pit installation, as poor bedding and backfill could lead to dif-ferential settlement around the valve pit, causing the heavy concrete collars to shift.The result can be damaged valve pits and/ or broken vacuum lines near the valve pit.

Recent systems have used fiberglass antiflotation collars (see Figure 3.14),whichfit around the tapered valve pit and rely on the soil burden to keep the pit fromfloating. The fiberglass collars not only eliminate the settlement problem, but aremuch less labor-intensive to install than concrete collars.

Safety is also a concern with the old style concrete rings. Concrete collars havemetal lifting rings used during the initial placement of the collar. There have beenreports of these lifting rings later failing, after being exposed to corrosive under-ground conditions. Not only can the valve pit be easily damaged should the collarslater be removed, but the workers are also vulnerable to injury during this process.Again, the lightweight fiberglass collars eliminate this problem.

VACUUM VALVES. Vacuum Valve. There are two different types of vacuumvalves-the piston type and the diaphragm type. While both types are used in Euro-pean and other countries, virtually all United States installations use the piston-typevalve manufactured by AIRVAC. For that reason, the discussion that follows willconcentrate on the piston-type valve.

The AIRVAC valve is vacuum-operated on opening and spring-assisted onclosing. System vacuum ensures positive valve seating. The valves have a 7.5-em (3-in.) full-port opening, are made of glass-filled polypropylene, and have stainless-steelshafts, delrin bearings, and elastomer seals (AIRVAC,200Sa). All materials of thevalve are chemically resistant to typical domestic wastewater constituents and gases.

Vacuum sewer design follows the industry standard that advocates the abilityto pass a 7.S-cm (3-in.) solid through any part of a wastewater collection system.


Also, some plumbing codes have provisions that prohibit restrictions less than7.5 em (3 in.) downstream of any toilet. For these reasons, a minimum valve size of7.5 em (3 in.) is recommended.

The driving force in a vacuum system is the pressure differential that existsbetween the atmosphere and vacuum in the system. This differential occurs when thevalve opens. As a result, the only place to impart energy in a vacuum system is at thevalve itself. Because vacuum sewers have a limited amount of energy available, anyloss through the valve further depletes this energy, resulting in less energy availablefor transport within the pipeline. This is especially critical when one considers thatthis loss occurs at each valve and during each valve cycle. For this reason, it is impor-tant to use a vacuum valve with a high flow coefficient (Cy) factor and, hence, a lowheadloss through the valve. The C, factor is the flowrate in liters per second (gallonsper minute), which would yield a headloss of 1 psi.

Controller/Sensor. The controller/sensor is the key component of the vacuumvalve. The device relies on three forces for its operation-pressure, vacuum, andatmosphere. As the wastewater level rises in the sump, it compresses air in the sensortube. This pressure initiates the opening of the valve, by overcoming spring tensionin the controller, and activates a three-way valve. Once opened, the three-way valveallows the controller/sensor to take the vacuum from the downstream side of thevalve and apply it to the actuator chamber to fully open the valve. The controller /sensor is capable of maintaining the valve fully open for a fixed period of time, whichis adjustable over a range of 3 to 10 seconds. After the preset time period has elapsed,atmospheric air is admitted to the actuator chamber, permitting spring-assistedclosing of the valve (AIRVAC,2005a).

The AlRVACvacuum valve controller was designed to give consistent timing,as set by the system operator. The AlRVAC valve pit was also designed so that avery repeatable, specific amount of liquid is withdrawn on each cycle (38 L [10gal]). With a consistent amount of air and a specific amount of liquid, theair-liquid ratio is kept consistent.

During system startup, AIRVACpersonnel will set the valve-controller timing,while explaining the reasoning as it applies to that system. In some circumstances, awhole system or small portions of a system may be designed to operate at higher air-to-liquid ratios. In this case, the controller can be adjusted to stay open longer in spe-cific high-lift areas. This longer timing will keep system lifts cleared, preventing


undesired vacuum loss. The ability to field-adjust the air-to-liquid ratio by adjustingthe controller timing has been cited by operators as a necessary feature.

The AIRVAC slip key is provided as standard equipment on the AIRVAC valve.The slip key allows the operator to remove and reinstall the controller withouthaving to unscrew bolts. This feature was the result of feedback from operators, whoexpressed the desire to be able to more easily remove and replace the controllers.

Breathers. As discussed earlier, the controller requires a source of atmospheric airto the actuator chamber, permitting spring-assisted closing of the vacuum valve.Without this air, the valve will stay in the open position. Two types of breathers havebeen used-external and in-sump breathers.

External Breather. Almost all systems installed before 2000 used an external breather(Figure 3.19). While relatively reliable, external breathers had two items that requiredattention. First, the entire breather piping system, from the dome to the connection atthe controller, must be water-tight. Second, the piping must slope toward the valve

GREEN PLASTICBREATHER DOME

FIBERGLASSVALVE PIT

ATMOSPHERIC

AIR IN-------0"'--'12' STANDARD LENGTH1-1/4" I.D X 3/8" WALLGREEN POLYURETHANETUBING

18"

ATMOSPHERIC AIR TOCONTROLLER (TO CLOSE VALVE)

3/4" BREATHER "T"

FIGURE 3.19 Early system external breather (courtesy of AIRVAC, Rochester, Indi-ana) (I.D. = internal diameter; 1 in. = 2.54 em).


pit setting. If not properly installed or maintained, the external breathers could allowwater to be directly pulled into the controller, resulting in the valve failing in theopen position.

While external breathers have been successfully used in many operating vacuumsystems, some problems with perceived aesthetics and vandalism have been experi-enced. Also, the external breather was consistently cited by operators as the singlelargest contributor to valve failures via "water in the controller". Because of this,most recent systems use the in-sump breather described in the next section.

In-Sump Breather-Recent Systems. In the late 1990s,AIRVACpatented a device thateliminated the external breather. This was called the in-sump breather(Figure 3.20). Asits name implies, this device uses atmospheric air from the sump to close the valve con-troller, rather than from an external source. In the event of low vacuum conditions, in

4"AIRINLET

ATMOSPHERIC AIRTO CONTROLLER (TOCLOSE VALVE)

ATMOSPHERIC AIR IN

IN-8UMP BREATHER

FIGURE3.20 Recent system in-sump breather (courtesy of AIRVAC, Rochester, Indi-ana) (1 in. = 2.54 em).


which the valve would not open, floats in the in-sump breather protect the controllerfrom unwanted liquid.

With the in-sump breather, the installation of the homeowner's building sewerbecomes more critical. A sag in the building sewer alignment will trap water and notallow the free flow of atmospheric air-an additional reason for redoing the buildingsewer at existing residences.

Appurtenances. Cycle Counter. To monitor the number of valve cycles, a cyclecounter can be used. This device mounts directly on the vacuum valve or the valvepit wall. Cycle counters typically are used where a large water use is expected, todetermine if the valve is reasonably capable of keeping up with the flow, or where anexcessive amount of infiltration or inflow in the building sewer is suspected.

Some entities use the cycle counter as a metering device. Knowing the numberof cycles and the approximate volume per cycle, one can estimate the amount ofwastewater through the vacuum valve over a given period. This would also allowa user to identify the amount of water used and discharged versus outdoor or otherconsumptive uses.

Others use the device as method of determining illegal storm connections. Theflow through the valve can be estimated and compared with metered water use.From this, it is possible to determine the approximate amount of extraneous waterentering the system.

Electronic Air Admission Control. The EAAC is an accessory for a typical7.5-cm(3-in.) vacuum valve, adding separate battery-operated controls (AIRVAC,200Sa).This device monitors the line vacuum and will open if the vacuum falls below apreset limit for an extended period of time. The opening of this valve injects largequantities of atmospheric air to boost liquid through various lifts and downstreamtowards the vacuum station. The end result is an increase of the available vacuumfor valve operation.

The primary purpose of the EAAC is to improve the transport characteristics of asystem that is already in operation. Occasionally, this device can be included as adesign feature when static losses are slightly in excess of the recommended 4 m (13ft).Because there are limits to the use of these devices, the vacuum manufacturer shouldbe involved in the placement and parameters for their use.


DIVISION VALVES AND CLEANOUTS. Division Valves. Division valvesare used on vacuum sewer mains, much as they are on water mains. Plug valves andresilient-seated gate valves have both been successfully used, although most systemsnow use gate valves. Typical locations are at branch-main intersections, at both sidesof a bridge crossing, at both sides of areas of unstable soil, and at periodic intervalson long routes. The intervals vary with the judgment of the engineers, but typicallyrange from 450 to 600m (1500to 2000ft).

The valves should be installed in a valve box conforming to local codes, with theoperating nut extended to a position where it is accessible with a standard valvewrench. The valves should be capable of sustaining a vacuum of 81 kPa (24 in. Hg).Contract specifications should call for a certified test from an independent laboratoryto verify this.

More recent designs have included a gauge tap, located on the downstream sideof each division valve (Figure 3.21). Its purpose is to allow periodic or trou-bleshooting vacuum monitoring by one person in the field.

Clean outs. Cleanouts, called access points in vacuum sewer terminology, havebeen used in the past. Their use is no longer recommended in systems with highvalve pit density, because access to the vacuum main can be gained at any valvepit. However, some state codes still require cleanouts to be installed at specifiedintervals. In these cases and in stretches where valve pits are non-existent, accesspoints should be constructed.

VACUUM STATION DESIGN. The Vacuum Station section described the pur-pose of the vacuum station, and Figure 3.9 showed its major components. Additionalinformation regarding the major station equipment follows.

Discharge Pumps. Materials. Duplicate pumps should be used, with each capableof delivering the design capacity at the specified head conditions. These are typicallyhorizontal, nonclog, centrifugal pumps. Because the pump is drawing from a tankunder vacuum, the NPSH calculations are especially criticaLA certification from thepump manufacturer that the pumps are suitable for use in a vacuum sewerage instal-lation is strongly recommended.

The wastewater pumps are the most susceptible component to submergence,so it would be wise to consider dry-pit submersible pumps for flood-prone areas.These pumps may operate in a dry pit under normal conditions and, if necessary,


VAlVE EXTENSIONWITH 2" SQUARE NUT

3/8' BARB ADAPTER WITH CAP

314" FPT, COMPRESSION FITTING,

314", SDR·7, 200 PSI,POLYETHYLENE TUBING

6" PVC PIPE EXTENSION

314" MPT, COMPRESSION FlTING,

DIVISION VAlVE OPTIONAl GAUGE TAP

FIGURE 3.21 Division valve with optional gauge tap (courtesy ofAIRVAC,Rochester, Indiana) (1 in. = 2.54em; 1 psi = 6.895kPa).

continue to operate while submersed. The obvious disadvantage is a motor cou-pled to the pump casing that is sealed, making maintenance more difficult. Sub-mersible pumps tend to require slightly higher NPSH requirements than theirnonsubmersible counterparts, so some special arrangements may be necessary tosatisfy this criterion also.

Equalizing lines are to be installed on each pump. Their purpose is to equalizethe liquid level on both sides of the impeller, so that air is removed (AIRVAC,2005a). This ensures that the impeller is filled with liquid, which allows the dis-charge pump to start without having to pump against the vacuum in the collectiontank. Clear PVC pipe is recommended for use, as small air leaks and blockages willbe clearly visible to the system operator. On small discharge pumps (generally lessthan 6.38 L/ s fIOOgpm]), the equalizing lines should be fitted with motorized fullport valves that close when the pumps are in operation.


Sizing. To size the discharge pumps, the following formula should be used:

Qdp = Qmax = Qa X peak factor (typical peak factors range from 3.0 to 4.0) (3.1)

Where Qdp = discharge pump capacity (LI s [gpm]),Qmax = peak flow (Lis [gpmj), and

Qa = average flow (Lis [gpm]).

The total dynamic head (TOH) is calculated using standard procedures for forcemains, However, head attributed to overcoming the vacuum in the collection tank(Hv) must also be considered, resulting in the following formula:

(3.2)

Where TOH = total dynamic head (m [ft]),Hs = static head (m [ft]),H, = friction head (m [ft]), andHv = vacuum head (m [ft]).

The H; value is typically 7 m (23 ft), which is roughly equivalent to 68 kPa (20 in.Hg) (typical upper operating value). Because the Hv will vary depending on the tankvacuum level (54 to 68 kPa [16 to 20 in. Hg]), with possible operation at much lowerand higher levels during problem periods), it is prudent to avoid a pump with a flatcapacity-head curve.

Where possible, horizontal wastewater pumps should be used, as they have lesssuction losses compared with vertical pumps. To reduce suction line friction losses,the pump suction line should be 5 cm (2 in.) larger than the discharge line.

Net positive suction head calculations are important in the discharge pumpselection process. Nomenclature and typical values are given in Table 3.7.

Figure 3.22 is a diagram for calculation of NPSHa in a vacuum system. To calcu-late NPSHa, the following formulas should be used:

NPSHa = havt + hs - h, - hvpa

havt = ha - Vmax

(3.3)

(3.4)

The NPSHa value must be greater than NPSHr• The NPSHa and TDH valuesshould be calculated for both the high and low vacuum operating levels and com-pared with the NPSHr at the corresponding point on the head-eapacity curve.


Hvpa (0.7B')

Hf (2')

Ha (33.4') Vmax (22.6' @ 20'Hg)

[

1" EQUALIZINGLINE OPEN ATALL TIMES_a_ .._._._._

NPSHa

NPSHr

Hs (1')

DESIGN FOR BOTH CONDITIONS•MAXIMUM TANK VACUUM (20"Hg)•LOWEST LIQUID LEVEL

SEWAGE PUMP

FIGURE 3.22 NPSHa calculation with typical values (courtesy of AIRVAC,Rochester, Indiana) (1 in. Hg = 3.377kPa; 1 ft = 0.3m).

Collection Tank.Materials. Carbon-steel, stainless-steel, and fiberglass tanks areacceptable. Carbon-steel and stainless-steel tanks should be of a welded constructionand fabricated from not less than 0.6-cm- (0.25-in.-) thick steel plates. The tanksshould be designed for a working pressure of 68 kPa (20 in. Hg) vacuum and testedto 95 kPa (28 in. Hg) vacuum. Fiberglass tanks may be substituted using the samespecifications. Fiberglass tanks are to have 1000kPa- (150psi-) rated flanges.


Term Definition

TABLE 3.7 Discharge pump NPSH calculation nomenclature (ft X 0.3048= m).

Typical value

Net positive suction head available, ft

Head available because of atmospheric pressure, ft

Head available because of atmospheric pressure atliquid level less vacuum in collection tank, ft

Maximum collection tank vacuum, ft

Depth of wastewater above pump centerline, ft

Absolute vapor pressure of wastewater at itspumping temperature, ft

Friction loss in suction pipes, ft

Net positive suction head required by the pumpselected, ft

(calculated)

33.9 at sea level33.2 at 500ft32.8 at 1000ft29.4at 4000ft

(calculated)

18.1 at 16-in. Hg22.6at 20-in.Hg

1.0 (minimum)

0.8

2.0(vertical pump)1.0 (horizontal pump)

Varies by pump

Carbon-steel tanks should be sand-blasted and painted. The internal coating typ-ically consists of two coats of epoxy primer suitable for immersion in wastewater.The external coating is one coat of epoxy primer and one coat of epoxy finish.

The tank should be furnished with the required number and size of openings,manways, and taps, as shown on the plans. In addition, the tank should be suppliedcomplete with sight glass and its associated valves.

It is recommended that only one vessel be used for most projects. Should thesingle tank concept result in a tank too large to transport, dual tanks are recom-mended, but this is very rare. Although a total tank failure would result in totalsystem outage, no such failures have ever been reported.

Sizing. Collection tanks are sized on peak flow to the vacuum station and to ensureadequate operating volume to prevent wastewater pump short cycles and emergency


storage volume. Using these criteria, the wastewater pumps will not operate morethan 4 times/h at minimum flow periods (2 starts per pump), or more than 7 times/Itat average flow (3.5 starts per pump) (AIRVAC, 2005a). This is represented by the fol-lowing formulas:

(3.5)

Where ~ = operating volume (m3 [gal]);Qa = average daily flow (LI s [gpm]) = Qnun/2;

Qnun = minimum flow (Lis [gpm]); andQdp = wastewater pump capacity (Lis [gpm]).

To the operating volume, a safety factor of 3.0 is applied, for emergency storage.An additional 1514 L (400 gal) is added to this subtotal, as a reserve volume withinthe tank, for moisture separation and vacuum pump reserve volume.

~t = 3~ + 400 (3.6)

Where ~t = collection tank size (m3 [gal]).

The minimum recommended collection tank size is 3800 L (1000 gal). After sizingthe operating volume, the designer should check to ensure that an excessive numberof pump starts per hour will not occur. This check should be performed for a waste-water inflow equal to one-half the pump capacity.

When designing the collection tank, the wastewater pump suction lines shouldbe placed at the lowest point on the tank and as far away as possible from the mainline inlets. The main line inlet elbows inside the tank should be turned at an angleaway from the pump suction openings.

Vacuum Pumps. Materials. Vacuum pumps may be either the sliding-vane or theliquid-ring type. In either case, the pumps should have a minimum (ultimate)vacuum of 98.9 kPa (29.3 in. Hg) at sea level. Even though the pumps operate onshort cycles, they should be capable of continuous operation.

Lubrication should be provided by an integral, fully recirculating oil supply. Theoil-separation system should also be integral. The entire pump, motor, and exhaustshould be factory-assembled and tested, with the unit mounted on vibration isola-tors, and should not require special mounting or foundation considerations.


Sizing. To size the vacuum pumps, the following empirical formula has been usedsuccessfully (AIRVAC,2005a):

Qvp= A X Qmax/7.5gal! cu £1*

"This equation is only presented in U.S. customary units per the reference.

Where Qvp= vacuum pump capacity (cfm), andQmax= peak flow (gpm).

The A value varies empirically with mainline length, as shown in Table 3.8. Theminimum recommended vacuum pump size is 4.3m3/min (150cu fr/min).

After the initial vacuum pump sizing is completed, a second calculation ismade, to ensure that the vacuum pump capacity is large enough for the connectedpipe volume. This calculation will show the amount of time it will take the selectedvacuum pumps to evacuate (pump-down) the collection piping from 54 to 68 kPa(16 to 20 in. Hg), assuming an operating range of 54 to 68 kPa (16 to 20 in. Hg).AIRVAC's 2005 Design Manual (AIRVAC,2005a) contains the formula needed tomake this calculation.

Some systems require only two vacuum pumps. In this case, redundancy isrequired, with each pump capable of providing 100%of the design capacity. In largersystems, where three, four, or even five vacuum pumps are required, the designcapacity must be met with one pump out of service. For example, if a design capacityof 25.5m3/min (900cu ft/rnin) is required, four 8.50-m3/min (300-cuft/min) pumpsare used (three duty pumps at 8.50-m3/min [300-cu ft/min] each, and one at 8.50-m3/min [300-cuft/min] standby).

(3.7)

Emergency (Standby) Generator. The standby generator should be capable of pro-viding 100% of standby power required for the station operation. It typically is

TABLE 3.8 The A factor for use in vacuum pump sizing.

Longest line length (m [ft]) A factor

o to 1524 (0 to 5000)

1524 to 2134 (5001 to 7000)

2134 to 3048 (7001 to 10 000)

3048 to 3658 (10 000 to 12 000)

>3658 (>12000)

6

7

8

9

11


located inside the station, although generators located outside the station in an enclo-sure are also acceptable.

In some cases, a municipal utility has a design for a pigtail connection on the out-side of the vacuum station, to allow for the connection of a mobile emergency gener-ator hookup. This design allows the utility to have some flexibility and mobility withits emergency generators, where the logistical distance between the utility's variouspump and vacuum stations make this concept feasible,

Station Piping. Wastewater, vacuum, and drain lines that are 10 em (4 in.) andlarger should be ductile iron, using American Water Works Association (DenverColorado)/ American National Standards Institute (ANSI) (Washington, D.C.)CllO/ A21.10 as standard. Pipe and fittings within the vacuum station should beflanged with ethylene propylene diene monomer gaskets. Exposed vacuum linesand other piping smaller than 10 em (4 in.) can be schedule 80 PVC, 304 stainless-steel, galvanized, or schedule 40 black iron. Building sanitary drains may be con-structed of schedule 40 PVC with drain-waste-vent (DWV) fittings.

The piping should be adequately supported to prevent sagging and vibration. Italso should be installed in a manner to permit expansion, venting, and drainage. Forfiberglass tanks, all piping must be supported, so that the tank flanges support noweight. Flange bolts should only be tightened to the manufacturer's recommenda-tions. Provisions must be allowed for inaccurate opening alignment.

All shutoff valves fitted within the collection station should be flanges-resilient-coated plug valves with circular ports. Check valves fitted to the vacuum piping areto be of the 57-kg (125-lb) bolted bonnet, rubber flapper, and horizontal swingvariety. Check valves are to be fitted with Buna-N soft seats. Check valves fitted tothe wastewater discharge piping are to be supplied with an external lever and weightto ensure positive closing. They should be fitted with soft rubber seats.

On the upstream side of each side of each vacuum sewer isolation valve, avacuum gauge of not less than 12.5-cm- (4.5-in.-) diameter should be installed.Gauges should be positioned so that they are easily viewed when the isolation valvesare operated. Diaphragm seals should not be used with compound gauges.

Electrical Controls. Control Panel. Rather than using a motor control center(MCC),most vacuum stations use a smaller control panel that is attached directly tothe equipment skid. These control panels are specifically designed for each station.

The control panel enclosure is to be National Electrical Manufacturers Associa-tion (NEMA) (Rosslyn, Virginia) type 12 and is generally mounted on the equipment


skid. A main disconnect switch, sized to handle the current draw for all relatedvacuum station equipment, is to be provided.

The panel includes motor starters for each motor. Either International Elec-trotechnical Commission (Geneva, Switzerland)-type or NEMA-rated motor startersare used. To reduce in-rush current, "soft-starts" or variable frequency drives can beused. The control panel also incorporates control relays or programmable logic con-trollers (PLCs), which then are connected to the various functions of the control panelsystem design.

Discharge pump level control relays are mounted inside the panel. Conductance-type level-control rods are mounted through the tank wall and set to specific heightsto maintain wastewater pump operation and to provide alarm functions.

The panel should include pilot lights and hand-off-auto switches. Hour-runmeters are included for the operator to track daily run times of the vacuum and dis-charge pumps.

Tomonitor system performance, a 7-day chart recorder is installed in the enclosure.The control panel should include a telephone alarm dialer, which monitors three alarmfunctions-low vacuum levels, high wastewater conditions, and power outages.

In small, simple vacuum station designs, the vacuum station skid manufacturerwires the control panel to each motor and control device. Larger, more complexdesigns require the contractor to wire the control panel to the related junction boxesprovided on the equipment.

Control panels can use either relay-logic or PLC logic. Some prefer the simplicityof relay-logic and may not want the sophistication of using a PLC. Others wish tohave more control over the system by changing the ladder logic through laptop com-puters. Many of the larger municipalities prefer this type of logic, as it matches othercontrols they may have with lift stations and treatment plants. Most recent systemsuse PLC logic.

Motor Control Center. In larger vacuum stations, the controls are typically house inan MCC. The MCC is to be manufactured, assembled, wired, and tested by the fac-tory in accordance with the latest NEMAIndustrial Control and Systems publications(http://www.nema.org/). The vertical section and the individual units shall bear a

_Underwriters Laboratories (UL) (Northbrook, Illinois) label, where applicable, as evi-dence of compliance with appropriate UL standards. Wiring inside the MCC is to beNEMA class II, type B. Where type B wiring is indicated, the terminal blocks shouldbe located in each section of the MCC.


The enclosure should be NEMA type 12, with gasketed doors. Vertical sectionsshall be constructed with steel divider sidesheet assemblies formed or otherwise fab-ricated to eliminate open framework between adjacent sections or full-length bolted-on sidesheet assemblies at the ends of the MCC.

The MCC should be assembled in such a manner that it is not necessary to haverear accessibility to remove any internal devices or components. All future spacesand wireways are to be covered by blank doors.

Level Controls. Seven probes inside the collection tank control the dischargepumps and alarms. These probes are 0.6-cm (0.25-in.) stainless steel with a PVCcoating. The seven positions are as follows:

(1) Ground probe,(2) Both discharge pumps stop,(3) Lead discharge pump start,(4) Lag discharge pump start,(5) High-level alarm,(6) Reset for probe number 7, and(7) High-level cutoff-stops all discharge pumps (auto position only) and

vacuum pumps (auto and manual positions).

Figure 3.23 gives approximate elevations of these probes in the collection tankrelative to the discharge pumps and incoming vacuum mains.

An acceptable alternative to the seven probes is a single capacitance-inductive-type probe capable of monitoring all seven set points. This type of pro"?erequires atransmitter / transducer to send a 4 to 20 rnA signal to the MCC.

Monitoring System. The fault monitoring system is used to alert the operator ofany irregularities, such as a low vacuum level. Fault monitoring systems include tele-phone dialers or other telemetry equipment, including radio-based SCADA systems,digital- or fiber-optic-based SCADA systems, and telephone-based SCADA commu-nications systems.

If a voice-communication-type, automatic-telephone-dialing alarm system isused, it should be mounted on a wall adjacent to the MCC. If the monitoring systemis to be housed within the MCC, provisions must be made to isolate the system frominterference. The system should be self-contained and capable of automatically mon-itoring up to four independent alarm conditions. The monitoring system should be


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provided with continuously charged batteries for standby operation in the event of apower outage.

Vacuum Gauges. All vacuum gauges should be specified to have a stainless-steelbourdon tube and socket and to be provided with 1.3-cm (0.5-in.) bottom outlets.Polypropylene or stainless-steel ball valves should be used as gauge cocks. The con-nection from the incoming main lines to the vacuum gauges should be made of PVCor chlorinated PVC pipe. Copper pipe is not to be used for this purpose.

Vacuum gauges should be provided on the collection tank in a position that iseasily viewed from the stairway leading to the basement. In addition, vacuumgauges should be provided on each incoming main line to the collection tank, imme-diately upstream of the isolation valve. These gauges should be in a position abovethe incoming main lines that is easily viewed from the operating position of the iso-lation valves.

Vacuum Recorder. The vacuum station control panel should contain a 7-day cir-cular chart recorder, with a minimum chart diameter of 30 em (12in.). The recordingrange should be 0 to 101 kPa (0 to 30 in. Hg) vacuum, with the zero position at thecenter of the chart. The chart recorder should have stainless-steel bellows.

Sump Valve. The basement of the vacuum station should be provided with a37.5 cm X 37.5 em X 30.0 em (15 in. X 15 in. X 12 in.) deep sump to collect wash-down water. A vacuum valve that is connected by piping to the collection tankempties this sump. A check valve and eccentric plug valve should be fittedbetween the sump valve and the collection tank.

Odor Control. Odor control for vacuum systems is typically associated with air-borne hydrogen sulfide within the vacuum pump exhaust. Various methods havebeen used to successfully eliminate hydrogen sulfide. Use of a biomass compost bed,designed in accordance with the u.s. EPAmanual Odor and Corrosion in Sanitary Sew-erage Systems (U.S.EPA, 1991b),has occurred for many years in various locals in theUnited States and in foreign countries and is recommended, assuming space is avail-able at the site.

The designer is cautioned that the thermophylic bacteria of the biomass are notviable at high temperatures (>48.9°C [120(FJ)and the effect thereon from the heat ofdischarge to the biomass should be considered. Thermophylic bacteria are suscep-tible to destruction at high heat, and the odor control system should address eitherheat dissipation or reduction.


Additional methods used include chemical neutralization, activated carbonabsorption systems, and absorption by manufactured biomass filters.

Noise and Heat Considerations. Noise generated by a vacuum station is typicallyassociated with the vacuum pumps. If desired, soundproofing features can be addedto the vacuum station. Of the 200+ vacuum stations in the United States, only a fewhave used this added protection.

The vacuum station generates heat, primarily from the wastewater pump motorsand the vacuum pumps. Obviously, this heat is added to ambient conditions and canadd up during the hottest periods of the day.

Vacuum stations are not typically air-conditioned, as long as adequate air-exchange rates are provided through ventilation equipment to prevent excessive heatbuildup. Allowing natural heat rising through roof-mounted vents with low air-intakes is recommended. The maximum recommended operating temperature forstation equipment is 40°C (104(F).

At site locations where summertime temperatures typically reach up to approxi-mately 36°C (97°F), the design engineer needs to give consideration to the heating,venting, and cooling requirements of the vacuum station. In these locations, experi-ence has shown that the interior temperatures of the vacuum station can easilyexceed 37.8°C(lOO(F),possibly resulting in overheating of motor control panels.

CONSTRUCTION CONSIDERATIONSConstruction of a vacuum sewer system is similar to other alternative collection sys-tems. Using small-diameter pipes in shallow trenches and having the ability to avoidunderground obstacles virtually at will makes this type of construction attractive tocontractors. However, there are certain inherent construction issues associated withvacuum sewers.

It is imperative that those with a thorough knowledge of vacuum sewer tech-nology perform inspection. The design of the system and its hydraulic limits must beunderstood by at least one member of the construction team.

VACUUM MAIN CONSTRUCTION. The use of chain-type trenches is some-times specified for service-line installation, where soil types allow, as they cause lessdisruption to the property owner's yard than does a backhoe. Rocky soils and someclayey soils that will not self-clean from the trencher teeth may be impractical to exca-vate using a trencher, although special designs have been successfully used for theformer condition.


Many contractors use a backhoe for the service-line excavation, because thissame equipment is required for the excavation of the valve pit, which typically islocated close to the main sewer. Many times, this results in overexcavation of the ser-vice-line trench. Overexcavation, coupled with the use of fittings that are typicallyrequired between the valve pit and main, can lead to future problems, if proper bed-ding and backfill materials are not used.

Because the native material and intended contractor equipment is not alwaysknown, it is recommended that the contract documents specify surrounding the pipewith imported pipe zone backfill. If local materials meet the required specifications,they may be substituted at a reduced cost.

To minimize damage to the vacuum sewer main caused by subsequent excava-tion, route markers are sometimes placed adjacent to the main, warning excavatorsof its presence. Accurate as-constructed plans are helpful in identifying the pipelinelocation. A cable buried with the main can be induced with a tone, so the main can befield-located using common utility-locating equipment. In other cases, a warningtape marked "vacuum sewer" is placed shallowly in the pipeline trench, to furthernotify excavators.

Line Changes. Unforeseen underground obstacles are a reality in sewer line con-struction. Water and gas lines, storm sewers, and culverts at unanticipated locationsall may present difficulties during construction. Natural underground conditions,such as rock, water, or sand also may present more problems than anticipated.

With the straight-line, constant-grade requirements of gravity sewer construc-tion, these obstacles often result in field changes. These field changes might includeinstalling an additional manhole and/ or removing and relaying part of the pipe at adifferent grade. The grade change could affect the depth and grade of the entiregravity sewer system. Another lift station may have to be installed. Alterations at thetreatment plant could be required. The end result is an increase in contract pricethrough change orders.

One key advantage of vacuum sewers is the flexibility they allow for line changesduring construction. Unforeseen underground obstacles typically can be avoidedsimply by going under, over, or around them. There may be cases where line changeswill be necessary because of hydraulic limitations. However, the likelihood of this isgreatly reduced when compared with conventional gravity sewers.

Line changes are made through the use of fittings. No gO-degree bends should beused invertical or horizontal line changes (AIRVAC, 2005a). Concrete thrust blockinggenerally is not necessary; however, compaction in the zone of abrupt change indirection is vital.


Grade Control. The ability to make grade changes to avoid obstacles is an advan-tage inherent with vacuum sewers, but the abuse of this freedom can result in majorproblems. Grade changes should not be made without a thorough evaluation of howthat change will affect overall system performance. This issue has been the cause ofconflicts between the contractor and the engineer in past vacuum projects. The engi-neer's inspector instinctively desires to eliminate lifts to improve the systemhydraulics, but this will result in a deeper installation. The contractor, on the otherhand, desires to add lifts, which will result in a shallower installation. As long as nei-ther party loses sight of the system's hydraulic limits and the effect on operationalcosts, a conflict does not have to take place.

Vacuum sewers must be laid with a positive slope toward the vacuum station.The only exception to this is where vertical profile changes (lifts) are made. The pipemust slope toward the vacuum station between lifts.

A minimum of 0.2%slope must be maintained at all times. Toensure this, a lasertypically is required during construction. The use of automatic levels also is accept-able, when handled by an experienced instrument operator. In areas where anobvious (>0.2%) downhill slope exists, the pipeline may follow the contour of theground. The engineer's inspector should routinely check the grade.

Horizontal Directional Drilling. The use of directional drilling has increasedgreatly in recent years. While significant advancements in this technology haveoccurred, it has not yet developed to the point where it can be used on a wide-scalebasis for vacuum sewers. The major reason is the inability of the equipment to main-tain such shallow (0.20%)slopes. At the time this manual went to publication, mostHDD companies felt that the minimum attainable slope was approximately 0.50%.Designing with a minimum slope of 0.50% instead of 0.20%would result in eitherdeeper vacuum mains or shorter line lengths before the static limit was reached. As aresult, a decision to allow HDD must be made before the design process.

Sags and summits are not acceptable in vacuum systems, as they can result indetrimental operating performance. For this reason, tolerances for directionaldrilling are the same as those required for open trenching. Those tolerances are typ-ically 0.02 m/30.5 m (0.05 ft/100 ft), or 0.05%. With a target slope of 0.20%, thismeans that the slope could vary from 0.15 to 0.25%.

The "minimum [15 m] 50 ft at 0.20% prior to a lift" rule applies to directionaldrilling, just as it does to open trenching. Contractors should not be permitted use theconcept of adding lift(s) to make up any difference in elevation that may occur by


TABLE 3.9 Requirements for using HDD to install vacuum sewers.

Item Requirement

System integrity

Installed pipe must meet slope tolerances(:to.02 m per 30.5 m [:to.OS ft per 100 ftl)

Are not acceptable

HDD firm must be able to electronically verify installedslopes at the time of installation

Installed pipe must be capable of passing daily andfinal vacuum tests

Must match up WIth manufacturer's products and othersystem components

Slope

Sags (bellies) and summits

Quality control

Pipe materials

completing a directional drill at a slope greater than 0.20%. Table 3.9 shows generalrequirements when HDD is used in vacuum sewer construction.

On noncritical lines, it may be possible to actually design for slopes greaterthan 0.20%, in areas where directional drilling is desired. This would not be a con-struction tolerance, but rather a design decision that allows for a more attainableslope, such as 0.50%.

Incorrect slopes or sags and summits have a larger negative affect when avacuum main is involved, rather than when a branch or service line is involved. Anincorrect slope on a service line would affect the operation of just that particular ser-vice line, whereas an incorrect slope on a vacuum main would affect not only thatmain, but all connected branch and service lines also.

Vacuum Testing During Construction. While a final vacuum test of the entire col-lection system ultimately will be performed to ensure the integrity of the collectionsystem, it is recommended that daily testing also be done. Doing so will increase thelikelihood of a successful final test, as the pipe layers will gain immediate feedbackconcerning their workmanship. Also, a leak is much easier to locate when only thesection installed that day is examined, rather than the entire system.

The duration of the daily vacuum test is 2 hours. The pipe laid that day shouldbe plugged and subjected to a vacuum of 74 kPa (22 in. Hg) and allowed to stabilizefor 15 minutes. The allowable loss is limited to no more than 1% vacuum pressure/h(approximately 1.7 kPa [0.5 in. HgJ). During the daily testing, all joints should be leftexposed. If any section of the sewer fails the test, it should be reworked before laying


new sections of sewer. Upon successfully passing the test, the day's constructionshould be backfilled before shift completion (AIRVAC,2005a).

Where climatic changes may occur during a vacuum test, it is recommended thatpipe temperature and atmospheric pressure be recorded at the beginning and end ofthe test, and the test results adjusted to correct for these changes.

VALVE PIT INSTALLATION. Pit Installation. Pit Location. The relationshipbetween the ground elevation where the pit setting will be situated and the elevationof the customer's building sewer dictates the depth of service required and the typeof pit required. The length of connecting lateral required must be considered to allowfor sufficient slope of the building sewer.

Prefabricated valve pits with fixed dimensions can sometimes make pit locationcritical. Moving the pit to a lower elevation, while allowing additional fall for thebuilding sewer, may result in lift being necessary to connect to the main. Moving thepit to a higher elevation may result in insufficient fall available for the buildingsewer. Each valve pit location should be evaluated for adequacy and verified by theengineer to the contractor before shipment of the valve pits.

It is good practice to boldly field-mark the location of the service lateral withproperly identified lath or stakes a few days before installation. This serves as areminder to the property owners about the intended location and may cause them torecognize some reason why the location should be changed. It also serves as a noticeto neighbors, if property lines are in doubt.

Pit Orientation. The orientation of the valve pit is a function of the number of houseconnections to the pit and the placement of the wye on the vacuum main. The 7.5-cm(3-in.) vacuum service lateral that exits the valve pit must always be at a 45-degreehorizontal angle to any of the lO-cm (4-in.) incoming gravity stub-out pipes (seeFigure 3.24).

Pit Installation. The installation of the valve pit is divided into two phases-at thecontractor's workshop and at the installation site. The AIRVACOperation, Installationand Maintenance Manual (2005b) provides detailed instruction for each of the fol-lowing tasks.

The times shown in Table 3.10 are attainable with desirable subsurface condi-tions, but can extend all the way to a full day if the installation location has a highgroundwater table that requires dewatering systems or where rock exists thatrequires specialized excavation.

PRIVATE RIGHT-OF-WAYPROPERTY(TYP.) (TYP.)

FLEXIBLE SERVICELATERAL CONNECTOR(TYP.)


6' MIN LENGTH GRAVITYSERVICE LATERAL WITHGLUED CAP 5' MIN(4" SINGLE SERVICE)(6" DOUBLE SERVICE)

METHOD "A"

I-1-

IMETHOD "B"

VALVE PITASSEMBLY


Where

TABLE 3.10 Valve pit installation tasks.

Task Time (hours)

Contractor'sworkshop

Cut the stub-out(s) to the required length

Install O-ring on pit bottom and bolt pit bottom to sump

Test sump (without holes) for tightness

0.15

0.35

0.250.75

Installationsite

Excavate pit to appropriate depthCut hole(s) in sump for stub-out(s)

Lower sump / pit bottom into hole and install stub-outs

Backfill and compact up to pit bottom

Test sump (with stub-outs) for tightness

Align and lower valve pit onto pit bottom

Install flotation collar, if requiredInsta1l7.5-cm (3-in.) flexible connector

Install cast iron frame and cover

Complete backfill

0.75

0.10

0.20

0.15

0.25

0.10

0.15

0.10

0.10

0.352.25

Connecting the Pit to the Main. Contractors typically have a crew installing mainlines and a second crew installing the on-lot services (i.e., valve pit settings). It iscommon for the line crew to install a wye fitting on the main to eventually accept theservice lateral piping from the pit crew. Typically, the pit crew installs the pit settingand then connects the pit to the main. The remaining work-connecting two fixedpoints at varying inverts and locations, but requiring rigid connection piping-canresult in the contractor using an excessive amount of fittings. Proper planning andcoordination between line and pit crews can minimize the use of fittings in the ser-vice lateral.

To eliminate potential problems caused by using rigid pipe and fittings to con-nect the valve pits to the main, AIRVACdeveloped a special product called the "flex-ible connector". The flexible connector uses a special7.5-cm (3-in.) PVC flexible hoseto give a degree of flexibility that allows for these difficult connections. Connectionsat both ends of the connector are the same as with PVC pipe. The use of a flexible ser-vice connector virtually eliminates stress-related leaks caused by poor workmanshipor ground settlement.


Many times, fittings are located within the pit excavation. This overexcavatedzone is one where a lack of compaction could easily lead to future settlement, whichcould lead to fitting failures.

Sump Tightness Test. It is important to test the sump portion of the valve pit forwater-tightness. The sumps are tested twice.

The first test is done at the contractor's workshop or at the construction site. Thisis done before any holes are cut into the sump. The purpose of this test is ensure thatthe sump is not defective or has not been damaged during shipment.

The second test is done after the complete valve pit has been installed at the site.This second test is to ensure that a tight seal has been achieved after the installationof the stub-out lines.

AIRVAC(2005b)provides detailed instructions for sump testing.

VACUUM STATION CONSTRUCTION. There is nothing unique about theconstruction of a vacuum station. Standard structural, mechanical, electrical, andplumbing practices that are used at other similar structures (treatment plants, lift sta-tions, etc.) are followed.

FINAL VACUUM TESTING AND SYSTEM STARTUP. Startup is typicallyconducted in two phases-the vacuum station and the collection system. The manu-facturer supplying the vacuum station equipment generally conducts all of the tasksrelated to the vacuum station startup. The contractor, with the manufacturer's help,is typically responsible for the collection system startup task.

Vacuum Station Startup. The supplying manufacturer's technician will performthe general startup tasks shown in Table 3.11. Detailed startup procedures are con-tained in the manufacturer's operation and maintenance manual.

After the collection system startup is completed, the technician will return thestation to its appropriate operational levels and then place the entire system in theautomatic mode for commissioning into active service.

Collection System Startup. Once the vacuum station has been checked success-fully, the technician will move on to the collection system. The isolation valvesbetween the collection tank in the station and the collection system incoming lineswill be opened, and the entire vacuum station and collection system will be placedunder vacuum. Startup related to the vacuum mains consists of vacuum testing andline flushing.


Startup tasks

TABLE 3.11 Vacuum station startup tasks.

Description

Vacuum pump test

Wastewater pump test

Electrical review

Level controls

Final vacuum test

Adjust operating vacuum range; set low-vacuum alarm level;verify vacuum pump capacityVerify motor rotation; verify pump capacity; check dischargepressure

Verify incoming voltages; verify that voltage and amperageare correct for all motors; operate system under various sce-narios to verify that all relays, sequencing, and alarms areworking properlySet level control probes for all control points

Isolate station from collection system and conduct final4-hour vacuum test

The supplying manufacturer's technician will perform the general startup tasksshown in Table3.12.Detailed startup procedures are contained in the manufacturer'soperation and maintenance manual.

Following the successful completion of line flushing, the technician will returnthe station to its appropriate operational levels and then place the entire system inthe automatic mode for commissioning into active service.

Vacuum Testing Summary. A summary of the various testing required is shown inTable3.13.

Startup tasks

TABLE 3.12 Collection system startup tasks.

Description

Complete 4-hourfinal vacuum test

Flush system

Isolate collection system from vacuum station and conductfinal4-hour vacuum tightness test

After a successful vacuum test, introduce water to the collec-tion system at the far extremities of the lines and other vari-ous points throughout the collection system to systematicallyflush any debris that may have entered the system duringconstruction to the station


TABLE 3.13 Vacuum testing.

BeginningTest test level Acceptableloss

What When duration (kPa[in.Hg]) (kPa[in.Hg])

Collectionpiping Daily 2 hours 74 (22) 1.7(0.5)

Collectionpiping Finaltest 4hours 74(22) 3 (1)

Vacuumstation Finaltest 4hours 74(22) 0(0)

CONSTRUCTION INSPECTION. General. Primary inspection responsibilitylies with the owner or their designated inspector, which typically is the design firm.To supplement this, the vacuum vendor may provide technical assistance in variousways, ranging from full-time direct technical services to training services. Not onlycan the vacuum vendor assist in ensuring proper installation, but they can also pro-vide immediate resolutions to unforeseen construction difficulties and provideadvice on whether lifts can be added or deleted. This helps minimize contractordowntime, which results in fewer change orders. Generally, the cost of adding avacuum vendor field representative is less than 1 to 2% of the total project cost.

Duties. It is not uncommon for the utility to require the supplying manufacture toassist the design engineer during the construction phase. This is because of theunique nature of the technology. Inthis case, the manufacturer's field representativewill assist the inspector with the following tasks.

Vacuum Main Installation. The inspector is to confirm that lines are installed asplanned. Grades, distances, and elevations are spot-checked, with particular atten-tion paid to vertical profile changes. The inspector should see that branch and servicelateral installations and horizontal direction changes are in compliance with estab-lished standards.

A check is made of the type of pipe and fittings being used, to ensure that theyare suitable for vacuum service and that proper certifications are in the contractor'spossession. Trench conditions are observed to ensure that adequate soil conditionsexist and that proper bedding and compaction are carried out in conjunction with thecontract documents.

The inspector also will watch the daily testing of vacuum sewer mains, to ensurecompliance with standards. Finally, the inspector must maintain a neat, legible, andaccurate set of as-built drawings and field notes.


Valve Pit Installation. The inspector will observe sump testing to see that it is con-ducted in accordance with manufacturer's standards. The inspector will also ensurethat all field cut penetrations are neatly cut, reasonably circular, and are located inaccordance with the manufacturer's recommendations.

Other duties include verification that the valve pit setting is placed in accordancewith construction drawings, the 7.S-cm(3-in.) service lateral is properly aligned withthe 7.5-cm (3-in.) suction pipe, the pit assembly is plumb and reasonably level, andthe pit depth is in accordance with design limits.

Finally, the inspector must maintain a neat, legible, and accurate set of as-builtdrawings, field notes, and valve pit installation forms.

Vacuum Station Construction. The inspector will observe the subsoil conditions,site drainage, dewatering operations, placement of concrete and steel reinforcement,and the proper use of water stop between concrete joints. The alignment of all wallpenetrations for vacuum mains and the force main will be checked and comparedwith the construction drawings and shop drawings. Routing of vacuum pumpexhaust lines will be checked. The inspector will observe and supervise the finalsystem test and station startup and provide documentation, as required.

Record Drawings. As is common with all system types, field changes will occurduring construction. The changes should be reflected on the record drawings, whichare commonly referred to as Has-built drawings". Certain information specific tovacuum systems is discussed below and should be included with the record drawings.

An index map showing the entire system should be included in the as-builtdrawings. Shown on this map will be all key components, line sizes, line identifica-tions, valve pit numbering and locations, and division valve locations. Detailed plansheets of each line of the collection system should be included, with dimensions nec-essary to allow the operator to locate the line and all related appurtenances.

Unique to a vacuum system is the need for an as-built hydraulic map. This issimilar to an index map, but includes special hydraulic information. This simple, butvital, information allows the operator to make intelligent decisions when trou-bleshooting the system.

Important information to include on the as-built hydraulic map is the following:

(1) Locations of every lift;(2) Amount of vacuum loss at key locations, such as the end of a line or the

intersection of a main and branch line; and


(3) Number of main branches, number of valves in each branch, and totalfootage (or volume) of pipe in each branch.

Another tool that is helpful to the operator is an as-built drawing of each valvepit setting. These drawings will show the location of the setting relative to some per-manent markers (house, power pole, etc.), orientation of the gravity stub-outs, depthof the stub-outs, and any other pertinent site-specific information. The operator mustupdate or supplement these records as new customers connect to the system.

BIDDING ISSUES-PROCUREMENT OPTIONS. There is no one bestmeans of procurement. Different bidding formats are necessary so that the best for agiven situation can be selected (WEF,1995).Some common bidding formats are

• Conventional open bid,

• Basel substitute bid, and

• Preselection of major equipment.

The 1995Water Environment Federation(r) (WEF) (Alexandria, VIrginia)document,Engineered Equipment Procurement Options to Ensure Project Quality, contains a very thor-ough discussion on each of the above procurement options. The reader is highly encour-aged to review this manual and use the method that best suits the situation.

OPERATION AND MAINTENANCECONSIDERATIONS

STAFFING REQUIREMENTS. Because they are mechanized, vacuum systemshave a reputation as being intensive in operation and maintenance. This may havebeen true of the early vacuum systems; however, information from system opera-tors suggests that the effort to operate and maintain a modern vacuum system istypically overstated.

One key to a successfully operating system is the combination of attitude,training, and skill of the system operator. An even more important consideration maybe the structure and organization of the maintenance staff. Maintenance staff thatdivide operating responsibility by system components (i.e., one division responsiblefor the vacuum station, another responsible for the vacuum mains, and a thirdresponsible for the valves) are rarely successful. Successful operations are those that


have at least one operator that is responsible for the entire system (see the Key to Suc-cessful Operation-The System Approach section).

OPERATOR TRAINING. It is desirable for the management entity to hire thesystem operator before or during the period when the system is under construction.This allows the operator to become familiar with the system, including the locations ofall lines, valve pits, division valves, and other key components. Also, the operator mayassist the construction inspector as a means ofbecoming more familiar with the system.

Further training may be offered by manufacturers at their facilities, and manage-ment should take advantage of it. By viewing a small-scale vacuum system thatincludes clear PVC pipe with various lift arrangements, trainees can watch the flowinside a clear pipe during a wide variety of vacuum and lift conditions. Faults can besimulated so that the trainee can gain troubleshooting experience. Manufacturersalso provide schooling, where the operator is taught valve operation and overhauland vacuum station maintenance.

The best training is actual operating experience. As sometimes happens, the bestknowledge is often gained from operating mistakes. This is especially true at startuptime. During this time, the engineer, who provided day-to-day inspection servicesduring construction, is gradually spending less time on the system. The operator isbusy setting vacuum valves and inspecting customer hookups. Complicating the sit-uation is the fact that the operating characteristics of the system continually change,until all of the customers are connected and all of the valves are fine-tuned. However,with the operator(s) being preoccupied with other tasks, this fine-tuning sometimesis not done, and problems develop. The biggest concern during this period is thatcommunity confidence in the vacuum system not be lost.

This training gap is present at the startup of virtually every vacuum system. Onesolution is for the engineer to budget a 3- to 6-month on-site training service duringthe startup period to aid the system operator in fine tuning and troubleshooting ofany early problems. The operator will benefit from the engineer's systematicapproach to problem solving. This most likely will instill a certain degree of confi-dence in the operator(s) concerning the system. Operator attitude is vital to the effi-cient operation of a vacuum or any mechanically based system.

KEY TO SUCCESSFUL OPERATION-THE SYSTEM APPROACH. Themajor components of a vacuum system-the interface valves, piping network, andvacuum station-are interrelated and must be designed to work as a system. Even


more importantly, they must be operated as a system, not as individual components.Making a change at the vacuum station affects not only the station components,

but also the hydraulics of the vacuum mains and the operation of the valves. Causeand effect can only be learned by understanding how the entire system works, andnot by concentrating solely on one particular component.

For this reason, the most successful systems are those that are operated by agroup with a single thought process. There is nothing wrong with several operatorsworking together, as long as they all know how the system responds to their actions.

MAINTENANCE. There are two major classifications of maintenance-normaland preventative maintenance, and emergency repairs or maintenance. A well-con-ceived asset-management program emphasizes the former and minimizes the latter.

Normal and Preventative Maintenance. Vacuum systems operate and must bemaintained 365 days/yo Variations in operation and maintenance workloads occur,making it imperative that preventative maintenance be planned and scheduled. Thiswill ensure that there is no idle time during nonpeak workload periods. Inspectionand maintenance planning and scheduling involves time, personnel, equipment,costs, work orders, and priorities.

A preventative maintenance schedule for all major equipment should be devel-oped. To initiate the preventative maintenance tasks, a work-order system should beestablished. This system identifies the required work, priority of task, and any spe-cial information, such as the tools or parts required for the job.

Vacuum Station. A properly designed vacuum station will be equipped with a faultmonitoring system, such as a telephone dialer or telemetry system. These systemsmonitor the operation of both the vacuum station and collection system and auto-matically notify the operator of low vacuum, high levels of wastewater in the collec-tion tank, and power outages.

Normal operation includes visiting each vacuum station daily. Some daily main-tenance procedures include the recording of pump running hours and oil and blocktemperature checks. Once an operator is familiar with the operating characteristics ofthe system, a simple visual check of the gauges and the charts in the station will pro-vide an adequate alert of any problems. This visual check and recording operatingdata generally takes approximately 30 minutes.

Daily, weekly, monthly, and semiannual tasks associated with the vacuum stationare shown in Table 3.14.


TABLE 3.14 Normal vacuum system operation and maintenance tasks andfrequencies.

Frequency Task

Daily

Visually check gauges/chartsRecord all pump run timesCheck oil level in vacuum pump sight glassTest cycle the AIRVAC sump valve in station

WeeklyTest cycle the AIRVAC sump valve in stationChange chart on chart recorderExercise generator

Monthly

Change oil and oil filters on vacuum pumpsRemove and clean inlet filters on vacuum pumpsTest all alarm systemsCheck all motor couplings and adjust if neededClean all sight glassesExercise all shutoff valves (vacuum station)

SemiannuallyExercise isolations valves (vacuum mains)Conduct external leak test on all vacuum valvesCheck valve timing and adjust, if needed

Preventative maintenance for the major equipment at the vacuum station shouldbe done in accordance with manufacturer's recommendations. In addition to theitems in Table 3.14,yearly (annual) maintenance might include removal from serviceand comprehensive inspection of the check valves, plug valves, vacuum pumps,wastewater pumps, generator, and telephone dialer.

Collection System Piping. On a typical day, the operator will not be required to visitthe collection system. Normal station gauge and chart readings are an indication thatthe collection system is fine.

Scheduled maintenance on the collection piping should be minimal. Areas wheredifficult or unusual conditions were encountered during construction should be vis-ited periodically.

At least once per year, the division valves should be checked. This is done bymoving the valve through the entire opening and closing cycle at least once. Thisprocedure is known as exercising and will keep valves in good operating condition.Inaddition, it will familiarize any new operating personnel with the location of allthese valves.


Vacuum Valves. Depending on a system's history of emergency-valve-breakdownmaintenance, periodic inspection may be required. As with pressure sewer systems,certain on-lot units are prone to more problems than the rest of the system.

Access to valves, for maintenance reasons, is gained by removing the manholecover on the valve pit. Routine maintenance is easily performed inside the standardvalve pit from the ground surface. The only tools required are a manhole cover pickand a sensor pipe puller, to drain any groundwater that may have accumulated in thevalve pit.

All vacuum valves should be inspected at least once each year (AIRVAC,2005b).They should be manually cycled to see that they are operating properly. The con-troller timing cycle should be recorded and compared with the original setting. Ifnecessary, the timing should be reset and recorded. The operator should check fordirt or water in the controller, valve, or tubing. If used, the above-ground ventscreens should be checked to see that they are clear of debris, spider webs, and so on.

Every 5 years, each controller should be removed and rebuilt (AIRVAC,2005b).For valves that cycle more frequently, the controller should be rebuilt every 3 yearsor 500 000 cycles. These would typically be valves installed in buffer tanks or otherhigh-use locations. The controller should be replaced with a spare and the removedunit returned to the owner's workshop. Rebuilding typically involves replacing theshaft seals, greasing the shaft, and cleaning all components.

Every 10 years, each vacuum valve should be removed, a spare put in its place,and the old valve returned to the workshop (AIRVAC,2005b). The valve should betaken apart and inspected for wear. If worn, the valve seat should be replaced, and anew shaft seal and bearing should be fitted during reassembly.

Table 3.15 summarizes the preventative maintenance tasks and their frequencies.

TABLE 3.15 Other preventative maintenance tasks and frequencies.

Frequency Task

Every year

Exercise division valves (station and vacuum mains)Inspect vacuum and wastewater pumps for wearVISual inspection of all valve pits and valvesCheck valve timing and adjust, if needed

Every 3 years Rebuild controller (buffer tank valves only)

Every 5 years Rebuild controller (most valves)

Every 10 years Rebuild valve


TABLE 3.16 Time requirements for rebuild tasks.

Maintenance Personnel LaborItem interval required (hours)

Physical inspection Everyyear 1person 0.50

Controller rebuild Every5 years 1personSanitize 0.25Rebuild 0.50Quality control tests 0.25--

LOa

Valverebuild Every 10years 1personSanitize 0.25Inspect 0.25Rebuild 1.00Quality control tests 0.25--

1.75

Typically, the operator will remove a valve or controller and replace it with aspare. The removal and replacement procedure takes approximately 5 to 10 minutes.The valve or controller is then taken to the maintenance shop, where rebuilding takesplace. The times required to rebuild controllers and valves are shown in Table 3.16.

Emergency Maintenance. Although very little effort is required on a day-to-daybasis, there will be times that emergency maintenance is necessary. This effort typicallyrequires more than one person, particularly when it involves searching for a malfunc-tioning valve. Many times, problems develop after normal working hours, requiringpersonnel to be called out on an overtime basis. Emergency or breakdown maintenancecan occur in the piping system, at the vacuum station, or at the vacuum valve.

Vacuum Station. Malfunctions at the vacuum station are generally caused by pump,motor, or electrical control breakdowns. The redundancy of most components allowsfor the continued operation of the system when this occurs.

Collection System Piping. Assuming proper design and construction, there is verylittle that can go wrong physically in the piping system. Occasionally, a line breakwill occur, as a result of excavation for other utilities or landslides, causing a loss ofsystem vacuum. By closing and opening division valves in a logical sequence in keyareas along the piping route, the operator can easily isolate the defective section.


Other potential problems include system waterlogging or even a complete lossof vacuum, which renders the entire collection system inoperable. Fortunately, theseinstances are very rare and typically short-lived. AIRVAC(2005b)provides detailedprocedures for correcting these system anomalies.

Vacuum Valves. Most emergency maintenance is related to malfunctioning vacuumvalves caused by either low system vacuum or extraneous water. While failure of thevalve is possible in either the open or closed position, virtually all (99%)occur in theopen position.

When open-position failure happens, a loss of system vacuum occurs, as thesystem is temporarily open to the atmosphere. The fault-monitoring system will rec-ognize this low-vacuum condition and alert the operator of the problem. A commoncause of failure in this position is the entrance of extraneous water into the controller.

Valve failures, if not located and corrected quickly, may cause failures in otherparts of the system. A valve that is hung open or that continuously cycles willcause the system vacuum to drop. If the vacuum pumps cannot keep up with thisvacuum loss, the result is insufficient vacuum to open other valves. This may leadto backups. When the vacuum is finally restored, a large amount of wastewater, inrelation to the amount of air, will be introduced to the system, possibly resultingin waterlogging.

A valve failing in the closed position will give the same symptoms as a blockedgravity line; that is, the customer will experience problems with toilet flushing orbackup of wastewater on the property. A phone call from the affected party makesidentification of this problem easy.

Some systems in Europe have used individual, hard-wired alarms at each valvepit. This practice is not done in the United States, as the costs of such systems gener-ally outweigh the benefits, especially considering the increased reliability of themodern vacuum valve. Future vacuum systems may include a wireless alarmsystem, as there has been some recent progress in the development of such systems.

SPARE PARTS INVENTORY. Valves and Valve Pits. For optimum operatingefficiency, it is necessary that a sufficient inventory of spare parts be kept. Some ofthe spare parts, such as fittings and pipe, can be purchased through local builder'ssupply companies. However, there are parts that are unique to vacuum systems thatcannot be purchased locally.For convenience, these spare parts often are included aspart of the construction contract.


Table 3.17 is a recommended list of spare parts. As previously described, faultyvalves and controllers are not repaired in place, but rather are removed and replacedwith a spare. The rebuilding procedure is then done at the maintenance facility. The3% spare valves and controllers and rebuild kits shown in Table 3.17 are for this pur-pose (i.e., for emergency maintenance).

The spare parts in Table 3.17 are not intended for use in the wholesale rebuildingof valves and controllers that is associated with the preventative maintenance pro-gram. For that, inexpensive rebuild kits are typically purchased by the operatingentity before this scheduled maintenance.

Vacuum Station. The vacuum station also requires spare parts. These range fromspare pump seals to fuses. Specialty items that should be considered are listed inTable 3.18.

TABLE 3.17 Spare parts list per every 100system valves.

Quantity p~3

3

3

3

3

611

3

3

6

122

4

312

3

3

7.5-cm (3-in.) vacuum valve

Sump breather unit assembly

Sump breather installation parts bag

Controller

Controller rebuild kit7.5-em (3-in.) no-hub couplingsO.97-cm(0.38-in.) clear vacuum tubing (1.8 m [6 ft] long)

1.6-em (O.63-in.)clear vacuum tubing (3.7 m [12 ft] long)

7.5-cm (3-in.) grommets15-em (6-in.) grommets

Vacuum valve rebuild kitsController mounting O-ring

Tube controller greaseTube vacuum valve grease

Surge suppressor

Tubing clampsController mounting keyCycle counters


TABLE 3.18 Vacuum station spare parts.

Quantity Item

57L (15 gal)

1

1

1

1

2

1

OilOverhaul kit (vacuum pump)

Filter kit (vacuum pump)

Motor-pump coupling set (vacuum pump)

Seal kit for wastewater pump

Motor coupling (wastewater pump)Gasket set (wastewater pump)

Special Tools. In addition to spare parts, certain specialty maintenance tools andequipment are needed and are listed in Table 3.19.

OPERATION AND MAINTENANCE MANUAL. To properly operate, avacuum sewer system requires proper training. Operation and maintenance manualsare a vital part of this training process. Problems arose in some of the early vacuumsystems because of the lack of such aids. Manufacturers and engineers are now rec-ognizing this fact and are reacting accordingly with improved technical assistanceand operation and maintenance manuals.

TABLE 3.19 Specialty tools and equipment (one set per project).

Quantity Item

1100

32

11

11

11

Portable vacuum chart recorders

Vacuum charts

Chart penso to 20 in," W.G. Magnehelic gauges

o to 50 in.* W.G. Magnehelic gauges

Sensor pipe pullerValve repair stand

No-hub torque wrenches

Vacuum gaugesController test box

*in. X 25.4 = mm.


While an operation and maintenance manual is a valuable tool, it should not beviewed as the ultimate solution to every problem. The efficiency of the systemdepends on the initiative, ingenuity, and sense of responsibility of the system's oper-ation and maintenance staff. Also, the manual should be constantly updated to reflectnew operational experience, updated equipment data, and previous problems andimplemented solutions.

Typical information that should be contained in the operation and maintenancemanual includes the following:

• Design data,

• Equipment manuals,

• Shop drawings,

• Permits and standards,

• Operation and control information,

• Personnel information,

• Records,

• Preventative maintenance schedules,

• Emergency operating and response program,

• Safety information, and

• Utility listings

RECORD KEEPING. Good records are important for the efficient, orderly opera-tion of the system. Pertinent and complete records provide a necessary aid to controlprocedures, as they are used as a basis of the system operation. The first step of anytroubleshooting procedure is an analysis of the records. A wealth of information iscontained in the basic records.

Records should be kept on all normal, preventative, and emergency maintenanceand on operating costs. These should be preserved and filed where they are readilyavailable to operating personnel. All records should be neat and accurate and madeat the time the data are obtained. It is good practice to summarize this data in a briefmonthly report and a more complete annual report. Ideally, the information can beentered into a computer program that can be accessed before the operation and main-tenance staff initiate a call.


Normal Maintenance Records. The following information should be recorded ona daily basis:

• Date and weather conditions;

• Personnel on duty;

• Routine duties performed;

• Operating range of vacuum pumps;

• Run-times of vacuum pumps, wastewater discharge pumps, and generator;

• Flow data;

• Complaints received and the remedies;

• Facilities visitors;

• Accidents or injuries;

• l1nusualconditions;and

• Alterations to the system.

Preventative Maintenance Records. Adequate records provide information thattells operational personnel when service was last performed on each system compo-nent and indicates approaching service or preventative maintenance requirements.Efficient scheduling of these maintenance tasks can be made, which avoid interfer-ence with other important aspects of system operation.

Results of periodic inspections should be kept. This includes a list of all potentialproblems, likely cause of these problems, repairs necessary to solve the problem, andrecommendations for future improvements to minimize recurrence.

Emergency Maintenance Records. Records should be kept concerning all emer-gency maintenance, including the following:

• Date and time of occurrence;

• Person(s) responding to problem;

• Description of problem;

• Remedy of problem, including total time to correct problem;

• Parts and equipment used; and

• Recommendations for future improvements.


Operating Cost Records. To ensure budget adequacy, it is very important to keepaccurate information concerning the costs of all operation and maintenance items.Costs include the following:

• Wages and fringe benefits,

• Power and fuel consumption,

• Utility charges,• Equipment purchases,

• Repair and replacement expenses, and

• Miscellaneous costs.

EVALUATION OF OPERATING SYSTEMS

OPERATING HISTORY OF VACUUM SEWERS. Early vacuum systems wereoften plagued with consistent operational problems. Small vacuum mains, improp-erly planned vacuum main profiles, too-large liquid slug volumes, and insufficientair all resulted in transport problems (Burns et al., 1973).Adding to the difficultieswas the fact that they were installed without sufficient field experience and withsystem components that were not yet fully reliable. In addition, operation and main-tenance guidelines were not yet available. Frequent service calls and high power billswere common during this era.

Several breakthroughs occurred in the 1980sthat led to significant improvementsin the technology.These included the introduction of the sawtooth profile, an improvedvalve controller, the use of gasketed pipe, and the use of larger pipe and vacuumpumps. Many feel that more progress was made in the vacuum sewer industry duringthis decade than in other time. Service calls were less frequent, systems were moreenergy-efficient,and the systems were becoming more reliable overall.

Improvements in the technology continued throughout the 1990s to the presentday. A better understanding of vacuum sewer hydraulics, improved system compo-nents, and established operation and maintenance guidelines have combined to leadto significant operational improvements.

Today's vacuum systems are significantly different than the systems of the 1970s.Efficiencyand reliability are the two areas where the most improvement has occurred.Continuing research and development is expected to further improve the technology.


OPERATION AND MAINTENANCE DATA: 2003 OPERATOR SURVEY.In 2003, a survey form was sent to selected operators of vacuum systems by AIRVAC(Rochester, Indiana). This was an internal survey conducted of systems operators togather operations and maintenance ata. Operators were told that the results wouldbe used in Alternative Sewer Systems but that their names (both the city name and theindividual system operator's name) would be kept confidential. An attempt wasmade to survey systems that would give a good cross-section of the technology. Theage of the system, topography, geographical location, and size were considered in theselection process. Operation and maintenance data from 22projects, with a total of 49operating systems, were gathered. This represents approximately 20% of the oper-ating systems in the United States.

Tobe consistent with the operation and maintenance data previously reported inthe 1991U.S. EPAManual, Alternative Wastewater Collection Systems (U.S.EPA,1991a),the survey requested information on labor, power, and service-call history.

For the labor component, operators were asked to break their maintenanceeffort into 3 categories: routine (day-to-day), preventative (planned/scheduled),and emergency (service calls). Adjustment to the raw data was required, in somecases, as several operators reported preventative maintenance as routine mainte-nance or vice versa. The data was reduced to the ranges and averages shown inTables 3.20 to 3.23.

Labor. The operator survey showed that labor associated with the vacuum stationwas relatively minor and predictable. Most viewed the labor effort for a vacuumstation as similar to that required for a lift station in a conventional system (seeTable 3.21).

Labor associated with the vacuum mains varied widely, as this was generally afunction of whether any major line problems occurred in the past year. While theupper values shown on Table 3.22 did occur, the vast majority of operators reportedfew, if any, problems with the vacuum mains. The average values are a more realisticview of a normally operating system.

For the labor associated with the vacuum valves, some operators reported pre-ventative maintenance as routine, and vice versa. Others reported no preventativemaintenance at all. The raw data was reduced, and the resulting ranges and averagesare shown in Table 3.23.


TABLE 3.20 2003 operator survey.

Number of Number ofNumber of vacuum vacuum House-to-pit Year

Project connections stations valves ratio operational

Plainville, Indiana 270 1 163 1.66 1975Westmoreland, Tennessee 1000 4 550 1.82 1979Fairmount, Maryland 238 1 159 1.50 1981Queen Anne's County, Maryland 6250 14 2299 2.72 1981White House, Tennessee 1177 2 575 2.05 1987Alton, Kentucky 430 4 210 2.05 1987Theresa, New York 237 1 141 1.68 1989Beallsville, Pennsylvania 235 1 127 1.85 1991Silver Lake, Indiana 492 2 192 2.56 1992Waverly, West Virginia 140 1 114 1.22 1992Montpelier, Ohio 300 1 120 2.30 1993Crystal Lake, Ohio 975 2 438 2.23 1994Pine Grove, West Virginia 380 1 184 2.07 1994York County, Virginia 2238 5 1049 2.13 1995Glen Park, New York 166 1 110 1.51 1995Wolcottville, Indiana 725 2 390 1.86 1996Crisfield, Maryland 258 1 162 1.59 1997Kotlik, Alaska 102 1 75 1.36 1998Jimmersontown, New York 200 1 98 2.04 1999Iron Mountain Lake, Missouri 368 1 241 1.53 2000Stanfield, North Carolina 190 1 129 1.47 2001Forest, Ohio 146 1 -65 2.25 2002

TABLE 3.21 Labor: vacuum station (from 2003 operator survey).

Range reported Average(hours/year/station) (hours/year/station)

Category Low High Average

Routine 100 600 250Preventative 0 90 50Emergency 0 85 30


TABLE 3.22 Labor: vacuum mains (from 2003 operator survey).

Range reported(hours/year/station)

Average(hours/year/station)

AverageCategory Low High

RoutinePreventative

Emergency

ooo

100

100

110

30

20

10

TABLE 3.23 Labor: vacuum valves (from 2003 operator survey).




RoutinePreventative

Emergency

0.20

0.00

0.10

0.90

1.00

1.35

0.50

0.40

0.60

Power. Inmost cases, the operators reported their power consumption in dollars(year 2003). Very few reported the unit charge for electricity (dollars per kilowatthour). An average cost of $0.07/kWh was assumed, and the power costs were con-verted to the power consumption figures shown in Table 3.24. Because of the largedisparity in power consumption between older and more recent systems, the datawas broken into 2 eras.

TABLE 3.24 Power consumption (from 2003 operator survey).




Pre-1990 systems

Post-1990 systems

430

200

570

400

500

300


TABLE 3.25 MTBSC trend.

Era Source MTBSC

6 systems (1970to 1989)49systems (1970to present)

1991U.S.EPAmanual (U.S.EPA,1991a)2003operator survey

2.2years5.1years

MEAN TIME BETWEEN SERVICE CALLS. The mean time between servicecalls (MTBSC) is calculated by dividing the number of valves by the number of ser-vice calls over a 1-year period. For example, a system with 500 valves that required50 service calls in a year would have an MTBSC of 10 years.

A U.S. EPA technology transfer seminar publication, prepared in 1977, detailed thefailure rate (MTBSC) of some of the early vacuum systems. Ingeneral, the MTBSC ofthe early systems ranged from less than 1 year to more than 8 years; all but one of thesystems had an MTBSC of less than 4 years (U.S. EPA, 1977). In the 1991 U.S. EPAmanual, Alternative Wastewater Collection Systems, the MTBSC of the 6 systems visitedranged from 1 year to 22.5 years (U.S.EPA, 1991a), with an average MTBSC of 2.2 years.

The 2003 operator survey showed a range of MTBSC values of 2 to 27 years, withthe average being 5.1 years. This survey included many of the early systems thathave lower MTBSC values. Even with these included, the overall MTBSC figure hasincreased over the years (Table 3.25).

HISTORICAL PROBLEMS. Each of the systems visited as part of the 1991 U.S.EPA manual (U.S. EPA, 1991a) effort experienced some type of problem thatdemanded operation and maintenance staff time. However, most were short-lived.The results of the 2003 operator survey indicate that many of these early problemshave vanished (see Table 3.26).

As is the case with other system types, extraneous water (infiltration and inflow)is the root cause of many problems, whether it is heat buildup in the station resultingfrom excessive pump run-times or problems with the valve controller resulting fromexcessive cycles. In a vacuum sewer system, the only potential source of infiltrationand inflow is the homeowner's building sewer, where even a small amount of infil-.tration and inflow can have a detrimental effect. Accepting flow from an existinggravity system, where infiltration and inflow is common, further exaggerates theproblems (see the following section).

TABLE 3.26 Summary of historical problems.


Pre-1990 systemsAs reported in theU.S. EPA manual (1991a)

Post~1990 systems

As in the2003 operator survey

Component defect

Broken controller springUnreliable controllerShaft/sealPlug valve

Isolated casesUntil the mid-1980sUntil the mid-1980sIsolated cases

No longer a problemNo longer a problemNo longer a problemNo longer a problem

Design shortcomings

Pump cavitationsLeaking check valvesOversized vacuum pumps

Isolated casesUntil mid-1980sMid-1980s

Not as frequent, but still a concernNo longer a problemNo longer a problem

Operator ErrorWastewater into vacuum pumps Fairly common

Construction related

More safeguards now, but still aconcern

Line breaksBroken fittingsConstruction debrisHeat in stationBroken cleanout

Common w / solvent weldCommon w / solvent weldCommon after startup

Isolated casesFairly common

Rarely with gasketed pipeRarelyNot as common because of opera-

tor trainingStill a concern when variable-fre-

quency drives are usedLess frequent with fewer

cleanouts

Equipment malfunction

Faulty level controlFaulty telephone dialer

Isolated casesIsolated cases

Rarely; improved technologyRarely; improved technology

Extraneous water

System waterlogging

Water in controllerInfiltration and inflow

More likely before sawtooth#1 component problem

Root cause of most problems

Less likely now, but still a con-cern

Less frequent, but still a concernStill the root of most problems


The number one component-related problem remains "water in the controller";however, the incidence rate of this happening has drastically decreased over time,as is evidenced by the increasing MTBSCvalues of the recent systems. Water in thecontroller is a byproduct of system problems that occur as a direct result of extra-neous water (infiltration and inflow) that is allowed to enter the system (see the fol-lowing section).

SITUATION TO AVOID-ACCEPTING FLOW FROM AN EXISTINGGRAVITY SYSTEM. Of all of the potentially bad situations that can occur, per-haps none is more damaging to a vacuum system than excessive flow that enters avacuum system via an existing gravity system. Problems ranging from sluggish, inef-ficient flow transport to temporary system failure have resulted. With new construc-tion, one can fairly accurately predict average and peak flow and design the vacuummains and vacuum station accordingly. By accepting flows from an existing system,another element is introduced to the equation-infiltration and inflow.

Should it be possible to accurately predict infiltration and inflow, this situationcan be considered, but still with caution. An analysis of the existing gravity systemmust be done. This would include having flow records that identify the magnitudeof flow that can be expected during normal periods and rain events (minimum 1 yearof flow data). Even then, should there be a large difference between normal dailyflow and flow during a rain event, it is recommended that the existing gravity flowbe handled by other means (AlRVAC,2005a).

SYSTEM COSTS

CONSTRUCTION COSTS. General. It is not uncommon for a wide range ofbids to be received on an engineer's first vacuum project. Contractors unfamiliarwith vacuum sewers may bid high simply because of the fear of the unknown.Equally possible is a contractor who bids too low because of an underestimation ofthe effort required. Once local contractors have experience with vacuum sewer con-struction, the spread between low and high bid narrows, and it becomes much easierfor the engineer to estimate the construction costs of future projects.

Many factors affect construction cost bids. Material surpluses or shortages, pre-vailing wage rates (depending on funding sources), local bidding climate, geographicarea, time of year, soundness of the design documents, and the design engineer's rep-utation are examples of these. Funding and regulatory requirements also playa part,


to the extent that the regulations may be a help or hindrance to the contractor.Because of the many variables, accurate cost-estimating guidelines are beyond thescope of this manuaL The following general guidelines, however, will be of help tothe estimator.

Note that figures shown in Tables 3.27 to 3.29 are estimated construction costsand do not include other project costs, such as legal, administrative, and engineeringcosts. Engineering costs, including design, construction, administration, and inspec-tion typically fall in the range of 20 to 25% of the overall project capital costs.

Vacuum Mains. Vacuum piping systems are best estimated using guidance fromwater system projects built in the same area, if similar materials and specificationsare used. The best situation is one where the water line project was designed and con-struction-managed by the same engineering firm that is producing the sewer costestimate, to facilitate the comparative similarities and differences. Some generalitiesbetween vacuum and other collection system types are shown in Table 3.30.

TABLE 3.27 Installed unit costs (4th quarter 2006)for vacuum sewer mains".

Item Unit cost (USD$)

lO-em (4-in.) mains

lS-em (6-in.) mains

20-cm (8-in.) mains

2S-em (10-in.) mains

$13.00 to $16.00/ lin ft

$16.00 to $21.00/ lin ft

$21.00 to $27.00/ lin ft

$27.00 to $37.00/ lin ft

*ft X 0.3048 = m; in. X 2S.4 = mm.

TABLE 3.28 Installed unit costs (4th quarter 2006) for valve pits and appurtenances.

Item Unit cost (USD$)

Standard valve pit (1.8 m [6 ft] deep)

Deep valve pit (2.4 m [8 ft] deep)

Single buffer tank

Dual buffer tank

Optional antiflotation collar

Optional flexible connector

Optional cycle counter

$3,500 to $4,000

$4,000 to $4,SOO

$4,500 to $5,000

$S,500 to $6,000

$160 to $185

$90 to $110

$2S0 to $300


TABLE 3.29 Installed unit costs (4th quarter 2006)for vacuum stations.

Number ofconnections

Totalcost (USD$)

Equipmentcost (USD$)

Buildingcost (USD$)

50 to 100

100 to 300

300 to 500

500 to 1000

1000 to 1500

$125,000 to $150,000

$150,000 to $200,000

$200,000 to $250,000

$250,000 to $300,000

$300,000 to $400,000

$125,000 to $150,000

$150,000 to $200,000

$200,000 to $275,000

$275,000 to $350,000

$350,000 to $450,000

$250,000 to $300,000

$300,000 to $400,000

$400,000 to $525,000

$525,000 to $650,000

$650,000 to $850,000

Installed costs for vacuum mains are very site-specific and vary widely from pro-ject to project. Items such as the existence of rock, unstable soils, or groundwater havea large effect on installed prices. The condition of the construction zone itself (i.e.,wide open space versus narrow construction) also affects the installed prices.

Experience has shown that vacuum main pricing typically falls somewherebetween gravity and pressure main pricing. Typically,they are closer to the pressureprices than they are to the gravity prices.

In the absence of better information, Table 3.27 provides a crude basis for costestimating that might suffice for planning purposes. This table was prepared byreviewing bid tabulations from numerous projects throughout the United States. Pipeprices include furnishing and installing the pipe, excavation, bedding, backfilling,compaction, vacuum testing, cleanup, and similar requirements. Not included areallowances for site-specific items, such as rock excavation or dewatering.

Valve Pits. Valve pit setting prices will vary, depending on the type and depth ofthe pit setting. Table 3.28 gives a range of installed unit prices for the various valvepit settings and appurtenances. The prices include furnishing and installing the valvepit setting, excavation, bedding, backfill, compaction, vacuum testing, and surfacerestoration.

It is typical for two or more houses to share a single valve pit setting, which nat-urally reduces the setting cost per house accordingly. This cost reduction is only frac-tionally reduced by additional costs that accrue because of previously describedadditional on-lot requirements.

Vacuum Stations. Table 3.29 gives a range of installed prices for typical vacuumstations. The equipment prices include the major station equipment (vacuum pumps,wastewater pump, collection tank, electrical controls, etc.).The building cost includessite work, structure, and station piping and plumbing.


TABLE 3.30 General cost comparison of vacuum mains with pressure and gravity sewer mains.

Installed cost ofvacuum main

Compared with: ($/lin £t)* Comments

Pressuresewer mains

10% higher forsame pipe size

Gravity mains As much as 60% lessfor same pipe size

Similar trench (narrow and shallow), but grade control ismore critical. Vacuum sewers do not require air-releases atsummits.Vacuum mains are typically 1 pipe size larger than pres-sure mains serving the same number of homes (i.e., Ifl-cm[4-in.] vacuum main versus 7.5-cm [3-in.] pressure main).

Narrower and shallower trench; the more difficult thesubsurface conditions, the larger the price savings.

Vacuum mains are typically 2 pipe sizes smaller thangravity mains serving the same number of homes (i.e., 10-em [4-in.] vacuum main versus 20-cm [8-in.] minimumgravity main).

*ft X 0.3048 = m.

Estimated costs do not include the cost of land required for the vacuum station.Additionally, the estimated costs do include additional building size and capital costsassociated with a standby generator and its appurtenances. Costs also vary widely,depending on the location of the project, control system requirements, the utility'srequirements and planning, and zoning requirements. These costs should be deter-mined on a case-by-case basis.

There is an economy of scale with vacuum stations, as Figure 3.25 shows. How-ever, the engineer should not be too tempted by this fact, but should keep theoptimum design approaches already described as primary guidance.

OPERATION AND MAINTENANCE COSTS. General. Fifteen years ago,very little historical operation and maintenance cost data existed on vacuum sewers.This lack of data led many to the conclusion that vacuum sewers must be intensivein operation and maintenance. A review of operating records of systems discussed inthis chapter suggests that previously published operation and maintenance figuresmay no longer apply. Reasons for this are twofold. First, the previous figures werebased on very limited data on a few early systems. Second, component improve-ments have resulted in significantly fewer service calls and lower operation andmaintenance costs.


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Operation and Maintenance Information from 1991 U.S. EPA Manual (U.S. EPA,1991a). The u.s. EPA conducted a study on alternative collection systems, includingvacuum sewers, in 1989 and 1990.Part of this effort included visits to operating sys-tems, to obtain information on operation and maintenance costs. The report con-taining this information, called the Alternative Wastewater Collection Systems Manual(EPA-625/1-91-024),was published in 1991 (U.S.EPA, 1991a).

It is important to note that a wide variety of projects were visited by U.S. EPA,including some of the earliest systems built and systems that use design concepts andsystem components other than those used by modern systems. As one would expect,the earliest systems had the highest operation and maintenance costs (see the Evalu-ation of Operating Systems section for discussion).

Basis of Operation and Maintenance Estimating Charts. Design advancements cou-pled with component improvements have led to modern vacuum systems that areoperated at much higher levels of reliability than their predecessors.

Information from the 1991U.S. EPAreport (1991a) and information from recent(post-1990) systems gathered from the 2003 operator survey was used in the forma-tion of the estimating tables that follow. For each particular operation and mainte-nance item, a cost range is given. With proper design, installation, and mainte-nance, the operation and maintenance costs at the lower end of the cost range canbe achieved.

A discussion of the typical operation and maintenance cost components thatmust be considered follows.

Labor. To estimate labor costs, the number of person-hours required is multipliedby the hourly rate. Fringe benefits are then added. The annual person-hour require-ments are made up of normal, preventative, and emergency maintenance. Judgmentmust be exercised in interpreting other projects for use in labor estimates (see fol-lowing section).

For most systems, normal maintenance does not require an operator 24 hours perday. Monitoring of the system is provided by the telephone dialer and/ or telemetrysystem. However, someone must at least be on call around-the-clock, in case the tele-phone dialer calls with a problem. In this respect, vacuum systems are unique. Veryfew problems in a vacuum system can go uncorrected for any length of time withoutcausing a cumulative effect. Therefore, rapid response time is a key requirement.

Typically, the normal workforce does preventative maintenance during off-peak working hours. As such, preventative maintenance is generally reported asnormal maintenance.


Often, emergency maintenance requires personnel after normal working hours.The result is overtime pay. Emergency maintenance typically requires two operatorsor one plus an assistant.

Table 3.31 provides a range of labor hours required per year. These factorswere based on an analysis of the operation and maintenance data from the 2003operator survey described in the Evaluation of Operating Systems section, whichincluded systems of all ages, including some of the earliest systems. The mid-range values shown in Table 3.31 represent the average of all of these systems,while the high and low values shown have been slightly modified to correct forunusually low or high figures that could skew the analysis. The values shownshould be considered as a realistic estimate for new systems with proper design,construction, and management.

When a full-time operator is to be hired, regardless of anticipated workload, thevalues in Table 3.31 should not be used. In this case, the estimated annual person-hour requirements should include the full-time hours of employment plus an esti-mate of the overtime (emergency maintenance) hours, taking into consideration thatovertime work generally requires two people. No allowance is needed for normal orpreventative maintenance, because these tasks can be performed during normalworking hours.

Effort to Operate a System-Actual versus Billable Time. The operatingutility's overall responsibilities should be considered when estimating labor costs.For example, the utility is likely to be responsible for other wastewater treatmentand/ or disposal facilities and possibly even water facilities. In these cases, oper-ating personnel are typically shared. At the end of the year, the time charged to theoperation of the vacuum system will relate exactly to the effort required (i.e., 1hour

TABLE 3.31 Labor estimating factors (based on 2003 operator survey).

Vacuum station Vacuum mains Vacuum valves(hours/year/station) (hours/year/station) (hours/year/station)

Range Mid-range Range Mid-range Range Mid-range

Normal 100to 400 250 20to 40 30 0.20to 0.80 0.50Preventative 20to 80 50 10to 30 20 0.20to 0.60 0.40Emergency 20 to 40 30 5to 15 10 0.20to 1.00 0.60

Total 140to 520 330 35 to 85 60 0.60to 2.40 1.50


per day for each vacuum station plus some hours charged for other preventativeand emergency maintenance). If the overall facilities are large enough to warrantmore than one shift, emergency work most likely will be done without overtimebeing required.

An entirely different situation exists for the utility operating nothing but avacuum system. Typically, a full-time operator is hired. This person charges 8hours/ d for the maintenance of the system, although, most days, much less time thanthis will be spent. Should a problem develop after normal working hours, this oper-ator will most likely will be paid overtime. Even though the primary operator andpart-time operator assistants will spend the same amount of actual vacuum sewermaintenance time as the staff with broader responsibilities above, the amount ofbilled time will appear to be entirely different.

The engineer should carefully analyze the client's overall management responsi-bilities, taking into consideration the possibility of shared duties, before making anestimate of the labor costs.

Power. Power is required for the vacuum pumps, the wastewater pumps, and theheating, lighting, and ventilation of the vacuum station. For planning purposes,values shown in Table 3.32 can be used to estimate the annual power consumptionfor the vacuum station.

Similar to the economy of scale in capital cost, there is an economy of scale per-taining to power costs. The smaller vacuum stations typically have the highest powerconsumption per connection, and the larger vacuum stations have the lowest powerconsumption per connection.

Utilities. Utilities at the vacuum station generally include water, telephone, andfuel. Water may be required for sinks and hose bibs. A telephone is required for thefault-monitoring system. Fuel may be required for the standby generator. The cost ofthese utilities generally is less than $85 per month (4th quarter 2006).

TABLE 3.32 Vacuum station power consumption estimating factors.

Range Monthly cost Monthly cost(kWh/year/connection) at $O.08/kWh at $O.10/kWh

Low 200 $1.33/month/ connection $1.66/month/ connection

High 400 $2.66/month/ connection $3.34/month/ connectionAve 300 $2.00/month/ connection $2.50/month/ connection


Clerical. This item includes wages for the clerical staff and billing costs, such asenvelopes and stamps. Like labor costs, the value of this item most likely will dependon whether the operating utility has an existing, ongoing operation that requiresoffice staff. If so, the total costs need to be allocated between the administrativeresponsibilities.

Transportation. Vehicle expenses to maintain the system will be incurred. For esti-mating purposes, a mileage rate multiplied by the estimated annual kilometers(miles) will suffice. This rate should include vehicle amortization, depreciation, taxes,and similar expenses.

Supplies/Maintenance. As with a conventional system, certain supplies will berequired. Restocking of spare parts and inventory is included in this item, as are oil,fuses, charts, and chart pens. The initial purchase of items on quantity discountshould be maximized, to take advantage of the lower unit costs when compared withsubsequent prices for replacement. Service contracts for emergency generators andfuel for the generators may also be included in this item.

Miscellaneous Expenses. Miscellaneous expenses include insurance and mainte-nance on the system structures and professional services (engineering, accounting,and legal) that may be required during the year.

Equipment Reconditioning and Replacement. A set-aside account should beestablished, to generate sufficient funds on an annual basis for major equipmentreconditioning and replacement. The annual cost of these needs is initially estimatedby dividing the replacement cost by the useful life. This amount is generally set asidein an interest-bearing account until needed. Present dollars can be used in the esti-mate, because the interest earnings most likely will offset inflation. Alternativemethods dictated by regulatory agencies also can be used. This annual cost estimateshould be reviewed regularly; to ensure that sufficient funds are available to keep thesystems running optimally. Table 3.33 lists the major equipment items and theiruseful life. .

Valves and controllers can be rebuilt very inexpensively (see the PreventativeMaintenance section). For this reason, reconditioning and replacement funds are notrequired for total replacement, but rather just for the rebuild costs. Table 3.34 showsthese operating costs.


TABLE3.33 Typical reconditioning and replacement costs for major equipment(4th quarter 2006).

Cost range(USD$)*

Expected life(years)

Annual reconditioningand replacement costs

($/year/vacuum station)

Vacuum pumps (2)

Discharge pumps (2)

Collection tank

Control panel

Miscellaneous equipment

$10,000 to $34,500

$6,000 to $19,200

$5,000 to $11,000

$5,000 to $21,200

$2,000 to $3,300

15 to 20

15 to 20

25 to 50

20 to 25

15 to 20

Typical range

$500 to $2,300

$300 to $1,280

$100 to $440

$200 to $1,060

$100 to $220

*Function of equipment size.

$1,200 to $5,300

TABLE3.34 Typical rebuilding costs for valves and controllers (4th quarter 2006).

Cost range(USD$)*

Rebuild frequency(years)

Annual reconditioningand replacements

($/year/valve)

Vacuum valves

Controller

$27.00 to $38.00

$27.00 to $38.00

8to 12

4to 6

Typical range

$2.25 to $4.75

$4.50 to $9.50

$6.75 to $14.25

SYSTEM MANAGEMENT CONSIDERATIONS

SEWER AUTHORITY RESPONSIBILITIES. Customer Connection toSystem. Table 3.35 shows the normal sequence of events, from construction of thesystem to home-hookup. Note that the contractor does not install the vacuum valveduring the construction phase (see discussion later in this section). When all contrac-tual obligations are fulfilled, the system is accepted by the utility, and the home-owners are notified that the system is ready.

Operating Personnel. Once all customers are connected, the utility's only focusshould be providing reliable, efficient service to their customers. To achieve this, theoperating personnel must be capable, dependable, and knowledgeable. Of utmost


TABLE 3.35 Normal sequence for connection.

Tasks Responsible party

Lines, pits, and vacuum station installed

Final4-hour vacuum test and line flushing

System acceptance and notification to homeownersthat system is ready

Building sewer and air-intake installed

Vacuum valve installed

Installation contractor

Installation contractor

Utility

Homeowner's plumber

Utility

importance is attitude. An operator who does not believe in the system will ulti-mately cause the system to operate below its potential, in terms of reliability andcosts. Conversely, an operator with a good attitude uses creativity to get more out ofthe system than was originally planned.

Sewer Use Ordinance. To operate any system at a high level of efficiency requiresa sewer use ordinance. This document sets consistent rules for all users to follow.Included are material specifications, minimum slope requirements, and air-intakelocations for the building sewer. Of extreme importance to the utility is a limitationof the use of the vacuum sewer to convey sanitary wastes only, as extraneous water(illegal discharges or infiltration and inflow) will cause operational problems.

An active program for the identification of extraneous water sources should bedeveloped. This may include smoke testing and dye testing, but the simplestapproach to quantify sources of extraneous water in a vacuum system is to usecycle counters. This device, when connected to an interface valve, will record thenumber of times the valve opens in a given period. Knowing that each cycle isapproximately 39 L (10 gal), the utility can estimate, based on water consumptionrecords, the number of cycles expected over that period. A count significantly inexcess of the expected number of cycles generally implies that extraneous water isentering the system.

The utility's other major concern during this full-operational phase is its respon-sibility for future extensions of the system. This includes proper planning, design,and construction of such extensions. The utility, in accordance with the provisions ofthe sewer use ordinance, is also responsible for implementing future connections tothe existing system.


Private versus Public Ownership of Equipment Serving House. There are twoissues to consider-(l) actual ownership of the valve/valve pit, and (2)maintenanceof the valve/valve pit. Because of the "system" nature of vacuum sewers, the main-tenance of a vacuum system, including the valve pit and the valve, must be done bythe utility. Improper maintenance at a single valve pit could affect the entire system,including the line hydraulics and operation of the station. Obviously, this wouldjeopardize the system and affect other customers. So, it is not prudent to put this inthe hands of the homeowner. The only way to guarantee maintenance of the valveand valve pit is by the utility actually owning it.

HOMEOWNER RESPONSIBILITIES. The homeowner's responsibility gener-ally begins at the end of the valve pit stub-out and includes the building sewer, air-intake, and any in-house needs.

Most utilities require the homeowner to replace the building sewer from thehouse foundation to the stub-out connection, because vacuum sewers are notdesigned to handle extraneous water. By accepting old, possibly defective buildingsewers, the utility would be taking a serious risk for increased operation and mainte-nance problems.

The homeowner is also responsible for the installation of the Itl-cm (4-in.) air-intake. The air-intake is necessary for the proper operation of the valve. It is desirablefor this to be located against a permanent structure, such as the house itself, a fence,or a wall.

The vacuum valve is not installed until the customer is ready to connect to thevalve pit setting (see Figure 3.26).It is common for the contractor to install the valvepit/sump, including all of the necessary piping, during collection system construc-tion. The valve is supplied to the utility for installation at a later date. In this manner,the utility can systematically install the valves as each customer requests connection.

In an effort to relieve the system owner from installing the vacuum valve,some engineers set up their bid documents to require the contractor to install thevacuum valve during construction. This is not recommended for the reasonsshown in Table 3.36.

A further complication may occur if a failed vacuum test is the result of a combi-nation of a valve leaking and a line leak(s). This could cause some real difficulties introubleshooting to determine where the problem really is and in subsequentlyassessing liability. Contractor liability versus manufacturer liability is clear-cut whenthe testing is done without the valve in place.


2 X 4 OR SIMILAR DEVICENEAR GRADE INDICATIONGEND OF GRAVITY STUB

AIRVAC VALVE PIT LOCATEDIN RIGHT-OF-WAY

7'-6" APPROX Ck

HOUSE

3" VACUUM LINEPREVIOUSLYINSTALLED ANDCONNECTED TO

/h-:I..--,=ct1:'::::1\i:::;~"",=<,"ri't~\l.'\'i'~r+f.j!~:.nT-;:::b"'.ffi=l=;;;j;;;;;;RZJ.!i-~m.-;~. ,.VACUUM STATION

APPROX. 4'-6" (CAN BEAS MUCH AS 6'-6" WITHDEEP SUMP)

LSTI"G GRAVITYU"EFROM RESIDENCE

4" OR 6" GRAVITY LINESTUBBED OUT ANDCAPPED OFF (MAY BEAS MANY AS FOURCONNECTIONS)

VALVE PIT INSTALLATION PRIOR TO HOME CONNECTION

20' MINIMUM IN NORTHERN CLIMATES

4" AIR INTAKE

HOUSE

OPTIONAL SCREEN

4" OR 6" GRAVITY LINEROUTED TO VACUUMVALVE PIT ANDCONNECTED TO EXISSTUB-DUT

VALVE PIT INSTALLATION WITH HOME CONNECTED

FIGURE 3.26 Valve pit installation with home connection (courtesy of AIRVAC, Rochester, Indi-ana) (1 in. = 2.54 em; 1 ft = 0.3 m).


Potential problem

TABLE 3.36 Potential problems if valve is installed during construction phase.

Reason

Pit collapse or implosion

Difficulty assigning blame if4-hour vacuum test fails

Homeowner may illegallyhook-up early

Cycling the vacuum valve without the homeowner'sbuilding sewer and lO-cm (4-in.) air-intake installed canresult in the bottom sump collapsing. This would requirethe pit to be re-excavated and replaced.

The intent of the final4-hour vacuum test is to test thecontractor's workmanship in installing the vacuumlines. Testing with the valve in place introduces onemore variable-the valve may leak. Failure resulting froma leaking valve is not the contractor's responsibility.

WIth a complete system available, the homeowner mayconnect to the valve pit without the utility's knowledge.This action would preclude the utility from doing thenormal inspection of the homeowner's gravity lateral,air-intake, and so on. This could lead to some seriousproblems, such as sump collapse, infiltration and inflowproblems, water in the controller, and so on.

All of the work required by the homeowner must be inspected by the utilitybefore the final connection. This ensures the proper and efficient operation of thesystem. Compliance with the sewer use ordinance is the only remaining user respon-sibility. Typical requirements include that the homeowner should not drive or buildover the valve pit and should protect the facilities from damage. Discharge of flam-mables, acids, and excessive amounts of grease, sanitary napkins, or other nonwaste-water items is forbidden. This requirement differs little from user ordinance require-ments for conventional sewers. Proper use of the system results in lower user chargesand improved reliability.

OTHER ENTITIES. During the planning, design, and construction of wastewatermanagement systems, there are many different entities involved. Two vitally impor-tant ones are the regulators and the engineer. It is during these times that critical deci-sions are made and details finalized.


Engineer. Historically, engineers have often viewed the startup of a wastewatersystem as their final involvement. While this attitude is economically understand-able, it is not acceptable where local management programs are minimal. Continuinginvolvement should be provided to help the utility develop an experience base withnewer systems that permits intelligent applications in the future.

The engineer should spend a significant amount of time assisting the utilityduring the startup of the system. Tests should be run and problems simulated to seeif the system is operating as designed. On a regular basis (often annually), the oper-ating records should be analyzed for budget sufficiency purposes. The institution ofenvironmental management systems practices can ensure that any problems andtheir solutions will be identified and addressed by the utility. In short, the engineershould be prepared to assist the utility in using the operating experience of thesystem to help develop improvements in future designs.

Regulatory Agencies. Likewise, regulatory agencies must, as part of their over-sight responsibilities, be aware of the potential effects of the operation of a new col-lection system on environmental compliance of the entire wastewater managementprogram. Information on problems, including causes and the remedies, should begathered by the utility for review by the regulatory agency. Cost and other datashould be obtained and used accordingly by the regulators in counseling futurepotential users of this type of collection system.

It is this present lack of useful capital and operational costs and other pertinentinformation that causes many engineers and regulatory agencies to shy away fromnew technologies. Continued use of conventional solutions that are well-known andcodified is far easier for regulators and engineers than seeking lower-cost, new solu-tions to solve wastewater pollution problems. Therefore, implementing new solu-tions, no matter how cost-effective, will continue to be difficult.

EDUCATION PROCESS. Before 1990,very little written documentation existedon vacuum sewers. Much of the recent growth in the vacuum sewer industry can beattributed to the ever-increasing amount of information regarding the technology.Sources of this information can be found in technical presentations by manufacturers,papers that have been presented at conferences, articles that have appeared in tradejournals, and factory and project tours. The intent of the factory tour conducted byvacuum manufacturers is to increase the comfort level of those considering the use ofvacuum technology. Specifically,the factory/project tour does the following:


• Provides a basic understanding of vacuum sewer system principles;

• Demonstrates the actual components used in a real system;

• Provides participants with firsthand knowledge of vacuum systems, by visitingthose who have designed, constructed, and operated these types of systems.

The primary reason for a utility or its engineer to attend a factory/project tour isto find out firsthand whether or not they, as responsible officials, can recommend thistechnology for their particular situation. Feedback from these groups indicates thatthe visit to the factory and/ or an operating system ultimately allowed them to makean intelligent, educated decision.

SAMPLE REGULATIONS FOR VACUUM SYSTEMSIn some parts of the world, there are established standards for vacuum sewers. Forexample, Australia has a 2004 standard called the Vacuum Sewerage Code of Aus-tralia (WSA 06-2004), and Europe has the 1997 European Norm Vacuum SewerageSystems Outside Buildings (EN-1091).

There is no national vacuum sewer system standard in the United States. Eventhough there are operating systems in 29 states, very few states have specific regula-tions regarding vacuum sewers. Because of the perceived difficulty of gaining regu-latory approval when no formal regulations exist, some engineers have even hesi-tated proposing vacuum sewers, despite the fact that the project may be an idealcandidate for the technology.

To create new regulations requires much technical writing, combined with aseries of public comment and review phases. Because the process can take much timeand effort, many states have chosen not to write new regulations for vacuum sewers.This is especially true of a state in which a vacuum sewer is being proposed for thefirst time, where it is hard to justify the time and effort necessary to create regulationsfor a technology that may only be used once.

Rather than creating new regulations, some states instead choose to refer to tech-nical publications as the basis for design review and approval. This is a similarmethod used by some for gravity sewer designs, which refer to the Ten State Stan-dards (Great Lakes, 1990).

The sample vacuum regulations that follow have been created for this purpose.These sample regulations incorporate the content of this manual, while using the TenStates Standards as a model for both format and content.


REQUIREMENTS FOR DESIGN OF VACUUM COLLECTION SYSTEM.General Requirements.

(1) The entire vacuum sewer system, including the individual valve pits, shallbe owned, operated, and maintained by a single operating entity.

(2) The vacuum piping network shall be designed with the intent to keep thebore of the entire pipeline open. Designs where sections of the pipeline arepurposely sealed are not allowed.

(3) The vacuum sewer system must be designed to remain operational duringthe loss of vacuum.

(4) For routine and emergency operation and maintenance of a vacuum sewersystem, the public entity responsible for the system shall have the right ofaccess to an adequate supply of spare valves, pumps, parts, and service.

(5) The vacuum sewer system air-to-liquid ratio shall be a minimum of twoparts air to one part liquid.

(6) The maximum static loss in the vacuum sewer system shall be 4 m (13 ft).These are the losses calculated from the furthest extremities of the vacuumsewer system to the vacuum pump station.• Vacuum loss from each lift is calculated by subtracting the pipe internal

diameter from the lift height (i.e., for a 0.3-m [l-ft] lift in a 10-cm [4-in.]pipe, vacuum loss = 0.3 m [1.0ft] - 0.1 m [0.33ft] = 0.2m [0.67ft]).

• When lifts are required in specific valve service lines or when concretebuffer tank valve pits have suction lifts in excess of 1.7m (5.5ft), the staticlosses shall be added to the losses for that main and shall not exceed 4 m(13 ft). In no case shall suction lifts from the bottom of the holding sumpto the valve centerline exceed 2.4 m (8 ft).

(7) The maximum friction loss in the vacuum sewer system shall be 1.5 ill (5 ft).• Friction losses for vacuum sewers installed at slopes between 0.20 and 2%

are cumulative for each "flow path" from the last valve on a line to thevacuum station. Friction losses for sewers installed in excess of 2% may beignored. Friction losses are to be calculated using the following formula:

f = 2.75 X 0.2083 X (100/C)1.85 X (Q1.85Id4.8655) (3.8)Where* f = friction losses (ft/lOO ft pipe),

C = pipe roughness coefficient (150for PVC pipe),Q = flow (gpm), andd = pipe internal diameter (in.).

*Units of measure are provided only in u.s. customary units per the orig-inal equation.


• Flow for a given section equals the mean flow for the section of pipe plusthe incoming flow. The mean flow for a section of pipe is the total flowdirectly connected to the section of pipe, divided by two. Peak flows mustbe used when calculating friction losses.

Design Requirements for the Vacuum Collection System.

(1) The general design configuration shall be based on the sawtooth pipelineprofile. Other vertical pipeline design profiles may be considered, if justifiedby appropriate engineering data, as part of the preliminary evaluation andengineering report.

(2) A minimum pipe diameter of 10 em (4 in.) is required for vacuum sewermains and gravity service laterals.

(3) Vacuum sewer lines must have a minimum slope of 0.20%. For profilechanges less than 38.1 m (125 ft) apart, the minimum fall between profilechanges is 0.08 m (0.25 ft).

(4) The maximum design flows (i.e., peak flows) for vacuum pipe sizing are asfollows:• 10-cm (4-in.) pipe shall be 2.4 L/s (38 gpm)• IS-em (6-in.) pipe shall be 6.62 L/s (105 gpm)• 20-cm (8-in.) pipe shall be 13.2 L/s (210 gpm)• 25-em (1O-in.)pipe shall be 23.7 L/ s (375 gpm)

(5) The maximum length of 10 em (4-in.) diameter lines, for anyone run, is609.6 m (2000 ft).

(6) Lift heights should be made according to the following guidelines. In nocase should a single lift exceed 0.9 m (3 ft) in height (see Table 3.37).

(7) A series of lifts should be made with consistent lift heights.(8) For changes in horizontal alignment, two 45-degree bends connected by a

short section of piping are required, rather than one 90-degree bend.(9) Isolation valves are required at every branch connection and at intervals no

greater than 457.2 m (1500 ft) on main lines. Resilient coated wedge gatevalves shall be used. The valves shall be installed with a valve box or otherapproved apparatus, to facilitate proper use of the valve.

(10) When vacuum mains or branches must ascend a hill, multiple lifts areplaced at a minimum distance of 6 m (20 ft) apart. Between each lift, vacuumlines are installed with a uniform slope, so that minimum fall is achievedbetween these lifts (minimum fall is 0.08 m [0.25 ft]).


TABLE 3.37 Lift height guidelines.

Pipe diameter Lift heights

Ill-em (4-in.) pipeIS-em (6-in.) pipe20-em (8-in.) pipe25-em (Ifl-in.) pipe

0.3 or 0.5 m (1.0 or 1.5 ft)0.3 or 0.5 m (1.0 or 1.5 ft)0.5 or 0.6 m (1.5 or 2.0 ft)0.5 or 0.6 m (1.5 or 2.0 ft)

(11) Where a lift or profile change is required in a branch sewer before enteringthe main, it should be made 6 m (20ft) or more from the main.

(12) For a series of lifts following a downward slope in excess of 0.20%, thevacuum sewer should be installed at a slope of 0.20%, for a minimum dis-tance of 15m (50ft).

(13) Recommended piping and fittings for the vacuum collection system is SDR21 PVC with push-on gasketed joints for piping 10em (4 in.) in diameter andlarger. Recommended piping and fittings for diameters less than 10 em (4in.) is SDR 21 PVC with push-on gasketed joints or schedule 40 PVC withsocket-type DWV fittings. Joints using DWV fittings on piping less than 10em (4 in.) shall be solvent-welded.

(14) Preferred configuration ofbranch connections to the main are made "above thetop" of the pipe using a vertical wye and, where applicable, a 45-degreebend.

Valve Pit Requirements.

(1) A single valve pit should serve a maximum of four EDUs, but no morethan a maximum EDU equivalent to 0.2 L/ s (3 gpm) (see number 3 in thissection). On a system-wide basis, the overall EDU to pit ratio shall notexceed 2.5:1.

(2) Inno case shall a single property or parcel be served by more than one valvepit, unless proper justification is provided to support multivalve pits.

(3) Peak flow to any valve pits is limited to a maximum of 0.2L/ s (3 gpm).(4) Vacuum valve pits installed within a road right-of-way or other area subject

to vehicular traffic shall be designed and installed to withstand appropriatetraffic loads.

(5) The valve pit arrangement shall have a receiving sump, with a minimum of189 L (50 gal) of storage. Arrangements without storage sumps are not rec-ommended.


(6) Vacuum valve pits shall be designed to prevent entrance of water in thesump and for the vacuum valve to remain fully operational if submerged.

(7) Vacuum valve pit locations shall be easily accessible, so that valves may beeasily removed and replaced.

(8) Air-intakes, a minimum of 10 em (4 in.) in diameter, shall be provided foreach individual gravity line and shall extend a minimum of 0.6 m (2 ft)above ground level and be protected against physical damage andflooding. Air-intakes shall be screened to prevent the entry of rodents,insects, and debris.

(9) Valve pits shall include gravity service connection stub-outs piping, towhich the sewer customer will ultimately connect. These are typically 10 em(4-in.) in diameter, schedule 40, PVC, or SDR 21 pipe. Length shall be fromthe pit location to the customer's property line or other lengths, as may berequired by the system owner.

Buffer Tank Requirements.

(1) Buffer tanks should be used instead of single valve pits under the followingconditions:• If there are nomesidential/ commercial or high flow inputs greater than 4

EDUs or 0.2-LI s (3-gpm) peak flow.• If there is no other practical method of serving the property by additional

vacuum mains and valve pits.(2) Buffer tanks may be also used to accept higher wastewater flows from

schools, apartments, nursing homes, or lift stations, or to accept flows froma number of grinder pumps (low-pressure sewer system).

(3) Buffer tanks must have an operating sump of no less than 38 L (10gal) at awastewater depth of 25 to 36 em (10to 14in.).

(4) No more than 25%of the total peak design flow on a system-wide basis mayenter through buffer tanks.

(5) No more than 50% of the total peak design flow may enter a single vacuummain through buffer tanks.

(6) One 7.5-cm (3-in.) vacuum valve shall be used for every 1 Lis (15 gpm) atpeak wastewater flow.For higher flows, the wastewater shall be admitted toa splitter manhole, which will evenly split and divert the flow to multiplevalve buffer tank units.


(7) Dual buffer tanks must be connected to a IS-em (6-in.) or larger vacuummain; where three or more valves are used, an 20-cm (8-in.)vacuum main orlarger is required.

(8) Buffer tanks are to be constructed of minimum 1.2m (4-ft)internal diameterprecast concrete manhole sections. All joints and connections on the buffertank must be water-tight. Aboveground venting of the vacuum valve mustbe installed, to ensure proper venting, in the event that the buffer tankbecomes filled with wastewater.

(9) Provisions must be included with the buffer tank design to allow for separa-tion of the valve access area from the sanitary wastewater storage area. Pro-visions shall also be made for maintenance personnel access.

Vacuum Valve Requirements.

(1) Vacuum valves shall have the ability to pass a 7.5-cm (3-in.) spherical solid,while matching the outside diameter of 7.5-cm(3-in.)SDR21 PVC pipe.

(2) Valves are to be vacuum-operated on opening and spring-assisted onclosing.

(3) Valve configuration shall be arranged so that the collection system vacuumensures positive valve seating. Valve plunger and shaft shall be arranged tobe completely out of the flow path when valve is in the open position.

(4) The valve shall be equipped with a sensor-controller that shall rely onatmospheric air and vacuum pressure from the downstream side of thevalve for its operation, thereby requiring no other power source. Thecontroller shall be capable of maintaining the valve fully open for a fixedperiod of time and shall be field-adjustable over a range of 3 to 10seconds.

(5) With the exception of the gravity lateral line air-intake, there shall be noother external sources of air necessary or permitted as a part of the valveassembly.

(6) An internal sump breather unit arrangement shall connect the valve con-troller to its air source and provide a means of ensuring that no liquid canenter the controller during system shutdowns and restarts. It shall also bearranged to prevent sump pressure from forcing the valve open during low-vacuum conditions and provide positive sump venting, regardless of trapsin the home gravity service line.


Relation to Water Mains.(1) Vacuum sewers shall be laid with at least 0.9 m (3 ft) of horizontal separa-

tion from any existing or proposed water main. This distance shall be mea-sured from the outside of pipe to the outside of pipe. Deviation to thisrequirement may be allowed on a case-by-case basis, if supported by appro-priate data from the design engineer.

(2) Vacuum sewers crossing over or under any existing or proposed water mainshall do so with a minimum of 15 em (6 in.) of separation. This distance shallbe measured from the outside of pipe to the outside of pipe. Deviation tothis requirement may be allowed on a case-by-case basis, if supported byappropriate data from the design engineer.

Vacuum Collection System Leak Testing.(1) At the end of each day's work, the completed vacuum mains and vacuum ser-

vice pit connections shall be tested. The completed portion of the system shallbe plugged and subjected to a vacuum of 74kPa (22in. Hg), allowed to stabi-lize for 15 minutes, and then monitored. A successful test shall record avacuum loss of less than 1%/h during the minimum testing period of 2 hours.

(2) The final completed vacuum mains and vacuum service pit connectionsshall be tested. The complete system shall be plugged and subjected to avacuum of 74 kPa (22 in. Hg), allowed to stabilize for 15 minutes, and thenmonitored. A successful test shall record a vacuum loss of less than 1%/hduring the minimum testing period of 4 hours.

(3) Changes in temperature and barometric pressure may affect the testingresults. As a result, it is recommend to not test the system just before achange in local weather conditions.

(4) Copies of all testing results shall be provided to the regulatory authoritiesupon completion of the vacuum collection system.

DESIGN OF VACUUM STATIONS. Vacuum Station General DesignRequirements.

(1) A minimum peak-flow-to-average-flow ratio of 3.5:1 is recommended forvacuum pump station component sizing.

(2) Standby power shall be capable of handling 110%peak loading. A standbygenerator is recommended for all vacuum stations.

206 Alternative SewerSystems

(3) A minimum of two pumping units shall be provided for both the vacuumpumps and the wastewater pumps, with each being capable of handlingpeak flow conditions with the other out of service.

(4) An alarm system, with the capability to notify staff operators remotely (i.e.,telemetry system), shall be provided. The monitoring system shall be pro-vided with continuously charged batteries for 24-hour standby operation, inthe event of a power outage.

(5) Certification is required from the pump manufacturer stating that the waste-water pumps are suitable for use in a vacuum sewer installation.

(6) Check valves are required on each wastewater pump discharge line.(7) Isolation valves are required between the vacuum collection tank, vacuum

pump(s), influent line, and raw wastewater discharge pipe.(8) Shutoff valves are required on both the wastewater pump suction and dis-

charge piping. Plug valves or resilient coated wedge gate valves shall beused. Butterfly valves are not recommended.

(9) Necessary pipe, fittings, and valves shall be provided, to allow for emer-gency pumping out of the vacuum collection tame

(10) Vacuum station piping and fittings 10 cm (4 in.) and larger shall be 150#ANSI flanged ductile iron. Piping and fittings less than 10 em (4 in.) shallbe schedule 80 PVC with solvent-welded joints.

(11) The ferrous metal components of the vacuum pump station shall be protec-tively coated to prevent corrosion.

(12) The vacuum station equipment shall be skid-mounted, fully assembled, andtested before shipment to the project site. Testing requirements shall be thevacuum system manufacturer's standard.

(13) It is recommended that the complete vacuum pump station equipmentskid(s) be labeled by the UL or other such equivalent agency.

Vacuum Station Component Sizing.

(1) Vacuum station design nomenclature is as shown in Table 3.38.(2) Wastewater discharge pumps shall be sized according to the following

formula:

Qdp = Qmax = Qa X peak factor (3.9)


TABLE 3.38 Vacuum station design nomenclature.

Term* Definition

Station peak flow (gpm)Station average flow (gpm)Station minimum flow (gpm)Discharge pump capacity (gpm)Vacuum pump capacity (cfm)Collection tank operating volume (gal)Collection tank volume (gal)System pump-down time (min)Piping system volume (gal)

Qmax

Qa

Qmin

Qdp

QvpVo

Vett

Vp

*cfm X 0.4719 = L/ s; gal X 3.785 = L; and gpm X 0.063 = L/ s.

(3) The total volume of the vacuum collection tank shall be three times the col-lection tank operating volume, plus 1514 L (400 gal), with a minimum sizeof 3785 L (1000 gal).

"V;,t = 3~ + 400

~ = 15Qmrn (Qdp - Qrnrn) + Qdp

Qrnrn = Q/2

Vacuum Pump Sizing.

(1) The minimum recommended vacuum pump size is 4.3 m3/min (150 cuft/min) (7460 W [10 hpJ).

(2) A two-step process shall be used to size the vacuum pumps. Vacuum pumpsshall first be sized according to peak flow. The adequacy of this initial sizingshall then be checked to see that the system pump-down time (t) is between1 and 3 minutes.• Preliminary sizing (based on peak flow). Vacuum pumps shall be sized to

handle the flow from the vacuum valves adjusted to a 2:1 air-liquid inlettime ratio, by using the following formula:

Q'17 = A X Qma,/7.5 gall ft3* (3.11)

*This equation is presented only in U.S. customary units as submitted.Where A varies empirically with mainline length, as shown in Table 3.8.


• Based on system volume. The adequacy of the selected vacuum pumpsshall be checked to see that system pump-down time (t) is between 1 and3minutes, according to the following formula:

0.045 X [2/3 Vp + (~t - Vo)]

t = Qvp

where Vp = total volume of the piping network in gallons (cu ft convertedto gallons)".

"Units of measure are presented only in U.S. customary units per the orig-inal equation.If t is less than 1 minute, smaller vacuum pumps must be used. If t isgreater than 3 minutes, either larger vacuum pumps or additionalvacuum pumps are required.For Qvp' use the combined rated capacity of all of the selected vacuumpump less 1 pump. For example, if 2 vacuum pumps are used, use therated capacity of 1 pump. If 3 pumps are used, use the combined ratingcapacity of 2 of the pumps; if 4 are used, use 3, and so on.

Instrumentation and Controls.(1) Instrumentation and control systems to provide operational functionality

shall be provided and incorporated to the vacuum station equipmentskid(s). Instrumentation systems shall be the manufacturers standard, as abase, with additional requirements as requested by the project owner. Provi-sions for automatic pump alternation must be included in the instrumenta-tion and control system.

(2) The instrumentation and control system shall bear the UL label, per therequirements of UL 508 and UL 508A.

Safety Ventilation of the Vacuum Pumping Station. Safety ventilation shallmeet the requirements of Chapter 40, Wastewater Pumping Stations, paragraph 42.7,Safety Ventilation, (GLUMRB,1990).

Miscellaneous Requirements.(1) Ingressl egress and general access to the facility and for equipment mainte-

nance shall meet local building code requirements.(2) Electrical equipment and installation shall meet the requirements of the

National Electrical Code and local building code.


(3) Adequate surge protection shall be provided.(4) Emergency power shall be provided at each vacuum station, either in the

form of a portable emergency generator or site-dedicated standby powergenerator system at the vacuum station.

(5) Adequate temperature control shall be provided for the main electricalequipment and primary power distribution.

(6) Potable water, power, and telephone service shall be provided to thevacuum pump station.

(7) Adequate security shall be provided to prevent unauthorized access.(8) Lightning protection, per local building code requirements, shall be pro-

vided.(9) Outdoor lighting for security shall be provided.

(10) Building shall meet local ordinances and building code, with respect tonoise abatement.

(11) Overhead crane shall be provided, for removal and replacement of equip-ment and components.

(12) General structural design shall meet local building code requirements.(13) In areas where high groundwater conditions can be expected, buoyancy of

the vacuum pump station shall be considered, and adequate protectionsshall be incorporated to the structural design.

(14) Exterior architectural design and site plan considerations shall meet thelocal planning and zoning requirements.

DESIGN OF WASTEWATER TRANSMISSION(FORCE MAIN) SYSTEM.

(1) Force main systems shall be sized to meet average day and peak flow condi-tions, as outlined in the engineering report.

(2) General force main system design requirements shall be in accordance withChapter 40, Wastewater Pumping Stations, Paragraph 48 (GLUMRB,1990),or other local regulations and codes.

CUSTOMER CONNECTIONS.

(1) Customers shall be connected to the vacuum system via gravity flow to thevacuum pit location. This shall be the vacuum-gravity interface point.


(2) The gravity lateral shall be in accordance with the following:• Lateral piping 15 em (6 in.) in diameter and smaller shall be installed

meeting the requirements of the plumbing sections of the local buildingcode.

• Lateral piping 20 em (8 in.) in diameter and larger shall be designed andinstalled per the requirements of Chapter 30, Design of Sewers (GLUMRB;1990),or other local regulations and codes governing conventional sanitarysewer system design and construction.

• Lateral piping shall be schedule 40 PVC or pressure-rated PVC (SDR 21or SDR26).Non-pressure-rated PVC is not acceptable.

(3) Individual gravity laterals shall be provided from the vacuum pit to eachcustomer.

(4) An intake shall be provided on each gravity lateral. The air-intake shall ter-minate above grade, with a gooseneck and a stainless-steel screen. Air-intake piping and fittings shall be the same diameter as the gravity lateral.Air-intake shall be located in a location to prevent damage to the piping.

(5) The customer connection lateral must be inspected and approved by thevacuum system owner or operating entity before final connection. Onceinspection of the customer's lateral connection has been completed and theconnections meets system requirements, the vacuum valve should beinstalled. Failure to confirm correct installation of the customer's gravityconnection could cause severe damage to the vacuum collection system.

REFERENCESAIRVAC(2005a)Design Manual; AIRVAC:Rochester, Indiana.

AIRVAC (200Sb) Operation, Installation and Maintenance Manual; AIRVAC:Rochester, Indiana,

Burns, B. C. et al. (1973) Method and Apparatus for Conveying Sewage. us.Patent 3,730,884,May 1.

Great Lakes-Upper Mississippi River Board of State Public Health and Environ-mental Managers (1990) Recommended Standards for Wastewater Facilities;Health Education Service: Albany, New York.


International Association of Plumbing and Mechanical Officials (2006) UniformPlumbing Code; International Association of Plumbing and Mechanical Offi-cials: Ontario, California.

U.S. Environmental Protection Agency (1977) Alternatives for Small WastewaterTreatment Systems, EPA-625/4-77-011, NTIS No. PB-299608; U.S. Environ-mental Protection Agency: Cincinnati, Ohio.

U.S. Environmental Protection Agency (1991a) Alternative Wastewater CollectionSystems, EPA-625/1-91-024; U.S. Environmental Protection Agency, Office ofResearch and Development, Office of Water: Washington, D.C.

U'.S.Environmental Protection Agency (1991b) Odor and Corrosion in SanitarySewer Systems; U.S. Environmental Protection Agency, Office of Water:Cincinnati, Ohio.

Water Environment Federation (1995) Engineered Equipment Procurement Optionsto Ensure Project Quality; Water Environment Federation: Alexandria, Virginia.

Index

301

A

Advantages, sewer alternatives, 105, 216Aeration, 257Air-intake, 138Air-liquid ratio, vacuum system, 118Air-release valves, 38, 229, 254AIRVAC, 15, 106Alignment, effluent sewers, 248Alternative collection technology,

advantages/ disadvantages, 5,7,9,11,27

Alternatives, comparison, 30Appurtenances, 40, 55, 60, 139, 143As-built drawings, 274Assessments, effluent sewers, 282Average daily flows, 45

B

Backwater valves, 137Bilfinger-Berger,106Breathers, 141Bronze swing-check valves, 58Buffer tanks, 132, 203Building sewers, 60, 137,225,243

cCarbon filters, 257Case studies, effluent sewers, 283Centralized wastewater system, 2Centrifugal pumps, 67Check valves, 60, 229Chemical addition, 257

Index 303

Cleanouts,building sewers, 60effluent sewers, 25, 228, 253pressure sewers, 37vacuum sewer, 144

Clerical costs, 192Collection system piping, emergency

maintenance, 170, 172Collection system startup, 163Collection tanks, 147Collector mains, 25, 228, 248, 276Commercial flows, 235Construction considerations, 80Construction costs

effluent sewers, 269,279pressure sewers, 90vacuum sewer system, 184

Constructiondocuments, 261effluent sewers, 261inspection, 165vacuum sewer, 156vacuum station, 163

Control panels, 151Controllers, 140Controls, 74,208Conventional sewers, planning, 27, 217Cooling, vacuum stations, 156Corrosion

effluent sewers, 255, 278materials of construction, 44pressure sewers, 80vacuum sewer, 120

Costs, effluent sewers, 269,279,281Costs, vacuum sewers, 184, 187


Customer connections, 193,209Cycle counter, vacuum valve, 143

D

Decentralized wastewater technologies,2

Depth of burial, effluent sewers, 250Design considerations, 45, 117, 125,200Design equations, effluent sewers, 238Design example, 286Design flows

effluent sewers, 232vacuum sewers, 117pressure sewers, 47

Designseptic tank effluent gravity system,

291septic tank effluent pumping

system, 295small-diameter gravity sewer, 291,

294Dexter, Oregon, STEG system, 283Diaphragm-type vacuum valve, 139Directional drilling, vacuum sewer, 158Discharge pumps, vacuum station, 144Distributed wastewater management, 2Division valves, vacuum sewer, 144Drawings, effluent sewers, 166, 256, 274

E

E~emem~corntructio~2MEducation, vacuum sewer management,

198

Effluent drain systems, 21Effluent sewer discharge, 29Effluent sewers, 21, 217, 219, 221, 223,Electric motors, 74Electrical controls, 151Electronic air admission control, 143Emergency generators, 150Emergency maintenance, 172, 177Emergency, pressure sewers, 89Engineer responsibilities, 198Equipment

costs, 192ownership, 195requirements, effluent sewers, 272substitutions, pressure sewers, 83spare parts inventory, 175

EVAC,108Excavation, effluent sewers, 222, 268External breathers, 141

F

Fiberglass settings, 132Flotation problems, 259Flow meters, 41, 56Flow, effluent sewer, 232Flowrates, vacuum sewer, 127Force main, design requirements, 209Freezing, effluent sewers, 258Friction losses, 121

G

Geometry, vacuum sewer, 124Glide, Oregon, STEP system, 284

Grade control, 158Grade, effluent sewers, 248Grinder-pump systems, 11, 42Growth inducements, 27

H

Hazen-Williams equation, 50Heating, 156Homeowner responsibilities, 94, 195Horizontal alignment, 248Horizontal directional drilling, 158House connections, 24House-to-pit sharing ratio, 132Hydraulic design, 230,238, 250Hydraulic grade line, 240Hydraulics, 45, 117Hydrogen sulfide control, 259

I

Industrial flows, 235Infiltration and inflow, 222, 235, 278Inspections, 165, 166Instrumentation, 208In-sump breathers, 142Integral valve pits, 131Interceptor tanks, 24, 225, 274Iseki,107Isolation valves, 37, 229

L

Labor, 179, 189Lapine, Oregon, STEG system, 285

Index 305

Leak testing, 205Level controls, 42, 153Lift stations, 25, 229Line changes, 157Lot facility plans, 262

M

Mainsconstruction costs, 185effluent sewers, 228, 248, 276pressure sewers, 51vacuum sewer, 123, 156

Maintenancecosts, 192effluent sewers, 274pressure sewers, 86vacuum sewer system, 169

Management considerations, 193,282Manholes, 25, 228Manning equation, 50Manufacturers, 106Material costs, 222Materials of construction

collection tanks, 147discharge pumps, 144interceptor tanks, 246pipes, 252pressure sewers, 44vacuum pumps, 149

Mean time between service calls, 182Minimum flow velocities, 49Monitoring systems, 153Motor control centers, 151


N

Noise, 156Normal maintenance, 169, 177

oObstructions, sewer maintenance, 276Odors

air-release valves, 38, 56effluent sewers, 255, 277pressure sewers, 80vacuum sewers, 120vacuum station, 155

Operation and maintenanceeffluent sewers, 270, 271, 281pressure sewers, 83, 93vacuum sewers, 103, 167, 175, 179,

187,193Operator training

effluent sewers, 273pressure sewers, 85vacuum sewers, 168

O-ring gasket pipe, 129

p

Peak flows, 46, 49Pipes

effluent sewer diameter, 250fittings, 39materials, 54, 129,252sizing, 51zone backfill, 54

Pipeline geometry, 51Pipelines, 51

Pipingemergency maintenance, 172normal maintenance, 170vacuum station, 151

Piston-type vacuum valve, 139Pits, vacuum sewer, 160Planning, system alternatives, 27Population density, 27Power costs, 191Power outages, 258Power, sewer operation, 181Pressure monitoring stations, 42Pressure sewers, 11Preventative maintenance, 88, 169, 177Probability method, 48Procurement options, 167Profiles, sewer, 28, 110, 257Progressing cavity pumps, 70Property plans, 262Public financing, 260Pumps

curves, 60discharge piping, 64disconnects, 42, 65electrical service, 67pressure sewer, 42solids handling, 42vaults, 42, 44, 61

Pumping rates, 46Pumping stations, ventilation, 208PVC swing-check valves, 58

R

Rational method, 48

Recordkeepingeffluent sewers, 277pressure sewers, 89vacuum sewers, 176

Rediweld Group, 106Regulations, 199Regulatory acceptance, 260Regulatory agency responsibilities, 198Rock excavation, 28Roediger, 107Roevac,107Routing, 51, 128

sSawtooth profile design, 118SCheduling/warranties, 83Self-cleaning velocity, 49Sensors, 140Septic tank effluent gravity (STEG)

sewers, 21, 215Septic tank effluent pumping (STEP)

sewers, 21, 215Septic tanks, 24, 247Service laterals, 25,129,227,246Service lines, 56, 57, 59,123,269Sewer authority responsibilities, 95, 193Sewer use ordinance, 194Site restoration, 264Situations to avoid, vacuum sewer, 119,

123,125,135,184Sizing

collection tank, 148discharge pumps, 146vacuum pumps, 150,207vacuum sewer, 124

Index 307

vacuum station components, 206Slopes, ground, 28Small-bore sewers, 21Small-diameter gravity (SDG) sewers,

21,215Soil filters, 257Spare parts, 86, 173, 273Specialty tools, 175Staffing requirements, 84, 167, 272Standards, sewer system, 199Standby generators, 150Startup, vacuum sewer system, 163Static losses, 121, 126Stub-outs, 135Subsurface problems, 28Sump tightness test, 163Sump valves, 155Supply costs, 192System approach, sewer operation, 168System components, 110,224,243System costs, 279System plan, 110Systems management, 94

T

Technology improvements, 131,129Transportation costs, 192Treatment requirements, 222Trenching, 54, 265Troubleshooting, 89,277

uUser charges, 93, 282User responsibilities, 282


Utility coordination, 256Utility costs, 191Utility responsibilities, 282

vVAC-Q-Tec, 108Vacuum sewers, 167, 200Vacuum gauges, 155Vacuum mains, 110, 156, 165, 185Vacuum pumps, 149,207Vacuum pumping station ventilation,

208Vacuum recorders, 155Vacuum stations

design, 144, 205components, 114,206construction, 163, 166, 186maintenance, 169, 172spare parts inventory, 174startup, 163

Vacuum systems, 15Vacuum testing, 159, 163

Vacuum transport, 118Vacuum valves, 139, 171, 173, 204Valve boxes, 40Valve pits

construction costs, 186design requirements, 202installation, 160, 166spare parts inventory, 173vacuum sewer, 110, 130, 132

Valves, 56, 57, 64, 173Vault covers, 64Vaults, 40Velocity requirements, 117,221, 238Ventilation, pumping station, 156, 208Vents, 25, 229Vertical alignment, 249

wWater conservation, 123Water lines, 256Water main design, 205

Excerpt from Water Environment Federation MOP FD-12 Scheduled for publication Spring 2008

3.10 Sample Regulations for Vacuum Systems

In some parts of the world, there are established standards for vacuum sewers. For

example, Australia has a 2004 standard called the Vacuum Sewerage Code of Australia

(WSA 06-2004) and Europe has the 1997 European Norm Vacuum Sewerage Systems

Outside Buildings (EN-1091).

There is no national vacuum sewer system standard in the U.S. Even though there are

operating systems in 29 states, very few states have specific regulations regarding

vacuum sewers. Due to the perceived difficulty of gaining regulatory approval when no

formal regulations exist, some engineers have even hesitated proposing vacuum,

despite the fact that the project may be an ideal candidate for the technology.

To create new regulations requires much technical writing combined with a series of

public comment and review phases. Because the process can take much time and

effort, many states have chosen not to write new regulations for vacuum. This is

especially true of a state where vacuum is being proposed for the first time, where it is

hard to justify the time and effort necessary to create regulations for a technology that

may only be used once.

Rather than creating new regulations, some states instead choose to refer to other

technical publications, such as the 1991 EPA Manual, as the basis for design review

and approval. This is a similar method used by some for gravity sewer designs, which

refer to the Ten State Standards (GLUMRB, 1990).

The sample vacuum regulations that follow have been created for this purpose. These

sample regulations incorporate the content of this manual, while using the Ten States

Standards as a model for both format and content.


SAMPLE REGULATIONS FOR VACUUM SYSTEMS 1.0 DESIGN OF VACUUM COLLECTION SYSTEMS 2.0 DESIGN OF VACUUM STATIONS 3.0 DESIGN OF SEWAGE TRANSMISSION (FORCEMAIN) SYSTEM 4.0 CUSTOMER CONNECTIONS 1.0 REQUIREMENTS FOR DESIGN OF VACUUM COLLECTION SYSTEM

1.1 General Requirements.

1.1.1 The entire vacuum sewer system, including the individual valve pits, shall

be owned, operated and maintained by a single operating entity.

1.1.2 The vacuum piping network shall be designed with the intent to keep the bore of the entire pipeline open. Designs where sections of the pipeline are purposely sealed are not allowed.

1.1.3 The vacuum sewer system must be designed to remain operational during loss of vacuum.

1.1.4 For routine and emergency operation and maintenance of a vacuum sewer

system, the public entity responsible for the system shall have the right of access, to an adequate supply of spare valves, pumps, parts and service.

1.1.5 Vacuum sewer system air to liquid ratio shall be a minimum of two (2)

parts air to one (1) part liquid.

1.1.6 The maximum static loss in the vacuum sewer system shall be thirteen (13) feet. These are the losses calculated from the furthest extremities of the vacuum sewer system to the vacuum pump station.

1.1.6.1 Vacuum loss due to each lift is calculated by subtracting the pipe

I.D. from the lift height (e.g., for a 1 foot lift in a 4 inch pipe, vacuum loss = 1.0’ – 0.33’ = 0.67’).

1.1.6.2 When lifts are required in specific valve service lines or when

concrete buffer tank valve pits have suction lifts in excess of 5.5 feet, the static losses shall be added to the losses for that main and shall not exceed 13 feet. In no case shall suction lifts from the bottom of the holding sump to the valve centerline exceed 8 feet.


1.1.7 The maximum friction loss in the vacuum sewer system shall be five (5)

feet.

1.1.7.1 Friction losses for vacuum sewers installed at slopes between 0.20% and 2% are cumulative for each “flow path” from the last valve on a line to the vacuum station. Friction losses for sewers installed in excess of 2% may be ignored. Friction losses are to be calculated using the following formula:

F = 2.75 X 0.2083 X (100/C)1.85 X (Q1.85/d4.8655), where

f = friction losses in feet per 100 feet of pipe C = pipe roughness coefficient (150 for PVC pipe) Q = flow in GPM* D = pipe I.D. in inches

1.1.7.2 Flow for a given section equals the mean flow for the section of

pipe plus the incoming flow. The mean flow for a section of pipe is the total flow directly connected to the section of pipe divided by two. Peak flows must be used when calculating friction losses.

1.2 Design requirements for the vacuum collection system.

1.2.1 The general design configuration shall be based on the sawtooth pipeline

profile. Other vertical pipeline design profiles may be considered if justified by appropriate engineering data as part of the preliminary evaluation and engineering report.

1.2.2 A minimum pipe diameter of four (4) inches is required for vacuum sewer

mains and gravity service laterals.

1.2.3 Vacuum sewer lines must have a minimum slope of 0.20%. For profile changes less than 125 feet apart, the minimum fall between profile changes is 0.25 feet.

1.2.4 The maximum design flows (i.e., peak flows) for vacuum pipe sizing are

as follows:

Four (4) inch pipe shall be 38 gpm Six (6) inch pipe shall be 105 gpm Eight (8) inch pipe shall be 210 gpm Ten (10) inch pipe shall be 375 gpm

1.2.5 The maximum length of four (4) inch diameter lines, for any one run, is two-thousand (2,000) feet.


1.2.6 Lift heights should be made according to the following guidelines. In no

case should a single lift exceed 3’ in height. Pipe Diameter Lift Heights

Four (4) inch pipe 1.0’ or 1.5’ Six (6) inch pipe 1.0’ or 1.5’ Eight (8) inch pipe 1.5’ or 2.0’ Ten (10) inch pipe 1.5’ or 2.0’

1.2.7 A series of lifts should be made with consistent lift heights.

1.2.8 For changes in horizontal alignment, two (2) forty-five (45) degree bends

connected by a short section of piping are required, rather than a ninety (90) degree bend.

1.2.9 Isolation valves are required at every branch connection and at intervals

no greater than one-thousand five-hundred (1,500) feet on main lines. Resilient coated wedge gate valves shall be used. The valves shall be installed with a valve box or other approved apparatus to facilitate proper use of the valve.

1.2.10 When vacuum mains or branches must ascend a hill, multiple lifts are

placed at a minimum distance of 20 feet apart. Between each lift, vacuum lines are installed with a uniform slope so that minimum fall is achieved between these lifts (minimum fall is 0.25 feet).

1.2.11 Where a lift or profile change is required in a branch sewer prior to

entering the main, it should be made 20 or more feet from the main. 1.2.12 For a series of lifts following a downward slope in excess of 0.20%, the

vacuum sewer should be installed at a slope of 0.20% for a minimum distance of 50 feet.

1.2.13 Recommended piping and fittings for the vacuum collection system is

SDR 21 PVC with push on gasketed joints for piping 4 inches in diameter and larger. Recommended piping and fittings for diameters less than 4 inches is SDR 21 PVC with push on gasketed joints or schedule 40 PVC with socket type DWV fittings. Joints utilizing DWV fittings on piping less than 4 inches shall be solvent welded.

1.2.14 Preferred configuration of branch connections to the main are made

“above the top” of the pipe using a vertical wye and where applicable a 45 degree bend.


1.3 Valve Pit requirements.

1.3.1 A single valve pit should serve a maximum of four (4) equivalent dwelling units (EDU’s) but no more than a maximum EDU equivalent to 3 gpm (see 1.3.3). On a system-wide basis, the overall EDU to pit ratio shall not exceed 2 ½:1.

1.3.2 In no case shall a single property or parcel be served by more than 1 valve

pit, unless proper justification is provided to support multi-valve pits. 1.3.3 Peak flow to any valve pits is limited to a maximum of 3 gpm.

1.3.4 Vacuum valve pits installed within road rights of way or other area subject

to vehicular traffic shall be designed and installed to withstand H20 traffic loads.

1.3.5 The valve pit arrangement shall have a receiving sump with a minimum of

50 gallons of storage. Arrangements without storage sumps are not recommended.

1.3.6 Vacuum valve pits shall be designed to prevent entrance of water in the

sump and for the vacuum valve to remain fully operational if submerged.

1.3.7 Vacuum valve pit locations shall be easily accessible so that valves may be easily removed and replaced.

1.3.8 Air intakes, a minimum of four (4) inches in diameter, shall be provided

for each individual gravity line and shall extend a minimum of two (2) feet above ground level and be protected against physical damage and flooding. Air intakes shall be screened to prevent the entry of rodents, insects and debris.

1.3.9 Valve pits shall include gravity service connection stub-outs piping to

which the sewer customer will ultimately connect. These are typically 4” diameter, Sch 40, PVC or SDR 21 pipe. Length shall be from the pit location to the customer’s property line or other lengths as may be required by the system owner.

1.4 Buffer Tank Requirements.

1.4.1 Buffer tanks should be utilized instead of single valve pits under the following conditions:


1.4.1.1 If there are non-residential/commercial or high flow inputs greater

than 4 equivalent dwelling units (EDU) or 3 gpm peak flow. 1.4.1.2 If there is no other practical method of serving the property by

additional vacuum mains and valve pits.

1.4.1 Buffer tanks may be also used to accept higher sewage flows from schools, apartments, nursing homes, lift stations or to accept flows from a number of grinder pumps (low pressure sewer system).

1.4.2 Buffer tanks must have an operating sump of no less than 10 gallons at a

sewage depth of 10” to 14”.

1.4.3 No more than 25% of the total peak design flow on a system-wide basis may enter through buffer tanks.

1.4.4 No more than 50% of the total peak design flow may enter a single

vacuum main through buffer tanks.

1.4.5 One 3” vacuum valve shall be used for every 15 gpm at peak sewage flow (see table below). For higher flows, the sewage shall be admitted to a splitter manhole, which will evenly split and divert the flow to multiple valve buffer tank units.

Recommended Design Flow Rates

For Valve Pits

Pit Type

Recommended Peak Flow (gpm)

Maximum * Peak Flow

(gpm)

Single Buffer Tank

3.1 - 15.0 gpm

30 gpm

Dual Buffer Tank

15.1 - 30.0 gpm

60 gpm

* Depending on static and friction loss, the overall

amount of peak flow entering the system through buffer tanks and

the exact location of the buffer tank, it may be possible to size a

particular buffer tank with the upper limits shown in this column.

Consult Design Engineer or Manufacturer for guidance.

1.4.6 Dual buffer tanks must be connected to a 6” or larger vacuum main; where

three (3) or more valves are used, an 8” vacuum main or larger is required.


1.4.7 Buffer tanks are to be constructed of minimum four (4) foot I.D. precast concrete manhole sections. All joints and connections on the buffer tank must be watertight. Above ground venting of the vacuum valve must be installed to insure proper venting in the event that the buffer tank becomes filled with sewage.

1.4.8 Provisions must be included with the buffer tank design to allow for

separation of the valve access area from the sanitary sewage storage area. Provisions shall also be made for maintenance personnel access.

1.5 Vacuum Valve Requirements.

1.5.1 Vacuum valves shall have the ability to pass a 3” spherical solid while matching the outside diameter of 3” SDR 21 PVC pipe.

1.5.2 Valves to be vacuum operated on opening and spring assisted on closing.

1.5.3 Valve configuration arranged so that the collection system vacuum ensures positive valve seating. Valve plunger and shaft arranged to be completely out of the flow path when valve is in open position.

1.5.4 The valve shall be equipped with a sensor-controller that shall rely on

atmospheric air and vacuum pressure from the downstream side of the valve for its operation, thereby requiring no other power source. The controller shall be capable of maintaining the valve fully open for a fixed period of time and shall be field adjustable over a range of 3 to 10 seconds.

1.5.5 With the exception of the gravity lateral line air intake , there shall be no

other external sources of air necessary or permitted as a part of the valve assembly.

1.5.6 An internal sump breather unit arrangement shall connect the valve

controller to its air source and provide a means of assuring no liquid can enter the controller during system shut downs and re-starts. It shall also be so arranged to prevent sump pressure from forcing the valve to open during low vacuum conditions and provide positive sump venting regardless of traps in the home gravity service line.


1.6 Relation to Water Mains.

1.6.1 Vacuum sewers shall be laid with at least three (3) feet of horizontal

separation from any existing or proposed water main. This distance shall be measured from outside of pipe to outside of pipe. Deviation to this requirement may be allowed on a case-by-case basis, if supported by appropriate data from the design engineer.

1.6.2 Vacuum sewers crossing over or under any existing or proposed water

main shall do so with a minimum of 6 inches of separation. This distance shall be measured from outside of pipe to outside of pipe. Deviation to this requirement may be allowed on a case-by-case basis, if supported by appropriate data from the design engineer.

1.7 Vacuum collection system leak testing.

1.7.1 At the end of each days work the completed vacuum mains and vacuum

service pit connections shall be tested. The completed portion of the system shall be plugged and subjected to a vacuum of 22 inches Hg, allowed to stabilize for 15 minutes then monitored. A successful test shall record a vacuum loss of less than 1% per hour during the minimum testing period of 2 hours.

1.7.2 The final completed vacuum mains and vacuum service pit connections

shall be tested. The complete system shall be plugged and subjected to a vacuum of 22 inches Hg, allowed to stabilize for 15 minutes then monitored. A successful test shall record a vacuum loss of less than 1% per hour during the minimum testing period of 4 hours.

1.7.3 Changes in temperature and barometric pressure may impact the testing

results. As a result it is recommend to not test the system just prior to a change in local weather conditions.

1.7.4 Copies of all testing results shall be provided to the regulatory authorities

upon completion of the vacuum collection system.


2.0 DESIGN OF VACUUM STATIONS

2.1 Vacuum Station General Design Requirements.

2.1.1 A minimum peak flow to average flow ratio of 3.5:1 is recommended for vacuum pump station component sizing.

2.1.2 Standby power shall be required capable of handling 110% peak loading.

A standby generator is recommended for all vacuum stations.

2.1.3 A minimum of two (2) pumping units shall be provided for both the vacuum pumps and the wastewater pumps, with each being capable of handling peak flow conditions with the other out of service.

2.1.4 An alarm system with capability to notify staff operators remotely (e.g.

telemetry system) shall be provided. The monitoring system shall be provided with continuously charged batteries for twenty-four (24) hour standby operation, in the event of a power outage.

2.1.5 Certification is required from the pump manufacturer stating that the

wastewater pumps are suitable for use in a vacuum sewer installation.

2.1.6 Check valves are required on each sewage pump discharge line.

2.1.7 Isolation valves are required between the vacuum collection tank, vacuum pump(s), influent line and raw sewage discharge pipe.

2.1.8 Shut-off valves are required on both the sewage pump suction and

discharge piping. Plug valves or resilient coated wedge gate valves shall be used. Butterfly valves are not recommended.

2.1.9 Necessary pipe, fittings and valves shall be provided to allow for

emergency pump out of the vacuum collection tank.

2.1.10 Vacuum station piping and fittings 4 inches and larger shall be 150 # ANSI flanged ductile iron. Piping and fittings less than 4 inches shall be Schedule 80 PVC with solvent welded joints.

2.1.11 The ferrous metal components of the vacuum pump station shall be

protectively coated to prevent corrosion.


2.1.12 The vacuum station equipment shall be skid mounted, fully assembled and

tested prior to shipment to the project site. Testing requirements shall be the vacuum system manufacturers standard.

2.1.13 It is recommended that the complete vacuum pump station equipment

skid(s) be labeled by Underwriters Laboratory (UL) or other such equivalent agency.

2.2 Vacuum station component sizing.

2.2.1 Vacuum Station Design nomenclature:

Term Definition Qmax Station peak flow (gpm) Qa Station average flow (gpm) Qmin Station minimum flow (gpm) Qdp Discharge pump capacity (gpm) Qvp Vacuum pump capacity (cfm) Vo Collection tank operating volume (gal) Vct Collection tank volume (gal) t System pump-down time (min) Vp Piping system volume (gal)

2.2.2 Sewage discharge pumps shall be sized according to the following

formula:

Qdp = Qmax = Qa x Peak Factor

2.2.3 Sizing of Vacuum Collection Tank

2.2.3.1 The total volume of the vacuum collection tank shall be three (3) times the collection tank operating volume plus four-hundred (400) gallons, with a minimum size of one-thousand (1,000) gallons.

Vct = 3Vo + 400

Vo = 15Qmin (Qdp – Qmin) + Qdp Qmin = Qa/2

2.2.4 Vacuum Pump Sizing.

2.2.4.1 The minimum recommended vacuum pump size is 150 cfm (10

hp).


2.2.4.2 A 2-step process shall be used to size the vacuum pumps.

Vacuum pumps shall first be sized according to peak flow. The adequacy of this initial sizing shall then be checked to see that the “system pump-down time (‘t’) is between 1 and 3 minutes.

2.2.4.2.1 Preliminary sizing (based on peak flow): Vacuum pumps

shall be sized to handle the flow from the vacuum valves adjusted to a 2:1 air/liquid inlet time ratio by using the following formula:

Q vp = A x Qmax/7.5 gal/ft3

Where “A” varies empirically with mainline length as shown below:

Longest line length (feet)

“A”

0 - 5,000 6 5,001 - 7,000 7 7,001 - 10,000 8 10,000 - 12,000 9

>12,000 11

2.2.4.2.2 Based on system volume: The adequacy of the selected vacuum pumps shall be checked to see that system pump-down time ‘t’ is between 1 and 3 minutes according to the following formula:

0.045 x [2/3 Vp + (Vct-Vo)]

t = Qvp

If ‘t’ is less than 1 minute, smaller vacuum pumps must be used. If ‘t’ is greater than 3 minutes, either larger vacuum pumps or additional vacuum pumps are required. NOTES Vp is the total volume of the piping network in gallons (ft3 converted to gallons).

For Qvp, use the combined rated capacity of all of the selected vacuum pump less 1 pump. For example, if 2 vacuum pumps are used, use the rated capacity of 1 pump. If 3 are used, use the combined rating capacity of 2 of the pumps; if 4 are used, use 3 and so on.


2.1.4 Instrumentation and Controls.

2.1.4.1 Instrumentation and control systems to provide operational

functionality shall be provided and incorporated into the vacuum station equipment skid(s). Instrumentation systems shall be the manufacturers standard as a base with additional requirements as requested by the project owner. Provisions for automatic pump alternation must be included in the instrumentation and control system.

2.1.4.2 The instrumentation and control system shall bear the UL label per

the requirements of UL 508 and UL 508A.

2.1.5 Safety ventilation of the vacuum pumping station.

2.1.5.1 Safety ventilation shall meet the requirements of Chapter 40,

Wastewater Pumping Stations, paragraph 42.7, Safety Ventilation, (GLUMRB).

2.1.6 Miscellaneous requirements.

2.1.6.1 Ingress/Egress and general access to the facility and for equipment

maintenance shall meet local building code requirements.

2.1.6.2 Electrical equipment and installation shall meet the requirements of the NEC and local building code.

2.1.6.3 Adequate surge protection shall be provided.

2.1.6.4 Emergency Power shall be provided at each vacuum station either in the form of a portable emergency generator or site dedicated standby power generator system at the vacuum station.

2.1.6.5 Adequate temperature control shall be provided for the main electrical equipment and primary power distribution.

2.1.6.6 Potable water, power and telephone service shall be provided to the vacuum pump station.

2.1.6.7 Adequate security shall be provided to prevent unauthorized access.


2.1.6.8 Lightning protection per local building code requirements shall be

provided.

2.1.6.9 Outdoor lighting for security shall be provided.

2.1.6.10 Building shall meet local ordinances and building code with respect to noise abatement.

2.1.6.11 Overhead crane shall be provided for removal and replacement of equipment and components.

2.1.6.12 General structural design shall meet local building code requirements.

2.1.6.13 In areas where high groundwater conditions can be expected, buoyancy of the vacuum pump station shall be considered, and adequate protections shall be incorporated into the structural design.

2.1.6.14 Exterior architectural design and site plan considerations shall meet the local planning and zoning requirements.

3.0 DESIGN OF SEWAGE TRANSMISSION (FORCE MAIN) SYSTEM

3.1 Force main systems shall be sized to meet average day and peak flow conditions as outline in the engineering report.

3.2 General force main system design requirements shall be in accordance with Chapter

40, Wastewater Pumping Stations, Paragraph 48 (GLUMRB), or other local regulations and codes.

4.0- CUSTOMER CONNECTIONS

4.1 Customers shall be connected to the vacuum system via gravity flow to the vacuum pit location. This shall be the vacuum/gravity interface point.

4.2 The gravity lateral shall be in accordance with the following:

4.2.1 Lateral piping 6 inches in diameter and smaller shall be installed meeting the

requirements of the plumbing sections of the local building code.

4.2.2 Lateral piping 8 inches in diameter and larger shall be designed and installed per the requirements of Chapter 30, Design of Sewers, (GLUMRB), or other local regulations and codes governing conventional sanitary sewer system design and construction.


4.2.3 Lateral piping shall be Sch 40 PVC or pressure rated PVC (SDR 21 or SDR

26). Non-pressure rated PVC is not acceptable.

4.3 Individual gravity laterals shall be provided from the vacuum pit to each customer.

4.4 An air intake shall be provided on each gravity lateral. The air intake shall terminate above grade with a gooseneck and a stainless steel screen. Air intake t piping and fittings shall be same diameter as gravity lateral. Air intake shall be located in a location to prevent damage to the piping.

4.5 The customer connection lateral must be inspected and approved by the vacuum

system owner or operating entity prior to final connection. Once inspection of the customer’s lateral connection has been completed and the connections meets system requirements the vacuum valve should be installed. Failure to confirm correct installation of the customer’s gravity connection could cause severe damage to the vacuum collection system.