USEPA Onsite Wastewater Treatment Systems Manual i

EPA/625/R-00/008

February 2002

Onsite Wastewater Treatment reatment reatmentreatment

Systems Manual Systems Manual Systems Manual

Wastewater Treatment

Systems Manual

Office of Water

Office of Research and Development

U.S. Environmental Protection Agency
Errata Sheet

Onsite Wastewater Treatment 

Systems Manual

EPA/625/R-00/008

June 2003

Page

Number

Errata

xi The following names were omitted from the list of contributors:

William C. Boyle, Ph.D.,PE, and Damann L. Anderson, Ayres

Associates.

3-29 In Table 3-19, in the row entitled “Phosphorus,” and the columns

entitled “Sand Filter Effluent” and “Foam or textile filter effluent” both

superscripts “4" should be “3" to correspond with footnote #3 below

the table. These numbers are not exponents.

4-33 The last paragraph on this page should be removed from the box and

moved to page 4-32, as the last paragraph of section 4.4.7. The

following should be added after the first sentence of that paragraph:

“However, siphons distribute wastewater to treatment media on

demand rather than via timed dosing approach, resulting in more

frequent dosing cycles during heavy use periods and fewer cycles

during off-peak times.”

TFS-8 Figure 2 should be disregarded. Peat is more generally used as 

media in a filter and is discussed in Section 4.7. 

TFS-23 Arrows above and below Figure 1 should be disregarded.

TFS-57 The headings for Table 2 should be: 

BOD (mg/L) TSS (mg/L) TKN (mg/L) TN (mg/L) Fecal Coliform

(CFU/100ml)

TFS-65 The second formula under Step 6 of Recirculating tank sizing should

be: Freeboard volume = (Qinf. + Qdose - Qeff.) x T 

Under conditions of peak flows (Qinf.>Qdose) there is no recycle flow so

Qeff=Qinf. Therefore the freeboard volume necessary is (Qinf.-Qdose)x 
USEPA Onsite Wastewater Treatment Systems Manual ii

Notice

This document has been reviewed in accordance with U.S. Environmental Protection Agency

policy and approved for publication. Mention of trade names or commercial products does not

constitute endorsement or recommendation for use.
USEPA Onsite Wastewater Treatment Systems Manual iii

Foreword

The U.S. Environmental Protection Agency is pleased to publish the “Onsite Wastewater Treatment

Systems Manual”. This manual provides up-to-date information on onsite wastewater treatment

system (OWTS) siting, design, installation, maintenance, and replacement. It reflects significant

advances that the expert community has identified to help OWTSs become more cost-effective and

environmentally protective, particularly in small suburban and rural areas.

In addition to providing a wealth of technical information on a variety of traditional and new

system designs, the manual promotes a performance-based approach to selecting and designing

OWTSs. This approach will enable States and local communities to design onsite wastewater

programs that fit local environmental conditions and communities’ capabilities. Further details on

the proper management of OWTSs to prevent system failures that could threaten ground and surface

water quality will be provided in EPA’s forthcoming “Guidelines for Management of Onsite/

Decentralized Wastewater Systems”. EPA anticipates that the performance-based approach to

selecting and managing appropriate OWTSs at both the watershed and site levels will evolve as

States and communities develop programs based on resources that need protection and

improvement.

Robert H. Wayland III, Director

Office of Wetlands, Oceans and Watersheds

E. Timothy Oppelt, Director

National Risk Management Research Laboratory
USEPA Onsite Wastewater Treatment Systems Manual iv
USEPA Onsite Wastewater Treatment Systems Manual v

Notice .................................................................................................................................................................. ii

Foreword ................................................................................................................................................................. iii

List of Figures ......................................................................................................................................................... vii

List of Tables ............................................................................................................................................................ ix

Acknowledgments ................................................................................................................................................... xi

Introduction ...........................................................................................................................................................xiii

Chapter 1. Background and use of onsite wastewater treatment systems ........................................................ 1-1

1.1 Introduction ................................................................................................................................................ 1–1

1.2 History of onsite wastewater treatment systems ......................................................................................... 1–2

1.3 Regulation of onsite wastewater treatment systems................................................................................... 1–3

1.4 Onsite wastewater treatment system use, distribution, and failure rate ..................................................... 1–4

1.5 Problems with existing onsite wastewater management programs ............................................................ 1–5

1.6 Performance-based management of onsite wastewater treatment systems .............................................. 1–10

1.7 Coordinating onsite system management with watershed protection efforts ......................................... 1–11

1.8 USEPA initiatives to improve onsite system treatment and management ............................................... 1–12

1.9 Other initiatives to assist and improve onsite management efforts ......................................................... 1–15

Chapter 2. Management of onsite wastewater treatment systems .................................................................... 2-1

2.1 Introduction ................................................................................................................................................ 2–1

2.2 Elements of a successful program ............................................................................................................... 2–3

2.3 Types of management entities .................................................................................................................... 2–6

2.4 Management program components .......................................................................................................... 2–13

2.5 Financial assistance for management programs and system installation ............................................... 2–41

Chapter 3. Establishing treatment system performance requirements ........................................................... 3-1

3.1 Introduction ................................................................................................................................................ 3–1

3.2 Estimating wastewater characteristics ........................................................................................................ 3–1

3.3 Estimating wastewater flow ........................................................................................................................ 3–2

3.4 Wastewater quality ...................................................................................................................................... 3–8

3.5 Minimizing wastewater flows and pollutants .......................................................................................... 3–10

3.6 Integrating wastewater characterization and other design information .................................................. 3–20

3.7 Transport and fate of wastewater pollutants in the receiving environment ............................................ 3–20

3.8 Establishing performance requirements ................................................................................................... 3–40

3.9 Monitoring system operation and performance ....................................................................................... 3–53

Chapter 4. Treatment processes and systems ..................................................................................................... 4-1

4.1 Introduction ................................................................................................................................................ 4–1

4.2 Conventional systems and treatment options ............................................................................................ 4–2

4.3 Subsurface wastewater infiltration .............................................................................................................. 4–2

4.4 Design considerations ................................................................................................................................. 4–6

4.5 Construction management and contingency options .............................................................................. 4–34

4.6 Septic tanks ............................................................................................................................................... 4–37

4.7 Sand/media filters ..................................................................................................................................... 4–48

4.8 Aerobic treatment units............................................................................................................................. 4–52

Table of Contents
USEPA Onsite Wastewater Treatment Systems Manual vi

Onsite wastewater treatment systems technology fact sheets

1 Continuous-Flow, Suspended-Growth Aerobic Systems (CFSGAS) ..................................................... TFS–1

2 Fixed-film processes............................................................................................................................... TFS–7

3 Sequencing batch reactor systems ....................................................................................................... TFS–13

4 Effluent disinfection processes............................................................................................................ TFS–17

5 Vegetated submerged beds and other high-specific-surface anaerobic reactors ................................ TFS–23

6 Evapotranspiration and evapotranspiration/infiltration ..................................................................... TFS–31

7 Stabilization ponds, FWS constructed wetlands, and other aquatic systems..................................... TFS–37

8 Enhanced nutrient removal—phosphorus........................................................................................... TFS–41

9 Enhanced nutrient removal—nitrogen ............................................................................................... TFS–45

10 Intermittent sand/media filters............................................................................................................ TFS–53

11 Recirculating sand/media filters......................................................................................................... TFS–61

12 Land treatment systems ....................................................................................................................... TFS–71

13 Renovation/restoration of subsurface wastewater infiltration systems (SWIS) ................................. TFS–77

Onsite wastewater treatment systems special issues fact sheets

1 Septic tank additives.............................................................................................................................SIFS–1

2 High-organic-strength wastewaters (including garbage grinders) .......................................................SIFS–3

3 Water softeners ......................................................................................................................................SIFS–7

4 Holding tanks and hauling systems......................................................................................................SIFS–9

Chapter 5. Treatment system selection ............................................................................................................... 5-1

5.1 Factors for selecting appropriate system design and size ......................................................................... 5–1

5.2 Design conditions and system selection ................................................................................................... 5–1

5.3 Matching design conditions to system performance ................................................................................ 5–1

5.4 Design boundaries and boundary loadings ............................................................................................... 5–3

5.5 Evaluating the receiving environment ...................................................................................................... 5–9

5.6 Mapping the site ...................................................................................................................................... 5–24

5.7 Developing the initial system design ...................................................................................................... 5–24

5.8 Rehabilitating and upgrading existing systems ..................................................................................... 5–32

Table of Contents, Cont’d.
USEPA Onsite Wastewater Treatment Systems Manual vii

Figures

Figure 1-1. Conventional onsite wastewater treatment system ......................................................................... 1–1

Figure 1-2. Typical single-compartment septic tank ......................................................................................... 1–3

Figure 1-3. Onsite treatment system distribution in the United States ............................................................. 1–5

Figure 1-4. Fate of water discharged to onsite wastewater treatment systems .................................................. 1–7

Figure 1-5. The watershed approach planning and management cycle .......................................................... 1–13

Figure 1-6. Large-capacity septic tanks and other subsurface discharges ...................................................... 1–14

Figure 2-1. Onsite wastewater management overlay zones example .............................................................. 2–18

Figure 2-2. Process for developing onsite wastewater management ............................................................... 2–20

Figure 3-1. Distribution of mean household daily per capita indoor water use ............................................... 3–5

Figure 3-2. Indoor water use percentage, including leakage, for 1,188 data logged homes ............................ 3–6

Figure 3-3. Daily indoor water use pattern for single-family residence ............................................................ 3–7

Figure 3-4. Peak wastewater flows for single-family home ............................................................................... 3–8

Figure 3-5. Average hourly distribution of total unfiltered BOD5 .................................................................. 3–10

Figure 3-6. Typical graywater reuse approach ................................................................................................. 3–19

Figure 3-7. Strategy for estimating wastewater flow and composition ........................................................... 3–21

Figure 3-8. Plume movement through the soil to the saturated zone .............................................................. 3–22

Figure 3-9. An example of effluent plume movement ..................................................................................... 3–25

Figure 3-10. Soil treatment zones....................................................................................................................... 3–26

Figure 3-11. Zinc sorption by clay as a function of pH ..................................................................................... 3–38

Figure 3-12. Example of compliance boundaries for onsite wastewater treatment systems............................. 3–40

Figure 3-13. Input and output components of the MANAGE assessment method ........................................... 3–44

Figure 3-14. Probability of environmental impact decision tree ....................................................................... 3–50

Figure 4-1. Conventional subsurface wastewater infiltration system................................................................ 4–2

Figure 4-2. Lateral view of conventional SWIS-based system .......................................................................... 4–5

Figure 4-3. Subsurface infiltration system design versus depth to a limiting condition ................................. 4–7

Figure 4-4. Raising the infiltration surface with a typical mound system ........................................................ 4–9

Figure 4-5. Schematic of curtain drain construction ......................................................................................... 4–9

Figure 4-6. Capacity chart for subsurface drains ............................................................................................. 4–11

Figure 4-7. Pathway of subsoil reaeration ........................................................................................................ 4–16

Figure 4-8. Distribution box with adjustable weir outlets .............................................................................. 4–19

Figure 4-9. Serial relief line distribution network and installation detail ...................................................... 4–19

Figure 4-10. Drop box distribution network ...................................................................................................... 4–21
USEPA Onsite Wastewater Treatment Systems Manual viii

Figure 4-11. Various gravelless systems ............................................................................................................ 4–21

Figure 4-12. Placement of leaching chambers in typical application ............................................................... 4–22

Figure 4-13. Typical pressurized distribution system layout ............................................................................ 4–23

Figure 4-14. Pressure manifold detail................................................................................................................. 4–24

Figure 4-15. Horizontal design for pressure distribution .................................................................................. 4–25

Figure 4-16. Rigid pipe pressure distribution networks with flushing cleanouts ............................................ 4–26

Figure 4-17. Pressure manifold and flexible drip lines prior to trench filling .................................................. 4–28

Figure 4-18. Emitter discharge rates versus in-line pressure ............................................................................. 4–29

Figure 4-19. Dripline layout on a site with trees ............................................................................................... 4–31

Figure 4-20. Pumping tank (generic) ................................................................................................................. 4–32

Figure 4-21. Profile of a single-compartment septic tank with outlet screen ................................................... 4–38

Figure 4-22. Two-compartment tank with effluent screen and surface risers .................................................... 4–40

Figure 4-23. Examples of septic tank effluent screens/filters............................................................................ 4–41

Figure 4-24. Tongue and groove joint and sealer .............................................................................................. 4–43

Figure 4-25. Underdrain system detail for sand filters ...................................................................................... 4–48

Figure 4-26. Schematics of the two most common types of sand media filters ................................................ 4–50

Figure 5-1. Preliminary design steps and considerations .................................................................................. 5–2

Figure 5-2. Performance (design) boundaries associated with onsite treatment systems ................................. 5–4

Figure 5-3. Subsurface wastewater infiltration system design/performance boundaries.................................. 5–5

Figure 5-4. Effluent mounding effect above the saturated zone ....................................................................... 5–8

Figure 5-5. General considerations for locating a SWIS on a sloping site ..................................................... 5–13

Figure 5-6. Landscape position features (see table 5-6 for siting potential) .................................................. 5–14

Figure 5-7. Conventional system layout with SWIS replacement area ........................................................... 5–15

Figure 5-8. Site evaluation/site plan checklist ................................................................................................ 5–16

Figure 5-9. Soil textural triangle ...................................................................................................................... 5–19

Figure 5-10. Types of soil structure .................................................................................................................... 5–20

Figure 5-11. Potential evaporation versus mean annual precipitation ............................................................. 5–24

Figure 5-12. Development of the onsite wastewater system design concept .................................................... 5–25

Figure 5-13. Onsite wastewater failure diagnosis and correction procedure .................................................... 5–33

Figures, Cont’d.
USEPA Onsite Wastewater Treatment Systems Manual ix

Tables

Table 1-1. Typical pollutants of concern from onsite wastewater treatment systems ..................................... 1–2

Table 1-2. Census of housing tables: sewage disposal, 1990 .......................................................................... 1–6

Table 1-3. Estimated onsite treatment system failure rates in surveyed states ................................................ 1–7

Table 2-1. Organizational approaches for managing onsite systems............................................................... 2–7

Table 2-2. Survey of state certification and licensing programs.................................................................... 2–33

Table 2-3. Components of an onsite system regulatory program .................................................................. 2–36

Table 2-4. Compliance assurance approaches................................................................................................ 2–38

Table 2-5. Example of functional responsibilities matrix ............................................................................. 2–42

Table 2-6. Funding options............................................................................................................................. 2–43

Table 2-7. Advantages and disadvantages of various funding sources......................................................... 2–47

Table 3-1. Summary of average daily residential wastewater flows ................................................................ 3–3

Table 3-2. Comparison of daily per capita indoor water use for 12 study sites .............................................. 3–4

Table 3-3. Residential water use by fixture or appliance ................................................................................. 3–5

Table 3-4. Typical wastewater flow rates from commercial sources ................................................................ 3–7

Table 3–5. Typical wastewater flow rates from institutional sources ............................................................... 3–8

Table 3-6. Typical wastewater flow rates from recreational facilities.............................................................. 3–9

Table 3-7. Constituent mass loadings and concentrations ............................................................................ 3–11

Table 3-8. Residential wastewater pollutant contributions by source .......................................................... 3–11

Table 3-9. Wastewater flow reduction methods.............................................................................................. 3–13

Table 3-10. Flow rates and flush volumes before and after U.S. Energy Policy Act ........................................ 3–14

Table 3-11. Wastewater flow reduction: water-carriage toilets and systems ................................................... 3–14

Table 3-12. Wastewater flow reduction: non-water-carriage toilets................................................................. 3–15

Table 3-13. Wastewater flow reduction: showering devices and systems ....................................................... 3–15

Table 3-14. Wastewater flow reduction: miscellaneous devices and systems ................................................. 3–16

Table 3-15. Reduction in pollutant loading achieved by eliminating garbage disposals ............................. 3–18

Table 3-16. Typical wastewater pollutants of concern ..................................................................................... 3–23

Table 3-17. Examples of soil infiltration system performance ........................................................................ 3–23

Table 3-18. Case study: septic tank effluent and soil water quality ................................................................ 3–28

Table 3-19. Wastewater constituents of concern and representative concentrations ...................................... 3–29

Table 3-20. Waterborne pathogens found in human waste and associated diseases ....................................... 3–32

Table 3-21. Typical pathogen survival times at 20 to 30 o

C ............................................................................ 3–33

Table 3-22. MCLs for selected organic chemicals in drinking water .............................................................. 3–35

Table 3-23. Case study: concentration of metals in septic tank effluent ........................................................ 3–36

Table 3-24. MCLs for selected inorganic chemicals in drinking water........................................................... 3–37

Table 3-25. Treatment performance requirements for New Shoreham, Rhode Island ..................................... 3–45
USEPA Onsite Wastewater Treatment Systems Manual x

Table 3-26. Resource listing, value ranking, and wastewater management schematic ................................... 3–46

Table 3-27. Proposed onsite system performance standards in various control zones.................................... 3–48

Table 3-28. Treatment performance standards in various control zones ......................................................... 3–48

Table 3-29. Nitrogen loading values used in the Buttermilk Bay assessment ................................................ 3–52

Table 3-30. Typical laboratory costs for water quality analysis ...................................................................... 3–61

Table 4-1. Commonly used treatment processes and optional treatment methods ......................................... 4–3

Table 4-2. Characteristics of typical SWIS applications.................................................................................. 4–5

Table 4-3. Suggested hydraulic and organic loading rates for sizing infiltration surfaces .......................... 4–12

Table 4-4. Geometry, orientation, and configuration considerations for SWISs ........................................... 4–16

Table 4-5. Distribution methods and applications ......................................................................................... 4–18

Table 4-6. Dosing methods and devices......................................................................................................... 4–23

Table 4-7. Pressure manifold sizing ................................................................................................................ 4–25

Table 4-8. Contingency options for SWIS malfunctions ............................................................................... 4–34

Table 4-9. Operation, maintenance, and monitoring activities ..................................................................... 4–36

Table 4-10. Characteristics of domestic septic tank effluent ........................................................................... 4–38

Table 4-11. Average septic tank effluent concentrations for selected parameters........................................... 4–39

Table 4-12. Average septic tank effluent concentrations from various commercial establishments .............. 4–39

Table 4-13. Septic tank capacities for one- and two-family dwellings ........................................................... 4–40

Table 4-14. Watertightness testing procedure/criteria for precast concrete tanks ........................................... 4–43

Table 4-15. Chemical and physical characteristics of domestic septage ........................................................ 4–46

Table 4-16. Single pass and recirculating filter performance .......................................................................... 4–53

Table 5-1. Types of mass loadings to subsurface wastewater infiltration systems .......................................... 5–6

Table 5-2. Potential impacts of mass loadings on soil design boundaries ...................................................... 5–7

Table 5-3. Types of mass loadings for point discharges to surface waters....................................................... 5–9

Table 5-4. Types of mass loadings for evapotranspiration systems ................................................................. 5–9

Table 5-5. Site characterization and assessment activities for SWIS applications........................................ 5–11

Table 5-6. SWIS siting potential vs. landscape position features.................................................................. 5–14

Table 5-7. Practices to characterize subsurface conditions through test pit inspection ............................... 5–18

Table 5-8. Example of a total cost summary worksheet to compare alternatives ......................................... 5– 31

Table 5-9. Common onsite wastewater treatment system failures ................................................................. 5–32

Table 5-10. General OWTS inspection and failure detection process ............................................................. 5–35

Table 5-11. Response of corrective actions on SWIS boundary mass loadings .............................................. 5–35

Tables, Cont’d.
USEPA Onsite Wastewater Treatment Systems Manual xi

This update of the 1980 Design Manual: Onsite Wastewater Treatment and Disposal Systems (see

information for wastewater treatment professionals in both the public and private sectors. This manual

is not intended to replace the previous manual, but rather to further explore and discuss recent

developments in treatment technologies, system design, and long-term system management.

The information in the chapters that follow is provided in response to several calls for a more focused

approach to onsite wastewater treatment and onsite system management. Congress has expressed interest

in the status of site-level approaches for treating wastewater, and the Executive Branch has issued

directives for moving forward with improving both the application of treatment technologies and

management of the systems installed.

The U.S. Environmental Protection Agency (USEPA) responded to this interest by convening a team

of subject matter experts from public agencies, private organizations, professional associations, and

the academic community. Two representatives from the USEPA Office of Water and a representative

from the Office of Research and Development coordinated the project team for this document. Close

coordination with the USEPA Office of Wastewater Management and other partners at the federal,

state, and local levels helped to ensure that the information in this manual supports and complements

other efforts to improve onsite wastewater management across the nation.

The principal authors of the document are Richard Otis of Ayres Associates; Jim Kreissl, Rod Frederick,

and Robert Goo of USEPA; Peter Casey of the National Small Flows Clearinghouse; and Barry

Tonning of Tetra Tech, Inc. Other persons who made significant contributions to the manual include

Robert Siegrist of the Colorado School of Mines; Mike Hoover of North Carolina State University;

Jean Caudill of the Ohio Department of Health; Bob Minicucci of the New Hampshire Department of

Environmental Services; Tom Groves of the New England Interstate Water Pollution Control Commission; Tom Yeager of Kennedy/Jenks Consultants; Robert Rubin of North Carolina State University; Pio Lombardo of Lombardo Associates; Dov Weitman and Joyce Hudson of USEPA; Lisa Brown,

Seldon Hall, Richard Benson, and Tom Long of the Washington Department of Health; David Pask

and Tricia Angoli of the National Small Flows Clearinghouse; James Davenport of the National

Association of Counties; Jim Watson of the Tennessee Valley Authority; John Austin of the U.S.

Agency for International Development; Pat Fleming of the U.S. Bureau of Land Management; James

Jacobsen of the Maine Department of Human Services; Richard Barror of the Indian Health Service;

Glendon Deal of the U.S. Department of Agriculture; Lisa Knerr, Jonathan Simpson, and Kay Rutledge

of Tetra Tech; Kenneth Pankow of Pankow Engineering; Linda Stein of Eastern Research Group;

Robert Adler, Charles Pycha, Calvin Terada, and Jonathon Williams of USEPA Region 10; Richard

Carr of the World Health Organization; Ralph Benson of the Clermont County, Ohio, General Health

District; Rich Piluk of the Anne Arundel, Maryland, county government; Jerry Nonogawa of the

Hawaii Department of Health; Tony Smithson of the Lake County, Illinois, Health Department;

Conrad G. Keyes, Jr., and Cecil Lue-Hing of the EWRI of ASCE; Robert E. Lee of the National Onsite

Wastewater Recycling Association; Anish Jantrania, private consultant; Larry Stephens of Stephens

Consultants; Bruce Douglass and Bill Heigis of Stone Engineering; Alan Hassett of Oak Hill Co.;

Steven Braband of Biosolutions, Inc.; Matt Byers of Zoeller Co.; Carl Thompson, Infiltrator Systems,

Inc.; Alex Mauck of EZ Drain; Bob Mayer of American Manufacturing; Rodney Ruskin of Geoflow;

Fred Harned of Netafim; Don Canada of the American Decentralized Wastewater Association, and

Michael Price, Norweco, Inc.

Graphics in the manual were provided by John Mori of the National Small Flows Clearinghouse,

Ayres Associates, and other sources. Regina Scheibner, Emily Faalasli, Krista Carlson, Monica Morrison,

Liz Hiett, and Kathryn Phillips of Tetra Tech handled layout and production; Martha Martin of Tetra

Tech edited the manual. The cover was produced by the National Small Flows Clearinghouse.

Acknowledgments

http://www.epa.gov/ORD/NRMRL/Pubs/625180012/625180012.htm) was developed to provide supplemental and new
USEPA Onsite Wastewater Treatment Systems Manual xii

Robert Goo, USEPA, Office of Wetlands (OW), Oceans

and Watersheds

Rod Frederick, USEPA, OW, Oceans and Watersheds

Eric Slaughter, USEPA, OW, Oceans and Watersheds

Jim Kreissl, USEPA, Office of Research and

Development (ORD)

Don Brown, USEPA, ORD

Robert Bastian, USEPA, Office of Wastewater

Management (OWM)

Charlie Vanderlyn, USEPA, OWM

Steve Hogye, USEPA, OWM

Joyce Hudson, USEPA, OWM

Joel Salter, USEPA, Office of Science and Technology

Philip Berger, USEPA, Office of Ground Water and

Drinking Water (OGWDW)

Howard Beard, USEPA, OGWDW

Robert Adler, USEPA Region 1

Charles Pycha, USEPA Region 5

Ernesto Perez, USEPA Region 6

Calvin Terada, USEPA Region 10

Danny Averett, U.S. Army Corps of Engineers

Ed Smith, USACE Research Laboratory

Rick Scholz, USACE Research Laboratory

John Austin, U.S. Agency for International Development

Patrick Fleming, National Park Service

Rick Barror, U.S. Public Health Service

Gary Morgan, USDA Rural Development Administration

Andree Duvarney, USDA Natural Resources

Conservation Service

Phil Mummert, Tennessee Valley Authority

Raymond Reid, Pan American Health Organization

Homero Silva, Organización Mundial de la Salud, Costa

Rica

Dennis Warner, World Health Organization

Tom Groves, New England Interstate Water Pollution

Control Commission

Paul Chase, DuPage County (Illinois) Health

Department

Douglas Ebelherr, Illinois Department of Public Health

Randy Clarkson, Missouri Department of Natural

Resources

Anish Janrania, Virginia Department of Health

Steve Steinbeck, North Carolina Department of Health

and Natural Resources

Ron Frey, Arizona Department of Environmental Quality

Mark Soltman, Washington State Department of Health

Alex Campbell, Ontario Ministry of Environment and

Approvals

Jerry Tyler, University of Wisconsin

Mike Hoover, North Carolina State University

Ruth Alfasso, Massachusetts Department of

Environmental Protection

Jerry Nunogawa, Hawaii Department of Health

Robert Siegrist, Colorado School of Mines

Rick Piluk, Anne Arundel County (Maryland) Health

Department

Gary Eckler, Erie County (Ohio) Sanitary Engineering

Department

Janet Rickabaugh, Clermont County (Ohio) Health

District

Jay Harrell, Mohave County (Arizona) Environmental

Health Division

Dan Smith, Coconino County (Arizona) Environmental

Health Services

Tom Yeager, Kennedy/Jenks Consultants

Richard Otis, Ayres Associates

Robert Mayer, American Manufacturing Co.

Hamilton Brown, National Association of Towns and

Townships

Larry Markham, National Environmental Health

Association

Robert Rubin, Water Environment Federation

Thomas McLane, American Society of Civil Engineers

Dan MacRitchie, American Society of Civil Engineers

Don Canada, American Decentralized Wastewater

Association

Naomi Friedman, National Association of Counties

Peter Casey, National Small Flows Clearinghouse

Tricia Angoli, national Small Flows Clearinghouse

Thomas Bruursema, National Sanitation Foundation

Review Team Members for the

Onsite Wastewater Treatment Systems Manual
USEPA Onsite Wastewater Treatment Systems Manual xiii

Background and Purpose

The U.S. Environmental Protection Agency (USEPA)

first issued detailed guidance on the design, construction, and operation of onsite wastewater treatment

systems (OWTSs) in 1980. Design Manual: Onsite

Wastewater Treatment and Disposal Systems (USEPA,

1980) was the most comprehensive summary of onsite

wastewater management since the U.S. Public Health

Service had published a guidance on septic tank

practice in 1967 (USPHS, 1967). The 1980 manual

focused on both treatment and “disposal” of wastewater in general accordance with the approach and

terminology in use at the time. The 1980 design

manual stressed the importance of site-specific soil,

landscape, ground water, and effluent characterization

and included soil percolation tests as one of several

site evaluation tools to be used in system design and

placement. The manual’s discussion of water conservation to reduce hydraulic flows, pollutant reduction

to minimize contaminant loading, and management

programs to oversee the full range of treatment

activities was especially important to the developing

field of onsite wastewater treatment in the United

States and other countries.

Technologies explored in the 1980 manual include

the conventional system (a septic tank with a subsurface wastewater infiltration system), alternating leach

fields, uniform distribution systems, intermittent sand

filters, aerobic units, disinfection technologies, and

evapotranspiration systems. The original manual also

contains guidance on dosing chambers, flow diversion methods for alternating beds, nutrient removal,

and disposal of residuals. Although much of that

information is still useful, advances in regional

planning, improvements in ground water and surface

water protection, and new technologies and management concepts necessitate further guidance for public

health districts, water quality agencies, planning

boards, and other audiences. In addition, the growing

national emphasis on management programs that

establish performance requirements rather than

prescriptive codes for the design, siting, installation,

operation, and maintenance of onsite systems underscores the importance of revising the manual to

address these emerging issues in public health and

water resource protection.

USEPA is committed to elevating the standards for

onsite wastewater management practice and removing

barriers that preclude widespread acceptance of onsite

treatment technologies. The purpose of this update of

the 1980 manual is to provide more comprehensive

information on management approaches, update

information on treatment technologies, and describe

the benefits of performance-based approaches to

system design. The management approaches suggested in this manual involve coordinating onsite

system planning and management activities with land

use planning and watershed protection efforts to

ensure that the impacts of onsite wastewater systems

are considered and controlled at the appropriate scale.

The management approaches described in this manual

support and are consistent with USEPA’s draft Guidelines for Management of Onsite/Decentralized

Wastewater Systems (USEPA, 2000). The incorporation of performance standards for management

programs and for system design and operation can

help ensure that no onsite system alternative presents

an unacceptable risk to public health or water

resources.

This manual contains overview information on

treatment technologies, installation practices, and

past performance. It does not, however, provide

detailed design information and is not intended as a

substitute for region- and site-specific program

criteria and standards that address conditions,

technologies, and practices appropriate to each

individual management jurisdiction. The information

in the following chapters provides an operational

framework for developing and improving OWTS

program structure, criteria, alternative designs, and

performance requirements. The chapters describe the

importance of planning to ensure that system densities are appropriate for prevailing hydrologic and

geologic conditions, performance requirements to

guide system design, wastewater characterization to

accurately predict waste strength and flows, site

evaluations that identify appropriate design and

performance boundaries, technology selection to

Introduction
USEPA Onsite Wastewater Treatment Systems Manual xiv

ensure that performance requirements are met, and

management activities that govern installation,

operation, maintenance, and remediation of failed

systems.

This manual is intended to serve as a technical

guidance for those involved in the design, construction, operation, maintenance, and regulation of onsite

systems. It is also intended to provide information to

policy makers and regulators at the state, tribal, and

local levels who are charged with responsibility for

developing, administering, and enforcing wastewater

treatment and management program codes. The

activities and functions described herein might also

be useful to other public health and natural resource

protection programs. For example, properly planned,

designed, installed, operated, and maintained onsite

systems protect wellhead recharge areas, drinking

water sources, watershed, estuaries, coastal zones,

aquatic habitat, and wetlands.

Finally, this manual is intended to emphasize the need

to improve cooperation and coordination among the

various health, planning, zoning, development,

utility, and resource protection programs operated by

public and private organizations. A watershed

approach to protecting public health and environmental resources requires an integrated operational

framework that encourages independent partners to

function cooperatively while each retains the ability

to satisfy internal programmatic and management

objectives. Integrating onsite wastewater management

processes with other activities conducted by public

and private entities can improve both the effectiveness and the efficiency of efforts to minimize the risk

onsite systems might present to health and ecological

resources.

Overview

Onsite wastewater treatment systems collect, treat, and

release about 4 billion gallons of treated effluent per

day from an estimated 26 million homes, businesses,

and recreational facilities nationwide (U.S. Census

Bureau, 1997). These systems, defined in this manual

as those serving fewer than 20 people, include

treatment units for both individual buildings and

small clusters of buildings connected to a common

treatment system. Recognition of the impacts of

onsite systems on ground water and surface water

quality (e.g., nitrate and bacteria contamination,

nutrient inputs to surface waters) has increased

interest in optimizing the systems’ performance.

Public health and environmental protection officials

now acknowledge that onsite systems are not just

temporary installations that will be replaced eventually by centralized sewage treatment services, but

permanent approaches to treating wastewater for

release and reuse in the environment. Onsite systems

are recognized as potentially viable, low-cost, longterm, decentralized approaches to wastewater treatment

if they are planned, designed, installed, operated, and

maintained properly (USEPA, 1997). NOTE: In

addition to existing state and local oversight, decentralized wastewater treatment systems that serve more

than 20 people might become subject to regulation

under the USEPA’s Underground Injection Control

Program, although EPA has proposed not to include

them (64FR22971:5/7/01).

Although some onsite wastewater management

programs have functioned successfully in the past,

problems persist. Most current onsite regulatory

programs focus on permitting and installation.

Few programs address onsite system operation and

maintenance, resulting in failures that lead to unnecessary costs and risks to public health and water

resources. Moreover, the lack of coordination among

agencies that oversee land use planning, zoning,

development, water resource protection, public health

initiatives, and onsite systems causes problems that

could be prevented through a more cooperative

approach. Effective management of onsite systems

requires rigorous planning, design, installation,

operation, maintenance, monitoring, and controls.

Public health and water resource impacts

State and tribal agencies report that onsite septic

systems currently constitute the third most common

source of ground water contamination and that these

systems have failed because of inappropriate siting or

design or inadequate long-term maintenance (USEPA,

1996a). In the 1996 Clean Water Needs Survey

(USEPA, 1996b), states and tribes also identified more

than 500 communities as having failed septic systems

that have caused public health problems. The discharge of partially treated sewage from malfunctioning onsite systems was identified as a principal or

contributing source of degradation in 32 percent of

all harvest-limited shellfish growing areas. Onsite

wastewater treatment systems have also contributed to

an overabundance of nutrients in ponds, lakes, and

coastal estuaries, leading to the excessive growth of

algae and other nuisance aquatic plants (USEPA,

1996b). In addition, onsite systems contribute to

contamination of drinking water sources. USEPA

estimates that 168,000 viral illnesses and 34,000

bacterial illnesses occur each year as a result of con-
USEPA Onsite Wastewater Treatment Systems Manual xv

sumption of drinking water from systems that rely on

improperly treated ground water. Malfunctioning

septic systems have been identified as one potential

source of ground water contamination (USEPA, 2000).

Improving treatment through performance

requirements

Most onsite wastewater treatment systems are of the

conventional type, consisting of a septic tank and a

subsurface wastewater infiltration system (SWIS). Site

limitations and more stringent performance requirements have led to significant improvements in the

design of wastewater treatment systems and how they

are managed. Over the past 20 years the OWTS

industry has developed many new treatment technologies that can achieve high performance levels on sites

with size, soil, ground water, and landscape limitations that might preclude installing conventional

systems. New technologies and improvements to

existing technologies are based on defining the

performance requirements of the system, characterizing wastewater flow and pollutant loads, evaluating

site conditions, defining performance and design

boundaries, and selecting a system design that

addresses these factors.

Performance requirements can be expressed as

numeric criteria (e.g., pollutant concentration or mass

loading limits) or narrative criteria (e.g., no odors or

visible sheen) and are based on the assimilative

capacity of regional ground water or surface waters,

water quality objectives, and public health goals.

Wastewater flow and pollutant content help define

system design and size and can be estimated by

comparing the size and type of facility with measured

effluent outputs from similar, existing facilities. Site

evaluations integrate detailed analyses of regional

hydrology, geology, and water resources with sitespecific characterization of soils, slopes, structures,

property lines, and other site features to further define

system design requirements and determine the

physical placement of system components.

Most of the alternative treatment technologies

applied today treat wastes after they exit the septic

tank; the tank retains settleable solids, grease, and oils

and provides an environment for partial digestion of

settled organic wastes. Post-tank treatment can

include aerobic (with oxygen) or anaerobic (with no

or low oxygen) biological treatment in suspended or

fixed-film reactors, physical/chemical treatment, soil

infiltration, fixed-media filtration, and/or disinfection. The application and sizing of treatment units

based on these technologies are defined by performance requirements, wastewater characteristics, and

site conditions.

Toward a more comprehensive approach

The principles of the 1980 onsite system design

manual have withstood the test of time, but much has

changed over the past 20 years. This manual incorporates much of the earlier guide but includes new

information on treatment technologies, site evaluation, design boundary characterization, and especially

management program functions. The manual is

organized by functional topics and is intended to be a

comprehensive reference. Users can proceed directly

to relevant sections or review background or other

information (see Contents).

Although this manual focuses on individual and

small, clustered onsite systems, state and tribal

governments and other management entities can use

the information in it to construct a framework for

managing new and existing large-capacity decentralized systems (those serving more than 20 people),

subject to regulation under state or local Underground

Injection Control (UIC) programs. The UIC program

was established by the Safe Drinking Water Act to

protect underground sources of drinking water from

contamination caused by the underground injection

of wastes. In most parts of the nation, the UIC program, which also deals with motor vehicle waste

disposal wells, large-capacity cesspools, and storm

water drainage wells, is managed by state or tribal

water or waste agencies with authority delegated by

USEPA.

The Class V UIC program and the Source Water

Protection Program established by the 1996 amendments to the federal Safe Drinking Water Act are

bringing federal and state drinking water agencies

into the field of onsite wastewater treatment and

management. Both programs will likely require more

interagency involvement and cooperation to characterize wastewater impacts on ground water resources

and to develop approaches to deal with real or

potential problems. States currently have permit-byrule provisions for large-capacity septic systems.

Overview of the revised manual

The first two chapters of this manual present overview and management information of special interest

to program administrators. Chapters 3, 4, and 5

contain technical information on wastewater characterization, site evaluation and selection, and treatment technologies and how to use them in develop-
USEPA Onsite Wastewater Treatment Systems Manual xvi

Overview of the Onsite Wastewater Treatment Systems Manual

ing a system design. Those three chapters are intended

primarily for engineers, soil scientists, permit writers,

environmental health specialists, site evaluators, and field

staff. Summaries of all the chapters appear below. The level

of detail provided in this manual is adequate for preliminary

system design and development of a management program.

References are provided for additional research and

information on how to incorporate local characteristics into

an optimal onsite management program.
USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 1-1

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1.1 Introduction

Onsite wastewater treatment systems (OWTSs)

have evolved from the pit privies used widely

throughout history to installations capable of

producing a disinfected effluent that is fit for

human consumption. Although achieving such a

level of effluent quality is seldom necessary, the

ability of onsite systems to remove settleable solids,

floatable grease and scum, nutrients, and pathogens

from wastewater discharges defines their importance

in protecting human health and environmental

resources. In the modern era, the typical onsite

system has consisted primarily of a septic tank and

a soil absorption field, also known as a subsurface

wastewater infiltration system, or SWIS (figure

1-1). In this manual, such systems are referred to as

conventional systems. Septic tanks remove most

settleable and floatable material and function as an

anaerobic bioreactor that promotes partial digestion

of retained organic matter. Septic tank effluent,

which contains significant concentrations of

pathogens and nutrients, has traditionally been

discharged to soil, sand, or other media absorption

fields (SWISs) for further treatment through

biological processes, adsorption, filtration, and

infiltration into underlying soils. Conventional

systems work well if they are installed in areas with

appropriate soils and hydraulic capacities; designed to

treat the incoming waste load to meet public health,

ground water, and surface water performance

standards; installed properly; and maintained to

ensure long-term performance.

Chapter 1:

Background and use of onsite wastewater treatment systems

These criteria, however, are often not met. Only

about one-third of the land area in the United States

has soils suited for conventional subsurface soil

absorption fields. System densities in some areas

exceed the capacity of even suitable soils to

assimilate wastewater flows and retain and transform their contaminants. In addition, many systems

are located too close to ground water or surface

waters and others, particularly in rural areas with

newly installed public water lines, are not designed

to handle increasing wastewater flows. Conventional onsite system installations might not be

adequate for minimizing nitrate contamination of

ground water, removing phosphorus compounds,

and attenuating pathogenic organisms (e.g.,

bacteria, viruses). Nitrates that leach into ground

1.1 Introduction

1.2 History of onsite wastewater treatment systems

1.3 Regulation of onsite wastewater treatment systems

1.4 Onsite wastewater treatment system use, distribution, and failure rate

1.5 Problems with existing onsite wastewater management programs

1.6 Performance-based management of onsite wastewater treatment systems

1.7 Coordinating onsite system management with watershed protection efforts

1.8 USEPA initiatives to improve onsite system treatment and management

1.9 Other initiatives to assist and improve onsite management efforts

Figure 1-1. Conventional onsite wastewater treatment system

Source: NSFC, 2000.
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-2 USEPA Onsite Wastewater Treatment Systems Manual

water used as a drinking water source can cause

methemoglobinemia, or blue baby syndrome, and

other health problems for pregnant women.

Nitrates and phosphorus discharged into surface

waters directly or through subsurface flows can

spur algal growth and lead to eutrophication and

low dissolved oxygen in lakes, rivers, and coastal

areas. In addition, pathogens reaching ground water

or surface waters can cause human disease through

direct consumption, recreational contact, or ingestion of contaminated shellfish. Sewage might also

affect public health as it backs up into residences or

commercial establishments because of OWTS

failure.

Nationally, states and tribes have reported in their

1998 Clean Water Act section 303(d) reports that

designated uses (e.g., drinking water, aquatic

habitat) are not being met for 5,281 waterbodies

because of pathogens and that 4,773 waterbodies

are impaired by nutrients. Onsite systems are one of

many known contributors of pathogens and nutrients

to surface and ground waters. Onsite wastewater

systems have also contributed to an overabundance

of nutrients in ponds, lakes, and coastal estuaries,

leading to overgrowth of algae and other nuisance

aquatic plants.

Threats to public health and water resources

(table 1-1) underscore the importance of instituting

management programs with the authority and

resources to oversee the full range of onsite system

activities—planning, siting, design, installation,

operation, monitoring, and maintenance. EPA has

issued draft Guidelines for Management of Onsite/

Decentralized Wastewater Systems (USEPA, 2000)

to improve overall management of OWTSs. These

guidelines are discussed in more detail in chapter 2.

1.2 History of onsite wastewater

treatment systems

King Minos installed the first known water closet

with a flushing device in the Knossos Palace in

Crete in 1700 BC. In the intervening 3,700 years,

societies and the governments that serve them have

sought to improve both the removal of human

wastes from indoor areas and the treatment of that

waste to reduce threats to public health and ecological resources. The Greeks, Romans, British, and

French achieved considerable progress in waste

removal during the period from 800 BC to AD

1850, but removal often meant discharge to surface

waters; severe contamination of lakes, rivers,

streams, and coastal areas; and frequent outbreaks

of diseases like cholera and typhoid fever.

By the late 1800s, the Massachusetts State Board of

Health and other state health agencies had documented links between disease and poorly treated

sewage and recommended treatment of wastewater

through intermittent sand filtration and land

application of the resulting sludge. The past

century has witnessed an explosion in sewage

treatment technology and widespread adoption of

centralized wastewater collection and treatment

services in the United States and throughout the

world. Although broad uses of these systems have

vastly improved public health and water quality in

urban areas, homes and businesses without centralized collection and treatment systems often conTable 1-1. Typical pollutants of concern in effluent from onsite wastewater treatment systems
USEPA Onsite Wastewater Treatment Systems Manual 1-3

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

tinue to depend on technologies developed more

than 100 years ago. Septic tanks for primary

treatment of wastewater appeared in the late 1800s,

and discharge of tank effluent into gravel-lined

subsurface drains became common practice during

the middle of the 20th century (Kreissl, 2000).

Scientists, engineers, and manufacturers in the

wastewater treatment industry have developed a wide

range of alternative technologies designed to address

increasing hydraulic loads and water contamination

by nutrients and pathogens. These technologies can

achieve significant pollutant removal rates. With

proper management oversight, alternative systems

(e.g., recirculating sand filters, peat-based systems,

package aeration units) can be installed in areas

where soils, bedrock, fluctuating ground water levels,

or lot sizes limit the use of conventional systems.

Alternative technologies typically are applied to the

treatment train beyond the septic tank (figure 1-2).

The tank is designed to equalize hydraulic flows;

retain oils, grease, and settled solids; and provide

some minimal anaerobic digestion of settleable

organic matter. Alternative treatment technologies

often provide environments (e.g., sand, peat, artificial

media) that promote additional biological treatment

and remove pollutants through filtration, absorption,

and adsorption. All of the alternative treatment

technologies in current use require more intensive

management and monitoring than conventional

OWTSs because of mechanical components, additional residuals generated, and process sensitivities

(e.g., to wastewater strength or hydraulic loading).

Replacing gravity-flow subsurface soil infiltration

beds with better-performing alternative distribution

technologies can require float-switched pumps and/

or valves. As noted in chapter 4, specialized

excavation or structures might be required to house

some treatment system components, including the

disinfection devices (e.g., chlorinators, ultraviolet

lamps) used by some systems. In addition, it is

often both efficient and effective to collect and

treat septic tank effluent from clusters of individual

sources through a community or cluster system

driven by gravity, pressure, or vacuum. These

devices also require specialized design, operation,

and maintenance and enhanced management

oversight.

1.3 Regulation of onsite

wastewater treatment systems

Public health departments were charged with

enforcing the first onsite wastewater “disposal”

laws, which were mostly based on soil percolation

tests, local practices, and past experience. Early

codes did not consider the complex interrelationships among soil conditions, wastewater characteristics, biological mechanisms, and climate and

Figure 1-2. Typical single-compartment septic tank with at-grade inspection ports and effluent screen

Source: NSFC, 2000.
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-4 USEPA Onsite Wastewater Treatment Systems Manual

prescribed standard designs sometimes copied from

jurisdictions in vastly different geoclimatic regions. In addition, these laws often depended on

minimally trained personnel to oversee design,

permitting, and installation and mostly untrained,

uninformed homeowners to operate and maintain

the systems. During the 1950s states began to adopt

laws upgrading onsite system design and installation practices to ensure proper functioning and

eliminate the threats posed by waterborne pathogens (Kreissl, 1982). Despite these improvements,

many regulations have not considered cumulative

ground water and surface water impacts, especially

in areas with high system densities and significant

wastewater discharges.

Kreissl (1982) and Plews (1977) examined changes

in state onsite wastewater treatment regulations

prompted by the publication of the first U.S. Public

Health Service Manual of Septic-Tank Practice in

1959. Plews found significant code revisions under

way by the late 1970s, mostly because of local

experience, new research information, and the need

to accommodate housing in areas not suited for

conventional soil infiltration systems. Kreissl found

that states were gradually increasing required

septic tank and drainfield sizes but also noted that

32 states were still specifying use of the percolation test in system sizing in 1980, despite its proven

shortcomings. Other differences noted among state

codes included separation distances between the

infiltration trench bottom and seasonal ground

water tables, minimum trench widths, horizontal

setbacks to potable water supplies, and maximum

allowable land slopes (Kreissl, 1982).

Although state lawmakers have continued to revise

onsite system codes, most revisions have failed to

address the fundamental issue of system performance in the context of risk management for both a

site and the region in which it is located. Prescribed

system designs require that site conditions fit

system capabilities rather than the reverse and are

sometimes incorrectly based on the assumption that

centralized wastewater collection and treatment

services will be available in the future. Codes that

emphasize prescriptive standards based on empirical relationships and hydraulic performance do not

necessarily protect ground water and surface water

resources from public health threats. Devising a

new regime for protecting public health and the

environment in a cost-effective manner will require

increased focus on system performance, pollutant

transport and fate and resulting environmental

impacts, and integration of the planning, design,

siting, installation, maintenance, and management

functions to achieve public health and environmental objectives.

1.4 Onsite wastewater treatment

system use, distribution, and

failure rate

According to the U.S. Census Bureau (1999),

approximately 23 percent of the estimated 115

million occupied homes in the United States are

served by onsite systems, a proportion that has

changed little since 1970. As shown in figure 1-3

and table 1-2, the distribution and density of homes

with OWTSs vary widely by state, with a high of

about 55 percent in Vermont and a low of around 10

percent in California (U.S. Census Bureau, 1990).

New England states have the highest proportion of

homes served by onsite systems: New Hampshire

and Maine both report that about half of all homes

are served by individual wastewater treatment

systems. More than a third of the homes in the

southeastern states depend on these systems,

including approximately 48 percent in North

Carolina and about 40 percent in both Kentucky

and South Carolina. More than 60 million people

depend on decentralized systems, including the

residents of about one-third of new homes and

more than half of all mobile homes nationwide

(U.S. Census Bureau, 1999). Some communities

rely completely on OWTSs.

A number of systems relying on outdated and

underperforming technologies (e.g., cesspools,

drywells) still exist, and many of them are listed

among failed systems. Moreover, about half of the

occupied homes with onsite treatment systems are

more than 30 years old (U.S. Census Bureau, 1997),

and a significant number report system problems. A

survey conducted by the U.S. Census Bureau

(1997) estimated that 403,000 homes experienced

septic system breakdowns within a

3-month period during 1997; 31,000 reported four

or more breakdowns at the same home. Studies

reviewed by USEPA cite failure rates ranging from

10 to 20 percent (USEPA, 2000). System failure

surveys typically do not include systems that might

be contaminating surface or ground water, a

situation that often is detectable only through site-
USEPA Onsite Wastewater Treatment Systems Manual 1-5

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

level monitoring. Figure 1-4 demonstrates ways

that effluent water from a septic system can reach

ground water or surface waters.

Comprehensive data to measure the true extent of

septic system failure are not currently collected by

any single organization. Although estimates of

system failure rates have been collected from 28

states (table 1-3), no state had directly measured its

own failure rate and definitions of failure vary

(Nelson et al., 1999). Most available data are the

result of incidents that directly affect public health

or are obtained from homeowners’ applications for

permits to replace or repair failing systems. The 20

percent failure rate from the Massachusetts time-oftransfer inspection program is based on an inspection of each septic system prior to home sale, which

is a comprehensive data collection effort. However,

the Massachusetts program only identifies failures

according to code and does not track ground water

contamination that may result from onsite system

failures.

In addition to failures due to age and hydraulic

overloading, OWTSs can fail because of design,

installation, and maintenance problems. Hydraulically functioning systems can create health and

ecological risks when multiple treatment units are

installed at densities that exceed the capacity of

local soils to assimilate pollutant loads. System

owners are not likely to repair or replace aging or

otherwise failing systems unless sewage backup,

septage pooling on lawns, or targeted monitoring

that identifies health risks occurs. Because ground

and surface water contamination by onsite systems

has rarely been confirmed through targeted monitoring, total failure rates and onsite system impacts

over time are likely to be significantly higher than

historical statistics indicate. For example, the

Chesapeake Bay Program found that 55 to 85 percent

of the nitrogen entering an onsite system can be

discharged into ground water (USEPA, 1993). A

1991 study concluded that conventional systems

accounted for 74 percent of the nitrogen entering

Buttermilk Bay in Massachusetts (USEPA, 1993).

1.5 Problems with existing onsite

wastewater management

programs

Under a typical conventional system management

approach, untrained and often uninformed system

owners assume responsibility for operating and

Figure 1-3. Onsite treatment system distribution in the United States Source: U.S. Census Bureau, 1990.
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-6 USEPA Onsite Wastewater Treatment Systems Manual

Table 1-2. Census of housing tables: sewage disposal, 1990

Source: U.S. Census Bureau, 1990.
USEPA Onsite Wastewater Treatment Systems Manual 1-7

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

Figure 1-4. Fate of water discharged to onsite wastewater treatment systems.

Table 1-3. Estimated onsite treatment system failure rates in surveyed states

Source: Nelson et al., 1999.

Source: Adapted from Venhuizen, 1995.
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-8 USEPA Onsite Wastewater Treatment Systems Manual

maintaining their relatively simple, gravity-based

systems. Performance results under this approach

can vary significantly, with operation and maintenance functions driven mostly by complaints or

failures. In fact, many conventional system failures

have been linked to operation and maintenance

failures. Typical causes of failure include unpumped

and sludge-filled tanks, which result in clogged

absorption fields, and hydraulic overloading caused

by increased occupancy and greater water use

following the installation of new water lines to replace

wells and cisterns. Full-time or high use of vacation

homes served by systems installed under outdated

practices or designed for part-time occupancy can

cause water quality problems in lakes, coastal bays,

and estuaries. Landscape modification, alteration of

the infiltration field surface, or the use of outdated

technologies like drywells and cesspools can also

cause contamination problems.

Newer or “alternative” onsite treatment technologies are more complex than conventional systems

and incorporate pumps, recirculation piping,

aeration, and other features (e.g., greater generation

of residuals) that require ongoing or periodic

monitoring and maintenance. However, the current

management programs of most jurisdictions do not

typically oversee routine operation and maintenance activities or detect and respond to changes in

wastewater loads that can overwhelm a system. In

addition, in many cases onsite system planning and

siting functions are not linked to larger ground

water and watershed protection programs. The

challenge for onsite treatment regulators in the new

millennium will be to improve traditional healthbased programs for ground water and surface water

protection while embracing a vigorous role in

protecting and restoring the nation’s watersheds.

The challenge is significant. Shortcomings in many

management programs have resulted in poor system

performance, public health threats, degradation of

surface and ground waters, property value declines,

and negative public perceptions of onsite treatment

as an effective wastewater management option.

(See examples in section 1.1.) USEPA (1987) has

identified a number of critical problems associated

with programs that lack a comprehensive management program:

• Failure to adequately consider site-specific

environmental conditions.

• Codes that thwart adaptation to difficult local

site conditions and are unable to accommodate

effective innovative and alternative technologies.

• Ineffective or nonexistent public education and

training programs.

• Failure to include conservation and potential

reuse of water.

• Ineffective controls on operation and maintenance of systems, including residuals (septage,

sludge).

• Failure to consider the special characteristics

and requirements of commercial, industrial, and

large residential systems.

• Weak compliance and enforcement programs.

• These problems can be grouped into three

primary areas: (1) insufficient funding and

public involvement; (2) inappropriate system

design and selection processes; and (3) poor

inspection, monitoring, and program evaluation

components. Management programs that do not

address these problems can directly and indirectly contribute to significant human health

risks and environmental degradation.

1.5.1 Public involvement and

education

Public involvement and education are critical to

successful onsite wastewater management. Engaging the public in wastewater treatment issues helps

build support for funding, regulatory initiatives,

and other elements of a comprehensive program.

Educational activities directed at increasing

general awareness and knowledge of onsite management efforts can improve the probability that

simple, routine operation and maintenance tasks

(e.g., inspecting for pooled effluent, pumping the

tank) are carried out by system owners. Specialized

training is required for system managers responsible for operating and maintaining systems with

more complex components. Even conventional,

gravity-based systems require routine pumping,

monitoring, and periodic inspection of sludge and

scum buildup in septic tanks. Failing systems can

cause public health risks and environmental

damage and are expensive to repair. System owners

should be made aware of the need for periodically

removing tank sludge, maintaining system compo-
USEPA Onsite Wastewater Treatment Systems Manual 1-9

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

nents, and operating systems within their design

limitations to help maximize treatment effectiveness and extend the life of the systems.

Information regarding regular inspections, pumping, ground water threats from chemicals, hydraulic overloading from roof runoff or other clear

water sources, pollutant loads from garbage disposal

units, drain field protection, and warning signs of

failing systems can be easily communicated. Flyers,

brochures, posters, news media articles, and other

materials have proven effective in raising awareness and increasing public knowledge of onsite

wastewater management issues (see Resources

section). Meetings with stakeholders and elected

officials and face-to-face training programs for

homeowners can produce better results when

actions to strengthen programs are required

(USEPA, 1994). Public involvement and education

programs are often overlooked because they require

resources, careful planning, and management and

can be labor-intensive. However, these efforts can

pay rich dividends in building support for the

management agency and improving system performance. Public education and periodic public input

are also needed to obtain support for developing

and funding a wastewater utility or other comprehensive management program (see chapter 2).

1.5.2 Financial support

Funding is essential for successful management of

onsite systems. Adequate staff is required to

implement the components of the program and

objectively enforce the regulations. Without money

to pay for planning, inspection, and enforcement

staff, these activities will not normally be properly

implemented. Financial programs might be needed

to provide loans or cost-share grants to retrofit or

replace failing systems. Statewide public financing

programs for onsite systems like the PENNVEST

initiative in Pennsylvania provide a powerful

incentive for upgrading inadequate or failed

systems (Pennsylvania Infrastructure Investment

Authority, 1997). Regional cost-share programs

like the Triplett Creek Project in Kentucky, which

provided funding for new septic tanks and drain

field repairs, are also effective approaches for

addressing failed systems (USEPA, 1997). Chapter 2 and the Resources section provide more

information on funding options for onsite systems

and management programs.

Managing onsite systems is particularly challenging

in small, unincorporated communities without paid

staff. Programs staffed by trained volunteers and

regional “circuit riders” can help deliver technical

expertise at a low cost in these situations. Developing a program uniquely tailored to each community

requires partnerships, ingenuity, commitment, and

perseverance.

1.5.3 Support from elected officials

In most cases the absence of a viable oversight

program that addresses the full range of planning,

design, siting, permitting, installation, operation,

maintenance, and monitoring activities is the main

reason for inadequate onsite wastewater system

management. This absence can be attributed to a

number of factors, particularly a political climate in

which the value of effective onsite wastewater

management is dismissed as hindering economic

development or being too restrictive on rural

housing development. In addition, low population

densities, low incomes, underdeveloped management entities, a history of neglect, or other unique

factors can impede the development of comprehensive management programs. Focusing on the public

health and water resource impacts associated with

onsite systems provides an important perspective

for public policy discussions on these issues.

Sometimes state and local laws prevent siting or

design options that could provide treatment and

recycling of wastewater from onsite systems. For

example, some state land use laws prohibit using

lands designated as resource lands to aid in the

development of urban uses. Small communities or

rural developments located near state resource

lands are unable to use those lands to address

onsite problems related to space restrictions, soil

limitations, or other factors (Fogarty, 2000).

The most arbitrary siting requirement, however, is

the minimum lot size restriction incorporated into

Note: This manual is not intended to be used to

determine appropriate or inappropriate uses of land. The

information the manual presents is intended to be used to

select appropriate technologies and management

strategies that minimize risks to human health and water

resources in areas that are not connected to centralized

wastewater collection and treatment systems.
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-10 USEPA Onsite Wastewater Treatment Systems Manual

many state and local codes. Lot size limits prohibit

onsite treatment system installations on nonconforming lots without regard to the performance

capabilities of the proposed system. Lot size

restrictions also serve as an inappropriate but de

facto approach to land use planning in many

localities because they are often seen as establishing

the allowable number of housing units in a development without regard to other factors that might

increase or decrease that number.

When developing a program or regulation, the

common tendency is to draw on experience from

other areas and modify existing management plans

or codes to meet local needs. However, programs

that are successful in one area of the country might

be inappropriate in other areas because of differences in economic conditions, environmental

factors, and public agency structures and objectives.

Transplanting programs or program components

without considering local conditions can result in

incompatibilities and a general lack of effectiveness. Although drawing on the experience of others

can save time and money, local planners and health

officials need to make sure that the programs and

regulations are appropriately tailored to local

conditions.

Successful programs have site evaluation, inspection, and monitoring processes to ensure that

regulations are followed. Programs that have poor

inspection and monitoring components usually

experience low compliance rates, frequent complaints, and unacceptable performance results. For

example, some states do not have minimum standards applicable to the various types of onsite

systems being installed or do not require licensing

of installers (Suhrer, 2000). Standards and enforcement practices vary widely among the states, and

until recently there has been little training for local

officials, designers, or installers.

USEPA has identified more effective management

of onsite systems as a key challenge for efforts to

improve system performance (USEPA, 1997). In its

Response to Congress on Use of Decentralized

Wastewater Treatment Systems, USEPA noted that

“adequately managed decentralized wastewater

treatment systems can be a cost-effective and longterm option for meeting public health and water

quality goals, particularly for small towns and rural

areas.”

In addition, the Agency found that properly

managed onsite systems protect public health and

water quality, lower capital and maintenance costs

for low-density communities, are appropriate for

varying site conditions, and are suitable for ecologically sensitive areas (USEPA, 1997). However,

USEPA identified several barriers to the increased

use of onsite systems, including the lack of adequate

management programs. Although most communities

have some form of management program in place,

there is a critical lack of consistency. Many management programs are inadequate, underdeveloped, or

too narrow in focus, and they might hinder widespread public acceptance of onsite systems as

viable treatment options or fail to protect health

and water resources.

1.6 Performance-based

management of onsite

wastewater treatment systems

Performance-based management approaches have

been proposed as a substitute for prescriptive

requirements for system design, siting, and operation. In theory, such approaches appear to be both

irresistibly simple and inherently logical. In

practice, however, it is often difficult to certify the

performance of various treatment technologies

under the wide range of climates, site conditions,

hydraulic loads, and pollutant outputs they are

subjected to and to predict the transport and fate of

those pollutants in the environment. Despite these

difficulties, research and demonstration projects

conducted by USEPA, the National Small Flows

Clearinghouse, the National Capacity Development

Project, private consultants and engineering firms,

academic institutions, professional associations, and

public agencies have collectively assembled a body

of knowledge that can provide a framework for

developing performance-based programs. Performance ranges for many alternative systems operating

under a given set of climatic, hydrological, site, and

wastewater load conditions have been established.

The site evaluation process is becoming more

refined and comprehensive (see chapter 5) and has

moved from simple percolation tests to a more

comprehensive analysis of soils, restrictive horizons,

seasonal water tables, and other factors. New

technologies that incorporate lightweight media,

recirculation of effluent, or disinfection processes

have been developed based on performance.
USEPA Onsite Wastewater Treatment Systems Manual 1-11

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

A performance-based management program makes

use of recent developments to select and size

system technologies appropriate for the estimated

flow and strength of the wastewater at the site

where treatment is to occur. For sites with appropriate soils, ground water characteristics, slopes, and

other features, systems with subsurface wastewater

infiltration systems (SWISs) might be the best

option. Sites with inadequate soils, high seasonal

water tables, or other restrictions require alternative approaches that can achieve performance

objectives despite restrictive site features. Selecting proven system designs that are sized to treat the

expected wastewater load is the key to this approach. Installing unproven technologies on

provisional sites is risky even if performance

monitoring is to be conducted because monitoring

is often expensive and sometimes inconclusive.

1.6.1 Prescriptive management programs

Onsite system management has traditionally been

based on prescriptive requirements for system

design, siting, and installation. Installation of a

system that “complies” with codes is a primary

goal. Most jurisdictions specify the type of system

that must be installed and the types and depth of

soils that must be present. They also require

mandatory setbacks from seasonally high water

tables, property lines, wells, surface waters, and

other landscape features. Some of these requirements (e.g., minimum setback distances from

streams and reservoirs) are arbitrary and vary

widely among the states (Curry, 1998). The prescriptive approach has worked well in some

localities but has severely restricted development

options in many areas. For example, many regions

do not have appropriate soils, ground water tables,

slopes, or other attributes necessary for installation

of conventional onsite systems. In Florida, 74 percent

of the soils have severe or very severe limitations

for conventional system designs, based on USDA

Natural Resources Conservation Service criteria

(Florida HRS, 1993).

1.6.2 Hybrid management programs

Some jurisdictions are experimenting with performance-based approaches while retaining prescriptive requirements for technologies that have proven

effective under a known range of site conditions.

These prescriptive/performance-based or “hybrid”

programs represent a practical approach to onsite

system management by prescribing specific sets of

technologies or proprietary systems for sites where

they have proven to be effective and appropriate.

Regulatory entities review and evaluate alternative

systems to see if they are appropriate for the site

and the wastewater to be treated. Performancebased approaches depend heavily on data from

research, wastewater characterization processes,

site evaluations, installation practices, and expected operation and maintenance activities, and

careful monitoring of system performance is

strongly recommended. Programs that allow or

encourage a performance-based approach must

have a strong management program to ensure that

preinstallation research and design and

postinstallation operation, maintenance, and

monitoring activities are conducted appropriately.

Representatives from government and industry are

supporting further development of management

programs that can adequately oversee the full range

of OWTS activities, especially operation and

maintenance. The National Onsite Wastewater

Recycling Association (NOWRA) was founded in

1992 to promote policies that improve the market

for onsite wastewater treatment and reuse products.

NOWRA has developed a model framework for

onsite system management that is based on performance rather than prescriptive regulations. The

framework endorses the adoption and use of

alternative technologies that achieve public health

and environmental protection objectives through

innovative technologies and comprehensive

program management. (NOWRA, 1999)

1.7 Coordinating onsite system

management with watershed

protection efforts

During the past decade, public and private entities

involved in protecting and restoring water resources

have increasingly embraced a watershed approach

to assessment, planning, and management. Under

this approach, all the land uses and other activities

and attributes of each drainage basin or ground

water recharge zone are considered when conducting monitoring, assessment, problem targeting, and

remediation activities (see figure 1-5). A watershed
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-12 USEPA Onsite Wastewater Treatment Systems Manual

approach incorporates a geographic focus, scientific

principles, and stakeholder partnerships.

Because onsite systems can have significant impacts

on water resources, onsite/decentralized wastewater

management agencies are becoming more involved

in the watershed protection programs that have

developed in their regions. Coordinating onsite

wastewater management activities with programs

and projects conducted under a watershed approach

greatly enhances overall land use planning and

development processes. A cooperative, coordinated

approach to protecting health and water resources

can achieve results that are greater than the sum of

the individual efforts of each partnering entity.

Onsite wastewater management agencies are

important components of watershed partnerships,

and their involvement in these efforts provides

mutual benefits, operating efficiencies, and public

education opportunities that can be difficult for

agencies to achieve individually.

1.8 USEPA initiatives to improve

onsite system treatment and

management

In 1996 Congress requested USEPA to report on the

potential benefits of onsite/decentralized wastewater

treatment and management systems, the potential

costs or savings associated with such systems, and

the ability and plans of the Agency to implement

additional alternative wastewater system measures

within the current regulatory and statutory regime.

A year later USEPA reported that properly managed

onsite/decentralized systems offer several advantages over centralized wastewater treatment facilities (USEPA, 1997; see http://www.epa.gov/owm/

decent/response/index.htm). The construction and

maintenance costs of onsite/decentralized systems

can be significantly lower, especially in low-density

residential areas, making them an attractive alternative for small towns, suburban developments,

remote school and institutional facilities, and rural

regions. Onsite/decentralized wastewater treatment

systems also avoid potentially large transfers of

water from one watershed to another via centralized collection and treatment (USEPA, 1997).

USEPA reported that both centralized and onsite/

decentralized systems need to be considered when

upgrading failing systems. The report concluded

that onsite/decentralized systems can protect public

health and the environment and can lower capital

and maintenance costs in low-density communities.

They are also appropriate for a variety of site

conditions and can be suitable for ecologically

sensitive areas (USEPA, 1997). However, the

Agency also cited several barriers to implementing

more effective onsite wastewater management

programs, including the following:

• Lack of knowledge and public misperceptions

that centralized sewage treatment plants

perform better, protect property values, and are

more acceptable than decentralized treatment

systems.

• Legislative and regulatory constraints and

prescriptive requirements that discourage local

jurisdictions from developing or implementing

effective management and oversight functions.

Model framework for onsite wastewater management Model framework for onsite wastewater management

✔ Performance requirements that protect human health and the environment.

✔ System management to maintain performance within the established performance requirements.

✔ Compliance monitoring and enforcement to ensure system performance is achieved and maintained.

✔ Technical guidelines for site evaluation, design, construction, and operation and acceptable prescriptive designs

for specific site conditions and use.

✔ Education/training for all practitioners, planners, and owners.

✔ Certification/licensing for all practitioners to maintain standards of competence and conduct.

✔ Program reviews to identify knowledge gaps, implementation shortcomings, and necessary corrective actions.

Source: NOWRA, 1999.
USEPA Onsite Wastewater Treatment Systems Manual 1-13

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

• Splitting of regulatory authority, which limits

the evaluation of alternatives, and a lack of

management programs that consolidate planning, siting, design, installation, and maintenance activities under a single entity with the

resources and authority to ensure that performance requirements are met and performance

is maintained.

• Liability laws that discourage innovation, as

well as cost-based engineering fees that

discourage investment in designing innovative,

effective, low-cost systems.

• Grant guidelines, loan priorities, and other

financial or institutional barriers that prevent

rural communities from accessing funds,

considering alternative wastewater treatment

approaches, or creating management entities

that span the jurisdictions of multiple agencies.

USEPA is committed to elevating the standards of

onsite wastewater management practice and removing barriers that preclude widespread acceptance of

onsite treatment technologies. In addition, the Agency

is responding to calls to reduce other barriers to

onsite treatment by improving access to federal

funding programs, providing performance information on alternative onsite wastewater treatment

technologies through the Environmental Technology

Verification program (see http://www.epa.gov/etv/)

and other programs, partnering with other agencies

to reduce funding barriers, and providing guidance

through cooperation with other public agencies and

private organizations. USEPA supports a number of

efforts to improve onsite treatment technology

design, application, and funding nationwide. For

example, the National Onsite Demonstration Project

(NODP), funded by USEPA and managed by the

National Small Flows Clearinghouse at West

Virginia University, was established in 1993 to

encourage the use of alternative, decentralized

wastewater treatment technologies to protect public

health and the environment in small and rural

communities (see http://www.nesc.wvu.edu).

In addition, USEPA is studying ground water

impacts caused by large-capacity septic systems,

which might be regulated under the Class V Underground Injection Control (UIC) program. Largecapacity septic systems serve multiple dwellings,

business establishments, and other facilities and are

used to dispose of sanitary and other wastes through

subsurface application (figure 1-6). Domestic and

most commercial systems serving fewer than 20

persons are not included in the UIC program (see

http://www.epa.gov/safewater/uic/classv.html for

exceptions and limitations), but some commercial

facilities serving fewer than 20 people may be

regulated. States and tribes with delegated authority

are studying possible guidance and other programs

that reduce water resource impacts from these

systems. USEPA estimates that there are more than

350,000 large-capacity septic systems nationwide.

USEPA also oversees the management and reuse or

disposal of septic tank residuals and septage

through the Part 503 Rule of the federal Clean

Water Act. The Part 503 Rule (see http://

www.epa.gov/ owm/bio/503pe/) established

requirements for the final use or disposal of sewage

sludge when it is applied to land to condition the

soil or fertilize crops or other vegetation, deposited

at a surface disposal site for final disposal, or fired

in a biosolids incinerator. The rule also specifies

other requirements for sludge that is placed in a

municipal solid waste landfill under Title 40 of the

Code of Federal Regulations (CFR), Part 258. The

Part 503 Rule is designed to protect public health

Source: Ohio EPA, 1997.

Figure 1-5. The watershed approach planning and management cycle
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-14 USEPA Onsite Wastewater Treatment Systems Manual

and the environment from any reasonably anticipated adverse effects of certain pollutants and

contaminants that might be present in sewage

sludge, and it is consistent with USEPA’s policy of

promoting the beneficial uses of biosolids.

USEPA has also issued guidance for protecting

wellhead recharge areas and assessing threats to

drinking water sources under the 1996 amendments

to the Safe Drinking Water Act (see http://

www.epa.gov/safewater/protect.html and http://

www.epa.gov/safewater/whpnp.html). State source

water assessment programs differ because they are

tailored to each state’s water resources and drinking

water priorities. However, each assessment must

include four major elements:

• Delineating (or mapping) the source water

assessment area

• Conducting an inventory of potential sources

of contamination in the delineated area

• Determining the susceptibility of the water

supply to those contamination sources

• Releasing the results of the determinations to

the public

Local communities can use the information collected in the assessments to develop plans to

protect wellhead recharge areas and surface waters

used as drinking water sources. These plans can

include local or regional actions to reduce risks

associated with potential contaminant sources,

prohibit certain high-risk contaminants or activities

in the source water protection area, or specify other

management measures to reduce the likelihood of

source water contamination. Improving the performance and management of onsite treatment systems

can be an important component of wellhead and

source water protection plans in areas where nitrate

contamination, nutrient inputs, or microbial

Figure 1-6. Large-capacity septic tanks and other subsurface

discharges subject to regulation under the Underground Injection

Control Program and other programs

Integrating public and private entities with watershed management Integrating public and private entities with watershed management

In 1991 the Keuka Lake Association established a watershed project to address nutrient, pathogen, and other

pollutant loadings to the upstate New York lake, which provides drinking water for more than 20,000 people and

borders eight municipalities and two counties. The project sought to assess watershed conditions, educate the

public on the need for action, and foster interjurisdictional cooperation to address identified problems. The

project team established the Keuka Watershed Improvement Cooperative as an oversight committee composed

of elected officials from the municipalities and counties. The group developed an 8-page intermunicipal

agreement under the state home rule provisions (which allow municipalities to do anything collectively that they

may do individually) to formalize the cooperative and recommend new laws and policies for onsite systems and

other pollutant sources.

Voters in each municipality approved the agreement by landslide margins after an extensive public outreach

program. The cooperative developed regulations governing onsite system permitting, design standards,

inspection, and enforcement. The regulations carry the force of law in each town or village court and stipulate

that failures must be cited and upgrades required. Inspections are required every 5 years for systems within

200 feet of the lake, and alternative systems must be inspected annually. The cooperative coordinates its

activities with state and county health agencies and maintains a geographic information system (GIS) database

to track environmental variables and the performance of new technologies. The program is financed by onsite

system permit fees, some grant funds, and appropriations from each municipality’s annual budget.

Source: Shephard, 1996.
USEPA Onsite Wastewater Treatment Systems Manual 1-15

Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

contaminants are identified as potential risks to

drinking water sources.

1.9 Other initiatives to assist and

improve onsite management

efforts

Financing the installation and management of

onsite systems can present a significant barrier for

homeowners and small communities. USEPA and

other agencies have developed loan, cost-share, and

other programs to help homeowners pay for new

systems, repairs, or upgrades (see chapter 2). Some

of the major initiatives are the Clean Water State Revolving Fund (CWSRF), the Hardship Grant Program,

the Nonpoint Source Pollution Program, USDA Rural

Development programs, and the Community

Development Block Grant (CDBG) program.

The CWSRF is a low-interest or no-interest loan

program that has traditionally financed centralized,

publicly owned treatment works across the nation

(see http://www.epa.gov/owm/finan.htm). The

program guidance, issued in 1997, emphasizes that

the fund can be used as a source of support for the

installation, repair, or upgrading of OWTSs in

small-town, rural, and suburban areas. The CWSRF

programs are administered by states and the

territory of Puerto Rico and operate like banks.

Federal and state contributions are used to capitalize the fund, which makes low- or no-interest loans

for important water quality projects. Funds are then

repaid to the CWSRFs over terms as long as 20

years. Repaid funds are recycled to support other

water quality projects. Projects that might be

eligible for CWSRF funding include new system

installations and replacement or modification of

existing systems. Also covered are costs associated

with establishing a management entity to oversee

onsite systems in a region, including capital outlays

(e.g., for pumper trucks or storage buildings).

Approved management entities include city and

county governments, special districts, public or

private utilities, and private for-profit or nonprofit

corporations.

The Hardship Grant Program of the CWSRF was

developed in 1997 to provide additional resources

for improving onsite treatment in low-income

regions experiencing persistent problems with

onsite treatment because of financial barriers. The

new guidance and the grant program responded to

priorities outlined in the Safe Drinking Water Act

Amendments of 1996 and the Clean Water Action

Plan, which was issued in 1998.

The Nonpoint Source Pollution Program provides

funding and technical support to address a wide

range of polluted runoff problems, including

contamination from onsite systems. Authorized

under section 319 of the federal Clean Water Act

and financed by federal, state, and local contributions, the program provides cost-share funding for

individual and community systems and supports

broader watershed assessment, planning, and

management activities. Demonstration projects

funded in the past have included direct cost-share

for onsite system repairs and upgrades, assessment

of watershed-scale onsite wastewater contributions

to polluted runoff, regional remediation strategy

development, and a wide range of other projects

dealing with onsite wastewater issues. (See http://

www.epa.gov/OWOW/NPS for more information.)

The USEPA Office of Wastewater Management

supports several programs and initiatives related to

onsite treatment systems, including development of

guidelines for managing onsite and cluster systems

(see http://www.epa.gov/own/bio.htm). The

disposition of biosolids and septage pumped from

septic tanks is also subject to regulation by state

and local governments (see chapter 4).

The U.S. Department of Agriculture provides grant

and loan funding for onsite system installations

through USDA Rural Development programs. The

Rural Housing Service program (see http://

www.rurdev.usda.gov/rhs/Individual/

ind_splash.htm) provides direct loans, loan

guarantees, and grants to low or moderate-income

individuals to finance improvements needed to

make their homes safe and sanitary. The Rural

Utilities Service (http:www.usda.gov/rus/water/

programs.htm) provides loans or grants to public

agencies, tribes, and nonprofit corporations seeking

to develop water and waste disposal services or

decrease their cost.

The U.S. Department of Housing and Urban

Development (HUD) operates the Community

Development Block Grant Program, which provides annual grants to 48 states and Puerto Rico.

The states and Puerto Rico use the funds to award

grants for community development to small cities
Chapter 1: Background and Use of Onsite Wastewater Treatment Systems

1-16 USEPA Onsite Wastewater Treatment Systems Manual

and counties. CDBG grants can be used for numerous activities, including rehabilitation of residential and nonresidential structures, construction of

public facilities, and improvements to water and

sewer facilities, including onsite systems. USEPA is

working with HUD to improve system owners’

access to CDBG funds by raising program awareness, reducing paperwork burdens, and increasing

promotional activities in eligible areas. (More

information is available at http://www.hud.gov/

cpd/cdbg.html.)

The Centers for Disease Control and Prevention

(CDC) of the U.S. Public Health Service (see http://

www.cdc.gov) conduct research and publish studies

on waterborne infectious disease outbreaks and

illness linked to nitrate contamination of ground

water, both of which have been linked to OWTSs,

among other causes. Disease outbreaks associated

with contaminated, untreated ground water and

recreational contact with water contaminated by

pathogenic organisms are routinely reported to the

CDC through state and tribal infectious disease

surveillance programs.

Individual Tribal Governments and the Indian

Health Service (IHS) handle Indian wastewater

management programs. The IHS Sanitation Facilities Construction Program, within the Division of

Facilities and Environmental Engineering of the

Office of Public Health, is supported by engineers,

sanitarians, technicians, clerical staff, and skilled

construction workers. Projects are coordinated

through the headquarters office in Rockville,

Maryland, and implemented through 12 area offices

across the nation. The program works cooperatively

with tribes and tribal organizations, USEPA, HUD,

the USDA’s Rural Utilities Service, and other

agencies to fund sanitation and other services

throughout Indian Country (see http://

www.ihs.gov/nonmedicalprograms/dfee/reports/

rpt1998.pdf).

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93-IRM-4(A). Water Environment Research

Foundation, Alexandria, VA.
USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 2-1

Chapter 2: Management of Onsite Wastewater Treatment Systems

2.1 Introduction

Effective management is the key to ensuring that

the requisite level of environmental and public

health protection for any given community is

achieved. It is the single most important factor in

any comprehensive wastewater management

program. Without effective management, even the

most costly and advanced technologies will not be

able to meet the goals of the community. Numerous

technologies are currently available to meet a broad

range of wastewater treatment needs. Without

proper management, however, these treatment

technologies will fail to perform as designed and

efforts to protect public health and the environment

will be compromised.

The literature on OWTSs is replete with case

studies showing that adequate management is

critical to ensuring that OWTSs are sited, designed,

installed, and operated properly. As USEPA

pointed out in its Response to Congress on Use of

Chapter 2:

Management of Onsite Wastewater Treatment Systems

Decentralized Wastewater Treatment Systems

(1997), “Few communities have developed organizational structures for managing decentralized

wastewater systems, although such programs are

required for centralized wastewater facilities and

for other services (e.g., electric, telephone, water,

etc).”

Good planning and management are inseparable.

The capacity of the community to manage any

given technology should be factored into the

decision-making process leading to the planning

and selection of a system or set of systems appropriate for the community. As Kreissl and Otis noted

in New Markets for Your Municipal Wastewater

Services: Looking Beyond the Boundaries (1999),

appropriate technologies should be selected based

on whether they are affordable, operable, and

reliable. The selection of individual unit processes

and systems should, at a minimum, be based on

those three factors. Although managing OWTSs is

obviously far more complicated than assessing

whether the systems are affordable, operable and

reliable, an initial screening using these criteria is a

critical element of good planning.

Historically, the selection and siting of OWTSs has

been an inconsistent process. Conventional septic

tank and leach field systems were installed based on

economic factors, the availability of adequate land

area, and simple health-based measures aimed only

at preventing direct public contact with untreated

wastewater. Little analysis was devoted to understanding the dynamics of OWTSs and the potential

impacts on ground water and surface waters. Only

recently has there been an understanding of the

issues and potential problems associated with

2.1 Introduction

2.2 Elements of a successful program

2.3 Types of management entities

2.4 Management program components

2.5 Financial assistance for management programs and system installation

In recognition of the need for a comprehensive

management framework that communities can use in

developing and improving OWTS management

programs, USEPA is publishing Guidelines for

Management of Decentralized Wastewater Systems

(see http://www.epa.gov/owm/decent/index.htm). At

the time of the publication of this manual, the final

guidelines and accompanying guidance manual are

almost complete. USEPA envisions that tribes, states,

local governments, and community groups will use the

management guidelines as a reference to strengthen

their existing onsite/decentralized programs. The

guidelines include a set of recommended program

elements and activities and model programs that OWTS

program managers can refer to in evaluating their

management programs.
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2-2 USEPA Onsite Wastewater Treatment Systems Manual

failing to manage OWTSs in a comprehensive,

holistic manner.

Many case studies and reports from across the

country provide documentation that a significant

number of OWTSs lack adequate management

oversight, which results in inadequate pollutant

treatment (USEPA, 2000). The lack of system

inventories in many communities makes the task of

system management even more challenging.

As a result of the perception that onsite/decentralized systems are inferior, old-fashioned, less

technologically advanced, and not as safe as

centralized wastewater treatment systems from both

an environmental and public health perspective,

many communities have pursued the construction

of centralized systems (collection systems and

sewage treatment plants). Centralized wastewater

collection and treatment systems, however, are not

the most cost-effective or environmentally sound

option for all situations (e.g., sewage treatment

plants can discharge high point source loadings of

pollutants into receiving waters). They are costly to

build and operate and are often infeasible or costprohibitive, especially in areas with low populations and dispersed households. Many communities

lack both the revenue to fund these facilities and

the expertise to manage the treatment operations. In

addition, centralized treatment systems can contribute to unpredicted growth and development that

might threaten water quality.

As development patterns change and increased

development occurs in rural areas and on the urban

fringe, many communities are evaluating whether

they should invest in centralized sewage treatment

plants or continue to rely on OWTSs. The availability of innovative and alternative onsite technologies and accompanying management strategies

now provides small communities with a practical,

cost-effective alternative to centralized treatment

plants. For example, analysis included in USEPA’s

Response to Congress on Use of Decentralized

Wastewater Treatment Systems (1997) shows that

the costs of purchasing and managing an OWTS or

a set of individual systems can be significantly (22

to 80 percent) less than the cost of purchasing and

managing a centralized system.

Regardless of whether a community selects more

advanced decentralized systems, centralized systems, or some combination of the two, a comprehensive management program is essential. As

USEPA noted in Wastewater Treatment/Disposal for

Small Communities (1992), effective management

strategies depend on carefully evaluating all

feasible technical and management alternatives and

selecting appropriate solutions based on the needs

of the community, the treatment objectives, the

economic capacity, and the political and legislative

climate.

The management tasks listed have become increasingly complex, especially given the need to develop

a management strategy based on changing priorities

primarily driven by new development activities.

Rapid urbanization and suburbanization, the

presence of other sources that might discharge

nutrients and pathogens, water reuse issues, increasingly stringent environmental regulations, and

recognition of the need to manage on a watershed

basis increase the difficulty of this task. Multiple

objectives (e.g., attainment of water quality criteria,

protection of ground water, efficient and affordable

wastewater treatment) now must be achieved to

reach the overarching goal of maintaining economically and ecologically sound communities.

Investment by small communities in collection and

treatment systems increases taxes and costs to

consumers—costs that might be reduced substantially by using decentralized wastewater treatment

systems. From a water resource perspective achieving these goals means that public health, contact

recreation activities, fisheries, shellfisheries,

drinking water resources, and wildlife need to be

protected or restored. From a practical standpoint,

achieving these goals requires that the management

entity develop and implement a program that is

consistent with the goal of simultaneously meeting

and achieving the requirements of the Safe Drinking Water Act, the Clean Water Act, the Endangered Species Act, and other applicable federal,

state, tribal, and local requirements.

Changing regulatory contexts point to scenarios in

which system selection, design, and replacement

will be determined by performance requirements

tied to water quality standards or maximum

contamination limits for ground water. Cumulative

effects analyses and antidegradation policies might

be used to determine the level of technology and

management needed to meet the communities’

resource management goals. Comprehensive
USEPA Onsite Wastewater Treatment Systems Manual 2-3

Chapter 2: Management of Onsite Wastewater Treatment Systems

coordinated management programs are needed to

meet this challenge. These programs require

interdisciplinary consultations among onsite system

management entities, water quality

agencies, land use planners, engineers, wildlife

biologists, public health specialists, and others to

ensure that these goals and objectives are efficiently

achieved with a minimum of friction or program

overlap.

Fortunately, there are solutions. Technologies that

can provide higher levels of pollutant reduction

than were practical in the past appear to be

emerging. Better monitoring and assessment

methods are now available to determine the

effectiveness of specific technologies. Remote

sensing is possible to help monitor and understand

system operation, and more sophisticated inspection tools are available to complement visual

septic tank/SWIS inspections.

2.2 Elements of a successful

program

The success or failure of an onsite wastewater

management program depends significantly on

public acceptance and local political support;

adequate funding; capable and trained technical and

field staff; and clear and concise legal authority,

regulations, and enforcement mechanisms (Ciotoli

and Wiswall, 1982). Management programs should

include the following critical elements:

• Clear and specific program goals

• Public education and outreach

• Technical guidelines for site evaluation, design,

construction, and operation/maintenance

• Regular system inspections, maintenance, and

monitoring

• Licensing or certification of all service providers

• Adequate legal authority, effective enforcement

mechanisms, and compliance incentives

• Funding mechanisms

• Adequate record management

• Periodic program evaluations and revisions

Although all of these elements should be present in a

successful management program, the responsibility

for administering the various elements might fall on

a number of agencies or entities. Regardless of the

size or complexity of the program, its components

must be publicly accepted, politically feasible,

fiscally viable, measurable, and enforceable.

Many of the program elements discussed in this

chapter are described in more detail in the other

chapters of this manual. The elements described in

detail in this chapter are those essential to the

selection and adoption of a management program.

2.2.1 Clear and specific program goals

Developing and meeting program goals is critical

to program success. Management programs typically focus on two goals—protection of public

health and protection of the environment. Each

onsite system must be sited, designed, and managed

to achieve these goals.

Public health protection goals usually focus on

preventing or severely limiting the discharge of

pathogens, nutrients, and toxic chemicals to ground

water. Surface water bodies, including rivers, lakes,

streams, estuaries, and wetlands, can also be

adversely affected by OWTSs. Program goals

should be established to protect both surface and

ground water resources.

Public participation opportunities during

program planning and implementation

• Agreement on basic need for program

• Participation on committees, e.g., finance, technical,

educational

• Selection of a consultant or expert (request for

proposal, selection committee, etc.)

• Choosing the most appropriate options from the

options identified by a consultant or expert

• Obtaining financing for the preferred option

• Identifying and solving legal questions and issues

• Providing input for the enforcement/compliance plan

• Implementation and construction
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2-4 USEPA Onsite Wastewater Treatment Systems Manual

2.2.2 Public education and outreach

Public education

Public participation in and support for planning,

design, construction, and operation and maintenance requirements are essential to the acceptance

and success of an onsite wastewater management

program. Public meetings involving state and local

officials, property owners, and other interested

parties are an effective way to garner support for

the program. Public meetings should include

discussions about existing OWTS problems and

cover issues like program goals, costs, financing,

inspection, and maintenance. Such meetings

provide a forum for identifying community

concerns and priorities so that they can be considered in the planning process. Public input is also

important in determining management and compliance program structure, defining the boundaries of

the program, and evaluating options, their relative

requirements and impacts, and costs.

Public outreach

Educating homeowners about the proper operation

and maintenance of their treatment systems is an

essential program activity. In most cases, system

owners or homeowners are responsible for some

portion of system operation and maintenance or

for ensuring that proper operation and maintenance occurs through some contractual agreement.

The system owner also helps to monitor system

performance. Increased public support and

program effectiveness can be promoted by educating the public about the importance of OWTS

management in protecting public health, surface

waters, ground water resources, and property

values.

Onsite system owners are often uninformed about

how their systems function and the potential for

ground water and surface water contamination

from poorly functioning systems. Surveys show

that many people have their septic tanks pumped

only after the system backs up into their homes or

yards. Responsible property owners who are

educated in proper wastewater disposal and maintenance practices and understand the consequences of

system failure are more likely to make an effort to

ensure their systems are in compliance with operation and maintenance requirements. Educational

materials for homeowners and training courses for

designers, site evaluators, installers, inspectors, and

operation/maintenance personnel can help reduce

the impacts from onsite systems by reducing the

number of failing systems, which potentially

reduces or eliminates future costs for the system

owner and the management program.

2.2.3 Technical guidelines for site

evaluation, design, and

construction

The regulatory authority (RA) should set technical

guidelines and criteria to ensure effective and

functioning onsite wastewater systems. Guidelines

for site evaluation, system design, construction,

operation/maintenance, and inspection are necessary to maintain performance consistency. Site

evaluation guidelines should be used to determine

the site’s capability to accept the expected wastewater volume and quality. Guidelines and standards on

system design ensure the system compatibility with

the wastewater characteristics to be treated and its

structural integrity over the life of the system.

Construction standards should require that systems

conform to the approved plan and use appropriate

construction methods, materials, and equipment.

2.2.4 Regular system operation,

maintenance, and monitoring

An OWTS should be operated and maintained to

ensure that the system performs as designed for its

service life. Both individual systems and sets of

systems within a delineated management area

should be monitored to ensure proper performance

and the achievement of public health and environmental goals. A combination of visual, physical,

bacteriological, chemical, and remote monitoring

approaches can be used to assess system performance. Specific requirements for reporting to the

appropriate regulatory agency should also be

defined in a management program. The right to

enter private property to access and inspect components of the onsite system is also an essential

element of an effective management program.
USEPA Onsite Wastewater Treatment Systems Manual 2-5

Chapter 2: Management of Onsite Wastewater Treatment Systems

2.2.5 Licensing or certification of

service providers

Service providers include system designers, site

evaluators, installers, operation/maintenance

personnel, inspectors, and septage pumpers/haulers.

A qualifications program that includes certification

or licensing procedures for service providers should

be incorporated into a management program.

Licensing can be based on examinations that assess

basic knowledge, skills, and experience necessary

to perform services. Other components include

requirements for continuing education, defined

service protocols, and disciplinary guidelines or

other mechanisms to ensure compliance and

consistency. Many states already have, or are

planning, certification programs for some service

providers. These and other existing licensing

arrangements should be incorporated when they

complement the objectives of the management

program.

2.2.6 Adequate legal authority, effective

enforcement mechanisms, and

compliance incentives

Onsite wastewater management programs need a

combination of legal authorities, enforcement

mechanisms, and incentives to ensure compliance

and achievement of program goals. To ensure

program effectiveness, some program mechanisms

should be enforceable. Although the types of

mechanisms management entities use will vary by

program, the following mechanisms should be

enforceable: construction and operating permits,

requirements for performance bonds to ensure

proper construction or system operation and

maintenance, and licensing/certification requirements to ensure that service providers have the

necessary skills to perform work on treatment

systems. Management entities should also have the

authority to carry out repairs or replace systems

and, ultimately, to levy civil penalties. Enforcement programs, however, should not be based

solely on fines if they are to be effective. Information stressing public health protection, the monetary benefits of a clean environment, and the

continued functioning of existing systems (avoidance of system replacement costs) can provide

additional incentives for compliance. Finally, it

should be recognized that the population served by

the management program must participate in and

support the program to ensure sustainability.

2.2.7 Funding mechanisms

Funding is critical to the functioning of an effective OWTS management program. Management

entities should ensure that there is adequate funding

available to support program personnel, education

and outreach activities, monitoring and evaluation,

and incentives that promote system upgrades and

replacement. Funding might also be needed for

new technology demonstrations and other program

enhancements.

2.2.8 Adequate record management

Keeping financial, physical, and operational

records is an essential part of a management

program. Accurate records of system location and

type, operation and maintenance data, revenue

generated, and compliance information are necessary to enhance the financial, operational, and

regulatory health of the management program.

Electronic databases, spreadsheets, and geographic

information systems can help to ensure program

effectiveness and appropriate targeting of program

resources. At a minimum, program managers

should maintain records of system permits, design,

size, location, age, site soil conditions, complaints,

inspection results, system repairs, and maintenance

schedules. This information should be integrated

with land use planning at a watershed or wellhead

protection zone scale.

2.2.9 Periodic program evaluations and

revisions

Management programs for onsite systems are

dynamic. Changing community goals, resources,

environmental and public health concerns, development patterns, and treatment system technologies

require that program managers—with public

involvement—regularly evaluate program effectiveness and efficiency. Program managers might

need to alter management strategies because of

suburban sprawl and the close proximity of centralized collection systems. Resource and staff limitations might also necessitate the use of service

providers or designated management entities to
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2-6 USEPA Onsite Wastewater Treatment Systems Manual

ensure that systems in a jurisdiction are adequately

managed.

2.3 Types of management entities

Developing, implementing, and sustaining a

management program requires knowledge of the

political, cultural, and economic context of the

community, the current institutional structure, and

available technologies. Also required are clearly

defined environmental and public health goals and

adequate funding. A management program should

be based on the administrative, regulatory, and

operational capacity of the management entity and

the goals of the community. In many localities,

partnerships with other entities in the management

area (watershed, county, region, state, or tribal

lands) are necessary to increase the capacity of the

management program and ensure that treatment

systems do not adversely affect human health or

water resources. The main types of management

entities are federal, state, and tribal agencies; local

government agencies; special-purpose districts and

public utilities; and privately owned and operated

management entities. Descriptions of the various

types of management entities are provided in the

following subsections.

2.3.1 Federal, state, tribal, and local

agencies

Federal, state, tribal, and local governments have

varying degrees of authority and involvement in the

development and implementation of onsite wastewater management programs. In the United States,

tribal, state, and local governments are the main

entities responsible for the promulgation and enforcement of OWTS-related laws and regulations. Many of

these entities provide financial and technical assistance. Tribal, state, and local authority determines the

degree of control these entities have in managing

onsite systems. General approaches and responsibilities are shown in table 2-1.

At the federal level, USEPA is responsible for

protecting water quality through the implementation of the Clean Water Act (CWA), the Safe

Drinking Water Act (SDWA), and the Coastal Zone

Act Reauthorization Amendments (CZARA). Under

these statutes, USEPA administers a number of

programs that affect onsite system management.

The programs include the Water Quality Standards

Program, the Total Maximum Daily Load Program, the Nonpoint Source Management Program,

the National Pollutant Discharge Elimination

System (NPDES) Program, the Underground

Injection Control (UIC) Program, and the Source

Water Protection Program. Under the CWA and the

Twelve problems that can aff lems that can affect OWTS management programs

1. Failure to adequately consider site-specific environmental conditions (site evaluations)

2. Codes that thwart system selection or adaptation to difficult local site conditions and that do not

allow the use of effective innovative or alternative technologies

3. Ineffective or nonexistent public education and training programs

4. Failure to include water conservation and reuse

5. Ineffective controls on operation and maintenance of systems

6. Lack of control over residuals management

7. Lack of OWTS program monitoring and evaluation, including OWTS inspection and monitoring

8. Failure to consider the special characteristics and requirements of commercial, industrial, and

large residential systems

9. Weak compliance and enforcement programs

10. Lack of adequate funding

11. Lack of adequate legal authority

12. Lack of adequately trained and experienced personnel

Source: Adapted from USEPA, 1986.
USEPA Onsite Wastewater Treatment Systems Manual 2-7

Chapter 2: Management of Onsite Wastewater Treatment Systems

Table 2-1. Organizational approaches, responsibilities, and other considerations for managing onsite systems

Source: Ciotoli and Wiswall, 1982.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-8 USEPA Onsite Wastewater Treatment Systems Manual

SDWA, USEPA has the authority to directly

regulate specific categories of onsite systems under

the UIC and NPDES programs. The CZARA

section 6217 Coastal Nonpoint Source Program

requires the National Oceanic and Atmospheric

Administration (NOAA) and USEPA to review and

approve upgraded state coastal nonpoint source

programs to meet management measures for new

and existing OWTSs. These measures address

siting, designing, installing, maintaining, and

protecting water quality. See chapter 1 for additional information and Internet web sites.

State and tribes might manage onsite systems

through various agencies. Typically, a state or tribal

public health office is responsible for managing

onsite treatment systems. Regulation is sometimes

centralized in one state or tribal government office

and administered from a regional or local state

office. In most states, onsite system management

responsibilities are delegated to the county or

municipal level. Where such delegation occurs, the

state might exercise varying degrees of local

program oversight.

Leadership and delegation of authority at the state

level are important in setting technical, management, and performance requirements for local

programs. In states where local governments are

responsible for managing onsite systems, state

authority often allows flexibility for local programs

to set program requirements that are appropriate

for local conditions and management structures as

long as the local program provides equal or greater

protection than that of state codes. Statewide

consistency can be promoted by establishing

• Administrative, managerial, and technological

requirements

• Performance requirements for natural resource

and public health protection

• Requirements for monitoring and laboratory

testing

• Education and training for service providers

• Technical, financial, and administrative support

• Periodic program reviews and evaluations

• Enforcement of applicable regulations

Many states set minimum system design and siting

requirements for onsite systems and are actively

involved in determining appropriate technologies.

Other states delegate some or all of this authority to

local governments. Some states retain the responsibility for the administrative or technical portions of

the onsite management program; in these states, the

local governments’ primary role is to implement

the state requirements.

2.3.2 Local government agencies

In many states, local governments have the responsibility for onsite wastewater program management. These local management programs are

administered by a variety of municipal, county, or

district-level agencies. The size, purpose, and

authority of county, township, city, or village

government units vary according to each state’s

statutes and laws. Depending on the size of the

jurisdiction and the available resources, an onsite

wastewater management program can be administered by a well-trained, fully staffed environmental

or public health agency or by a board composed of

local leaders. In some states, some or most of the

responsibility for onsite system management is

delegated by the legislature to local governments.

In states with “home rule” provisions, local units of

government have the authority to manage onsite

systems without specific delegation by the state

legislature. Some local home rule governments also

have the power to enter into multiple agency or

jurisdictional agreements to jointly accomplish any

home rule function without any special authority

from the state (Shephard, 1996).

County governments can be responsible for a

variety of activities regarding the management of

onsite systems. A county can assume responsibility

for specific activities, such as OWTS regulation,

within its jurisdiction, or it can supplement and

support existing state, city, town, or village wastewater management programs with technical,

financial, or administrative assistance. Counties can

provide these services through their normal operational mechanisms (e.g., a county department or

agency), or they can establish a special district to

provide designated services to a defined service

area. County agency responsibilities might include

• Adoption of state minimal requirements or

development of more stringent requirements

• Planning, zoning, and general oversight of

proposed development

• Review of system designs, plans, and installation practices
USEPA Onsite Wastewater Treatment Systems Manual 2-9

Chapter 2: Management of Onsite Wastewater Treatment Systems

• Permitting of systems and construction oversight

• Inspection, monitoring, and enforcement

• Reports to public and elected officials

Township, city, or village governments can be

responsible for planning, permitting, and operating onsite wastewater facilities and enforcing

applicable regulations. The precise roles and

responsibilities of local governments depend on

the preferences, capabilities, and circumstances

of each jurisdiction. Because of the variability in

state enabling legislation and organizational

structures, the administrative capacity, jurisdiction, and authority of local entities to manage

onsite wastewater systems vary considerably.

2.3.3 Special-purpose districts and

public utilities

The formation of special-purpose districts and

public utilities is usually enabled by state law to

provide public services that local governments do

not or cannot provide. A special-purpose district

or public utility is a quasigovernmental entity

established to provide specific services or to

conduct activities specified by the enabling

legislation. Special districts (e.g., sanitation

districts) provide single or multiple services, such

as managing planning and development activities,

conducting economic development programs,

improving local conditions, and operating drinking

water and wastewater treatment facilities. The

territory serviced by this entity is variable and can

include a single community, a portion of a community, a group of communities, parts of several

communities, an entire county, or a regional area.

State enabling legislation usually outlines the

authority, structure, and operational scope of the

district, including service area, function, organizational structure, financial authority, and performance criteria.

Special-purpose districts and public utilities are

usually given sufficient financial authority to apply

for or access funds, impose service charges, collect

fees, impose special assessments on property, and

issue revenue or special assessment bonds. Some

special-purpose districts have the same financing

authority as municipalities, including the authority

to levy taxes and incur general obligation debt.

These districts are usually legal entities that might

enter into contracts, sue, or be sued. There might

be situations where eminent domain authority is

needed to effectively plan and implement onsite

programs. Special-purpose districts and public

Sanitation district management of onsite systems: New Mexico

Onsite systems in the community of Peña Blanca, New Mexico, are managed by the Peña Blanca Water and

Sanitation District, which is organized under state statutes that require a petition signed by 25 percent of the

registered voters and a public referendum before a district may be formed. Once formed, water and sanitation

districts in New Mexico are considered subdivisions of the state and have the power to levy and collect ad

valorem taxes and the right to issue general obligation and revenue bonds.

Residents and public agency officials in Peña Blanca sought to improve the management of systems in the

community after a 1985 study found that 86 percent of existing systems required upgrades, repair, or

replacement. The water and sanitation district was designated as the lead agency for managing OWTSs

because it already provided domestic water service to the community and had an established administrative

structure. The sanitation district relies on the New Mexico Environment Department to issue permits and monitor

installation, while the district provides biannual pumping services through an outside contractor for a monthly fee

of $10.64 for a 1,000-gallon tank. The district also supervises implementation of the community’s onsite system

ordinance, which prohibits untreated and unauthorized discharges, lists substances that might not be discharged

into onsite systems (e.g., pesticides, heavy metals), and provides for sampling and testing. Penalties for

noncompliance are set at $300 per violation and not more than 90 days imprisonment. Liens might be placed on

property for nonpayment of pumping fees.

The program has been in operation since 1991 and serves nearly 200 homes and businesses. Septage pooling

on ground surfaces, a problem identified in the 1985 study, has been eliminated.

Source: Rose, 1999.
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2-10 USEPA Onsite Wastewater Treatment Systems Manual

utilities will most likely have to work closely with

state or local authorities when program planning or

implementation requires the use of this authority.

Special districts and public utilities can be an effective

option for managing onsite systems. The special

district and public utility models have been adopted

successfully in many states. A good example is the

creation of water districts and sanitation districts,

which are authorized to manage and extend potable

water lines and extend sewerage service in areas

near centralized treatment plants. The development

of onsite system management functions under the

authority of existing sanitation districts provides

support for planning, installation, operation,

maintenance, inspection, enforcement, and financing of these programs. Traditional onsite management entities (e.g., health departments) can partner

with sanitation or other special districts to build a

well-integrated program. For example, a health

department could retain its authority to approve

system designs and issue permits while the sanitation district could assist with regional planning and

conduct inspection, maintenance, and remediation/

repair activities.

In some areas, special districts or public utilities

have been created to handle a full range of management activities, from regional planning and system

permitting to inspection and enforcement. In 1971

the City of Georgetown, California, developed and

implemented a comprehensive, community-wide

onsite management program in the Lake Auburn

Trails subdivision (Shephard, 1996). The district

does not own the onsite systems in the subdivision

but is empowered by the state and county governments to set performance requirements, review and

approve system designs, issue permits, oversee

construction, access treatment system sites to

conduct monitoring, and provide routine maintenance. The initial permit fees were approximately

$550. Annual fees in 1995 were approximately

$170 per dwelling and $80 for undeveloped lots

(Shephard, 1996).

Onsite management districts or public utilities,

whether wholly or partially responsible for system

oversight, can help ensure that treatment systems

are appropriate for the site and properly planned,

designed, installed, and maintained. Typical goals

for the management district or utility might include

• Providing appropriate wastewater collection/

treatment service for every residence or business

• Integrating wastewater management with land

use and development policies

• Managing the wastewater treatment program at

a reasonable and equitable cost to users

Management districts and public utilities generally

are authorized to generate funds from a variety of

sources for routine operation and maintenance,

inspections, upgrades, and monitoring and for

future development. Sources of funds can include

initial and renewable permit fees, monthly service

charges, property assessments, and special fees.

Onsite wastewater management districts that are

operated by or closely allied with drinking water

supply districts can coordinate collection of system

service charges with monthly drinking water bills

in a manner similar to that used by centralized

wastewater treatment plants. Although some homeowners might initially resist fees and other charges

that are necessary to pay for wastewater management services, outreach information on the efficiencies, cost savings, and other benefits of cooperative management (e.g., financial support for

system repair, upgrade, or replacement and no-cost

pumping and maintenance) can help to build

support for comprehensive programs. Such support

is especially needed if a voter referendum is

required to create the management entity. When

creating a new district, public outreach and stakeholder involvement should address the following

topics:

• Proposed boundaries of the management district

• Public health and natural resource protection

issues

• Problems encountered under the current management system

• Performance requirements for treatment systems

• Onsite technologies appropriate for specific site

conditions

• Operation and maintenance requirements for

specific system types

• Septage treatment and sewage treatment plant

capacity to accept septage

• Cost estimates for management program components

• Program cost and centralized system management cost comparisons
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Chapter 2: Management of Onsite Wastewater Treatment Systems

• Potential program partners and inventory of

available resources

• Proposed funding source(s)

• Compliance and enforcement strategies

• Legal, regulatory, administrative, and managerial actions to create, develop, or establish the

management entity

Another type of special district is the public

authority. A public authority is a corporate body

chartered by the state legislature with powers to

own, finance, construct, and operate revenueproducing public facilities. A public authority can

be used in a variety of ways to construct, finance,

and operate public facilities, including OWTSs.

It should be noted that some state codes restrict or

disallow a managed group of special districts from

managing onsite systems. In other cases, clear legal

authority for program staff to enter private property to perform inspections and correct problems

has not been provided. These limitations can be

addressed through special legislation authorizing

the creation of entities with explicit onsite management responsibilities. Laws and regulations can also

be changed to provide special districts the authority

to manage onsite systems and to conduct inspection, maintenance, and remediation activities.

2.3.4 Privately owned and operated

management entities

Private sector management entities are another

option for ensuring OWTS are properly managed.

These entities are often responsible for system

design, installation, operation, and maintenance. In

some cases, these private firms also serve as the

sole management entity; for example, a firm might

manage an onsite system program for a residential

subdivision as a part of a public-private partnership. Several options exist for public/private

partnerships in the management of onsite systems.

OWTS management programs can contract with

private firms to perform clearly defined tasks for

which established protocols exist, such as site

evaluation, installation, monitoring/inspection, or

maintenance. An example of such an arrangement

would be to contract with a licensed/certified

provider, such as a trained septage pumper/hauler

who could be responsible for system inspection,

maintenance, and record keeping. Another example

would be the case where treatment systems in

residential subdivisions are serviced by a private

entity and operated under a contract with the

subdivision or neighborhood association.

Private for-profit corporations or utilities that

manage onsite systems are often regulated by the

state public utility commission to ensure continuDevelopment company creates a service district in Colorado

The Crystal Lakes Development Company has been building a residential community 40 miles northwest of Fort

Collins, Colorado, since 1969. In 1972 the company sponsored the creation of the Crystal Lakes Water and

Sewer Association to provide drinking water and sewage treatment services. Membership in the association is

required of all lot owners, who must also obtain a permit for onsite systems from the Larimer County Health

Department. The association enforces county health covenants, aids property owners in the development of

onsite water and wastewater treatment systems, monitors surface and ground water, and has developed

guidelines for inspecting onsite water and wastewater systems. System inspections are conducted at the time

of property transfer.

The association conducts preliminary site evaluations for proposed onsite systems, including inspection of a

backhoe pit excavated by association staff with equipment owned by the association. The county health

department has also authorized the association to design proposed systems. The association currently

manages systems for more than 100 permanent dwellings and 600 seasonal residences. Management services

are provided for all onsite systems in the development, including 300 holding tanks, 7 community vault toilets,

recreational vehicle dump stations, and a cluster system that serves 25 homes on small lots and the

development’s lodge, restaurant, and office buildings. The association is financed by annual property owner

dues of $90 to $180 and a $25 property transfer fee, which covers inspections.

Source: Mancl, 1999.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-12 USEPA Onsite Wastewater Treatment Systems Manual

ous, acceptable service at reasonable rates. Service

agreements are usually required to ensure private

organizations will be financially secure, provide

adequate service, and be accountable to their

customers. These entities can play a key role in

relieving the administrative and financial burden on

local government by providing system management

services. It is likely that in the future private firms

will build, own, and operate treatment systems and

be subject only to responsible administrative

oversight of the management entity.

2.3.5 Regulatory authorities and

responsible management entities

Most regulatory authorities (e.g., public health

departments and water quality authorities) lack

adequate funding, staff, and technical expertise to

develop and implement comprehensive onsite

system management programs. Because of this lack

of resources and trained personnel, program

managers across the country are considering or

implementing alternative management structures

that delegate responsibility for specified management program elements to other entities. Hoover

and Beardsley (2000) recommend that management

entities develop alliances with public and private

organizations to establish environmental quality

goals, evaluate treatment system performance

information, and promote activities that ensure

onsite system management programs meet performance requirements.

English and Yeager (2001) have proposed the

formation of responsible management entities

(RMEs) to ensure the performance of onsite and

other decentralized (cluster) wastewater treatment

systems. RMEs are defined as legal entities that

have the technical, managerial, and financial

capacity to ensure viable, long-term, cost-effective

centralized management, operation, and maintenance of all systems within the RME’s jurisdiction.

Viability is defined as the capacity of the RME to

protect public health and the environment efficiently and effectively through programs that focus

on system performance rather than adherence to

prescriptive guidelines (English and Yeager, 2001).

RMEs can operate as fully developed management

programs under existing oversight programs (e.g.,

health departments, sanitation districts) in states

with performance-based regulations, and they are

usually defined as comprehensive management

entities that have the managerial, technical, and

financial capacity to ensure that proposed treatment

system applications will indeed achieve clearly

defined performance requirements. System technology performance information can be ranked along

a continuum that gives greater weight to confirmatory studies, peer-reviewed assessments, and third

party analysis of field applications. Under this

approach, unsupported performance assertions by

vendors and results from limited field studies

Responsibilities of a Comprehensive Onsite e Onsite e Onsite Wastewater Management Program

• Power to propose legislation and establish and enforce program rules and regulations

• Land use planning involvement, review and approval of system designs, permit issuance

• Construction and installation oversight

• Routine inspection and maintenance of all systems

• Management and regulation of septage handling and disposal

• Local water quality monitoring

• Administrative functions (e.g., bookkeeping, billing)

• Grant writing, fund raising, staffing, outreach

• Authority to set rates, collect fees, levy taxes, acquire debt, issue bonds, make purchases

• Authority to obtain easements for access to property, enforce regulations, require repairs

• Education, training, certification, and licensing programs for staff and contractors

• Record keeping and database maintenance

Source: NSFC, 1996.
USEPA Onsite Wastewater Treatment Systems Manual 2-13

Chapter 2: Management of Onsite Wastewater Treatment Systems

receive less emphasis in management entity evaluations of proposed treatment technologies (Hoover

and Beardsley, 2001).

Management responsibilities can be assigned to an

entity designated by the state or local government

to manage some or all of the various elements of

onsite wastewater programs. The assignment of

management responsibilities to a comprehensive

RME or to some less-comprehensive management

entity (ME) appears to be a practical solution to the

dilemma of obtaining adequate funding and

staffing to ensure that critical management activities occur. The use of an RME, however, makes

developing and implementing an onsite management program more complex. Increased coordination and planning are necessary to establish an

effective management program. All of the management program activities described below can be

performed by an RME; some may be executed by a

management entity with a smaller scope of capabilities. In jurisdictions where management program responsibilities are delegated to an RME, the

regulatory authority (RA; e.g., local health department) must oversee the RME to ensure that the

program achieves the comprehensive public health

and environmental goals of the community. Depending on state and local codes, a formal agreement or some other arrangement between the RME

and the RA might be required for RME execution

of some program elements, such as issuing permits.

The accompanying text insert, adapted from the

National Small Flows Clearinghouse (1996),

contains an example of activities that a comprehensive RME typically must incorporate into its

management program. It should be noted that the

involvement of an ME to perform some management program tasks or an RME to perform the full

range of management tasks should be tailored to

each local situation. Given the evolving nature of

onsite wastewater management programs, activities

in some cases might be performed by an RME,

such as an onsite system utility or private service

provider. In other cases, these responsibilities might

be divided among several state or local government

agencies, such as the local public health department, the regional planning office, and the state

water quality agency. Changes in management

strategies (movement toward performance-based

approaches, institution of model management

structures) have resulted in the addition of other

responsibilities, which are discussed later in this

section.

When a less-comprehensive ME conducts a specified set of these activities, the RA usually retains

the responsibility for managing some or all of the

following activities:

• Defining management responsibilities for the

RA and the ME

• Overseeing the ME

• Issuing permits

• Inspecting onsite systems

• Responding to complaints

• Enforcement and compliance actions

• Monitoring receiving water quality (surface and

ground water)

• Regulation of septage handling and disposal

• Licensing and certification programs

• Keeping records and managing databases for

regulatory purposes

• Coordinating local and regional planning efforts

The RA, however, will often delegate to the ME

the responsibility for implementing some of the

activities listed above. The activities delegated to

the ME will be determined by the capacity of the

ME to manage specific activities, the specific

public health and environmental problems to be

addressed by the ME, and the RA’s legal authority

to delegate some of those activities. For example, if

the ME is an entity empowered to own and operate

treatment systems in the service area, the ME

typically would be responsible for all aspects of

managing individual systems, including setting

fees, designing and installing systems, conducting

inspections, and monitoring those systems to ensure

that the RA’s performance goals are met. Otis,

McCarthy, and Crosby (2001) have presented a

framework appropriate for performance management that illustrates the concepts discussed above.

2.4 Management program

components

Developing and implementing an effective onsite

wastewater management program requires that a

systematic approach be used to determine necessary

program elements. Changes and additions to the
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-14 USEPA Onsite Wastewater Treatment Systems Manual

management program should be based on evaluations of the program to determine whether the

program has adequate legal authorities, funding,

and management capacity to administer both

existing and new OWTSs and respond to changing

environmental and public health priorities and

advances in OWTS technologies.

The management program elements described in

the following sections are common to the most

comprehensive onsite management programs (e.g.,

RMEs). USEPA recognizes that states and local

governments are at different stages along the

continuum of developing and implementing

comprehensive management programs that address

their communities’ fiscal, institutional, environmental, and public health goals.

2.4.1 Authority for regulating and

managing onsite treatment

systems

Onsite wastewater program managers should

identify all legal responsibilities of the RA that

might affect the implementation of an effective

program. Legal responsibilities can be found in

state and local statutes, regulations, local codes,

land use laws, and planning requirements. Other

legal mechanisms such as subdivision covenants,

private contracts, and homeowner association rules

might also affect the administration of the program. In many jurisdictions, legal authorities that

do not specifically refer to onsite programs and

authorities, such as public nuisance laws, state

water quality standards, and public health laws,

might be useful in implementing the program. A

typical example would be a situation where the

public health agency charged with protecting

human health and preventing public nuisances

interprets this mandate as sufficient authorization to

require replacement or retrofit of onsite system that

have surface seepage or discharges.

The extent and interpretation of authority assigned

to the RA will determine the scope of its duties, the

funding required for operation, and the personnel

necessary to perform its functions. In many jurisdictions, the authority to perform some of these

activities might be distributed among multiple RAs.

Typical A ypical Aypical Authorities of a Regulator ities of a Regulatory Authority

• Develop and implement policy and regulations

• Provide management continuity

• Enforce regulations and program requirements through fines or incentives

• Conduct site and regional-scale evaluations

• Require certification or licensing of service providers

• Oversee system design review and approval

• Issue installation and operating permits

• Oversee system construction

• Access property for inspection and monitoring

• Inspect and monitor systems and the receiving environment

• Finance the program through a dedicated funding source

• Charge fees for management program services (e.g., permitting, inspections)

• Provide financial or cost-share assistance

• Issue and/or receive grants

• Develop or disseminate educational materials

• Provide training for service providers and staff

• Conduct public education and involvement programs

• Hire, train, and retain qualified employees
USEPA Onsite Wastewater Treatment Systems Manual 2-15

Chapter 2: Management of Onsite Wastewater Treatment Systems

Where this is the case, the organizations involved

should have the combined authority to perform all

necessary activities and should coordinate their

activities to avoid program gaps, redundancy, and

inefficiency. In some cases, the RA might delegate

some of these responsibilities to an ME. When a

comprehensive set of responsibilities are delegated

to an RME, the RA should retain oversight and

enforcement authority to ensure compliance with

legal, performance, and other requirements.

Each state or local government has unique organizational approaches for managing onsite wastewater

systems based on needs, perceptions, and circumstances. It is vitally important that the authorizing

legislation, regulations, or codes allow the RAs and

MEs to develop an institutional structure capable of

fulfilling mandates through adoption of appropriate

technical and regulatory programs. A thorough

evaluation of authorized powers and capabilities at

various levels and scales is necessary to determine

the scope of program authority, the scale at which

RAs and MEs can operate, and the processes they

must follow to enact and implement the management program. Involving stakeholders who represent public health entities, environmental groups,

economic development agencies, political entities,

and others in this process can ensure that the lines

and scope of authority for an onsite management

program are well understood and locally supported.

In some cases, new state policies or regulations

must be implemented to allow for recognition of

onsite MEs.

2.4.2 Onsite wastewater management

program goals

Developing and implementing an effective management program requires first establishing program

goals. Program goals should be selected based on

public health, environmental, and institutional

factors and public concerns. Funding availability,

institutional capability, and the need to protect

consumers and their interests typically affect the

selection of program goals and objectives. One or

more entities responsible for public health and

environmental protection, such as public health and

water quality agencies, can determine the goals.

The development of short- and long-term comprehensive goals will most likely require coordination

among these entities. Community development and

planning agencies as well as residents should also

play a role in helping to determine appropriate

goals.

Traditionally, the main goals of most onsite

management programs have been to reduce risks to

public health (e.g., prevent direct public contact

with sewage and avoid pathogenic contamination of

ground water and surface waters); abate public

nuisances (e.g., odors from pit privies and cesspools); and provide cost-effective wastewater

treatment systems and management programs.

More recently, there has been an increased focus on

preventing OWTS-related surface and ground

water quality degradation and impacts on aquatic

habitat. Program goals have been expanded to

address nutrients, toxic substances, and a broader

set of public health issues regarding pathogens.

Onsite wastewater-related nutrient enrichment

leading to algae blooms and eutrophication or low

dissolved oxygen levels in surface waters is of

concern, especially in waters that lack adequate

assimilative capacity, such as lakes and coastal

embayments or estuaries. The discharge of toxic

substances into treatment systems and eventually

into ground water has also become a more prominent concern, especially in situations where onsite/

decentralized treatment systems are used by commercial or institutional entities like gasoline service

stations and nursing homes. The potential impacts

from pathogens discharged from OWTS on shellfisheries and contact recreation activities have also

moved some OWTS program managers to adopt

goals to protect these resources.

Historically, in many jurisdictions the public health

agency has had the primary role in setting program

goals. Without documented health problems

implicating onsite systems as the source of

problem(s), some public health agencies have had

little incentive to strengthen onsite management

programs beyond the goals of ensuring there was

no direct public contact with sewage or no obvious

drinking water-related impacts, such as bacterial or

chemical illnesses like methemoglobinemia (“blue

baby syndrome”). The availability of more advanced assessment and monitoring methodologies

and technologies and a better understanding of

surface water and ground water interactions,

however, has led to an increased focus on protecting water quality and aquatic habitat. As a result, in

many states and localities, water quality agencies

have become more involved in setting onsite
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-16 USEPA Onsite Wastewater Treatment Systems Manual

program goals and managing onsite wastewater

programs. Some water quality agencies (e.g.,

departments of natural resources), however, lack

direct authority or responsibility to regulate onsite

systems. This lack of authority points to the need

for increased coordination and mutual goal setting

among health agencies that have such authority.

Regardless of which agency has the legal authority

to manage onsite systems, there is the recognition

that both public health and water quality goals need

to be incorporated into the management program’s

mission. Achievement of these goals requires a

comprehensive watershed-based approach to ensure

that all of the program’s goals are met. Partnerships

with multiple agencies and other entities are often

required to integrate planning, public health

protection, and watershed protection in a meaningful way. Because of the breadth of the issues

affecting onsite system management, many programs depend on cooperative relationships with

planning authorities, environmental protection and

public health agencies, universities, system manufacturers, and service providers to help determine

appropriate management goals and objectives.

2.4.3 Public health and resource

protection goals

OWTS programs should integrate the following

types of goals: public health protection, abatement

of nuisances, ground and surface water resource

protection, and aquatic ecosystem protection.

Setting appropriate program goals helps onsite

program managers determine desired performance

goals for treatment systems and influence siting,

design, and management criteria and requirements.

Examples of more detailed goals follow.

Public health protection goals:

• Reduce health risk due to sewage backup in

homes.

• Prevent ground water and well water contamination due to pathogens, nitrates, and toxic

substances.

• Prevent surface water pollution due to pathogens, nutrients, and toxic substances.

• Protect shellfish habitat and harvest areas from

pathogenic contamination and excessive nutrients

• Prevent sewage discharges to the ground surface

to avoid direct public contact.

• Minimize risk from reuse of inadequately

treated effluent for drinking water, irrigation, or

other uses.

• Minimize risk from inadequate management of

septic tank residuals.

• Minimize risk due to public access to system

components.

Public nuisance abatement goals:

• Eliminate odors caused by inadequate plumbing

and treatment processes.

• Eliminate odors or other nuisances related to

transportation, reuse, or disposal of OWTS

residuals (septage).

Environmental protection goals:

• Prevent and reduce adverse impacts on water

resources due to pollutants discharged to onsite

systems, e.g., toxic substances.

• Prevent and reduce nutrient overenrichment of

surface waters.

• Protect sensitive aquatic habitat and biota

2.4.4 Comprehensive planning

Comprehensive planning for onsite systems has

three important components: (1) establishing and

implementing the management entity, (2) establishing internal planning processes for the management

entity, and (3) coordination and involvement in the

broader land-use planning process. Comprehensive

The Department of Environmental Resources and

Health Department in Maryland’s Prince George’s

County worked together to develop geographic

information system (GIS) tools to quantify and

mitigate nonpoint source nutrient loadings to the

lower Patuxent River, which empties into the

Chesapeake Bay. The agencies developed a

database of information on existing onsite systems,

including system age, type, and location, with

additional data layers for depth to ground water

and soils. The resulting GIS framework allows users

to quantify nitrogen loadings and visualize likely

impacts under a range of management scenarios.

Information from GIS outputs is provided to

decision makers for use in planning development

and devising county management strategies.

Source: County Environmental Quarterly, 1997.
USEPA Onsite Wastewater Treatment Systems Manual 2-17

Chapter 2: Management of Onsite Wastewater Treatment Systems

planning provides a mechanism to ensure that the

program has the necessary information to function

effectively.

It is necessary to ensure that onsite management

issues are integrated into decisions regarding future

growth and development. An effective onsite wastewater management program should be represented

in the ongoing land use planning process to ensure

achievement of the goals of the program and to

assist planners in avoiding the shortcomings of past

planning efforts, which generally allowed the

limitations of conventional onsite technologies to

drive some land use planning decisions. Such

considerations are especially important in situations

where centralized wastewater treatment systems are

being considered as an alternative or adjunct to

onsite or cluster systems. Comprehensive planning

and land use zoning are typically interrelated and

integrated: the comprehensive planning process

results in the development of overarching policies

and guidance, and the land use zoning process

provides the detailed regulatory framework to

implement the comprehensive plan. Honachefsky

(2000) provides a good overview of comprehensive

planning processes from an ecological perspective.

In general, the comprehensive plan can be used to

set the broad environmental protection goals of the

community, and the zoning ordinance(s) can be

used to

• Specify performance requirements for individual or clustered systems installed in

unsewered areas, preferably by watershed and/or

subwatershed.

• Limit or prevent development on sensitive

natural resource lands or in critical areas.

• Encourage development in urban growth areas

serviced by sewer systems, if adequate capacity

exists.

• Factor considerations such as system density,

hydraulic and pollutant loadings, proximity to

water bodies, soil and hydrogeological conditions, and water quality/quantity into planning

and zoning decisions.

• Restore impaired resources.

Integrating comprehensive planning and zoning

programs with onsite wastewater program management also can provide a stronger foundation for

determining and requiring the appropriate level of

treatment needed for both the individual site and

the surrounding watershed or subwatershed. The

integrated approach thus allows the program

manager to manage both existing and new onsite

systems from a cumulative loadings perspective or

performance-based approach that is oriented toward

the protection of identified resources. Local health

departments (regulatory authorities) charged with

administering programs based on prescriptive codes

typically have not had the flexibility or the reComprehensive planning program elements

• Define management program boundaries.

• Select management entity(ies).

• Establish human health and environmental protection goals.

• Form a planning team composed of management staff and local stakeholders.

• Identify internal and external planning resources and partners.

• Collect information on regional soils, topography, rainfall, and water quality and quantity.

• Identify sensitive ecological areas, recreational areas, and water supply protection areas.

• Characterize and map past, current, and future development where OWTSs are necessary.

• Coordinate with local sewage authorities to identify current and future service areas and determine treatment

plant capacity to accept septage.

• Identify documented problem areas and areas likely to be at risk in the future.

• Prioritize and target problem areas for action or future action.

• Develop performance requirements and strategies to deal with existing and possible problems.

• Implement strategy; monitor progress and modify strategy if necessary.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-18 USEPA Onsite Wastewater Treatment Systems Manual Figure 2-1. Onsite wastewater management overlay zones example Source: Heigis and Douglas, 2000.
USEPA Onsite Wastewater Treatment Systems Manual 2-19

Chapter 2: Management of Onsite Wastewater Treatment Systems

sources to deviate from zoning designations and as

a result often have had to approve permits for

developments where onsite system-related impacts

were anticipated. Coordinating onsite wastewater

management with planning and zoning activities

can ensure that parcels designated for development

are permitted based on a specified level of onsite

system performance that considers site characteristics and watershed-level pollutant loading analyses.

To streamline this analytical process, some management programs designate overlay zones in which

specific technologies or management strategies are

required to protect sensitive environmental resources. These overlay zones may be based on soil

type, topography, geology, hydrology, or other site

characteristics (figure 2-1). Within these overlay

zones, the RA may have the authority to specify

maximum system densities, system design requirements, performance requirements, and operation/

maintenance requirements. Although the use of

overlay zones may streamline administrative

efforts, establishing such programs involves the use

of assumptions and generalizations until a sufficient

number of site-specific evaluations are available to

ensure proper siting and system selection.

Internally, changes in program goals, demographics, and technological advances require information

and coordination to ensure that the short- and longterm goals of the program can continue to be met.

Many variables affect the internal planning process,

including factors such as the locations and types of

treatment systems within the jurisdictional area, the

present or future organizational and institutional

structure of the management entity, and the funding

available for program development and implementation.

The box “Performance-based program elements”

(page 2-21) provides guidance for planning processes undertaken by an onsite/decentralized

wastewater management entity. At a minimum, the

onsite management entity should identify and

delineate the planning region, develop program

goals, and coordinate with the relevant public

health, resource protection, economic development,

and land-use planning agencies.

Figure 2-2 shows a process that might be useful in

developing and implementing a performance-based

program whose objectives are to protect specific

resources or achieve stated public health objectives.

2.4.5 Performance requirements

Many state and local governments are currently

adopting or considering the use of performance

requirements to achieve their management goals.

The management entity can use performance

requirements to establish specific and measurable

standards for the performance of onsite systems

that are necessary to achieve the required level of

environmental or public health protection for an

identified management area and resource. All onsite

wastewater management programs are based to

varying degrees on this concept. Traditional

programs have elected to use prescriptive siting,

design, and setback requirements to dictate where

and when conventional septic tank/SWIS systems

are appropriate. The prescriptive standards were

based on the presumption that systems sited and

designed to these standards would protect public

health. In most cases, this assumption provided an

adequate level of protection, but the prescriptions

often were based on standards adopted by others

and not based on scientific evaluations of the site

conditions of the community using them. As a

result, many programs based on prescriptive

requirements do not adequately protect the

resource. (See chapter 5 for more detailed information about performance-based approaches.) The

NOWRA Model Framework for Unsewered Wastewater Infrastructure, discussed in chapter 1, also

provides a model for the development of performance-based programs (Walsh et al., 2001; see

http://www.nowra.org).

Performance requirements provide the onsite

system regulatory agency with an objective basis to

oversee siting, system selection and design, installation, maintenance, and monitoring of OWTS in

order to protect an identified resource or achieve a

stated public health goal. In jurisdictions where

performance requirements are used, the regulatory

agency should not conduct site evaluations and

specify system designs because of potential conflict

of interest issues regarding enforcement and

compliance; that is, the agency would be evaluating

the performance of systems it designed and sited.

The role of the regulatory agency in such a situation should be to establish performance requirements and provide oversight of management,

operation, maintenance, and other activities conducted by private contractors or other entities.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-20 USEPA Onsite Wastewater Treatment Systems Manual

Figure 2-2. Process for developing onsite wastewater management
USEPA Onsite Wastewater Treatment Systems Manual 2-21

Chapter 2: Management of Onsite Wastewater Treatment Systems

Where appropriate, prescriptive guidelines for

siting, design, and operation that are accepted by

the management entity as meeting specific performance requirements for routine system applications

can be appended to local codes or retained to avoid

cost escalation and loss of qualified service providers (Otis et al., 2001). Designating performance

requirements for areas of a management district

with similar environmental sensitivities and site

conditions can provide property owners with

valuable information on performance expectations

and their rationale (Otis et al., 2001). Performance

standards can be determined based on the need to

protect a site-specific resource, such as residential

drinking wells, or they can be based on larger-scale

analyses intended to manage cumulative OWTS

pollutant loadings (e.g., to protect a lake or

estuary from nutrient enrichment).

Implementation of performance-based programs

might result in increased management expenditures

due to the need for staff to conduct site or areawide

(e.g., watersheds, subwatersheds, or other geographic areas) evaluations, inspect, and monitor

system performance as necessary. Service provider

training, the evaluation and approval of new or

alternative system designs, public outreach efforts

to establish public support for this approach, and

new certification/licensing or permit programs will

also increase program costs. These increases can

usually be recovered through permit/license fees.

Also, system owners will be responsible for

operation and maintenance costs. The following

box contains a recommended list of elements for a

performance-based program.

2.4.6 Performance requirements and

the watershed approach

USEPA encourages the use of performance requirements on a watershed, subwatershed, or source

water protection zone basis. These are useful

natural units on which to develop and implement

performance-based management strategies. In

situations where jurisdictional boundaries cross

watershed, subwatershed, or source water recharge

boundaries, interagency coordination might be

needed. Setting performance requirements for

individual watersheds, subwatersheds, or source

water areas allows the program manager to determine and allocate cumulative hydraulic and pollutant loads to ensure that the goals of the community

can be met. To do so, an analysis to determine

whether the cumulative pollutant or hydraulic

loadings can be assimilated by the receiving

environment without degrading the quality of the

resource or use is necessary. There is some uncertainty in this process, and program managers

should factor in a margin of safety to account for

errors in load and treatment effectiveness estimates.

(Refer to chapter 3 for more information on

estimating treatment effectiveness.)

Onsite systems are typically only one of many

potential sources of pollutants that can negatively

affect ground or surface waters. In most cases other

Performance-based program elements

• Obtain or define legal authority to enact management regulations.

• Identify management area.

• Identify program goals.

• Identify specific resource areas that need an additional level of protection, e.g., drinking water

aquifers, areas with existing water quality problems, and areas likely to be at risk in the future.

• Establish performance goals and performance requirements for the management area and specific

watersheds, subwatersheds, or source water protection areas.

• Define performance boundaries and monitoring protocols.

• Determine and set specific requirements for onsite systems based on protecting specific

management areas and achieving of a specified level of treatment (e.g., within a particular

subbasin, there will be no discharge that contains more than 1.0 mg/L of total phosphorus).

• Develop or acquire information on alternative technologies, including effectiveness information and

operation and maintenance requirements (see chapter 4).

• Develop a review process to evaluate system design and system components (see chapter 5).
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-22 USEPA Onsite Wastewater Treatment Systems Manual

site level; that is, the individual OWTS must meet

the performance requirements at the treatment

performance boundary or the point of compliance.

It should be noted that the performance-based

approach is a useful program tool both to prevent

degradation of a water resource and to restore a

degraded resource. Additional information on

antidegradation is available in USEPA’s Water

Quality Standards Handbook. (See http://

www.epa.gov/waterscience/library/wqstandards/

handbook.pdf. For general information on the

USEPA Water Quality Standards Program, see

http://www.epa.gov/OST/standards/.) The Clean

Water Act Section 303(d) program (Total Maximum Daily Load [TMDL] program) has published

numerous documents and technical tools regarding

the development and implementation of pollutant

load allocations. This information can be found at

http://www.epa.gov/owow/tmdl/. (NOTE: The

identification of other pollutant sources and the

analyses of loadings and modeling related to

TMDL are beyond the scope of this document.)

The text above contains a list of steps that the OWTS

program manager should consider in developing

performance requirements at a watershed scale.

Establishing performance requirements at a watershed scale

Establishing performance requirements involves a sequential set of activities at both the landscape level and

the site level. The following steps describe the general process of establishing performance requirements for

onsite systems:

• Identify receiving waters (ground water, surface waters) for OWTS effluent.

• Define existing and planned uses for receiving waters (e.g., drinking water, recreation, habitat).

• Identify water quality standards associated with designated uses (check with state water agency).

• Determine types of OWTS-generated pollutants (e.g., nutrients, pathogens) that might affect use.

• Identify documented problem areas and areas likely to be at risk in the future.

• Determine whether OWTS pollutants pose risks to receiving waters.

• If there is a potential risk,

– Estimate existing and projected OWTS contributions to total pollutant loadings.

– Determine whether OWTS pollutant loadings will cause or contribute to violations of water quality or

drinking water standards.

– Establish maximum output level (mass or concentration in the receiving water body) for specified

OWTS effluent pollutants based on the cumulative load analysis of all sources of pollutant(s) of

concern.

– Define performance boundaries for measurement of OWTS effluent and pollutant concentrations to

achieve watershed- and site-level pollutant loading goals.

sources of OWTS-generated pollutants (primarily

nutrients and pathogens), such as agricultural

activities or wildlife, are also present in the watershed or subwatershed. To properly calculate the

cumulative acceptable OWTS-generated pollutant

loadings for a given watershed or subwatershed, all

other significant sources of the pollutants that

might be discharged by onsite systems should be

identified. This process requires coordination

between the onsite program manager and the

agencies responsible for assessing and monitoring

both surface waters and ground water. Once all

significant sources have been identified, the relative

contributions of the pollutants of concern from

these sources should be determined and pollutant

loading allocations made based on factors the

community selects. State water quality standards

and drinking source water protection requirements

are usually the basis for this process. Once loading

allocations have been made for all of the significant

contributing sources, including onsite systems, the

OWTS program manager needs to develop or

revise the onsite program to ensure that the overall

watershed-level goals of the program are met.

Cumulative loadings from onsite systems must be

within the parameters set under the loading allocations, and public health must be protected at the
USEPA Onsite Wastewater Treatment Systems Manual 2-23

Chapter 2: Management of Onsite Wastewater Treatment Systems

The use of a watershed-based approach also affords

the water quality and onsite program managers

some flexibility in determining how to most costeffectively meet the goals of the community. Given

the presence of both onsite systems and other

sources of pollutants of concern, evaluations can be

made to determine the most cost-effective means of

achieving pollutant load reductions. For example,

farmer or homeowner nutrient management

education might result in significant loading

reductions of nitrogen that could offset the need to

require expensive, more technically advanced

onsite systems designed for nitrogen removal.

Watershed-level evaluations, especially in cases

where new and refined monitoring methods are

employed, might also negate the need for system

upgrade or replacement in some watersheds. For

example, new genetic tracing methods can provide

the water quality program manager with a reliable

tool to differentiate between human sources of

fecal coliform and animal contributions, both

domestic and wild (see chapter 3). The use of these

new methods can be expensive, but they might

provide onsite program managers with a means of

eliminating onsite systems as a significant contributing source of pathogens.

Onsite program managers have legitimate concerns

regarding the adoption of a performance-based

approach. The inherent difficulty of determining

cumulative loadings and their impacts on a watershed,

the technical difficulties of monitoring the impacts

of OWTS effluent, the evaluation of new technologies and the potential costs, staffing and expertise

needed to implement a performance-based program

can make this option more costly and difficult to

implement. (NOTE: In general, the RA should not

have the responsibility for monitoring systems

other than conducting random quality assurance

inspections. Likewise, the RA should not have the

primary responsibility of evaluating new or alternative technologies. Technologies should be evaluated

by an independent entity certified or licensed to

conduct such evaluations, such as an RME.)

Prescriptive regulatory codes that specify technologies for installation under a defined set of site

conditions have worked reasonably well in the past

in many localities. The use of this approach, in

which baseline design requirements and treatment

effectiveness are estimated based on the use of the

specified technology at similar sites, will continue

to be a key component of most management

programs because it is practical, efficient, and easy

to implement. Programs based purely on prescriptive

requirements, however, might not consistently

provide the level of treatment needed to protect

community water resources and public health.

Many programs using prescriptive requirements are

based on empirical relationships that do not necessarily result in appropriate levels of treatment. Sitespecific factors can also result in inadequate

treatment of OWTS effluent where a prescriptive

approach is used. Political pressure to approve

specific types of systems for use on sites where

Performance requirements in Texas

In 1996 Texas eliminated percolation test requirements

for onsite systems and instituted new performance

requirements for alternative systems (e.g., drip

systems, intermittent sand filters, leaching chambers).

Site evaluations in Texas are now based on soil and site

analyses, and service providers must be certified. These

actions were taken after onsite system installations

nearly tripled between 1990 and 1997.

Source: Texas Natural Resource Conservation

Commission, 1997.

Arizona’s performance-based technical standards

In 2001 Arizona adopted a rule containing technical standards for

onsite systems with design flows less than 24,000 gallons per day

(Arizona Administrative Code, Title 18, Chapters 5, 9, 11, and 14). Key

provisions of the rule include site investigation requirements,

identification of site limitations, design adjustments for better-thanprimary treatment to overcome site limitations, and design criteria and

nominal performance values for more than 20 treatment or effluent

dispersal technologies. Applications for proposed systems are required

to contain wastewater characterization information, technology

selections that address site limitations, soil treatment calculations, and

effluent dispersal area information. Technology-specific general ground

water discharge permits required under the new rule specify design

performance values for TSS, BOD, total coliforms, and TN. Products

with satisfactory third-party performance verification data might receive

additional credits for continuing performance improvement. The

Arizona rule contains important elements of performance-based and

hybrid approaches through adoption of performance values and

specific use criteria for certain systems.

Source: Swanson, 2001.
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2-24 USEPA Onsite Wastewater Treatment Systems Manual

prescriptive criteria are not met is another factor

that leads to the installation of inadequate systems.

2.4.7 Implementing performance

requirements through a hybrid

management approach

RAs often adopt a “hybrid” approach that includes

both prescriptive and performance elements. To set

appropriate performance requirements, cumulative

load analyses should be conducted to determine the

assimilative capacity of the receiving environment(s). This process can be costly, time-consuming, and controversial when water resource characterization data are incomplete, absent, or contested.

Because of these concerns, jurisdictions might elect

to use prescriptive standards in areas where it has

been determined that onsite systems are not a

significant contributing source of pollutants or in

areas where onsite systems are not likely to cause

water quality problems. Prescriptive designs might

also be appropriate and practical for sites where

previous experience with specified OWTS designs

has resulted in the demonstration of adequate

performance (Ayres Associates, 1993).

In those areas where problems due to pollutants

typically found in OWTS discharges have been

identified and in areas where there is a significant

threat of degradation due to OWTS discharges

(e.g., source water protection areas, recreational

swimming areas, and estuaries), performance

requirements might be appropriate. The use of a

performance-based approach allows jurisdictions to

prioritize their resources and efforts to target

collections of systems within an area or subwatershed or individual sites within a jurisdictional area.

2.4.8 Developing and implementing

performance requirements

OWTS performance requirements should be

developed using risk-based analyses on a watershed

or site level. They should be clear and quantifiable

to allow credible verification of system performance through compliance monitoring. Performance requirements should at a minimum include

stipulations that no plumbing backups or ground

surface seepage may occur and that a specified

level of ground/surface water quality must be

maintained at some performance boundary, such as

the terminus of the treatment train, ground water

Florida’s performance-based per mance-based per mance-based permit prog mit progmit prog mit program

Florida adopted provisions for permitting residential performance-based treatment systems in September 2000.

The permit regulations, which can be substituted for provisions governing the installation of onsite systems under

existing prescriptive requirements, apply to a variety of alternative and innovative methods, materials, processes,

and techniques for treating onsite wastewaters statewide. Discharges under the performance-based permit

program must meet treatment performance criteria for secondary, advanced secondary, and advanced wastewater

treatment, depending on system location and the proximity of protected water resources. Performance

requirements for each category of treatment are as follows:

• Secondary treatment: annual arithmetic mean for BOD and TSS < 20 mg/L, annual arithmetic mean for fecal

coliform bacteria < 200 cfu/100 mL.

• Advanced secondary treatment: annual arithmetic mean for BOD and TSS < 10 mg/L, annual arithmetic mean

for total nitrogen < 20 mg/L, annual arithmetic mean for total phosphorus < 10 mg/L, annual arithmetic mean for

fecal coliform bacteria < 200 cfu/100 mL.

• Advanced wastewater treatment: annual arithmetic mean for BOD and TSS < 5 mg/L, annual arithmetic mean

for total nitrogen

< 3 mg/L, annual arithmetic mean for total phosphorus < 1 mg/L, fecal coliform bacteria count for any one

sample < 25 cfu/100 mL.

Operation and maintenance manuals, annual operating permits, signed maintenance contracts, and biannual

inspections are required for all performance-based systems installed under the new regulation. The operating

permits allow for property entry, observation, inspection, and monitoring of treatment systems by state health

department personnel.

Source: Florida Administrative Code, 2000.
USEPA Onsite Wastewater Treatment Systems Manual 2-25

Chapter 2: Management of Onsite Wastewater Treatment Systems

surface, property line, or point of use

(e.g., water supply well, recreational surface water,

aquatic habitat area; see chapter 5).

If prescriptive designs are allowed under a performance-based program, these systems should be

proven capable of meeting the same performance

requirements as a system specifically designed for

that site. Under this approach, the management

entity should determine through experience (monitoring and evaluation of the prescribed systems on

sites with similar site characteristics) that the

system will perform adequately to meet stated

performance requirements given sufficiently

frequent operating inspections and maintenance.

Performance monitoring might be difficult and

costly. Although plumbing backups and ground

surface seepage can be easily and inexpensively

observed through visual monitoring, monitoring

the receiving environment (surface receiving waters

and ground water) might be expensive and complicated. Monitoring of ground water is confounded

by the difficulty of locating and sampling subsurface effluent plumes. Extended travel times,

geologic factors, the presence of other sources of

ground water recharge and pollutants, and the

dispersal of OWTS pollutants in the subsurface all

complicate ground water monitoring.

To avoid extensive sampling of ground water and

surface waters, especially where there are other

contributing sources of pollutants common to

OWTS discharges, performance requirements can

be set for the treated effluent at a designated

performance boundary before release into the

receiving environment (refer to chapters 3 and 5).

Adjustments for the additional treatment, dispersion, and dilution that will occur between the

performance boundary and the resource to be

protected should be factored into the performance

requirements. For example, pretreated wastewater

is typically discharged to unsaturated soil, through

which it percolates before it reaches ground water.

The performance requirement should take into

account the treatment due to physical (filtration),

biological, and chemical processes in the soil, as

well as the dispersion and dilution that will occur

in the unsaturated soil and ground water prior to

the point where the standard is applied.

As a practical matter, performance verification of

onsite systems can be relaxed for identified types of

systems that the RA knows will perform as anticipated. Service or maintenance contracts or other

legal mechanisms might be prerequisites to waiving

or reducing monitoring requirements or inspections. The frequency and type of monitoring will

depend on the management program, the technologies employed, and watershed- and site-specific

factors. Monitoring and evaluation might occur at

or near the site and include receiving environment

or water quality monitoring and monitoring to

ascertain hydraulic performance and influent flows.

In addition, the OWTS management program needs

to be evaluated to ascertain whether routine maintenance is occurring and whether individual systems

and types of systems are operating properly.

Chapter 4 contains descriptions of most of the

onsite wastewater treatment processes currently in

use. OWTS program managers developing and

implementing performance-based programs will

often need to conduct their own site-specific

evaluations of these treatment options. The text box

that follows documents one approach used to

cooperatively evaluate innovative or alternative

wastewater treatment technologies. Many tribal,

state, and local programs lack the capability to

continually evaluate new and innovative technology

alternatives and thus depend on regional evaluations and field performance monitoring to provide

a basis on which to develop their programs.

2.4.9 Public education, outreach, and

involvement

Public education and outreach are critical aspects of

an onsite management program to ensure public

support for program development, implementation,

and funding. In addition, a working understanding

of the importance of system operation and maintenance is necessary to help ensure an effective

program. In general the public will want to know

the following:

• How much will it cost the community and the

individual?

• Will the changes mean more development in my

neighborhood? If so, how much?

• Will the changes prevent development?

• Will the changes protect our resources (drinking

waters, shellfisheries, beaches)?
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-26 USEPA Onsite Wastewater Treatment Systems Manual

• How do the proposed management alternatives

relate to the above questions?

A public outreach and education program should

focus on three components—program audience,

information about the program, and public outreach media. An effective public outreach program

makes information as accessible as possible to the

public by presenting the information in a nontechnical format. The public and other interested

parties should be identified, contacted, and consulted early in the process of making major decisions or proposing significant program changes.

Targeting the audience of the public outreach and

education program is important for both maximizing public participation and ensuring public

confidence in the management program. For onsite

wastewater system management programs, the

audiences of a public outreach and education

program can vary and might include:

• Homeowners

• Manufacturers

• Installers

• System operators and maintenance contractors

• Commercial or industrial property owner

• Public agency planners

• Inspectors

• Site evaluators

• Public

• Students

• Citizen groups and homeowner neighborhood

associations

• Civic groups such as the local Chamber of

Commerce

• Environmental groups

Onsite management entities should also promote

and support the formation of citizen advisory

groups composed of community members to build

or enhance public involvement in the management

program. These groups can play a crucial role in

representing community interests and promoting

support for the program.

Typical public outreach and education program

information includes:

• Promoting water conservation

• Preventing household and commercial/industrial

hazardous waste discharges

• Benefits of the onsite management program

Public outreach and education programs use a

variety of media options available for information

dissemination, including:

• Local newspapers

• Radio and TV

• Speeches and presentations

• Exhibits and demonstrations

• Conferences and workshops

• Public meetings

A cooperative approach for approving innovative/alternative designs in New England

The New England Interstate Water Pollution Control Commission is a forum for consultation and cooperative action

among six New England state environmental agencies. NEIWPCC has adopted an interstate process for reviewing

proposed wastewater treatment technologies. A technical review committee composed of representatives from New

England state onsite wastewater programs and other experts evaluates innovative or alternative technologies or

system components that replace part of a conventional system, modify conventional operation or performance, or

provide a higher level of treatment than conventional onsite systems.

Three sets of evaluation criteria have been developed to assess proposed replacement, modification, or advanced

treatment units. Review teams from NEIWPCC assess the information provided and make determinations that are

referred to the full committee. The criteria are tailored for each category but in general include:

• Treatment system or treatment unit size, function, and applicability or placement in the treatment train.

• Structural integrity, composition, durability, strength, and corresponding independent test results.

• Life expectancy and costs including comparisons with conventional systems/units.

• Availability and cost of parts, service, and technical assistance.

• Test data on prior installations or uses, test conditions, failure analysis, and tester identity.

Source: New England Interstate Water Pollution Control Commission, 2000.
USEPA Onsite Wastewater Treatment Systems Manual 2-27

Chapter 2: Management of Onsite Wastewater Treatment Systems

• School programs

• Local and community newsletters

• Reports

• Direct mailings, e.g., flyers with utility bills

2.4.10 Site evaluation

Evaluating a proposed site in terms of its environmental conditions (climate, geology, slopes, soils/

landscape position, ground water and surface water

aspects), physical features (property lines, wells,

hydrologic boundaries structures), and wastewater

characteristics (anticipated flow, pollutant content,

waste strength) provides the information needed to

size, select, and site the appropriate wastewater

treatment system. In most cases (i.e., under current

state codes and lower-level management entity

structures) RAs issue permits—legal authorizations

to install and operate a particular system at a

specific site—based on the information collected

and analyses performed during the site evaluation.

(NOTE: Detailed wastewater characterization

procedures are discussed in chapter 3; site evaluation processes are presented in section 5.5.)

2.4.11 System design criteria and

approval process

Performance requirements for onsite systems can

be grouped into two general categories—numeric

requirements and narrative criteria. Numeric

requirements set measurable concentration or mass

loading limits for specific pollutants (e.g., nitrogen

or pathogen concentrations). Narrative requirements describe acceptable qualitative aspects of the

wastewater (e.g., sewage surface pooling, odor). A

numerical performance requirement might be that

all septic systems in environmentally sensitive areas

must discharge no more than 5 pounds of nitrogen

per year, or that concentrations of nitrogen in the

effluent may be no greater than 10 mg/L. Some of

the parameters for which performance requirements

are commonly set for OWTSs include:

• Fecal coliform bacteria (an indicator of pathogens)

• Biochemical oxygen demand (BOD)

• Nitrogen (total of all forms, i.e., organic,

ammonia, nitrite, nitrate)

• Phosphorus (for surface waters)

• Nuisance parameters (e.g., odor, color)

Under a performance-based approach, performance

requirements, site conditions, and wastewater

characterization information drive the selection of

treatment technologies at each site. For known

technologies with extensive testing and field data,

the management agency might attempt to institute

performance requirements prescriptively by

designating system type, size, construction practices, materials to be used, acceptable site conditions, and siting requirements. For example, the

Arizona Department of Environmental Quality has

adopted a rule that establishes definitions, permit

requirements, restrictions, and performance criteria

for a wide range of conventional and alternative

treatment systems. (Swanson, 2001). Alaska

requires a 2-foot-thick sand liner when the receiving soil percolates at a rate faster than 1 minute per

inch (Alaska Administrative Code, 1999). At a

minimum, prescriptive system design criteria

Site evaluation program elements

• Establish administrative processes for permit/site

evaluation applications.

• Establish processes and policies for evaluating site

conditions (e.g., soils, slopes, water resources).

• Develop and implement criteria and protocols for

wastewater characterization.

• Determine level of skill and training required for site

evaluators.

• Establish licensing/certification programs for site

evaluators.

• Offer training opportunities as necessary.

Performance requirements and system design in

Massachusetts

Massachusetts onsite regulations identify certain wellhead protection

areas, public water supply recharge zones, and coastal embayments

as nitrogen-sensitive areas and require OWTSs in those areas to meet

nitrogen loading limitations. For example, recirculating sand filters or

equivalent technologies must limit total nitrogen concentrations in

effluent to no more than 25 mg/L and remove at least 40 percent of

the influent nitrogen load. All systems in nitrogen-sensitive areas must

discharge no more than 440 gallons of design flow per acre per day

unless system effluent meets a nitrate standard of 10 mg/L or other

nitrogen removal technologies or attenuation strategies are used.

Source: Massachusetts Environmental Code, Title V.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-28 USEPA Onsite Wastewater Treatment Systems Manual

should consider the following. (See chapter 5 for

details.)

• Wastewater characterization and expected

effluent volumes.

• Site conditions (e.g., soils, geology, ground

water, surface waters, topography, structures,

property lines).

• System capacity, based on estimated peak and

average daily flows.

• Location of tanks and appurtenances.

• Tank dimensions and construction materials.

• Alternative tank effluent treatment units and

configuration.

• Required absorption field dimensions and

materials.

• Requirements for alternative soil absorption

field areas.

• Sizing and other acceptable features of system

piping.

• Separation distances from other site features.

• Operation and maintenance requirements (access

risers, safety considerations, inspection points).

• Accommodations required for monitoring.

2.4.12 Construction and installation

oversight authority

A comprehensive construction management program will ensure that system design and specifications are followed during the construction process.

If a system is not constructed and installed properly, it is unlikely to function as intended. For

example, if the natural soil structure is not preserved during the installation process (if equipment

compacts infiltration field soils), the percolation

potential of the infiltration field can be significantly reduced. Most early failures of conventional

onsite systems’ soil absorption fields have been

attributed to hydraulic overloading (USEPA,

1980). Effective onsite system management

programs ensure proper system construction and

installation through construction permitting,

inspection, and certification programs.

Construction should conform to the approved plan

and use appropriate methods, materials, and

equipment. Mechanisms to verify compliance with

performance requirements should be established to

ensure that practices meet expectations. Typical

existing regulatory mechanisms that ensure proper

installation include reviews of site evaluation

procedures and findings and inspections of systems

during and after installation, i.e., before cover-up

and final grading. A more effective review and

inspection process should include

• Predesign meeting with designer, owner, and

contractor

• Preconstruction meeting with designer, owner,

and contractor

• Field verification and staking of each system

component

• Inspections during and after construction

• Issuance of a permit to operate system as

designed and built

Simplified incorporation of system design requirements

into a regulatory program: the Idaho approach

Idaho bypasses cumbersome legislative processes when making

adjustments to its onsite system design guidelines by referencing a

technical manual in the regulation that is not part of the state

regulation. Under this approach, new research findings, new

technologies, or other information needed to improve system design

and performance can be incorporated into the technical guidance

without invoking the regulatory rulemaking process. The regulations

contain information on legal authority, responsibilities, permit

processes, septic tanks, and conventional systems. The reference

guidance manual outlines types of alternative systems that can be

installed, technical and design considerations, soil considerations, and

operation and maintenance requirements.

Source: Adapted from NSFC, 1995b.

Construction oversight program elements

• Establish preconstruction review procedure for site

evaluation and system design.

• Determine training and qualifications of system

designers and installers.

• Establish designer and installer licensing and

certification programs.

• Define and codify construction oversight

requirements.

• Develop certification process for overseeing and

approving system installation.

• Arrange training opportunities for service providers

as necessary
USEPA Onsite Wastewater Treatment Systems Manual 2-29

Chapter 2: Management of Onsite Wastewater Treatment Systems

Construction oversight inspections should be conducted at several stages during the system installation process to ensure compliance with regulatory

requirements. During the construction process,

inspections before and after backfilling should verify

compliance with approved construction documents

and procedures. An approved (i.e., licensed or

certified) construction oversight inspector, preferably the designer of the system, should oversee

installation and certify that it has been conducted

and recorded properly. The construction process for

soil-based systems must be flexible to accommodate weather events because construction during

wet weather can compact soils in the infiltration

field or otherwise alter soil structure.

2.4.13 Operation and maintenance

requirements

A recurring weakness of many existing OWTS

management programs has been the failure to

ensure proper operation and maintenance of

installed systems. Few existing oversight agencies

conduct inspections to verify basic system performance, and many depend on uninformed, untrained

system owners to monitor tank residuals buildup,

schedule pumping, ensure that flow distribution is

occurring properly, check pumps and float

switches, inspect filtration media for clogging, and

perform other monitoring and maintenance tasks.

Complaints to the regulatory authority or severe

and obvious system failures often provide the only

formal notification of problems under present

codes. Inspection and other programs that monitor

system performance (e.g., Critical Point Monitoring; see chapter 3) can help reduce the risk of

premature system failure, decrease long-term

investment costs, and lower the risk of ground

water or surface water contamination (Eliasson et

al., 2001; Washington Department of Health,

1994).

Various options are available to implement operation and maintenance oversight programs. These

range from purely voluntary (e.g., trained

homeowners responsible for their system operation

and maintenance activities) to more sophisticated

operating permit programs and ultimately to

programs administered by designated RMEs that

conduct all management/maintenance tasks. In

general, voluntary maintenance is possible only

where systems are nonmechanical and gravitybased and located in areas with very low population

densities. The level of management should increase

if the system is more complex or the resource(s) to

be protected require a higher level of performance.

Alarms (onsite and remote) should be considered to

alert homeowners and service providers that system

malfunction might be occurring. In addition to

simple float alarms, several manufacturers have

developed custom-built control systems that can

program and schedule treatment process events,

remotely monitor system operation, and notify

technicians by pager or the Internet of possible

problems. New wireless and computer protocols,

cellular phones, and personal digital assistants are

being developed to allow system managers to

remotely monitor and assess operation of many

systems simultaneously (Nawathe, 2000), further

enhancing the centralized management of OWTSs

in outlying locations. Using such tools can save

considerable travel and inspection time and focus

Operation, maintenance, and residuals management program elements

• Establish guidelines or permit program for operation and maintenance of systems.

• Develop reporting system for operation and maintenance activities.

• Circulate operation and maintenance information and reminders to system owners.

• Develop operation and maintenance inspection and compliance verification program.

• Establish licensing/certification programs for service providers.

• Arrange for training opportunities as necessary.

• Establish procedures for follow-up notices or action when appropriate.

• Establish reporting and reminder system for monitoring system effluent.

• Establish residuals (septage) management requirements, manifest system, and disposal/use

reporting.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-30 USEPA Onsite Wastewater Treatment Systems Manual

field personnel on systems that require attention or

regular maintenance. Telemetry panels at the

treatment site operating through existing or dedicated phone lines can be programmed to log and

report information such as high/low water alarm

warnings, pump run and interval times, water level

readings in tanks/ponds, amperage drawn by system

pumps, and other conditions. Operators at a

centralized monitoring site can adjust pump run

cycles, pump operation times, alarm settings, and

high-level pump override cycles (Stephens, 2000).

Some management entities have instituted comprehensive programs that feature renewable/

revocable operating permits, mandatory inspections or disclosure (notification/inspection) upon

property transfer (e.g., Minnesota, Wisconsin,

Massachusetts), and/or periodic monitoring by

licensed inspectors. Renewable operating permits

might require system owners to have a contract with a

certified inspection/maintenance contractor or

otherwise demonstrate that periodic inspection and

maintenance procedures have been performed for

permit renewal (Wisconsin Department of Commerce,

2001). Minnesota, Wisconsin, Massachusetts, and

some counties (e.g., Cayuga and other counties in

New York, Washtenaw County in Michigan) require

that sellers of property disclose or verify system

performance (e.g., disclosure statement, inspection

by the local oversight entity or other approved

inspector) prior to property transfer. Financial

incentives usually aid compliance and can vary from

small fines for poor system maintenance to preventing

the sale of a house if the OWTS is not functioning

properly. Inspection fees might be one way to

cover or defray these program costs. Lending

institutions nationwide have influenced the adoption

of a more aggressive approach toward requiring

system inspections before home or property loans

are approved. In some areas, inspections at the time

of property transfer are common despite the

absence of regulatory requirements. This practice is

incorporated into the loan and asset protection

policies of local banks and lending firms.

RAs, however, should recognize that reliance on

lending institutions to ensure that proper inspections occur can result in gaps. Property transfers

without lending institution involvement might

occur without inspections. In addition, in cases

where inspections are conducted by private

individuals reporting to the lending agents, the

inspectors might not have the same degree of

accountability that would occur in jurisdictions that

have mandatory requirements for state or local

licensing or certification of inspectors. RAs should

require periodic inspections of systems based on

system design life, system complexity, and

changes in ownership.

Wisconsin’s new Private Onsite Wastewater Treatment System rule (see http://www.commerce.

state.wi.us/SB/SB-POWTSProgram.html)

requires management plans for all onsite treatment

systems. The plans must include information and

procedures for maintaining the systems in accordance with the standards of the code as designed

and approved. Any new or existing system that is

not maintained in accordance with the approved

management plan is considered a human health

hazard and subject to enforcement actions. The

maintenance requirements are specified in the code.

All septic tanks are to be pumped when the combined sludge and scum volume equals one-third of

the tank volume. Existing systems have the added

requirement of visual inspections every 3 years for

Onsite system disclosure requirements in Minnesota

Minnesota law requires that before signing an agreement to sell or transfer real property, a seller must disclose to

a buyer in writing the status and location of all septic systems on the property, including existing or abandoned

systems. If there is no onsite treatment system on the property, the seller can satisfy the disclosure requirement

by making such a declaration at the time of property transfer. The disclosure must indicate whether the system is

in use and whether it is, to the seller’s knowledge, in compliance with applicable laws and rules. A map indicating

the location of the system on the property must also be included. A seller who fails to disclose the existence or

known status of a septic system at the time of sale and who knew or had reason to know the existence or known

status of a system might be liable to the buyer for costs relating to bringing the system into compliance, as well

as reasonable attorney’s fees incurred in collecting the costs from the seller. An action for collection of these

sums must be brought within 2 years of the closing date.

Source: Minnesota Statutes, 2000.
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wastewater ponding on the ground surface. Only

persons certified by the department may perform

the inspections or maintenance. Systems requiring

maintenance more than once annually require

signed maintenance contracts and a notice of

maintenance requirements on the property deed.

The system owner or designated agent of the owner

must report to the department each inspection or

maintenance action specified in the management

plan at its completion (Wisconsin Department of

Commerce, 2001).

2.4.14 Residuals management

requirements

The primary objective of residuals management is

to establish procedures and rules for handling and

disposing of accumulated wastewater treatment

system residuals to protect public health and the

environment. These residuals can include septage

removed from septic tanks and other by-products

of the treatment process (e.g., aerobic-unit-generated

sludge). When planning a program a thorough

knowledge of legal and regulatory requirements

regarding handling and disposal is important. In

general, state and local septage management

programs that incorporate land application or burial

of septage must comply with Title 40 of the U.S.

Code of Federal Regulations (CFR), Parts 503 and

257. Detailed guidance for identifying, selecting,

developing, and operating reuse or disposal sites

for septage can be found in the USEPA Process

Design Manual: Land Application of Sewage

Sludge and Domestic Septage (USEPA, 1995c),

which is posted on the Internet at http://

www.epa.gov/ORD/WebPubs/sludge.pdf. Additional information is provided in Domestic Septage

Regulatory Guidance (USEPA, 1993b), posted at

http://www.epa.gov/oia/tips/scws.htm. Another

document useful to practitioners and small communities is the Guide to Septage Treatment and

Disposal (USEPA, 1994).

States and municipalities typically establish other

public health and environmental protection regulations for residuals handling, transport, treatment, and

reuse/disposal. In addition to regulations, practical

Requiring pump-outs to ensure proper

maintenance

Periodic pumping of septic tanks is now required by law

in some jurisdictions and is becoming established

practice for many public and private management

entities. In 1991 Fairfax County, Virginia, amended its

onsite systems management code to require pumping

at least every 5 years. The action, which was based on

provisions of the Chesapeake Bay Preservation Act,

was accompanied by public outreach notices and news

articles. System owners must provide the county health

department with a written notification within 10 days of

pumpout. A receipt from the pumpout contractor, who

must be licensed to handle septic tank residuals, must

accompany the notification.

Source: Fairfax County Health Department, 1995.

Installer and designer permitting in New Hampshire

Onsite system designers and installers in New Hampshire have been required to obtain state-issued permits since

1979. The New Hampshire’s Department of Environmental Services Subsurface Systems Bureau issues the

permits, which must be renewed annually. Permits are issued after successful completion of written examinations.

The designer’s test consists of three written sections and a field test for soil analysis and interpretation. The

installers must pass only one written examination.

The tests are broad and comprehensive, and they assess the candidate’s knowledge of New Hampshire’s codified

system design, regulatory setbacks, methods of construction, types of effluent disposal systems, and new

technology. Completing the three tests designers must take requires about 5 hours. The passing grade is

80 percent. The field test measures competency in soil science through an analysis of a backhoe pit,

determination of hydric soils, and recognition of other wetland conditions. The 2-hour written exam for installers

measures understanding of topography, regulatory setbacks, seasonal high water table determination, and

acceptable methods of system construction.

Sources: Bass, 2000; New Hampshire Department of Environmental Services, 1991.
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2-32 USEPA Onsite Wastewater Treatment Systems Manual

limitations such as land availability, site conditions,

buffer zone requirements, hauling distances, fuel

costs, and labor costs play a major role in evaluating

septage reuse/disposal options. These options

generally fall into three basic categories—land

application, treatment at a wastewater treatment

plant, and treatment at a special septage treatment

plant (see chapter 4). The initial steps in the

residuals reuse/disposal decision-making process are

characterizing the quality of the septage and determining

potential adverse impacts associated with various reuse/

disposal scenarios. In general, program officials strive to

minimize exposure of humans, animals, ground water,

and ecological resources to the potentially toxic or

hazardous chemicals and pathogenic organisms

found in septage. Other key areas of residuals

management programs include tracking or manifest

systems that identify septage sources, pumpers,

transport equipment, final destinations, and treatment methods, as well as procedures for controlling

human exposure to residuals, including vector

control, wet weather runoff management, and

limits on access to disposal sites. (Refer to chapter 4 for more details.)

2.4.15 Certification and licensing of

service providers and program

staff

Certification and licensing of service providers such

as septage haulers, designers, installers, and maintenance personnel can help ensure management program effectiveness and compliance and reduce the

administrative burden on the RA. Certification and

licensing of service providers is an effective means of

ensuring that a high degree of professionalism and

experience is necessary to perform specified activities.

Maine instituted a licensing program for site evaluators in 1974 and saw system failure rates drop to

insignificant levels (Kreissl, 1982). The text box that

follows provides a list of activities that management

entities should consider in setting up certification and

licensing programs or requirements.

RAs should establish minimum criteria for licensing/certification of all service providers to ensure

protection of health and water resources. Maine

requires that site evaluators be licensed (certified)

and that designers of systems treating more than

RA/ME activities for training, certifying, and

licensing service providers

• Identify tasks that require in-house or contractor

certified/licensed professionals.

• Develop certification and/or licensing program based

on performance requirements.

• Establish process for certification/licensing

applications and renewals if necessary.

• Develop database of service providers, service

provider qualifications and contact information.

• Establish education, training, and experience

requirements for service providers.

• Develop or identify continuing training opportunities

for service providers.

• Circulate information on available training to service

providers.

• Update service provider database to reflect verified

training participation/performance.

Statewide training institute for onsite professionals in North Carolina

http://www.soil.ncsu.edu/swetc/subsurf etc/subsurface/

subsurface.htm

North Carolina State University and other partners in the state developed the Subsurface Wastewater

System Operator Training School (see http://www.soil.ncsu.edu/swetc/subsurface/

subsurface.htm) in response to state rules requiring operators of some systems (e.g., large systems

and those using low-pressure pipe, drip irrigation, pressure-dosed sand filter, or peat biofilter

technologies) to be certified. The school includes classroom sessions on wastewater characteristics,

laws, regulations, permit requirements, and the theory and concepts underlying subsurface treatment

and dispersal systems. Training units also cover the essential elements of operating small and large

mechanical systems, with field work in alternative system operation at NCSU’s field laboratory.

Participants receive a training manual before they arrive for the 3-day training course. Certification of

those successfully completing the educational program is handled by the Water Pollution Control

System Operators Certification Commission, an independent entity that tests and certifies system

operators throughout North Carolina.

Source: NCSU, 2001
USEPA Onsite Wastewater Treatment Systems Manual 2-33

Chapter 2: Management of Onsite Wastewater Treatment Systems

2,000 gallons per day or systems with unusual

wastewater characteristics be registered professional

engineers. Prerequisites for applying for a site

evaluator permit and taking the certification

examination are either a degree in engineering,

soils, geology, or a similar field plus 1 year of

experience or a high school diploma or equivalent

and 4 years of experience (Maine Department of

Human Services, 1996). State certification and

licensing programs are summarized in table 2-2.

Table 2-2. Survey of state certification and licensing programs

Source: Noah, 2000.
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2-34 USEPA Onsite Wastewater Treatment Systems Manual

2.4.16 Education and training programs

for service providers and

program staff

Onsite system RAs, RMEs, and service provider

staff should have the requisite level of training and

experience to effectively assume necessary program

responsibilities and perform necessary activities.

Professional programs are typically the mechanism

for ensuring the qualifications of these personnel.

They usually include licensing or certification

elements, which are based on required coursework

or training; an assessment of knowledge, skills, and

professional judgment; past experience; and

demonstrated competency. Most licensing programs

require continuing education through recommended

or required workshops at specified intervals. For

example, the Minnesota program noted previously

requires 3 additional days of training every 3 years.

Certification programs for inspectors, installers,

and septage haulers provide assurance that systems

are installed and maintained properly. States are

beginning to require such certification for all

service providers to ensure that activities the

providers conduct comply with program requirements. Violation of program requirements or poor

performance can lead to revocation of certification

and prohibitions on installing or servicing onsite

systems. This approach, which links professional

performance with economic incentives, is highly

effective in maintaining compliance with onsite

program requirements. Programs that simply

register service providers or fail to take disciplinary

action against poor performers cannot provide the

same level of pressure to comply with professional

and technical codes of behavior.

Some certification and licensing programs for those

implementing regulations and performing site

evaluations require higher educational achievement.

For example, Kentucky requires a 4-year college

degree with 24 hours of science coursework,

completion of a week-long soils characterization

class, and another week of in-service training for

all permit writers and site evaluators (Kentucky

Revised Statutes, 2001). Regular training sessions

are also important in keeping site evaluators,

permit writers, designers, and other service personnel effective. For example, the Minnesota Cooperative Extension Service administers 3-day workshops on basic and advanced inspection and maintenance practices, which are now required for

certification in 35 counties and most cities in the

state (Shephard, 1996). Comprehensive training

programs have been developed in other states,

including West Virginia and Rhode Island.

Sixteen states have training centers. For more

information on training programs for onsite

wastewater professionals, including a calendar of

planned training events and links to training

providers nationwide, visit the web site of the

National Environmental Training Center for Small

Communities at West Virginia University at http://

www.estd.wvu.edu/netc/

NSF onsite wastewater inspector accreditation program

NSF International has developed an accreditation program designed to verify the proficiency of persons

performing inspections of existing OWTSs. The accreditation program includes written and field tests and provides

credit for continuing education activities. Inspectors who pass the tests and receive accreditation are listed on the

NSF International web site and in the NSF Listing Book, which is circulated among industry, government, and

other groups.

The accreditation process includes four components. A written examination, conducted at designated locations

around the country, covers a broad range of topics related to system inspections, including equipment, evaluation

procedures, troubleshooting, and the NSF International Certification Policies. The field examination includes an

evaluation of an existing OWTS. An ethics statement, required as part of the accreditation, includes a pledge by

the applicant to maintain a high level of honesty and integrity in the performance of evaluation activities. Finally,

the continuing education component requires requalification every 5 years through retesting or earning

requalification credits by means of training or other activities.

To pass the written examination, applicants must answer correctly at least 75 of the 100 multiple-choice questions

and score at least 70 percent on the field evaluation. A 30-day wait is required for retesting if the applicant fails

either the written or field examination.

Source: Noah, 2000.
USEPA Onsite Wastewater Treatment Systems Manual 2-35

Chapter 2: Management of Onsite Wastewater Treatment Systems

NETCSC_curricula.html. For links to state onsite

regulatory agencies, codes, and other information,

visit http://www.estd.wvu.edu/nsfc/

NSFC_links.html.

2.4.17 Inspection and monitoring

programs to verify and assess

system performance

Routine inspections should be performed to ascertain system effectiveness. The type and frequency

of inspections should be determined by the size of

the area, site conditions, resource sensitivity, the

complexity and number of systems, and the resources of the RA or RME. The RA should ensure

that correct procedures are followed.

Scheduling inspections during seasonal rises in

ground water levels can allow monitoring of

performance during “worst case” conditions. A site

inspection program can be implemented as a system

owner training program, an owner/operator contract program with certified operators, or a routine

program performed by an RME. A combination of

visual, physical, bacteriological, chemical, and

remote monitoring and modeling can be used to

assess system performance. Specific requirements

for reporting to the appropriate regulatory agency

should be clearly defined for the management

program. Components of an effective inspection,

monitoring, operation, and maintenance program

include

• Specified intervals for required inspections

(e.g., every 3 months, every 2 years, at time of

property transfer or change of use).

• Legal authority to access system components for

inspections, monitoring, and maintenance.

• Monitoring of overall operation and performance, including remote sensing and failure

reporting for highly mechanical and complex

systems.

• Monitoring of receiving environments at

compliance boundaries to meet performance

requirements.

• Review of system use or flow records, (e.g.,

water meter readings).

• Required type and frequency of maintenance for

each technology.

• Identification, location, and analysis of system

failures.

• Correction schedules for failed systems through

retrofits or upgrades.

• Record keeping on systems inspected, results,

and recommendations.

Inspection programs are often incorporated into

comprehensive management programs as part of a

Providing legal access for inspections in

Colorado

Colorado regulations state that “the health officer or his/

her designated agent is authorized to enter upon

private property at reasonable times and upon

reasonable notice . . . to conduct required tests, take

samples, monitor compliance, and make inspections.”

Source: NSFC, 1995a.

Inspection and monitoring program elements

• Develop/maintain inventory of all systems in management area (e.g., location, age, owner, type, size).

• Establish schedule, parameters, and procedures for system inspections.

• Determine knowledge level required of inspectors and monitoring program staff.

• Ensure training opportunities for all staff and service providers.

• Establish licensing/certification program for inspectors.

• Develop inspection program (e.g., owner inspection, staff inspection, contractor inspection).

• Establish right-of-entry provisions to gain access for inspection or monitoring.

• Circulate inspection program details and schedules to system owners.

• Establish reporting system and database for inspection and monitoring program.

• Identify existing ground water and surface water monitoring in area and determine supplemental monitoring

required.
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2-36 USEPA Onsite Wastewater Treatment Systems Manual

seamless approach that includes planning site

evaluation, design, installation, operation, maintenance, and monitoring. For example, the Town of

Paradise, California, established an onsite wastewater management program in Butte County in 1992

after voters rejected a sewage plant proposal for a

commercial area (NSFC, 1996). The program

manages 16,000 systems through a system of

installation permits, inspections, and operating

permits with terms up to 7 years. Operating permit

fees are less than $15 per year and are included in

monthly water bills. Regular inspections, tank

pumping, and other maintenance activities are

conducted by trained, licensed service providers,

who report their activities to program administrators. Paradise is one of the largest unsewered

incorporated towns in the nation.

Outreach programs to lending institutions on the

benefits of requiring system inspections at the time

of property transfer can be an effective approach

for identifying and correcting potential problems

and avoiding compliance and enforcement actions.

Many lending institutions across the nation require

system inspections as part of the disclosure requirements for approving home or property loans. For

example, Washington State has disclosure provisions for realtors at the point of sale, and many

lending institutions have incorporated onsite system

performance disclosure statements into their loan

approval processes (Soltman, 2000)

Source: Adapted from Ciotoli and Wiswall, 1982; USEPA, 2000. Table 2-3. Components of an onsite system regulatory program
USEPA Onsite Wastewater Treatment Systems Manual 2-37

Chapter 2: Management of Onsite Wastewater Treatment Systems

2.4.18 Compliance, enforcement, and

corrective action programs

Requiring corrective action when onsite systems

fail or proper system maintenance does not occur

helps to ensure that performance goals and requirements will be met. Compliance and enforcement

measures are more acceptable to system owners and

the public when the RA is clear and consistent

regarding its mission, regulatory requirements, and

how the mission relates to public health and water

resource protection. An onsite wastewater compliance and enforcement program should be based on

reasonable and scientifically defensible regulations,

promote fairness, and provide a credible deterrent

to those who might be inclined to skirt its provisions. Regulations should be developed with

community involvement and provided in summary

or detailed form to all stakeholders and the public

at large through education and outreach efforts.

Service provider training programs are most

effective if they are based on educating contractors

and staff on technical and ecological approaches for

complying with regulations and avoiding known

and predictable enforcement actions. Table 2-3

describes the components of a regulatory program

for onsite/decentralized systems.

Various types of legal instruments are available to

formulate or enact onsite system regulations.

Regulatory programs can be enacted as ordinances,

management constituency agreements, or local or

state codes, or simply as guidelines. Often, local

health boards or other units of government can

modify state code requirements to better address

local conditions. Local ordinances that promote

performance-based approaches can reference

technical design manuals for more detailed criteria

on system design and operation. Approaches for

enforcing requirements and regulations of a

management program can include

• Response to complaints

• Performance inspections

• Review of required documentation and reporting

• Issuance of violation notices

• Consent orders and court orders

• Formal and informal hearings

• Civil and criminal actions or injunctions

• Condemnation of systems and/or property

• Correcting system failures

• Restriction of real estate transactions (e.g.,

placement of liens)

• Issuance of fines and penalties

Some of these approaches can become expensive or

generate negative publicity and provide little in

terms of positive outcomes if public support is not

present. Involvement of stakeholders in the development of the overall management program helps

ensure that enforcement provisions are appropriate

for the management area and effectively protect

human health and water resources. Stakeholder

involvement generally stresses restoration of

performance compliance rather than more formal

punitive approaches.

Information on regional onsite system performance, environmental conditions, management

approaches by other agencies, and trends analyses

might be needed if regulatory controls are increased. Most states establish regulatory programs

and leave enforcement of these codes up to the

local agencies. Table 2-4 contains examples of

enforcement options for onsite management

programs.

A regulatory program focused on achieving

performance requirements rather than complying

with prescriptive requirements places greater

responsibilities on the oversight/permitting agency,

service providers (site evaluator, designer, contractor, and operator), and system owners. The management entity should establish credible performance standards and develop the competency to

review and approve proposed system designs that a

manufacturer or engineer claims will meet established standards. Continuous surveillance of the

performance of newer systems should occur

Corrective action program elements

• Establish process for reporting and responding to

problems (e.g., complaint reporting, inspections).

• Define conditions that constitute a violation of

program requirements.

• Establish inspection procedures for reported

problems and corrective action schedule.

• Develop a clear system for issuing violation notices,

compliance schedules, contingencies, fines, or other

actions to address uncorrected violations.
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2-38 USEPA Onsite Wastewater Treatment Systems Manual

through an established inspection and compliance

program. The service providers should be involved

in such programs to ensure that they develop the

knowledge and skills to successfully design, site,

build, and/or operate the treatment system within

established performance standards. Finally, the

management entity should develop a replicable

process to ensure that more new treatment technologies can be properly evaluated and appropriately managed.

2.4.19 Data collection, record keeping,

and reporting

Onsite wastewater management entities require a

variety of data and other information to function

effectively. This information can be grouped in the

following categories:

• Environmental assessment information: climate,

geology, topography, soils, slopes, ground water

and surface water characterization data (including direction of flow), land use/land cover

information, physical infrastructure (roads,

water lines, sewer lines, commercial development, etc.).

• Planning information: existing and proposed

development, proposed water or sewer line

extensions, zoning classifications, population

trends data, economic information, information

regarding other agencies or entities involved in

onsite wastewater issues.

• Existing systems information: record of site

evaluations conducted and inventory of all

existing onsite systems, cluster systems, package

plants, and wastewater treatment plants, including location, number of homes/facilities served

and size (e.g., 50-seat restaurant, 3-bedroom

Table 2-4. Compliance assurance approaches

Source: Ciotoli and Wiswall, 1982.
USEPA Onsite Wastewater Treatment Systems Manual 2-39

Chapter 2: Management of Onsite Wastewater Treatment Systems

home), system owner and contact information,

location and system type, design and site

drawings (including locations of property lines,

wells, water resources), system components

(e.g., concrete or plastic tank, infiltration lines

or leaching chambers), design hydraulic capacity, performance expectations or effluent

requirements (if any), installation date, maintenance records (e.g., last pumpout, repair,

complaints, problems and actions taken, names

of all service providers), and septage disposal

records. Many states and localities lack accurate

system inventories. USEPA (2000) recommends

the establishment and continued maintenance of

accurate inventories of all OWTSs within a

management entity’s jurisdiction as a basic

requirement of all management programs.

• Administrative information: personnel files

(name, education/training, work history, skills/

expertise, salary rate, job review summaries),

financial data (revenue, expenses, debts and debt

service, income sources, cost per unit of service

estimates), service provider/vendor data (name,

contact information, certifications, licenses, job

performance summaries, disciplinary actions,

work sites, cost record), management program

initiatives and participating entities, program

development plans and milestones, septage

management information, and available resources.

Data collection and management are essential to

program planning, development, and implementation. The components of a management information system include database development, data

collection, data entry, data retrieval and integration,

data analysis, and reporting. A variety of software

is commercially available for managing system

inventory data and other information. Electronic

databases can increase the ease of collecting,

storing, retrieving, using, and integrating data after

the initial implementation and learning curve have

been overcome. For example, if system locations

Record keeping and reporting program

elements

Establish a database structure and reporting systems,

at a minimum, for

• Environmental assessments

• Planning and stakeholder involvement functions

• Existing systems

• Staff, service providers, financial, and other

administrative functions

• Inspection and monitoring program, including

corrective actions required

• Septage and residuals management, including

approved haulers, disposal sites, and manifest

system records

Use of onsite system tracking software in the Buzzards Bay watershed

The Buzzards Bay Project is a planning and technical assistance initiative sponsored by the state

environmental agency’s Coastal Zone Management Program. The Buzzards Bay Project was the first National

Estuary Program in the country to develop a watershed Comprehensive Conservation and Management Plan,

which the Governor and USEPA approved in 1991. The primary focus of the Buzzards Bay management plan is

to provide financial and technical assistance to Buzzards Bay municipalities to address nonpoint source

pollution and facilitate implementation of Buzzards Bay Management Plan recommendations. The Buzzards Bay

Project National Estuary Program provided computers and a software package to municipal boards of health in

the watershed to enable better tracking of septic system permits, inspection results, and maintenance

information. The software, along with the user’s manual and other information, can be downloaded from the

Internet to provide easy access for jurisdictions interested in its application and use (see http:// http://

www.buzzardsba uzzardsbauzzardsbay.org/septrfct.htm). .org/septrfct.htm This approach is designed to help towns and cities reduce the time they

spend filing, retrieving, and maintaining information through a system that can provide—at the click of a

mouse—relevant data on any lot in the municipality. The software program can also help towns respond to

information requests more effectively, process permit applications more quickly, and manage new inspection

and maintenance reporting requirements more efficiently.

Source: Buzzards Bay Project National Estuary Program, 1999.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-40 USEPA Onsite Wastewater Treatment Systems Manual

are described in terms of specific latitude and

longitude coordinates, a data layer for existing

onsite systems can be created and overlaid on

geographic information system (GIS) topographic

maps. Adding information on onsite wastewater

hydraulic output, estimated mass pollutant loads,

and transport times expected for specified

hydrogeomorphic conditions can help managers

understand how water resources become contaminated and help target remediation and prioritization

actions. Models can also be constructed to predict

impacts from proposed development and assist in

setting performance requirements for onsite

systems in development areas.

System inventories are essential elements for

management programs, and most jurisdictions

maintain databases of new systems through their

permitting programs. Older systems (those installed

before 1970), however, are often not included in

the system inventories. Some onsite management

programs or other entities conduct inventories of

older systems when such systems are included in a

special study area. For example, Cass County and

Crow Wing County in Minnesota have developed

projects to inventory and inspect systems at more

than 2,000 properties near lakes in the north-central

part of the state (Sumption, personal communication, 2000). The project inventoried systems that

were less than 5 years old but did not inspect them

unless complaint or other reports indicated possible

problems. Costs for inventorying and inspecting

234 systems in one lake watershed totaled $9,000, or

nearly $40 per site (Sumption, personal communication, 2000). Mancl and Patterson (2001) cite a cost

of $30 per site inspection at Lake Panorama, Iowa.

Some data necessary for onsite system management

might be held and administered by other agencies.

For example, environmental or planning agencies

often collect, store, and analyze land and water

resource characterization data. Developing data

sharing policies with other entities through cooperative agreements can help all organizations

involved with health and environmental issues

improve efficiency and overall program performance. The management agency should ensure that

data on existing systems are available to health and

water resource authorities so their activities and

analyses reflect this important aspect of public

health and environmental protection.

2.4.20 Program evaluation criteria and

procedures

Evaluating the effectiveness of onsite management

program elements such as planning, funding,

enforcement, and service provider certification can

provide valuable information for improving

programs. A regular and structured evaluation of

any program can provide critical information for

program managers, the public, regulators, and

decision makers. Regular program evaluations

should be performed to analyze program methods

and procedures, identify problems, evaluate the

potential for improvement through new technologies or program enhancements, and ensure funding

is available to sustain programs and adjust program

goals. The program evaluation process should

include

• A tracking system for measuring success

and for evaluating and adapting program

components

• Processes for comparing program achievements

to goals and objectives

• Approaches for adapting goals and objectives if

internal or external conditions change

• Processes for initiating administrative or legal

actions to improve program functioning

• An annual report on the status, trends, and

achievements of the management program

• Venues for ongoing information exchange

among program stakeholders

A variety of techniques and processes can be used

to perform program evaluations to assess administrative and management elements. The method

chosen for each program depends on local circumstances, the type and number of stakeholders involved, and the level of support generated by

management agencies to conduct a careful, unbiased,

detailed review of the program’s success in protecting

health and water resources. Regardless of the

method selected, the program evaluation should be

performed at regular intervals by experienced staff,

and program stakeholders should be involved.

A number of state, local, and private organizations

have implemented performance-based management

programs for a wide range of activities, from state

budgeting processes to industrial production

operations. The purpose of these programs is
USEPA Onsite Wastewater Treatment Systems Manual 2-41

Chapter 2: Management of Onsite Wastewater Treatment Systems

twofold: linking required resources with management objectives and ensuring continuous improvement. Onsite management programs could also ask

partnering entities to use their experience to help

develop and implement in-house evaluation processes.

2.5 Financial assistance for

management programs and

system installation

Most management programs do not construct or

own the systems they regulate. Homeowners or

other private individuals usually pay a permit fee to

the agency to cover site evaluation and permitting

costs and then finance the installation, operation,

maintenance, and repair of their systems themselves. During recent years, however, onsite

management officials and system owners have

become increasingly supportive of centralized operation,

maintenance, and repair services. In addition, some

management programs are starting to provide

assistance for installation, repair, or replacement in the

form of cost-share funding, grants, and low-interest

loans. Some communities have elected to make a

transition from individual systems to a clustered

approach to capitalize on the financial and other

benefits associated with the joint use of lagoons,

drain fields, and other system components linked by

gravity, vacuum, or low-pressure piping. Developers of

cluster systems, which feature individual septic tanks

and collective post-tank treatment units, have been

particularly creative and aggressive in obtaining

financing for system installation.

Funding for site evaluation, permitting, and

enforcement programs is generally obtained from

permit fees, property assessments (e.g., health district

taxes), and allocations from state legislatures for

environmental health programs. However, many

jurisdictions have discovered that these funding

sources do not adequately support the full range of

planning, design review, construction oversight,

inspection and monitoring, and remediation functions

that constitute well-developed onsite management

programs. Urbanized areas have supplemented

funding for their management programs with fees

paid by developers, monthly wastewater treatment

service fees (sometimes based on metered water

use), property assessment increases, professional

licensing fees, fines and penalties, and local general

fund appropriations. This section includes an

overview of funding options for onsite system

management programs.

2.5.1 Financing options

Two types of funding are usually necessary for

installation and management of onsite wastewater

systems. First, initial funding is required to pay for

any planning and construction costs, which include

legal, administrative, land acquisition, and engineering costs. Once the construction is complete,

additional funding is needed to finance the ongoing

operation and maintenance, as well as to pay for

the debt service incurred from borrowing the initial

funds. Table 2-6 lists potential funding sources and

the purposes for which the funds are typically used.

As indicated in the table, each funding source has

advantages and disadvantages. Decision makers

must choose the funding sources that best suit their

community.

Primary sources of funds include

• Savings (capital reserve)

• Grants (state, federal)

• Loans (state, federal, local)

• Bond issues (state, local)

• Property assessments

Publicly financed support for centralized wastewater treatment services has been available for

decades from federal, state, and local sources.

Since 1990 support for public funding of onsite

treatment systems has been growing. The following

section summarizes the most prominent sources of

Performance-based budgeting in Texas

Since 1993 state agencies in Texas have been required

to develop a long-term strategic plan that includes a

mission statement, goals for the agency, performance

measures, an identification of persons served by the

agency, an analysis of the resources needed for the

agency to meet its goals, and an analysis of expected

changes in services due to changes in the law. Agency

budget line items are tied to performance measures

and are available for review through the Internet.

Information on the budgeting process in Texas is

available from the Texas Legislative Budget Board at

http://www.lbb.state.tx.us.

Source: Texas Senate Research Center, 2000.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-42 USEPA Onsite Wastewater Treatment Systems Manual

Suggested approach for conducting a formal program evaluation

Form a program evaluation team

Define the goals, objectives, and operational elements Define the goals, objectives, and operational elements 

Review the program components checklist and feedback

Identify program components or elements in need of improvement Identify program components or elements in need of improvement

Communicate suggested improvements to program managers

Form a program evaluation team composed of management program staff, service providers, public health

agency representatives, environmental protection organizations, elected officials, and interested citizens.

Define the goals, objectives, and operational elements of the various onsite management program

components. This can be done simply by using a checklist to identify which program components currently exist.

Table 2-5 provides an excellent matrix for evaluating the management program.

Review the program components checklist and feedback collected from staff and stakeholders to determine

progress toward goals and objectives, current status, trends, cost per unit of service, administrative processes

used, and cooperative arrangements with other entities.

Identify program components or elements in need of improvement, define actions or amount and type of

resources required to address deficient program areas, identify sources of support or assistance, discuss

proposed program changes with the affected stakeholders, and implement recommended improvement actions.

Communicate suggested improvements to program managers to ensure that the findings of the evaluation

are considered in program structure and function.

Table 2-5. Example of Functional Responsibilities Matrix

*Management functions that require local agency input.
USEPA Onsite Wastewater Treatment Systems Manual 2-43

Chapter 2: Management of Onsite Wastewater Treatment Systems

grant, loan, and loan guarantee funding and outline

other potential funding sources.

2.5.2 Primary funding sources

The following agencies and programs are among the

most dependable and popular sources of funds for

onsite system management and installation programs.

Clean Water State Revolving Fund

The Clean Water State Revolving Fund, or CWSRF

(see http://www.epa.gov/owm/finan.htm), is a

low- or no-interest loan program that has traditionally financed centralized sewage treatment plants

across the nation. Program guidance issued in 1997

emphasized that the fund could be used as a source

of support for the installation, repair, or upgrading

of onsite systems in small towns, rural areas, and

suburban areas. The states and the territory of

Puerto Rico administer CWSRF programs, which

operate like banks. Federal and state contributions

are used to capitalize the fund programs, which

make low- or no-interest loans for water quality

projects. Funds are then repaid to the CWSRF over

terms as long as 20 years. Repaid funds are recycled to fund other water quality projects. Projects

a Principal and interest payment (debt service) on various loans used for initial financing.

Sources: Ciotoli and Wiswall, 1982, 1986; Shephard, 1996.

Table 2-6. Funding options
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-44 USEPA Onsite Wastewater Treatment Systems Manual

that might be eligible for CWSRF funding include

new system installations and replacement or

modification of existing systems. Costs associated

with establishing a management entity to oversee

onsite systems in a region, including capital outlays

(e.g., for trucks on storage buildings), may also be

eligible. Approved management entities include

city and county governments, special districts,

public or private utilities, and private for-profit or

nonprofit corporations.

U.S. Department of Agriculture Rural

Development programs

U.S. Department of Agriculture Rural Development programs provide loans and grants to low and

moderate-income persons. State Rural Development offices administer the programs; for state

office locations, see http://www.rurdev.usda.gov/

recd_map.html. A brief summary of USDA Rural

Development programs is provided below.

Rural Housing Service

The Rural Housing Service Single-Family Housing

Program (http://www.rurdev.usda.gov/rhs/Individual/ind_splash.htm) provides homeownership

opportunities to low- and moderate-income rural

Americans through several loan, grant, and loan

guarantee programs. The program also makes

funding available to individuals to finance vital

improvements necessary to make their homes safe

and sanitary. The Direct Loan Program (section

502) provides individuals or families direct financial assistance in the form of a home loan at an

affordable interest rate. Most loans are to families

with incomes below 80 percent of the median

income level in the communities where they live.

Applicants might obtain 100 percent financing to

build, repair, renovate, or relocate a home, or to

purchase and prepare sites, including providing

water and sewage facilities. Families must be

without adequate housing but be able to afford the

mortgage payments, including taxes and insurance.

These payments are typically within 22 to 26

percent of an applicant’s income. In addition,

applicants must be unable to obtain credit elsewhere yet have reasonable credit histories. Elderly

and disabled persons applying for the program may

have incomes up to 80 percent of the area median

income.

Home Repair Loan and Grant Program

For very low-income families that own homes in

need of repair, the Home Repair Loan and Grant

Program offers loans and grants for renovation.

Money might be provided, for example, to repair a

leaking roof; to replace a wood stove with central

heating; or to replace a pump and an outhouse with

running water, a bathroom, and a waste disposal

system. Homeowners 62 years and older are

eligible for home improvement grants. Other lowincome families and individuals receive loans at a

1 percent interest rate directly from the Rural

Housing Service. Loans of up to $20,000 and

grants of up to $7,500 are available. Loans are for

up to 20 years at 1 percent interest.

Rural Utilities Service

The Rural Utilities Service (http://www.usda.gov/

rus/water/programs.htm) provides assistance for

public or not-for-profit utilities, including wastewater management districts. Water and waste

disposal loans provide assistance to develop water

and waste disposal systems in rural areas and towns

with a population of 10,000 or less. The funds are

available to public entities such as municipalities,

counties, special-purpose districts, Indian tribes,

and corporations not operated for profit. The

program also guarantees water and waste disposal

loans made by banks and other eligible lenders.

Water and Waste Disposal Grants can be accessed to

reduce water and waste disposal costs to a reasonable level for rural users. Grants might be made for

up to 75 percent of eligible project costs in some

cases.

Financial assistance program elements

• Determine program components or system aspects

that require additional financial assistance.

• Identify financial resources available for system

design, installation, operation, maintenance, and

repair.

• Research funding options (e.g., permit or user fees,

property taxes, impact fees, fines, grants/loans).

• Work with stakeholder group to execute or establish

selected funding option(s).
USEPA Onsite Wastewater Treatment Systems Manual 2-45

Chapter 2: Management of Onsite Wastewater Treatment Systems

Rural Business-Cooperative Service

The Rural Business-Cooperative Service (http://

www.rurdev.usda.gov/rbs/busp/b&i_gar.htm)

provides assistance for businesses that provide

services for system operation and management.

Business and Industry Guaranteed Loans can be

made to help create jobs and stimulate rural

economies by providing financial backing for rural

businesses. This program provides guarantees up to

90 percent of a loan made by a commercial lender.

Loan proceeds might be used for working capital,

machinery and equipment, buildings and real

estate, and certain types of debt refinancing.

Assistance under the Guaranteed Loan Program is

available to virtually any legally organized entity,

including a cooperative, corporation, partnership,

trust or other profit or nonprofit entity, Indian tribe

or federally recognized tribal group, municipality,

county, or other political subdivision of a state.

Community Development Block Grants

The U.S. Department of Housing and Urban

Development (HUD) operates the Community

Development Block Grant (CDBG) program, which

provides annual grants to 48 states and Puerto Rico.

The states and Puerto Rico use the funds to award

grants for community development to smaller cities

and counties. CDBG grants may be used for

numerous activities, including rehabilitating

residential and nonresidential structures, constructing public facilities, and improving water and

sewer facilities, including onsite systems. USEPA

is working with HUD to improve access to CDBG

funds for treatment system owners by raising

program awareness, reducing paperwork burdens,

and increasing promotional activities in eligible

areas. More information is available at http://

www.hud.gov/cpd/cdbg.html.

Nonpoint Source Pollution Program

Clean Water Act section 319 (nonpoint source

pollution control) funds can support a wide range

of polluted runoff abatement, including onsite

wastewater projects. Authorized under section 319

of the federal Clean Water Act and financed by

federal, state, and local contributions, these projects

provide cost-share funding for individual and

community systems and support broader watershed

assessment, planning, and management activities.

Projects funded in the past have included direct

cost-share for onsite system repairs and upgrades,

assessment of watershed-scale onsite system

contributions to polluted runoff, regional

remediation strategy development, and a wide

range of other programs dealing with onsite

wastewater issues. For example, a project conducted by the Gateway District Health Department

in east-central Kentucky enlisted environmental

science students from Morehead State University to

collect and analyze stream samples for fecal

coliform “hot spots.” Information collected by the

students was used to target areas with failing

systems for cost-share assistance or other

remediation approaches (USEPA, 1997b). The

Rhode Island Department of Environmental

Management developed a user-friendly system

inspection handbook with section 319 funds to

improve system monitoring practices and then

developed cost-share and loan programs to help

system owners pay for needed repairs (USEPA,

1997). For more information, see http://

www.epa.gov/OWOW/NPS/.

2.5.3 Other funding sources

Other sources of funding include state finance

programs, capital reserve or savings funds, bonds,

PENNVEST: Financing onsite wastewater systems in the Keystone State

The Pennsylvania Infrastructure Investment Authority (PENNVEST) provides low-cost financing for systems on

individual lots or within entire communities. Teaming with the Pennsylvania Housing Finance Agency and the

state’s Department of Environmental Protection, PENNVEST created a low-interest onsite system loan program

for low- to moderate-income (150 percent of the statewide median household income) homeowners. The $65

application fee is refundable if the project is approved. The program can save system owners $3,000 to $6,000

in interest payments on a 15-year loan of $10,000. As of 1999 PENNVEST had approved 230 loans totaling $3.5

million. Funds for the program come from state revenue bonds, special statewide referenda, the state general

fund, and the State Revolving Fund.

Source: PADEP, 1998.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-46 USEPA Onsite Wastewater Treatment Systems Manual

Funding systems and management in

Massachusetts

The Commonwealth of Massachusetts has developed

three programs that help finance onsite systems and

management programs. The loan program provides

loans at below-market rates. A tax credit program

provides a tax credit of up to $4,500 over 3 years to

defray the cost of system repairs for a primary

residence. Finally, the Comprehensive Community

Septic Management Program provides funding for longterm community, regional, or watershed-based

solutions to system failures in sensitive environmental

areas. Low-interest management program loans of up

to $100,000 are available.

Source: Massachusetts DEP, 2000.

certificates of participation, notes, and property

assessments. Nearly 20 states offer some form of

financial assistance for installation of OWTSs,

through direct grants, loans, or special project costshare funding. Capital reserve or savings funds are

often used to pay for expenses that might not be

eligible for grants or loans, such as excess capacity

for future growth. Capital reserve funds can also be

used to assist low- and moderate-income households with property assessment or connection fees.

Bonds usually finance long-term capital projects

such as the construction of OWTSs. States, municipalities, towns, townships, counties, and special

districts issue bonds. The two most common types

of bonds are general obligation bonds, which are

backed by the faith and credit of the issuing

government, and revenue bonds, which are supported by the revenues raised from the beneficiaries

of a service or facility. General obligation bonds

are rarely issued for wastewater treatment facilities

because communities are often limited in the

amount of debt they might incur. These bonds are

generally issued only for construction of schools,

libraries, municipal buildings, and police or fire

stations.

Revenue bonds are usually not subject to debt

limits and are secured by repayment through user

fees. Issuing revenue bonds for onsite projects

allows a community to preserve the general obligation borrowing capacity for projects that do not

generate significant revenues. A third and less

commonly used bond is the special assessment

bond, which is payable only from the collection of

special property assessments. Some states administer state bond banks, which act as intermediaries

between municipalities and the national bond

market to help small towns that otherwise would

have to pay high interest rates to attract investors or

would be unable to issue bonds. State bond banks,

backed by the fiscal security of the state, can issue

one large, low-interest bond that funds projects in a

number of small communities

Communities issue Certificates of Participation

(COPs) to lenders to spread out costs and risks of

loans to specific projects. If authorized under state

law, COPs can be issued when bonds would exceed

debt limitations. Notes, which are written promises

to repay a debt at an established interest rate, are

similar to COPs and other loan programs. Notes are

used mostly as a short-term mechanism to finance

construction costs while grant or loan applications

are processed. Grant anticipation notes are secured

by a community’s expectation that it will receive a

grant. Bond anticipation notes are secured by the

community’s ability to sell bonds.

Finally, property assessments might be used to

recover capital costs for wastewater facilities that

benefit property owners within a defined area. For

example, property owners in a specific neighbor-
USEPA Onsite Wastewater Treatment Systems Manual 2-47

Chapter 2: Management of Onsite Wastewater Treatment Systems

Table 2-7. Advantages and disadvantages of various funding sources

Source: USEPA,1994.
Chapter 2: Management of Onsite Wastewater Treatment Systems

2-48 USEPA Onsite Wastewater Treatment Systems Manual

hood could be assessed for the cost of installing

sewers or a cluster treatment system. Depending on

the amount of the assessment, property owners

might pay it all at once or pay in installments at a

set interest rate. Similar assessments are often

charged to developers of new residential or commercial facilities if the developers are not required

to install wastewater treatment systems approved by

the local regulatory agency. Funding for ongoing

management of onsite systems in newly developed

areas should be considered when these assessments

are calculated.

Although funds from grants, special projects, and

other one-time sources can help initiate special

projects or develop new functions, support for

onsite management over the long term should come

from sources that can provide continuous funding

(table 2-7). Monthly service fees, property assessments, regular general fund allocations, and permit/

licensing fees can be difficult to initiate but provide

the most assurance that management program

activities can be supported over the long term.

Securing public acceptance of these financing

mechanisms requires stakeholder involvement in

their development, outreach programs that provide

a clear picture of current problems and expected

benefits, and an appropriate matching of community resources with management program need.

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Chapter 2: Management of Onsite Wastewater Treatment Systems

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USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 3-1

Chapter 3: Establishing Treatment System Performance Requirements

3.1 Introduction

This chapter outlines essential steps for characterizing wastewater flow and composition and provides

a framework for establishing and measuring

performance requirements. Chapter 4 provides

information on conventional and alternative

systems, including technology types, pollutant

removal effectiveness, basic design parameters,

operation and maintenance, and estimated costs.

Chapter 5 describes treatment system design and

selection processes, failure analysis, and corrective

measures.

This chapter also describes methods for establishing

and ensuring compliance with wastewater treatment

performance requirements that protect human

health, surface waters, and ground water resources.

The chapter describes the characteristics of typical

domestic and commercial wastewaters and discusses

approaches for estimating wastewater quantity and

quality for residential dwellings and commercial

establishments. Pollutants of concern in wastewaters are identified, and the fate and transport of

these pollutants in the receiving environment are

discussed. Technical approaches for establishing

performance requirements for onsite systems, based

on risk and environmental sensitivity assessments,

are then presented. Finally, the chapter discusses

performance monitoring to ensure sustained

protection of public health and water resources.

Chapter 3:

Establishing treatment system performance requirements

3.2 Estimating wastewater

characteristics

Accurate characterization of raw wastewater,

including daily volumes, rates of flow, and associated

pollutant load, is critical for effective treatment

system design. Determinating treatment system

performance requirements, selecting appropriate

treatment processes, designing the treatment

system, and operating the system depends on an

accurate assessment of the wastewater to be treated.

There are basically two types of onsite system

wastewaters—residential and nonresidential.

Single-family households, condominiums, apartment houses, multifamily households, cottages, and

resort residences all fall under the category of

residential dwellings. Discharges from these

dwellings consist of a number of individual waste

streams generated by water-using activities from a

variety of plumbing fixtures and appliances.

Wastewater flow and quality are influenced by the

type of plumbing fixtures and appliances, their

extent and frequency of use, and other factors such

as the characteristics of the residing family, geographic location, and water supply (Anderson and

Siegrist, 1989; Crites and Tchobanoglous, 1998;

Siegrist, 1983).

A wide variety of institutional (e.g., schools),

commercial (e.g., restaurants), and industrial

3.1 Introduction

3.2 Estimating wastewater characteristics

3.3 Estimating wastewater flow

3.4 Wastewater quality

3.5 Minimizing wastewater flows and pollutants

3.6 Integrating wastewater characterization and other design information

3.7 Transport and fate of wastewater pollutants in the receiving environment

3.8 Establishing performance requirements
Chapter 3: Establishing Treatment System Performance Requirements

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establishments and facilities fall into the

nonresidential wastewater category. Wastewatergenerating activities in some nonresidential establishments are similar to those of residential dwellings.

Often, however, the wastewater from nonresidential

establishments is quite different from that from of

residential dwellings and should be characterized

carefully before Onsite Wastewater Treatment

System (OWTS) design. The characteristics of

wastewater generated in some types of nonresidential

establishments might prohibit the use of conventional systems without changing wastewater loadings

through advanced pretreatment or accommodating

elevated organic loads by increasing the size of the

subsurface wastewater infiltration system (SWIS).

Permitting agencies should note that some commercial and large-capacity septic systems (systems

serving 20 or more people, systems serving commercial facilities such as automotive repair shops)

might be regulated under USEPA’s Class V Underground Injection Control Program (see http://

www.epa.gov/safewater/uic/classv.html).

In addition, a large number of seemingly similar

nonresidential establishments are affected by subtle

and often intangible influences that can cause

significant variation in wastewater characteristics.

For example, popularity, price, cuisine, and

location can produce substantial variations in wastewater flow and quality among different restaurants

(University of Wisconsin, 1978). Nonresidential

wastewater characterization criteria that are easily

applied and accurately predict flows and pollutant

loadings are available for only a few types of

establishments and are difficult to develop on a

national basis with any degree of confidence. Therefore, for existing facilities the wastewater to be

treated should be characterized by metering and

sampling the current wastewater stream. For many

existing developments and for almost any new

development, however, characteristics of nonresidential wastewaters should be estimated based on

available data. Characterization data from similar

facilities already in use can provide this information.

3.3 Estimating wastewater flow

The required hydraulic capacity for an OWTS is

determined initially from the estimated wastewater

flow. Reliable data on existing and projected flows

should be used if onsite systems are to be designed

properly and cost-effectively. In situations where

onsite wastewater flow data are limited or unavailable, estimates should be developed from water

consumption records or other information. When

using water meter readings or other water use

records, outdoor water use should be subtracted to

develop wastewater flow estimates. Estimates of

outdoor water use can be derived from discussions

with residents on car washing, irrigation, and other

outdoor uses during the metered period under

review, and studies conducted by local water

utilities, which will likely take into account climatic

and other factors that affect local outdoor use.

Accurate wastewater characterization data and

appropriate factors of safety to minimize the

possibility of system failure are required elements

of a successful design. System design varies

considerably and is based largely on the type of

establishment under consideration. For example,

daily flows and pollutant contributions are usually

expressed on a per person basis for residential

dwellings. Applying these data to characterize

residential wastewater therefore requires that a

second parameter, the number of persons living in

the residence, be considered. Residential occupancy

is typically 1.0 to 1.5 persons per bedroom; recent

census data indicate that the average household size

is 2.7 people (U.S. Census Bureau, 1998). Local

census data can be used to improve the accuracy of

design assumptions. The current onsite code

practice is to assume that maximum occupancy is

2 persons per bedroom, which provides an estimate

that might be too conservative if additional factors

of safety are incorporated into the design.

For nonresidential establishments, wastewater flows

are expressed in a variety of ways. Although per

person units may also be used for nonresidential

wastewaters, a unit that reflects a physical characteristic of the establishment (e.g., per seat, per meat

served, per car stall, or per square foot) is often

used. The characteristic that best fits the wastewater

characterization data should be employed (University of Wisconsin, 1978).

When considering wastewater flow it is important

to address sources of water uncontaminated by

wastewater that could be introduced into the

treatment system. Uncontaminated water sources

(e.g., storm water from rain gutters, discharges

from basement sump pumps) should be identified

and eliminated from the OWTS. Leaking joints,
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Chapter 3: Establishing Treatment System Performance Requirements

cracked treatment tanks, and system damage caused

by tree roots also can be significant sources of clear

water that can adversely affect treatment performance. These flows might cause periodic hydraulic

overloads to the system, reducing treatment effectiveness and potentially causing hydraulic failure.

3.3.1 Residential wastewater flows

Average daily flow

The average daily wastewater flow from typical

residential dwellings can be estimated from indoor

water use in the home. Several studies have evaluated residential indoor water use in detail (Anderson and Siegrist, 1989; Anderson et al., 1993;

Brown and Caldwell, 1984; Mayer et al., 1999). A

summary of recent studies is provided in table 3-1.

These studies were conducted primarily on homes

in suburban areas with public water supplies.

Previous studies of rural homes on private wells

generally indicated slightly lower indoor water use

values. However, over the past three decades there

has been a significant increase in the number of

suburban housing units with onsite systems, and it

has recently been estimated that the majority of

OWTSs in the United States are located in suburban metropolitan areas (Knowles, 1999). Based on

the data in table 3-1, estimated average daily

wastewater flows of approximately 50 to 70 gallons

per person per day (189 to 265 liters per person per

day) would be typical for residential dwellings

built before 1994.

In 1994 the U.S. Energy Policy Act (EPACT)

standards went into effect to improve water use

efficiency nationwide. EPACT established national

flow rates for showerheads, faucets, urinals, and

water closets. In 2004 and again in 2007 energy use

standards for clothes washers will go into effect,

and they are expected to further reduce water use

by those appliances. Homes built after 1994 or

retrofitted with EPACT-efficient fixtures would

have typical average daily wastewater flows in the

40 to 60 gallons/person/day range. Energy- and

water-efficient clothes washers may reduce the per

capita flow rate by up to 5 gallons/person/day

(Mayer et al., 2000).

Of particular interest are the results of the Residential End Uses of Water Study (REUWS), which

was funded by the American Water Works Association Research Foundation (AWWARF) and 12

water supply utilities (Mayer et al., 1999). This

study involved the largest number of residential

water users ever characterized and provided an

evaluation of annual water use at 1,188 homes in

12 metropolitan areas in North America. In addition, detailed indoor water use characteristics of

approximately 100 homes in each of the 12 study

areas were evaluated by continuous data loggers

and computer software that identified fixturespecific end uses of water. Table 3-2 provides the

Table 3-1. Summary of average daily residential wastewater flowsa
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average daily per capita indoor water use by study

site for the 1,188 homes. The standard deviation

data provided in this table illustrate the significant

variation of average daily flow among residences. The

median daily per capita flow ranged from 54 to 67

gallons/person/day (204 to 253 liters/person/day) and

probably provides a better estimate of average daily

flow for most homes given the distribution of mean

per capita flows in figure 3-1 (Mayer et al., 2000).

This range might be reduced further in homes with

EPACT-efficient fixtures and appliances.

Individual activity flows

Average daily flow is the average total flow generated

on a daily basis from individual wastewatergenerating activities in a building. These activities

typically include toilet flushing, showering and

bathing, clothes washing and dishwashing, use of

faucets, and other miscellaneous uses. The average

flow characteristics of several major residential waterusing activities are presented in table 3-3. These data

were derived from some 1 million measured indoor

water use events in 1,188 homes in 12 suburban

areas as part of the REUWS (Mayer et al., 1999).

Figure 3-2 illustrates these same data graphically.

One of the more important wastewater-generating

flows identified in this study was water leakage

from plumbing fixtures. The average per capita

leakage measured in the REUWS was 9.5 gallons/

person/day (35.0 liters/person/day). However, this

value was the result of high leakage rates at a

relatively small percentage of homes. For example,

the average daily leakage per household was 21.9

gallons (82.9 liters) with a standard deviation of

54.1 gallons (204.8 liters), while the median

leakage rate was only 4.2 gallons/house/day (15.9

liters/house/day). Nearly 67 percent of the homes

in the study had average leakage rates of less than

10 gallons/day (37.8 liters/day), but 5.5 percent of

the study homes had leakage rates that averaged

more than 100 gallons (378.5 liters) per day. Faulty

toilet flapper valves and leaking faucets were the

primary sources of leaks in these high-leakage-rate

homes. Ten percent of the homes monitored

accounted for 58 percent of the leakage measured.

This result agrees with a previous end use study

where average leakage rates of 4 to 8 gallons/

person/day (15.1 to 30.3 liters/person/day) were

measured (Brown and Caldwell, 1984). These data

point out the importance of leak detection and

repair during maintenance or repair of onsite

Table 3-2. Comparison of daily per capita indoor water use for 12 study sites
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Chapter 3: Establishing Treatment System Performance Requirements

Table 3-3. Residential water use by fixture or appliancea,b

Source: Mayer et al., 1999.

Figure 3-1. Distribution of mean household daily per capita indoor water use for 1,188 data-logged homes
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systems. Leakage rates like those measured in the

REUWS could significantly increase the hydraulic

load to an onsite wastewater system and might

reduce performance.

Maximum daily and peak flows

Maximum and minimum flows and instantaneous

peak flow variations are necessary factors in

properly sizing and designing system components.

For example, most of the hydraulic load from a

home occurs over several relatively short periods of

time (Bennett and Lindstedt, 1975; Mayer et al.,

1999; University of Wisconsin, 1978). The system

should be capable of accepting and treating normal

peak events without compromising performance.

For further discussion of flow variations, see

section 3.3.3.

3.3.2 Nonresidential wastewater flows

For nonresidential establishments typical daily

flows from a variety of commercial, institutional,

and recreational establishments are shown in tables

3-4 to 3-6 (Crites and Tchobanoglous, 1998;

Tchobanoglous and Burton, 1991). The typical

values presented are not necessarily an average of

the range of values but rather are weighted values

based on the type of establishment and expected

use. Actual monitoring of specific wastewater flow

and characteristics for nonresidential establishments

is strongly recommended. Alternatively, a similar

establishment located in the area might provide

good information. If this approach is not feasible,

state and local regulatory agencies should be

consulted for approved design flow guidelines for

nonresidential establishments. Most design flows

provided by regulatory agencies are very conservative estimates based on peak rather than average

daily flows. These agencies might accept only their

established flow values and therefore should be

contacted before design work begins.

Figure 3-2. Indoor water use percentage, including leakage, for 1,188 data logged homesa

a

 gpcd = gallons per capita (person) per day

Source: Mayer et al. 1999.
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Chapter 3: Establishing Treatment System Performance Requirements

3.3.3 Variability of wastewater flow

Variability of wastewater flow is usually characterized by daily and hourly minimum and maximum

flows and instantaneous peak flows that occur

during the day. The intermittent occurrence of

individual wastewater-generating activities can

create large variations in wastewater flows from

residential or nonresidential establishments. This

variability can affect gravity-fed onsite systems by

potentially causing hydraulic overloads of the

system during peak flow conditions. Figure 3-3

illustrates the routine fluctuations in wastewater

flows for a typical residential dwelling.

Wastewater flow can vary significantly from day to

day. Minimum hourly flows of zero are typical for

Table 3-4. Typical wastewater flow rates from commercial sourcesa,b

Figure 3-3. Daily indoor water use pattern for single-family residence

Source: University of Wisconsin, 1978.
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residential dwellings. Maximum hourly flows as

high as 100 gallons (380 L/hr) (Jones, 1976;

Watson et al., 1967) are not unusual given the

variability of typical fixture and appliance usage

characteristics and residential water use demands.

Hourly flows exceeding this rate can occur in cases

of plumbing fixture failure and appliance misuse

(e.g., broken pipe or fixture, faucets left running).

Wastewater flows from nonresidential establishments are also subject to wide fluctuations over

time and are dependent on the characteristics of

water-using fixtures and appliances and the business characteristics of the establishment (e.g., hours

of operation, fluctuations in customer traffic).

The peak flow rate from a residential dwelling is

a function of the fixtures and appliances present

and their position in the plumbing system configuration. The peak discharge rate from a given

fixture or appliance is typically around 5 gallons/

minute (19 liters/minute), with the exception of

the tank-type toilet and possibly hot tubs and

bathtubs. The use of several fixtures or appliances

simultaneously can increase the total flow rate

above the rate for isolated fixtures or appliances.

However, attenuation occurring in the residential

drainage system tends to decrease peak flow rates

observed in the sewer pipe leaving the residence.

Although field data are limited, peak discharge

rates from a single-family dwelling of 5 to 10

gallons/minute (19 to 38 liters/minute) can be

expected. Figure 3-4 illustrates the variability in

peak flow from a single home.

3.4 Wastewater quality

The qualitative characteristics of wastewaters

generated by residential dwellings and nonresidential establishments can be distinguished by their

physical, chemical, and biological composition.

Because individual water-using events occur

intermittently and contribute varying quantities of

Table 3-5. Typical wastewater flow rates from institutional sources a

Source: University of Wisconsin, 1978.

Figure 3-4. Peak wastewater flows for single-family home
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Chapter 3: Establishing Treatment System Performance Requirements

pollutants, the strength of residential wastewater

fluctuates throughout the day (University of

Wisconsin, 1978). For nonresidential establishments,

wastewater quality can vary significantly among

different types of establishments because of differences in waste-generating sources present, water

usage rates, and other factors. There is currently a

dearth of useful data on nonresidential wastewater

organic strength, which can create a large degree of

uncertainty in design if facility-specific data are not

available. Some older data (Goldstein and Moberg,

1973; Vogulis, 1978) and some new information

exists, but modern organic strengths need to be

verified before design given the importance of this

aspect of capacity determination.

Wastewater flow and the type of waste generated

affect wastewater quality. For typical residential

sources peak flows and peak pollutant loading rates

do not occur at the same time (Tchobanoglous and

Burton, 1991). Though the fluctuation in wastewater quality (see figure 3-5) is similar to the water

use patterns illustrated in figure 3-3, the fluctuations in wastewater quality for an individual home

are likely to be considerably greater than the

multiple-home averages shown in figure 3-5.

Table 3-6. Typical wastewater flow rates from recreational facilitiesa
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Shaw, 1970). For shopping centers, the average

characteristics determined by one study found

BOD5

 average concentrations of 270 milligrams/

liter, with suspended solids concentrations of 337

milligrams/liter and grease concentrations of 67

milligrams/liter (Hayashida, 1975).

More recent characterizations of nonresidential

establishments have sampled septic tank effluent,

rather than the raw wastewater, to more accurately

identify and quantify the mass pollutant loads

delivered to the components of the final treatment

train (Ayres Associates, 1991; Siegrist et al., 1984).

Because of the variability of the data, for establishments where the waste-generating sources are

significantly different from those in a residential

dwelling or where more refined characterization

data might be appropriate, a detailed review of the

pertinent literature, as well as wastewater sampling

at the particular establishment or a similar establishment, should be conducted.

3.5 Minimizing wastewater flows

and pollutants

Minimizing wastewater flows and pollutants

involves techniques and devices to (1) reduce water

use and resulting wastewater flows and (2) decrease

the quantity of pollutants discharged to the waste

stream. Minimizing wastewater volumes and

pollutant concentrations can improve the efficiency

of onsite treatment and lessen the risk of hydraulic

or treatment failure (USEPA, 1995). These methOWTSs should be designed to accept and process

hydraulic flows from a residence (or establishment)

while providing the necessary pollutant removal

efficiency to achieve performance goals. The

concentrations of typical pollutants in raw residential wastewaters and average daily mass loadings

are summarized in table 3-7. Residential water-using

activities contribute varying amounts of pollutants to

the total wastewater flow. Table 3-8 contains a

summary of the average mass loading of several

key pollutants from the sources identified in table 3-7.

If the waste-generating sources present at a particular nonresidential establishment are similar to those

of a typical residential dwelling, an approximation

of the pollutant mass loadings and concentrations in

the wastewater can be derived using the residential

wastewater quality data for those categories presented in tables 3-7 and 3-8. However, the results

of previous studies have demonstrated that in many

cases nonresidential wastewater is considerably

different from residential wastewater. Restaurant

wastewater, for example, contains substantially

higher levels of organic matter, solids, and grease

compared to typical residential wastewater (Siegrist

et al., 1984; University of Wisconsin, 1978).

Restaurant wastewater BOD5

 concentrations

reported in the literature range from values similar

to those for domestic waste to well over 1,000

milligrams/liter, or 3.5 to 6.5 times higher than

residential BOD5

. Total suspended solids and grease

concentrations in restaurant wastewaters were

reported to be 2 to 5 times higher than the concentrations in domestic wastewaters (Kulesza, 1975;

Figure 3-5. Average hourly distribution of total unfiltered BOD5

Source: University of Wisconsin, 1978.
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Chapter 3: Establishing Treatment System Performance Requirements

Table 3-7. Constituent mass loadings and concentrations in typical residential wastewater a

Table 3-8. Residential wastewater pollutant contributions by source a,b
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3-12 USEPA Onsite Wastewater Treatment Systems Manual

ods have been developed around two main strategies—wastewater flow reduction and pollutant

mass reduction. Although this section emphasizes

residential flows, many of the concepts are applicable to nonresidential establishments. (For more

information on both residential and nonresidential

water use reduction, see http://www.epa.gov/OW/

you/intro.html.)

3.5.1 Minimizing residential wastewater

volumes

The most commonly reported failure of residential

OWTS infiltration systems is hydraulic overloading. Hydraulic overloads can be caused by wastewater flow or pollutant loads that exceed system

design capacity. When more water is processed than

an OWTS is designed to handle, detention time

within the treatment train is reduced, which can

decrease pollutant removal in the tank and overload

the infiltration field. Reducing water use in a

residence can decrease hydraulic loading to the

treatment system and generally improve system

performance. If failure is caused by elevated

pollutant loads, however, other options should be

considered (see chapter 5).

Indoor residential water use and resulting wastewater flows are attributed mainly to toilet flushing,

bathing, and clothes washing (figure 3-2). Toilet

use usually accounts for 25 to 30 percent of indoor

water use in residences; toilets, showers, and

faucets in combination can represent more than 70

percent of all indoor use. Residential wastewater

flow reduction can therefore be achieved most

dramatically by addressing these primary indoor

uses and by minimizing wastewater flows from

extraneous sources. Table 3-9 presents many of the

methods that have been applied to achieve wastewater flow reduction.

Eliminating extraneous flows

Excessive water use can be reduced or eliminated

by several methods, including modifying water use

habits and maintaining the plumbing system

appropriately. Examples of methods to reduce

water use include

• Using toilets to dispose of sanitary waste only

(not kitty litter, diapers, ash tray contents, and

other materials.)

• Reducing time in the shower

• Turning off faucets while brushing teeth or

shaving

• Operating dishwashers only when they are full

• Adjusting water levels in clothes washers to

match loads; using machine only when full

• Making sure that all faucets are completely

turned off when not in use

• Maintaining plumbing system to eliminate leaks

These practices generally involve changes in water

use behavior and do not require modifying of

plumbing or fixtures. Homeowner education

programs can be an effective approach for modifying water use behavior (USEPA, 1995). Wastewater flow reduction resulting from eliminating

wasteful water use habits will vary greatly depending on past water use habits. In many residences,

significant water use results from leaking plumbing

fixtures. The easiest ways to reduce wastewater

flows from indoor water use are to properly

maintain plumbing fixtures and repair leaks when

they occur. Leaks that appear to be insignificant,

such as leaking toilets or dripping faucets, can

generate large volumes of wastewater. For

example, a 1/32-inch (0.8 millimeters) opening at

40 pounds per square inch (207 mm of mercury) of

pressure can waste from 3,000 to 6,000 gallons

(11, 550 to 22,700 liters) of water per month. Even

apparently very slow leaks, such as a slowly

dripping faucet, can generate 15 to 20 gallons

(57 to 76 liters) of wastewater per day.

Reducing wastewater flow

Installing indoor plumbing fixtures that reduce

water use and replacing existing plumbing fixtures

or appliances with units that use less water are

successful practices that reduce wastewater flows

(USEPA, 1995). Recent interest in water conservation has been driven in some areas by the absence

of adequate source water supplies and in other areas

by a desire to minimize the need for expensive

wastewater treatment. In 1992 Congress passed the

U.S. Energy Policy Act (EPACT) to establish

national standards governing the flow capacity of

showerheads, faucets, urinals, and water closets for

the purpose of national energy and water conservation (table 3-10). Several states have also implemented specific water conservation practices
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Chapter 3: Establishing Treatment System Performance Requirements

(USEPA, 1995; for case studies and other information, see http://www.epa.gov/OW/you/intro.html.

Several toilet designs that use reduced volumes of

water for proper operation have been developed.

Conventional toilets manufactured before 1994

typically use 3.5 gallons (13.2 liters) of water per

flush. Reduced-flow toilets manufactured after

1994 use 1.6 gallons (6.1 liters) or less per flush.

Though studies have shown an increased number of

flushes with reduced-flow toilets, potential savings

of up to 10 gallons/person/day (37.8 liters/person/

day) can be achieved (Aher et al., 1991; Anderson

et al., 1993; Mayer et al., 1999, 2000). Table 3-11

contains information on water carriage toilets and

systems; table 3-12 contains information on nonwater-carriage toilets. The reader is cautioned that

not all fixtures perform well in every application

and that certain alternatives might not be acceptable

to the public.

The volume of water used for bathing varies

considerably based on individual habits. Averages

indicate that showering with common showerheads

using 3.0 to 5.0 gallons/minute (0.19 to 0.32 liters/

second) amounts to a water use of 10 to 12.5

Table 3-9. Wastewater flow reduction methods

Sources: Adapted from USEPA, 1992, 1995.
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Table 3-11. Wastewater flow reduction: water-carriage toilets and systems a

Table 3-10. Comparison of flow rates and flush volumes before and after U.S. Energy Policy Act
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Chapter 3: Establishing Treatment System Performance Requirements

Table 3-12. Wastewater flow reduction: non-water-carriage toilets a

Table 3-13. Wastewater flow reduction: showering devices and systems a
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gallons/person/day (37.9 to 47.3 liters/person/day).

Table 3-13 provides an overview of showering

devices available to reduce wastewater flows

associated with shower use. A low-flow

showerhead can reduce water flow through the

shower by 2 or 3 gallons/minute (0.13 to 0.19

liters/second), but if the user stays in the shower

twice as long because the new showerhead does not

provide enough pressure or flow to satisfy showering preferences, projected savings can be negated.

Indoor water use can also be reduced by installing

flow reduction devices or faucet aerators at sinks

and basins. More efficient faucets can reduce water

use from 3 to 5 gallons/minute (0.19 to 0.32 liters/

second) to 2 gallons/minute (0.13 liters/second),

and aerators can reduce water use at faucets by as

much as 60 percent while still maintaining a strong

flow. Table 3-14 provides a summary of wastewater flow reduction devices that can be applied to

water use at faucets.

Reducing water pressure

Reducing water pressure is another method for

reducing wastewater flows. The flow rate at faucets

and showers is directly related to the water pressure

in the water supply line. The maximum water flow

from a fixture operating on a fixed setting can be

Source: Adapted from USEPA, 1992.

reduced by reducing water pressure. For example, a

reduction in pressure from 80 pounds per square

inch (psi) (414 cm Hg) to 40 psi (207 cm Hg) can

reduce the flow rate through a fully opened faucet

by about 40 percent. Reduced pressure has little

effect on the volume of water used by fixtures that

operate on a fixed volume of water, such as toilets

and washing machines, but it can reduce wastewater flows from sources controlled by the user

(e.g., faucets, showerheads).

3.5.2 Reducing mass pollutant loads in

wastewater

Pollutant mass loading modifications reduce the

amount of pollutants requiring removal or treatment in the OWTS. Methods that may be applied

for reducing pollutant mass loads include modifying product selection, improving user habits, and

eliminating or modifying certain fixtures. Household products containing toxic compounds, commonly referred to as “household hazardous waste,”

should be disposed of properly to minimize threats

to human health and the environment. For more

information on disposal options and related issues,

visit the USEPA Office of Solid Waste’s Household

Hazardous Waste web site at http://www.epa.gov/

epaoswer/non-hw/muncpl/hhw.htm.

Table 3-14. Wastewater flow reduction: miscellaneous devices and systems
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Selecting cleaning agents and household

chemicals

Toilet flushing, bathing, laundering, washing

dishes, operating garbage disposals, and general

cleaning are all activities that can include the use of

chemicals that are present in products like disinfectants and soaps. Some of these products contribute

significant quantities of pollutants to wastewater

flows. For example, bathing, clothes washing, and

dish washing contribute large amounts of sodium to

wastewater. Before manufacturers reformulated

detergents, these activities accounted for more than

70 percent of the phosphorus in residential flows.

Efforts to protect water quality in the Chesapeake

Bay, Great Lakes, and major rivers across the

nation led to the first statewide bans on phosphorus

in detergents in the 1970s, and other states issued

phosphorus bans throughout the 1980s. The new

low-phosphorus detergents have reduced phosphorus loadings to wastewater by 40 to 50 percent

since the 1970s.

The impacts associated with the daily use of

household products can be reduced by providing

public education regarding the environmental

impacts of common household products. Through

careful selection of cleaning agents and chemicals,

pollution impacts on public health and the environment associated with their use can be reduced.

Improving user habits

Everyday household activities generate numerous

pollutants. Almost every commonly used domestic

product—cleaners, cosmetics, deodorizers, disinfectants, pesticides, laundry products, photographic

products, paints, preservatives, soaps, and medicines—contains pollutants that can contaminate

ground water and surface waters and upset biological treatment processes in OWTSs (Terrene Institute, 1995). Some household hazardous waste

(HHW) can be eliminated from the wastewater

stream by taking hazardous products to HHW

recycling/reuse centers, dropping them off at HHW

collection sites, or disposing of them in a solid

waste form (i.e., pouring liquid products like paint,

cleaners, or polishes on newspapers, allowing them

to dry in a well-ventilated area, and enclosing them

in several plastic bags for landfilling) rather than

dumping them down the sink or flushing them

down the toilet. Improper disposal of HHW can

best be reduced by implementing public education

and HHW collection programs. A collection

program is usually a 1-day event at a specific site.

Permanent programs include retail store drop-off

programs, curbside collection, and mobile facilities.

Establishing HHW collection programs can significantly reduce the amount of hazardous chemicals in

the wastewater stream, thereby reducing impacts on

the treatment system and on ground water and

surface waters.

Stopping the practice of flushing household wastes

(e.g., facial tissue, cigarette butts, vegetable

peelings, oil, grease, other cooking wastes) down

the toilet can also reduce mass pollutant loads and

decrease plumbing and OWTS failure risks.

Homeowner education is necessary to bring about

these changes in behavior. Specific homeowner

information is available from the National Small

Flows Clearinghouse at http://www.estd.wvu.edu/

nsfc/NSFC_septic_news.html.

Improving onsite system performance by

improving user habits

The University of Minnesota Extension Service’s Septic

System Owner’s Guide recommends the following

practices to improve onsite system performance:

• Do not use “every flush” toilet bowl cleaners.

• Reduce the use of drain cleaners by minimizing the

amount of hair, grease, and food particles that go

down the drain.

• Reduce the use of cleaners by doing more

scrubbing with less cleanser.

• Use the minimum amount of soap, detergent, and

bleach necessary to do the job.

• Use minimal amounts of mild cleaners and only as

needed.

• Do not drain chlorine-treated water from swimming

pools and hot tubs into septic systems.

• Dispose of all solvents, paints, antifreeze, and

chemicals through local recycling and hazardous

waste collection programs.

• Do not flush unwanted prescription or over-thecounter medications down the toilet.

Adapted from University of Minnesota, 1998.
Chapter 3: Establishing Treatment System Performance Requirements

3-18 USEPA Onsite Wastewater Treatment Systems Manual

Eliminating use of garbage disposals

Eliminating the use of garbage disposals can

significantly reduce the amount of grease, suspended

solids, and BOD in wastewater (table 3-15). Reducing

the amount of vegetable and other food-related

material entering wastewater from garbage disposals can also result in a slight reduction in nitrogen

and phosphorus loads. Eliminating garbage disposal

use also reduces the rate of sludge and scum

accumulation in the septic tank, thus reducing the

frequency of required pumping. OWTSs, however,

can accommodate garbage disposals by using larger

tanks, SWISs, or alternative system designs. (For

more information, see Special Issue Fact Sheets 2

and 3 in the Chapter 4 Fact Sheets section.)

Using graywater separation approaches

Another method for reducing pollutant mass

loading to a single SWIS is segregating toilet

waste flows (blackwater) from sink, shower,

washing machine, and other waste flows

(graywater). Some types of toilet systems provide

separate handling of human excreta (such as the

non-water-carriage units in table 3-14). Significant quantities of suspended solids, BOD, nitrogen, and pathogenic organisms are eliminated

from wastewater flows by segregating body wastes

from the OWTS wastewater stream through the

use of composting or incinerator toilets. This

approach is more cost-effective for new homes,

homes with adequate crawl spaces, or mobile or

modular homes. Retrofitting existing homes,

especially those with concrete floors, can be

expensive. (For more information on graywater

reuse, see Special Issue Fact Sheet 4 in the

Chapter 4 Fact Sheets section and http://

www.epa.gov/OW/you/chap3.html.)

Graywaters contain appreciable quantities of

organic matter, suspended solids, phosphorus,

grease, and bacteria (USEPA, 1980a). Because of

the presence of significant concentrations of

bacteria and possibly pathogens in graywaters from

bathing, hand washing, and clothes washing, caution

should be exercised to ensure that segregated

graywater treatment and discharge processes occur

below the ground surface to prevent human contact.

In addition, siting of graywater infiltration fields

should not compromise the hydraulic capacity of

treatment soils in the vicinity of the blackwater

infiltration field.

3.5.3 Wastewater reuse and recycling

systems

Many arid and semiarid regions in the United

States have been faced with water shortages,

creating the need for more efficient water use

practices. Depletion of ground water and surface

water resources due to increased development,

irrigation, and overall water use is also becoming a

growing concern in areas where past supplies have

been plentiful (e.g., south Florida, central Georgia). Residential development in previously rural

areas has placed additional strains on water supplies

and wastewater treatment facilities. Decentralized

wastewater management programs that include

onsite wastewater reuse/recycling systems are a

viable option for addressing water supply shortages

and wastewater discharge restrictions. In municipalities where water shortages are a recurring

problem, such as communities in California and

Arizona, centrally treated reclaimed wastewater has

been used for decades as an alternative water

supply for agricultural irrigation, ground water

recharge, and recreational waters.

Wastewater reuse is the collection and treatment of

wastewater for other uses (e.g., irrigation, ornamental ponds, and cooling systems). Wastewater

recycling is the collection and treatment of

wastewater and its reuse in the same water-use

scheme, such as toilet and urinal flushing

(Tchobanoglous and Burton, 1991). Wastewater

reuse/recycling systems can be used in individual

homes, clustered communities, and larger institutional facilities such as office parks and recreational facilities. The Grand Canyon National

Park has reused treated wastewater for toilet

flushing, landscape irrigation, cooling water, and

Table 3-15. Reduction in pollutant loading achieved by eliminating

garbage disposals
USEPA Onsite Wastewater Treatment Systems Manual 3-19

Chapter 3: Establishing Treatment System Performance Requirements

Figure 3-6. Typical graywater reuse approach boiler feedstock since 1926, and other reuse

systems are gaining acceptance (Tchobanoglous

and Burton, 1991). Office buildings, schools, and

recreational facilities using wastewater reuse/

recycling systems have reported a 90 percent

reduction in water use and up to a 95 percent

reduction in wastewater discharges (Burks and

Minnis, 1994).

Wastewater reuse/recycling systems reduce potable

water use by reusing or recycling water that has

already been used at the site for nonpotable purposes, thereby minimizing wastewater discharges.

The intended use of wastewater dictates the degree

of treatment necessary before reuse. Common

concerns associated with wastewater reuse/recycling

systems include piping cross-connections, which

could contaminate potable water supplies with

wastewater, difficulties in modifying and integrating potable and nonpotable plumbing, public and

public agency acceptance, and required maintenance of the treatment processes.

A number of different onsite wastewater reuse/

recycling systems and applications are available.

Some systems, called combined systems, treat and

reuse or recycle both blackwater and graywater

(NAPHCC, 1992. Other systems treat and reuse or

recycle only graywater. Figure 3-6 depicts a typical

graywater reuse approach. Separating graywater

and blackwater is a common practice to reduce

pollutant loadings to wastewater treatment systems

(Tchobanoglous and Burton, 1991).

3.5.4 Factors of safety in

characterization estimates

Conservative predictions or factors of safety are

typically used to account for potential variability

in wastewater characteristics at a particular

dwelling or establishment. These predictions

attempt to ensure adequate treatment by the onsite

system without requiring actual analysis of the

variability in flow or wastewater quality. However, actual measurement of wastewater flow and

quality from a residential dwelling or nonresidential establishment always provides the most

accurate estimate for sizing and designing an

OWTS. Metering daily water use and analyzing a

set of grab samples to confirm wastewater

strength estimates are often substituted for direct

measurement of concentrations because of cost

considerations.

Minimum septic tank size requirements or minimum design flows for a residential dwelling may

be specified by onsite codes (NSFC, 1995). Such

stipulations should incorporate methods for the

conservative prediction of wastewater flow. It is

important that realistic values and safety factors

be used to determine wastewater characteristics in

order to design the most cost-effective onsite

system that meets performance requirements.

Factors of safety can be applied indirectly by the

choice of design criteria for wastewater characteristics and occupancy patterns or directly through an

overall factor. Most onsite code requirements for

system design of residential dwellings call for

estimating the flow on a per person or per bedroom

basis. Codes typically specify design flows of 100 to

150 gallons/bedroom/day (378 to 568 liters/bedroom/

day), or 75 to 100 gallons/person/day (284 to 378

liters/person/day), with occupancy rates of between

1.5 and 2 persons/bedroom (NSFC, 1995).

For example, if an average daily flow of 75 gallons/person/day (284 liters/person/day) and an

occupancy rate of 2 persons per bedroom were the

selected design units, the flow prediction for a

three-bedroom home would include a factor of

safety of approximately 2 when compared to

typical conditions (i.e., 70 gallons/person/day and

1 person/bedroom). In lieu of using conservative

design flows, a direct factor of safety (e.g., 2)

may be applied to estimate the design flow from a
Chapter 3: Establishing Treatment System Performance Requirements

3-20 USEPA Onsite Wastewater Treatment Systems Manual

residence or nonresidential establishment. Multiplying the typical flow estimated (140 gallons/

day) by a safety factor of 2 yields a design flow

of 280 gallons/day (1,058 liters/day). Factors of

safety used for individual systems will usually be

higher than those used for larger systems of 10

homes or more.

Great care should be exercised in predicting

wastewater characteristics so as not to accumulate

multiple factors of safety that would yield unreasonably high design flows and result in unduly high

capital costs. Conversely, underestimating flows

should be avoided because the error will quickly

become apparent if the system overloads and

requires costly modification.

3.6 Integrating wastewater

characterization and other

design information

Predicting wastewater characteristics for typical

residential and nonresidential establishments can be a

difficult task. Following a logical step-by-step

procedure can help simplify the characterization

process and yield more accurate wastewater characteristic estimates. Figure 3-7 is a flow chart that

illustrates a procedure for predicting wastewater

characteristics. This strategy takes the reader through

the characterization process as it has been described

in this chapter. The reader is cautioned that this

flowchart is provided to illustrate one simple

strategy for predicting wastewater characteristics.

Additional factors to consider, such as discrepancies

between literature values for wastewater flow and

quality and/or the need to perform field studies,

should be addressed based on local conditions and

regulatory requirements.

In designing wastewater treatment systems, it is

recommended that designers consider the most

significant or limiting parameters, including those

that may be characterized as outliers, when

considering hydraulic and mass pollutant treatment requirements and system components. For

example, systems that will treat wastewaters with

typical mass pollutant loads but hydraulic loads

that exceed typical values should be designed to

handle the extra hydraulic input. Systems designed for facilities with typical hydraulic loads

but atypical mass pollutant loads (e.g., restaurants,

grocery stores, or other facilities with highstrength wastes) should incorporate pretreatment

units that address the additional pollutant loadings, such as grease traps.

3.7 Transport and fate of

wastewater pollutants in the

receiving environment

Nitrate, phosphorus, pathogens, and other contaminants are present in significant concentrations in

most wastewaters treated by onsite systems. Although most can be removed to acceptable levels

under optimal system operational and performance

conditions, some may remain in the effluent exiting

the system. After treatment and percolation of the

wastewater through the infiltrative surface biomat

and passage through the first few inches of soil, the

wastewater plume begins to migrate downward

until nearly saturated conditions exist. The worst

case scenario occurs when the plume is mixing with

an elevated water table. At that point, the wastewater plume will move in response to the prevailing

hydraulic gradient, which might be lateral, vertical,

or even a short distance upslope if ground water

mounding occurs (figure 3-8). Moisture potential,

soil conductivity, and other soil and geological

characteristics determine the direction of flow.

Further treatment occurs as the plume passes

through the soil. The degree of this additional

treatment depends on a host of factors (e.g.,

residence time, soil mineralogy, particle sizes).

Permit writers should consider not only the

performance of each individual onsite system but

also the density of area systems and overall

hydraulic loading, the proximity of water resources, and the collective performance of onsite

systems in the watershed. Failure to address these

issues can lead ultimately to contamination of

lakes, rivers, streams, wetlands, coastal areas, or

ground water. This section examines key wastewater pollutants, their impact on human health and

water resources, how they move in the environment, and how local ecological conditions affect

wastewater treatment.
USEPA Onsite Wastewater Treatment Systems Manual 3-21

Chapter 3: Establishing Treatment System Performance Requirements

Figure 3-7. Strategy for estimating wastewater flow and composition
Chapter 3: Establishing Treatment System Performance Requirements

3-22 USEPA Onsite Wastewater Treatment Systems Manual

3.7.1 Wastewater pollutants of concern

Environmental protection and public health agencies are becoming increasingly concerned about

ground water and surface water contamination

from wastewater pollutants. Toxic compounds,

excessive nutrients, and pathogenic agents are

among the potential impacts on the environment

from onsite wastewater systems. Domestic wastewater contains several pollutants that could cause

significant human health or environmental risks if

not treated effectively before being released to the

receiving environment.

A conventional OWTS (septic tank and SWIS) is

capable of nearly complete removal of suspended

solids, biodegradable organic compounds, and fecal

coliforms if properly designed, sited, installed,

operated, and maintained (USEPA, 1980a, 1997).

These wastewater constituents can become pollutants in ground water or surface waters if treatment

is incomplete. Research and monitoring studies

have demonstrated removals of these typically

found constituents to acceptable levels. More

recently, however, other pollutants present in

wastewater are raising concerns, including nutrients

(e.g., nitrogen and phosphorus), pathogenic

parasites (e.g., Cryptosporidum parvum, Giardia

lamblia), bacteria and viruses, toxic organic

compounds, and metals. Their potential impacts on

ground water and surface water resources are

summarized in table 3-16. Recently, concerns have

been raised over the movement and fate of a

variety of endocrine disrupters, usually from use of

pharmaceuticals by residents. No data have been

developed to confirm a risk at this time.

3.7.2 Fate and transport of pollutants

in the environment

When properly designed, sited, constructed, and

maintained, conventional onsite wastewater treatment technologies effectively reduce or eliminate

most human health or environmental threats posed

by pollutants in wastewater (table 3-17). Most

traditional systems rely primarily on physical,

biological, and chemical processes in the septic

tank and in the biomat and unsaturated soil zone

below the SWIS (commonly referred to as a leach

field or drain field) to sequester or attenuate

pollutants of concern. Where point discharges to

surface waters are permitted, pollutants of concern

should be removed or treated to acceptable, permitspecific levels (levels permitted under the National

Pollutant Discharge Elimination System of the

Clean Water Act) before discharge.

Figure 3-8. Plume movement through the soil to the saturated zone.

Source: Adapted from NSFC, 2000.
USEPA Onsite Wastewater Treatment Systems Manual 3-23

Chapter 3: Establishing Treatment System Performance Requirements

Table 3-16. Typical wastewater pollutants of concern

Table3-17. Examples of soil infiltration system performance

Source: Adapted from USEPA, 1992.
Chapter 3: Establishing Treatment System Performance Requirements

3-24 USEPA Onsite Wastewater Treatment Systems Manual

Onsite systems can fail to meet human health and

water quality objectives when fate and transport of

potential pollutants are not properly addressed.

Failing or failed systems threaten human health if

pollutants migrate into ground waters used as

drinking water and nearby surface waters used for

recreation. Such failures can be due to improper

siting, inappropriate choice of technology, faulty

design, poor installation practices, poor operation, or

inadequate maintenance. For example, in highdensity subdivisions conventional septic tank/SWIS

systems might be an inappropriate choice of technology because leaching of nitrate-nitrogen could result

in nitrate concentrations in local aquifers that exceed

the drinking water standard. In soils with excessive

permeability or shallow water tables, inadequate

treatment in the unsaturated soil zone might allow

pathogenic bacteria and viruses to enter the ground

water if no mitigating measures are taken. Poorly

drained soils can restrict reoxygenation of the subsoil

and result in clogging of the infiltrative surface.

A number of factors influence the shape and

movement of contaminant plumes from OWTSs.

Climate, soils, slopes, landscape position, geology,

regional hydrology, and hydraulic load determine

whether the plume will disperse broadly and deeply

or, more commonly, migrate in a long and relatively narrow plume along the upper surface of a

confining layer or on the surface of the ground

water. Analyses of these factors are key elements in

understanding the contamination potential of

individual or clustered OWTSs in a watershed or

ground water recharge area.

Receiving environments and contaminant

plume transport

Most onsite systems ultimately discharge treated

water to ground water. Water beneath the land

surface occurs in two primary zones, the aerated or

vadose zone and the saturated (groundwater) zone.

Interstices in the aerated (upper) vadose zone are

unsaturated, filled partially with water and partially

with air. Water in this unsaturated zone is referred to

as vadose water. In the saturated zone, all interstices

are filled with water under hydrostatic pressure.

Water in this zone is commonly referred to as

ground water. Where no overlying impermeable

barrier exists, the upper surface of the ground water

is called the water table. Saturation extends slightly

above the water table due to capillary attraction but

water in this “capillary fringe” zone is held at less

than atmospheric pressure.

Onsite wastewater treatment system performance

should be measured by the ability of the system to

discharge a treated effluent capable of meeting

public health and water quality objectives established for the receiving water resource. Discharges

from existing onsite systems are predominantly to

ground water but they might involve direct (point

source) or indirect (nonpoint source) surface water

discharges in some cases. Ground water discharges

usually occur through soil infiltration. Point source

discharges are often discouraged by regulatory

agencies because of the difficulty in regulating

many small direct, permitted discharges and the

potential for direct or indirect human contact with

wastewater. Nonpoint source surface water discharges usually occur as base flow from ground

water into watershed surface waters. In some cases

regional ground water quality and drinking water

wells might be at a lesser risk from OWTS discharges than nearby surface waters because of the

depth of some aquifers and regional geology.

The movement of subsurface aqueous contaminant

plumes is highly dependent on soil type, soil

layering, underlying geology, topography, and

rainfall. Some onsite system setback/separation

codes are based on plume movement models or

measured relationships that have not been supported by recent field data. In regions with moderate to heavy rainfall, effluent plumes descend

relatively intact as the water table is recharged

from above. The shape of the plume depends on

the soil and geological factors noted above, the

uniformity of effluent distribution in the SWIS, the

orientation of the SWIS with respect to ground

water flow and direction, and the preferential flow

that occurs in the vadose and saturated zones (Otis,

2000).

In general, however, plumes tend to be long,

narrow, and definable, exhibiting little dispersion

(figure 3-9). Some studies have found SWIS

plumes with nitrate levels exceeding drinking water

standards (10 mg/L) extending more than 328 feet

(100 meters) beyond the SWIS (Robertson, 1995).

Mean effluent plume dispersion values used in a

Florida study to assess subdivision SWIS nitrate

loadings over 5 years were 60 feet, 15 feet, and 1.2

feet for longitudinal, lateral, and vertical disper-
USEPA Onsite Wastewater Treatment Systems Manual 3-25

Chapter 3: Establishing Treatment System Performance Requirements

sion, respectively (Florida HRS, 1993). A study

that examined SWIS plume movement in a shallow, unconfined sand aquifer found that after 12

years the plume had sharp lateral and vertical

boundaries, a length of 426 feet (130 meters), and

a uniform width of about 32.8 feet (10 meters)

(Robertson, 1991). At another site examined in that

study, a SWIS constructed in a similar carbonatedepleted sand aquifer generated a plume with

discrete boundaries that began discharging into a

river 65.6 feet (20 meters) away after 1.5 years of

system operation.

Given the tendency of OWTS effluent plumes to

remain relatively intact over long distances (more

than 100 meters), dilution models commonly used

in the past to calculate nitrate attenuation in the

vadose zone are probably unrealistic (Robertson,

1995). State codes that specify 100-foot separation

distances between conventional SWIS treatment

units and downgradient wells or surface waters

should not be expected to always protect these

resources from dissolved, highly mobile contaminants such as nitrate (Robertson, 1991). Moreover,

published data indicate that viruses that reach

groundwater can travel at least 220 feet (67 meters)

vertically and 1,338 feet (408 meters) laterally in

some porous soils and still remain infective (Gerba,

1995). One study noted that fecal coliform bacteria

moved 2 feet (0.6 meter) downward and 50 feet

(15 meters) longitudinally 1 hour after being

injected into a shallow trench in saturated soil on a

14 percent slope in western Oregon (Cogger,

1995). Contaminant plume movement on the

surface of the saturated zone can be rapid, especially under sloping conditions, but it typically

slows upon penetration into ground water in the

saturated zone. Travel times and distances under

unsaturated conditions in more level terrain are

likely much less.

Ground water discharge

A conventional OWTS (septic tank and SWIS)

discharges to ground water and usually relies on the

unsaturated or vadose zone for final polishing of

the wastewater before it enters the saturated zone.

The septic tank provides primary treatment of the

wastewater, removing most of the settleable solids,

greases, oils, and other floatable matter and anaerobic liquifaction of the retained organic solids. The

biomat that forms at the infiltrative surface and

within the first few centimeters of unsaturated soil

below the infiltrative field provides physical,

chemical, and biological treatment of the SWIS

effluent as it migrates toward the ground water.

Because of the excellent treatment the SWIS provides, it is a critical component of onsite systems

that discharge to ground water. Fluid transport from

the infiltrative surface typically occurs through three

zones, as shown in figure 3-10 (Ayres Associates,

1993a). In addition to the three zones, the figure

shows a saturated zone perched above a restrictive

horizon, a site feature that often occurs.

Pretreated wastewater enters the SWIS at the

surface of the infiltration zone. A biomat forms in

this zone, which is usually only a few centimeters

thick. Most of the physical, chemical, and biological treatment of the pretreated effluent occurs in

this zone and in the vadose zone. Particulate matter

in the effluent accumulates on the infiltration surface

and within the pores of the soil matrix, providing a

Figure 3-9. An example of effluent plume movement Source: NSFC, 1998.
Chapter 3: Establishing Treatment System Performance Requirements

3-26 USEPA Onsite Wastewater Treatment Systems Manual

source of carbon and nutrients to the active biomass.

New biomass and its metabolic by-products accumulate in this zone. The accumulated biomass, particulate matter, and metabolic by-products reduce the

porosity and the infiltration rate through them.

Thus, the infiltration zone is a transitional zone

where fluid flow changes from saturated to unsaturated flow. The biomat controls the rate at which the

pretreated wastewater moves through the infiltration

zone in coarse- to medium-textured soils, but it is

less likely to control the flow through fine-textured

silt and clay soils because they may be more restrictive to flow than the biomat.

Below the zone of infiltration lies the unsaturated or

vadose zone. Here the effluent is under a negative

pressure potential (less than atmospheric) resulting

from the capillary and adsorptive forces of the soil

matrix. Consequently, fluid flow occurs over the

surfaces of soil particles and through finer pores of

the soil while larger pores usually remain air-filled.

This is the most critical fluid transport zone because

the unsaturated soil allows air to diffuse into the

open soil pores to supply oxygen to the microbes

that grow on the surface of the soil particles. The

negative soil moisture potential forces the wastewater into the finer pores and over the surfaces of the

soil particles, increasing retention time, absorption,

filtration, and biological treatment of the wastewater.

From the vadose zone, fluid passes through the

capillary fringe immediately above the ground

water and enters the saturated zone, where flow

occurs in response to a positive pressure gradient.

Treated wastewater is transported from the site by

fluid movement in the saturated zone. Mixing of

treated water with ground water is somewhat

limited because ground water flow usually is

laminar. As a result, treated laminar water can

remain as a distinct plume at the ground water

interface for some distance from its source

(Robertson et al., 1989). The plume might descend

into the ground water as it travels from the source

because of recharge from precipitation above.

Dispersion occurs, but the mobility of solutes in the

plume varies with the soil-solute reactivity.

Water quality-based performance requirements for

ground water discharging systems are not clearly

defined by current codes regulating OWTSs.

Primary drinking water standards are typically

required at a point of use (e.g., drinking water well)

but are addressed in the codes only by requirements

that the infiltration system be located a specified

horizontal distance from the wellhead and vertical

distance from the seasonal high water table. Nitratenitrogen is the common drinking water pollutant of

concern that is routinely found in ground water

below conventional SWISs. Regions with karst

terrain or sandy soils are at particular risk for rapid

movement of bacteria, viruses, nitrate-nitrogen, and

other pollutants to ground water. In addition,

geological conditions that support “gaining streams”

(streams fed by ground water during low-flow

conditions) might result in OWTS nutrient or

pathogen impacts on surface waters if siting or

design criteria fail to consider these conditions.

Surface water discharge

Direct discharges to surface waters require a permit

issued under the National Pollutant Discharge

Elimination System (NPDES) of the Clean Water

Figure 3-10. Soil treatment zones

Source: Ayres Associates, 1993a.
USEPA Onsite Wastewater Treatment Systems Manual 3-27

Chapter 3: Establishing Treatment System Performance Requirements

Act. The NPDES permitting process, which is

administered by all but a few states, defines

discharge performance requirements in the form of

numerical criteria for specific pollutants and

narrative criteria for parameters like color and

odor. The treated effluent should meet water

quality criteria before it is discharged. Criteriabased standards may include limits for BOD5

, TSS,

fecal coliforms, ammonia, nutrients, metals, and

other pollutants, including chlorine, which is often

used to disinfect treated effluent prior to discharge.

The limits specified vary based on the designated

use of the water resource (e.g., swimming, aquatic

habitat, recreation, potable water supply), state

water classification schemes (Class I, II, III, etc.),

water quality criteria associated with designated

uses, or the sensitivity of aquatic ecosystems—

especially lakes and coastal areas—to eutrophication. Surface water discharges are often discouraged for individual onsite treatment systems,

however, because of the difficulty in achieving

regulatory oversight and surveillance of many

small, privately operated discharges.

Atmospheric discharge

Discharges to the atmosphere also may occur

through evaporation and transpiration by plants.

Evapotranspiration can release significant volumes

of water into the atmosphere, but except for areas

where annual evaporation exceeds precipitation

(e.g., the American Southwest), evapotranspiration

cannot be solely relied on for year-round discharge.

However, evapotranspiration during the growing

season can significantly reduce the hydraulic

loading to soil infiltration systems.

Contaminant attenuation

Performance standards for ground water discharge

systems are usually applied to the treated effluent/

ground water mixture at some specified point away

from the treatment system (see chapter 5). This

approach is significantly different from the effluent

limitation approach used with surface water

discharges because of the inclusion of the soil

column as part of the treatment system. However,

monitoring ground water quality as a performance

measure is not as easily accomplished. The fate and

transport of wastewater pollutants through soil

should be accounted for in the design of the overall

treatment system.

Contaminant attenuation (removal or inactivation

through treatment processes) begins in the septic

tank and continues through the distribution piping

of the SWIS or other treatment unit components,

the infiltrative surface biomat, the soils of the

vadose zone, and the saturated zone. Raw wastewater composition was discussed in section 3.4 and

summarized in table 3-7. Jantrania (1994) found

that chemical, physical, and biological processes in

the anaerobic environment of the septic tank produce

effluents with TSS concentrations of 40 to 350 mg/

L, oil and grease levels of 50 to 150 mg/L, and total

coliform counts of 106

 to 108 per 100 milliliters.

Although biofilms develop on exposed surfaces as

the effluent passes through piping to and within the

SWIS, no significant level of treatment is provided

by these growths. The next treatment site is the

infiltrative zone, which contains the biomat. Filtration, microstraining, and aerobic biological decomposition processes in the biomat and infiltration zone

remove more than 90 percent of the BOD and

suspended solids and 99 percent of the bacteria

(University of Wisconsin, 1978).

As the treated effluent passes through the biomat

and into the vadose and saturated zones, other

treatment processes (e.g., filtration, adsorption,

precipitation, chemical reactions) occur. The

following section discusses broadly the transport

and fate of some of the primary pollutants of

concern under the range of conditions found in

North America. Table 3-18 summarizes a case

study that characterized the septic tank effluent and

soil water quality in the first 4 feet of a soil

treatment system consisting of fine sand. Results

for other soil types might be significantly different.

Note that mean nitrate concentrations still exceed

the 10 mg/L drinking water standard even after the

wastewater has percolated through 4 feet of fine

sand under unsaturated conditions.

Biochemical oxygen demand and total

suspended solids

Biodegradable organic material creates biochemical

oxygen demand (BOD), which can cause low

dissolved oxygen concentrations in surface water,

create taste and odor problems in well water, and

cause leaching of metals from soil and rock into

ground water and surface waters. Total suspended

solids (TSS) in system effluent can clog the infiltrative surface or soil interstices, while colloidal solids
Chapter 3: Establishing Treatment System Performance Requirements

3-28 USEPA Onsite Wastewater Treatment Systems Manual

cause cloudiness in surface waters. TSS in direct

discharges to surface waters can result in the development of sludge layers that can harm aquatic

organisms (e.g., benthic macro invertebrates).

Systems that fail to remove BOD and TSS and are

located near surface waters or drinking water wells

may present additional problems in the form of

pathogens, toxic pollutants, and other pollutants.

Under proper site and operating conditions, however, OWTSs can achieve significant removal rates

(i.e., greater than 95 percent) for biodegradable

organic compounds and suspended solids. The risk

of ground water contamination by BOD and TSS

(and other pollutants associated with suspended

solids) below a properly sited, designed, constructed, and maintained SWIS is slight (Anderson

et al., 1994; University of Wisconsin, 1978). Most

settleable and floatable solids are removed in the

septic tank during pretreatment. Most particulate

BOD remaining is effectively removed at the

infiltrative surface and biomat. Colloidal and

dissolved BOD that might pass through the biomat

are removed through aerobic biological processes

in the vadose zone, especially when uniform dosing

and reoxygenation occur. If excessive concentrations of BOD and TSS migrate beyond the tank

because of poor maintenance, the infiltrative

Table 3-18. Case study: septic tank effluent and soil water quality a
USEPA Onsite Wastewater Treatment Systems Manual 3-29

Chapter 3: Establishing Treatment System Performance Requirements

surface can clog and surface seepage of wastewater

or plumbing fixture backup can occur.

Nitrogen

Nitrogen in raw wastewater is primarily in the

form of organic matter and ammonia. After the

septic tank, it is primarily (more than 85 percent)

ammonia. After discharge of the effluent to the

infiltrative surface, aerobic bacteria in the biomat

and upper vadose zone convert the ammonia in the

effluent almost entirely to nitrite and then to nitrate.

Nitrogen in its nitrate form is a significant ground

water pollutant. It has been detected in urban and

rural ground water nationwide, sometimes at levels

exceeding the USEPA drinking water standard of 10

mg/L (USGS, 1999). High concentrations of nitrate

(greater than 10 mg/L) can cause methemoglobinemia or “blue baby syndrome,” a disease in infants

that reduces the blood’s ability to carry oxygen, and

problems during pregnancy. Nitrogen is also an

important plant nutrient that can cause excessive

algal growth in nitrogen-limited inland (fresh)

waters and coastal waters, which are often limited in

available nitrogen. High algal productivity can block

sunlight, create nuisance or harmful algal blooms,

and significantly alter aquatic ecosystems. As algae

die, they are decomposed by bacteria, which can

deplete available dissolved oxygen in surface waters

and degrade habitat conditions.

Nitrogen contamination of ground water below

infiltration fields has been documented by many

investigators (Anderson et al., 1994; Andreoli et

al., 1979; Ayres Associates, 1989, 1993b, c; Bouma

et al., 1972; Carlile et al., 1981; Cogger and

Table 3-19. Wastewater constituents of concern and representative concentrations in the effluent of various treatment units

Source: Siegrist, 2001 (after Siegrist et al., 2000).
Chapter 3: Establishing Treatment System Performance Requirements

3-30 USEPA Onsite Wastewater Treatment Systems Manual

Carlile, 1984; Ellis and Childs, 1973; Erickson and

Bastian, 1980; Gibbs, 1977a, b; Peavy and

Brawner, 1979; Peavy and Groves, 1978; Polta,

1969; Preul, 1966; Reneau, 1977, 1979; Robertson

et al., 1989, 1990; Shaw and Turyk, 1994; Starr

and Sawhney, 1980; Tinker, 1991; Uebler, 1984;

Viraraghavan and Warnock, 1976a, b, c; Walker et

al., 1973a, b; Wolterink et al., 1979). Nitratenitrogen concentrations in ground water were

usually found to exceed the drinking water standard

of 10 mg/L near the infiltration field. Conventional

soil-based systems can remove some nitrogen from

septic tank effluent (table 3-19), but high-density

installation of OWTSs can cause contamination of

ground or surface water resources. When nitrate

reaches the ground water, it moves freely with little

retardation. Denitrification has been found to be

significant in the saturated zone only in rare

instances where carbon or sulfur deposits are

present. Reduction of nitrate concentrations in

ground water occurs primarily through dispersion

or recharge of ground water supplies by precipitation (Shaw and Turyk, 1994).

Nitrogen can undergo several transformations in

and below a SWIS, including adsorption, volatilization, mineralization, nitrification, and denitrification. Nitrification, the conversion of ammonium

nitrogen to nitrite and then nitrate by bacteria

under aerobic conditions, is the predominant

transformation that occurs immediately below the

infiltration zone. The negatively charged nitrate ion

is very soluble and moves readily with the percolating soil water.

Biological denitrification, which converts nitrate to

gaseous forms of nitrogen, can remove nitrate from

percolating wastewater. Denitrification occurs

under anaerobic conditions where available electron

donors such as carbon or sulfur are present. Denitrifying bacteria use nitrate as a substitute for

oxygen when accepting electrons. It has been

generally thought that anaerobic conditions with

organic matter seldom occur below soil infiltration

fields. Therefore, it is has been assumed that all the

nitrogen applied to infiltration fields ultimately

leaches to ground water (Brown et al., 1978;

Walker et al., 1973a, b). However, several studies

indicate that denitrification can be significant.

Jenssen and Siegrist (1990) found in their review

of several laboratory and field studies that approximately 20 percent of nitrogen is lost from wastewater percolating through soil. Factors found to

favor denitrification are fine-grained soils (silts and

clays) and layered soils (alternating fine-grained

and coarser-grained soils with distinct boundaries

between the texturally different layers), particularly

if the fine-grained soil layers contain organic

material. Jenssen and Siegrist concluded that

nitrogen removal below the infiltration field can be

enhanced by placing the system high in the soil

profile, where organic matter in the soil is more

likely to be present, and by dosing septic tank

effluent onto the infiltrative surface to create

alternating wetting and drying cycles. Denitrification can also occur if ground water enters surface

water bodies through organic-rich bottom sediments. Nitrogen concentrations in ground water

were shown to decrease to less than 0.5 mg/L after

passage through sediments in one Canadian study

(Robertson et al., 1989, 1990).

It is difficult to predict removal rates for wastewater-borne nitrate or other nitrogen compounds in

the soil matrix. In general, however, nitrate concentrations in SWIS effluent can and often do

exceed the 10 mg/L drinking water standard. Shaw

and Turyk (1994) found nitrate concentrations

ranging from 21 to 108 mg/L (average of 31 to 34

mg/L) in SWIS effluent plumes analyzed as part of

a study of 14 pressure-dosed drain fields in sandy

soils of Wisconsin. The limited ability of conventional SWISs to achieve enhanced nitrate reductions and the difficulty in predicting soil nitrogen

removal rates means that systems sited in drinking

water aquifers or near sensitive aquatic areas should

incorporate additional nitrogen removal technologies prior to final soil discharge.

Phosphorus

Phosphorus is also a key plant nutrient, and like

nitrogen it contributes to eutrophication and

dissolved oxygen depletion in surface waters,

especially fresh waters such as rivers, lakes, and

ponds. Monitoring below subsurface infiltration

systems has shown that the amount of phosphorus

leached to ground water depends on several factors:

the characteristics of the soil, the thickness of the

unsaturated zone through which the wastewater

percolates, the applied loading rate, and the age of

the system (Bouma et al., 1972; Brandes, 1972;

Carlile et al., 1981, Childs et al., 1974; Cogger and

Carlile, 1984; Dudley and Stephenson, 1973; Ellis

and Childs, 1973; Erickson and Bastian, 1980;

Gilliom and Patmont, 1983; Harkin et al., 1979;
USEPA Onsite Wastewater Treatment Systems Manual 3-31

Chapter 3: Establishing Treatment System Performance Requirements

Jones and Lee, 1979; Whelan and Barrow, 1984).

The amount of phosphorus in ground water varies

from background concentrations to concentrations

equal to that of septic tank effluent. However,

removals have been found to continue within

ground water aquifers (Carlile et al., 1981; Childs

et al., 1974; Cogger and Carlile, 1984; Ellis and

Childs, 1973; Gilliom and Patmont, 1983; Rea and

Upchurch, 1980; Reneau, 1979; Reneau and Pettry,

1976; Robertson et al., 1990).

Retardation of phosphorus contamination of surface

waters from SWISs is enhanced in fine-textured

soils without continuous macropores that would

allow rapid percolation. Increased distance of the

system from surface waters is also an important

factor in limiting phosphorus discharges because of

greater and more prolonged contact with soil

surfaces. The risk of phosphorus contamination,

therefore, is greatest in karst regions and coarsetextured soils without significant iron, calcium, or

aluminum concentrations located near surface waters.

The fate and transport of phosphorus in soils are

controlled by sorption and precipitation reactions

(Sikora and Corey, 1976). At low concentrations

(less than 5 mg/L), the phosphate ion is chemisorbed onto the surfaces of iron and aluminum

minerals in strongly acid to neutral systems and on

calcium minerals in neutral to alkaline systems. As

phosphorus concentrations increase, phosphate

precipitates form. Some of the more important

precipitate compounds formed are strengite,

FePO4

.

2H2O; variscite, AlPO4

.

2H2O; dicalcium

phosphate, CaHPO4

.

2H2O; octacalcium phosphate,

Ca4

H(PO4)3

.

3H2O; and hydroxyapatite, Ca10

(PO4)6(OH2). In acidic soils, phosphate sorption

probably involves the aluminum and iron compounds; in calcareous or alkaline soils, calcium

compounds predominate.

Estimates of the capacity of the soil to retain

phosphorus are often based on sorption isotherms

such as the Langmuir model (Ellis and Erickson,

1969; Sawney, 1977; Sawney and Hill, 1975;

Sikora and Corey, 1976; Tofflemire and Chen,

1977). This method significantly underestimates

the total retention capacity of the soil (Anderson et

al., 1994; Sawney and Hill, 1975; Sikora and

Corey, 1976; Tofflemire and Chen, 1977). This is

because the test measures the chemi-sorption

capacity but does not take into account the slower

precipitation reactions that regenerate the chemisorption sites. These slower reactions have been

shown to increase the capacity of the soil to retain

phosphorus by 1.5 to 3 times the measured capacity

calculated by the isotherm test (Sikora and Corey,

1976; Tofflemire and Chen, 1977). In some cases

the total capacity has been shown to be as much as

six times greater (Tofflemire and Chen, 1977).

These reactions can take place in unsaturated or

saturated soils (Ellis and Childs, 1973; Jones and

Lee, 1977a, b; Reneau and Pettry, 1976; Robertson

et al., 1990; Sikora and Corey, 1976).

The capacity of the soil to retain phosphorus is

finite, however. With continued loading, phosphorus movement deeper into the soil profile can be

expected. The ultimate retention capacity of the

soil depends on several factors, including its

mineralogy, particle size distribution, oxidationreduction potential, and pH. Fine-textured soils

theoretically provide more sorption sites for

phosphorus. As noted above, iron, aluminum, and

calcium minerals in the soil allow phosphorus

precipitation reactions to occur, a process that can

lead to additional phosphorus retention. Sikora and

Corey (1976) estimated that phosphorus penetration

into the soil below a SWIS would be 52 centimeters per year in Wisconsin sands and 10 centimeters

per year in Wisconsin silt loams.

Nevertheless, knowing the retention capacity of the

soil is not enough to predict the travel of phosphorus from subsurface infiltration systems. Equally

important is an estimate of the total volume of soil

that the wastewater will contact as it percolates to

and through the ground water. Fine-textured,

unstructured soils (e.g., clays, silty clays) can be

expected to disperse the water and cause contact

with a greater volume of soil than coarse, granular

soils (e.g., sands) or highly structured fine-textured

soils (e.g., clayey silts) having large continuous

pores. Also, the rate of water movement and the

degree to which the water’s elevation fluctuates are

important factors.

There are no simple methods for predicting phosphorus removal rates at the site level. However,

several landscape-scale tools that provide at least

some estimation of expected phosphorus loads from

clusters of onsite systems are available. The

MANAGE assessment method, which is profiled in

section 3.9.1, is designed to estimate existing and

projected future (build-out) nutrient loads and to

identify “hot spots” based on land use and cover
Chapter 3: Establishing Treatment System Performance Requirements

3-32 USEPA Onsite Wastewater Treatment Systems Manual

(see http://www.epa.gov/owow/watershed/

Proceed/joubert.html; http://www.edc.uri.edu/

cewq/manage.html). Such estimates provide at

least some guidance in siting onsite systems and

considering acceptable levels of both numbers and

densities in sensitive areas.

Pathogenic microorganisms

Pathogenic microorganisms found in domestic

wastewater include a number of different bacteria,

viruses, protozoa, and parasites that cause a wide

range of gastrointestinal, neurological, respiratory,

renal, and other diseases. Infection can occur

through ingestion (drinking contaminated water;

incidental ingestion while bathing, skiing, or

fishing), respiration, or contact (table 3-20). The

occurrence and concentration of pathogenic microorganisms in raw wastewater depend on the sources

contributing to the wastewater, the existence of

infected persons in the population, and environmental factors that influence pathogen survival

rates. Such environmental factors include the

following: initial numbers and types of organisms,

temperature (microorganisms survive longer at

lower temperatures), humidity (survival is longest

at high humidity), amount of sunlight (solar

radiation is detrimental to survival), and additional

soil attenuation factors, as discussed below. Typical

ranges of survival times are presented in table 3-21.

Among pathogenic agents, only bacteria have any

potential to reproduce and multiply between hosts

(Cliver, 2000). If temperatures are between 50 and

80 degrees Fahrenheit (10 to 25 degrees Celsius)

Table 3-20. Waterborne pathogens found in human waste and associated diseases
USEPA Onsite Wastewater Treatment Systems Manual 3-33

Chapter 3: Establishing Treatment System Performance Requirements

and nutrients are available, bacterial numbers may

increase 10- to 100-fold. However, such multiplication is usually limited by competition from other,

better-adapted organisms (Cliver, 2000).

Enteric bacteria are those associated with human

and animal wastes. Once the bacteria enter a soil,

they are subjected to life process stresses not

encountered in the host. In most nontropical

regions of the United States, temperatures are

typically much lower; the quantity and availability

of nutrients and energy sources are likely to be

appreciably lower; and pH, moisture, and oxygen

conditions are not as likely to be conducive to

long-term survival. Survival times of enteric

bacteria in the soil are generally reduced by higher

temperatures, lower nutrient and organic matter

content, acidic conditions (pH values of 3 to 5),

lower moisture conditions, and the presence of

indigenous soil microflora (Gerba et al., 1975).

Potentially pathogenic bacteria are eliminated faster

at high temperatures, pH values of about 7, low

oxygen content, and high dissolved organic substance content (Pekdeger, 1984). The rate of

bacterial die-off approximately doubles with each

10-degree increase of temperature between 5 and

30 o

C (Tchobanoglous and Burton, 1991). Observed survival rates for various potential pathogenic bacteria have been found to be extremely

variable. Survival times of longer than 6 months

can occur at greater depths in unsaturated soils

where oligotrophic (low-nutrient) conditions exist

(Pekdeger, 1984).

The main methods of bacterial retention in unsaturated soil are filtration, sedimentation, and adsorption (Bicki et al., 1984; Cantor and Knox, 1985;

Gerba et al., 1975). Filtration accounts for the most

retention. The sizes of bacteria range from 0.2 to 5

microns (μm) (Pekdeger, 1984; Tchobanoglous and

Burton, 1991); thus, physical removal through

filtration occurs when soil micropores and surface

water film interstices are smaller than this. Filtration of bacteria is enhanced by slow permeability

rates, which can be caused by fine soil textures,

unsaturated conditions, uniform wastewater distribution to soils, and periodic treatment system

resting. Adsorption of bacteria onto clay and

organic colloids occurs within a soil solution that

has high ionic strength and neutral to slightly acid

pH values (Canter and Knox, 1985).

Normal operation of septic tank/subsurface infiltration systems results in retention and die-off of

most, if not all, observed pathogenic bacterial

indicators within 2 to 3 feet (60 to 90 centimeters)

of the infiltrative surface (Anderson et al., 1994;

Ayres Associates, 1993a, c; Bouma et al., 1972;

McGauhey and Krone, 1967). With a mature

biomat at the infiltrative surface of coarser soils,

most bacteria are removed within the first 1 foot

(30 centimeters) vertically or horizontally from the

trench-soil interface (University of Wisconsin,

1978). Hydraulic loading rates of less than 2

inches/day (5 centimeters/day) have also been

found to promote better removal of bacteria in

septic tank effluent (Ziebell et al., 1975). Biomat

Table 3-21. Typical pathogen survival times at 20 to 30 oC
Chapter 3: Establishing Treatment System Performance Requirements

3-34 USEPA Onsite Wastewater Treatment Systems Manual

formation and lower hydraulic loading rates

promote unsaturated flow, which is one key to soilbased removal of bacteria from wastewater. The

retention behavior of actual pathogens in unsaturated soil might be different from that of the

indicators (e.g., fecal coliforms) that have been

measured in most studies.

Failure to properly site, design, install, and/or

operate and maintain subsurface infiltration systems

can result in the introduction of potentially pathogenic bacteria into ground water or surface waters.

Literature reviews prepared by Hagedorn (1982)

and Bicki et al. (1984) identify a number of

references that provide evidence that infiltrative

surfaces improperly constructed below the ground

water surface or too near fractured bedrock correlate with such contamination. Karst geology and

seasonally high water tables that rise into the

infiltrative field can also move bacteria into ground

water zones. Once in ground water, bacteria from

septic tank effluent have been observed to survive

for considerable lengths of time (7 hours to 63

days), and they can travel up to and beyond 100

feet (30 meters) (Gerba et al., 1975).

Viruses are not a normal part of the fecal flora.

They occur in infected persons, and they appear in

septic tank effluent intermittently, in varying

numbers, reflecting the combined infection and

carrier status of OWTS users (Berg, 1973). It is

estimated that less than 1 to 2 percent of the stools

excreted in the United States contain enteric viruses

(University of Wisconsin, 1978). Therefore, such

viruses are difficult to monitor and little is known

about their frequency of occurrence and rate of

survival in traditional septic tank systems. Once an

infection (clinical or subclinical) has occurred,

however, it is estimated that feces may contain 106

to 1010 viral particles per gram (Kowal, 1982).

Consequently, when enteric viruses are present in

septic tank effluent, they might be present in

significant numbers (Anderson et al., 1991; Hain

and O’Brien, 1979; Harkin et al., 1979; Vaughn

and Landry, 1977; Yeager and O’Brien, 1977).

Some reduction (less than 1 log) of virus concentrations in wastewater occurs in the septic tank.

Higgins et al. (2000) reported a 74 percent decrease in

MS2 coliphage densities, findings that concurs with

those of other studies (Payment et al., 1986; Roa,

1981). Viruses can be both retained and inactivated in

soil; however, they can also be retained but not

inactivated. If not inactivated, viruses can accumulate in soil and subsequently be released due to

changing conditions, such as prolonged peak

OWTS flows or heavy rains. The result could be

contamination of ground water. Soil factors that

decrease survival include warm temperatures, low

moisture content, and high organic content. Soil

factors that increase retention include small particle

size, high moisture content, low organic content,

and low pH. Sobsey (1983) presents a thorough

review of these factors. Virus removal below the

vadose zone might be negligible in some geologic

settings. (Cliver, 2000).

Most studies of the fate and transport of viruses in

soils have been columnar studies using a specific

serotype, typically poliovirus 1, or bacteriophages

(Bitton et al., 1979; Burge and Enkiri, 1978;

Drewry, 1969, 1973; Drewry and Eliassen, 1968;

Duboise et al., 1976; Goldsmith et al., 1973; Green

and Cliver, 1975; Hori et al., 1971; Lance et al.,

1976; Lance et al., 1982; Lance and Gerba, 1980;

Lefler and Kott, 1973, 1974; Nestor and Costin,

1971; Robeck et al., 1962; Schaub and Sorber, 1977;

Sobsey et al., 1980; Young and Burbank, 1973;

University of Wisconsin, 1978). The generalized

results of these studies indicate that adsorption is the

principal mechanism of virus retention in soil.

Increasing the ionic strength of the wastewater

enhances adsorption. Once viruses have been retained,

inactivation rates range from 30 to 40 percent per day.

Various investigations have monitored the transport

of viruses through unsaturated soil below the

infiltration surface has been monitored by (Anderson et al., 1991; Hain and O’Brien, 1979; Jansons

et al., 1989; Schaub and Sorber, 1977; Vaughn and

Landry, 1980; Vaughn et al., 1981; Vaughn et al.,

1982, 1983; Wellings et al., 1975). The majority of

these studies focused on indigenous viruses in the

wastewater and results were mixed. Some serotypes

were found to move more freely than others. In

most cases viruses were found to penetrate more

than 10 feet (3 meters) through unsaturated soils.

Viruses are less affected by filtration than bacteria

(Bechdol et al., 1994) and are more resistant than

bacteria to inactivation by disinfection (USEPA,

1990). Viruses have been known to persist in soil

for up to 125 days and travel in ground water for

distances of up to 1,339 feet (408 meters). However, monitoring of eight conventional individual

home septic tank systems in Florida indicated that

2 feet (60 centimeters) of fine sand effectively
USEPA Onsite Wastewater Treatment Systems Manual 3-35

Chapter 3: Establishing Treatment System Performance Requirements

removed viruses (Anderson et al., 1991; Ayres

Associates, 1993c). Higgins (2000) reported 99

percent removal of virus particles within the first 1

foot (30.5 centimeters) of soil.

Recent laboratory and field studies of existing

onsite systems using conservative tracers (e.g.,

bromide ions) and microbial surrogate measures

(e.g., viruses, bacteria) found that episodic breakthroughs of virus and bacteria can occur in the

SWIS, particularly during early operation (Van

Cuyk et al., 2001). Significant (e.g., 3-log) removal

of viruses and near complete removal of fecal

bacteria can be reasonably achieved in 60 to 90

centimeters of sandy media (Van Cuyk et al., 2001).

Inactivation of pathogens through other physical,

chemical, or biological mechanisms varies considerably. Protozoan cysts or oocysts are generally killed

when they freeze, but viruses are not. Ultraviolet

light, extremes of pH, and strong oxidizing agents

(e.g., hypochlorite, chlorine dioxide, ozone) are also

effective in killing or inactivating most pathogens

(Cliver, 2000). Korich (1990) found that in demandfree water, ozone was slightly more effective than

chlorine dioxide against Cryptosporidium parvum

oocysts, and both were much more effective than

chlorine or monochloramine. C. parvum oocysts were

found to be 30 times more resistant to ozone and

14 times more resistant to chlorine dioxide than are

Giardia lamblia cysts (Korich et al., 1990).

Toxic organic compounds

A number of toxic organic compounds that can

cause neurological, developmental, or other

problems in humans and interfere with biological

processes in the environment can be found in septic

tank effluent. Table 3-22 provides information on

potential health effects from selected organic

chemicals, along with USEPA maximum containment levels for these pollutants in drinking water.

The toxic organics that have been found to be the

most prevalent in wastewater are 1,4-dichlorobenzene, methylbenzene (toluene), dimethylbenzenes

(xylenes), 1,1-dichloroethane, 1,1,1-

trichloroethane, and dimethylketone (acetone).

These compounds are usually found in household

products like solvents and cleaners.

No known studies have been conducted to determine toxic organic treatment efficiency in singlefamily home septic tanks. A study of toxic organics

in domestic wastewater and effluent from a community septic tank found that removal of lowmolecular-weight alkylated benzenes (e.g., toluene,

Table 3-22. Maximum contaminant levels (MCLs) for selected organic chemicals in drinking water
Chapter 3: Establishing Treatment System Performance Requirements

3-36 USEPA Onsite Wastewater Treatment Systems Manual

xylene) was noticeable, whereas virtually no

removal was noted for higher-molecular-weight

compounds (DeWalle et al., 1985). Removal

efficiency was observed to be directly related to

tank detention time, which is directly related to

settling efficiency.

The behavior of toxic organic compounds in unsaturated soil is not well documented. The avenues of

mobility available to toxic organics include those

which can transport organics in both gaseous and

liquid phases. In the gaseous phase toxic organics

diffuse outward in any direction within unobstructed

soil voids; in the liquid phase they follow the movement of the soil solution. Because of their nonpolar

nature, certain toxic organics are not electrochemically retained in unsaturated soil. Toxic organics can

be transformed into less innocuous forms in the soil

by indigenous or introduced microorganisms. The

biodegradability of many organic compounds in the

soil depends on oxygen availability. Halogenated

straight-chain compounds, such as many chlorinated solvents, are usually biodegraded under

anaerobic conditions when carbon dioxide replaces

oxygen (Wilhelm, 1998). Aromatic organic compounds like benzene and toluene, however, are

biodegraded primarily under aerobic conditions. As

for physical removal, organic contaminants are

adsorbed by solid organic matter. Accumulated

organic solids in the tank and in the soil profile,

therefore, might be important retainers of organic

contaminants. In addition, because many of the

organic contaminants found in domestic wastewater

are relatively volatile, unsaturated conditions in

drain fields likely facilitate the release of these

compounds through gaseous diffusion and volatilization (Wilhelm, 1998).

Rates of movement for the gaseous and liquid

phases depend on soil and toxic organic compound

type. Soils having fine textures, abrupt interfaces

of distinctly different textural layers, a lack of

fissures and other continuous macropores, and low

moisture content retard toxic organic movement

(Hillel, 1989). If gaseous exchange between soil

and atmosphere is sufficient, however, appreciable

losses of low-molecular-weight alkylated benzenes

such as toluene and dimethylbenzene (xylene) can

be expected because of their relatively high vapor

pressure (Bauman, 1989). Toxic organics that are

relatively miscible in water (e.g., methyl tertiary

butyl ether, tetrachloroethane, benzene, xylene) can

be expected to move with soil water. Nonmiscible

toxic organics that remain in liquid or solid phases

(chlorinated solvents, gasoline, oils) can become

tightly bound to soil particles (Preslo et al., 1989).

Biodegradation appears to be an efficient removal

mechanism for many volatile organic compounds.

Nearly complete or complete removal of toxic

organics below infiltration systems was found in

several studies (Ayres Associates, 1993a, c;

Robertson, 1991; Sauer and Tyler, 1991).

Some investigations have documented toxic organic

contamination of surficial aquifers by domestic

wastewater discharged from community infiltration

fields (Tomson et al., 1984). Of the volatile

organic compounds detected in ground water

samples collected in the vicinity of subsurface

infiltration systems, Kolega (1989) found trichloromethane, toluene, and 1,1,1-trichloroethane most

frequently and in some of the highest concentrations. Xylenes, dichloroethane, and dichloromethane were also detected.

Table 3-23. Case study: concentration of metals in septic tank effluenta
USEPA Onsite Wastewater Treatment Systems Manual 3-37

Chapter 3: Establishing Treatment System Performance Requirements

Once toxic organics reach an aquifer, their movement generally follows the direction of ground

water movement. The behavior of each within an

aquifer, however, can be different. Some stay near the

surface of the aquifer and experience much lateral

movement. Others, such as aliphatic chlorinated

hydrocarbons, experience greater vertical movement

because of their heavier molecular weight (Dagan and

Bresler, 1984). Based on this observation, 1,4-

dichlorobenzene, toluene, and xylenes in septic tank

effluent would be expected to experience more lateral

than vertical movement in an aquifer; 1,1-dichloroethane, 1,1,1-trichloroethane, dichloromethane, and

trichloromethane would be expected to show more

vertical movement. Movement of toxic organic

compounds is also affected by their degree of solubility in water. Acetone, dichloromethane, trichloromethane, and 1,1-dichloroethane are quite soluble in

water and are expected to be very highly mobile;

1,1,1-trichloroethane, toluene, and 1,2-dimethylbenzene (o-xylene) are expected to be moderately

mobile; and 1,3-dimethylbenzene (m-xylene), 1,4-

dimethylbenzene (p-xylene), and 1,4-dichlorobenzene

are expected to have low mobility (Fetter, 1988).

System design considerations for removing toxic

organic compounds include increasing tank retention time (especially for halogenated, straight-chain

compounds like organic solvents), ensuring greater

vadose zone depths below the SWIS, and placing

the infiltration system high in the soil profile,

where higher concentrations of organic matter and

oxygen can aid the volatilization and treatment of

aromatic compounds. It should be noted that

significantly high levels of toxic organic compounds

can cause die-off of tank and biomat microorganisms, which could reduce treatment performance.

Onsite systems that discharge high amounts of toxic

organic compounds might be subject to USEPA’s

Class V Underground Injection Control Program

(see http://www.epa.gov/safewater.uic.html).

Metals

Metals like lead, mercury, cadmium, copper, and

chromium can cause physical and mental developmental delays, kidney disease, gastrointestinal

illnesses, and neurological problems. Some information is available regarding metals in septic tank

effluent (DeWalle et. al. 1985). Metals can be

present in raw household wastewater because many

commonly used household products contain metals.

Aging interior plumbing systems can contribute

lead, cadmium, and copper (Canter and Knox,

1985). Other sources of metals include vegetable

matter and human excreta. Several metals have been

found in domestic septage, confirming their presence

in wastewater. They primarily include cadmium,

copper, lead, and zinc (Bennett et al., 1977; Feige et

al., 1975; Segall et al., 1979). OWTSs serving

nonresidential facilities (e.g., rural health care

facilities, small industrial facilities) can also experience metal loadings. Several USEPA priority

pollutant metals have been found in domestic septic

tank effluent (Whelan and Titmanis, 1982). The

most prominent metals were nickel, lead, copper,

Table 3-24. Maximum contaminant levels (MCLs) for selected inorganic chemicals in drinking water
Chapter 3: Establishing Treatment System Performance Requirements

3-38 USEPA Onsite Wastewater Treatment Systems Manual

zinc, barium, and chromium. A comparison of

mean concentrations of metals in septic tank

effluent as found in one study (table 3-23) with the

USEPA maximum contaminant levels for drinking

water noted in table 3-24 reveals a potential for

contamination that might exceed drinking water

standards in some cases.

The fate of metals in soil is dependent on complex

physical, chemical, and biochemical reactions and

interactions. The primary processes controlling the

fixation/mobility potential of metals in subsurface

infiltration systems are adsorption on soil particles

and interaction with organic molecules. Because the

amount of naturally occurring organic matter in the

soil below the infiltrative surface is typically low,

the cation exchange capacity of the soil and soil

solution pH control the mobility of metals below

the infiltrative surface. Acidic conditions can

reduce the sorption of metals in soils, leading to

increased risk of ground water contamination

(Evanko, 1997; Lim et al., 2001). (See figure 3-11.)

It is likely that movement of metals through the

unsaturated zone, if it occurs at all, is accomplished

by movement of organic ligand complexes formed at

or near the infiltrative surface (Canter and Knox,

1985; Matthess, 1984).

Information regarding the transport and fate of

metals in ground water can be found in hazardous

waste and soil remediation literature (see http://

www.gwrtac.org/html/Tech_eval.html#METALS).

One study attempted to link septic tank systems to

metal contamination of rural potable water supplies,

but only a weak correlation was found (Sandhu et

al., 1977). Removal of sources of metals from the

wastewater stream by altering user habits and

implementing alternative disposal practices is

recommended. In addition, the literature suggests

that improving treatment processes by increasing

septic tank detention times, ensuring greater

unsaturated soil depths, and improving dose and

rest cycles may decrease risks associated with metal

loadings from onsite systems (Chang, 1985;

Evanko, 1997; Lim et al., 2001).

Surfactants

Surfactants are commonly used in laundry detergents

and other soaps to decrease the surface tension of

water and increase wetting and emulsification.

Surfactants are the largest class of anthropogenic

organic compounds present in raw domestic wastewater (Dental et al., 1993). Surfactants that survive

treatment processes in the septic tank and subsequent treatment train can enter the soil and mobilize otherwise insoluble organic pollutants. Surfactants have been shown to decrease adsorption — and

even actively desorb — the pollutant trichlorobenzene

from soils (Dental, 1993). Surfactants can also change

soil structure and alter wastewater infiltration rates.

Surfactant molecules contain both strongly hydrophobic and strongly hydrophilic properties and thus

tend to concentrate at interfaces of the aqueous

system including air, oily material, and particles.

Surfactants can be found in most domestic septic tank

effluents. Since 1970 the most common anionic

surfactant used in household laundry detergent is

linear alkylbenzenesulfonate, or LAS. Whelan and

Titmanis (1982) found a range of LAS concentrations from 1.2 to 6.5 mg/L in septic tank effluent.

Dental (1993) cited studies finding concentrations of LAS in raw wastewater ranging from

3 mg/L to 21 mg/L.

Because surfactants in wastewater are associated

with particulate matter and oils and tend to concentrate in sludges in wastewater treatment plants

(Dental, 1993), increasing detention times in the

tank might aid in their removal. The behavior of

surfactants in unsaturated soil is dependent on

surfactant type. It is expected that minimal retention

of anionic and nonionic surfactants occurs in unsaturated soils having low organic matter content. HowSource: Lim et al., 2001. ever, the degree of mobility is subject to soil

Figure 3-11. Zinc sorption by clay as a function of pH at various

loading concentrations (in 0.05 M NaCl medium)
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Chapter 3: Establishing Treatment System Performance Requirements

solution chemistry, organic matter content of the

soil, and rate of degradation by soil microorganisms.

Soils with high organic matter should favor

retention of surfactants because of the lipophilic

component of surfactants. Surfactants are readily

biodegraded under aerobic conditions and are more

stable under anaerobic conditions. Substantial attenuation of LAS in unsaturated soil beneath a subsurface

infiltration system has been demonstrated (Anderson

et al., 1994; Robertson et al., 1989; Shimp et al.,

1991). Cationic surfactants strongly sorb to cation

exchange sites of soil particles and organic matter

(McAvoy et al., 1991). Thus, fine-textured soils and

soils having high organic matter content will generally favor retention of these surfactants.

Some investigations have identified the occurrence

of methylene blue active substance (MBAS) in

ground water (Perlmutter and Koch, 1971; Thurman

et al., 1986). The type of anionic surfactant was not

specifically identified. However, it was surmised

that the higher concentrations noted at the time of

the study were probably due to use of alkylbenzenesulfonate (ABS), which is degraded by

microorganisms at a much slower rate than LAS.

There has also been research demonstrating that all

types of surfactants might be degraded by microorganisms in saturated sediments (Federle and

Pastwa, 1988). No investigations have been found

that identify cationic or nonionic surfactants in

ground water that originated from subsurface

wastewater infiltration systems. However, because

of concerns over the use of alkylphenol

polyethoxylates, studies of fate and transport of this

class of endocrine disrupters are in progress.

Summary

Subsurface wastewater infiltration systems are

designed to provide wastewater treatment and

dispersal through soil purification processes and

ground water recharge. Satisfactory performance is

dependent on the treatment efficiency of the

pretreatment system, the method of wastewater

distribution and loading to the soil infiltrative

surface, and the properties of the vadose and

saturated zones underlying the infiltrative surface.

The soil should have adequate pore characteristics,

size distribution, and continuity to accept the daily

volume of wastewater and provide sufficient soilwater contact and retention time for treatment before

the effluent percolates into the ground water.

Ground water monitoring below properly sited,

designed, constructed, and operated subsurface

infiltration systems has shown carbonaceous

biochemical oxygen demand (CBOD), suspended

solids (TSS), fecal indicators, metals, and surfactants

can be effectively removed by the first 2 to 5 feet

of soil under unsaturated, aerobic conditions.

Phosphorus and metals can be removed through

adsorption, ion exchange, and precipitation reactions, but the capacity of soil to retain these ions is

finite and varies with soil mineralogy, organic

content, pH, reduction-oxidation potential, and

cation exchange capacity. Nitrogen removal rates

vary significantly, but most conventional SWISs do

not achieve drinking water standards (i.e., 10 mg/L)

for nitrate concentrations in effluent plumes.

Evidence is growing that some types of viruses are

able to leach with wastewater from subsurface

infiltration systems to ground water. Longer

retention times associated with virus removal are

achieved with fine-texture soil, low hydraulic

loadings, uniform dosing and resting, aerobic subsoils, and high temperatures. Toxic organics appear

to be removed in subsoils, but further study of the

fate and transport of these compounds is needed.

Subsurface wastewater infiltration systems do

affect ground water quality and therefore have the

potential to affect surface water quality (in areas

with gaining streams, large macropore soils, or

karst terrain or in coastal regions). Studies have

shown that after the treated percolate enters ground

water it can remain as a distinct plume for as much

as several hundred feet. Concentrations of nitrate,

dissolved solids, and other soluble contaminants

can remain above ambient ground water concentrations within the plume. Attenuation of solute

concentrations is dependent on the quantity of

natural recharge and travel distance from the

source, among other factors. Organic bottom

sediments of surface waters appear to provide some

retention or removal of wastewater contaminants if

the ground water seeps through those sediments to

enter the surface water. These bottom sediments

might be effective in removing trace organic

compounds, endotoxins, nitrate, and pathogenic

agents through biochemical activity, but few data

regarding the effectiveness and significance of

removal by bottom sediments are available.

Public health and environmental risks from properly sited, designed, constructed, and operated
Chapter 3: Establishing Treatment System Performance Requirements

3-40 USEPA Onsite Wastewater Treatment Systems Manual

septic tank systems appear to be low. However,

soils with excessive permeability (coarse-texture

soil or soil with large and continuous pores), low

organic matter, low pH, low cation exchange

capacities, low oxygen-reduction potential, high

moisture content, and low temperatures can increase health and environmental risks under certain

circumstances.

3.8 Establishing performance

requirements

As noted in chapter 2, the OWTS regulatory

authority and/or management entity establishes

performance requirements to ensure future compliance with the public health and environmental

objectives of the community. Performance requirements are based on broad goals such as eliminating

health threats from contact with effluent or direct/

indirect ingestion of effluent contaminants. They are

intended to meet standards for water quality and

public health protection and can be both quantitative (total mass load or concentration) or qualitative (e.g., no odors or color in discharges to surface

waters). Compliance with performance requirements

is measured at a specified performance boundary (see

chapter 5), which can be a physical boundary or a

property boundary. Figure 3-12 illustrates performance and compliance boundaries and potential

monitoring sites in a cutaway view of a SWIS.

Design boundaries are where conditions abruptly

change. A design boundary can be at the intersection

of unit processes or between saturated and unsaturated

soil conditions (e.g., the delineation between the

infiltrative, vadose, and ground water zones) or at

another designated location, such as a drinking water

well, nearby surface water, or property boundary.

Performance requirements for onsite treatment

systems should be established based on water

quality standards for the receiving resource and the

assimilative capacity of the environment between

the point of the wastewater release to the receiving

environment and the performance boundary

designated by the management entity or regulatory

authority. Typically, the assimilative capacity of the

receiving environment is considered part of the

treatment system to limit costs in reaching the desired

performance requirement or water quality goals (see

figure 3-12). The performance boundary is usually a

specified distance from the point of release, such as a

property boundary, or a point of use, such as a

drinking water well or surface water with designated uses specified by the state water agency.

Achievement of water quality objectives requires

that treatment system performance consider the

assimilative capacity of the receiving environment.

If the assimilative capacity of the receiving environment is overlooked because of increases in

pollutant loadings, the treatment performance of

onsite systems before discharge to the soil should

increase. OWTSs serving high-density clusters of

homes or located near sensitive receiving waters

might be the subject of more stringent requirements

than those serving lower-density housing farther

from sensitive water resources.

Performance requirements for onsite systems

should be based on risk assessments that consider

the hazards of each potential pollutant in the

wastewater to be treated, its transport and fate,

potential exposure opportunities, and projected

effects on humans and environmental resources. A

variety of governmental agencies have already

established water quality standards for a wide range

of surface water uses. These include standards for

protecting waters used for recreation, aquatic life

support, shellfish propagation and habitat, and

drinking water. In general, these standards are

based on risk assessment processes and procedures

that consider the designated uses of receiving

waters, the hazard and toxicity of the pollutants,

Figure 3-12. Example of compliance boundaries for onsite

wastewater treatment systems
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Chapter 3: Establishing Treatment System Performance Requirements

the potential for human and ecosystem exposure,

and the estimated impacts of exposure. Although

federally mandated ground water quality standards

(maximum contaminant levels; see tables in section

3.8) are currently applicable only to drinking water

supply sources, some states have adopted similar

local ground water quality standards (see sidebar).

Local needs or goals need to be considered when

performance requirements are established. Watershed- or site-specific conditions might warrant

lower pollutant discharge concentrations or mass

pollutant limits than those required by existing

water quality standards. However, existing water

quality standards provide a good starting point for

selecting appropriate OWTS performance requirements. The mass of pollutants that should be

removed by onsite treatment systems can be

determined by estimating the mass of cumulative

OWTS pollutants discharged to the receiving

waters and calculating the assimilative capacity of

the receiving waters. Mass pollutant loads are

usually apportioned among the onsite systems and

other loading sources (e.g., urban yards and

landscaped areas, row crop lands, animal feeding

operations) in a ground water aquifer or watershed.

3.8.1 Assessing resource vulnerability

and receiving water capacity

Historically, conventional onsite systems have been

designed primarily to protect human health. Land

use planning has affected system oversight requirements, but environmental protection has been a

Nitrogen contributions from onsite systems

The San Lorenzo River basin in California is served primarily by onsite wastewater treatment systems. Since

1985 the Santa Cruz County Environmental Health Service has been working with local stakeholders to develop a

program for inspecting all onsite systems, assessing pollutant loads from those systems, and correcting identified

problems. Studies conducted through this initiative included calculations of nutrient inputs to the river from onsite

systems. According to the analyses performed by the county and its contractors, 55 to 60 percent of the nitrate

load in the San Lorenzo River during the summer months came from onsite system effluent. Assumptions

incorporated into the calculations included an average septic tank effluent total nitrogen concentration of 50 mg/L,

per capita wastewater generation of 70 gallons per day, and an average house occupancy of 2.8 persons. Nitrogen

removal was estimated at 15 percent for SWISs in sandy soils and 25 percent for SWISs in other soils.

Source: Ricker et al., 1994.

Performance requirements of mance requirements of Wisconsin’ Wisconsin’s ground w round water quality r ater quality rule

Wisconsin was one of the first states to promulgate ground water standards. Promulgated in 1985, Wisconsin’s

ground water quality rule establishes both public health and public welfare ground water quality standards for

substances detected in or having a reasonable probability of entering the ground water resources of the state.

Preventive action and enforcement limits are established for each parameter included in the rule. The preventive

action limits (PALs) inform the Department of Natural Resources (DNR) of potential threats to ground water quality.

When a PAL is exceeded, the Department is required to take action to control the contamination so that the

enforcement limit is not reached. For example, nitrate-nitrogen is regulated through a public health standard. The

PAL for nitrate is 2 mg/L (nitrogen), and its enforcement limit is 10 mg/L (nitrogen). If the PAL is exceeded, the

DNR requires a specific control response based on an assessment of the cause and significance of the elevated

concentration. Various responses may be required, including no action, increased monitoring, revision of

operational procedures at the facility, remedial action, closure, or other appropriate actions that will prevent further

ground water contamination.

Source: State of Wisconsin Administrative Code, Chapter NR 140.
Chapter 3: Establishing Treatment System Performance Requirements

3-42 USEPA Onsite Wastewater Treatment Systems Manual

tertiary objective, at best, for most regulatory

programs. Human health protection is assumed (but

not always ensured) by infiltrating septic tank

effluent at sufficiently low rates into moderately

permeable, unsaturated soils downgradient and at

specified distances from water supply wells. Site

evaluations are performed to assess the suitability

of proposed locations for the installation of conventional systems. Criteria typically used are estimated

soil permeability (through soil analysis or percolation tests), unsaturated soil depth above the seasonally high water table, and horizontal setback

distances from wells, property lines, and dwellings

(see chapter 5).

OWTS codes have not normally considered increased pollutant loads to a ground water resource

(aquifer) due to higher housing densities, potential

contamination of water supplies by nitrates, or the

environmental impacts of nutrients and pathogens

on nearby surface waters. Preserving and protecting

water quality require more comprehensive evaluations of development sites proposed to be served by

onsite systems. A broader range of water contaminants and their potential mobility in the environment should be considered at scales that consider

both spatial (site vs. region) and temporal (existing

vs. planned development) issues (see tables 3-20 to

3-24). Some watershed analyses are driven by

TMDLs (Total Maximum Daily Loads established

under section 303 of the Clean Water Act) for

interconnected surface waters, while others are

driven by sole source aquifer or drinking water

standards.

Site suitability assessments

Some states have incorporated stricter site suitability and performance requirements into their OWTS

permit programs. Generally, the stricter requirements were established in response to concerns over

nitrate contamination of water supplies or nutrient

inputs to surface waters. For example, in Massachusetts the Department of Environmental Protection has designated “nitrogen-sensitive areas” in

which new nitrogen discharges must be limited.

Designation of these areas is based on ecological

sensitivity and relative risk of threats to drinking

water wells.

Multivariate rating approaches: DRASTIC

Other approaches are used that typically involve

regional assessments that inventory surface and

ground water resources and rate them according to

their sensitivity to wastewater impacts. The ratings

are based on various criteria that define vulnerability. One such method is DRASTIC (see sidebar).

DRASTIC is a standardized system developed by

USEPA to rate broad-scale ground water vulnerability using hydrogeologic settings (Aller et al.,

1987). The acronym identifies the hydrogeologic

factors considered: depth to ground water, (net)

recharge, aquifer media, soil media, topography

(slope), impact of the vadose zone media, and

(hydraulic) conductivity of the aquifer. This

method is well suited to geographic information

system (GIS) applications but requires substantial

amounts of information regarding the natural

resources of a region to produce meaningful

results. Landscape scale methods and models are

excellent planning tools but might have limited

utility at the site scale. These approaches should be

Massachusetts’ requirements for nitrogen-sensitive areas

Nitrogen-sensitive areas are defined in state rules as occurring within Interim Wellhead Protection Areas, 1-year

recharge areas of public water supplies, nitrogen-sensitive embayments, and other areas that are designated as

nitrogen-sensitive based on scientific evaluations of the affected water body (310 Code of Massachusetts

Regulations 15.000, 1996). Any new construction using onsite wastewater treatment in these designated areas

must abide by prescriptive standards that limit design flows to a maximum of 440 gallons per day of aggregated

flows per acre. Exceptions are permitted for treatment systems with enhanced nitrogen removal capability. With

enhanced removal, the maximum design flow may be increased. If the system is an approved alternative system

or a treatment unit with a ground water discharge permit that produces an effluent with no more than 10 mg/L of

nitrate, the design flow restrictions do not apply.

Source: Title V, Massachusetts Environmental Code.
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Chapter 3: Establishing Treatment System Performance Requirements

supported and complemented by other information

collected during the site evaluation (see chapter 5).

GIS overlay analysis: MANAGE

A simpler GIS-based method was developed by the

University of Rhode Island Cooperative Extension

Service (see http://www.edc.uri.edu/cewq/

manage.html). The Method for Assessment,

Nutrient-loading, and Geographic Evaluation

(MANAGE) uses a combination of map analyses

that incorporates landscape features, computergenerated GIS and other maps, and a spreadsheet to

estimate relative pollution risks of proposed land

uses (Joubert et al., 1999; Kellogg et al., 1997).

MANAGE is a screening-level tool designed for

areawide assessment of entire aquifers, wellhead

protection areas, or small watersheds (figure 3-13).

Local knowledge and input are needed to identify

critical resource areas, refine the map data, and

select management options for analysis. Community decision makers participate actively in the

assessment process (see sidebar).

The spreadsheet from the MANAGE application

extracts spatial and attribute data from the national

Soil Survey Geographic (SSURGO) database

(USDA, 1995; see http://www.ftw.nrcs.usda.gov/

ssur_data.html) and Anderson Level III Land

Cover data (Anderson, 1976) through the Rhode

Island GIS system. The soils are combined into

hydrologic groups representing the capability of the

soils to accept water infiltration, the depth to the

water table, and the presence of hydraulically

restrictive horizons. Estimates of nutrient loadings

are made using published data and simplifying

assumptions. The spreadsheet estimates relative

pollutant availability, surface water runoff pollutant

concentrations, and pollutant migration to ground

water zones without attempting to model fate and

transport mechanisms, which are highly uncertain.

From these data the spreadsheet calculates a

hydrologic budget, estimates nutrient loading, and

summarizes indicators of watershed health to create

a comprehensive risk assessment for wastewater

management planning. (For mapping products

available from the U.S. Geological Survey, see

http://www.nmd.usgs.gov/.)

MANAGE generates three types of assessment

results that can be displayed in both map and chart

form: (1) pollution “hot spot” mapping of potential

high-risk areas, (2) watershed indicators based on

land use characteristics (e.g., percent of impervious

area and forest cover), and (3) nutrient loading in

the watershed based on estimates from current

research of sources, and generally assumed fates of

nitrogen and phosphorus (Joubert et al., 1999).

It is important to note that before rules, ordinances,

or overlay zones based on models are enacted or

established, the models should be calibrated and

verified with local monitoring information collected over a year or more. Only models that

accurately and consistently approximate actual

event-response relationships should serve as the

basis for management action. Also, the affected

population must accept the model as the basis for

both compliance and possible penalties.

Value analysis and vulnerability assessment

Hoover et al. (1998) has proposed a more subjective vulnerability assessment method that emphasizes public input. This approach considers risk

Using GIS tools to characterize potential water quality threats in Colorado

Summit County, Colorado, developed a GIS to identify impacts that OWTS-generated nitrates might have on

water quality in the upper Blue River watershed. The GIS was developed in response to concerns that increasing

residential development in the basin might increase nutrient loadings into the Dillon Reservoir. Database

components entered into the GIS included geologic maps, soil survey maps, topographic features, land parcel

maps, domestic well sampling data, onsite system permitting data, well logs, and assessors’ data. The database

can be updated with new water quality data, system maintenance records, property records, and onsite system

construction permit and repair information. The database is linked to the DRASTIC ground water vulnerability

rating. The approach is being used to identify areas that have a potential for excessive contamination by nitratenitrogen from OWTSs. These assessments could support onsite system placement and removal decisions and

help prioritize water quality improvement projects.

Source: Stark et al., 1999.
Chapter 3: Establishing Treatment System Performance Requirements

3-44 USEPA Onsite Wastewater Treatment Systems Manual

Source: Kellogg et al., 1997.

assessment methods and management control

strategies for both ground waters and surface

waters. It uses three components of risk assessment

and management, including consideration of

• Value of ground and surface water as a public

water supply or resource

• Vulnerability of the water supply or resource

• Control measures for addressing hazards

The first part of the onsite risk assessment and

management approach involves a listing of all the

ground water and surface water resources in a

region or community (table 3-26). Through

community meetings consensus is developed on the

relative perceived value of each identified resource

and the potential perceived consequences of

contamination. For example, a community might

determine that shellfish waters that are open to

public harvesting are less important than public

drinking water supply areas but more important

than secondary recreational waters that might be

used for body contact sports. This ranking is used

to create a table that shows the relative importance

of each resource (table 3-26 and case study).

The second part of this risk assessment process is

development of a vulnerability assessment matrix.

One potential measure of pollution vulnerability is

the ability of pollutants to move vertically from the

point of release to the water table or bedrock.

Figure 3-13. Input and output components of the MANAGE assessment method
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Chapter 3: Establishing Treatment System Performance Requirements

Table 3-25. Treatment performance requirements for New Shoreham, Rhode Island

Application of the MANAGE tool to establish perf lish perflish performance requirements

The town of New Shoreham, Rhode Island, is a popular vacation resort on a 6,400-acre island 10 miles off the southern coast of the state.

The permanent population is approximately 800, but during the summer the population swells to as many as 10,000 overnight visitors and

another 3,000 daily tourists. Proper wastewater management is a serious concern on the island. A publicly owned treatment works serves

the town’s harbor/commercial/business district, but 85 percent of the permanent residents and 54 percent of the summer population are

served by OWTSs, many of which ultimately discharge to the island’s sole source aquifer. Protection of this critical water resource is vital to

the island’s residents and tourism-based economy.

The University of Rhode Island (URI) Cooperative Extension Service’s MANAGE risk analysis model was used to identify potential sources

of ground water contamination (Kellogg et al., 1997). The model was also used to analyze potential ground water impacts at build-out

assuming current zoning. This projection was used to compare the relative change in pollution risk under future development scenarios

including the use of alternative technologies that provide better removal of nitrogen and pathogens. Onsite treatment systems were

estimated to contribute approximately 72 percent of the nitrogen entering ground water recharge areas. The model indicated that nitrogen

removal treatment technologies could effectively maintain nitrogen inputs at close to existing levels even with continued growth. It also

showed that nitrogen removal technologies were not necessary throughout the island but would be most beneficial in “hot spots” where the

risk of system failure and pollutant delivery to sensitive areas was the greatest.

The town adopted a wastewater management ordinance that mandated regular inspections of onsite systems by a town inspector (Town of

New Shoreham, 1996, 1998). It also established septic tank pumping schedules and other maintenance requirements based on inspection

results. Inspection schedules have the highest priority in public drinking water supply reservoirs, community wellhead protection zones, and

“hot spots” such as wetland buffers. Because the town expected to uncover failed and substandard systems, zoning standards were

developed for conventional and alternative OWTS technologies to ensure that new and reconstructed systems would be appropriate for

difficult sites and critical resource areas (Town of New Shoreham, 1998). A type of site vulnerability matrix was developed in cooperation

with URI Cooperative Extension using key site characteristics—depth to seasonally high water table, presence of restrictive layers, and

excessively permeable soils (Loomis et al., 1999). The matrix was used to create a vulnerability rating that is used to establish the level of

treatment needed to protect water quality in that watershed or critical resource area.

Three treatment levels were established: T1, primary treatment with watertight septic tanks and effluent screens; T2N, nitrogen removal

required to meet < 19 mg/L; and T2C, fecal coliform removal < 1,000 MPN/100 mL (table 3-25). The town provides a list of specific stateapproved treatment technologies considered capable of meeting these standards. By the year 2005, cesspools and failing systems must be

upgraded to specified standards. In addition, all septic tanks must be retrofitted with tank access risers and effluent screens.

Source: Loomis et al., 1999.
Chapter 3: Establishing Treatment System Performance Requirements

3-46 USEPA Onsite Wastewater Treatment Systems Manual Table 3-26. Resource listing, value ranking, and wastewater management schematic
USEPA Onsite Wastewater Treatment Systems Manual 3-47

Chapter 3: Establishing Treatment System Performance Requirements

Important criteria considered include the thickness

of the unsaturated soil layer and the properties of

the soil. The vulnerability assessment matrix

(table 3-26) identifies areas of low, moderate, high,

or extreme vulnerability depending on soil conditions.

For example, vulnerability might be “extreme” for

coarse or sandy soils with less than 2 feet of

vertical separation between the ground surface and

the water table or bedrock. Vulnerability might be

“low” for clay-loam soils with a vertical separation

of greater than 6 feet and low permeability. Each

resource specified in the first part of the risk

assessment process can be associated with each

vulnerability category. A more detailed discussion

of ground water vulnerability assessment is provided

in Groundwater Vulnerability Assessment: Predicting

Relative Contamination Potential under Conditions

of Uncertainty (National Research Council, 1993).

The third and final part of the risk assessment

process is developing a management matrix that

specifies a control measure for each vulnerability

category relative to each resource (tables 3-27,

3-28). Several categories of management control

measures (e.g., stricter performance requirements

for OWTSs) might be referenced depending on the

value and vulnerability of the resource. Generally,

each management control measure would define

• Management entity requirements for each

control measure

• System performance and resource impact

monitoring requirements for each vulnerable

category

• Types of acceptable control measures based on

the vulnerability and value of the resource

• Siting flexibility allowed for each control

measure

• Performance monitoring requirements for each

control measure and vulnerability category

Probability of impact approach

Otis (1999) has proposed a simplified “probability

of environmental impact” approach. This method

was developed for use when resource data are

insufficient and mapping data are unavailable for

a more rigorous assessment. The approach is

presented in the form of a decision tree that

considers mass loadings to the receiving environment (ground water or surface water), population

density, and the fate and transport of potential

pollutants to a point of use (see following case

study and figure 3-14). The decision tree (figure

3-14) estimates the relative probability of water

resource impacts from wastewater discharges

generated by sources in the watershed. Depending

on the existing or expected use of the water

resource, discharge standards for the treatment

systems can be established. The system designer

can use these discharge standards to assemble an

appropriate treatment train.

Resource value ranking and wastewater management Resource value ranking and wastewater management

A northern U.S. unsewered coastal community was concerned about the impacts onsite treatment systems might

have on its ground water resources (Hoover et al., 1998). Public water in the community is derived exclusively

from ground water. The extended recharge zone for the community well fields is also a water supply source in the

community. Other resources in the community include regionally important sand and gravel glacial outwash

aquifers, public beaches, shellfish habitat in shallow surface waters, nutrient-sensitive surface waters, low-yield

glacial till aquifers, and other surface waters used as secondary recreational waters.

Through public meetings, the community identified and ranked the various water resources according to their

perceived value. After ranking, the vulnerability of each resource to pollution from onsite treatment systems was

estimated. The vulnerability ratings were based on the thickness of the unsaturated zone in the soil, the rate of

water movement through the soil, and the capability of the soil to attenuate pollutants (table 3-25). For each

rating, a control zone designation was assigned (R5, R4, R3, R2, or R1). The criteria used for the vulnerability

ratings were documented in the community’s wastewater management plan. Control measures were established

for each control zone. In this instance, specific wastewater treatment trains were prescribed for use in each

control zone based on the depth of the unsaturated soil zone (tables 3-26 and 3-27). The treatment standards are

TS1 = primary treatment, TS2 = secondary treatment, TS3 = tertiary treatment, TS4 = nutrient reduction, and TS5

= tertiary treatment with disinfection.
Chapter 3: Establishing Treatment System Performance Requirements

3-48 USEPA Onsite Wastewater Treatment Systems Manual

Assimilative capacity is a volume-based (parts of

pollutant per volume of water) measurement of the

ability of water to decrease pollutant impacts

through dilution. Threshold effects levels are

usually established by state, federal, or tribal water

quality standards, which assign maximum concentrations of various pollutants linked to designated

uses of the receiving waters (e.g., aquatic habitat,

drinking water source, recreational waters). Because wastewater pollutants of concern (e.g.,

nitrogen compounds, pathogens, phosphorus) can

come from a variety of non-OWTS sources,

characterization of all pollutant sources and potential pathways to receiving waters provides important information to managers seeking to control or

reduce elevated levels of contaminants in those

Table 3-27. Proposed onsite system treatment performance standards in various control zones

Assessment and modeling through

quantitative analysis

Numeric performance requirements for onsite

wastewater treatment systems can be derived by

quantifying the total pollutant assimilative capacity

of the receiving waters, estimating mass pollutant

loads from non-OWTS sources, and distributing

the remaining assimilative capacity among onsite

systems discharging to the receiving waters.

Consideration of future growth, land use and

management practices, and a margin of safety

should be included in the calculations to ensure that

estimation errors favor protection of human health

and the environment.

Table 3-28. Treatment performance standards in various control zones
USEPA Onsite Wastewater Treatment Systems Manual 3-49

Chapter 3: Establishing Treatment System Performance Requirements

waters. For example, the mass balance equation

used to predict nitrate-nitrogen (or other soluble

pollutant) concentrations in ground water and

surface waters is

As the examples above indicate, there are a wide

range of approaches for assessing water resource

vulnerability and susceptibility to impacts from

onsite wastewater treatment systems. Other methodologies include risk matrices similar to those

summarized above and complex contaminant

transport models, including Qual2E, SWMM, and

BASINS, the EPA-developed methodology for

integrating point and nonpoint source pollution

assessments (see http://www.epa.gov/ow/compendium/toc.htm for more information on BASINS

and other water quality modeling programs).

Establishing performance requirements by assessing the probability of impact Establishing performance requirements by assessing the probability of impact

The “probability of impact” method estimates the probability that treated water discharged from an onsite system

will reach an existing or future point of use in an identified water resource. By considering the relative probability

of impact based on existing water quality standards (e.g., drinking water, shellfish water, recreational water),

acceptable treatment performance standards can be established. The pollutants and their concentrations or mass

limits to be stipulated in the performance requirements will vary with the relative probability of impact estimated,

the potential use of the water resource, and the fate and transport characteristics of the pollutant.

As an example, the assessment indicates that a ground water supply well that provides water for drinking without

treatment might be adversely affected by an onsite system discharge. Soils are assumed to be of acceptable

texture and structure, with a soil depth of 3 feet. Nitrate-nitrogen and fecal coliforms are two wastewater pollutants

that should be addressed by the performance requirements for the treatment system (i.e., constructed

components plus soil). With a relative probability of impact estimated to be “high,” the regulatory authority

considers it reasonable to require the treatment system to achieve drinking water standards for nitrate and fecal

coliforms before discharge to the saturated zone. The drinking water standards for nitrate and fecal coliforms in

drinking water are 10 mg/L for nitrate and zero for fecal coliforms. Considering the fate of nitrogen in the soil, it

can be expected that any of the nitrogen discharged by the pretreatment system will be converted to nitrate in the

unsaturated zone of the soil except for 2 to 3 mg/L of refractory organic nitrogen. Because nitrate is very soluble

and conditions for biological denitrification in the soil cannot be relied on, the performance standard for the onsite

system is 12 mg/L of total nitrogen (10 mg/L of nitrite + 2 mg/L of refractory organic nitrogen) prior to soil

discharge. In the case of fecal coliforms, the natural soil is very effective in removing fecal indicators where

greater than 2 feet of unsaturated natural soil is present. Therefore, no fecal coliform standard is placed on the

pretreatment (i.e., constructed) system discharge because the standard will be met after soil treatment and before

final discharge to the saturated zone.

If the probability of impact is estimated to be “moderate” or “low,” only the nitrogen treatment standard would

change. If the probability of impact is “moderate” because travel time to the point of use is long, dispersion and

dilution of the nitrate in the ground water is expected to reduce the concentration in the discharge substantially.

Therefore, the treatment standard for total nitrogen can be safely raised, perhaps to 20 to 30 mg/L of nitrogen. If

the probability of impact is “low,” no treatment standard for nitrogen is necessary.

If the probability of impact is “high” but the point of ground water use at risk is an agricultural irrigation well, no

specific pollutants in residential wastewater are of concern. Therefore, the treatment required need be no more

than that provided by a septic tank.

Source: Otis, 1999.

Annual nitrogen loading from

all sources in

lb/yr x 454,000 mg/lb

Nitrate-nitrogen (mg/L) =

Annual water recharge volume

from all

sources in liters
Chapter 3: Establishing Treatment System Performance Requirements

3-50 USEPA Onsite Wastewater Treatment Systems Manual Figure 3-14. Probability of environmental impact decision tree (see key) Source: Otis, 1999.
USEPA Onsite Wastewater Treatment Systems Manual 3-51

Chapter 3: Establishing Treatment System Performance Requirements

Environmental sensitivity assessment key (for figure 3-14).
Chapter 3: Establishing Treatment System Performance Requirements

3-52 USEPA Onsite Wastewater Treatment Systems Manual

Estimating nitrogen loadings and impacts for Buttermilk Bay, Massachusetts

In Buttermilk Bay, a 530-acre shallow coastal bay at the northern end of Buzzards Bay in Massachusetts, elevated nitrogen levels

associated with onsite systems and land use in the watershed have contributed to nuisance algal growth and declines in eelgrass beds in

some areas. An investigation in the early 1990s supported by the New England Interstate Water Pollution Control Commission and

USEPA established a critical (maximum allowable) nitrogen loading rate of 115,600 pounds per year by identifying an appropriate

ecological effects threshold (the nitrogen concentration associated with significant ecological impacts, or 0.24 mg/L in nitrogen-sensitive

Buttermilk Bay) and considering both the size and recharge rate of the bay:

Critical Loading Rate (pounds per year) =

Threshold nitrogen concentration x volume x number of annual water body recharges =

240 milligrams of N per cubic meter x 2,996,000 cubic meters x 73 annual recharges =

52,489,920,000 milligrams of N / 454,000 milligrams in one pound =

115,617 pounds per year = critical loading rate for nitrogen

After establishing the critical nitrogen loading rate, the watershed assessment team sought to quantify annual nitrogen loads discharged

to the bay under existing conditions. Loading values for various sources of nitrogen in the watershed were estimated and are presented

in table 3-29. For the purposes of estimating nitrogen contributions from onsite systems, it was assumed that the total nitrogen

concentration in onsite treated effluent was 40 mg/L and the per capita flow was 55 gallons per day. [It should be noted that nitrogen

concentrations in onsite system treated effluent commonly range between 25 and 45 mg/L for soil-based systems, though some

researcher have found higher effluent concentrations. In general, SWIS nitrogen removal rates range between 10 and 20 percent (Van

Cuyk et al., 2001) for soil-based systems. Mechanized systems designed for nitrogen removal can achieve final effluent N concentrations

as low as 10-25 mg/L.]

Using the research-based assumptions and estimates summarized in the table, the assessment team estimated that total current

nitrogen loadings totaled about 91,053 lb/yr. Onsite wastewater treatment systems represented a significant source (74 percent) of the

overall nitrogen input, followed by lawn fertilizers (15 percent) and cranberry bogs (7 percent).

The final part of the Buttermilk Bay analysis involved projecting the impact of residential build-out on nitrogen loads to the bay. With a

critical (maximum allowable) nitrogen loading rate of 115,617 lb/yr and an existing loading rate of 91,053 lb/yr, planners had only a

24,564 lb/yr cushion with which to work. Full residential build-out projections generated nitrogen loading rates that ranged from 96,800 lb/

yr to 157,500 lb/yr. Regional planners used this information to consider approaches for limiting nitrogen loadings to a level that could be

safely assimilated by the bay. Among a variety of options that could be considered under this scenario are increasing performance

requirements for onsite systems, decreasing system densities, limiting the total number of new residences with onsite systems in the bay

watershed, and reducing nitrogen inputs from other sources (e.g., lawn fertilizers, cranberry bogs).

Table 3-29. Nitrogen loading values used in the Buttermilk Bay assessment
USEPA Onsite Wastewater Treatment Systems Manual 3-53

Chapter 3: Establishing Treatment System Performance Requirements

3.8.2 Establishing narrative or

numerical performance

requirements

Performance requirements should reflect acceptable

environmental impacts and public health risks based

on assessment methods such as those described in

the preceding section. They should specify observable or measurable requirements in narrative or

numerical form. Conventional onsite treatment

systems (septic tanks with SWISs) have used

narrative requirements such as prohibitions on

wastewater backup in plumbing fixtures or effluent

pooling on the ground surface. These requirements

are measurable through observation but address

only some specific public health issues. An example

of a narrative performance requirement that

addresses potential environmental impacts is the

Town of Shoreham’s requirement for specifically

approved treatment trains for environmentally

sensitive areas (see sidebar and table 3-26 in

preceding section). Compliance is determined by

whether the required treatment processes are in

place; water quality monitoring is not involved.

The regulating agencies assume that the water

quality objectives are achieved if these narrative

performance requirements are met. Although there

is merit in this approach, some additional steps

(e.g., operation and maintenance monitoring,

targeted water quality monitoring) would be

included in a more comprehensive program.

Numerical performance requirements specify the

critical parameters of concern (e.g., nitrate,

phosphorus, fecal coliforms), the maximum

allowable concentration or mass pollutant/flow

discharge permitted per day, and the point at which

the requirements apply. Examples of numerical

performance requirements include Massachusetts’

requirement for limited volume discharges (measured in gallons per day) in designated nitrogensensitive areas or a water quality standard for

nitrogen of 25 mg/L, to be met at the property

boundary. Unlike the narrative requirements,

numerical performance requirements provide more

assurance that the public health and water quality

goals are being met.

3.9 Monitoring system operation

and performance

Performance monitoring of onsite treatment

systems serves several purposes. Its primary

purpose is to ensure that treatment systems are

operated and maintained in compliance with the

performance requirements. It also provides performance data useful in making corrective action

decisions and evaluating areawide environmental

impacts for land use and wastewater planning.

Historically, performance monitoring of onsite

treatment systems has not been required. Regulatory agencies typically limit their regulatory

Onsite system inspection/maintenance guidance for Rhode Island

The Rhode Island Department of Environmental Management published in 2000 the Septic System Checkup, an

inclusive guide to inspecting and maintaining septic systems. The handbook, available to the public, is written for

both lay people and professionals in the field. The guide is an easy-to-understand, detailed protocol for inspection

and maintenance and includes newly developed state standards for septic system inspection and maintenance. It

describes two types of inspections: a maintenance inspection to determine the need for pumping and minor

repairs, and a functional inspection for use during property transfers. The handbook also includes detailed

instructions for locating septic system components, diagnosing in-home plumbing problems, flow testing and dye

tracing, and scheduling inspections. Several Rhode Island communities, including New Shoreham, North

Kingstown and Glocester, currently use Septic System Checkup as their inspection standard. The University of

Rhode Island offers a training course for professionals interested in becoming certified in the inspection

procedures.

The handbook is available free on-line at http://www.state.ri.us/dem/regs/water/isdsbook.pdf ater/isdsbook.pdf. ater/isdsbook.pdf Individual spiralbound copies can be purchased for $10 with inspection report forms or $7 for the manual without forms from

DEM’s Office of Technical and Customer Assistance at 401.222.6822.

Source: Rhode Island Department of Environmental Management.
Chapter 3: Establishing Treatment System Performance Requirements

3-54 USEPA Onsite Wastewater Treatment Systems Manual

control primarily to system siting, design, and

construction and certification of site evaluators,

designers, and other service providers. System

performance is largely ignored by the regulatory

authority or management entity or addressed

through sometimes weak owner education and

voluntary compliance programs until a hydraulic

failure is reported or observed (see chapters 2 and 5).

OWTS oversight agencies typically exert regulatory control by conducting the site evaluation and

reviewing the proposed design for compliance with

administrative code prescriptions for proven

systems. If the site characteristics and selected

system design meet the prescriptions in the code, a

construction permit is issued for installation by a

certified contractor. The regulatory authority or

management entity usually performs a pre-coverup

inspection before final approval is given to use the

system. At that point the regulatory authority

typically relinquishes any further oversight of the

system until a hydraulic failure is observed or

reported. The owner may be given educational

materials and instructions describing the system and

what maintenance should be performed, but routine

operation and maintenance is left up to the owner.

Tank pumping or other routine maintenance tasks

are seldom required or even tracked by the regulatory authority or management entity for information purposes. Regular inspections of systems are

usually not mandated.

This regulatory approach might be adequate for the

degree of risk to human health and the environment

posed by isolated and occasional hydraulic failures.

Where onsite treatment is used in moderate-tohigh-density suburban and seasonal developments,

however, it has not proven to be adequate, particularly where treatment failures can be expected to

significantly affect ground water and surface water

quality. Onsite system failure rates across the nation

range as high as 10 percent or more in some areas

(see Section 1.3). In cases where high system

densities or system age indicates the likelihood of

ground or surface water contamination, incorporation of mandated performance monitoring into

OWTS management programs is strongly recommended. In 2000 USEPA issued suggested guidelines for onsite system management programs.

Draft Guidelines for Management of Onsite/

Decentralized Wastewater Systems (USEPA, 2000b)

provides an excellent framework for developing a

comprehensive management program that considers

the full range of issues involved in OWTS planning, siting, design, installation, operation, maintenance, monitoring, and remediation (see chapter 2).

Local OWTS regulatory and management agencies

in many areas are embracing more rigorous operation, maintenance, and inspection programs to deal

with problems caused by aging systems serving

developments built before 1970, poor maintenance

due to homeowner indifference or ignorance, and

regional hydraulic or pollutant overloads related to

high-density OWTS installations. Operation and

maintenance management programs adopted by

these agencies consist mostly of an integrated

performance assurance system that inventories new

and existing systems, establishes monitoring or

inspection approaches, requires action when

systems fail to operate properly, and tracks all

activities to ensure accountability among regulatory

program staff and system owners. (See chapter 2

and Draft Guidelines for Management of Onsite/

Decentralized Wastewater Systems at http://

www.epa.gov/owm/decent/index.htm for more

information and examples.)

3.9.1 Operating permits

Periodic review of system performance is necessary to

ensure that systems remain in compliance with

established performance requirements after they are

installed. Thus, regulatory agencies need to maintain

rigorous, perpetual oversight of systems to ensure

periodic tank pumping, maintenance of system

components, and prompt response to problems that

may present threats to human health or water resources. Some jurisdictions are fulfilling this responsibility by issuing renewable/revocable operating

permits. The permit stipulates conditions that the

system must meet before the permit can be renewed

(see sidebar). The duration of such permits might

vary. For example, shorter-term permits might be

issued for complex treatment systems that require

more operator attention or to technologies that are less

proven (or with which the regulatory authority has

less comfort). The owner is responsible for documenting and certifying that permit conditions have

been met. If permit conditions have not been met, a

temporary permit containing a compliance schedule

for taking appropriate actions may be issued. Failure

to meet the compliance schedule can result in fines or

penalties.
USEPA Onsite Wastewater Treatment Systems Manual 3-55

Chapter 3: Establishing Treatment System Performance Requirements

3.9.2 Monitoring programs

Monitoring individual or regional onsite system

performance may include performance inspections

(see Chapter 2 and Draft Management Guidelines

for Onsite/Decentralized Wastewater Systems),

water quality sampling at performance boundaries,

drinking water well monitoring, and assessment of

problem pollutant concentrations (pathogens,

nitrate, phosphorus) in nearby surface waters. In

general, monitoring of system performance seeks to

ascertain if onsite systems are meeting performance

requirements, i.e., protecting public health and

water quality. Assessing the sensitivity of water

resources to potential pollutant loadings from

onsite systems helps in developing performance

requirements and the monitoring methods and

sampling locations that might be used.

Monitoring system performance through water

quality sampling is difficult for conventional onsite

systems because the infiltration field and underlying soil are part of the treatment system. The

percolate that enters the ground water from the

infiltration system does not readily mix and

disperse in the ground water. It can remain as a

distinct, narrow plume for extended distances from

the system (Robertson et al., 1991). Locating this

plume for water quality sampling is extremely

difficult, and the cost involved probably does not

warrant this type of monitoring except for large

systems that serve many households or commercial

systems constructed over or near sensitive ground

water and surface water resources (see chapter 5).

Monitoring of onsite treatment systems is enhanced

considerably by the inclusion of inspection and

sampling ports at performance boundaries (e.g.,

between treatment unit components) and the final

discharge point. Other methods of monitoring such

as simple inspections of treatment system operation

or documentation of required system maintenance

Onsite system oper Onsite system operating per ating perating permits in St. mits in St.mits in St. Louis County, Minnesota

St. Louis County, located in the northeastern region of Minnesota, extends from the southwestern tip of Lake

Superior north to the Canadian border. The physical characteristics of the region are poorly suited for application

of traditional onsite treatment systems. Many of the soils are very slowly permeable lacustrine clays, shallow to

bedrock, and often near saturation. The existing state minimum code restricts onsite systems to sites featuring

permeable soils with sufficient unsaturated depths to maintain a 3-foot separation distance to the saturated zone.

To allow the use of onsite treatment, the county has adopted performance requirements that may be followed in

lieu of the prescriptive requirements where less than 3 feet of unsaturated, permeable soils are present. In such

cases the county requires that the owner continuously demonstrate and certify that the system is meeting the

performance requirements. This is achieved through the issuance of renewable operating permits for higherperformance alternative treatment systems. The operating permit is based on evaluation of system performance

rather than design prescription and includes the following:

✔ System description

✔ Environmental description

✔ Site evaluation documentation

✔ Performance requirements

✔ System design, construction plan, specifications, and construction drawings

✔ Maintenance requirements

✔ Monitoring requirements (frequency, protocol, and reporting)

✔ Contingency plan to be implemented if the system fails to perform to requirements

✔ Enforcement and penalty provisions

The permit is issued for a limited term, typically 5 years. Renewal requires that the owner document that the

permit requirements have been met. If the documentation is not provided, a temporary permit is issued with a

compliance schedule. If the compliance schedule is not met, the county has the option of reissuing the temporary

permit and/or assessing penalties. The permit program is self-supporting through permit fees.
Chapter 3: Establishing Treatment System Performance Requirements

3-56 USEPA Onsite Wastewater Treatment Systems Manual

might be sufficient and more cost-effective than

water quality sampling at a performance boundary.

The Critical Point Monitoring (CPM) approach

being developed in Washington State provides a

systematic approach to choosing critical locations

to monitor specific water quality parameters

(Eliasson et al., 2001). The program is most

suitable for responsible management entities

operating comprehensive management programs.

CPM provides an appropriate framework for

monitoring treatment train components, though it

should be recognized that evaluations of overall

system effectiveness—and compliance with

Monitor Monitoring requirements in ing requirements in Washington ashingtonashington

The Department of Health of the state of Washington has adopted a number of monitoring requirements that

OWTS owners must meet (Washington Department of Health, 1994). Because such requirements place additional

oversight responsibilities on management agencies, additional resources are needed to ensure compliance.

Among the requirements are the following:

The system owner is responsible for properly operating and maintaining the system and must

• Determine the level of solids and scum in the septic tank once every 3 years.

• Employ an approved pumping service provider to remove the septage from the tank when the level of solids

and scum indicates that removal is necessary.

• Protect the system area and the reserve area from cover by structures or impervious material, surface

drainage, soil compaction (for example, by vehicular traffic or livestock), and damage by soil removal and

grade alteration.

• Keep the flow of sewage to the system at or below the approved design both in quantity and waste strength.

• Operate and maintain alternative systems as directed by the local health officer.

• Direct drains, such as footing or roof drains away from the area where the system is located.

Local health officers in Washington also perform monitoring duties, including the following;

• Providing operation and maintenance information to the system owner upon approval of any installation, repair,

or alteration of a system.

• Developing and implementing plans to monitor all system performance within areas of special concern1;

initiating periodic monitoring of each system by no later than January 1, 2000, to ensure that each system

owner properly maintains and operates the system in accordance with applicable operation and maintenance

requirements; disseminating relevant operation and maintenance information to system owners through

effective means routinely and upon request; and assisting in distributing educational materials to system

owners.

Finally, local health officers may require the owner of the system to perform specified monitoring, operation, or

maintenance tasks, including the following:

• Using one or more of the following management methods or another method consistent with the following

management methods for proper operation and maintenance: obtain and comply with the conditions of a

renewable or operational permit; employ a public entity eligible under Washington state statutes to directly or

indirectly manage the onsite system; or employ a private management entity, guaranteed by a public entity

eligible under Washington state statutes or sufficient financial resources, to manage the onsite system.

• Evaluating any effects the onsite system might have on ground water or surface water.

• Dedicating easements for inspections, maintenance, and potential future expansion of the onsite system.

1 “Areas of special concern” are areas where the health officer or department determines additional requirements

might be necessary to reduce system failures or minimize potential impacts upon public health. Examples include

shellfish habitat, sole source aquifers, public water supply protection areas, watersheds of recreational waters,

wetlands used in food production, and areas that are frequently flooded.

Source: Washington Department of Health, 1994.
USEPA Onsite Wastewater Treatment Systems Manual 3-57

Chapter 3: Establishing Treatment System Performance Requirements

performance requirements—should be based on

monitoring at the performance boundaries (see

chapter 5).

Elements of a monitoring program

Any monitoring program should be developed

carefully to ensure that its components consider

public health and water quality objectives, regulatory authority / management entity administrative

and operational capacity, and the local political,

social, and economic climate. Critical elements for

a monitoring program include

• Clear definition of the parameters to be monitored and measurable standards against which

the monitoring results will be compared.

• Strict protocols that identify when, where, and

how monitoring will be done, how results will

be analyzed, the format in which the results will

be presented, and how data will be stored.

• Quality assurance and quality control measures

that should be followed to ensure credible data.

System inspections

Mandatory inspections are an effective method for

identifying system failures or systems in need of

corrective actions. Inspections may be required at

regular intervals, at times of property transfer or

changes in use of the property, or as a condition to

obtain a building permit for remodeling or expansion. Twenty-three states now require some form of

inspection for existing OWTSs (NSFC, 1999). The

OWTS regulatory authority or management entity

State of Massachusetts’ onsite treatment system inspection program

Massachusetts in 1996 mandated inspections of OWTSs to identify and address problems posed by failing

systems (310 CMR 15.300, 1996). The intent of the program is to ensure the proper operation and maintenance of

all systems. A significant part of the program is the annual production of educational materials for distribution to

the public describing the importance of proper maintenance and operation of onsite systems and the impact

systems can have on public health and the environment.

Inspections are required at the time of property transfer, a change in use of the building, or an increase in discharges to the system.

Systems with design flows equal to or greater than 10,000 gpd require annual inspections. Inspections are to be performed only by persons

approved by the state. The inspection criteria are established by code and must include

✔ A general description of system components, their physical layout, and horizontal setback distances from

property lines, buildings, wells, and surface waters.

✔ Description of the type of wastewater processed by the system (domestic, commercial, or industrial).

✔ System design flow and daily water use, if metered.

✔ Description of the septic tank, including age, size, internal and external condition, water level, etc.

✔ Description of distribution box, dosing siphon, or distribution pump, including evidence of solids carryover,

clear water infiltration, and equal flow division, and evidence of backup, if any.

✔ Description of the infiltration system, including signs of hydraulic failure, condition of surface vegetation,

level of ponding above the infiltration surface, other sources of hydraulic loading, depth to seasonally high

water table, etc.

A system is deemed to be failing to protect public health, safety, and the environment if the septic tank is made

of steel, if the OWTS is found to be backing up, if it is discharging directly or indirectly onto the surface of the

ground, if the infiltration system elevation is below the high ground water level elevation, or if the system

components encroach on established horizontal setback distances.

The owner must make the appropriate upgrades to the system within 2 years of discovery. The owner’s failure to

have the system inspected as required or to make the necessary repairs constitutes a violation of the code.

Source: Title V, Massachusetts Environmental Code.
Chapter 3: Establishing Treatment System Performance Requirements

3-58 USEPA Onsite Wastewater Treatment Systems Manual

should collect information on new systems (system

owner, contact information, system type, location,

design life and capacity, recommended service

schedule) at the time of permitting and installation.

Inventories of existing systems can be developed by

consulting wastewater treatment plant service area

maps, identifying areas not served by publicly

owned treatment works (POTWs), and working

with public and private utilities (drinking water,

electricity, and solid waste service providers) to

develop a database of residents and contact information. Telephone, door-to-door, or mail surveys

can be used to gather information on system type,

tank capacity, installation date, last date of service

(e.g., pumping, repair), problem incidents, and

other relevant information.

Minnesota, Massachusetts, Wisconsin, and a

number of counties and other jurisdictions require

disclosure of system condition or assurances that

they are functioning properly at the time of property transfer (see sidebar). Assurances are often in

the form of inspection certificates issued by county

health departments, which have regulatory jurisdiction over OWTSs. Clermont County, Ohio, developed an OWTS owner database by cross-referencing water line and sewer service customers. Contact

information from the database was used for a mass

mailing of information on system operation and

maintenance and the county’s new inspection

program to 70 percent of the target audience. Other

approaches used in the Clermont County outreach

program included advisory groups, homeowner

education meetings, news media releases and

interview programs, meetings with real estate

agents, presentations at farm bureau meetings,

displays at public events, and targeted publications

(Caudill, 1998).

Effluent quality requirements in Minnesota

St. Louis County, Minnesota, has established effluent standards for onsite systems installed on sites that do not

have soils meeting the state’s minimum requirements. Many of the soils in the county do not meet the minimum

3-foot unsaturated soil depth required by the state code. To allow for development the county has adopted a

performance code that establishes effluent requirements for systems installed where the minimums cannot be

met. Where the natural soil has an unsaturated depth of less than 3 feet but more than 1 foot, the effluent

discharged to the soil must have no more than 10,000 fecal coliform colonies per 100 mL. On sites with 1 foot of

unsaturated soil or less, the effluent must have no more than 200 fecal coliform colonies per 100 mL. These

effluent limits are monitored prior to final discharge at the infiltrative surface but recognize treatment provided by

the soil. If hydraulic failure occurs, the county considers the potential risk within acceptable limits. The

expectation is that any discharges to the surface will meet at least the primary contact water quality requirements

of 200 fecal coliform colonies per 100 mL. Other requirements, such as nutrient limitations, may be established

for systems installed in environmentally sensitive areas.

Documenting wastewater mig ater migater migration to streams in Nor ation to streams in Northern Virginia VirginiaVirginia

The Northern Virginia Planning District Commission uses commercially available ultraviolet light bulbs and cotton

swatches to screen for possible migration of residential wastewater into area streams. The methodology is based

on the presence of optical brighteners in laundry detergents, which are invisible to the naked eye but glow under

“black” lights. The brighteners are very stable in the environment and are added to most laundry soaps. They are

readily absorbed onto cotton balls or cloth swatches, which can be left in the field for up to two weeks. Users

must ensure that the absorbent medium is free from optical brighteners prior to use.

Although the methodology is acceptable for screening-level analysis, it does not detect wastewater inputs from

buildings that do not have laundry facilities and does not verify the presence of other potential contaminants (e.g.,

bacteria, nitrogen compounds). Despite these shortcomings, the approach is inexpensive, effective, and a good

tool for screening and public education.

Source: Northern Virginia Regional Commission, 1999.
USEPA Onsite Wastewater Treatment Systems Manual 3-59

Chapter 3: Establishing Treatment System Performance Requirements

The Town of Shoreham, Rhode Island, adopted a

similar inspection program by ordinance in 1996

(Loomis et al., 1999). The ordinance mandates

regular inspection of all systems by a town inspector. Septage pumping schedules and other maintenance requirements are based on the results of the

inspection. Factors considered in the inspections

include site characteristics, system technology and

design, system use, and condition. The ordinance

allows the town to prioritize inspection schedules in

critical resource areas such as public wellheads and

high-risk areas determined to be prone to onsite

system failure. It also authorizes the town to assess

fees, levy fines, and track the inspections.

Prescribed maintenance

Where specific unit processes or treatment trains

have satisfactorily demonstrated reliable performance through a credible testing program, some

programs assume that identical processes or treatment trains will perform similarly if installed under

similar site-specific conditions. The system would

need to be managed according to requirements of

the designer/manufacturer as outlined in the

operation and maintenance manual to maximize the

potential for assured performance. Therefore, some

states monitor system maintenance as an alternative

to water quality-based performance monitoring.

The method of monitoring varies. In several states

the owner must contract with the equipment

manufacturer or certified operator to provide

operation and maintenance services. If the owner

severs the contract, the contractor is obligated to

notify the state regulatory authority or other

management entity. Failure to maintain a contract

with an operator is a violation of the law. Other

states require that the owner provide certified

documentation that required maintenance has been

performed in accordance with the system management plan. Requiring the owner to provide periodic

documentation helps to reinforce the notion that the

owner is responsible for the performance of the

system. Chapter 2 provides additional information

on prescriptive and other approaches to monitoring,

operation, and maintenance.

Water quality sampling and bacterial

source tracking

OWTS effluent quality sampling is a rigorous and

expensive method of onsite system compliance

monitoring. Such programs require that certain

water quality criteria be met at designated locations

after each treatment unit (see chapter 5). Sampling

pretreated effluent before discharge to the soil

requires an assumption of the degree of treatment

that will occur in the soil. Therefore, the performance requirements used to determine compliance

should be adjusted to credit soil treatment. Unfortunately, some incomplete or inaccurate data equate

travel time in all types of soil to pollutant removals

under various conditions. Even when better data

are available, it is often difficult to match condiBiochemical application of a bacterial source tracking methodology

Researchers from Virginia Tech analyzed antibiotic resistance in fecal streptococci to determine the sources of

bacteria found in streams in rural Virginia. The team first developed a database of antibiotic resistance patterns for

7,058 fecal streptococcus isolates from known human, livestock, and wildlife sources in Montgomery County,

Virginia. Correct fecal streptococcus source identification averaged 87 percent for the entire database and ranged

from 84 percent for deer isolates to 93 percent for human isolates. A field test of the database yielded an overall

bacteria source accuracy rate of 88 percent, with an accuracy rate of at least 95 percent for differentiation

between human and animal sources.

The approach was applied to a watershed improvement project on Page Brook in Clarke County, Virginia, to

determine the impacts of a cattle exclusion fencing and alternative stock watering project. Pre-project bacterial

analyses showed heavy bacteria contamination from cattle sources (more than 78 percent), with smaller

proportions from waterfowl, deer, and unidentified sources (about 7 percent each). After the fencing and alternative

stock watering stations were installed, fecal coliform levels from all sources declined by an average of 94 percent,

from 15,900/100 mL to 960/100 mL. Analysis of bacteria conducted after the project also found that cattle-linked

isolates decreased to less than 45 percent of the total.

Source: Hagedorn et al., 1999.
Chapter 3: Establishing Treatment System Performance Requirements

3-60 USEPA Onsite Wastewater Treatment Systems Manual

tions at the site from which the data were derived

to the soils, geology, water resources, slopes,

topography, climate, and other conditions present at

the site under consideration. Effluent monitoring

should be undertaken only when the potential risk

to human health and the environment from system

failure is great enough to warrant the cost of

sampling and analysis or when assessment information is needed to establish performance requirements or identify technologies capable of protecting valued water resources.

Ground water sampling is the most direct method

of compliance monitoring. However, because of the

difficulty of locating monitoring wells in the

effluent plume it has historically been used only for

compliance monitoring of large infiltration systems. If performance standards are to be used in the

future, ground water monitoring will become more

commonplace despite its cost because it is the only

true determinant of compliance with risk assessment criteria and values. Installing small-diameter

drop tubes at various depths at strategic

downgradient locations can provide a cost-effective

approach for continuous sampling.

Monitoring of the unsaturated zone has been

conducted as an alternative to ground water monitoring. This method avoids the problem of locating

narrow contaminant plumes downgradient of the

infiltration system, but allowances should be made

in parameter limits to account for dispersion and

treatment that could occur in the saturated zone. To

obtain samples, suction lysimeters are used. Porous

cups are installed in the soil at the desired sample

depth, and a vacuum is applied to extract the

sample. This type of sampling works reasonably

well for some dissolved inorganic chemical species

but is not suitable for fecal indicators (Parizek and

Lane, 1970; Peters and Healy, 1988). Use of this

method should be based on a careful evaluation of

whether the method is appropriate for the parameters to be monitored because it is extremely

expensive and proper implementation requires

highly skilled personnel.

Water quality sampling of lakes, rivers, streams,

wetlands, and coastal embayments in areas served

by OWTSs can provide information on potential

resource impacts caused by onsite systems. Concentrations of nitrogen, phosphorus, total and fecal

coliforms, and fecal streptococci are often measured to determine possible impacts from system

effluent. Unless comprehensive source sampling

that characterizes OWTS pollutant contributions is

in place, however, it is usually difficult to attribute

elevated measurements of these parameters directly

to individual or clustered OWTSs. Despite this

difficulty, high pollutant concentrations often

generate public interest and provide the impetus

necessary for remedial actions (e.g., tank pumping;

voluntary water use reduction; comprehensive

system inspections; system repairs, upgrades,

replacements) that might be of significant benefit.

Tracer dye tests of individual systems, infrared

photography, and thermal imaging are used in

many jurisdictions to confirm direct movement of

treated or partially treated wastewater into surface

waters. Infrared and thermal photography can show

areas of elevated temperature and increased chlorophyll concentrations from wastewater discharges.

Areas with warmer water during cold months or

high chlorophyll during warm months give cause

for further investigation (Rouge River National Wet

Weather Demonstration Project, 1998). The

Arkansas Health Department has experimented with

helicopter-mounted infrared imaging equipment to

detect illicit discharges and failed systems around

Lake Conway with some success (Eddy, 2000),

though these and other monitoring approaches

(e.g., using tracers such as surfactants, laundry

whiteners, and caffeine) are not typical and are still

undergoing technical review.

Recently, some success has been demonstrated by

advanced bacterial source tracking (BST) methodologies, which identify bacteria sources (humans,

cattle, dogs, cats, wildlife) through molecular or

biochemical analysis. Molecular (genotype) assessments match bacteria collected at selected sampling

points with bacteria from known mammalian

sources using ribotype profiles, intergenetic DNA

sequencing, ribosomal DNA genetic marker profile

analyses, and other approaches (Bernhard and

Field, 2000; Dombek et al., 2000; Parveen et al.,

1999). Biochemical (phenotype) assessments of

bacteria sources conduct similar comparisons

through analysis of antibiotic resistance in known

and unknown sources of fecal streptococci

(Hagedorn et al., 1999), coliphage serological

differentiation, nutritional pattern analysis, and

other methods. In general, molecular methods seem

to offer the most precise identification of specific
USEPA Onsite Wastewater Treatment Systems Manual 3-61

Chapter 3: Establishing Treatment System Performance Requirements

types of sources (animal species), but are costly,

time-consuming, and not yet suitable for largescale use. The precision of most biochemical

approaches appears to be somewhat less than

molecular methods, but analyte costs are lower,

processing times are shorter, and large numbers of

samples can be assayed in shorter time periods

(Virginia Tech, 2001). It has been suggested that

biochemical methods be used to screen large

numbers of bacterial isolates for likely sources

followed by an analysis of a subset of the isolates

through molecular approaches to validate the

findings. (For more information, see http://

www.bsi.vt.edu/biol_4684/BST/BST.html).

Finally, some OWTS management agencies use

fecal coliform/fecal streptococci (FC/FS) ratios as a

screening tool to detect the migration of poorly

treated effluent to inland surface waters. Under this

approach, which is effective only if samples are

taken near the source of contamination, the number

of fecal coliforms in a sample volume is divided by

the number of fecal streptococci in an equal sample

volume. If the quotient is below 0.7, the bacteria

sources are most likely animals. Quotients above

4.0 indicate a greater likelihood of human sources

of bacteria, while values between 0.7 and 4.0

indicate a mix of human and animal sources.

Several factors should be considered when using

the FC/FS screening approach:

• Bacterial concentrations can be highly variable

if the pH is outside the 4.0 to 9.0 range

• Faster die-off rates of fecal coliforms will alter

the ratio as time and distance from contaminant

sources increase

• Pollution from several sources can alter the ratio

and confuse the findings

• Ratios are of limited value in assessing bays,

estuaries, marine waters, and irrigation return

waters

Sampling and analysis costs vary widely across the

nation and are influenced by factors such as the

number of samples to be collected and assessed,

local business competition, and sample collection,

handling, and transport details. Because of variability in price and the capacity of local agencies to

handle sample collection, transport, and analysis,

several cost estimates should be solicited. Some

example analytical costs are provided in table 3-30.

Because of the cost and difficulty of monitoring,

underfunded management agencies have often

opted to focus their limited resources on ensuring

that existing systems are properly operated and

maintained and new systems are appropriately

planned, designed, installed, operated, and maintained. They have relied on limited water quality

monitoring of regional ground water and surface

waters to provide an indication of regional onsite

system performance. Additional site-specific

monitoring is recommended, however, where

drinking water or valued surface water resources

are threatened.

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Resources Research Institute report no. 083.

New Mexico State University, New Mexico

Water Resources Research Institute, Las

Cruces, NM.

Young, R.H.F., and N.C. Burbank, Jr. 1973. Virus

removal in Hawaiian soils. Journal of the

American Water Works Association 65:698-704.

Ziebell, W.A., D.H. Nero, J.F. Deininger, and E.

McCoy. 1975. Use of bacteria in assessing

waste treatment and soil disposal systems. In

Home Sewage Disposal, Proceedings of the

National Home Sewage Disposal Symposium,

December 19-20, 1974, Chicago, IL, pp. 58-

63. American Society of Agricultural

Engineers, St. Joseph, MI.
Chapter 3: Establishing Treatment System Performance Requirements

3-74 (Click Here to Return to Bookmarks Page) USEPA Onsite Wastewater Treatment Systems Manual
USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 4-1

Chapter 4: Treatment Processes and Systems

4.1 Introduction

This chapter contains information on individual

onsite/decentralized treatment technologies or unit

processes. Information on typical application,

design, construction, operation, maintenance, cost,

and pollutant removal effectiveness is provided for

most classes of treatment units and their related

processes. This information is intended to be used

in the preliminary selection of a system of treatment unit processes that can be assembled to

achieve predetermined pollutant discharge concentrations or other specific performance requirements. Complete design specifications for unit

processes and complete systems are not included in

the manual because of the number of processes and

process combinations and the wide variability in

their application and operation under various site

conditions. Designers and others who require more

detailed technical information are referred to such

sources.

Chapter 4 is presented in two main sections. The

first section contains information about conventional (soil-based or subsurface wastewater infiltration) systems, referred to as SWISs in this document. Both gravity-driven and mechanized SWISs

are covered in this section of chapter 4. The second

section contains a general introduction to sand

filters (including other media), and a series of fact

sheets on treatment technologies, alternative

systems (e.g., fixed-film and suspended growth

systems, evapotranspiration systems, and other

applications), and special issues pertaining to the

design, operation, and maintenance of onsite

wastewater treatment systems (OWTSs). This

Chapter 4

Treatment processes and systems

approach was used because the conventional system

is the most economical and practical system type

that can meet performance requirements in many

applications.

The first section is further organized to provide

information about the major components of a

conventional system. Given the emphasis in this

manual on the design boundary (performancebased) approach to system design, this section was

structured to lead the reader through a discussion of

system components by working backwards from

the point of discharge to the receiving environment

to the point of discharge from the home or other

facility served by the onsite system. Under this

approach, soil infiltration issues are discussed first,

the distribution piping to the infiltration system

including graveless sytems is addressed next, and

matters related to the most common preliminary

treatment device, the septic tank, are covered last.

The fact sheets in the second section of this chapter

describe treatment technologies and discuss special

issues that might affect system design, performance, operation, and maintenance. These treatment technologies are often preceded by a septic

tank and can include a subsurface wastewater

infiltration system. Some treatment technologies

may be substituted for part or all of the conventional system, though nearly all alternative approaches include a septic tank for each facility

being served. Fact sheets are provided for the more

widely used and successful treatment technologies,

such as sand filters and aerobic treatment units.

4.1 Introduction

4.2 Conventional systems and treatment options

4.3 Subsurface wastewater infiltration

4.4 Design considerations

4.5 Construction management and contingency options

4.6 Septic tanks

4.7 Sand/media filters

4.8 Aerobic Treatment Units
Chapter 4: Treatment Processes and Systems

4-2 USEPA Onsite Wastewater Treatment Systems Manual

The component descriptions provided in this

chapter are intended to assist the reader in screening components and technologies for specific

applications. Chapter 5 presents a strategy and

procedures that can be used to screen and select

appropriate treatment trains and their components

for specific receiver sites. The reader should review

chapter 5 before selecting system components.

4.2 Conventional systems and

treatment options

The three primary components of a conventional

system (figure 4-1) are the soil, the subsurface

wastewater infiltration system (SWIS; also called a

leach field or infiltration trench), and the septic

tank. The SWIS is the interface between the

engineered system components and the receiving

ground water environment. It is important to note

that the performance of conventional systems relies

primarily on treatment of the wastewater effluent

in the soil horizon(s) below the dispersal and

infiltration components of the SWIS. Information

on SWIS siting, hydraulic and mass loadings,

design and geometry, distribution methods, and

construction considerations is included in this

chapter. The other major component of a conventional system, the septic tank, is characterized by

describing its many functions in an OWTS.

Treatment options include physical, chemical, and

biological processes. Use of these options is

determined by site-specific needs. Table 4-1 lists

common onsite treatment processes and methods

that may be used alone or in combination to

assemble a treatment train capable of meeting

established performance requirements. Special

issues that might need to be addressed in OWTS

design include treatment of high-strength wastes

(e.g., biochemical oxygen demand and grease from

schools and restaurants), mitigation of impacts

from home water softeners and garbage disposals,

management of holding tanks, and additives (see

related fact sheets).

4.3 Subsurface wastewater

infiltration

Subsurface wastewater infiltration systems (SWISs)

are the most commonly used systems for the

treatment and dispersal of onsite wastewater.

Infiltrative surfaces are located in permeable,

unsaturated natural soil or imported fill material so

wastewater can infiltrate and percolate through the

underlying soil to the ground water. As the wastewater infiltrates and percolates through the soil, it

is treated through a variety of physical, chemical,

and biochemical processes and reactions.

Many different designs and configurations are used,

but all incorporate soil infiltrative surfaces that are

located in buried excavations (figure 4-1). The

primary infiltrative surface is the bottom of the

excavation, but the sidewalls also may be used for

infiltration. Perforated pipe is installed to distribute

the wastewater over the infiltration surface. A porous

Figure 4-1. Conventional subsurface wastewater infiltration system
USEPA Onsite Wastewater Treatment Systems Manual 4-3

Chapter 4: Treatment Processes and Systems

Table 4-1. Commonly used treatment processes and optional treatment methods
Chapter 4: Treatment Processes and Systems

4-4 USEPA Onsite Wastewater Treatment Systems Manual

medium, typically gravel or crushed rock, is placed

in the excavation below and around the distribution

piping to support the pipe and spread the localized

flow from the distribution pipes across the excavation

cavity. Other gravelless or “aggregate-free” system

components may be substituted. The porous

medium maintains the structure of the excavation,

exposes the applied wastewater to more infiltrative

surface, and provides storage space for the wastewater within its void fractions (interstitial spaces,

typically 30 to 40 percent of the volume) during peak

flows with gravity systems. A permeable geotextile

fabric or other suitable material is laid over the porous

medium before the excavation is backfilled to prevent

the introduction of backfill material into the porous

medium. Natural soil is typically used for backfilling,

and the surface of the backfill is usually slightly

mounded and seeded with grass.

Subsurface wastewater infiltration systems provide

both dispersal and treatment of the applied wastewater. Wastewater is transported from the infiltration

system through three zones (see chapter 3). Two of

these zones, the infiltration zone and vadose zone, act

as fixed-film bioreactors. The infiltration zone, which

is only a few centimeters thick, is the most biologically active zone and is often referred to as the

“biomat.” Carbonaceous material in the wastewater is

quickly degraded in this zone, and nitrification occurs

immediately below this zone if sufficient oxygen is

present. Free or combined forms of oxygen in the soil

must satisfy the oxygen demand generated by the

microorganisms degrading the materials. If sufficient

oxygen is not present, the metabolic processes of the

microorganisms can be reduced or halted and both

treatment and infiltration of the wastewater will be

adversely affected (Otis, 1985). The vadose (unsaturated) zone provides a significant pathway for oxygen

diffusion to reaerate the infiltration zone (Otis, 1997,

Siegrist et al., 1986). Also, it is the zone where most

sorption reactions occur because the negative moisture

potential in the unsaturated zone causes percolating

water to flow into the finer pores of the soil, resulting

in greater contact with the soil surfaces. Finally, much

of the phosphorus and pathogen removal occurs in

this zone (Robertson and Harman, 1999; Robertson et

al., 1998; Rose et al., 1999; Yates and Yates, 1988).

4.3.1 SWIS designs

There are several different designs for SWISs.

They include trenches, beds, seepage pits, at-grade

systems, and mounds. SWIS applications differ in

their geometry and location in the soil profile.

Trenches have a large length-to-width ratio, while

beds have a wide, rectangular or square geometry.

Seepage pits are deep, circular excavations that rely

almost completely on sidewall infiltration. Seepage

pits are no longer permitted in many jurisdictions

because their depth and relatively small horizontal

profile create a greater point-source pollutant

loading potential to ground water than other

geometries. Because of these shortcomings, seepage

pits are not recommended in this manual.

Infiltration surfaces may be created in natural soil

or imported fill material. Most traditional systems

are constructed below ground surface in natural

soil. In some instances, a restrictive horizon above

a more permeable horizon may be removed and the

excavation filled with suitable porous material in

which to construct the infiltration surface (Hinson

et al., 1994). Infiltration surfaces may be constructed at the ground surface (“at-grades”) or

elevated in imported fill material above the natural

soil surface (“mounds”). An important difference

between infiltration surfaces constructed in natural

soil and those constructed in fill material is that a

secondary infiltrative surface (which must be

considered in design) is created at the fill/natural

soil interface. Despite the differences between the

types of SWISs, the mechanisms of treatment and

dispersal are similar.

4.3.2 Typical applications

Subsurface wastewater infiltration systems are

passive, effective, and inexpensive treatment

systems because the assimilative capacity of many

soils can transform and recycle most pollutants

found in domestic and commercial wastewaters.

SWISs are the treatment method of choice in rural,

unsewered areas. Where point discharges to surface

waters are not permitted, SWISs offer an alternative if ground water is not closely interconnected

with surface water. Soil characteristics, lot size, and

the proximity of sensitive water resources affect the

use of SWISs. Table 4-2 presents characteristics for

typical SWIS applications and suggests applications

to avoid. Local codes should be consulted for

special requirements, restrictions, and other

relevant information.
USEPA Onsite Wastewater Treatment Systems Manual 4-5

Chapter 4: Treatment Processes and Systems

4.3.3 Typical performance

Results from numerous studies have shown that

SWISs achieve high removal rates for most wastewater pollutants of concern (see chapter 3) with the

notable exception of nitrogen. Biochemical oxygen

demand, suspended solids, fecal indicators, and

surfactants are effectively removed within 2 to 5

feet of unsaturated, aerobic soil (figure 4-2).

Phosphorus and metals are removed through

adsorption, ion exchange, and precipitation reactions. However, the retention capacity of the soil is

finite and varies with soil mineralogy, organic

content, pH, redox potential, and cation exchange

capacity. The fate of viruses and toxic organic

compounds has not been well documented (Tomson

et al., 1984). Field and laboratory studies suggest

that the soil is quite effective in removing viruses,

but some types of viruses apparently are able to

leach from SWISs to the ground water. Finetextured soils, low hydraulic loadings, aerobic

subsoils, and high temperatures favor destruction of

viruses and toxic organics. The most significant

documented threats to ground water quality from

SWISs are nitrates. Wastewater nitrogen is nearly

completely nitrified below properly operating

SWISs. Because nitrate is highly soluble and

environments favoring denitrification in subsoil are

limited, little removal occurs (see chapter 3).

Chlorides also leach readily to ground water

because they, too, are highly soluble and are

nonreactive in soil.

Figure 4-2. Lateral view of conventional SWIS-based system

Source: Bouma, 1975.

a

Avoid when possible.

Source: Adapted from WEF, 1990.

Table 4-2. Characteristics of typical SWIS applications
Chapter 4: Treatment Processes and Systems

4-6 USEPA Onsite Wastewater Treatment Systems Manual

Dispersion of SWIS percolate in the ground water

is often minimal because most ground water flow is

laminar. The percolate can remain for several

hundred feet as a distinct plume in which the solute

concentrations remain above ambient ground water

concentrations (Robertson et al., 1989, Shaw and

Turyk, 1994). The plume descends in the ground

water as the ground water is recharged from the

surface, but the amount of dispersion of the plume

can be variable. Thus, drinking water wells some

distance from a SWIS can be threatened if they are

directly in the path of a percolate plume.

4.4 Design considerations

Onsite wastewater treatment system designs vary

according to the site and wastewater characteristics

encountered. However, all designs should strive to

incorporate the following features to achieve

satisfactory long-term performance:

• Shallow placement of the infiltration surface

(< 2 feet below final grade)

• Organic loading comparable to that of septic

tank effluent at its recommended hydraulic

loading rate

• Trench orientation parallel to surface contours

• Narrow trenches (< 3 feet wide)

• Timed dosing with peak flow storage

• Uniform application of wastewater over the

infiltration surface

• Multiple cells to provide periodic resting,

standby capacity, and space for future repairs or

replacement

Based on the site characteristics, compromises to

ideal system designs are necessary. However, the

designer should attempt to include as many of the

above features as possible to ensure optimal longterm performance and minimal impact on public

health and environmental quality.

4.4.1 Placement of the infiltration

surface

Placement of a SWIS infiltration surface may be

below, at, or above the existing ground surface (in

an in-ground trench, at grade, or elevated in a

mound system). Actual placement relative to the

original soil profile at the site is determined by

desired separation from a limiting condition

(figure 4-3). Treatment by removal of additional

pollutants during movement through soils and the

potential for excessive ground water mounding will

control the minimum separation distance from a

limiting condition. The depth below final grade is

affected by subsoil reaeration potential. Maximum

delivery of oxygen to the infiltration zone is most

likely when soil components are shallow and

narrow and have separated infiltration areas.

(Erickson and Tyler, 2001).

4.4.2 Separation distance from a

limiting condition

Placement of the infiltration surface in the soil

profile is determined by both treatment and hydraulic performance requirements. Adequate

separation between the infiltration surface and any

saturated zone or hydraulically restrictive horizon

within the soil profile (secondary design boundary

as defined in section 5.3.1) must be maintained to

achieve acceptable pollutant removals, sustain

aerobic conditions in the subsoil, and provide an

adequate hydraulic gradient across the infiltration

zone. Treatment needs (performance requirements)

establish the minimum separation distance, but the

potential for ground water mounding or the

availability of more permeable soil may make it

advantageous to increase the separation distance by

raising the infiltration surface in the soil profile.

Most current onsite wastewater system codes

require minimum separation distances of at least 18

inches from the seasonally high water table or

saturated zone irrespective of soil characteristics.

Generally, 2- to 4-foot separation distances have

proven to be adequate in removing most fecal

coliforms in septic tank effluent (Ayres Associates,

1993). However, studies have shown that the

applied effluent quality, hydraulic loading rates,

and wastewater distribution methods can affect the

unsaturated soil depth necessary to achieve acceptable wastewater pollutant removals. A few studies

have shown that separation distances of 12 to 18

inches are sufficient to achieve good fecal coliform

removal if the wastewater receives additional

pretreatment prior to soil application (Converse and

Tyler, 1998a, 1998b; Duncan et al., 1994). However, when effluents with lower organic and
USEPA Onsite Wastewater Treatment Systems Manual 4-7

Chapter 4: Treatment Processes and Systems

oxygen-demanding content are applied to the

infiltration surface at greater hydraulic loading

rates than those typically used for septic tank

effluents (during extended periods of peak flow),

treatment efficiency can be lost (Converse and

Tyler, 1998b, Siegrist et al., 2000).

Reducing the hydraulic loading rate or providing

uniform distribution of the septic tank effluent has

been shown to reduce the needed separation

distance (Bomblat et al., 1994; Converse and Tyler,

1998a; Otis, 1985; Siegrist et al., 2000; Simon and

Reneau, 1987). Reducing both the daily and

instantaneous hydraulic loading rates and providing

uniform distribution over the infiltration surface

can help maintain lower soil moisture levels.

Lower soil moisture results in longer wastewater

retention times in the soil and causes the wastewater to flow though the smaller soil pores in the

unsaturated zone, both of which enhance treatment

and can reduce the necessary separation distance.

Based only on hydraulics, certain soils require

different vertical separation distances from ground

water to avoid hydrologic interference with the

infiltration rate. From a treatment standpoint,

required separation distances are affected by dosing

pattern, loading rate, temperature, and soil characteristics. Uniform, frequent dosing (more than 12

times/day) in coarser soils maximizes the effectiveness of biological, chemical, and physical treatment

mechanisms. To offset inadequate vertical separation, a system designer can raise the infiltration

surface in an at-grade system or incorporate a

mound in the design. If the restrictive horizon is a

high water table and the soil is porous, the water

table can be lowered through the use of drainage

tile or a curtain drain if the site has sufficient relief

to promote surface discharge from the tile piping.

For flat terrain with porous soils, a commercial

system has been developed and is being field tested.

It lowers the water table with air pressure, thereby

avoiding any aesthetic concerns associated with a

raised mound on the site. Another option used

where the terrain is flat and wet is pumped drainage surrounding the OWTS (or throughout the

subdivision) to lower the seasonal high water table

and enhance aerobic conditions beneath the

Figure 4-3. Suggested subsurface infiltration system design versus depth (below the original ground surface) to a

limiting condition

Source: Otis, 2001.
Chapter 4: Treatment Processes and Systems

4-8 USEPA Onsite Wastewater Treatment Systems Manual

drainfield. These systems must be properly operated by certified operators and managed by a public

management entity since maintenance of off-lot

portions of the drainage network will influence

performance of the SWIS.

The hydraulic capacity of the site or the hydraulic

conductivity of the soil may increase the minimum

acceptable separation distance determined by

treatment needs. The soil below the infiltration

surface must be capable of accepting and transmitting the wastewater to maintain the desired unsaturated separation distance at the design hydraulic

loading rate to the SWIS. The separation distance

necessary for satisfactory hydraulic performance is

a function of the permeability of the underlying

soil, the depth to the limiting condition, the

thickness of the saturated zone, the percentage of

rocks in the soil, and the hydraulic gradient.

Ground water mounding analyses may be necessary

to assess the potential for the saturated zone to rise

and encroach upon the minimum acceptable

separation distance (see section 5.4). Raising the

infiltration surface can increase the hydraulic

capacity of the site by accommodating more

mounding. If the underlying soil is more slowly

permeable than soil horizons higher in the profile,

it might be advantageous to raise the infiltration

surface into the more permeable horizon where

higher hydraulic loading rates are possible (Hoover

et al., 1991; Weymann et al., 1998). A shallow

infiltration system covered with fill or an at-grade

system can be used if the natural soil has a shallow

permeable soil horizon (Converse et al., 1990;

Penninger, and Hoover, 1998). If more permeable

horizons do not exist, a mound system constructed

of suitable sand fill (figure 4-4) can provide more

permeable material in which to place the infiltration surface.

4.4.3 Depth of the infiltration surface

The depth of the infiltration surface is an important

consideration in maintaining adequate subsoil

aeration and frost protection in cold climates. The

maximum depth should be limited to no more than

3 to 4 feet below final grade to adequately reaerate

the soil and satisfy the daily oxygen demand of the

applied wastewater. The infiltrative surface depth

should be less in slowly permeable soils or soils

with higher ambient moisture. Placement below

this depth to take advantage of more permeable

soils should be resisted because reaeration of the

soil below the infiltration surface will be limited.

In cold climates, a minimum depth of 1 to 2 feet

may be necessary to protect against freezing.

Porous fill material can be used to provide the

necessary cover even with an elevated (at-grade or

mound) system if it is necessary to place the

infiltration surface higher.

4.4.4 Subsurface drainage

Soils with shallow saturated zones sometimes can

be drained to allow the infiltration surface to be

placed in the natural soil. Curtain drains, vertical

drains, underdrains, and mechanically assisted

commercial systems can be used to drain shallow

water tables or perched saturated zones. Of the

three, curtain drains are most often used in onsite

wastewater systems to any great extent. They can

be used effectively to remove water that is perched

over a slowly permeable horizon on a sloping site.

However, poorly drained soils often indicate other

soil and site limitations that improved drainage

alone will not overcome, so the use of drainage

enhancements must be carefully considered. Any

sloping site that is subject to frequent inundation

during prolonged rainfall should be considered a

candidate for upslope curtain drains to maintain

unsaturated conditions in the vadose zone.

Curtain drains are installed upslope of the SWIS to

intercept the permanent and perched ground water

flowing through the site over a restrictive horizon.

Perforated pipe is laid in the bottom of upslope

trenches excavated into the restrictive horizon. A

durable, porous medium is placed around the

piping and up to a level above the estimated

seasonally high saturated zone. The porous medium

intercepts the ground water and conveys it to the

drainage pipe (figure 4-5). To provide an outfall

for the drain, one or both ends of the pipe are

extended downslope to a point where it intercepts

the ground surface. When drainage enhancements

are used, the outlet and boundary conditions must

be carefully evaluated to protect local water

quality.

The drain should avoid capture of the SWIS

percolate plume and ground water infiltrating from

below the SWIS or near the end of the drain. A

separation distance between the SWIS and the drain

that is sufficient to prevent percolate from the
USEPA Onsite Wastewater Treatment Systems Manual 4-9

Chapter 4: Treatment Processes and Systems

SWIS from entering the drain should be maintained. The vertical distance between the bottom of

the SWIS and the drain and soil permeability

characteristics should determine this distance. As

the vertical distance increases and the permeability

decreases, the necessary separation distance increases. A 10-foot separation is used for most

applications. Also, if both ends of the drain cannot

be extended to the ground surface, the upslope end

should be extended some distance along the surface

contour beyond the end of the SWIS. If not done,

ground water that seeps around the end of the drain

can render the drain ineffective. Similar cautions

should be observed when designing and locating

outlet locations for commercial systems on flat

sites.

The design of a curtain drain is based on the

permeability of the soil in the saturated zone, the

size of the area upslope of the SWIS that contributes water to the saturated zone, the gradient of the

drainage pipe, and a suitable outlet configuration.

Figure 4-4. Raising the infiltration surface with a typical mound system.

Curtain

Drain

Fill

Material

Perched

Water

Table Gravel Filled

Above High

Water Table

Drainage Pipe

Impermeable Layer

Absorption

Trenches

Fill

Figure 4-5. Schematic of curtain drain construction Source: USEPA, 1980

Source: ASAE, Converse and Tyler, 1998b.
Chapter 4: Treatment Processes and Systems

4-10 USEPA Onsite Wastewater Treatment Systems Manual

If the saturated hydraulic conductivity is low and

the drainable porosity (the percentage of pore space

drained when the soil is at field capacity) is small,

even effectively designed curtain drains might have

limited effect on soil wetness conditions. Penninger

et al. (1998) illustrated this at a site with a silty

clay loam soil at field capacity that became completely re-saturated with as little as 1-inch of

precipitation. Figure 4-6 provides a useful design

chart that considers most of these parameters. For

further design guidance, refer to the U.S. Department of Agriculture’s Drainage of Agricultural

Land (USDA, 1973).

4.4.5 Sizing of the infiltration surface

The minimum acceptable infiltration surface area is

a function of the maximum anticipated daily

wastewater volume to be applied and the maximum

instantaneous and daily mass loading limitations of

the infiltration surface (see chapter 5). Both the

bottom and sidewall area of the SWIS excavation

can be infiltration surfaces; however, if the sidewall

is to be an active infiltration surface, the bottom

surface must pond. If continuous ponding of the

infiltration surface persists, the infiltration zone

will become anaerobic, resulting in loss of hydraulic capacity. Loss of the bottom surface for infiltration will cause the ponding depth to increase over

time as the sidewall also clogs (Bouma, 1975; Keys

et al., 1998; Otis, 1977). If allowed to continue,

hydraulic failure of the system is probable. Therefore, including sidewall area as an active infiltration surface in design should be avoided. If

sidewall areas are included, provisions should be

made in the design to enable removal of the ponded

system from service periodically to allow the

system to drain and the biomat to oxidize naturally.

Design flow

An accurate estimation of the design flow is critical

to infiltration surface sizing. For existing buildings

where significant changes in use are not expected,

water service metering will provide good estimates

for design. It is best to obtain several weeks of

metered daily flows to estimate daily average and

peak flows. For new construction, water use

metering is not possible and thus waste flow

projections must be made based on similar establishments. Tables of “typical” water use or wastewater flows for different water use fixtures, usage

patterns, and building uses are available (see

section 3.3.1). Incorporated into these guidelines

are varying factors of safety. As a result, the use of

these guides typically provides conservatively high

estimates of maximum peak flows that may occur

only occasionally. It is critical that the designer

recognizes the conservativeness of these guides and

how they can be appropriately adjusted because of

their impacts on the design and, ultimately, performance of the system.

Curtain drain design

Curtain drain design (see preceding figures) is dependent on the size of the contributing drainage area, the

amount of water that must be removed, the soil’s hydraulic properties, and the available slope of the site.

The contributing drainage area is estimated by outlining the capture zone on a topographic map of the site.

Drainage boundaries are determined by extending flow lines perpendicular to the topographic contours upslope

from the drain to natural divides (e.g., ridge tops) or natural or man-made “no-flow” boundaries (e.g., rock

outcrops, major roads). The amount of water that must be removed is an estimate of the volume of precipitation

that would be absorbed by the soil after a rainfall event. This is called the drainage coefficient, which is expressed

as the depth of water to be removed over a specified period of time, typically 24 hours. Soil structure, texture,

bulk density, slope, and vegetated cover all affect the volume of water to be drained.

The slope of the drain can be determined after the upslope depth of the drain invert and the outfall invert are

established. These can be estimated from the topographic map of the site. The contributing drainage area, water

volume to be removed, and slope of the drain are estimated. Figure 4-6 can be used to determine the drain

diameter. For example, the diameter of a curtain drain that will drain an area upslope of 50 acres with a drainage

coefficient of 3⁄4 inch on a slope of 5 percent would be 8 inches (see figure). At 0.5 percent, the necessary drain

diameter would be 12 inches.
USEPA Onsite Wastewater Treatment Systems Manual 4-11

Chapter 4: Treatment Processes and Systems

DRAINAGE CHART FOR

CORRUGATED PLASTIC DRAINAGE TUBING

GRADE IN CENTIMETERS PER METER

0.05 0.1 0.2 0.3 0.4 0.5 1.0 2.0 3.0 5.04.0 10

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0.05 0.1 0.2 0.3 0.4 0.5 1.0 2.0 3.0 4.0 5.0 10

GRADE IN FEET PER 100 FEET

Space between lines is the range of drain

capacity for the size shown between lines

V= velocity in feet per second

n=0.015

DISCHARGE IN CUBIC FEET PER SECOND

ACRES

DRAINED

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1200

1200

1500

1500

1500

2000

2000

2000

2500

2500

3000

3000

3500

4000

4500

3

3

3

2

2

1” 3”

8

1”

2

3”

4

DRAINAGE

COEFFICIENT

30”

24”

20”

18”

16”

14”

12”

10”

8”

6”

5”

4”

V=2

V=1

V=1.4

V=3

V=4

V=5

V=6

V=8

V=10

V=15

180

Figure 4-6. Capacity chart for subsurface drains

Source: USDA, 1973.
Chapter 4: Treatment Processes and Systems

4-12 USEPA Onsite Wastewater Treatment Systems Manual

Table 4-3. Suggested hydraulic and organic loading rates for sizing infiltration surfaces

Source: Adapted from Tyler, 2000.

Infiltration surface loading limitations

Infiltration surface hydraulic loading design rates

are a function of soil morphology, wastewater

strength, and SWIS design configuration. Hydraulic loadings are traditionally used to size infiltration

surfaces for domestic septic tank effluent. In the

past, soil percolation tests determined acceptable

hydraulic loading rates. Codes provided tables that

correlated percolation test results to the necessary

infiltration surface areas for different classes of

soils. Most states have supplemented this approach

with soil morphologic descriptions. Morphologic

features of the soil, particularly structure, texture,

and consistence, are better predictors of the soil’s

hydraulic capacity than percolation tests (Brown et

al., 1994; Gross et al., 1998; Kleiss and Hoover,

1986; Simon and Reneau, 1987; Tyler et al., 1991;

Tyler and Converse, 1994). Although soil texture

analysis supplemented the percolation test in most

states by the mid-1990s, soil structure has only

recently been included in infiltrative surface sizing

tables (table 4-3). Consistence, a measure of how

well soils form shapes and stick to other objects, is

an important consideration for many slowly

permeable soil horizons. Expansive clay soils that

become extremely firm when moist and very sticky

or plastic when wet (exhibiting firm or extremely

firm consistence) are not well suited for SWISs.

Not all soil conditions are represented in table 4-3,

which is a generic guide to the effects of soil

properties on the performance of SWISs. Also
USEPA Onsite Wastewater Treatment Systems Manual 4-13

Chapter 4: Treatment Processes and Systems

available are many other state and local guides that

include loadings for soils specific to local geomorphology. North Carolina, for example, uses the

long-term acceptance rate (LTAR) for soil loadings, which is the volume of wastewater that can be

applied to a square foot of soil each day over an

indefinite period of time such that the effluent

from the onsite system is absorbed and properly

treated (North Carolina DEHNR, 1996). In the

North Carolina rules, LTAR and loading rate values

are the same.

Increasingly, organic loading is being used to size

infiltration surfaces. Based on current understanding of the mechanisms of SWIS operation, organic

loadings and the reaeration potential of the subsoil

to meet the applied oxygen demand are critical

considerations in successful SWIS design. Anaerobic conditions are created when the applied oxygen

demand exceeds what the soil is able to supply by

diffusion through the vadose zone (Otis, 1985,

1997; Siegrist et al., 1986). The facultative and

anaerobic microorganisms that are able to thrive in

this environment are less efficient in degrading the

waste materials. The accumulating waste materials

and the metabolic by-products cause soil clogging

and loss of infiltrative capacity.

Further, higher forms of soil fauna that would help

break up the biomat (e.g., worms, insects, nonwetland plants) and would be attracted to the

carbon and nutrient-rich infiltration zone are

repelled by the anoxic or anaerobic environment. If

wastewater application continues without ample

time to satisfy the oxygen demand, hydraulic

failure due to soil clogging occurs. Numerous

studies have shown that wastewaters with low BOD

concentrations (e.g., < 50 mg/L) can be applied to

soils at rates 2 to 16 times the typical hydraulic

loading rate for domestic septic tank effluent (Jones

and Taylor, 1965; Laak, 1970, 1986; Louden et al.,

1998; Otis, 1985; Siegrist and Boyle, 1987; Tyler

and Converse, 1994).

The comparatively higher hydraulic loadings that

highly treated wastewater (highly treated in terms

of TSS, ammonium-nitrogen, and BOD) may

permit should be considered carefully because the

resulting rapid flow through the soil may allow

deep penetration of pathogens (Converse and Tyler,

1998a, 1998b; Siegrist et al., 2000; Siegrist and

Van Cuyk, 2001b; Tyler and Converse, 1994). The

trench length perpendicular to ground water

movement (footprint) should remain the same to

minimize system impacts on the aquifer.

Unfortunately, well-tested organic loading rates for

various classes of soils and SWIS design configurations have not been developed. Most organic

loading rates have been derived directly from the

hydraulic loadings typically used in SWIS design

by assuming a BOD5

 concentration (see box and

table 4-3). The derived organic loading rates also

incorporate the implicit factor of safety found in

the hydraulic loading rates. Organic loadings do

appear to have less impact on slowly permeable

soils because the resistance of the biomat that forms

at the infiltrative surface presents less resistance to

infiltration of the wastewater than the soil itself

(Bouma, 1975). For a further discussion of SWIS

performance under various environmental conditions, see Siegrist and Van Cuyk, 2001b.

Constituent mass loadings

Constituent mass loadings may be a concern with

respect to water quality. For example, to use the

soil’s capacity to adsorb and retain phosphorus

when systems are located near sensitive surface

waters, a phosphorus loading rate based on the soil

adsorption capacity might be selected as the

controlling rate of wastewater application to the

infiltration surface to maximize phosphorus

removal. Placement of the effluent distribution

piping high in the soil profile can promote greater

phosphorus removal because the permeability of

medium- and fine-textured soils tends to decrease

with depth and because the translocation of aluminum and iron—which react with phosphorus to

form insoluble compounds retained in the soil

matrix—occurs in some sandy soils, with the

maximum accumulation usually above 45 cm

(Mokma et al., 2001). Many lakes are surrounded

by sandy soils with a low phosphorus adsorption

capacity. If effluent distribution systems are

installed below 45 cm in these sandy soils, less

phosphorus will be removed from the percolating

effluent. In the case of a soluble constituent of

concern such as nitrate-nitrogen, a designer might

decide to reduce the mass of nitrate per unit of

application area. This would have the effect of

increasing the size of the SWIS footprint, thereby

reducing the potential concentration of nitrate in

the ground water immediately surrounding the

SWIS (Otis, 2001).
Chapter 4: Treatment Processes and Systems

4-14 USEPA Onsite Wastewater Treatment Systems Manual

4.4.6 Geometry, orientation, and

configuration of the infiltration

surface

The geometry, orientation, and configuration of the

infiltration surface are critical design factors that

affect the performance of SWISs. They are important for promoting subsoil aeration, maintaining an

acceptable separation distance from a saturated

zone or restrictive horizon, and facilitating construction. Table 4-4 lists the design considerations

discussed in this section.

Geometry

The width and length of the infiltration surface are

important design considerations to improve performance and limit impacts on the receiving environment. Trenches, beds, and seepage pits (or dry

wells) are traditionally used geometries. Seepage

pits can be effective for wastewater dispersal, but

they provide little treatment because they extend

deep into the soil profile, where oxygen transfer

and treatment are limited and the separation

distance to ground water is reduced. They are not

recommended for onsite wastewater treatment and

are not included as an option in this manual.

Width

Infiltration surface clogging and the resulting loss

of infiltrative capacity are less where the infiltration surface is narrow. This appears to occur

because reaeration of the soil below a narrow

infiltration surface is more rapid. The dominant

pathway for oxygen transport to the subsoil appears

to be diffusion through the soil surrounding the

infiltration surface (figure 4-7). The unsaturated

zone below a wide surface quickly becomes

anaerobic because the rates of oxygen diffusion are

too low to meet the oxygen demands of biota and

organics on the infiltration surface. (Otis, 1985;

Siegrist et al., 1986). Therefore, trenches perform

better than beds. Typical trench widths range from

1 to 4 feet. Narrower trenches are preferred, but

soil conditions and construction techniques might

limit how narrow a trench can be constructed. On

sloping sites, narrow trenches are a necessity

because in keeping the infiltration surface level, the

uphill side of the trench bottom might be excavated

into a less suitable soil horizon. Wider trench

infiltration surfaces have been successful in atgrade systems and mounds probably because the

engineered fill material and elevation above the

natural grade promote better reaeration of the fill.

Factors of safety in infiltration surface sizing

Sizing of onsite wastewater systems for single-family homes is typically based on the estimated peak daily flow

and the “long term acceptance rate” of the soil for septic tank effluent. In most states, the design flow is based on

the number of bedrooms in the house. A daily flow of 150 gallons is commonly assumed for each bedroom. This

daily flow per bedroom assumes two people per bedroom that generate 75 gpd each. Bedrooms, rather than

current occupancy, are used for the basis of SWIS design because the number of occupants in the house can

change.

Using this typical estimating procedure, a three-bedroom home would have a design flow of 150 gpd/bedroom x 3

bedrooms or 450 gpd. However, the actual daily average flow could be much less. Based on the 1990 census, the

average home is occupied by 2.8 persons. Each person in the United States generates 45 to 70 gpd of domestic

wastewater. Assuming these averages, the average daily flow would be 125 to 195 gpd or 28 to 44 percent of the

design flow, respectively. Therefore, the design flow includes an implicit factor of safety of 2.3 to 3.6. Of course,

this factor of safety varies inversely with the home occupancy and water use.

Unfortunately, the factors of safety implicitly built into the flow estimates are seldom recognized. This is

particularly true in the case of the design hydraulic loading rates, which were derived from existing SWISs. In

most codes, the hydraulic loading rates for sand are about 1.0 to 1.25 gpd/ft2. Because these hydraulic loading

rates assume daily flows of 150 gpd per bedroom, they are overestimated by a factor of 2.3 to 3.6. Fortunately,

these two assumptions largely cancel each other out in residential applications, but the suggested hydraulic

loading rates often are used to size commercial systems and systems for schools and similar facilities, where the

ratios between design flows and actual daily flows are closer to 1.0. This situation, combined with a lack of useful

information on allowable organic loading rates, has resulted in failures, particularly for larger systems where

actual flow approximates design.
USEPA Onsite Wastewater Treatment Systems Manual 4-15

Chapter 4: Treatment Processes and Systems

However, infiltration bed surface widths of greater

than 10 feet are not recommended because oxygen

transfer and clogging problems can occur (Converse and Tyler, 2000; Converse et al., 1990).

Length

The trench length is important where downslope

linear loadings are critical, ground water quality

impacts are a concern, or the potential for ground

water mounding exists. In many jurisdictions,

trench lengths have been limited to 100 feet. This

restriction appeared in early codes written for

gravity distribution systems and exists as an artifact

with little or no practical basis when pressure

distribution is used. Trench lengths longer than 100

feet might be necessary to minimize ground water

impacts and to permit proper wastewater drainage

from the site. Long trenches can be used to reduce

the linear loadings on a site by spreading the

Comparing hydraulic and organic mass loadings for a restaurant wastewater Comparing hydraulic and organic mass loadings for a restaurant wastewater

Infiltration surface sizing traditionally has been based on the daily hydraulic load determined through experience

to be acceptable for the soil characteristics. This approach to sizing fails to account for changes in applied

wastewater strength. Since soil clogging has been shown to be dependent on applied wastewater strength, it

might be more appropriate to size infiltration surfaces based on organic mass loadings.

To illustrate the impact of the different sizing methods, sizing computations for a restaurant are compared. A

septic tank is used for pretreatment prior to application to the SWIS. The SWIS is to be constructed in a sandy

loam with a moderate, subangular blocky structure. The suggested hydraulic loading rate for domestic septic tank

effluent on this soil is 0.6 gpd/ft2

 (table 4-3). The restaurant septic tank effluent has the following characteristics:

BOD5 800 mg/L

TSS 200 mg/L

Average daily flow 600 gpd

Infiltration area based on hydraulic loading: Infiltration area based on hydraulic loading:

Area = 600 gpd/0.6 gpd/ft2 = 1,000 ft2

Infiltration area based on organic loading: Infiltration area based on organic loading:

At the design infiltration rate of 0.6 gpd/ft2

 recommended for domestic septic tank effluent, the equivalent organic

loading is (assuming a septic tank BOD5 effluent concentration of 150 mg/L)

Organic Loading = 150 mg/L x 0.6 gpd/ft2 x (8.34 lb/mg/L x 10-6 gal)

= 7.5 x 10-4 lb BOD5

/ft2-d

Assuming 7.5 x 10-4 lb BOD5/ft2-d as the design organic loading rate,

Area = (800 mg-BOD5/L x 600 gpd x 8.34 lbs/mg/L x 10-6 gal)

 (7.5 x 10-4 lb BOD5/ft2-d)

 = 4.0 lb BOD5/d = 5337 ft2 (a 540% increase)

 (7.5 x 10-4 lb BOD5

/ft2-d)

Impact of a 40% water use reduction on infiltration area sizing

Based on hydraulic loading,

Area = (1 – 0.4) x 600 gpd = 600 ft2

 0.6 gpd/ft2

Based on organic loading (note the concentration of BOD5 increases with water conservation but the mass of

BOD5 discharged does not change),

Area = (800 mg-BOD5/L x 600 gpd) x (8.34 lb/mg/L x 10-6 gal)

 [(1 – 0.4) x 600 gpd] x (7.5 x 10-4 lb BOD5/ft2-d)

 = 4.0 lb BOD5/d = 5337 ft2 (an 890% increase)

 (7.5 x 10-4 lb BOD5

/ft2-d)
Chapter 4: Treatment Processes and Systems

4-16 USEPA Onsite Wastewater Treatment Systems Manual

Figure 4-7. Pathway of subsoil reaeration

Source: Ayres Associates, 2000

Table 4-4. Geometry, orientation, and configuration considerations for SWISs
USEPA Onsite Wastewater Treatment Systems Manual 4-17

Chapter 4: Treatment Processes and Systems

wastewater loading parallel to and farther along the

surface contour. With current distribution/dosing

technology, materials, and construction methods,

trench lengths need be limited only by what is

practical or feasible on a given site. Also, use of

standard trench lengths, e.g., X feet of trench/BR,

is discouraged because it restricts the design options

to optimize performance for a given site condition.

Height

The height of the sidewall is determined primarily

by the type of porous medium used in the system,

the depth of the medium needed to encase the

distribution piping, and/or storage requirements for

peak flows. Because the sidewall is not included as

an active infiltration surface in sizing the infiltration area, the height of the sidewall can be minimized to keep the infiltration surface high in the

soil profile. A height of 6 inches is usually sufficient for most porous aggregate applications. Use

of a gravelless system requires a separate analysis

to determine the height based on whether it is an

aggregate-free (empty chamber) design or one that

substitutes a lightweight aggregate for washed

gravel or crushed stone.

Orientation

Orientation of the infiltration surface(s) becomes

an important consideration on sloping sites, sites

with shallow soils over a restrictive horizon or

saturated zone, and small or irregularly shaped lots.

The long axes of trenches should be aligned

parallel to the ground surface contours to reduce

linear contour hydraulic loadings and ground water

mounding potential. In some cases, ground water

or restrictive horizon contours may differ from

surface contours because of surface grading or the

soil’s morphological history. Where this occurs,

consideration should be given to aligning the

trenches with the contours of the limiting condition

rather than those of the surface. Extending the

trenches perpendicular to the ground water gradient

reduces the mass loadings per unit area by creating

a “line” source rather than a “point” source along

the contour. However, the designer must recognize

that the depth of the trenches and the soil horizon

in which the infiltration surface is placed will vary

across the system. Any adverse impacts this might

have on system performance should be mitigated

through design adjustments.

Configuration

The spacing of multiple trenches constructed

parallel to one another is determined by the soil

characteristics and the method of construction. The

sidewall-to-sidewall spacing must be sufficient to

enable construction without damage to the adjacent

trenches. Only in very tight soils will normally

used spacings be inadequate because of high soil

wetness and capillary fringe effects, which can

limit oxygen transfer. It is important to note that

the sum of the hydraulic loadings to one or more

trenches or beds per each unit of contour length

(when projected downslope) must not exceed the

estimated maximum contour loading for the site.

Also, the finer (tighter) the soil, the greater the

trench spacing should be to provide sufficient

oxygen transfer. Quantitative data are lacking, but

Camp (1985) reported a lateral impact of more

than 2.0 meters in a clay soil.

Given the advantages of lightweight gravelless

systems in terms of potentially reduced damage to

the site’s hydraulic capacity, parallel trenches may

physically be placed closer together, but the

downslope hydraulic capacity of the site and the

natural oxygen diffusion capacity of the soil cannot

be exceeded.

4.4.7 Wastewater distribution onto the

infiltration surface

The method and pattern of wastewater distribution

in a subsurface infiltration system are important

design elements. Uniform distribution aids in

maintaining unsaturated flow below the infiltration

surface, which results in wastewater retention times

in the soil that are sufficiently long to effect

treatment and promote subsoil reaeration. Uniform

distribution design also results in more complete

utilization of the infiltration surface.

Gravity flow and dosing are the two most commonly used distribution methods. For each method,

various network designs are used (table 4-5).

Gravity flow is the most commonly used method

because it is simple and inexpensive. This method

discharges effluent from the septic tank or other

pretreatment tank directly to the infiltration surface

as incoming wastewater displaces it from the

tank(s). It is characterized by the term “trickle

flow” because the effluent is slowly discharged

over much of the day. Typically, tank discharges
Chapter 4: Treatment Processes and Systems

4-18 USEPA Onsite Wastewater Treatment Systems Manual

are too low to flow throughout the distribution

network. Thus, distribution is unequal and localized overloading of the infiltration surface occurs

with concomitant poor treatment and soil clogging

(Bouma, 1975; McGauhey and Winneberger, 1964;

Otis, 1985; Robeck et al., 1964).

Dosing, on the other hand, accumulates the wastewater effluent in a dose tank from which the water

is periodically discharged under pressure in “doses”

to the infiltration system by a pump or siphon. The

pretreated wastewater is allowed to accumulate in

the dose tank and is discharged when a predetermined water level, water volume, or elapsed time is

reached. The dose volumes and discharge rates are

usually such that much of the distribution network

is filled, resulting in more uniform distribution

over the infiltration surface. Dosing outperforms

gravity-flow systems because distribution is more

uniform. In addition, the periods between doses

provide opportunities for the subsoil to drain and

reaerate before the next dose (Bouma et al., 1974;

Hargett et al., 1982; Otis et al., 1977). However,

which method is most appropriate depends on the

specific application.

Gravity flow

Gravity flow can be used where there is a sufficient

elevation difference between the outlet of the

pretreatment tank and the SWIS to allow flow to

and through the SWIS by gravity. Gravity flow

systems are simple and inexpensive to construct but

are the least efficient method of distribution.

Distribution is very uneven over the infiltration

surface, resulting in localized overloading (Converse, 1974; McGauhey and Winneberger, 1964;

Otis et al., 1978; University of Wisconsin, 1978).

Until a biomat forms on the infiltration surface to

slow the rate of infiltration, the wastewater residence time in the soil might be too short to effect

good treatment. As the biomat continues to form on

the overloaded areas, the soil surface becomes

clogged, forcing wastewater effluent to flow

through the porous medium of the trench until it

reaches an unclogged infiltration surface. This

phenomenon, known as “progressive clogging,”

occurs until the entire infiltration surface is ponded

and the sidewalls become the more active infiltration surfaces. Without extended periods of little or

no flow to allow the surface to dry, hydraulic

failure becomes imminent. Although inefficient,

these systems can work well for seasonal homes

with intermittent use or for households with low

occupancies. Seasonal use of SWISs allows the

infiltration surface to dry and the biomat to oxidize, which rejuvenates the infiltration capacity.

Low occupancies result in mass loadings of wastewater constituents that are lower and less likely to

exceed the soil’s capacity to completely treat the

effluent.

Perforated pipe

Four-inch-diameter perforated plastic pipe is the

most commonly used distribution piping for

Table 4-5. Distribution methods and applications.
USEPA Onsite Wastewater Treatment Systems Manual 4-19

Chapter 4: Treatment Processes and Systems

gravity flow systems. The piping is generally

smooth-walled rigid polyvinyl chloride (PVC), or

flexible corrugated polyethylene (PE) or acrylonitrile-butadiene-styrene (ABS). One or two rows of

holes or slots spaced 12 inches apart are cut into the

pipe wall. Typically, the piping is laid level in

gravel (figure 4-1) with the holes or slots at the

bottom (ASTM, undated). One distribution line is

used per trench. In bed systems, multiple lines are

installed 3 to 6 feet apart.

Distribution box

Distribution boxes are used to divide the wastewater effluent flow among multiple distribution lines.

They are shallow, flat bottomed, watertight structures with a single inlet and individual outlets

provided at the same elevation for each distribution

line. An above-grade cover allows access to the

inside of the box. The “d-box” must be laid level

on a sound, frost-proof footing to divide the flow

evenly among the outlets. Uneven settlement or

frost heaving results in unequal flow to the lateral

lines because the outlet hole elevations cease to be

level. If this occurs, adjustments must be made to

reestablish equal division of flow. Several devices

can be used. Adjustable weirs that can level the

outlet inverts and maintain the same length of weir

per outlet are one option. Other options include

designs that allow for leveling of the entire box

(figure 4-8). The box can also be used to take

individual trenches out of service by blocking the

outlet to the distribution lateral or raising the outlet

weir above the weir elevations for the other outlets.

Because of the inevitable movement of d-boxes,

their use has been discouraged for many years

(USPHS, 1957). However, under a managed care

system with regular adjustment, the d-box is

acceptable.

Serial relief line

Serial relief lines distribute wastewater to a series

of trenches constructed on a sloping site. Rather

than dividing the flow equally among all trenches

as with a distribution box, the uppermost trench is

loaded until completely flooded before the next

(lower) trench receives effluent. Similarly, that

trench is loaded until flooded before discharge

occurs to the next trench, and so on. This method

of loading is accomplished by installing “relief

lines” between successive trenches (figure 4-9).

Figure 4-8. Distribution box with adjustable weir outlets

Figure 4-9. Serial relief line distribution network and installation

detail

Source: USEPA, 1980.

Source: Ayres Associates.
Chapter 4: Treatment Processes and Systems

4-20 USEPA Onsite Wastewater Treatment Systems Manual

The relief lines are simple overflow lines that

connect one trench to the adjacent lower trench.

They are solid-wall pipes that connect the crown of

the upper trench distribution pipe with the distribution pipe in the lower trench. Successive relief lines

are separated by 5 to 10 feet to avoid shortcircuiting. This method of distribution makes full

hydraulic use of all bottom and sidewall infiltration

surfaces, creates the maximum hydrostatic head

over the infiltration surfaces to force the water into

the surrounding soil, and eliminates the problem of

dividing flows evenly among independent trenches.

However, because continuous ponding of the

infiltration surfaces is necessary for the system to

function, the trenches suffer hydraulic failure more

rapidly and progressively because the infiltration

surfaces cannot regenerate their infiltrative capacity.

Drop box

Drop box distribution systems function similarly to

relief line systems except that drop boxes are used

in place of the relief lines. Drop boxes are installed

for each trench. They are connected in manifolds to

trenches above and below (figure 4-10). The outlet

invert can be placed near the top of each trench to

force the trench to fill completely before it discharges to the next trench if a serial distribution

mode of operation is desired. Solid-wall pipe is

used between the boxes.

The advantage of this method over serial relief

lines is that individual trenches can be taken out of

service by attaching 90 degree ells to the outlets

that rise above the invert of the manifold connection to the next trench drop box. It is easier to add

additional trenches to a drop box system than to a

serial relief line network. Also, the drop box

system may be operated as an alternating trench

system by using the 90 degree ells on unused lines.

With this and the serial distribution system, the

designer must carefully evaluate the downslope

capacity of the site to ensure that it will not be

overloaded when the entire system or specific

trench combinations are functioning.

Gravelless wastewater dispersal systems

Gravelless systems have been widely used. They

take many forms, including open-bottomed chambers, fabric-wrapped pipe, and synthetic materials

such as expanded polystyrene foam chips (figure 4-11). Some gravelless drain field systems use

large-diameter corrugated plastic tubing covered

with permeable nylon filter fabric not surrounded

by gravel or rock. The area of fabric in contact

with the soil provides the surface for the septic tank

effluent to infiltrate the soil. The pipe is a minimum of 10 to 12 inches (25.4 to 30.5 centimeters)

in diameter covered with spun bonded nylon filter

fabric to distribute water around the pipe. The pipe

is placed in a 12- to 24-inch (30.5- to 61-centimeter)-wide trench. These systems can be installed in

areas with steep slopes with small equipment and in

hand-dug trenches where conventional gravel

systems would not be possible.

Reduced sizing of the infiltration surface is often

promoted as another advantage of the gravelless

system. This is based primarily on the premise that

gravelless systems do not “mask” the infiltration

surface as gravel does where the gravel is in direct

contact with the soil. Proponents of this theory

claim that an infiltration surface area reduction of

50 percent is warranted. However, these reductions

are not based on scientific evidence though they

have been codified in some jurisdictions (Amerson

et al., 1991; Anderson et al., 1985; Carlile and

Osborne, 1982; Effert and Cashell, 1987). Although gravel masking might occur in porous

medium applications, reducing the infiltration

surface area for gravelless systems increases the

BOD mass loading to the available infiltration

surface. Many soils might not be able to support

the higher organic loading and, as a result, more

severe soil clogging and greater penetration of

pollutants into the vadose zone and ground water

can occur (University of Wisconsin, 1978), negating the benefits of the gravelless surface.

A similar approach must be taken with any contaminant in the pretreatment system effluent that

must be removed before it reaches ground water or

nearby surface waters. A 50 percent reduction in

infiltrative surface area will likely result in less

removal of BOD, pathogens, and other contaminants in the vadose zone and increase the presence

and concentrations of contaminants in effluent

plumes. The relatively confined travel path of a

plume provides fewer adsorption sites for removal

of adsorbable contaminants (e.g., metals, phosphorus, toxic organics). Because any potential reductions in infiltrative surface area must be analyzed in

a similar comprehensive fashion, the use of
USEPA Onsite Wastewater Treatment Systems Manual 4-21

Chapter 4: Treatment Processes and Systems

Figure 4-10. Drop box distribution network

Source: National Small Flows Clearinghouse.

Figure 4-11. Various gravelless systems

Source: USEPA, 1980
Chapter 4: Treatment Processes and Systems

4-22 USEPA Onsite Wastewater Treatment Systems Manual

gravelless medium should be treated similarly to

potential reductions from increased pretreatment

and better distribution and dosing concepts.

Despite the cautions stated above, the overall

inherent value of lightweight gravelless systems

should not be ignored, especially in areas where

gravel is expensive and at sites that have soils that

are susceptible to smearing or other structural

damage during construction due to the impacts of

heavy machinery on the site. In all applications

where gravel is used (see SWIS Media in the

following section), it must be properly graded and

washed. Improperly washed gravel can contribute

fines and other material that can plug voids in the

infiltrative surface and reduce hydraulic capability.

Gravel that is embedded into clay or fine soils

during placement can have the same effect.

Leaching chambers

A leaching chamber is a wastewater treatment

system that consists of trenches or beds and one or

more distribution pipes or open-bottomed plastic

chambers. Leaching chambers have two key

functions: to disperse the effluent from septic tanks

and to distribute this effluent throughout the

trenches. A typical leaching chamber consists of

several high-density polyethylene injection-molded

arch-shaped chamber segments. A typical chamber

has an average inside width of 15 to 40 inches (38

to 102 centimeters) and an overall length of 6 to 8

feet (1.8 to 2.4 meters). The chamber segments are

usually 1-foot high, with wide slotted sidewalls.

Depending on the drain field size requirements, one

or more chambers are typically connected to form

an underground drain field network.

Typical leaching chambers (figure 4-12) are

gravelless systems that have drain field chambers

with no bottoms and plastic chamber sidewalls,

available in a variety of shapes and sizes. Use of

these systems sometimes decreases overall drain

field costs and may reduce the number of trees that

must be removed from the drain field lot.

About 750,000 chamber systems have been installed

over the past 15 years. Currently, a high percentage

of new construction applications use lightweight

plastic leaching chambers for new wastewater

treatment systems in states like Colorado, Idaho,

North Carolina, Georgia, Florida, and Oregon. The

gravel aggregate traditionally used in drain fields

can have large quantities of mineral fines that also

clog or block soil pores. Use of leaching chambers

avoids this problem. Recent research sponsored by

manufacturers shows promising results to support

reduction in sizing of drain fields through the use

of leaching chambers without increased hydraulic

and pollutant penetration failures (Colorado School

of Mines, 2001; Siegrist and Vancuyk, 2001a, 2001b).

These studies should be continued to eventually yield

rational guidelines for proper sizing of these systems

based on the type of pretreatment effluent to be

received (septic tank effluent, effluent from filters

or aerobic treatment units, etc.), as well as different

soil types and hydrogeological conditions. Many

states offer drain field sizing reduction allowances

when leaching chambers are used instead of

conventional gravel drain fields.

Because leaching chamber systems can be installed

without heavy equipment, they are easy to install

Figure 4-12. Placement of leaching chambers in typical application

Source: Hoover et al., 1996.
USEPA Onsite Wastewater Treatment Systems Manual 4-23

Chapter 4: Treatment Processes and Systems

and repair. These high-capacity, open-bottom drain

field systems can provide greater storage than

conventional gravel systems and can be used in

areas appropriate for gravel aggregate drain fields.

Leaching systems can operate independently and

require little day-to-day maintenance. Their

maintenance requirements are comparable to those

of aggregate trench systems.

The lightweight chamber segments available on the

market stack together compactly for efficient

transport. Some chambers interlock with ribs

without fasteners, cutting installation time by

more than 50 percent reused and conventional

gravel/pipe systems. Such systems can be reused

and relocated if the site owner decides to build

on another drain field site. A key disadvantage of

leaching chambers compared to gravel drain

fields is that they can be more expensive if a

low-cost source of gravel is readily available.

Porous media should be placed along the chamber

sidewall area to a minimum compacted height of

8 inches above the trench bottom. Additional backfill

is placed to a minimum compacted height of 6 to12

inches above the chamber, depending on the chamber

strength. Individual chamber trench bottoms should

be leveled in all directions and follow the contour of

the ground surface elevation without any dams or

other water stops. The manufacturer’s installation

instructions should be followed, and systems should

be installed by an authorized contractor.

Dosed flow distribution

Dosed-flow distribution systems are a significant

improvement over gravity-flow distribution systems.

The design of dosed-flow systems (figure 4-13)

includes both the distribution network and the

dosing equipment (see table 4-6). Dosing achieves

better distribution of the wastewater effluent over

the infiltration surface than gravity flow systems and

provides intervals between doses when no wastewater

is applied. As a result, dosed-flow systems reduce the

rate of soil clogging, more effectively maintain

unsaturated conditions in the subsoil (to effect good

treatment through extended residence times and

increased reaeration potential), and provide a means

to manage wastewater effluent applications to the

infiltration system (Hargett et al., 1982). They can be

used in any application and should be the method of

choice. Unfortunately, they are commonly perceived

to be less desirable because they add a mechanical

Table 4-6. Dosing methods and devices.

Source: National Small Flows Clearinghouse

Figure 4-13. Typical pressurized distribution system layout
Chapter 4: Treatment Processes and Systems

4-24 USEPA Onsite Wastewater Treatment Systems Manual

component to an otherwise “passive” system and

add cost because of the dosing equipment. The

improved performance of dosed-flow systems over

gravity flow systems should outweigh these perceived

disadvantages, especially when a management

entity is in place. It must be noted, however, that if

dosed infiltration systems are allowed to pond, the

advantages of dosing are lost because the bottom

infiltration surface is continuously inundated and

no longer allowed to rest and reaerate. Therefore,

there is no value in using dosed-flow distribution in

SWISs designed to operate ponded, such as systems

that include sidewall area as an active infiltration

surface or those using serial relief lines.

Perforated pipe

Four-inch perforated pipe networks (with or

without d-boxes or pressure manifolds) that receive

dosed-flow applications are designed no differently

than gravity-flow systems. Many of the advantages

of dosing are lost in such networks, however,

because the distribution is only slightly better than

that of gravity-flow systems (Converse, 1974).

Pressure manifold

A pressure manifold consists of a large-diameter

pipe tapped with small outlet pipes that discharge

to gravity laterals (figure 4-14). A pump pressurizes the manifold, which has a selected diameter to

ensure that pressure inside the manifold is the same

at each outlet. This method of flow division is

more accurate and consistent than a distribution

box, but it has the same shortcoming since flow

after the manifold is by gravity along each distribution lateral. Its most common application is to

divide flow among multiple trenches constructed at

different elevations on a sloping site.

Table 4-7 can be used to size a pressure manifold

for different applications (see sidebar). This table was

developed by Berkowitz (1985) to size the manifold

diameter based on the spacing between pressure lateral

taps, the lateral tap diameter, and the number of

lateral taps. The hydraulic computations made to

develop the table set a maximum flow differential

between laterals of 5 percent. The dosing rate is

determined by calculating the flow in a single lateral

tap assuming 1 to 4 feet of head at the manifold

outlets and multiplying the result by the number of

lateral taps. The Hazen-Williams equation for pipe

flow can be used to make this calculation.

Pressure distribution is typically constructed of

Schedule 40 PVC pipe (figure 4-15). The lateral

taps are joined by tees. They also can be attached

by tapping (threading) the manifold pipe, but the

manifold pipe must be Schedule 80 to provide a

thicker pipe wall for successful tapping. Valves on

each pressure tap are recommended to enable each

line to be taken out of service as needed by closing

the appropriate valve. This allows an opportunity

to manage, rest, or repair individual lines. To

prevent freezing, the manifold can be drained back

to the dose tank after each dose. If this is done, the

volume of water that will drain from the manifold

and forcemain must be added to the dose volume to

achieve the desired dose.

Rigid pipe pressure network

Rigid pipe pressure distribution networks are used

to provide relatively uniform distribution of

Figure 4-14. Pressure manifold detail
USEPA Onsite Wastewater Treatment Systems Manual 4-25

Chapter 4: Treatment Processes and Systems

Figure 4-15. Horizontal design for pressure distribution

Source: Washington Department of Health, 1998.

wastewater effluent over the entire infiltration

surface simultaneously during each dose. They are

well suited for all dosed systems. Because they

deliver the same volume of wastewater effluent per

linear length of lateral, they can be used to dose

multiple trenches of unequal length. Although rigid

pipe pressure networks can be designed to deliver

equal volumes to trenches at different elevations

(Mote, 1984; Mote et al., 1981; Otis, 1982), these

situations should be avoided. Uniform distribution

is achieved only when the network is fully pressurized. During filling and draining of the network,

the distribution lateral at the lowest elevation

receives more water. This disparity increases with

increasing dosing frequency. As an alternative on

sloping sites, the SWIS could be divided into

multiple cells, with the laterals in each cell at the

same elevation. If this is not possible, other

distribution designs should be considered.

The networks consist of solid PVC pipe manifolds

that supply water to a series of smaller perforated

PVC laterals (figure 4-16). The laterals are designed to discharge nearly equal volumes of

Table 4-7. Pressure manifold sizing

Source: Adapted from Berkowitz, 1985.
Chapter 4: Treatment Processes and Systems

4-26 USEPA Onsite Wastewater Treatment Systems Manual

wastewater from each orifice in the network when

fully pressurized. This is accomplished by maintaining a uniform pressure throughout the network

during dosing. The manifolds and laterals are sized

relative to the selected orifice size and spacing to

achieve uniform pressure. A manual flushing

mechanism should be included to enable periodic

flushing of slimes and other solids that accumulate

in the laterals.

Design of dosed flow systems

A simplified method of network design has been

developed (Otis, 1982). Lateral and manifold

sizing is determined using a series of graphs and

tables after the designer has selected the desired

orifice size and spacing and the distal pressure in

the network (typically 1 to 2 feet of head). These

graphs and tables were derived by calculating the

change in flow and pressure at each orifice between

the distal and proximal ends of the network. The

method is meant to result in discharge rates from

the first and last orifices that differ by no more

than 10 percent in any lateral and 15 percent across

the entire network. However, subsequent testing of

field installations indicated that the design model

overestimates the maximum lateral length by as

much as 25 percent (Converse and Otis, 1982).

Therefore, if the graphs and tables are used, the

maximum lateral length for any given orifice size

and spacing should not exceed 80 percent of the

maximum design length suggested by the lateral

sizing graphs. In lieu of using the graphs and

tables, a spreadsheet could be written using the

equations presented and adjusting the orifice

discharge coefficient.

Pressure manifold design

A SWIS consisting of 12 trenches of equal length is to be constructed on a slope. To divide the septic tank

effluent equally among the 12 trenches, a pressure manifold is to be used. The lateral taps are to be spaced 6

inches apart on one side of the manifold.

Table 4-7 can be used to size the manifold. Looking down the series of columns under the Single-sided manifold,

up to sixteen 1⁄2-inch taps could be made to a 4-inch manifold. Therefore, a 4-inch manifold would be acceptable. If

3⁄4- or 1-inch taps were used, a 6-inch manifold would be necessary.

Using the orifice equation, the flow from each lateral tap can be estimated by assuming an operating pressure in

the manifold:

Q = Ca(2gh)2

where Q is the lateral discharge rate, C is a dimensionless coefficient that varies with the characteristics of the

orifice (0.6 for a sharp-edged orifice), a is the area of the orifice, g is the acceleration due to gravity, and h is the

operating pressure within the manifold. In English units using a 0.6 orifice coefficient, this equation becomes

Q = 11.79 d2hd

1/2

where Q is the discharge rate in gallons per minute, d is the orifice diameter in inches, and h is the operating

pressure in feet of water.

Assuming 1⁄2-inch taps with a operating pressure of 3 feet of water, the discharge rate from each outlet is

Q = 11.79 (1⁄2)2 31/2 = 5.1 gpm

Thus, the pump must be capable of delivering 12 x 5.1 gpm or approximately 60 gpm against an operating

pressure of 3 feet of water plus the static lift and friction losses incurred in the forcemain to the pressure

manifold.

Septic Tank

Pumping (Dosing)

Chamber

Effluent

Pump

Small Diameter

Pressure Distribution

Cleanout

Figure 4-16. Rigid pipe pressure distribution networks with flushing

cleanouts
USEPA Onsite Wastewater Treatment Systems Manual 4-27

Chapter 4: Treatment Processes and Systems

To achieve uniform distribution, the density of

orifices over the infiltration surface should be as

high as possible. However, the greater the number

of orifices used, the larger the pump must be to

provide the necessary dosing rate. To reduce the

dosing rate, the orifice size can be reduced, but the

smaller the orifice diameter, the greater the risk of

orifice clogging. Orifice diameters as small as 1/8

inch have been used successfully with septic tank

effluent when an effluent screen is used at the

septic tank outlet. Orifice spacings typically are 1.5

to 4 feet, but the greater the spacing, the less

uniform the distribution because each orifice

represents a point load. It is up to the designer to

achieve the optimum balance between orifice

density and pump size.

The dose volume is determined by the desired

frequency of dosing and the size of the network.

Often, the size of the network will control design.

During filling and draining of the network at the

start and end of each dose, the distribution is less

uniform. The first holes in the network discharge

more during initial pressurization of the network,

and the holes at the lowest elevation discharge

more as the network drains after each dose. To

Design procedure for rigid pipe pressure distribution network

The simplified design procedure for rigid pipe pressure networks as presented by Otis (1982) includes the

following steps:

1. Lay out the proposed network.

2. Select the desired orifice size and spacing. Maximize the density of orifices over the infiltration surface,

keeping in mind that the dosing rate increases as the orifice size increases and the orifice spacing

decreases.

3. Determine the appropriate lateral pipe diameter compatible with the selected orifice size and spacing using a

spreadsheet or sizing charts from Otis (1982).

4. Calculate the lateral discharge rate using the orifice discharge equation (0.48 discharge coefficient or 80

percent of 0.6).

5. Determine the appropriate manifold size based on the number, spacing, and discharge rate of the laterals

using a spreadsheet or sizing table from Otis (1982).

6. Determine the dose volume required. Use either the minimum dose volume equal to 5 times the network

volume or the expected daily flow divided by the desired dosing frequency, whichever is larger.

7. Calculate the minimum dosing rate (the lateral discharge times the number of laterals).

8. Select the pump based on the required dosing rate and the total dynamic head (sum of the static lift, friction

losses in the forcemain to the network, and the network losses, which are equal to 1.3 times the network

operating pressure).

minimize the relative difference in discharge

volumes, the dose volume should be greater than

five times the volume of the distribution network

(Otis, 1982). A pump or siphon can be used to

pressurize the network.

Dripline pressure network

Drip distribution, which was derived from drip

irrigation technology, was recently introduced as a

method of wastewater distribution. It is a method

of pressure distribution capable of delivering small,

precise volumes of wastewater effluent to the

infiltration surface. It is the most efficient of the

distribution methods and is well suited for all types

of SWIS applications. A dripline pressure network

consists of several components:

• Dose tank

• Pump

• Prefilter

• Supply manifold

• Pressure regulator (when turbulent, flow

emitters are used)
Chapter 4: Treatment Processes and Systems

4-28 USEPA Onsite Wastewater Treatment Systems Manual

• Dripline

• Emitters

• Vacuum release valve

• Return manifold

• Flush valve

• Controller

The pump draws wastewater effluent from the dose

tank, preferably on a timed cycle, to dose the

distribution system. Before entering the network,

the effluent must be prefiltered through mechanical

or granular medium filters. The former are used

primarily for large SWIS systems. The backflush

water generated from a self-cleaning filter should

be returned to the headworks of the treatment

system. The effluent enters the supply manifold

that feeds each dripline (figure 4-17). If turbulent

flow emitters are used, the filtered wastewater must

first pass through a pressure regulator to control the

maximum pressure in the dripline. Usually, the

dripline is installed in shallow, narrow trenches 1 to 2

feet apart and only as wide as necessary to insert

the dripline using a trenching machine or vibratory

plow. The trench is backfilled without any porous

medium so that the emitter orifices are in direct

contact with the soil. The distal ends of each

dripline are connected to a return manifold. The

return manifold is used to regularly flush the

dripline. To flush, a valve on the manifold is

opened and the effluent is flushed through the

driplines and returned to the treatment system

headworks.

Because of the unique construction of drip distribution systems, they cause less site disruption during

installation, are adaptable to irregularly shaped lots

or other difficult site constraints, and use more of

the soil mantle for treatment because of the shallow

depth of placement. Also, because the installed cost

per linear foot of dripline is usually less than the

cost of conventional trench construction, dripline

can be added to decrease mass loadings to the

infiltration surface at lower costs than other

distribution methods. Because of the equipment

required, however, drip distribution tends to be

more costly to construct and requires regular

operation and maintenance by knowledgeable

individuals. Therefore, it should be considered for

use only where operation and maintenance support

is ensured.

The dripline is normally a 1⁄2-inch-diameter flexible

polyethylene tube with emitters attached to the

inside wall spaced 1 to 2 feet apart along its length.

Because the emitter passageways are small, friction

losses are large and the rate of discharge is low

(typically from 0.5 to nearly 2 gallons per hour).

Two types of emitters are used. One is a “turbulentflow” emitter, which has a very long labyrinth.

Flow through the labyrinth reduces the discharge

pressure nearly to atmospheric rates. With increasing in-line pressure, more wastewater can be forced

through the labyrinth. Thus, the discharges from

turbulent flow emitters are greater at higher

pressures (figure 4-18). To more accurately control

the rate of discharge, a pressure regulator is

installed in the supply manifold upstream of the

dripline. Inlet pressures from a minimum of 10 psi

to a maximum of 45 psi are recommended. The

second emitter type is the pressure-compensating

Figure 4-17. Pressure manifold and flexible drip lines

prior to trench filling

Source: Ayres Associates.
USEPA Onsite Wastewater Treatment Systems Manual 4-29

Chapter 4: Treatment Processes and Systems

emitter. This emitter discharges at nearly a constant

rate over a wide range of in-line pressures (figure 4-18).

Head losses through driplines are high because of

the small diameter of the tubing and its in-line

emitters, and therefore dripline lengths must be

limited. Manufacturers limit lengths at various

emitter spacings. With turbulent flow emitters, the

discharge from each successive emitter diminishes

in response to pressure loss created by friction or

by elevation changes along the length of the

dripline. With pressure-compensating emitters, the

in-line pressure should not drop below 7 to 10 psi

at the final emitter. The designer is urged to work

with manufacturers to ensure that the system meets

their requirements.

Pressure-compensating emitters are somewhat more

expensive but offer some important advantages

over turbulent-flow emitters for use in onsite

wastewater systems. Pressure-compensating

dripline is better suited for sloping sites or sites

with rolling topography where the dripline cannot

be laid on contour. Turbulent-flow emitters discharge more liquid at lower elevations than the

same emitters at higher elevations. The designer

should limit the difference in discharge rates

between emitters to no more than 10 percent. Also,

because the discharge rates are equal when under

pressure, monitoring flow rates during dosing of a

pressure-compensating dripline network can

provide an effective way to determine whether

leaks or obstructions are present in the network or

emitters. Early detection is important so that simple

and effective corrective actions can be taken.

Usually, injection of a mild bleach solution into the

dripline is effective in restoring emitter performance if clogging is due to biofilms. If this action

proves to be unsuccessful, other corrective actions

are more difficult and costly. An additional advantage of pressure-compensating emitters is that

pressure regulators are not required. Finally, when

operating in their normal pressure range, pressurecompensating emitters are not affected by soil

water pressure in structured soils, which can cause

turbulent-flow emitters to suffer reduced dosing

volumes.

Controlling clogging in drip systems

With small orifices, emitters are susceptible to

clogging. Particulate materials in the wastewater,

soil particulates drawn into an emitter when the

dripline drains following a dose, and biological

slimes that grow within the dripline pose potential

clogging problems. Also, the moisture and nutrients

discharged from the emitters may invite root

intrusion through the emitter. Solutions to these

problems lie in both the design of the dripline and

the design of the distribution network. Emitter

hydrodynamic design and biocide impregnation of

the dripline and emitters help to minimize some of

these problems. Careful network design is also

necessary to provide adequate safeguards. Monitoring allows the operator to identify other problems

such as destruction from burrowing animals.

To control emitter clogging, appropriate engineering controls must be provided. These include

prefiltration of the wastewater, regular dripline

flushing, and vacuum release valves on the network. Prefiltration of the effluent through granular

or mechanical filters is necessary. These filters

should be capable of removing all particulates that

could plug the emitter orifices. Dripline manufacturers recommend that self-cleaning filters be designed

to remove particles larger than 100 to 115 microns.

Despite this disparate experience, pretreatment with

filters is recommended in light of the potential cost

of replacing plugged emitters. Regular cleaning of

the filters is necessary to maintain satisfactory

performance. The backflush water should be

returned to the head of the treatment works.

Figure 4-18. Turbulent-flow and pressure-compensating emitter

discharge rates versus in-line pressure
Chapter 4: Treatment Processes and Systems

4-30 USEPA Onsite Wastewater Treatment Systems Manual

The dripline must be flushed on a regular schedule

to keep it scoured of solids. Flushing is accomplished by opening the flush valve on the return

manifold and increasing the pumping rate to

achieve scouring velocity. Each supplier recommends a velocity and procedure for this process.

The flushing rate and volume must include water

losses (discharge) through the emitters during the

flushing event. Both continuous flushing and timed

flushing are used. However, flushing can add a

significant hydraulic load to the treatment system

and must be considered in the design. If intermittent flushing is practiced, flushing should be

performed at least monthly.

Aspiration of soil particles is another potential

emitter clogging hazard. Draining of the network

following a dosing cycle can create a vacuum in the

network. The vacuum can cause soil particles to be

aspirated into the emitter orifices. To prevent this

from occurring, vacuum relief valves are used. It is

best to install these at the high points of both the

supply and return manifolds.

Placement and layout of drip systems

When drip distribution was introduced, the approach to sizing SWISs using this distribution

method was substantially different from that for

SWISs using other distribution methods. Manufacturer-recommended hydraulic loading rates were

expressed in terms of gallons per day per square

foot of drip distribution footprint area. Typically,

the recommended rates were based on 2-foot

emitter and dripline spacing. Therefore, each

emitter would serve 4 square feet of footprint area.

Because the dripline is commonly plowed into the

soil without surrounding it with porous medium,

the soil around the dripline becomes the actual

infiltration surface. The amount of infiltration

surface provided is approximately 2/3 to 1 square

foot per 5 linear feet of dripline. As a result, the

wastewater loading rate is considerably greater than

the hydraulic loadings recommended for traditional

SWISs. Experience has shown however, that the

hydraulic loading on this surface can be as much as

seven times higher than that of traditional SWIS

designs (Ayres Associates, 1994). This is probably

due to the very narrow geometry, higher levels of

pretreatment, shallow placement, and intermittent

loadings of the trenches, all of which help to

enhance reaeration of the infiltration surface.

The designer must be aware of the differences

between the recommended hydraulic loadings for

drip distribution and those customarily used for

traditional SWISs. The recommended drip distribution loadings are a function of the soil, dripline

spacing, and applied effluent quality. It is necessary

to express the hydraulic loading in terms of the

footprint area because the individual dripline trenches

are not isolated infiltration surfaces. If the emitter

and/or dripline spacing is reduced, the wetting

fronts emanating from each emitter could overlap

and significantly reduce hydraulic performance. Therefore, reducing the emitter and/or dripline spacing should

not reduce the overall required system footprint.

Reducing the spacing might be beneficial for irrigating small areas of turf grass, but the maximum daily

emitter discharge must be reduced proportionately by

adding more dripline to maintain the same footprint

size. Using higher hydraulic loading rates must be

carefully considered in light of secondary boundary

loadings, which could result in excessive ground

water mounding (see chapter 5). Further, the instantaneous hydraulic loading during a dose must be

controlled because storage is not provided in the

dripline trench. If the dose volume is too high, the

wastewater can erupt at the ground surface.

Layout of the drip distribution network must be

considered carefully. Two important consequences

of the network layout are the impacts on dose

pump sizing necessary to achieve adequate flushing

flows and the extent of localized overloading due

to internal dripline drainage. Flushing flow rates

are a function of the number of manifold/dripline

connections: More connections create a need for

greater flushing flows, which require a larger

pump. To minimize the flushing flow rate, the

length of each dripline should be made as long as

possible in accordance with the manufacturer’s

recommendations. To fit the landscape, the dripline

can be looped between the supply and return

manifolds (figure 4-19). Consideration should also

be given to dividing the network into more than

one cell to reduce the number of connections in an

individual network. A computer program has been

developed to evaluate and optimize the hydraulic

design for adequate flushing flows of dripline

networks that use pressure-compensating emitters

(Berkowitz and Harman, 1994).

Internal drainage that occurs following each dose

or when the soils around the dripline are saturated
USEPA Onsite Wastewater Treatment Systems Manual 4-31

Chapter 4: Treatment Processes and Systems

can cause significant hydraulic overloading to

lower portions of the SWIS. Following a dose

cycle, the dripline drains through the emitters. On

sloping sites, the upper driplines drain to the lower

driplines, where hydraulic overloading can occur.

Any free water around the dripline can enter

through an emitter and drain to the lowest elevation. Each of these events needs to be avoided as

much as possible through design. The designer can

minimize internal drainage problems by isolating

the driplines from each other in a cell, by aligning

the supply and return manifolds with the site’s

contours. A further safeguard is to limit the number

of doses per day while keeping the instantaneous

hydraulic loadings to a minimum so the dripline

trench is not flooded following a dose. This tradeoff is best addressed by determining the maximum

hydraulic loading and adjusting the number of

doses to fit this dosing volume.

Freezing of dripline networks has occurred in

severe winter climates. Limited experience indicates

that shallow burial depths together with a lack of

uncompacted snow cover or other insulating

materials might lead to freezing. In severe winter

climates, the burial depth of dripline should be

increased appropriately and a good turf grass

established over the network. Mulching the area the

winter after construction or every winter should be

considered. Also, it is good practice to install the

vacuum release valves below grade and insulate the

air space around them. Although experience with

drip distribution in cold climates is limited, these

safeguards should provide adequate protection.

Dosing methods

Two methods of dosing have been used (table 4-6).

With on-demand dosing, the wastewater effluent

rises to a preset level in the dose tank and the pump

or siphon is activated by a float switch or other

mechanism to initiate discharge (figure 4-20).

During peak-flow periods, dosing is frequent with

little time between doses for the infiltration system

to drain and the subsoil to reaerate. During lowflow periods, dosing intervals are long, which can

be beneficial in controlling biomat development

but is inefficient in using the hydraulic capacity of

the system.

Figure 4-19. Dripline layout on a site with trees

Source: Adapted from American Manufacturing, 2001.
Chapter 4: Treatment Processes and Systems

4-32 USEPA Onsite Wastewater Treatment Systems Manual

Timed dosing overcomes some of the shortcomings

of on-demand dosing. Timers are used to turn the

pump on and off at specified intervals so that only

a predetermined volume of wastewater is discharged

with each dose. Timed dosing has two distinct

advantages over on-demand dosing. First, the doses

can be spaced evenly over the entire 24-hour day to

optimize the use of the soil’s treatment capacity.

Second, the infiltration system receives no more

than its design flow each day. Clear water infiltration, leaking plumbing fixtures, or excessive water

use are detected before the excess flow is discharged

to the infiltration system because the dose tank will

eventually fill to its high water alarm level. At that

point, the owner has the option of calling a septage

pumper to empty the tanks or activating the pump to

dose the system until the problem is diagnosed and

corrected. Unlike on-demand dosing, timed dosing

requires that the dose tank be sized to store peak

flows until they can be pumped (see sidebar).

Dosing frequency and volume are two important

design considerations. Frequent, small doses are

preferred over large doses one or two times per

day. However, doses should not be so frequent that

distribution is poor. This is particularly true with

either of the pressure distribution networks. With

pressure networks, uniform distribution does not

occur until the entire network is pressurized. To

ensure pressurization and to minimize unequal

discharges from the orifices during filling and

draining, a dose volume equal to five times the

network volume is a good rule of thumb. Thus,

doses can be smaller and more frequent with dripline

networks than with rigid pipe networks because the

volume of drip distribution networks is smaller.

4.4.8 SWIS media

A porous medium is placed below and around SWIS

distribution piping to expand the infiltration surface

area of the excavation exposed to the applied wastewater. This approach is similar in most SWIS designs,

except when drip distribution or aggregate-free

designs are used. In addition, the medium also

supports the excavation sidewalls, provides storage of

peak wastewater flows, minimizes erosion of the

infiltration surface by dissipating the energy of the

influent flow, and provides some protection for the

piping from freezing and root penetration.

Traditionally, washed gravel or crushed rock,

typically ranging from 3⁄4 to 21⁄2 inches in diameter, has been used as the porous medium. The

rock should be durable, resistant to slaking and

dissolution, and free of fine particles. A hardness

of at least 3 on the Moh’s scale of hardness is

suggested. Rock that can scratch a copper penny

without leaving any residual meets this criterion.

It is important that the medium be washed to

remove fine particles. Fines from insufficiently

washed rock have been shown to result in significant reductions in infiltration rates (Amerson et

al., 1991). In all applications where gravel is

used, it must be properly graded and washed.

Improperly washed gravel can contribute fines and

other material that can plug voids in the infiltrative surface and reduce hydraulic capability.

Gravel that is embedded into clay or fine soils

during placement can have the same effect.

In addition to natural aggregates, gravelless systems

have been widely used as alternative SWIS medium

(see preceding section). These systems take many

forms, including open-bottomed chambers, fabricwrapped pipe, and synthetic materials such as

expanded polystyrene foam chips, as described in

the preceding section. Systems that provide an open

chamber are sometimes referred to as “aggregatefree” systems, to distinguish them from others that

substitute lightweight medium for gravel or stone.

These systems provide a suitable substitute in

locales where gravel is not available or affordable.

Some systems (polyethylene chambers and lightFigure 4-20. Pumping tank (generic)

Source: Purdue University, 1990
USEPA Onsite Wastewater Treatment Systems Manual 4-33

Chapter 4: Treatment Processes and Systems

Dose tank sizing for timed dosing

Timed dosing to a SWIS is to be used in an onsite system serving a restaurant in a summer resort area. Timed

dosing will equalize the flows, enhancing treatment in the soil and reducing the required size of the SWIS.

The restaurant serves meals from 11 a.m. to 12 midnight Tuesday through Saturday and from

9 a.m. to 2 p.m. Sundays. The largest number of meals is served during the summer weekends. The restaurant is

closed on Mondays. The metered water use is as follows:

Average weekly water use (summer) 17,500 gal

Peak weekend water use (4 p.m. Friday to 2 p.m. Sunday) 9,500 gal

The dose tank will be sized to equalize flows over a 7-day period. The dosing frequency is to be six times daily or

one dose every 4 hours. Therefore, the dose volume will be

Dose volume = 17,500 gal/wk  ̧ (7 d/wk x 6 doses/day) = 417 gal/dose

The necessary volume of the dose tank to store the peak flows and equalize the flow to the SWIS over the 7-day

week can be determined graphically.

The accumulated water use over the week and the daily dosing rate (6 doses/day x 417 gal/dose = 2,500 gpd) is

plotted on the graph. Lines parallel to the dosing rate are drawn tangent to points 1 and 2 representing the

maximum deviations of the water use line above and below the dosing rate line. The volume represented by the

difference between the two parallel lines is the tank volume needed to achieve flow equalization. A 4,500-gallon

tank would be required.

Both siphons and pumps can be used for dosing distribution networks. Only drip distribution networks cannot be

dosed by siphons because of the higher required operating pressures and the need to control instantaneous

hydraulic loadings (dose volume). Siphons can be used where power is not available and elevation is adequate to

install the siphon sufficiently above the distribution network to overcome friction losses in the forcemain and

network. Care must be taken in their selection and installation to ensure proper performance. Also, owners must

be aware that siphon systems require routine monitoring and occasional maintenance. “Dribbling” can occur when

the siphon bell becomes saturated, suspending dosing and allowing the wastewater effluent to trickle out under

the bell. Dribbling can occur because of leaks in the bell or a siphon out of adjustment. Today, pumps are favored

over siphons because of the greater flexibility in site selection and dosing regime.

Source: Ayres Associates.
Chapter 4: Treatment Processes and Systems

4-34 USEPA Onsite Wastewater Treatment Systems Manual

weight aggregate systems) can also offer substantial

advantages in terms of reduced site disruption over

the traditional gravel because their light weight

makes them easy to handle without the use of

heavy equipment. These advantages reduce labor

costs, limit damage to the property by machinery,

and allow construction on difficult sites where

conventional medium could not reasonably be used.

4.5 Construction management and

contingency options

Onsite wastewater systems can and do fail to

perform at times. To avoid threats to public health

and the environment during periods when a system

malfunctions hydraulically, contingency plans

should be made to permit continued use of the

system until appropriate remedial actions can be

taken. Contingency options should be considered

during design so that the appropriate measures are

designed into the original system. Table 4-8 lists

common contingency options.

4.5.1 Construction considerations

Construction practices are critical to the performance of SWISs. Satisfactory SWIS performance

depends on maintaining soil porosity. Construction activities can significantly reduce the porosity

and cause SWISs to hydraulically fail soon after

being brought into service. Good construction

practices should carefully consider site protection

before and during construction, site preparation,

and construction equipment selection and use.

Good construction practices for at-grade and

mound systems can be found elsewhere (Converse

and Tyler, 2000; Converse et al., 1990). Many of

them, however, are similar to those described in

the following subsections.

Site protection

Construction of the onsite wastewater system is

often only one of many construction activities that

occur on a property. If not protected against

intrusion, the site designated for the onsite system

can be damaged by other, unrelated construction

Table 4-8. Contingency options for SWIS malfunctions
USEPA Onsite Wastewater Treatment Systems Manual 4-35

Chapter 4: Treatment Processes and Systems

activities. Therefore, the site should be staked and

roped off before any construction activities begin

to make others aware of the site and to keep traffic

and materials stockpiles off the site.

The designer should anticipate what activities will

be necessary during construction and designate

acceptable areas for them to occur. Site access

points and areas for traffic lanes, material stockpiling, and equipment parking should be designated

on the drawings for the contractor.

Site preparation

Site preparation activities include clearing and

surface preparation for filling. Before these activities are begun, the soil moisture should be determined. In nongranular soils, compaction will occur

if the soil is near its plastic limit. This can be tested

by removing a sample of soil and rolling it between

the palms of the hands. If the soil fails to form a

“rope” the soil is sufficiently dry to proceed.

However, constant care should be taken to avoid

soil disturbance as much as possible.

Clearing

Clearing should be limited to mowing and raking

because the surface should be only minimally

disturbed. If trees must be removed, they should be

cut at the base of the trunk and removed without

heavy machinery. If it is necessary to remove the

stumps, they should be ground out. Grubbing of

the site (mechanically raking away roots) should be

avoided. If the site is to be filled, the surface

should be moldboard- or chisel-plowed parallel to

the contour (usually to a depth of 7 to 10 inches)

when the soil is sufficiently dry to ensure maximum vertical permeability. The organic layer

should not be removed. Scarifying the surface with

the teeth of a backhoe bucket is not sufficient.

Excavation

Excavation activities can cause significant reductions in soil porosity and permeability (Tyler et al.,

1985). Compaction and smearing of the soil

infiltrative surface occur from equipment traffic

and vibration, scraping actions of the equipment, and

placement of the SWIS medium on the infiltration

surface. Lightweight backhoes are most commonly

used. Front-end loaders and blades should not be used

because of their scraping action. All efforts should

be made to avoid any disturbance to the exposed

infiltration surface. Equipment should be kept off

the infiltration field. Before the SWIS medium is

installed, any smeared areas should be scarified and

the surface gently raked. If gravel or crushed rock

is to be used for SWIS medium, the rock should be

placed in the trench by using the backhoe bucket

rather than dumping it directly from the truck. If

damage occurs, it might be possible to restore the

area, but only by removing the compacted layer. It

might be necessary to remove as much as 4 inches

of soil to regain the natural soil porosity and

permeability (Tyler et al., 1985). Consequences of

the removal of this amount of soil over the entire

infiltration surface can be significant. It will reduce

the separation distance to the restrictive horizon

and could place the infiltration surface in an

unacceptable soil horizon.

To avoid potential soil damage during construction,

the soil below the proposed infiltration surface

elevation must be below its plastic limit. This

should be tested before excavation begins. Also,

excavation should be scheduled only when the

infiltration surface can be covered the same day to

avoid loss of permeability from wind-blown silt or

raindrop impact. Another solution is to use lightweight gravelless systems, which reduce the

damage and speed the construction process.

Before leaving the site, the area around the site

should be graded to divert surface runoff from the

SWIS area. The backfill over the infiltration

surface should be mounded slightly to account for

settling and eliminate depressions over the system

that can pond water. Finally, the area should be

seeded and mulched.

4.5.2 Operation, maintenance, and

monitoring

Subsurface wastewater infiltration systems require

little operator intervention. Table 4-9 lists typical

operation, maintenance, and monitoring activities

that should be performed. However, more complex

pretreatment, larger and more variable flows, and

higher-risk installations increase the need for

maintenance and monitoring. More information is

provided in the USEPA draft Guidelines for Onsite/

Decentralized Wastewater Systems (2000) and in the

chapter 4 fact sheets.
Chapter 4: Treatment Processes and Systems

4-36 USEPA Onsite Wastewater Treatment Systems Manual

4.5.3 Considerations for large and

commercial systems

Designs for systems treating larger flows follow the

same guidelines used for residential systems, but they

must address characteristics of the wastewater to be

treated, site characteristics, infiltration surface sizing,

and contingency planning more comprehensively.

Wastewater characteristics

Wastewaters from cluster systems serving multiple

homes or commercial establishments can differ

substantially in flow pattern and waste strength from

wastewaters generated by single family residences.

The ratio of peak to average daily flow from residential clusters is typically much lower than what is

typical from single residences. This is because the

moderating effect associated with combining multiple

water use patterns reduces the daily variation in flow.

Commercial systems, on the other hand, can vary

significantly in wastewater strength. Typically,

restaurants have high concentrations of grease and

BOD, laundromats have high sodium and suspended

solids concentrations, and toilet facilities at parks

and rest areas have higher concentrations of BOD,

TSS, and nitrogen. These differences in daily flow

patterns and waste strengths must be dealt with in

the design of SWISs. Therefore, it is important to

characterize the wastewater fully before initiating

design (see chapter 3).

Site characteristics

The proposed site for a SWIS that will treat wastewater from a cluster of homes or a commercial

establishment must be evaluated more rigorously

than a single-residence site because of the larger

volume of water that is to be applied and the

greater need to determine hydraulic gradients and

direction. SWIS discharges can be from 10 to more

than 100 times the amount of water that the soil

infiltration surface typically receives from precipitation. For example, assume that an area receives an

average of 40 inches of rainfall per year. Of that, less

than 25 percent (about 10 inches annually) infiltrates

and even less percolates to the water table. A wastewater infiltration system is designed to infiltrate

0.4 to 1.6 inches per day, or 146 to 584 inches per

year. Assuming actual system flows are 30 percent

of design flows, this is reduced to 44 to 175 inches

per year even under this conservative approach.

The soils associated with small systems can usually

accommodate these additional flows. However,

systems that treat larger flows load wastewaters to

the soil over a greater area and might exceed the

site’s capacity to accept the wastewater. Restrictive

horizons that may inhibit deep percolation need to

Table 4-9. Operation, maintenance, and monitoring activities
USEPA Onsite Wastewater Treatment Systems Manual 4-37

Chapter 4: Treatment Processes and Systems

be identified before design. Ground water mounding

analysis should be performed to determine whether

the hydraulic loading to the saturated zone (secondary design boundary), rather than the loading to the

infiltration surface, controls system sizing (see Chapter 5). If the secondary boundary controls design, the

size of the infiltration surface, its geometry, and even

how wastewater is applied will be affected.

Infiltration surface sizing

Selection of the design flow is a very important

consideration in infiltration surface sizing. State

codified design flows for residential systems

typically are 2 to 5 times greater than the average

daily flow actually generated in the home. This

occurs because the design flow is usually based on

the number of bedrooms rather than the number of

occupants. As a result, the actual daily flow is often

a small fraction of the design flow.

This is not the case when the per capita flows for

the population served or metered flows are used as

the design flow. In such instances, the ratio of

design flow to actual daily flow can approach

unity. This is because the same factors of safety are

typically not used to determine the design flow. In

itself, this is not a problem. The problem arises

when the metered or averaged hydraulic loading

rates are used to size the infiltration surface. These

rates can be more than two times what the soil

below the undersized system is actually able to

accept. As a result, SWISs would be significantly

undersized. This problem is exacerbated where the

waste strength is high.

To avoid the problem of undersizing the infiltration

surface, designs must compensate in some way.

Factors of safety of up to 2 or more could be

applied to accurate flow estimates, but the more

common practice is to design multiple cells that

provide 150 to 200 percent of the total estimated

infiltration surface needed. Multiple cells are a

good approach because the cells can be rotated into

service on a regular schedule that allows the cells

taken out of service to rest and rejuvenate their

hydraulic capacity. Further, the system provides

standby capacity that can be used when malfunctions occur, and distribution networks are smaller

to permit smaller and more frequent dosing,

thereby maximizing oxygen transfer and the

hydraulic capacity of the site. For high-strength

wastewaters, advanced pretreatment can be specified or the infiltration surface loadings can be

adjusted (see Special Issue Fact Sheet 4).

Contingency planning

Malfunctions of systems that treat larger flows can

create significant public health and environmental

hazards. Therefore, adequate contingency planning

is more critical for these systems than for residential systems. Standby infiltration cells, timed

dosing, and flow monitoring are key design

elements that should be included. Also, professional

management should be required.

4.6 Septic tanks

The septic tank is the most commonly used wastewater pretreatment unit for onsite wastewater systems.

Tanks may be used alone or in combination with

other processes to treat raw wastewater before it is

discharged to a subsurface infiltration system. The

tank provides primary treatment by creating quiescent

conditions inside a covered, watertight rectangular,

oval, or cylindrical vessel, which is typically buried.

In addition to primary treatment, the septic tank stores

and partially digests settled and floating organic solids

in sludge and scum layers. This can reduce the sludge

and scum volumes by as much as 40 percent, and it

conditions the wastewater by hydrolyzing organic

molecules for subsequent treatment in the soil or by

other unit processes (Baumann et al., 1978). Gases

generated from digestion of the organics are vented

back through the building sewer and out of the house

plumbing stack vent. Inlet structures are designed to

limit short circuiting of incoming wastewater across

the tank to the outlet, while outlet structures (e.g., a

sanitary “tee” fitting) retain the sludge and scum

layers in the tank and draw effluent only from the

clarified zone between the sludge and scum layers.

The outlet should be fitted with an effluent screen

(commonly called a septic tank filter) to retain larger

solids that might be carried in the effluent to the

SWIS, where it could contribute to clogging and

eventual system failure. Inspection ports and manways

are provided in the tank cover to allow access for

periodically removing the tank contents, including the

accumulated scum and sludge (figure 4-21). A

diagram of a two-compartment tank is shown later

in this section.

Septic tanks are used as the first or only pretreatment step in nearly all onsite systems regardless of
Chapter 4: Treatment Processes and Systems

4-38 USEPA Onsite Wastewater Treatment Systems Manual

daily wastewater flow rate or strength. Other

mechanical pretreatment units may be substituted for

septic tanks, but even when these are used septic

tanks often precede them. The tanks passively

provide suspended solids removal, solids storage

and digestion, and some peak flow attenuation.

4.6.1 Treatment

A septic tank removes many of the settleable solids,

oils, greases, and floating debris in the raw wastewater, achieving 60 to 80 percent removal

(Baumann et al., 1978; Boyer and Rock, 1992;

University of Wisconsin, 1978). The solids removed

are stored in sludge and scum layers, where they

undergo liquefaction. During liquefaction, the first

step in the digestion process, acid-forming bacteria

partially digest the solids by hydrolyzing the

proteins and converting them to volatile fatty acids,

most of which are dissolved in the water phase. The

volatile fatty acids still exert much of the biochemical

oxygen demand that was originally in the organic

suspended solids. Because these acids are in the

dissolved form, they are able to pass from the tank in

the effluent stream, reducing the BOD removal

efficiency of septic tanks compared to primary sedimentation. Typical septic tank BOD removal efficiencies are 30 to 50 percent (Boyer and Rock, 1992;

University of Wisconsin, 1978; see table 4-10). Complete digestion, in which the volatile fatty acids are

converted to methane, could reduce the amount of BOD

released by the tank, but it usually does not occur to a

significant extent because wastewater temperatures in

septic tanks are typically well below the optimum

temperature for methane-producing bacteria.

Gases that form from the microbial action in the

tank rise in the wastewater column. The rising gas

bubbles disturb the quiescent wastewater column,

which can reduce the settling efficiency of the tank.

They also dislodge colloidal particles in the sludge

blanket so they can escape in the water column. At

the same time, however, they can carry active anaerobic and facultative microorganisms that might help

to treat colloidal and dissolved solids present in the

wastewater column (Baumann and Babbit, 1953).

Septic tank effluent varies naturally in quality

depending on the characteristics of the wastewater

and condition of the tank. Documented effluent

quality from single-family homes, small communities and cluster systems, and various commercial

septic tanks is presented in tables 4-10 through 4-12.

Table 4-10. Characteristics of domestic septic tank effluent

Figure 4-21. Profile of a single-compartment septic

tank with outlet screen
USEPA Onsite Wastewater Treatment Systems Manual 4-39

Chapter 4: Treatment Processes and Systems

4.6.2 Design considerations

The primary purpose of a septic tank is to provide

suspended solids and oil/grease removal through

sedimentation and flotation. The important factor

to achieving good sedimentation is maintaining

quiescent conditions. This is accomplished by

providing a long wastewater residence time in the

septic tank. Tank volume, geometry, and compartmentalization affect the residence time.

Volume

Septic tanks must have sufficient volume to provide

an adequate hydraulic residence time for sedimentation. Hydraulic residence times of 6 to 24 hours have

been recommended (Baumann and Babbitt, 1953:

Kinnicutt et al., 1910). However, actual hydraulic

residence times can vary significantly from tank to

tank because of differences in geometry, depth, and

inlet and outlet configurations (Baumann and Babbitt,

1953). Sludge and scum also affect the residence

time, reducing it as the solids accumulate.

Table 4-12. Average septic tank effluent concentrations of selected parameters from various commercial establishmentsa

Table 4-11. Average septic tank effluent concentrations for selected parameters from small community and cluster systems
Chapter 4: Treatment Processes and Systems

4-40 USEPA Onsite Wastewater Treatment Systems Manual

Most state and national plumbing codes specify the

tank volume to be used based on the building size

or estimated peak daily flow of wastewater. Table

4-13 presents the tank volumes recommended in

the International Private Sewage Disposal Code

specified for one- and two-family residences (ICC,

1995). The volumes specified are typical of most

local codes, but in many jurisdictions the minimum

tank volume has been increased to 1,000 gallons or

more. For buildings other than one- or two-family

residential homes, the rule of thumb often used for

sizing tanks is to use two to three times the estimated design flow. This conservative rule of thumb

is based on maintaining a 24-hour minimum

hydraulic retention time when the tank is ready for

pumping, for example, when the tank is one-half to

two-thirds full of sludge and scum.

Geometry

Tank geometry affects the hydraulic residence time

in the tank. The length-to-width ratio and liquid

depth are important considerations. Elongated tanks

with length-to-width ratios of 3:1 and greater have

been shown to reduce short-circuiting of the raw

wastewater across the tank and improve suspended

solids removal (Ludwig, 1950). Prefabricated tanks

generally are available in rectangular, oval, and

cylindrical (horizontal or vertical) shapes. Vertical

cylindrical tanks can be the least effective because

of the shorter distance between the inlets and

outlets. Baffles are recommended.

Among tanks of equal liquid volumes, the tank

with shallower liquid depths better reduces peak

outflow rates and velocities, so solids are less likely

to remain in suspension and be carried out of the

tank in the effluent. This is because the shallow

tank has a larger surface area. Inflows to the tank

cause less of a liquid rise because of the larger

surface area. The rate of flow exiting the tank

(over a weir or through a pipe invert) is proporFigure 4-22. Two-compartment tank with effluent screen and surface risers

Source: Washington Department of Health, 1998.

Table 4-13. Septic tank capacities for one- and twofamily dwellings (ICC, 1995).
USEPA Onsite Wastewater Treatment Systems Manual 4-41

Chapter 4: Treatment Processes and Systems

tional to the height of the water surface over the

invert (Baumann et al., 1978; Jones, 1975). Also,

the depth of excavation necessary is reduced with

shallow tanks, which helps to avoid saturated

horizons and lessens the potential for ground water

infiltration or tank flotation. A typically specified

minimum liquid depth below the outlet invert is 36

inches. Shallower depths can disturb the sludge

blanket and, therefore, require more frequent

pumping.

Compartmentalization

Compartmentalized tanks (figure 4-23) or tanks

placed in series provide better suspended solids

removal than single-compartment tanks alone,

although results from different studies vary

(Baumann and Babbitt, 1953; Boyer and Rock,

1992; Weibel et al., 1949, 1954; University of

Wisconsin, 1978). If two compartments are used,

better suspended solids removal rates are achieved

if the first compartment is equal to one-half to twothirds the total tank volume (Weibel et al., 1949,

1954). An air vent between compartments must be

provided to allow both compartments to vent. The

primary advantage of these configurations is when

gas generated from organic solids digestion in the

first compartment is separated from subsequent

compartments.

Inlets and outlets

The inlet and outlet of a septic tank are designed to

enhance tank performance. Their respective invert

elevations should provide at least a 2- to 3-inch

drop across the tank to ensure that the building

sewer does not become flooded and obstructed

during high wastewater flows (figure 4-24). A clear

space of at least 9 inches should be provided above

the liquid depth (outlet invert) to allow for scum

storage and ventilation. Both the inlet and outlet

are commonly baffled. Plastic sanitary tees are the

most commonly used baffles. Curtain baffles

(concrete baffles cast to the tank wall and fiberglass

or plastic baffles bolted to the tank wall) have also

been used. The use of gasket materials that achieve

a watertight joint with the tank wall makes plastic

sanitary tees easy to adjust, repair, or equip with

effluent screens or filters. The use of a removable,

cleanable effluent screen connected to the outlet is

strongly recommended.

The inlet baffle is designed to prevent shortcircuiting of the flow to the outlet by dissipating

the energy of the influent flow and deflecting it

downward into the tank. The rising leg of the tee

should extend at least 6 inches above the liquid

level to prevent the scum layer from plugging the

inlet. It should be open at the top to allow venting

of the tank through the building sewer and out the

plumbing stack vent. The descending leg should

extend well into the clear space between the sludge

and scum layers, but not more than about 30 to 40

percent of the liquid depth. The volume of the

descending leg should not be larger than 2 to 3

gallons so that it is completely flushed to expel

floating materials that could cake the inlet. For this

reason, curtain baffles should be avoided.

Figure 4-23. Examples of septic tank effluent screens/filters

Source: Adapted from various manufacturers’ drawings.
Chapter 4: Treatment Processes and Systems

4-42 USEPA Onsite Wastewater Treatment Systems Manual

The outlet baffle is designed to draw effluent from

the clear zone between the sludge and scum layers.

The rising leg of the tee should extend 6 inches

above the liquid level to prevent the scum layer

from escaping the tank. The descending leg should

extend to 30 or 40 percent of the liquid depth.

Effluent screens (commonly called septic tank

filters), which can be fitted to septic tank outlets,

are commercially available. Screens prevent solids

that either are buoyant or are resuspended from the

scum or sludge layers from passing out of the tank

(figures 4-22 and 4-23). Mesh, slotted screens, and

stacked plates with openings from 1/32 to 1/8 inch

are available. Usually, the screens can be fitted into

the existing outlet tee or retrofitted directly into the

outlet. An access port directly above the outlet is

required so the screen can be removed for inspection and cleaning.

Quality-assured, reliable test results have not shown

conclusively that effluent screens result in effluents

with significantly lower suspended solids and BOD

concentrations. However, they provide an excellent,

low-cost safeguard against neutral-buoyancy solids

and high suspended solids in the tank effluent

resulting from solids digestion or other upsets.

Also, as the effluent screens clog over time, slower

draining and flushing of home fixtures may alert

homeowners of the need for maintenance before

complete blockage occurs.

Tank access

Access to the septic tank is necessary for pumping

septage, observing the inlet and outlet baffles, and

servicing the effluent screen. Both manways and

inspection ports are used. Manways are large

openings, 18 to 24 inches in diameter or square. At

least one that can provide access to the entire tank

for septage removal is needed. If the system is

compartmentalized, each compartment requires a

manway. They are located over the inlet, the outlet,

or the center of the tank. Typically, in the past

manway covers were required to be buried under

state and local codes. However, they should be

above grade and fitted with an airtight, lockable

cover so they can be accessed quickly and easily.

Inspection ports are 8 inches or larger in diameter

and located over both the inlet and the outlet unless

a manway is used. They should be extended above

grade and securely capped.

(CAUTION: The screen should not be removed for

inspection or cleaning without first plugging the

outlet or pumping the tank to lower the liquid level

below the outlet invert. Solids retained on the screen

can slough off as the screen is removed. These

solids will pass through the outlet and into the

SWIS unless precautions are taken. This caution

should be made clear in homeowner instructions

and on notices posted at the access port.)

Septic tank designs for large wastewater flows do

not differ from designs for small systems. However, it is suggested that multiple compartments or

tanks in series be used and that effluent screens be

attached to the tank outlet. Access ports and

manways should be brought to grade and provided

with locking covers for all large systems.

Construction materials

Septic tanks smaller than 6,000 gallons are typically premanufactured; larger tanks are constructed

in place. The materials used in premanufactured

tanks include concrete, fiberglass, polyethylene,

and coated steel. Precast concrete tanks are by far

the most common, but fiberglass and plastic tanks

are gaining popularity. The lighter weight fiberglass and plastic tanks can be shipped longer

distances and set in place without cranes. Concrete

tanks, on the other hand, are less susceptible to

collapse and flotation. Coated steel tanks are no

longer widely used because they corrode easily.

Tanks constructed in place are typically made of

concrete.

Tanks constructed of fiberglass-reinforced polyester

(FRP) usually have a wall thickness of about 1/4

inch (6 millimeters). Most are gel- or resin-coated

to provide a smooth finish and prevent glass fibers

from becoming exposed, which can cause wicking.

Polyethylene tanks are more flexible than FRP

tanks and can deform to a shape of structural

weakness if not properly designed. Concrete tank

walls are usually about 4 inches thick and reinforced with no. 5 rods on 8-inch (20-centimeter)

centers. Sulfuric acid and hydrogen sulfide, both of

which are present in varying concentrations in

septic tank effluent, can corrode exposed rods and

the concrete itself over time. Some plastics (e.g.,

polyvinyl chloride, polyethylene, but not nylon)

are virtually unaffected by acids and hydrogen

sulfide (USEPA, 1991).
USEPA Onsite Wastewater Treatment Systems Manual 4-43

Chapter 4: Treatment Processes and Systems

Quality construction is critical to proper performance. Tanks must be properly designed, reinforced, and constructed of the proper mix of

materials so they can meet anticipated loads

without cracking or collapsing. All joints must be

watertight and flexible to accommodate soil

conditions. For concrete tank manufacturing, a

“best practices manual” can be purchased from the

National Pre-Cast Concrete Association (NPCA,

1998). Also, a Standard Specification for Precast

Concrete Septic Tanks (C 1227) has been published

by the American Society for Testing and Materials

(ASTM, 1998).

Watertightness

Watertightness of the septic tank is critical to the

performance of the entire onsite wastewater system.

Leaks, whether exfiltrating or infiltrating, are

serious. Infiltration of clear water to the tank from

the building storm sewer or ground water adds to

the hydraulic load of the system and can upset

subsequent treatment processes. Exfiltration can

threaten ground water quality with partially treated

wastewater and can lower the liquid level below the

outlet baffle so it and subsequent processes can

become fouled with scum. Also, leaks can cause the

tank to collapse.

Tank joints should be designed for watertightness.

Two-piece tanks and tanks with separate covers

should be designed with tongue and groove or lap

joints (figure 4-24). Manway covers should have

similar joints. High-quality, preformed joint sealers

should be used to achieve a watertight seal. They

should be workable over a wide temperature range

and should adhere to clean, dry surfaces; they must

not shrink, harden, or oxidize. Seals should meet

the minimum compression and other requirements

prescribed by the seal manufacturer. Pipe and

inspection port joints should have cast-in rubber

boots or compression seals.

Septic tanks should be tested for watertightness

using hydrostatic or vacuum tests, and manway

risers and inspection ports should be included in the

test. The professional association representing the

materials industry of the type of tank construction

(e.g., the National Pre-cast Concrete Association)

should be contacted to establish the appropriate

testing criteria and procedures. Test criteria for

precast concrete are presented in table 4-14.

4.6.3 Construction considerations

Important construction considerations include tank

location, bedding and backfilling, watertightness,

and flotation prevention, especially with nonconcrete tanks. Roof drains, surface water runoff,

and other clear water sources must not be routed to

the septic tank. Attention to these considerations

Table 4-14. Watertightness testing procedure/criteria for precast concrete tanks

Figure 4-24. Tongue and groove joint and sealer

Source: Ayres Associates
Chapter 4: Treatment Processes and Systems

4-44 USEPA Onsite Wastewater Treatment Systems Manual

will help to ensure that the tank performs as

intended.

Location

The tank should be located where it can be accessed

easily for septage removal and sited away from

drainage swales or depressions where water can

collect. Local codes must be consulted regarding

minimum horizontal setback distances from

buildings, property boundaries, wells, water lines,

and the like.

Bedding and backfilling

The tank should rest on a uniform bearing surface.

It is good practice to provide a level, granular base

for the tank. The underlying soils must be capable

of bearing the weight of the tank and its contents.

Soils with a high organic content or containing

large boulders or massive rock edges are not

suitable.

After setting the tank, leveling, and joining the

building sewer and effluent line, the tank can be

backfilled. The backfill material should be freeflowing and free of stones larger than 3 inches in

diameter, debris, ice, or snow. It should be added in

lifts and each lift compacted. In fine-textured soils

such as silts, silt loams, clay loams, and clay,

imported granular material should be used. This is

a must where freeze and thaw cycles are common

because the soil movement during such cycles can

work tank joints open. This is a significant concern

when using plastic and fiberglass tanks.

The specific bedding and backfilling requirements

vary with the shape and material of the tank. The

manufacturer should be consulted for acceptable

materials and procedures.

Watertightness

All joints must be sealed properly, including tank

joints (sections and covers if not a monolithic

tank), inlets, outlets, manways, and risers (ASTM,

1993; NPCA, 1998). The joints should be clean

and dry before applying the joint sealer. Only highquality joint sealers should be used (see previous

section). Backfilling should not proceed until the

sealant setup period is completed. After all joints

have been made and have cured, a watertightness

test should be performed (see table 4-14 for precast

concrete tanks). Risers should be tested.

Flotation prevention

If the tank is set where the soil can be saturated,

tank flotation may occur, particularly when the

tank is empty (e.g., recently pumped dose tanks or

septic tank after septage removal). Tank manufacturers should be consulted for appropriate

antiflotation devices.

4.6.4 Operation and maintenance

The septic tank is a passive treatment unit that

typically requires little operator intervention.

Regular inspections, septage pumping, and periodic

cleaning of the effluent filter or screen are the only

operation and maintenance requirements. Commercially available microbiological and enzyme

additives are promoted to reduce sludge and scum

accumulations in septic tanks. They are not necessary for the septic tank to function properly when

treating domestic wastewaters. Results from studies

to evaluate their effectiveness have failed to prove

their cost-effectiveness for residential application.

For most products, concentrations of suspended

solids and BOD in the septic tank effluent increase

upon their use, posing a threat to SWIS performance. No additive made up of organic solvents or

strong alkali chemicals should be used because they

pose a potential threat to soil structure and ground

water.

Inspections

Inspections are performed to observe sludge and

scum accumulations, structural soundness, watertightness, and condition of the inlet and outlet

baffles and screens. (Warning: In performing

inspections or other maintenance, the tank should

not be entered. The septic tank is a confined space

and entering can be extremely hazardous because of

toxic gases and/or insufficient oxygen.)

Sludge and scum accumulations

As wastewater passes through and is partially

treated in the septic tank over the years, the layers

of floatable material (scum) and settleable material

(sludge) increase in thickness and gradually reduce

the amount of space available for clarified waste-
USEPA Onsite Wastewater Treatment Systems Manual 4-45

Chapter 4: Treatment Processes and Systems

water. If the sludge layer rises to the bottom of the

effluent T-pipe, solids can be drawn through the

effluent port and transported into the infiltration

field, increasing the risk of clogging. Likewise, if

the bottom of the thickening scum layer moves

lower than the bottom of the effluent T-pipe, oils

and other scum material can be drawn into the

piping that discharges to the infiltration field.

Various devices are commercially available to

measure sludge and scum depths. The scum layer

should not extend above the top or below the

bottom of either the inlet or outlet tees. The top of

the sludge layer should be at least 1 foot below the

bottom of either tee or baffle. Usually, the sludge

depth is greatest below the inlet baffle. The scum

layer bottom must not be less than 3 inches above

the bottom of the outlet tee or baffle. If any of

these conditions are present, there is a risk that

wastewater solids will plug the tank inlet or be

carried out in the tank effluent and begin to clog

the SWIS.

Structural soundness and watertightness

Structural soundness and watertightness are best

observed after the septage has been pumped from

the tank. The interior tank surfaces should be

inspected for deterioration, such as pitting,

spalling, delamination, and so forth and for cracks

and holes. The presence of roots, for example,

indicates tank cracks or open joints. These observations should be made with a mirror and bright

light. Watertightness can be checked by observing

the liquid level (before pumping), observing all

joints for seeping water or roots, and listening for

running or dripping water. Before pumping, the

liquid level of the tank should be at the outlet

invert level. If the liquid level is below the outlet

invert, exfiltration is occurring. If it is above, the

outlet is obstructed or the SWIS is flooded. A

constant trickle from the inlet is an indication that

plumbing fixtures in the building are leaking and

need to be inspected.

Baffles and screens

The baffles should be observed to confirm that they

are in the proper position, secured well to the

piping or tank wall, clear of debris, and not

cracked or broken. If an effluent screen is fitted to

the outlet baffle, it should be removed, cleaned,

inspected for irregularities, and replaced. Note that

effluent screens should not be removed until the

tank has been pumped or the outlet is first plugged.

Septic tank pumping

Tanks should be pumped when sludge and scum

accumulations exceed 30 percent of the tank

volume or are encroaching on the inlet and outlet

baffle entrances. Periodic pumping of septic tanks

is recommended to ensure proper system performance and reduce the risk of hydraulic failure. If

systems are not inspected, septic tanks should be

pumped every 3 to 5 years depending on the size of

the tank, the number of building occupants, and

household appliances and habits (see Special Issues

Fact Sheets). Commercial systems should be

inspected and/or pumped more frequently, typically

annually. There is a system available that provides

continuous monitoring and data storage of changes

in the sludge depth, scum or grease layer thickness,

liquid level, and temperature in the tank. Longterm verification studies of this system are under

way. Accumulated sludge and scum material stored

in the tank should be removed by a certified,

licensed, or trained service provider and reused or

disposed of in accordance with applicable federal,

state, and local codes. (Also see section 4.5.5.)

4.6.5 Septage

Septage is an odoriferous slurry (solids content of

only 3 to 10 percent) of organic and inorganic

material that typically contains high levels of grit,

hair, nutrients, pathogenic microorganisms, oil, and

grease (table 4-15). Septage is defined as the entire

contents of the septic tank—the scum, the sludge,

and the partially clarified liquid that lies between

them—and also includes pumpings from aerobic

treatment unit tanks, holding tanks, biological

(“composting”) toilets, chemical or vault toilets,

and other systems that receive domestic wastewaters. Septage is controlled under the federal regulations at 40 CFR Part 503. Publications and other

information on compliance with these regulations

can be found at http://www.epa.gov/oia/tips/

scws.htm.

Septage also may harbor potentially toxic levels of

metals and organic and inorganic chemicals. The

exact composition of septage from a particular

treatment system is highly dependent upon the type

of facility and the activities and habits of its users.
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4-46 USEPA Onsite Wastewater Treatment Systems Manual

For example, oil and grease levels in septage from

food service or processing facilities might be many

times higher than oil and grease concentrations in

septage from residences (see Special Issues Fact

Sheets). Campgrounds that have separate graywater

treatment systems for showers will likely have

much higher levels of solids in the septage from the

blackwater (i.e., toilet waste) treatment system.

Septage from portable toilets might have been

treated with disinfectants, deodorizers, or other

chemicals.

Septage management programs

The primary objective of a septage management

program is to establish procedures and rules for

handling and disposing of septage in an affordable

manner that protects public health and ecological

resources. When planning a program it is important

to have a thorough knowledge of legal and regulatory requirements regarding handling and disposal.

USEPA (1994) has issued regulations and guidance

that contain the type of information required for

developing, implementing, and maintaining a

septage management program. Detailed guidance

for identifying, selecting, developing, and operating reuse or disposal sites for septage is provided in

Process Design Manual: Surface Disposal of

Sewage Sludge and Domestic Septage (USEPA,

1995b), which is on the Internet at http://

www.epa.gov/ORD/WebPubs/sludge.pdf. Additional information can be found in Domestic

Septage Regulatory Guidance (USEPA, 1993), at

http://www.epa.gov/oia/tips/scws.htm.

States and municipalities typically establish public

health and environmental protection regulations for

septage management (pumping, handling, transport, treatment, and reuse/disposal). Key components of septage management programs include

tracking or manifest systems that identify acceptable septage sources, pumpers, transport equipment, final destination, and treatment, as well as

procedures for controlling human exposure to

septage, including vector control, wet weather

runoff, and access to disposal sites.

Septage treatment/disposal: land

application

The ultimate fate of septage generally falls into

three basic categories—land application, treatment

at a wastewater treatment plant, or treatment at a

special septage treatment plant. Land application is

the most commonly used method for disposing of

septage in the United States. Simple and costeffective, land application approaches use minimal

energy and recycle organic material and nutrients

back to the land. Topography, soils, drainage

patterns, and agricultural crops determine which

type of land disposal practice works best for a

given situation. Some common alternatives are

surface application, subsurface incorporation, and

burial. Disposal of portable toilet wastes mixed

with disinfectants, deodorizers, or other chemicals

at land application sites is not recommended. If

possible, these wastes should be delivered to the

collection system of a wastewater treatment plant to

avoid potential chemical contamination risks at

septage land application sites. Treatment plant

operators should be consulted so they can determine when and where the septage should be added

to the collection system.

When disposing of septage by land application,

appropriate buffers and setbacks should be provided between application areas and water resources (e.g., streams, lakes, sinkholes). Other

considerations include vegetation type and density,

slopes, soils, sensitivity of water resources, climate,

Table 4-15. Chemical and physical characteristics of domestic

septage
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Chapter 4: Treatment Processes and Systems

and application rates. Agricultural products from

the site must not be directly consumed by humans.

Land application practices include the following:

Spreading by hauler truck or farm equipment

In the simplest method, the truck that pumps the

septage takes it to a field and spreads it on the soil.

Alternatively, the hauler truck can transfer its

septage load into a wagon spreader or other specialized spreading equipment or into a holding facility

at the site for spreading later.

Spray irrigation

Spray irrigation is an alternative that eliminates the

problem of soil compaction by tires. Pretreated

septage is pumped at 80 to 100 psi through nozzles

and sprayed directly onto the land. This method

allows for septage disposal on fields with rough

terrain.

Ridge and furrow irrigation

Pretreated septage can be transferred directly into

furrows or row crops. The land should be relatively

level.

Subsurface incorporation of septage

This alternative to surface application involves

placing untreated septage just below the surface.

This approach reduces odors and health risks while

still fertilizing and conditioning the soil. The

method can be applied only on relatively flat land

(less than 8 percent slope) in areas where the

seasonally high water table is at least 20 inches.

Because soil compaction is a concern, no vehicles

should be allowed to drive on the field for 1 to 2

weeks after application. Subsurface application

practices include the following:

• Plow and furrow irrigation: In this simple

method, a plow creates a narrow furrow 6 to 8

inches (15 to 20 centimeters) deep. Liquid

septage is discharged from a tank into the

furrow, and a second plow covers the furrow.

• Subsurface injection: A tillage tool is used to

create a narrow cavity 4 to 6 inches (10 to 15

centimeters) deep. Liquid septage is injected

into the cavity, and the hole is covered.

Codisposal of septage in sanitary landfills

Because of the pollution risks associated with

runoff and effluent leaching into ground water,

landfill disposal of septage is not usually a viable

option. However, some jurisdictions may allow

disposal of septage/soil mixtures or permit other

special disposal options for dewatered septage

(sludge with at least 20 percent solids). Septage or

sludge deposited in a landfill should be covered

immediately with at least 6 inches of soil to control

odors and vector access (USEPA, 1995b). (Note:

Codisposal of sewage sludge or domestic septage at

a municipal landfill is considered surface disposal

and is regulated under 40 CFR Part 258.)

Septage treatment/disposal: treatment

plants

Disposal of septage at a wastewater treatment plant

is often a convenient and cost-effective option.

Addition of septage requires special care and

handling because by nature septage is more concentrated than the influent wastewater stream at the

treatment plant. Therefore, there must be adequate

capacity at the plant to handle and perhaps temporarily store delivered septage until it can be fed into

the treatment process units. Sites that typically

serve as the input point for septage to be treated at

a wastewater treatment plant include the following:

Upstream sewer manhole

This alternative is viable for larger sewer systems

and treatment plants. Septage is added to the

normal influent wastewater flow at a receiving

station fitted with an access manhole.

Treatment plant headworks

The septage is added at the treatment plant upstream of the inlet screens and grit chambers. The

primary concern associated with this option is the

impact of the introduced wastes on treatment unit

processes in the plant. A thorough analysis should

be conducted to ensure that plant processes can

accept and treat the wastes while maintaining

appropriate effluent pollutant concentrations and

meeting other treatment requirements. In any

event, the treatment plant operator should be

consulted before disposal.
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4-48 USEPA Onsite Wastewater Treatment Systems Manual

Sludge-handling process

To reduce loading to the liquid stream, the septage

can be sent directly to the sludge-handling process.

Like the headworks option, the impact on the

sludge treatment processes must be carefully

analyzed to ensure that the final product meets

treatment and other requirements.

Treatment at a special septage treatment plant

This method of septage disposal is usually employed in areas where land disposal or treatment at

a wastewater treatment plant is not a feasible

option. There are few of these facilities, which

vary from simple lagoons to sophisticated plants

that mechanically and/or chemically treat septage.

Treatment processes used include lime stabilization,

chlorine oxidation, aerobic and anaerobic digestion,

composting, and dewatering using pressure or

vacuum filtration or centrifugation. This is the

most expensive option for septage management and

should be considered only as a last resort.

Public outreach and involvement

Developing septage treatment units or land application sites requires an effective public outreach

program. Opposition to locating these facilities in

the service area is sometimes based about incomplete or inaccurate information, fear of the unknown, and a lack of knowledge on potential

impacts. Without an effective community-based

program of involvement, even the most reasonable

plan can be difficult to implement. Traditional

guidance on obtaining public input in the development of disposal or reuse facilities can be found in

Process Design Manual: Surface Disposal of

Sewage Sludge and Domestic Septage (USEPA,

1995b), which is on the Internet at http://

www.epa.gov/ORD/WebPubs/sludge.pdf.

Additional information can be found in Domestic

Septage Regulatory Guidance (USEPA, 1993),

posted at http://www.epa.gov/oia/tips/scws.htm.

General guidance on developing and implementing

a public outreach strategy is available in Getting In

Step: A Guide to Effective Outreach in Your

Watershed, published by the Council of State

Governments (see chapter 2) and available at http:/

/www.epa.gov/owow/watershed/outreach/

documents/.

4.7 Sand/media filters

Sand (or other media) filters are used to provide

advanced treatment of settled wastewater or septic

tank effluent. They consist of a lined (lined with

impervious PVC liner on sand bedding) excavation

or watertight structure filled with uniformly sized

washed sand (the medium) that is normally placed

over an underdrain system (figure 4-25). These

contained media filters are also known as packed

bed filters. The wastewater is dosed onto the

surface of the sand through a distribution network

and is allowed to percolate through the sand to the

underdrain system. The underdrain collects the

filtrate for further processing, recycling, or discharging to a SWIS. Some “bottomless” designs

directly infiltrate the filtered effluent into the soil

below.

4.7.1 Treatment mechanisms and filter

design

Sand filters are essentially aerobic, fixed-film

bioreactors used to treat septic tank effluent. Other

very important treatment mechanisms that occur in

sand filters include physical processes such as

straining and sedimentation, which remove suspended solids within the pores of the media, and

chemical adsorption of dissolved pollutants (e.g.,

phosphorus) to media surfaces. The latter phenomenon tends to be finite because adsorption sites

become saturated with the adsorbed compound, and

it is specific to the medium chosen. Bioslimes from

the growth of microorganisms develop as attached

films on the sand particle surfaces. The microorganisms in the slimes absorb soluble and colloidal waste

materials in the wastewater as it percolates around

the sand surfaces. The absorbed materials are

incorporated into new cell mass or degraded under

aerobic conditions to carbon dioxide and water.

Figure 4-25. Underdrain system detail for sand filters



 

  
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Chapter 4: Treatment Processes and Systems

Most of the biochemical treatment occurs within

approximately 6 inches (15 centimeters) of the

filter surface. As the wastewater percolates through

this active layer, carbonaceous BOD and ammonium-nitrogen are removed. Most of the suspended

solids are strained out at the filter surface. The

BOD is nearly completely removed if the wastewater retention time in the sand media is sufficiently

long for the microorganisms to absorb and react

with waste constituents. With depleting carbonaceous BOD in the percolating wastewater, nitrifying microorganisms are able to thrive deeper in this

active surface layer, where nitrification will readily

occur.

To achieve acceptable treatment, the wastewater

retention time in the filter must be sufficiently long

and reaeration of the media must occur to meet the

oxygen demand of the applied wastewater. The

pore size distribution and continuity of the filter

medium, the dose volume, and the dosing frequency are key design and operating considerations

for achieving these conditions. As the effective size

and uniformity of the media increases, the

reaeration rate increases, but the retention time

decreases. Treatment performance might decline if

the retention time is too short. If so, it may be

necessary to recirculate the wastewater through the

filter several times to achieve the desired retention

time and concomitant treatment performance.

Multiple small dose volumes that do not create a

saturated wetting front on the medium can be used

to extend residence times. If saturated conditions

are avoided, moisture tensions within the medium

will remain high, which will redistribute the

applied wastewater throughout the medium,

enhancing its contact with the bioslimes on the

medium. The interval between doses provides time

for reaeration of the medium to replenish the

oxygen depleted during the previous dose.

Filter surface clogging can occur with finer media

in response to excessive organic loadings. Biomass

increases can partially fill the pores in the surface

layer of the sand. If the organic loadings are too

great, the biomass will increase to a point where

the surface layer becomes clogged and is unable to

accept further wastewater applications. However, if

the applied food supply is less than that required by

resident microorganisms, the microorganisms are

forced into endogenous respiration; that is, they

begin to draw on their stored metabolites or

surrounding dead cells for food. If the microorganisms are maintained in this growth phase, net

increases of biomass do not occur and clogging can

be minimized.

Chemical adsorption can occur throughout the

medium bed, but adsorption sites in the medium

are usually limited. The capacity of the medium to

retain ions depends on the target constituent, the

pH, and the mineralogy of the medium. Phosphorus is one element of concern in wastewater that

can be removed in this manner, but the number of

available adsorption sites is limited by the characteristics of the medium. Higher aluminum, iron, or

calcium concentrations can be used to increase the

effectiveness of the medium in removing phosphorus. Typical packed bed sand filters are not efficient units for chemical adsorption over an extended period of time. However, use of special

media can lengthen the service (phosphorus removal) life of such filters beyond the normal, finite

period of effective removal.

Filter designs

Sand filters are simple in design and relatively

passive to operate because the fixed-film process is

very stable and few mechanical components are

used. Two types of filter designs are common,

“single-pass” and “recirculating” (figure 4-26).

They are similar in treatment mechanisms and

performance, but they operate differently. Singlepass filters, historically called “intermittent” filters,

discharge treated septic tank effluent after one pass

through the filter medium (see Fact Sheet 10).

Recirculating filters collect and recirculate the

filtrate through the filter medium several times

before discharging it (see Fact Sheet 11). Each has

advantages for different applications.

Single-pass filters

The basic components of single-pass filters (see

Fact Sheet 10) include a dose tank, pump and

controls (or siphon), distribution network, and the

filter bed with an underdrain system (figure 4-25).

The wastewater is intermittently dosed from the

dose tank onto the filter through the distribution

network. From there, it percolates through the sand

medium to the underdrain and is discharged. Ondemand dosing has often been used, but timed

dosing is becoming common.
Chapter 4: Treatment Processes and Systems

4-50 USEPA Onsite Wastewater Treatment Systems Manual

To create the wastewater retention times necessary

for achieving desired treatment results, single-pass

filters must use finer media than that typically used

in recirculating filters. Finely sized media results in

longer residence times and greater contact between

the wastewater and the media surfaces and their

attached bioslimes. BOD removals of greater than

90 percent and nearly complete ammonia removal

are typical (Darby et al., 1996; Emerick et al., 1997;

University of Wisconsin, 1978). Single-pass filters

typically achieve greater fecal coliform removals

than recirculating filters because of the finer media

and the lower hydraulic loading. Daily hydraulic

loadings are typically limited to 1 to 2 gpd/ft2, depending on sand size, organic loading, and especially the number of doses per day (Darby et al.,

1996).

Figure 4-26. Schematics of the two most common types of sand media filters
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Chapter 4: Treatment Processes and Systems

Recirculating filters

The basic components of recirculating filters (see

Fact Sheet 11) are a recirculation/dosing tank,

pump and controls, a distribution network, a filter

bed with an underdrain system, and a return line

fitted with a flow-splitting device to return a

portion of the filtrate to the recirculation/dosing

tank (figure 4-26). The wastewater is dosed to the

filter surface on a timed cycle 1 to 3 times per

hour. The returned filtrate mixes with fresh septic

tank effluent before being returned to the filter.

Media types

Many types of media are used in packed bed filters.

Washed, graded sand is the most common medium.

Other granular media used include gravel, anthracite, crushed glass, expanded shale, and bottom ash

from coal-fired power plants. Bottom ash has been

studied successfully by Swanson and Dix (1987).

Crushed glass has been studied (Darby et al., 1996;

and Emerick et al., 1997), and it was found to

perform similarly to sand of similar size and

uniformity. Expanded shale appears to have been

successful in some field trials in Maryland, but the

data are currently incomplete in relation to longterm durability of the medium.

Foam chips, peat, and nonwoven coarse-fiber

synthetic textile materials have also been used.

These are generally restricted to proprietary units.

Probably the most studied of these is the peat filter,

which has become fairly common in recent years.

Depending on the type of peat used, the early performance of these systems will produce an effluent with

a low pH and a yellowish color. This is accompanied by some excellent removal of organics and

microbes, but would generally not be acceptable as

a surface discharge (because of low pH and visible

color). However, as a pretreatment for a SWIS,

low pH and color are not a problem. Peat must

meet the same hydraulic requirements as sand (see

Fact Sheets 10 and 11). The primary advantage of

the proprietary materials, the expanded shale, and to

some degree the peat is their light weight, which

makes them easy to transport and use at any site.

Some short-term studies of nonwoven fabric filters

have shown promise (Roy and Dube, 1994).

System manufacturers should be contacted for

application and design using these materials.

4.7.2 Applications

Sand media filters may be used for a broad range

of applications, including single-family residences,

large commercial establishments, and small communities. They are frequently used to pretreat

wastewater prior to subsurface infiltration on sites

where the soil has insufficient unsaturated depth

above ground water or bedrock to achieve adequate

treatment. They are also used to meet water quality

requirements before direct discharge to a surface

water. They are used primarily to treat domestic

wastewater, but they have been used successfully in

treatment trains to treat wastewaters high in organic

materials such as those from restaurants and

supermarkets. Single pass filters are most frequently used for smaller applications and sites

where nitrogen removal is not required. Recirculating filters are used for both large and small flows

Performance of sand and other filters

Twelve innovative treatment technologies were installed to replace failed septic systems in the Narragansett Bay

watershed, which is both pathogen- and nitrogen-sensitive. The technologies installed consisted of an at-grade

recirculating sand filter, single pass sand filters, Maryland-style recirculating sand filters, foam biofilters, and a

recirculating textile filter. The treatment performance of these systems was monitored over an 18-month period. In

the field study, TSS and BOD5 concentrations were typically less than 5 mg/L for all sand filter effluent and less

than 20 mg/L for both the foam biofilter and textile filter effluents. Single pass sand filters achieved substantial

fecal coliform reductions, reaching mean discharge levels ranging from 200 to 520 colonies per 100 mL for all 31

observations. The at-grade recirculating sand filter achieved the highest total nitrogen reductions of any

technology investigated and consistently met the Rhode Island state nitrogen removal standard (a TN reduction of

50 percent or more and a TN concentration of 19 mg/L or less) throughout the study.

Source: Loomis et al., 2001.
Chapter 4: Treatment Processes and Systems

4-52 USEPA Onsite Wastewater Treatment Systems Manual

and are frequently used where nitrogen removal is

necessary. Nitrogen removal of up to 70 to 80

percent can be achieved if an anoxic reactor is used

ahead of the recirculation tank, where the nitrified

return filtrate can be mixed with the carbon-rich

septic tank effluent (Anderson et al., 1998; Boyle

et al., 1994; Piluk and Peters, 1994).

4.7.3 Performance

The treatment performance of single-pass and

recirculating filters is presented in table 4-16. The

medium used was sand or gravel as noted. Recirculating sand filters generally match or outperform

single-pass filters in removal of BOD, TSS, and

nitrogen. Typical effluent concentrations for

domestic wastewater treatment are less than 10 mg/

L for both BOD and TSS, and nitrogen removal is

approximately 50 percent. Single-pass sand filters

can also typically produce an effluent of less than

10 mg/L for both BOD and TSS. Effluent is nearly

completely nitrified, but some variability can be

expected in nitrogen removal capability. Pell and

Nyberg (1989) found typical nitrogen removals of

18 to 33 percent with their intermittent sand filter.

Fecal coliform removal is somewhat better in

single pass filters. Removals range from 2 to 4 logs

in both types of filters. Intermittent sand filter fecal

coliform removal is a function of hydraulic loading; removals decrease as the loading rate increases

above 1 gpm/ft2 (Emerick et al., 1997).

Effluent suspended solids from sand filters are

typically low. The medium retains the solids. Most

of the organic solids are ultimately digested. Gravel

filters, on the other hand, do not retain solids as

well.

excessive solids buildup due to the lack of periodic

sludge pumping and removal. In such cases, the

solids storage capacity of the final settling compartment might be exceeded, which results in the

discharge of solids into the effluent. ATU performance and effluent quality can also be negatively

affected by the excessive use of toxic household

chemicals. ATUs must be properly operated and

maintained to ensure acceptable performance.

4.8 Aerobic treatment units

Aerobic treatment units (ATUs) refer to a broad

category of pre-engineered wastewater treatment

devices for residential and commercial use. ATUs

are designed to oxidize both organic material and

ammonium-nitrogen (to nitrate nitrogen), decrease

suspended solids concentrations and reduce pathogen concentrations.

A properly designed treatment train that incorporates an ATU and a disinfection process can provide

a level of treatment that is equivalent to that level

provided by a conventional municipal biological

treatment facility. The AUT, however, must be

properly designed, installed, operated and maintained.

Although most ATUs are suspended growth devices, some units are designed to include both

suspended growth mechanisms combined with

fixed-growth elements. A third category of ATU is

designed to provide treatment entirely through the

use of fixed-growth elements such as trickling

filters or rotating biological contactors (refer to

sheets 1 through 3). Typical ATU’s are designed

using the principles developed for municipal-scale

wastewater treatment and scaled down for residential or commercial use.

Most ATUs are designed with compressors or

aerators to oxygenate and mix the wastewater.

Partial pathogen reduction is achieved. Additional

disinfection can be achieved through chlorination,

UV treatment, ozonation or soil filtration. Increased nutrient removal (denitrification) can be

achieved by modifying the treatment process to

provide an anaerobic/anoxic step or by adding

treatment processes to the treatment train.

4.8.1 Treatment mechanisms

ATUs may be designed as continuous or batch flow

systems (refer to fact sheets 1 through 3). The

simplest continuous flow units are designed with no

flow equalization and depend upon aeration tank

volume and/or baffles to reduce the impact of

hydraulic surges. Some units are designed with

flow-dampening devices, including air lift or floatcontrolled mechanical pumps to transfer the

wastewater from the aeration tank to a clarifier.

Other units are designed with multiple-chambered

tanks to attenuate flow. The batch (fill and draw)

flow system design eliminates the problem of

hydraulic variation. Batch systems are designed to

collect and treat wastewater over a period of time.
USEPA Onsite Wastewater Treatment Systems Manual 4-53

Chapter 4: Treatment Processes and Systems

Table 4-16. Single pass and recirculating filter performance.

a Single-family home filters. bRestaurant (grease and oil inf/eff = 119/<1 mg/L respectively). c Small community treating average

15,000 gpd of septic tank effluent. d 1 gpd/ft2 = 4 cm/day = 0.04m3/m2×day. e 1 lb BOD/1000ft2×day = 0.00455 kg/m2×day
Chapter 4: Treatment Processes and Systems

4-54 USEPA Onsite Wastewater Treatment Systems Manual

Pumps are used to discharge the settled effluent at

the end of the cycle (usually one day). Fixed film

treatment plants typically are operated as continuous flow systems.

Oxygen is transferred by diffused air, sparged

turbine, or surface entrainment devices. When

diffused air systems are used, blowers or compressors are used to force the air through diffusers near

the bottom of the tank. The sparged turbine is

typically designed with a diffused air source and an

external mixer, e.g., a submerged flat-bladed

turbine. The sparged turbine is more complex than

the simple diffused air system. A variety of surface

entrainment devices aerate and mix the wastewater.

Air is entrained and circulated in the mixed liquor

through violent agitation from mixing or pumping.

The separation of process-generated solids by

clarification or filtration is a critical design factor

for successful ATU performance. Most ATUs are

designed to rely on the process of simple gravity

separation to remove most of the solids. Some

systems include effluent filters within the clarifier

to further screen and retain solids in the treatment

plant. Gas deflection barriers and scum baffles are

a part of some designs and are a simple way to

keep floating solids away from the weir area.

Properly managed uplow clarifiers can improve

separation.

4.8.2 Design Considerations

ATU’s are typically rated by hydraulic capacity and

organic and solids loadings. ATU daily treatment

volumes may range from 400 gpd to a maximum

of 1,500 gpd. ATUs typically can be used to treat

residential wastewaters with influent concentrations

which have 100 mg/L to 300 mg/L total organic

compounds and 100 mg/L to 350 mg/L total

suspended solids. Design flows are generally set by

local sanitary codes for residential and commercial

dwellings using methods described in Section 3.3.

ATU’s should be equipped with audio and visual

alarms to warn of compressor/aerator failure and

high water. These alarms alert the owner and/or

service provider of service issues that require

immediate attention.

ATU’s should be constructed of noncorrosive

materials, including reinforced plastics and

fiberglass, coated steel, and reinforced concrete.

Buried ATU’s must be designed to provide easy

access to mechanical parts, electrical control

systems, and appurtenances requiring maintenance

such as weirs, air lift pump lines, etc. ATU’s

installed above ground should be properly housed

to protect against severe climatic conditions.

Installation should be in accordance with manufacturers’ specifications.

Appurtenances should be constructed of corrosionfree materials including polyethylene plastics. Air

diffusers are usually constructed of PVC or ceramic

stone. Mechanical components must be either

waterproofed and/or protected from the elements.

Because blowers, pumps, and other prime movers

can be subject to harsh environments and continuous operation, they should be designed for heavy

duty use. Proper housing can reduce blower noise.

4.8.3 Applications

ATUs are typically integrated in a treatment train to

provide additional treatment before the effluent is

discharged to a SWIS. ATU-treatment trains can

also be designed to discharge to land and surface

waters; ATU discharge is suitable for drip irrigation

if high quality effluent is consistently maintained

through proper management. Although some

jurisdictions allow reductions in vertical separation

distances and/or higher soil infiltration rates when

ATUs are used, consideration must be given to the

potential impacts of higher hydraulic and pollutant

loadings. Increased flow through the soil may

allow deeper penetration of pathogens and

decreased treatment efficiency of other pollutants

(see sections 4.4.2 and 4.4.5).

4.8.4 Performance

Managed ATU effluent quality is typically

characterized as 25 mg/L or less CBOD5 and 30

mg/L or less TSS. Fecal coliform counts are

typically 3-4 log # / 100 ml (Table 3-19) when the

ATUs are operated at or below their design flows

and the influent is typical domestic sewage.

Effluent nutrient levels are dependent on influent

concentrations, climate, and operating conditions.

Other wastewater characteristics may influence

performance. Cleaning agents, bleach, caustic
USEPA Onsite Wastewater Treatment Systems Manual 4-55

Chapter 4: Treatment Processes and Systems

agents, floating matter, and other detritus can plug

or damage equipment. Temperature will affect

process efficiency, i.e., treatment efficiency

generally will improve as the temperature

increases.

Owners should be required by local sanitary codes

or management program requirements to maintain

ongoing service agreements for the life of the

system. ATU’s should be inspected every three

months to help ensure proper operation and

treatment effectiveness. Many ATU manufacturers

offer a two-year warranty with an optional service

agreement after the warranty expires. Inspections

generally include visual checks of hoses, wires,

leads and contacts, testing of alarms, examination

of the mixed liquor, cleaning of filters, removal of

detritus, and inspection of the effluent. ATU’s

should be pumped when the mixed-liquor (aerator)

solids are above 6,000 mg/L or the final settler is

more than 1/3 full of settled solids.

4.8.5 Risk management

ATU’s should be designed to protect the treatment

capability of the soil dispersal system and also to

sound alarms or send signals to the management

entity (owners and/or service providers) when

inspection or maintenance is needed. All biological

systems are sensitive to temperature, power

interruptions, influent variability, and shock

loadings of toxic chemicals. Successful operation

of ATUs depends on adherence to manufacturers’

design and installation requirements and good

management that employs meaningful measurements of system performance at sufficiently

frequent intervals to ascertain changes in system

function. Consistent performance depends on a

stable power supply, an intact system as designed,

and routine maintenance to ensure that components

and appurtenances are in good order. ATU’s, like

all other onsite wastewater treatment technologies,

will fail if they are not designed, installed, or

operated properly. Vigilance on the part of owners

and service providers is essential to ensure ATUs

are operated and maintained to function as

designed.

4.8.6 Costs

Installed ATU costs range from $2500 to $9000

installed. Pumping may be necessary at any time

due to process upsets, or every eight to twelve

months, depending on influent quality, temperature

and type of process. Pumping could cost from

$100-to-$300, depending on local requirements.

Aerators/compressors last about three to five years

and cost from $300 to $500 to replace.

Many communities require service contracts.

These contracts typically range in cost between

$100 and $400 per year, depending on the options

and features the owners choose. The high end

includes pumping costs. Power requirements are

generally quoted at around $200/year.

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USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 4-65

Chapter 4: Treatment Processes and Systems
Introduction

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Onsite Wastewater Treatment Systems

Technology Fact Sheet 1

Continuous-Flow, SuspendedGrowth Aerobic Systems (CFSGAS)

Description

The activated sludge process is an aerobic suspended-growth process that maintains a relatively high population of microorganisms (biomass) by recycling settled biomass back to the treatment process. The biomass converts soluble and colloidal biodegradable organic matter and some inorganic compounds into cell mass and metabolic end products. The biomass

is separated from the wastewater through settling in a clarifier for recycling or wasting to sludge handling processes.

Preliminary treatment to remove settleable solids and floatable materials is usually provided by a septic tank or other

primary treatment device. Most onsite designs are capable of providing significant ammonia oxidation and effective

removal of organic matter.

The basic system consists of a number of interrelated components (as shown in figure 1):

• An aeration tank or basin.

• An oxygen source and equipment to disperse atmospheric or pressurized air or oxygen into the aeration

tank at a rate sufficient to always maintain positive

dissolved oxygen.

• A means to appropriately mix the aeration basin and

ensure suspension of the biomass (usually accomplished by the aeration system).

• A clarifier to separate the biomass from the treated

effluent and collect settled biomass for recycling to the

aeration basin.

Several modifications of this basic process are commercially available. These include different aeration devices; different

means of sludge collection and recycling to the aerator; the use of coarse membrane filters in lieu of, or in addition to, the

clarifier; and process enhancement through the addition of an inert media area on which biofilms can grow. The addition

of surfaces where biota can become attached and grow increases the capacity of the system (increased organic loading

possible). This last modification is the most significant enhancement and is described below.

The combined fixed-film/suspended growth process is sometimes referred to as a class of treatment processes called

coupled contact aeration, enhanced, or high biomass systems. To enhance performance and increase the capacity of the

aeration tank, an inert support medium is added to the aeration tank. This allows a fixed film of biomass to attach and

grow on the medium to augment the suspended microbial population, providing more biomass to feed on wastewater

constituents (figure 2). Synthetic trickling filter media, loops of fiber bundles, and a variety of different plastic surface

configurations can be suspended in the aeration tank. Advantages include increased active microbial mass per unit volume,

enhanced potential for nitrification, reduced suspended solids loading to the clarifier, improved solids separation characteristics, reduced sludge production, and resilience under variable influent conditions.

Figure 1. A basic CFSGAS configuration
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Typical application

These systems are usually preceded by a septic

tank and followed by a subsurface wastewater

infiltration system (SWIS). Despite some claims

of reduced SWIS sizing when compared to the

conventional septic tank pretreatment, the

designer is cautioned to consider ground water

protection. These systems should be applied only

where onsite system management services are

available. For surface water discharge, the system

must be followed by disinfection at a minimum

to consistently meet discharge standards. However, some subsurface (non-human-contact) reuse may be implemented without further treatment. High biomass systems

can be a low-cost means of upgrading existing overloaded CFSGAS units that currently do not meet BOD or nitrification

goals. They can also compete directly with conventional designs because they have greater stability in handling highly

variable loadings.

Design assumptions

The extended aeration type of CFSGAS is the most commonly used design. At present there is no generic information on

design parameters for fixed film activated sludge systems. Package plants are delivered based on design flow rates. A

conservative design approach for extended aeration systems is presented in table 1. The inert medium should support

additional biomass and add to the total system microbial mass. Because the increase in microbial population is difficult to

measure, any “credits” for this addition would have to be based on empirical observation. Claims for significantly decreased sludge production, increased oxygen transfer efficiency, and improved settleability of the sludge have not been

universally proved. However, a number of successful installations for onsite and small municipal systems have been in

operation throughout the world for more than 10 years (Mason, 1977; Rogella et al., 1988; Rusten et al., 1987).

Suspended

solids

separation

Influent Effluent

Sludge recycling

Aeration

tank

Air

Treatment

media

Figure 2. An enhanced CFSGAS or “high biomass” system

Table 1-1. Design parameters for CFSGAS extended aeration package plants

a

TSS in aeration tank.

b

Organic loading (pounds of BOD per day) to aeration tank volatile fraction of MLSS.

c

Power input per cubic foot of tank volume.
Introduction

TFS-3

Onsite package treatment units (see figure 3) should be constructed of noncorrosive materials, such as coated concrete,

plastic, fiberglass, or coated steel. Units may be stand-alone or manufactured to drop into a compartmented septic tank.

Some units are installed aboveground on a concrete slab with proper housing to protect against severe climatic conditions.

Units may also be buried underground as long as easy access is provided to all mechanical parts, electrical control systems,

and water surfaces. All electrical components should follow NEC code and be waterproof and/or housed from the elements. If airlift pumps are used, large-diameter units should be provided to avoid clogging. Blowers, pumps, and other

mechanical devices should be designed for continuous use because they will be abused by climatic conditions and the

corrosive atmosphere within the treatment environment. Easy access to all moving parts should be provided for routine

maintenance. An effective alarm system should be employed. Typical land area requirements for package plants are

modest.

For engineered package plants, final clarifier designs should be conservative for high MLSS and poor settleability of

biomass. Because of the potential for bulking sludge, secondary clarifiers should be equipped with surface skimming

devices to remove greases and floating solids, as well as efficient screens.

Performance

Well-operated CFSGAS extended aeration units that are well operated can achieve BOD concentrations ranging from 10 to

50 mg/L and TSS concentrations ranging from 15 to 60 mg/L. Some studies (Brewer et al., 1978; Hutzler et al., 1978)

have indicated poorer performance owing to surge flows, variable loading, and inadequate maintenance. Nitrification can

also be significant in these aeration units during warmer periods. Some nitrogen removal can be achieved by denitrification, which can remove 30 to 40 percent of the total nitrogen (TN) under optimum conditions. Average total nitrogen

effluent concentrations in residential extended aeration units range from 17 to 40 mg/L. Fecal coliform and virus removal

has been reported in the range of 1 to 2 logs.

High biomass systems have produced BOD and TSS effluents of 5 to 40 mg/L. Although they are less dependent on

temperature than the extended aeration CFSGAS, temperature does have an impact on their seasonal capability to nitrify

the influent ammonium-nitrogen to nitrate-nitrogen. All CFSGAS systems do an excellent job of removing toxic organics

and heavy metals. Most CFSGAS systems do not remove more than a small percentage of phosphorus (10 to 20 percent)

and nitrogen (15 to 25 percent).

Figure 3. Components of a typical aerobic treatment unit

Vent

Outlet

Settling

Compartment

Sludge Return Mixing Return

Electric

Motor

Air

Discharge

Inlet

Aeration

Compartment
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Management requirements

CFSGAS systems must be managed and maintained by trained personnel rather than homeowners to perform acceptably.

Power requirements vary from 2.5 to 10 kWh/day. They should be inspected at least every 2 to 3 months. During these

inspections, excess solids pumping should be based on the mixed liquor measurements. It is estimated that an effective

program will require between 12 and 28 person-hours annually, in addition to analytical testing of the effluent, where

required. Management contracts should be in place for the life of the system. Common operational problems with extended aeration systems are provided in table 2. Residuals generated will vary from 0.6 to 0.9 lb TSS per lb BOD removed, over and above the normal septic tank sludge produced.

Risk management issues

CFSGAS systems require effluent disinfection at a minimum to meet surface discharge or any surface reuse water quality

requirements. They are quite sensitive to temperature, interruption of electric supply, influent variability, or shock loadings of toxic chemicals. The septic tank helps protect these units from the latter problems. Aesthetically, noise from the

blowers is the major irritant, while odors can be significant during power outages or organic overloading periods. High

biomass units are more resistant to the above impacts. The systems are not well suited to seasonal use because of long

start-up times.

Costs

The installed costs of package plants are highly variable but are usually less than $10,000. Operation and maintenance (O/

M) costs are primarily dependent on local power and labor costs, varying from $400 to $600 per year in most cases.

References

Ayres Associates. 1998. Florida Keys Onsite Wastewater Nutrient Reduction Systems Demonstration Project. Contract no.

LP 988. Florida Department of Health Onsite Sewage Program, Tallahassee, FL.

Brewer, W.S., J. Lucas, and G. Prascak. 1978. An evaluation of the performance of household aerobic sewage treatment

units. Journal of Environmental Health 41(2):82-84.

Table 1-2. Common operational problems of extended aeration package plants
Introduction

TFS-5

Converse, J.C., and M.M. Converse. 1998. Pump Chamber Effluent Quality Following Aerobic Units and Sand Filters

Serving Residences. In Proceedings of the Eighth National Symposium on Individual and Small Community Sewage

Systems. American Society of Agricultural Engineers, Orlando, FL.

Englehardt, J.D., and R.C. Ward. 1986. Operation and maintenance requirements for small flow treatment systems.

Journal of the Water Pollution Control Federation 58(10).

Hutzler, N.L., L. Waldorf, and J. Fancy. 1978. Performance of Aerobic Treatment Units. In Proceedings of the Second

National Home Sewage Treatment Symposium. American Society of Agricultural Engineers, Chicago, IL.

Kellam, J.G., et al. 1993. Evaluation of Performance of Five Aerated Package Treatment Systems. Bull. 178. Virginia

Water Resources Research Center, Blacksburg, VA.

Mason, D.G. 1977. A Unique Biological Treatment System for Small Plants. Paper presented at the 50th Water Pollution

Control Federation Conference, Philadelphia, PA.

Midwest Plan Service. 1982. On-site Domestic Sewage Disposal Handbook. Midwest Plan Service, University of

Minnesota, St. Paul, MN.

Otis, R.J., and W.C Boyle. 1976. Performance of single household treatment units. Journal of Environmental Engineering

Division, ASCE, 102, EE1, 175.

Otis R.J., et al. 1975. The Performance of Household Wastewater Treatment Units under Field Conditions. In Proceedings

of the Third National Home Sewage Disposal Symposium. American Society of Agricultural Engineers, Chicago, IL.

Rogella, F., J. Sibony, G. Boisseau, and M. Benhomme. 1988. Fixed Biomass to Upgrade Activated Sludge. Paper

presented at 61st Annual Water Pollution Control Federation Conference, Philadelphia, PA.

Rusten, B., M.J. Tetreault, and J.F. Kreissl. 1987. Assessment of Phased Isolation Ditch Technologies for Nitrogen

Control. In Proceedings of the Seventh European Sewage and Refuse Symposium, pp. 279-291, Munich, Germany.

Tchobanoglous, G., and F. Burton. 1991. Wastewater Engineering. 3rd ed. McGraw-Hill, Inc., New York.

U.S. Environmental Protection Agency (USEPA). 1978. Management of Small Waste Flows. Small Scale Waste

Management Project. EPA 600/2-78-173. National Technical Information Service PB 286 474.

U.S. Environmental Protection Agency (USEPA). 1980. Design Manual: Onsite Wastewater Treatment and Disposal

Systems. EPA 625/1-80-012. U.S. Environmental Protection Agency, Office of Water Programs, Washington, DC.
TFS-6 
Introduction

(Click Here to Return to Bookmarks Page) TFS-7

Onsite Wastewater Treatment Systems

Technology Fact Sheet 2

Fixed-Film Processes

Description

Fixed-film systems (FFS) are biological treatment processes that employ a medium such as rock, plastic, wood, or other

natural or synthetic solid material that will support biomass on its surface and within its porous structure. At least two

types of fixed-film systems may be considered —those in which the medium is held in place and is stationary relative to

fluid flow (trickling filter) and those in which the medium is in motion relative to the wastewater (e.g., rotating biological

disk). A third classification includes dual-process systems that encompass both fixed and suspended biomass together or

in series. This approach is covered in Fact Sheet No. 1 on continuous-flow suspended-growth aerobic systems

(CFSGAS).

Trickling filter systems are typically constructed as beds of media through which wastewater flows. Oxygen is normally

provided by natural or forced ventilation. Flow distributors or sprayers distribute the wastewater evenly onto the surface

of the medium. As the wastewater moves by gravity through the medium, soluble and colloidal organic matter is metabolized by the biofilm that forms on the medium. Excess biomass sloughs from the medium and is carried with the treated

wastewater to the clarifier, where the solids settle and separate from the treated effluent. At this point the treated wastewater may be discharged or recycled back to the filter medium for further treatment (figure 1).

A fixed-film biological treatment process that employs rotating disks that move within the wastewater is referred to as a

rotating biological contactor (RBC). Developed in the late 1960s, the RBC employs a plastic medium configured as disks

and mounted on a horizontal shaft. The shafts are rotated slowly (1 to 2 rpm) by mechanical or compressed air drive. For

a typical aerobic RBC, approximately 40 percent of the medium is immersed in the wastewater. Anoxic or anaerobic RBCs

(far less common) are fully immersed in the wastewater. Wastewater flows through the medium by simple displacement

and gravity. Biomass continuously sloughs from the disks, and some suspended biomass develops within the wastewater

channels through which the disks rotate, making the addition of a secondary clarifier necessary. The rotation of the disks

exposes the attached biomass to atmospheric air and wastewater. Oxygen is supplied by natural surface transfer to the

biomass. Some oxygenation of the wastewater is also created by turbulence at the disk-water interface. The use of

Septic

tank

Fixed-film

reactor

Influent

Clarifier

Effluent

Optional sludge return

Optional recirculation of effluent

Figure 1. Trickling filter treatment system
TFS-8

exposed and submerged stages in multiple tanks to create aerobic and anoxic conditions may be employed where nitrogen

removal is required.

Commercially available modifications primarily address the media employed, the configuration of the tankage, and the

mechanical supporting systems (e.g., supplemental aeration, programmable cycling, etc.). Some FFS sludges are wasted

directly by pumping of the clarifier, whereas others convey all excess solids back to the pretreatment stage (septic tank)

for subsequent removal. Lightweight synthetic media have greater surface area and are easier to install. Numerous variations ranging from extruded foam to high-specific-surface PVC and other plastic shapes are available commercially.

Typical applications

Fixed-film systems (FFS) are an alternative to CFSGAS for reducing biochemical oxygen demand (BOD) and total

suspended solids (TSS) from septic tank effluent to meet a higher effluent standard (figure 2). Like CFSGAS, they can

meet secondary effluent standards (30 mg/L of BOD and TSS), but they would need a minimum of effluent disinfection to

be acceptable for surface water discharges. They might meet onsite water reuse requirements as long as the effluent is

distributed below the ground surface. Some data support the potential for soil absorption field infiltrative surface reduction

following FFS, but caution is urged regarding ground water quality protection from use of such reductions. FFS can also

be used as part of a nutrient reduction treatment train (see Facts Sheet No. 8 and No. 9 on nutrient removal). FFS provide

an aerobic oxidation step in those sequences.

Source: Bord Na Mona, 1999.

Design assumptions

Design guidelines for fixed-film systems are given in table 1. FFS package units should be constructed of noncorrosive

materials. Some are installed aboveground on a concrete slab with proper housing to anticipate local climatic conditions.

The units may also be buried underground as long as access is provided to all mechanical parts, control systems,

underdrains, distribution system, and water surfaces. All electric components must meet NEC code and should be waterproofed and housed from the elements. If natural ventilation is required for aeration, proper design and construction must

be considered to ensure adequate oxygen transfer. Pumps, drives (for rotating units), and other mechanical devices should

be designed for continuous heavy-duty use and climatic conditions. Access and drainage capability should be provided to

underdrains and distribution systems because they may become clogged over time. Alarms that alert homeowners or

management entities should be provided to warn of system malfunctions.

Figure 2. Fixed-film system using peat moss as a treatment medium
Introduction

TFS-9

Onsite RBC package units should also be constructed of noncorrosive materials. Disk shafts and bearings and drives should

be designed for heavy-duty use since they will be abused by the corrosive atmosphere generated by treatment processes

and climatic conditions. Access should be provided to bearings, drives, and disks for maintenance. RBC units should be

covered and insulated against cold weather and sunlight. Proper ventilation of the unit is necessary to ensure adequate

oxygen transfer.

Performance

Typical trickling filters and rotating medium systems currently available should be capable of producing effluent BOD and

TSS concentrations of 5 to 40 mg/L. System reliability is somewhat better than suspended growth package plants because

of the more effective capture and control of suspended solids. Nitrification is achievable at low loading rates in warm

climates. Factors affecting performance include influent wastewater characteristics, hydraulic and organic loading, medium type, maintenance of optimal dissolved oxygen levels, and recirculation rates. The process is characteristically

vulnerable to climatic conditions because of the cooling effect of the wastewater as it passes through the medium. Proper

insulation, reduced effluent recirculation, and improved distribution techniques can lessen the impact of cold temperatures.

Limited denitrification has been noted in nitrifying filters when oxygenation is poor and within dead zones (anaerobic

portions) of the filter. Fecal coliform reductions are 1 to 2 logs. Nitrogen removal varies from 0 to 35 percent, while

phosphorus removal of 10 to 15 percent might be expected.

Combined fixed-growth/suspended-growth package units are commercially available and are generally valuable in treating

high-strength wastewaters. These “high-biomass” units can be organically loaded at much higher rates than either fixedfilm or flow-through suspended growth systems. They are covered in the fact sheet on CFSGAS.

Management needs

With proper management, RBC package plants are reliable and should pose no unacceptable risks to the homeowner or the

environment. If not properly managed, however, the process can result in either premature failure of subsurface systems

or environmental damage through the production of poor-quality effluent that may pose public health risks. Odors and

filter flies may also create an environmental nuisance. Although there are benefits to RBCs, they do not come without

Table 1. Design parameters for fixed-film systems

a Loading rates for RBC are expressed per 1,000 ft2

 of total disk surface. b Sludge generated is in addition to solids removed in septic tank.
TFS-10

some cost. The mechanical complexity of some proprietary systems causes them to require more management attention.

Additional management is needed when disinfection and surface discharge are used.

The manufacturer normally fixes the pumping and recirculation rates for fixed-film systems, and the rates require minimal

adjustments once performance objectives are attained. Sludge wasting from the clarifier to the septic tank is normally

fixed by timer setting and requires occasional adjustment to avoid biomass buildup. Where mechanical or diffused aeration

is employed, complexity and required frequency of inspection increase. The most frequent need is to remove solids from

the distribution system. Other maintenance requirements are listed in table 2.

Fixed-film units are also operation and maintenance intensive. Startup of the unit does not require seeding with bacterial

cultures and may require 6 to 12 weeks for effective performance depending on the season. This makes them unsuitable

for seasonal application. Most operating parameters in package systems cannot be controlled by the operator. The process

is less labor-intensive than extended aeration (CFSGAS) systems, but it also requires semiskilled management personnel.

Based on limited data on these systems, it is estimated that 4 to 12 person-hours per year plus analytical services should be

sufficient. If disinfection is required, see Technology Fact Sheet 4. Power requirements depend on the package system

selected but may range from 1 to 8 kW-h/day. Sludge production is 0.6 to 1.0 lb TSS/lb BOD removed over and above

normal septic tank sludge (septage) production. Long power outages can be particularly damaging to RBC units, and any

FFS will become odiferous under these conditions.

Inspections are recommended three to four times per year, with septage pumping (solid wasting) as needed based on

inspection results. Routine maintenance requirements for onsite fixed-film systems are provided in table 2; certain tasks

may not be required based on system design. For example, servicing of the final clarifier may be less critical if solids

separated in the clarifier are returned to the primary settling chamber (septic tank). Field experience on operation and

maintenance for these units has not been as well documented as for CFSGAS.

Table 2. Suggested maintenance for onsite fixed-film package plants
Introduction

TFS-11

Risk management

Fixed-film systems also require a minimum of effluent disinfection to meet surface water discharge requirements. They

are more susceptible to extreme cold than CFSGAS but less sensitive to shock loading and influent variability. A prolonged

interruption of electric supply will result in odors. Filter flies may also be a nuisance with these systems if vents are not

properly screened.

Costs

Observed costs are highly variable depending on climate, location, onsite aesthetic requirements, and many other factors.

The cost of power should be in the range of $100 per year for RBC units and $35 per year for trickling filters. Capital

(installed) costs of $9,000 to $14,000 are typical. A management contract (estimated at about $100 to $200 per year) is

recommended.

References

Hutzler, N.L., L. Waldorf, and J. Fancy. 1978. Performance of Aerobic Treatment Units. In Proceedings of the Second

National Home Sewage Treatment Symposium. American Society of Agricultural Engineers, Chicago, IL.

Otis, R.J., and W. C. Boyle. 1976. Performance of single household treatment units. Journal of Environmental

Engineering Division, American Society of Civil Engineers, 102, EE1, 175.

Otis, R.J., et al. 1975. The Performance of Household Wastewater Treatment Units under Field Conditions. In

Proceedings of the Third National Home Sewage Disposal Symposium, American Society of Agricultural Engineers,

Chicago, IL, p.191.

Tchobanoglous, G., and F. Burton. 1991. Wastewater Engineering. 3rd ed. McGraw-Hill, Inc., New York.

Water Environment Federation. 1998. Design of Municipal Wastewater Treatment Plants. Manual of Practice no. 8. 4th ed.

Water Environment Federation, Alexandria, VA.

Water Pollution Control Federation (WCPF). 1988. O & M of Trickling Filters, RBCs, and Related Processes. Manual of

Practice OM-10. Water Pollution Control Federation, Alexandria, VA.

U.S. Environmental Protection Agency (USEPA). 1984. Design Information on Rotating Biological Contactors. EPA-600/

2-84-106. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1984. Review of Current RBC Performance and Design Procedures.

EPA-600/2-85-033. U.S. Environmental Protection Agency, Cincinnati, OH.
TFS-12 
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 3

Sequencing Batch Reactor Systems

Description

The sequencing batch reactor (SBR) process is a sequential suspended growth (activated sludge) process in which all major

steps occur in the same tank in sequential order (figure 1). There are two major classifications of SBRs: the intermittent

flow (IF) or “true batch reactor,” which employs all the steps in figure 1, and the continuous flow (CF) system, which does

not follow these steps. Both have been used successfully at a variety of U.S. and worldwide installations. SBRs can be

designed and operated to enhance removal of nitrogen, phosphorus, and ammonia, in addition to removing TSS and BOD.

The intermittent flow SBR accepts influent only at specified intervals and, in general, follows the five-step sequence.

There are usually two IF units in parallel. Because this system is closed to influent flow during the treatment cycle, two

units may be operated in parallel, with one unit open for intake while the other runs through the remainder of the cycles.

In the continuous inflow SBR, influent flows continuously during all phases of the treatment cycle. To reduce shortcircuiting, a partition is normally added to the tank to separate the turbulent aeration zone from the quiescent area.

Figure 1. Sequencing batch reactor (SBR) design principle
TFSThe SBR system is typically found in packaged configurations for onsite and small community or cluster applications. The

major components of the package include the batch tank, aerator, mixer, decanter device, process control system (including

timers), pumps, piping, and appurtenances. Aeration may be provided by diffused air or mechanical devices. SBRs are

often sized to provide mixing as well and are operated by the process control timers. Mechanical aerators have the added

value of potential operation as mixers or aerators. The decanter is a critical element in the process. Several decanter

configurations are available, including fixed and floating units. At least one commercial package employs a thermal

processing step for the excess sludge produced and wasted during the “idle” step. The key to the SBR process is the control

system, which consists of a combination of level sensors, timers, and microprocessors. Programmable logic controllers can

be configured to suit the owner’s needs. This provides a precise and versatile means of control.

Typical applications

SBR package plants have found application as onsite systems in some states and counties where they are allowed by code.

They are normally used to achieve a higher degree of treatment than a continuous-flow, suspended-growth aerobic system

(CFSGAS) unit by eliminating impacts caused by influent flow fluctuations. For discharge to surface waters, they must

meet effluent permit limits on BOD, TSS, and possibly ammonia. Additional disinfection is required to meet effluent fecal

coliform requirements. For subsurface discharge, they can be used in situations where infiltrative surface organic loadings

must be reduced. There are data showing that a higher quality effluent may reduce soil absorption field area requirements.

The process may be used to achieve nitrification as well as nitrogen and phosphorus removal prior to surface and subsurface discharge. (See Fact Sheets 8 and 9.)

Design assumptions

Typical IF system design information is provided in table 1. With CF-type SBRs, a typical cycle time is 3 to 4 hours, with

50 percent of that cycle devoted to aeration (step 2), 25 percent to settling (step 3), and 25 percent to decant (step 4). With

both types, downstream or subsequent unit processes (e.g., disinfection) must be designed for greater capacity (because the

effluent flow is several times the influent flow during the decant period) or an equalization tank must be used to permit a

consistent flow to those processes.

Onsite package units should be constructed of noncorrosive materials, such as coated concrete, plastic, fiberglass, or coated

steel. Some units are installed aboveground on a concrete slab with proper housing to protect against local climatic

concerns. The units can also be buried underground as long as easy access is provided to all mechanical parts, electrical

control systems, and water surfaces. All electric components should meet NEC code and should be waterproofed and/or

sheltered from the elements. If airlift pumps are used, large-diameter pipes should be provided to avoid clogging. Blowers, pumps, and other mechanical devices should be designed for continuous heavy-duty use. Easy access to all moving

Table 1. Design parameters for IF-type SBR treatment systems

a

 Cycle times should be tuned to effluent quality requirements, wastewater flow, and other site constraints.

14
TFSparts must be provided for routine maintenance. An effective alarm system should be installed to alert homeowners or

management entities of malfunctions. The area requirements for SBR package plants are similar to those in Fact Sheets 1 and 2.

Performance

With appropriate design and operation, SBR plants have been reported to produce high quality BOD and TSS effluents.

Typical ranges of CBOD5

 (carbonaceous 5-day BOD) are from 5 to 15 mg/L. TSS ranges from 10 to 30 mg/L in welloperated systems. FC removal of 1 to 2 logs can be expected. Normally, nitrification can be attained most of the time

unless cold temperatures persist. The SBR systems produce a more reliable effluent quality than CFSGAS or FFS owing

to the random nature of the wastewater generated from an individual home. The CF/SBR is also capable of meeting

secondary effluent standards (30 mg/L of CBOD and TSS), but more subject to upset by randomly generated wastewaters

than the IF/SBR (Ayers Associates, 1998) if short-circuiting cannot be minimized.

Management needs

Long-term management (including operation and maintenance) of SBRs through homeowner service contracts or local

management programs is an important component of the operation and maintenance program. Homeowners do not

typically possess the skills needed or the desire to learn to perform proper operation and maintenance. In addition, homeowner neglect, ignorance, or interference (e.g., disabling alarm systems) has contributed to operational malfunctions. No

wasting of biomass should be practiced until a satisfactory concentration has developed. Intensive surveillance by qualified

personnel is desirable during the first months of startup.

Most operating parameters in SBR package systems can be controlled by the operator. Time clock controls may be used to

regulate cycle times for each cycle, adjusted for and depending on observed performance. Alarm systems that warn of

aerator system failure and/or pump failure are essential.

Inspections are recommended three to four times per year; septage pumping (solids wasting) is dependent upon inspection

results. Routine maintenance requirements for onsite SBRs are given below. Operation and maintenance requires semiskilled personnel. Based on field experience, 5 to 12 person-hours per year, plus analytical services, are required. The

process produces 0.6 to 0.9 lb TSS/lb BOD removed and requires between 3.0 and 10 kWh/day for operation. Operating

Table 2. Suggested maintenance for sequencing batch reactor package plants

15
TFSpersonnel prefer these systems to CFSGAS for their simplicity of O/M tasks. The key operational components are the

programmer and the decanter, and these must be maintained in proper working order. The primary O/M tasks are provided

in table 2.

Risk management issues

With proper management, a package SBR system is reliable and should pose no unacceptable risks to the homeowner or

the environment. If neglected, however, the process can result in environmental damage through production of poorquality effluent that may pose public health risks and can result in the premature failure of subsurface systems. Odor and

noise may also create some level of nuisance. SBRs are less susceptible to flow and quality loading changes than other

aerobic biological systems, but they are still not suitable for seasonal applications. They are similarly susceptible to

extreme cold and should be buried and/or insulated in areas subjected to these extremes. Local authorities can provide

guidance on climatic effects on equipment and how to prevent them. The controller should be located in a heated environment. Long power outages can result in odors and effluent degradation, as is the case with other aerobic biological

systems.

Costs

For residential applications, typical system equipment costs are $7,000 to $9,000. Installation costs vary depending on site

conditions; installation costs between $1,500 and $3,000 are typical for uncomplicated sites with good access. It should be

noted that additional system components (e.g., subsurface infiltration system) will result in additional costs.

Annual operation and maintenance costs include electricity use (<$300/year), sludge removal (>$100/year), and equipment

servicing. (Some companies are providing annual service contracts for these units for $250 to $400.) Actual costs will vary

depending on the location of the unit and local conditions.

References

Arora, M.L., et al. 1985. Technology evaluation of sequencing batch reactors. Journal of the Water Pollution Control

Federation 57:867.

Ayres Associates. 1998. Florida Keys Onsite Wastewater Nutrient Reduction Systems Demonstration Project. HRS Contract

No. LP988. Florida Department of Health, Gainesville, FL.

Buhr, H.O., et al. 1984. Making full use of step feed capability. Journal of the Water Pollution Control Federation 56:325.

Deeny, K.J., and J.A. Heidman. 1991. Implementation of Sequencing Batch Reactor Technology in the United States.

Paper presented at the 64th Annual Meeting of the Water Pollution Control Federation, Toronto, Canada.

Eikum, A.S., and T. Bennett. 1992. New Norwegian Technology for Treatment of Small Flows. In Proceedings of Seventh

Northwest Onsite Wastewater Treatment Short Course, ed. R.W. Seabloom. University of Washington, Seattle.

U.S. Environmental Protection Agency (USEPA). 1986. Summary Report, Sequencing Batch Reactors. EPA 625/8-86-

001. Technology Transfer, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1987. Analysis of a Full-Scale SBR Operation at Grundy Center, Iowa.

EPA 600/J-87-065. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1993. Process Design Manual for Nitrogen Removal. EPA 625/R-93-

010. U.S. Environmental Protection Agency, Cincinnati, OH.

Water Environment Federation. 1998. Design of Municipal Wastewater Treatment Plants. Manual of Practice No. 8. Water

Environment Federation, Alexandria, VA.

16 
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 4

Effluent Disinfection Processes

Description

The process of disinfection destroys pathogenic and other microorganisms in wastewater. A number of important waterborne pathogens are found in the United States, including some bacteria species, protozoan cysts, and viruses. All pretreatment processes used in onsite wastewater management remove some pathogens, but data are scant on the magnitude

of this destruction. The two methods described in this section, chlorination and ultraviolet irradiation, are the most commonly used (figure 1). Currently, the effectiveness of disinfection is measured by the use of indicator bacteria, usually

fecal coliform. These organisms are excreted by all warm-blooded animals, are present in wastewater in high numbers,

tend to survive in the natural environment as long as or longer than many pathogenic bacteria, and are easy to detect and

quantify.

A number of methods can be used to disinfect wastewater. These include chemical agents, physical agents, and irradiation. For onsite applications, only a few of these methods have proven to be practical (i.e., simple, safe, reliable, and costeffective). Although ozone and iodine can be and have been used for disinfection, they are less likely to be employed

because of economic and engineering difficulties.

Chlorine

Chlorine is a powerful oxidizing agent and has been used as an effective disinfectant in water and wastewater treatment

for a century. Chlorine may be added to water as a gas (Cl2

) or as a liquid or solid in the form of sodium or calcium

hypochlorite, respectively. Because the gas can present a significant safety hazard and is highly corrosive, it is not recommended for onsite applications. Currently, the solid form (calcium hypochlorite) is most favored for onsite applications.

When added to water, calcium hypochlorite forms hypochlorous acid (HOCl) and calcium hydroxide (hydrated lime,

Ca(OH)2

). The resulting pH increase promotes the formation of the anion, OCl-, which is a free form of chlorine. Because

of its reactive nature, free chlorine will react with a number of reduced compounds in wastewater, including sulfide,

ferrous iron, organic matter, and ammonia. These nonspecific side reactions result in the formation of combined chlorine

(chloramines), chloro-organics, and chloride, the last two of which are not effective as disinfectants. Chloramines are

weaker than free chlorine but are more stable. The difference between the chlorine residual in the wastewater after some

Figure 1. Generic disinfection diagram
TFS-18

time interval (free and combined chlorine) and the initial dose of chlorine is referred to as chlorine demand. The 15-

minute chlorine demand of septic tank effluent may range from 30 to 45 mg/L as Cl; for biological treatment effluents,

such as systems in Technology Fact Sheets 1, 2, and 3, it may range from 10 to 25 mg/L; and for sand filtered effluent, it

may be 1 to 5 mg/L (Technology Fact Sheets 10 and 11).

Calcium hypochlorite is typically dosed to wastewater in an onsite treatment system using a simple tablet feeder device

(figure 2). Wastewater passes through the feeder and then flows to a contact tank for the appropriate reaction. The

product of the contact time and disinfectant residual concentration (Ct) is often used as a parameter for design of the

system. The contact basin should be baffled to ensure that short-circuiting does not occur. Chlorine and combined

chlorine residuals are highly toxic to living organisms in the receiving water. Because overdosing (ecological risk) and

underdosing (human health risk) are quite common with the use of tablets, long swales/ditches are recommended prior to

direct discharge to sensitive waters.

Use of simple liquid sodium hypochlorite (bleach) feeders is more reliable but requires more frequent site visits by operators. These systems employ aspirator or suction feeders that can be part of the pressurization of the wastewater, causing

both the pump and the feeder to require inspection and calibration. These operational needs should be met by centralized

management or contracted professional management.

Ultraviolet irradiation

The germicidal properties of ultraviolet (UV) irradiation have been recognized for many years. UV is germicidal in the

wavelength range of 250 to 270 nm. The radiation penetrates the cell wall of the organism and is absorbed by cellular

materials, which either prevents replication or causes the death of the cell. Because the only UV radiation effective in

destroying the organism is that which reaches it, the water must be relatively free of turbidity. Because the distance over

which UV light is effective is very limited, the most effective disinfection occurs when a thin film of the water to be

treated is exposed to the radiation. The quantity of UV irradiation required for a given application is measured as the

radiation intensity in microWatt-seconds per square centimeter (mW-s/cm2). For each application, wastewater transmittance, organisms present, bulb and sleeve condition, and a variety of other factors will have an impact on the mW-s/cm2

required to attain a specific effluent microorganism count per 100 mL. The most useful variable that can be readily

controlled and monitored is Total Suspended Solids. TSS has a direct impact on UV disinfection, which is related to the

level of pretreatment provided.

Figure 2. Example of a stack-feed chlorinator
TFS-19

Many commercial UV disinfection systems (figure 3) are

available in the marketplace. Each has its own approach to how

the wastewater contacts UV irradiation, such as the type of

bulb (medium or low pressure; medium, low, or high intensity), the type of contact chamber configuration (horizontal or

vertical), or the sleeve material separating the bulb from the

liquid (quartz or teflon). All can be effective, and the choice

will usually be driven by economics.

Typical applications

Disinfection is generally required in three onsite-system

circumstances. The first is after any process that is to be

surface discharged. The second is before a SWIS where there

is inadequate soil (depth to ground water or structure too

porous) to meet ground water quality standards. The third is

prior to some other immediate reuse (onsite recycling) of

effluent that stipulates some specific pathogen requirement

(e.g., toilet flushing or vegetation watering).

Design assumptions

Chlorination units must ensure that sufficient chlorine release occurs (depending on pretreatment) from the tablet chlorinator. These units have a history of erratic dosage, so frequent attention is required. Performance is dependent on pretreatment, which the designer must consider. At the point of chlorine addition, mixing is highly desirable and a contact chamber

is necessary to ensure maximum disinfection. Working with chlorinator suppliers, designers should try to ensure consistent dosage capability, maximize mixing usually by chamber or head loss, and provide some type of pipe of sufficient

length to attain effective contact time before release. Tablets are usually suspended in open tubes that are housed in a

plastic assembly designed to increase flow depth (and tablet exposure) in proportion to effluent flow. Without specific

external mixing capability, the contact pipe (large-diameter Schedule 40 PVC) is the primary means of accomplishing

disinfection. Contact time in these pipes (often with added baffles) is on the order of 4 to 10 hours, while dosage levels are

in excess of those stated in table 1 for different pretreatment qualities and pH values. The commercial chlorination unit is

generally located in a concrete vault with access hatch to the surface. The contact pipe usually runs from the vault toward

the next step in the process or discharge location. Surface discharges to open swales or ditches will also allow for dechlorination prior to release to a sensitive receiving water.

Figure 3. Wastewater flow in a quartz UV unit

Table 1. Chlorine disinfection dose (in mg/L) design guidelines for onsite applications

Note: Contact time = 1 hour at average flow and temperature 20 °C. Increase contact time to 2 hours at 10 °C and 8 hours at 5

°C for comparable efficiency. Dose = mg/L as Cl. Doses assume typical chlorine demand and are conservative estimates based

on fecal coliform data.
TFS-20

The effectiveness of UV disinfection is dependent

upon UV power (table 2), contact time, liquid film

thickness, wastewater absorbance, wastewater

turbidity, system configuration, and temperature.

Empirical relationships are used to relate UV power

(intensity at the organism boundary) and contact

time. Table 2 gives a general indication of the dose

requirements for selected pathogens. Since effective

disinfection is dependent on wastewater quality as

measured by turbidity, it is important that pretreatment provide a high degree of suspended and colloidal solids removal.

Commercially available UV units that permit internal contact times of 30 seconds at peak design flows for the onsite

system can be located in insulated outdoor structures or in heated spaces of the structure served, both of which must

protect the unit from dust, excessive heat, freezing, and vandals. Ideally, the unit should also provide the necessary UV

intensity (e.g., 35,000 to 70,000 mW-s/cm2) for achieving fecal coliform concentrations of about 200 CFU/100 mL. The

actual dosage that reaches the microbes will be reduced by the transmittance of the wastewater (e.g., continuous-flow

suspended-growth aerobic systems [CFSGAS] or fixed-film systems [FFS] transmittance of 60 to 65 percent). Practically,

septic tank effluents cannot be effectively disinfected by UV, whereas biological treatment effluents can meet a standard of

200 cfu/100 mL with UV. High-quality reuse standards will require more effective pretreatment to be met by UV disinfection. No additional contact time is required. Continuous UV bulb operation is recommended for maximum bulb service

life. Frequent on/off sequences in response to flow variability will shorten bulb life. Other typical design parameters are

presented in table 2.

Performance

There are few field studies of tablet chlorinators, but those that exist for post-sand-filter applications show fecal coliform

reductions of 2 to 3 logs/100 mL. Another field study of tablet chlorinators following biological treatment units exceeded a

standard of 200 FC/100 mL

93 percent of the time. No chlorine residual was present in 68 percent of the samples. Newer units managed by the

biological unit manufacturer fared only slightly better. Problems were related to TSS accumulation in the chlorinator, tablet

caking, failure of the tablet to drop into the sleeve, and failure to maintain the tablet supply. Sodium hypochlorite liquid feed

systems can provide consistent disinfection of sand filter effluents (and biological system effluents) if the systems are

managed by a utility.

Data for UV disinfection for onsite systems are also inadequate to perform a proper analysis. However, typical units

treating sand filter effluents have provided more than 3 logs of FC removal and more than 4 logs of poliovirus removal.

Since this level of pretreatment results in a very low final FC concentration (<100 CFU/100 mL), removals depend more

on the influent concentration than inherent removal capability. This is consistent with several large-scale water reuse

studies that show that filtered effluent can reach

essentially FC-free levels (<1 CFU/100 mL) with UV

dosage of about 100 mW-s/cm2

, while higher (but

attainable) effluent FC levels require less dosage to

filtered effluent (about 48 mW-s/cm2

) than is required

by aerobic unit effluent (about 60 mW-s/cm2

). This

can be attributed to TSS, turbidity, and transmittance

(table 3). Average quartz tube transmittance is about

75 to 80 percent.

Table 2. Typical ultraviolet (UV) system design parameters

Table 3. Typical (UV) transmittance values for water

Source: USEPA, 1986.
TFS-21

Management needs

Chlorine addition by tablet feeders is likely to be the most practical method for chlorine addition for onsite applications.

Tablet feeders are constructed of durable, corrosion-free plastics and are designed for in-line installation. Tablet chlorinators come as a unit similar to figure 2. If liquid bleach chlorinators are used, they would be similarly constructed. That

unit is placed inside a vault that exits to the contact basin. The contact basin may be plastic, fiberglass, or a length of

concrete pipe placed vertically and outfitted with a concrete base. Baffles should be provided to prevent short-circuiting of

the flow. The contact basin should be covered to protect against the elements, but it should be readily accessible for

maintenance and inspection.

The disinfection system should be designed to minimize operation and maintenance requirements, yet ensure reliable

treatment. For chlorination systems, routine operation and maintenance would include servicing the tablet or solution

feeder equipment, adding tablets or premixed solution, adjusting flow rates, cleaning the contact tank, and collecting and

analyzing effluent samples for chlorine residuals. Caking of tablet feeders may occur and will require appropriate maintenance. Bleach feeders must be periodically refilled and checked for performance. Semiskilled technical support should be

sufficient, and estimates of time are about 6 to 10 hours per year. There are no power requirements for gravity-fed

systems. Chemical requirements are estimated to be about 5 to 15 pounds of available chlorine per year for a family of

four. During the four or more inspections required per year, the contact basin may need cleaning if no filter is located

ahead of the unit. Energy requirements for a gravity-fed system are nil. If positively fed by aspirator/suction with pumping, the disinfection unit and alarms for pump malfunctions will use energy and require inspection. Essentially unskilled

(but trained) labor may be employed. Safety issues are minimal and include wearing of proper gloves and clothing during

inspection and tablet/feeder work.

Commercially available package UV units are available for onsite applications. Most are self-contained and provide lowpressure mercury arc lamps encased by quartz glass tubes. The unit should be installed downstream of the final treatment

process and protected from the elements. UV units must be located near a power source and should be readily accessible

for maintenance and inspection. Appropriate controls for the unit must be corrosion-resistant and enclosed in accordance

with electrical codes.

Routine operation and maintenance for UV systems involves semiskilled technician support. Tasks include cleaning and

replacing the UV lamps and sleeves, checking and maintaining mechanical equipment and controls, and monitoring the UV

intensity. Monitoring would require routine indicator organism analysis. Lamp replacement (usually annually) will depend

upon the equipment selected, but lamp life may range from 7,500 to 13,000 hours. Based on limited operational experience,

it is estimated that 10 to 12 hours per year would be required for routine operation and maintenance. Power requirements

may be approximately 1 to 1.5 kWh/d. Quartz sleeves will require alcohol or other mildly acidic solution at each (usually

four per year) inspection.

Whenever disinfection is required, careful attention to system operation and maintenance is necessary. Long-term management, through homeowner-service contracts or local management programs, is an important component of the operation

and maintenance program. Homeowners do not possess the skills needed to perform proper servicing of these units, and

homeowner neglect, ignorance, or interference may contribute to malfunctions.

Risk management issues

With proper management, the disinfection processes cited above are reliable and should pose little risk to the homeowner.

As mentioned above, a potentially toxic chlorine residual may have an important environmental impact if it persists at high

concentrations in surface waters. By-products of chlorine reactions with wastewater constituents may also be toxic to

aquatic species. If dechlorination is required prior to surface discharge, reactors containing sulfur dioxide, sodium bisulfate, sodium metabisulfate, or activated carbon can be employed. If the disinfection processes described above are improperly managed, the processes may not deliver the level of pathogen destruction that is anticipated and may result in some

risk to downstream users of the receiving waters. The systems described are compact and require modest attention.

Chlorination does not inherently require energy input; UV irradiation and dosage pumps do consume some energy
TFS-22 

(>1kWh/day). Both processes will require skilled technical support for the monitoring of indicator organisms in the

process effluents.

Chlorination systems respond to flow variability if the tablets are feeding correctly. UV does not do so and is designed for

the highest flow scenario, thus overdosing at lower flows since there is no danger in doing so. Toxic loads are unlikely to

affect either system, but TSS can affect both. Inspections must include all pretreatment steps. UV is more sensitive to

extreme temperatures than chlorination, and must be housed appropriate to the climate. In extremely cold climates, the UV

system can be housed inside the home with minimal danger to the inhabitants. Power outages will terminate UV disinfection and pressurized pumps for both systems, while causing few problems for gravity-fed chlorination units. There should

be no odor problems during these outages.

Costs

Installed costs of a complete tablet chlorination unit are about $400 to $500 for the commercial chlorinator unit and

associated materials and $800 to $1,200 for installation and housing. Operation and maintenance would consist of tablets

($30 to $50 per year), labor ($75 to $100 per year), and miscellaneous repairs and replacements ($15 to $25 per year), in

addition to any analytical support required.

Installed costs of UV units and associated facilities are $1,000 to $2,000. O/M costs include power ($35 to $40 per year),

semiskilled labor ($50 to $100 per year), and lamp replacement ($70 to $80 per year), plus any analytical support.

References

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1981. Evaluation of Onsite Wastewater Treatment and Disposal Options.

EPA 600/S2-81-178. NTIS No. PB-82-101-635. National Technical Information Service, Cincinnati, OH.

Crites, R., and G. Tchobanoglous. 1998. Small and Decentralized Wastewater Management Systems. WCB/McGraw-Hill,

San Francisco, CA.

Hanzon, B.D., and R. Vigilia. 1999. Just the facts. Water Environment and Technology November 1999, 34-42.

Scheible, O.K. 1987. Development of a rationally based design protocol for the ultraviolet light disinfection process.

Journal of the Water Pollution Control Federation 59:25.

University of Wisconsin. 1978. Management of Small Waste Flows. EPA 600/2-78-173. Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1980. Design Manual: Onsite Wastewater Treatment and Disposal

Systems. EPA 625/1-80-0012. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1986. Municipal Wastewater Disinfection Design Manual. EPA 625/1-

86-021. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1992.Ultraviolet Disinfection Technology Assessment. EPA-832/R-92-

004. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

Water Environment Federation. 1998. Design of Municipal Wastewater Treatment Plants, 3d ed. Alexandria, VA.

White, G.C. 1992. The Handbook of Chlorination and Alternative Disinfectants. 3d ed. Van Nostrand Reinhold, New

York.
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 5

Vegetated Submerged Beds and

Other High-Specific-Surface

Anaerobic Reactors

Description

A high-specific-surface anaerobic reactor (figure 1) is any tank or cavity filled with solid media through which wastewater

flows with a high hydraulic retention time (HRT). In onsite treatment the two primary types are vegetated submerged beds

(VSBs) and anaerobic upflow filters (AUFs). The first is characterized by horizontal flow and prolific growth of macrophytes on the surface. The second comes in a variety of forms from upflow sludge blanket systems and fixed media

anaerobic filters to partially fluidized beds of fine media. Both have long HRTs, produce anaerobic effluents, generally

treat either high-strength or minimally pretreated wastewater, and usually require some form of posttreatment to meet

surface discharge or water reuse requirements.

The primary removal mechanisms in all of these systems are physical, that is, floculation, sedimentation, and adsorption.

Anaerobic biological reactions are extremely slow and do not have a significant impact on soluble BOD until HRTs

become quite long. Some toxic organic compounds may be reduced through these mechanisms and chemical precipitation

(e.g., sulfides) at shorter HRTs.

VSBs, as shown in figure 2, usually follow a septic tank and remove most of the suspended and larger colloidal particles,

BOD, organic forms of nitrogen, and other particles. Although they are frequently identified as subsurface constructed

wetlands, they do not fit the strict definition of a constructed wetland.

Three types of AUFs can be used as pretreatment devices for high-strength wastewater and some onsite pretreatment

applications in the United States. They are in shown in figures 3, 4, and 5. Figure 3, with a rock medium, is the most

typical U.S. application.

Figure 1. Generic high-specific surface anaerobic reactor

Influent Effluent

Top

Treatment Media
TFS-24

Figure 2. Elements of a vegetated submerged bed (VSB) system

Pretreated

(settled)

influent

Liner

Inlet zone Top slope (media)

Outlet zone

Effluent

Variable-level

outlet

Treatment zone

Source: Toms Creek Project, VA.
TFS-25

Figure 3. Schematic of the upflow anaerobic filter process

Effluent

Influent

Treatment

edia

Flow

distributor

Drain

Recycle

Biogas

m

Biogas

Effluent

Sludge

blanket

Sludge

bed

Influent

Gas/solids

separator

Figure 4. Schematic of the upflow anaerobic sludge blanket process
TFS-26

Figure 5. Schematic of the anaerobic fluidized bed process

Typical applications

AUFs are widely used in hot climates where domestic wastewaters are several times higher in strength than U.S. wastewaters. These systems can reduce high BOD and TSS to levels that can be readily treated by typical aerobic processes such

as suspended and fixed growth aerobic units or recirculating/intermittent media filters. International literature contains

numerous references to the three types of AUFs and their valuable contributions to water pollution abatement. Anaerobic

rock upflow filters (figure 6) are also used to lower septic tank effluent BOD and TSS concentrations prior to discharge to

the subsurface wastewater infiltration system (SWIS).

Figure 6. Anaerobic upflow filter
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VSBs are extremely popular in the United States because of their aesthetic features and their ability to meet basic (secondary) effluent standards when treating septic tank effluent. Until recently they were purported to be capable of nitrification

and nutrient removal at economically competitive HRTs. Since they are largely anaerobic, this would be biochemically

impossible. However, they are fully capable of meeting secondary BOD and TSS standards. They are also sometimes used

before a SWIS and can meet the same effluent TSS and BOD standards as aerobic units (Technology Fact Sheets 1, 2, and

3). VSBs can be considered as pretreatment units regarding SWIS design requirements. They do not, however, remove

more than 2 logs of fecal coliform and would likely require disinfection for direct surface discharge. They also require

some form of aeration to meet effluent standards for dissolved oxygen (DO). These VSBs will capture rainfall and

snowmelt, effluent standards for requiring adjustment to designs of SWIS following these units.

Both VSBs and AUFs are being used in rural areas in combination with aerobic processes to remove significant amounts of

nitrogen through denitrification. These processes are included in the nutrient removal fact sheets.

Design assumptions

VSB design guidance for small communities is provided in table 1. In the first few months of operation, excellent phosphorus removal will occur until the rock medium becomes saturated with phosphorus and breakthrough occurs. (Note:

USEPA guidance on design of VSBs can be found in Manual: Constructed Wetlands Treatment of Municipal Wastewater,

posted at http://www.epa.gov/ordntrnt/ord/nrmrl/pubs/2001/wetlands/625r99010.pdf)

Except for the anaerobic upflow rock filter, AUFs are rarely employed for U.S. onsite applications. Since the primary

purpose of these systems is to improve the BOD and TSS of septic tank effluent, they are essentially physical processes.

Therefore, they must be designed to maximize their flocculation and sedimentation functions. Limited field studies

Table 1. Summary of VSB design guidance

a

 Use after primary sedimentation (e.g., septic tank, Imhoff tank, primary clarifier); not recommended

for use after ponds because of problems with algae.
TFS-28

indicate that successful removal of particulate BOD and TSS could be obtained with an average HRT between 16 and 24

hours, rounded media size of 1 to 2 inches or greater, and a means of periodically draining excess accumulated solids from

the bottom of the unit. At higher temperatures, some partial digestion of accumulated organic solids occurs. This liquefaction may by accompanied by gas production. The amount and makeup of that gas depend on pH, wastewater constituents

(e.g., protein, lipids, carbohydrates), sulfate, alkalinity, and other constituents.

Performance

VSB systems can treat septic tank effluent to a BOD of 20 to 30 mg/L, depending on the organic loading rate chosen. The

VSB effluent TSS is almost always less than 30 mg/L. Some removal of all constituents (e.g., heavy metals, organic

nitrogen and organic phosphorus, pesticides, and other toxic organics) can also be expected. Over and above these

removals, there will be some small percentage of dissolved organic removal owing to anaerobic biological activity.

Rock AUFs after septic tanks have not been widely studied, but they appear to remove TSS by as much as 55 percent from

septic tank effluent, while removing a similar percent of the BOD. Actual removals will depend on the specific fractions

of particulate, colloidal, and soluble matter in the septic tank effluent. Little soluble or fine particulate removal is likely.

Both systems will remove pathogens, with VSBs capable of removing from 1 to 3 logs (design average = 2 logs), while

AUF removal is estimated to be closer to 1 log because of shorter HRTs.

Management needs

All of these anaerobic systems are passive in nature and require minimal O/M activity. AUF units may be constructed

aboveground, but they usually are below the ground surface to provide insulation and protect against severe climatic

conditions. The solid medium can be a coarse gravel or one of many commercially available synthetic media that will not

easily clog with biomass. Access to inlet and outlet systems should be provided for purposes of cleaning and servicing. An

easily accessible means to drain the unit and an effective alarm system should be provided.

VSB units are generally aesthetically pleasing additions to the landscape if sufficient area is available for their application. It

is estimated that fewer than 4 hours per year will be required for O/M tasks, which will involve inspecting the system and

making any adjustments required. Therefore, until more information becomes available, a site visit schedule of three to

four times a year is suggested.

Residuals generate in VSB systems at a slow rate. Although the system inlet where most solids accumulate can be excavated or piped for high-pressure removal, it is more likely that a replacement system would be built after the service life of

the original system ends.

AUF units will require periodic flushing of accumulated solids and inspection of inlet and outlet systems. If solids are

allowed to accumulate, the filter may clog or release high solids “events” to the SWIS. This will clog the infiltrative

surface or the distribution system. Therefore, a site visit schedule of three to four times per year is suggested until more

information becomes available. This would entail from 6 to 8 hours per year of labor. Disposal and transport of excess

solids will require similar management to septage.

Risk management issues

VSB systems can usually handle the flow variations likely to occur from residential sources, as well as toxic shock loads

and power outages. Reed and colleagues (1995) proposed some models to support the view that insulation provided by

dead vegetation (litter) on the surface should aid these systems during typical winters in northern climates. The potential

for odor is low for properly sized systems.

AUF systems should also accommodate typical flow variations, toxic shocks, and power outages. They should be insulated

from cold weather. AUFs are inherently odor and corrosion generators, so corrosion-resistant materials should be employed.

Odor (hydrogen sulfide) production may require the use of an odor-control system (e.g., soil filters) to deodorize off-gases.
TFS-29

Costs

VSB systems for onsite application will cost about $20 per square foot (USEPA, 1999). Almost half of that cost is for the

media, while excavation, liner, plants, control structures, and piping make up the rest. Operation and maintenance costs

would run less than $100 per year if these services are professionally provided.

AUF systems are likely to cost about $1,000 to $1,500 per house, primarily related to the cost of the tank and related

containment features. O/M costs would run around $200 per year, including solids transport as required.

References

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1979. Evaluation of Onsite Wastewater Treatment and Disposal Options.

EPA 600/s2-81-178. U.S. Environmental Protection Agency, Cincinnati, OH.

Cowlter, J.B., S. Soneda, and M.B. Ettinger. 1957. Anaerobic contact process for sewage disposal. Sewage and Industrial

Wastes Journal 29(4):468-477.

Crites, R., and G. Tchobanoglous. 1998. Small and Decentralized Wastewater Management Systems. WCB McGraw-Hill,

San Francisco, CA.

DeRenzo, D.J. 1977. Energy from Bioconversion of Waste Materials. Noyes Data Corporation, Park Ridge, NJ.

Hamilton, J. 1975. Treatment of Septic Tank Effluent with an Anaerobic Filter. Master’s of Science in Civil Engineering

thesis, University of Washington, Seattle.

Hamilton, J. 1976. Proceedings of Northwest Onsite Wastewater Disposal Short Course. University of Washington, Seattle.

Jewell, W.J. 1987. Anaerobic sewage treatment. Journal of Environmental Science and Technology 21(1):14- 21.

Kennedy, J.C. 1979. Performance of Anaerobic Filters and Septic Tanks Applied to the Treatment of Residential

Wastewater. Master’s thesis, University of Washington, Seattle.

Lombardo & Associates, Inc. 1983. Design Report. Anaerobic Upflow Filters. Newton, MA.

Netter, R., E. Stubner, P.A. Wildner, and I. Sekoulov. 1993. Treatment of septic tank effluent in a subsurface biofilter.

Water Science Technology 28(10):117-124.

Reed, S.C., R.W. Crites, and E.J. Middlebrooks. 1995. Natural Systems for Waste Management and Treatment. McGraw

Hill, Inc, New York.

Switzenbaum, M.S. 1985. Proceedings of Seminar/Workshop-anaerobic Treatment of Sewage. Report No. Env.E. 88-85-5.

University of Massachusetts, Amherst, MA.

Thaulow, H. 1974. Use of Anaerobic Filters for Onsite Treatment of Household Wastewater. Master’s thesis, University of

Washington, Seattle.

U.S. Environmental Protection Agency (USEPA). 1992. Wastewater Treatment/Disposal for Small Communities. EPA 625/

R-92-005. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1993a. Nitrogen Control Manual. EPA 625/R-93/0010. U.S.

Environmental Protection Agency, Office of Research and Development, Washington, DC.

U.S. Environmental Protection Agency (USEPA). 1993b. Subsurface Flow Constructed Wetlands for Wastewater

Treatment: A Technology Assessment. EPA 832-R-93-008. U.S. Environmental Protection Agency, Office of Water,

Washington, DC.

U.S. Environmental Protection Agency (USEPA). 1999. Manual: Constructed Wetlands Treatment of Municipal

Wastewater. EPA 625/R-99/010. U.S. Environmental Protection Agency, Cincinnati, OH.
TFS-30 
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 6

Evapotranspiration and

Evapotranspiration/Infiltration

Description

Onsite evapotranspiration wastewater treatment systems are designed to disperse effluent exclusively by evapotranspiration. Evapotranspiration (ET) is defined as the combined effect of water removal from a medium by direct evaporation

and by plant transpiration. The evapotranspiration/infiltration (ETI) process is a subsurface system designed to dispose of

effluent by both evapotranspiration and infiltration into the soil. Both of these systems are preceded by primary pretreatment units (e.g., septic tank) to remove settleable and floatable solids. The influent to the ET or ETI units enters through

a series of distribution pipes to a porous bed. In ET systems, a liner is placed below the bed to prevent water loss via

infiltration unless the soil is impermeable. The surface of the sand bed is planted with water-tolerant plants. Effluent is

drawn up through fine media by capillary wicking and evaporated or transpired into the atmosphere. In ETI systems,

effluent is allowed to percolate into the underlying soil.

Modifications to ET and ETI systems include mechanical evaporating devices and a broad array of different designs and

means of distribution, storage of excess influent, wicking, and containment or infiltration prevention. Some newer studies

are using drip irrigation with distribution to forested areas with purported success.

Typical applications

ET and ETI systems are best suited for arid (evaporation exceeds precipitation) climates. If ETI is selected, soil percolation is also an important consideration. Both systems are often selected when site characteristics dictate that conventional

methods of effluent disposal are not appropriate (e.g., unprotected sole source aquifer, high water table or bedrock, tight

soils, etc.).

Although these systems normally follow septic tanks, additional pretreatment may be employed to minimize clogging of

the ET/ETI system piping and media. They are sometimes used as alternative systems during periods when normal

disposal methods are inoperable, for example, spray or other surface irrigation. Also, these systems have been widely used

for seasonal homes in areas where year-round application of ET/ETI is not practical and conventional methods are not

feasible. Year-round ET systems (see figure 1) require large surface areas and are most feasible in the areas shown on

figure 2. ETI systems can be employed to reduce the infiltrative burden on the site during the growing season. Such

applications can also result in some reduction in nutrients, which are transferred to the overlying vegetation (USEPA,

1999).
TFS-32

Figure 1. Cross section of a generic evapotranspiration bed (adapted from NSFC)

Figure 2. Relative suitability for evapotranspiration systems
TFS-33

Design assumptions

The design evapotranspiration rate is site specific. Some areas are arid (precipitation < evaporation) but lack the solar

radiation or wind velocities necessary to efficiently evaporate wastewater throughout the year. Therefore, simple use of

well-known evaporation estimates like Pentman, Blaney-Criddle, and Jensen-Haise will not likely be satisfactory. In fact,

historically, the definition of workable ET rates for an area has been a trial and error process, which is further complicated

by the system design and the plants used. The primary variables that have an impact on the potential ET rate are climate,

cover soil, and vegetation. The most important system variables, which control the movement of wastewater to the

surface, are media and the depth to saturated (stored) water. Most published designs are suspect because they store the

wastewater so deep that the wicking properties of the fill and the area (voids) through which water must rise restrict

delivery of water to the surface where it is evaporated.

Present ET system designs normally employ 20-mil polyethylene liners where the soil is too permeable and ground water

contamination is likely. Most employ distribution systems placed in 12 inches of gravel (0.75 to 2.5 inches) at the bottom

of the bed. Spacing of the distribution pipes is 4 to 12 feet, with lower values preferred for better distribution. Wicking is

accomplished by a 2- to 2.5-foot layer of sand (0.1 millimeter) and a loamy soil-sand mix to raise the water to the surface

or a thin layer of soil at the surface. Most have employed the formula:

A = nQ/ET – P

where:

A = surface area required to evaporate the wastewater

n = coefficient, which varies from 1 to 1.6

Q = annual flow volume

ET = annual evapotranspiration rate

P = annual precipitation rate

Each of these factors is open to some degree of interpretation. Because these systems are large and expensive, there has

been a tendency to minimize their design size and cost, resulting in significant failure rates. Typical ET estimates range

from 0.01 to 2.0 centimeters per day. The contribution of plants has remained a matter of controversy. ET bed sizing has

varied from 3,000 to 10,000 square feet and higher. A water balance based on at least 10 years of data is calculated to

provide sufficient storage for nonsurfacing operations or to estimate nonatmospheric volumes to be infiltrated.

The modern use of shallow trenches for SWIS is strongly related to the maximization of ET, and such systems could be

classified as ETI systems. Further, the use of shallow serial distribution where topographic relief is available is a classic

application of the ETI concept, that is, shallow trenches close to the surface, full of wastewater, with only a short wicking

distance to the evaporative surface. Such a system fulfills all the described features of an ideal ETI system. Similarly, drip

irrigation uses the shallowest of all SWIS burial requirements and, by nature, maximizes ET potential.

Performance

There have been few studies of ET and ETI systems. Most ET system studies have been less than impressive. In most

cases the fault has been related to poor design assumptions, for example, over-estimating the ET potential of shrubs and

trees planted on the surface and of the overall potential of ET itself. Poor system design has been somewhat offset by

leaking liners that give the appearance that the system is performing adequately. Inadequate wicking has been overcome by

raising water levels. However, better ET assessment and more rational designs will improve performance at increased

costs.

ETI systems have generally worked well, but no scientific studies have been performed to verify this observation. ETI

systems do fail when the ET contribution is overestimated, but many times the placement of the wastewater higher in the

soil profile offsets that error by increasing the infiltrative capacity of the site.
TFS-34

Management needs

ET systems are very sensitive to variations in construction techniques. Poor construction can defeat their utility through

poor liner installation, poor placement and choice of wicking media, compaction, and inadequate surface drainage mitigation.

Operation and maintenance requirements are minimal, often consisting of simply mowing the grass on the surface.

Replanting cover crops to improve cold season performance has been suggested but offers little return. Shrubs or small

trees planted on the ET system generally improve active (warm) season ET and hinder ET in the dormant (cold) season.

Therefore, the O/M needs of the system should be limited to two to three short visits to observe and record the water

height in the observation well. These tasks require about 1 to 2 hours per year of unskilled labor. No energy is required.

ET system salt buildup, if not diluted by precipitation, may require some media replacement after 5 to 10 years of operation depending on water supply characteristics. There are no known safety issues with these systems as long as they are

fenced or otherwise isolated from children’s play areas.

ETI systems are very similar to SWIS systems, and their management requirements are similar to those of ET systems.

Because ETI systems infiltrate wastewater, they have ground water and surface water contamination concerns like those of

other SWIS designs, and they may require monitoring of effluent impacts depending on the uses of ground water and

performance standards to protect them.

Risk management issues

Because ET systems are large, there may be some visual aesthetic problems. Odors are usually not a problem, but they can

be on occasion. Flow peaks during low ET periods could result in overflows, thus leading to the usual restriction for yearround ET use in areas where ET does not exceed precipitation by more than 2 inches per month. These systems do not

function when their surface freezes. They are typically unaffected by power outages since they are generally fed by

gravity. Toxics also have no impact unless they are phytotoxic and would then kill the surface vegetation.

Costs

Because of their large size and specific media (and often liner) requirements, ET systems are generally expensive, reinforcing their use as a “last resort” alternative. Installed costs of $10,000 to $15,000 and higher are possible depending on

climate and location. O/M costs are relatively low, on the order of $20 to $30 per year, but they could increase if the

system fills and requires pumping. ETI systems have capital and O/M costs similar to a SWIS.

References

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1979. Evaluation of Existing and Potential Technologies for Onsite

Wastewater Treatment and Disposal. EPA 600/S2-81-178. U.S. Environmental Protection Agency, Cincinnati, OH.

Beck, A.F. 1979. Evapotranspiration bed design. Journal of Environmental Engineering Division-American Society of Civil

Engineers 105(2): 411-415.

Frank, W.L. 1996. Engineering parameters in the design of evapotranspiration beds. Water and Engineering Management

November, 31-37.

Ingham, A.T. 1987. Guidelines for Evapotranspiration Systems. State Water Resources Control Board, State of California.

Sacramento, CA.

Lomax, K.M., et al. 1978. Guidelines for Evapotranspiration Systems. State Water Resources Control Board, State of

California. Sacramento, CA.

National Small Flows Clearinghouse (NSFC). 1998. Evapotranspiration Systems Fact Sheet. Cooperative Agreement

CX825652, U.S. Environmental Protection Agency, Washington, DC.
TFS-35

National Small Flows Clearinghouse (NSFC). 2000. Evapotranspiration systems. Pipeline 11(1).

Peters, E.C. 1988. An Evaluation of Enhanced ET Onsite Sewage Treatment and Disposal Systems. Master’s thesis,

University of Maryland, College Park.

Salvato, J.A. 1982. Rational design of evapotranspiration bed. Journal of Electrical Engineering-American Society of Civil

Engineers 109(3):646-660.

U.S. Environmental Protection Agency (USEPA). 1999. Manual: Constructed Wetlands Treatment of Municipal

Wastewaters. EPA/625/R-99/010. U.S. Environmental Protection Agency, Cincinnati, OH.

Victoria (AUS)-Environmental Protection Agency. 1980. The Use of Transpiration Beds for Domestic Wastewater

Disposal. EPA Report No. 104. Melbourne, Australia.

Wheeter, D.W. 1979. The Use of Evapotranspiration as a Means of Wastewater Disposal. Research Report No. 73.

Tennessee Water Resources Research Center, University of Tennessee, Knoxville.
TFS-36 (Click Here to Return to Bookmarks Page)
Introduction

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Onsite Wastewater Treatment Systems

Technology Fact Sheet 7

Stabilization Ponds, FWS

Constructed Wetlands, and Other

Aquatic Systems

Description

Aquatic systems are large basins filled with wastewater undergoing some combination of physical, chemical, and/or

biological treatment processes that render the wastewater more acceptable for discharge to the environment (figure 1).

They are not widely used because they tend to be large in area, require some form of fencing to minimize human health

risk, often require supplemental treatment

before discharge or reuse, and are approved in

only a few states.

Stabilization ponds (lagoons) have many

forms, but the facultative lagoon is the most

widely used. Aerated lagoons are often preferred because of their smaller size requirements. Anaerobic lagoons and maturation

ponds are not used in the United States for

onsite application by design. In some areas,

lagoons must be lined according to codes,

which further limits their application. Facultative lagoons are large in size, perform best when segmented into at least

three cells, obtain necessary oxygen for treatment by surface reaeration from the atmosphere, combine sedimentation of

particulates with biological degradation, and produce large quantities of algae, which limits the utility of their effluent

without further treatment. A typical

facultative lagoon is shown in figure 2.

Aerated lagoons use mechanical

equipment to enhance and intensify the

biodegradation rate. They do not

produce the intense algal load on

downstream processes and have smaller

areal requirements than facultative

systems.

Free water surface (FWS) constructed

wetlands have also been used, though rarely, for similar reasons. These systems perform best when divided into a minimum of three zones, the first and last being fully vegetated with macrophytes (cattails or bulrushes) and the middle having

an open water surface, which performs like a facultative lagoon. In the first zone, the influent is suspended and colloidal

solids are flocculated and settled under anaerobic or anoxic conditions. The second zone reaerates the anaerobic wastewater

Figure 1. Generic aquatic lagoon system

Figure 2. Generic facultative lagoon
TFS-38

to provide oxygen for aerobic biodegradation and possible nitrification before the final-zone flocculation and sedimentation (and denitrification) steps. An FWS constructed wetland is shown in figure 3.

Typical applications

Facultative lagoon systems, like evaporative (ET) systems, are not widely used for onsite wastewater treatment. They are

large in size, are expensive to build, perform only a portion of the treatment necessary to permit surface discharge or reuse

(table 1), and produce large concentrations of algae, which negates their use as direct pretreatment before soil infiltration.

They have been used in a few states as an alternative system when a subsurface wastewater infiltration system (SWIS) is

not feasible, usually to discharge without further treatment to surface waters, which is generally unacceptable under

normal circumstances. In some states intermittent discharge lagoons are required. Storage volume is for all cold weather

months (4 to 6 months), making the size of these systems too large for most applications.

Aerated lagoons require far less land and could theoretically be used in place of aerobic biological treatment, but they

cannot be buried and insulated in northern climates like those units. They could be used in southern climates as pretreatment for SWISs and would otherwise have similar features to the fact sheets that describe those systems.

FWS constructed wetlands reliably produce an advanced secondary effluent and can be employed for significant nitrogen

removal. They require large land areas, similar to facultative lagoons. Their effluent quality is excellent for SWIS application, but they require disinfection for surface discharge and many reuse options. They have a highly desirable appearance,

which often makes them the preferred alternative for owners with sufficient land areas.

Figure 3. Elements of a free water surface (FWS) constructed wetland

Table 1. Typical design guidance
Introduction

TFS-39

All of these aquatic systems should be placed after the septic tank and before the disinfection or SWIS steps in the treatment train.

Performance

Facultative lagoons are capable of 75 to 95 percent BOD removal, but TSS removal varies widely because of algal

growth. During nonalgal periods, up to 90 percent TSS removal is possible, but during warm seasons TSS removal can be

negligible. In summer months 80 percent of the ammonia-nitrogen is nitrified, total nitrogen removal can reach 60

percent, and total phosphorus removal can approach 50 percent. Very long detention times in hot climates can reduce fecal

coliforms to levels that can often meet surface water discharge standards, but typical U.S. retention times reduce fecal

coliforms by 2 to 3 logs/100 mL.

Aerated ponds have removal capabilities similar to facultative lagoons, except that TSS removal is more consistent with

aerobic biological systems (20 to 60 mg/L). Nitrification of ammonia-nitrogen can be nearly complete in warm seasons,

while cold weather will halt that process. Some minimal phosphorus and nitrogen removal (10 to 20 percent) can be

anticipated. Fecal coliform removal of 1 to 2 logs/100 mL is likely.

FWS systems can produce effluent BOD and TSS of 20 to 30 mg/L and can reduce nitrogen significantly. TP reduction is

generally minor and similar to that of lagoons. Fecal coliform removals of about 2 to 3 logs (99 to 99.9 percent) can be

expected.

Management needs

Aquatic systems are normally excavated in natural soil and constructed with earthen dikes. They may or may not be lined,

depending on soil type. Sufficient freeboard (up to 2 feet) must be provided to prevent topping during high winds. In

some cases the lagoon may act as a percolation pond, allowing effluent to infiltrate into the underlying soil. When used,

mechanical aeration devices must be installed and fixed in place. Aerators may be mounted on piers or floats. Appropriate

controls and electrical connections must be provided. Inlets should be located as far away from outlets as possible, and

both should be accessible for normal maintenance. Piping and pumps, as required, should be of corrosion-resistant materials, and pumps should be readily accessible. Fencing will normally be required to restrict access by the public.

The operation and maintenance of aquatic systems is typically minimal. Some attention must be paid to flow monitoring

and adjustments, as required. Inlet and outlet structures, berms, and surface blockages should be inspected and maintained.

The use of mechanical aeration will require operation and maintenance tasks but less than those for extended aeration

systems. Sludge management is relatively simple, since sludge builds up very slowly over a period of 10 to 15 years.

Pretreatment of lagoon influent by septic tanks will greatly reduce sludge accumulations in the lagoon. Requiring septic

tanks for individual homes or facilities served by lagoon cluster systems is recommended. Monitoring of effluent quality

for parameters of interest should be provided.

Only aerobic ponds require energy and semiskilled operators. Energy costs are in the range of $150 to $250 per year, and

labor costs would be $200 to $250 per year. Sludge production would be similar to aerobic units. Facultative lagoons and

FWS systems require nonskilled O/M personnel to visit the facility two to three times per year. Sludge removal will be

required every 10 to 15 years at most. A fence around the facility usually satisfies safety needs.

Risk management issues

Aquatic systems, particularly facultative lagoons and FWS constructed wetlands, are large, passive systems that are

minimally affected by flow variations, extreme cold, and power outages. The risk of drowning can be mitigated only with

restricted access, such as that provided by fencing and signage. Aerated lagoons are negatively affected by toxic loads,

extreme cold, and power outages. Both lagoon types can be overloaded, resulting in odors, and the aerated type may

become odorous during power outages. Poorly maintained facultative lagoons and FWS systems can become sources of
TFS-40 

vector problems such as mosquito infestations. FWS systems can be negatively affected by extended toxic discharges, but

their aesthetic image is extremely positive.

Costs

Capital costs for a facultative lagoon for an individual home would be in the range of $2,500 to $7,500, whereas an

aerated lagoon should cost somewhat more. An FWS system would cost $2,000 to $4,000.

Operation and maintenance costs for the facultative lagoon and FWS systems should be less than $100 per year, whereas

O/M costs for the aerated lagoon (including power) would be $350 to $500 per year.

References

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1979. Evaluation of Onsite Wastewater Treatment and Disposal Options.

EPA 600/S2-81-178. U.S. Environmental Protection Agency, Cincinnati, OH.

National Small Flows Clearinghouse. 1996. Summary of Onsite System in the United States, 1993. National Small Flows

Clearinghouse Publication, Morgantown, WV.

Reed, S.C., R.W. Crites, and E.J. Middlebrooks. 1995. Natural Systems for Waste Management and Treatment McGrawHill, New York, NY.

U.S. Environmental Protection Agency (USEPA). 1983. Design Manual: Municipal Wastewater Stabilization Ponds. EPA

625/1-83/015. Center for Environmental Research Information, U.S. Environmental Protection Agency, Cincinnati,

OH.

U.S. Environmental Protection Agency (USEPA). 1992. Manual: Wastewater Treatment Disposal for Small Communities.

EPA 625/R-92/005. U.S. Environmental Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1999. Manual: Constructed Wetlands Treatment of Municipal

Wastewaters. EPA 625/R-99/010. U.S. Environmental Protection Agency, Cincinnati, OH.

Water Environment Federation. 1990. Natural Systems for Wastewater Treatment: Manual of Practice FD-16. Water

Environment Federation, Alexandria, VA.
(Click Here to Return to Bookmarks Page) TFS-41

Onsite Wastewater Treatment Systems

Technology Fact Sheet 8

Enhanced Nutrient Removal—

Phosphorus

Description

There are a large number of processes that can reduce nitrogen and a few that can reduce phosphorus. Most of these

phosphorus removal processes are additions to other pretreatment processes that enhance the overall removal of phosphorus

(figure 1). The degree of nutrient removal, the cost, and the O/M difficulty of these combinations quickly reduce the

number of systems that are likely to be implemented for onsite nutrient removal. The removal of phosphorus is of concern

where effluents may enter surface waters via direct

surface discharge or subsurface flow through

fractured bedrock, and in soils where little phosphorus exchange would take place (see chapter 3).

Phosphorus is a key element in the eutrophication

of natural or impounded freshwater bodies and

some estuarine waters.

Few phosphorus removal processes are well

developed for onsite wastewater systems application. Those that have been successfully applied

generally fall into the categories of chemical, physical, and biological systems. The controlled addition of chemicals such

as aluminum, iron, and calcium compounds with subsequent flocculation and sedimentation has had only limited success

because of inadequate operation and maintenance of mechanical equipment problems and excessive sludge production.

Physical and chemical processes such as ion exchange and precipitation of phosphates have been tried, but with limited

success. Most notable successes have come with special filter materials that are naturally high in their concentration of the

above chemicals, but their service lives are finite. Studies of high-iron sands and high-aluminum muds indicate that 50 to

95 percent of the phosphorus can be removed. However, the life of these systems has yet to be determined, after which the

filter media will have to be removed and replaced. Use of supplemental iron powder mixed with natural sands is also being

researched. All calcareous sands and other sands with high concentrations of these three elements will exhibit high phosphorus removal rates for some finite periods. Typical calcium-containing U.S. sands will essentially exhaust their capacity

in 3 to 6 months, after which they will remove only particulate-based organic phosphorus or about 10 to 20 percent of the

phosphorus contained in the wastewater.

One other practical way to minimize phosphorus discharges to the environment is the use of low-phosphate or phosphatefree detergents, which can reduce the wastewater P concentration from 7 to 8 mg/L to 5 to 6 mg/L. In terms of P movement from the SWIS to nearby waters, such a change could add 30 to 40 percent to the site’s service life in attenuating or

containing P from movement away from the SWIS. Of all the options, this may be the simplest, but concerns over public

acceptance of these detergents as cleaning agents persist.

The only other known P-removal approach is the use of biological treatment systems. All aerobic treatment systems

described in other fact sheets have the natural ability to remove 10 to 20 percent of the influent phosphorus, which is

connected to the organic form in the biological reactors and wasted with excess sludge. Certain processes such as the

Figure 1. Phosphorus removal
TFS-42

sequencing batch reactor (SBR) can improve on this removal by proper sequencing of aeration periods. Other aerobic

biological units can similarly upgrade their phosphorus removal performance by the addition of anaerobic steps up to an

effluent limit of 1 to 2 mg P/L, but the data to support the onsite applications of these upgrade technologies are lacking.

Typical application

Phosphorus is rarely designed to be removed in onsite pretreatment because most soils have the innate ability to adsorb

the nutrient for many years before it begins to migrate to nearby ground or surface waters. However, as onsite system sites

age, there is the potential for serious environmental degradation, as witnessed by the thousands of inland lakes where older,

onsite development is increasingly being cited as the primary reason for lake eutrophication.

Therefore, the most likely P-reduction systems that will be applied are iron-rich intermittent sand filter (ISF) media,

sequencing batch reactors (SBR), and phosphate-free detergents. Other systems will surely be developed, especially

upgraded aerobic treatment systems, but these three systems are most representative of current phosphorus reduction

programs.

Design assumptions

For special filter media, the design assumptions would be the same as those for an intermittent sand filter (ISF) with

adjustment to the hydraulic and phosphorus areal rates because they might differ from conventional systems. Hydraulic

loadings for one successful study are essentially 3 cm/day, and the TP loading is 0.16g/m2/day. The major unknown is the

life of the special P-adsorption media. Most high-calcium sands become saturated in a few months, but one specific case

has reported 2.5 years. Generally, these sands are not cost-effective. High-iron sands and crushed bricks are being studied

and show longer durations of P-removal effectiveness, but definitive service lives are as yet unknown. The use of “red

mud” and iron oxide powder mixed with sands and placed below the infiltrative surface in the SWIS has been successful,

but the life of such media and the difficulty of replacement make these concepts less attractive unless the former is in the

range of 20 years. Red mud (a bauxite mining by-product) must constitute at least 30 percent of the total volume of the

filter bed. In a SWIS, the material must be mixed with the natural soil to a depth of 1 foot (0.3 m) below the infiltrative

surface to attain high P-removal efficiency. Specific depths of mixed soils and loading rates have not been clearly delineated.

SBRs are capable of phosphorus removals greater than the typical CFSGAS, which can range from 20 to 40 percent. This

is best accomplished by the “true” SBR (IF), but also by continuous feed (CF) SBRs if designed to do so. The IF type

must not aerate during the fill stage in order to remove greater amounts of TP. The CF type must have a no-aeration

section immediately following the recycle point to accomplish similar goals. Such designs are capable of reaching effluent

TP in the range of 1.0 mg/L. The only onsite CF test available did not employ this sequence and removed only about 30

percent of the TP. Sludge wasting requirements are severe and limit the performance of this alternative.

Because carbon-to-phosphorus ratios in septic tank effluent are generally favorable (typically, 150 mg/L BOD to 7mg/L

TP), the anoxic/anaerobic first stages (combined with appropriate organic loading rates and HRTs, as noted in the SBR

fact sheet) can result in significant TP removal. Typically, this mode of SBR operation should also remove most of the

nitrogen. All the phosphorus removal options require noncorrosive materials of construction, appropriate alarms and

sensing systems, and regular management by semiskilled staff.

Performance

The systems described above, in concert with low- or non-phosphate detergent use, are capable of removing phosphorus to

an effluent value of 1 to 2 mg/L with proper maintenance. Subsequent travel through the soil’s vadose zone would further

enhance TP concentrations to very low ambient values. Direct discharge (after disinfection) would meet most surface

discharge requirements.

Phosphorus removal should be provided in sensitive surface water areas if direct surface discharging systems are used, or

if SWISs are located in noncalcareous, low-iron or low-aluminum soils in close proximity to or directly influencing

sensitive surface waters.
TFS-43

Management needs

The use of low- or non-phosphate detergents would generally be a regional responsibility. Management of a high-iron or a

high-aluminum filter would be similar to that required for ISFs. Flows and dosing rates should be checked on each O/M

visit, along with annual recalibration of dosing pumps and monitoring of TP in the effluent. At least two visits per year

are suggested to manage these systems (or 8 hours per year).

The SBR option is exactly the same as in the SBR fact sheet or three to four visits per year by semiskilled personnel (6 to

12 hours), with electrical usage of 3 to 10 kWh/day. The SBR will produce an additional 0.6 to 1.0 lb TSS/lb BOD

removed, over and above the solids captured in the septic tank.

Risk management issues

The two treatment systems described above are relatively unaffected by wide flow variations. The SBR can be seriously

impaired by the toxic shocks but not the enhanced ISF. Both should be safe from extremely cold climates if properly

insulated, but the SBR will suffer reduced biochemical efficiency in such extremes. Power outages will affect the SBR,

producing odors and poor efficiency for some time after power restoration. The enhanced filter will also be interrupted

because of dosing pump failure, but it should not experience odors or subsequent impairment.

Costs

Enhanced TP-removal filters will have cost characteristics similar to conventional ISFs except in the initial and subsequent

replacement of the enhanced media. Such a system may have an initial media cost increment of at least 1.2 and possibly

2.0 or larger, and an annual additional O/M cost related to more frequent media replacement. For example, a 5-year life

would mean that a substantial replacement charge would be incurred every 5 years, equating to several hundred dollars per

year in O/M cost over and above the normal O/M cost of $250 to $400 per year. The capital cost would vary between

$5,000 and $11,000.

The SBR would exhibit similar capital ($9,000 to $12,000 per year) and O/M ($650 to $800 per year) costs as provided in

Technology Fact Sheet 3.

References

Ayres Associates. 1997. Florida Keys Onsite Wastewater Nutrient Reduction Systems (OWNRS) Demo Project Control

Testing Facility: 2nd Quarter Status Report. Report to Florida Department of Health under Contact No. LPQ988 and

U.S. Environmental Protection Agency under Contract No. X994394-93-0. Ayres Associates, Madison, WI.

Brandes, M. 1977. Effective phosporus removal by adding alum to septic tank. Journal of Water Pollution Control

Federation 49:2285-2296.

Ho, G.E., K. Mathew, and R.A. Gibbs. 1992. Nitrogen and phosphorus removal from sewage effluent in amended soil

columns. Water Resources 26(3):295-300.

Irvine, R.L., L.H. Ketchum, Jr., M.L. Arora, and E.F. Barth. 1985. An organic loading study of full-scale SBRs. Journal

of Water Pollution Control Federation 57(8):847-853.

National Small Flows Clearinghouse. 1999. Benzie County, Michigan, NODPI project completed. Small Flows 13(4):10-

11.

U.S. Environmental Protection Agency (USEPA). 1987. Phosphorus Removal Design Manual. EPA 625/1-87/001. U.S.

Environmental Protection Agency, Water Engineering Research Laboratory, Cincinnati, OH.
TFS-44 
Introduction

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Onsite Wastewater Treatment Systems

Technology Fact Sheet 9

Enhanced Nutrient Removal—

Nitrogen

Description

Nitrogen is a pollutant of concern for a number of reasons. Nitrogen in the ammonia form is toxic to certain aquatic

organisms. In the environment, ammonia is oxidized rapidly to nitrate, creating an oxygen demand and low dissolved

oxygen in surface waters. Organic and inorganic forms of nitrogen may cause eutrophication (i.e., high productivity of

algae) problems in nitrogen-limited freshwater lakes and in estuarine and coastal waters. Finally, high concentrations of

nitrate can harm young children when ingested.

Ammonia oxidation (nitrification) occurs in some of the processes described in previous fact sheets, and is dependent upon

oxygen availability, organic biochemical oxygen demand (BOD), and hydraulic loading rates. Nitrogen removal by means

of volatilization, sedimentation, and denitrification may also occur in some of the systems and system components. The

amount of nitrogen removed (figure 1) is dependent upon process design and operation. Processes that remove 25 to 50

percent of the total nitrogen include aerobic biological systems and media filters, especially recirculating filters (Technology Fact Sheet 11). Enhanced nitrogen removal systems can be categorized by their mode of removal. Wastewater separation systems, which remove toilet wastes and garbage grinding, are capable of 80 to 90 percent nitrogen removal. Physical-chemical systems such as ion exchange, volatilization, and membrane processes, are capable of similar removal rates.

Ion exchange resins remove NH4

-N or NO3- N. Membrane processes employ a variety of membranes and pressures that all

have a significant reject flow rate. Volatilization is generally significant only in facultative lagoon systems where ammonia

volatilization can be significant. The vast majority of practical nitrogen-removal systems employ nitrification and denitrification biological reactions. Most notable of these are recirculating sand filters (RSFs) with enhanced anoxic modifications, sequencing batch reactors (SBR), and an array

of aerobic nitrification processes combined with an

anoxic/anaerobic process to perform denitrification.

Some of the combinations are proprietary. Any

fixed-film or suspended-growth aerobic reactor can

perform the aerobic nitrification when properly

loaded and oxygenated. A variety of upflow (AUF),

downflow, and horizontal-flow anaerobic reactors

can perform denitrification if oxygen is absent, a

degradable carbon source (heterotrophic) is provided, and other conditions (e.g., temperature, pH,

etc.) are acceptable.

The most commonly applied and effective nitrogen-removal systems are biological toilets or segregated plumbing options

and/or nitrification-denitrification process combinations. A more complete list is described below, along with accompanying schematic diagrams.

Figure 1. Nitrogen removal systems
TFS-46

Source separation systems

Source separation relies on isolating toilet wastes or blackwater from wastewater. This requires separate interior collection

systems. Two source separation systems were identified: blackwater holding tank with low-volume-discharge toilets and

graywater septic tank system, and non-water-carriage toilets and graywater septic tank system (figure 2). These types of

toilets are discussed in chapter 3.

Blackwater holding tank with low-volume-discharge toilets and graywater septic tank system

Blackwater discharged directly to a holding tank requires periodic removal for offsite treatment. Graywater wastes can be

discharged to a conventional septic tank or subsurface infiltration system.

Non-water-carriage toilets and graywater septic tank system

Excreta is discharged to non-water-carriage toilets to promote bulk reduction and decomposition. Biological and incineration toilets are the most common methods of accomplishing this. Non-water-carriage toilets that use these processes are

commercially available. The remaining graywater wastes can be discharged to a conventional septic tank subsurface

infiltration system.

Physical/chemical treatment systems

Two types of physical/chemical treatment systems, ion exchange and reverse osmosis, appear to have some promise for

single home use, although neither is in use at present (figure 9-3).

Figure 2. Source separation systems: A. blackwater holding tank with low-volume discharge toilets and graywater septic

tank system; B. non-water-carriage toilet and graywater septic tank system

Blackwater Holding

tank

Offsite

treatment

Graywater Septic

tank

Subsurface

infiltration

Human

excreta

Non-water

carriage

toilet

Offsite

treatment

Graywater Septic

tank

Subsurface

infiltration

A.

B.
Introduction

TFS-47

Ion exchange

Two types of systems may be employed: cationic or anionic exchange systems. In the cationic system, the ammonium in

septic tank effluent is removed. Clinoptilolite, a naturally occurring zeolite that has excellent selectivity for ammonium

over most other cations in wastewater, can be used as an exchange medium. In the anionic system, septic tank effluent

must be nitrified prior to passage through the exchange unit. Strong-base anion resins can be employed as an exchange

medium for nitrate. Both systems require resin regeneration offsite.

Reverse osmosis

This system requires pretreatment to remove much of the organic and inorganic suspended solids in wastewater. Pretreated

wastewater stored under pressure is fed to a chamber containing a semipermeable membrane that allows separation of ions

and molecules before disposal. Large volumes of waste brine are generated and must be periodically removed for offsite

treatment.

Biological treatment systems

A number of onsite treatment systems use biological denitrification for removal of nitrogen from wastewater. These

systems have received the most scrutiny with respect to development and performance monitoring. However, more

development and performance monitoring will be necessary to refine the performance consistency and improve understanding of operation processes and mechanisms (see figure 4).

Figure 3. Physical/chemical systems: A. cation (NH4

+) exchange; B. anion (NO3-

) exchange; C. reverse osmosis

Wastewater Septic

tank

Subsurface

infiltration Lift

station

Ion

exchange

unit

Surface

discharge

(optional)

Wastewater

Aerobic

pretreatment

Subsurface

infiltration Lift

station

Ion

exchange

unit

Surface

discharge

(optional)

Offsite

regeneration

Offsite

regeneration

Wastewater Pretreatment unit

Subsurface

infiltration

Surface

discharge

(optional)

Lift

station

Pressure

tank

Filter

cartridge

Osmosis

membrane

Brine

storage

Offsite

treatment

A

B.

C.
TFS-48

Figure 4. Biological systems: A. an aerobic/anaerobic trickling filter package plant; B. sequencing batch reactor (SBR)

design principle; C. ISF with AUF; D. source separation, treatment, recombination; E. recirculating sand filter with septic

tank option; F. recirculating sand filter with anaerobic filter and carbon source
Introduction

TFS-49

Aerobic/anaerobic trickling filter package plant

These commercial systems use synthetic media trickling filters that receive wastewater from overlying sprayheads for

aerobic treatment and nitrification. Filtrate returns to the anaerobic zone to mix with either septic tank contents or incoming septic tank effluent and undergoes denitrification. A portion of the filtered effluent (equal to the influent flow) is

discharged for disposal or further treatment.

Sequencing batch reactor (SBR)

If sufficient hydraulic retention time (HRT) is provided to permit nitrification during the “react” phase of the SBR cycle

and if the fill stage is anoxic for a sufficient HRT, the system can remove significant amounts of nitrogen and phosphorus.

The SBR design is essentially the same as is described in the SBR fact sheet, while operationally the conditions noted

above must be maintained.

Intermittent sand filters with anaerobic filters

Nitrification is provided in the ISF, while denitrification is provided in either the preceding septic tank with recirculation

or a separate anaerobic filter. A vegetated submerged bed (VSB) (“subsurface flow wetland”) may be substituted for the

anaerobic filter.

Source separation, treatment, and recombination

One commercial system employs this sequence where blackwater (toilet wastewater), after settling in a separate tank, is

aerobically treated with an ISF to nitrify the majority of the nitrogen before it is recombined with settled greywater in an

anaerobic upflow filter (AUF) for denitrification.

Figure 4. (continued)
TFS-50

Recirculating sand filters combined with anaerobic/anoxic filters

RSF systems normally remove 40 to 50 percent of influent nitrogen. To enhance this capability, they can be combined

with a greater supply of carbon, time, and mixing than is normally available from the conventional recirculation tank. The

anaerobic/anoxic options include recycling to the septic tank, better mixing, and longer HRT in a separate UF or VSB, or

adding supplemental carbon (e.g., methanol, ethanol) to enhance the potential of the denitrification step.

Typical applications

Nitrogen removal is increasingly being required when onsite systems are on or near coastal waters or over sensitive,

unconfined aquifers used for drinking water. Nitrogen removal systems generally are located last in the treatment train

prior to SWIS disposal and may be followed by disinfection when the system must discharge to surface waters. Usually,

the minimum total nitrogen standard that can be regularly met is about 10 mg/L. Aerobic biological systems should not be

employed at seasonal facilities.

Design assumptions

A myriad of potential systems exist for enhanced nitrogen removal, and all of the major unit processes of such systems are

described elsewhere. Also, since waste stream modification is covered in chapter 3, only the most promising, developed

options are discussed in this fact sheet. Of the options discussed, granular media filters or aerobic biological systems

(usually combined with an anaerobic upflow filter or the original septic tank process) are discussed in more detail.

Some salient design considerations that are not covered in other fact sheets or text include the following:

• Autotrophic denitrification in packed-bed sulfur reactors (variation on AUF) has been successfully demonstrated, but

the need for additional alkalinity and the production of a high sulfate effluent have thus far limited the process.

• Denitrification improves with increased HRT in the recirculation tank, better mixing, and a pH between 7 and 8.

• Use of greywater as the degradable carbon source for denitrification limits the degree of denitrification attainable

owing to reduced nitrogen content and low carbon-to-nitrogen ratio. The latter should exceed 5:1 for good denitrification.

• Use of synthetic anionic exchange resins appears impractical at this time. Cationic exchange of NH4

-N with

clinoptilolite is feasible but very expensive because of the regeneration management costs. Both may be subject to

fouling and clogging problems.

• Membranes present a major problem given the volume of the reject stream, which must be collected and frequently

trucked to a site that will accept it for disposal.

• The use of beds of carbon-rich materials below SWIS leach lines could be a promising concept if the hydraulic

matching problems are solved and the bed service life can be extended for 10 years or more.

• Accessibility, size of the holding tank, and availability of residual management facilities are significant design considerations in blackwater separation systems.

• Recycling to the septic tank may affect solids and grease removal in the tank and cause poor mixing of the nitrified

stream with the septic tank contents. This could raise the oxidation-reduction potential (ORP) of the mixture above the

normal range for an anoxic zone that accomplishes denitrification. Recycling to the second compartment of a

multicompartment tank is suggested at a ratio of less than 2.5 to 1 with a contact time of greater than 2 days.

• An AUF used for enhanced denitrification should be loaded with between 0.06 and 0.3 1b COD/ft3

 per day and have

an HRT of at least 24 hours (preferably 36 or more hours). It can be filled with large (> 2 inches) rocks or synthetic

media. A vegetated submerged bed (VSB) can be substituted for an AUF and may contribute some labile carbon to aid

the process.
Introduction

TFS-51

• SBR design for nitrogen and phosphorus removal is essentially similar, but the amount of labile carbon required is

greater (6 to 8 mg/LCOD/ mg/L of TKN to be denitrified).

• Modern microprocessor controls make very complex process combinations possible to remove nitrogen, but overall

simplicity is still desirable and requires less O/M sophistication.

• To attain full (>85 percent) nitrification, fixed-film systems cannot be loaded above 3 to 6 g BOD/m3

 per day or 6 to

12 g BOD/m3

 per day for rock and plastic media, respectively.

Performance

Some expected sustainable performance ranges for the most likely

combinations of nitrogen removal

processes are given in table 1. Some

of the nitrogen-removal systems

could be combined with source

separation and product substitution

(low-phosphate detergents) for a

maximum reduction in nitrogen

where extreme measures might be

required. However, the removals

would not be additive owing to the

changes in wastewater characteristics.

Management needs

Management needs for most unit processes are covered in other fact sheets. Source separation is feasible only for new

homes, as it would be prohibitively expensive for existing homes. AUF systems are different from the fact sheet in that

they must have HRTs greater than 2 days to enable anaerobic biological denitrification to be effective. This will add to O/

M tasks by requiring regular flushing of excess biological growth. Some separation and removal would require regular

inspection and maintenance of non-water-carriage toilets and periodic removal and proper disposal of excess solids from

these units and from holding tanks.

Risk management issues

Of the most likely systems shown in the table, few are extremely susceptible to upset by hydraulic loading variations.

However, soluble toxic shocks could affect any AUF, SBR, or fixed-film nitrification system. Extreme cold will also have

an impact on these systems. However, the ISF, RSF, and AUF systems have been the most resilient unit processes (excluding source separation) when properly housed and insulated. Power outages will affect all of the treatment systems. Reliability would be greatest for those that incorporate filters and less for the SBR and fixed-film systems.

Costs

The capital and total costs of most of the nitrogen removal systems are very site specific, but non-water-carriage toilet

source separation (assuming new homes) is the least expensive (low-water-use fixtures and holding tanks would add about

$4,000 to $6,000). The biological combinations would be more expensive, and the physical/chemical systems would likely

be the most expensive. Multiple units will generally increase costs, while the use of gravity transfer between processes will

reduce them.

The additional O/M associated with an AUF involves flushing and disposal of excess flushed solids. If methanol is employed to enhance denitrification, additional O/M is required for the feeding system.

Table 1. Typical N-removal ranges for managed systems

a

Commercially available systems.

Note: RSF = recirculating sand filters; AUF = anaerobic upflow filter; ST = septic tank; FFS = fixed-film system;

SBR = sequencing batch reactor; SS = source separation; TT = treatment applied to both systems; R =

recombined; ISF = intermittent sand filter.
TFS-52 

References

Ayres Associates. 1991. Onsite Nitrogen Removal Systems: Phase I. Report to Wisconsin DILHR, Madison, WI.

Ayres Associates. 1997. Florida Keys Wastewater Nutrient Reduction Systems Demo Project: 2nd Quarter Report. Report

to Florida Department of Health and U.S. Environmental Protection Agency. Florida Department of Health,

Tallahassee, FL.

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1979. Evaluation of Existing and Potential Technologies for Onsite

Wastewater Treatment and Disposal. EPA 600/S2/81/178. Cincinnati, OH.

Boyle, W.C., R.J. Otis, R.A. Apfel, R.W. Whitmeyer, J.C. Converse, B. Burkes, M.J. Bruch, Jr., and M. Anders. 1994.

Nitrogen Removal from Domestic Wastewater in Unsewered Areas. In Proceedings of the Seventh On-Site Wastewater

Treatment Conference. American Society of Agricultural Engineering, St. Joseph, MI.

Katers, J.F., and A.E. Zanoni. 1998. Nitrogen removal. Journal of Water Environment and Technology 10(3):32-36.

Lamb, B., A.J. Gold, G. Loomis, and C. McKiel. 1987. Evaluation of Nitrogen Removal Systems for Onsite Sewage

Disposal. In Proceedings of Fifth On-Site Wastewater Treatment Conference. American Society of Agricultural

Engineering, St. Joseph, MI.

U.S. Environmental Protection Agency (USEPA). 1993. Nitrogen Control Manual. EPA 625/R-93/010. U.S.

Environmental Protection Agency, Office of Research and Development, Cincinnati, OH.

Venhuizen, D. LCRA onsite demonstration project for nitrogen removal and water reclamation. Unpublished but available

from D. Venhuizen, P.E., 21 Cotton Gin Road, Uhland, TX 78640.

Whitmyer, R.W., R.A. Apfel, R.J. Otis, and R.L. Meyer. 1991. Overview of Individual Onsite Nitrogen Removal Systems.

In Proceedings of Sixth On-Site Wastewater Treatment Conference. American Society of Agricultural Engineering, St.

Joseph, MI.

Winkler, E.S., and P.L.M.Veneman. 1991. A Denitrification System for Septic Tank Effluent Using Sphagnum Peat Moss.

In Proceedings of Sixth On-Site Wastewater Treatment Conference, American Society of Agricultural Engineering,

St. Joseph, MI.
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 10

Intermittent Sand/Media Filters

Description

The term intermittent sand filter (ISF) is used to describe a variety of packed-bed filters of sand or other granular materials available on the market. Sand filters provide advanced secondary treatment of settled wastewater or septic tank effluent. They consist of a lined (e.g., impervious PVC liner on sand bedding) excavation or structure filled with uniform

washed sand that is placed over an underdrain system (see figure 1). The wastewater is dosed onto the surface of the sand

through a distribution network and allowed to percolate through the sand to the underdrain system. The underdrain system

collects the filter effluent for further processing or discharge.

Sand filters are aerobic, fixed-film bioreactors. Other treatment mechanisms that occur in sand filters include physical

processes, such as straining and sedimentation, that remove suspended solids within the pores of the media. Also, chemical

adsorption of pollutants onto media surfaces plays a finite role in the removal of some chemical constituents (e.g., phosphorus). Bioslimes from the growth of microorganisms develop as films on the sand particle surfaces. The microorganisms

in the slimes absorb soluble and colloidal waste materials in the wastewater as it percolates over the sand surfaces. The

adsorbed materials are incorporated into a new cell mass or degraded under aerobic conditions to carbon dioxide and

water.

Most biochemical treatment occurs within approximately 6 inches of the filter surface. As the wastewater percolates

through this layer, suspended solids and carbonaceous biochemical oxygen demand (BOD) are removed. Most suspended

Figure 1. Generic, open intermittent sand filter
TFS-54

solids are strained out at the filter surface. The BOD is nearly completely removed if the wastewater retention time in the

sand media is sufficiently long for the microorganisms to absorb wastewater constituents. With depleting carbonaceous

BOD in the percolating wastewater, nitrifying microorganisms are able to thrive deeper in the surface layer where nitrification will readily occur.

Chemical adsorption can occur throughout the media bed. Adsorption sites in the media are usually limited, however. The

capacity of the media to retain ions depends on the target constituent, the pH, and the mineralogy of the media. Phosphorous is one element of concern in wastewater that can be removed in this manner, but the number of available adsorption

sites is limited by the characteristics of the media.

The basic components of intermittent sand filters include a dosing tank, pump and controls (or siphon), distribution

network, and the filter bed with an underdrain system (see figure 1). The wastewater is intermittently dosed from the

dosing tank onto the filter through the distribution network. From there, it percolates through the sand media to the

underdrain and is discharged. On-demand dosing is usually used, but timed dosing is becoming common.

There are a large number of variations in ISF designs. For example, there are different means of distribution, underdrain

designs, housing schemes and, most notably, media choices. Many types of media are used in single-pass filters. Washed,

graded sand is the most common. Other granular media used include gravel, crushed glass, and bottom ash from coal-fired

power plants. Foam chips (polystyrene), peat, and coarse-fiber synthetic textile materials have also been used. These media

are generally restricted to proprietary units. System manufacturers should be contacted for application and design using

these materials.

There are also related single-pass designs, which are not covered in this fact sheet. These include lateral flow designs and

upflow-wicking concepts, both of which use physical removal concepts closer to the concepts described in the fact sheet on

anaerobic upflow filters and vegetated submerged beds. These processes are not discussed herein but may exhibit some

pollutant removal mechanisms that are described here. Simple gravity-fed, buried sand filters are not discussed because

their performance history is unsatisfactory.

Applications

Sand filters can be used for a broad range of applications, including single-family residences, large commercial establishments, and small communities. Sand filters are frequently used to pretreat septic tank effluent prior to subsurface infiltration onsite where the soil has insufficient unsaturated depth above ground water or bedrock to achieve adequate treatment.

They are also used to meet water quality requirements (with the possible exception of fecal coliform removal) before

direct discharge to a surface water. Sand filters are used primarily to treat domestic wastewater, but they have been used

successfully in treatment trains to treat wastewaters high in organic materials such as those from restaurants and supermarkets. Single-pass ISF filters are most frequently used for smaller applications and sites where nitrogen removal is not

required. However, they can be combined with anaerobic processes to reduce nitrogen significantly. Many studies have

shown that ISF-treated onsite wastewaters can reduce clogging of the infiltrative surface by many times when compared

with septic-tank effluents. However, be careful to evaluate the overall loading of pollutants and pathogens to the underlying aquifer and nearby surface waters before considering significant SWIS sizing reductions.

Design

ISF filter design starts with the selected media. The media characteristics determine the necessary filter area, dose volumes, and dosing frequency. Availability of media for a specific application should be determined before completing the

detailed design. Typical specifications, mass loadings, and media depths are presented in table 1. The sand or gravel

selected should be durable with rounded grains. Only washed material should be used. Fine particles passing the U.S. No.

200 sieve (less than 0.074 mm) should be limited to less than 3 percent by weight. Other granular media that have been

used are bottom ash, expanded clay, expanded shale, and crushed glass. These media should remove BOD and TSS similar

to sand and gravel for similar effective sizes, uniformity, and grain shape. Newer commercial media such as textile

materials have had limited testing, but based on early testing should be expected to perform as well as the above types.
TFS-55

Traditionally, sand filters have

been designed based on hydraulic

loadings. However, since these

filters are primarily aerobic

biological treatment units, it is

more appropriate that they be

designed based on organic loadings. Unfortunately, insufficient

data exist to establish well-defined

organic loading rates. Experience

presently suggests that BOD5

loadings on sand media should not

exceed about 5 lb/1,000 ft3 per day

(0.024 kg/m2

 per day) where the

effective size is near 1.0 mm and

the dosing rate is at least 12 times

per day.

Higher hydraulic and organic

loadings have been described in

several studies, but the long-term

viability of the systems loaded at

those higher organic loads has not

yet been fully verified. The values

in the table are thus considered

conservative and may be subject to

increases as more quality-assured

data become available.

Dosing volume and frequency

have been shown to be the critical design variables. Small dose volumes are preferred because the flow through the porous

media will occur under unsaturated conditions with higher moisture tensions. Better wastewater media contact and longer

residence times occur under these conditions. Smaller dose volumes are achieved by increasing the number of doses per

day. It has been suggested that each dose should be <0.5 cm (based on media surface perpendicular to infiltration direction) to fully nitrify the effluent in an ISF. This would limit maximum daily hydraulic loading to 12 cm/d, or 3 gpd/ft2

, if

the maximum frequency of daily dosing is accepted as 24 (or hourly) as supported by the literature. Media characteristics

can limit the number of doses possible. Reaeration of the media must occur between doses. As the effective size of the

media decreases, the time for drainage and reaeration of the media increases.

Distribution network characteristics will also limit the number of doses possible. The primary characteristics are the

volume, pressure, orifice sizes, and spacing. To achieve uniform distribution over the filter surface, minimum dose

volumes are necessary and can vary with the distribution method selected. Therefore, if the dose volume dictated by the

distribution network design is too high, the network should be redesigned. Since the dose volume is a critical operating

parameter, the method of distribution and design of the distribution system should be considered carefully.

Distribution methods used include rigid pipe pressure networks with orifices or spray nozzles, drip distribution, and

surface flooding, which is no longer recommended for small ISFs (see chapter 4). Rigid pipe pressure networks are the

most commonly used method. Both orifices and spray nozzles are used. The use of spray nozzles is usually limited to

recirculating filters because nozzle fouling from suspended solids is less likely than with undiluted septic tank effluent.

Since the minimum dose volume required to achieve uniform distribution is five times the rigid pipe volume, the filter can

be divided into multiple cells that are loaded individually so the distribution networks can be smaller to reduce the dose

volume needed for uniform distribution. Optimum designs minimize the dose each time the system is dosed. Drip distribution is being used increasingly because the minimum dose volumes are much less than the volumes of rigid pipe networks.

Table 1. Specifications, mass loadings, and depth for single-pass intermittent sand filters

a

 1 gpd/ft2 = 4 cm/day = 0.04 m3/m2 per day b

 1 lb BOD/1000 ft2 per day = 0.00455 kg/m2 per day
TFS-56

Source: Converse and Tyler, 1998.

The underdrain system is placed on the floor of the tank or lined excavation. Ends of the underdrains should be brought to

the surface of the filter and fitted with cleanouts that can be used to clean the biofilms underdrain, if necessary. The

underdrain outlet is cut in the basin wall such that the drain invert is at the floor elevation and the filter can be completely

drained. The underdrain outlet invert elevation must be sufficiently above the recirculation tank inlet to accommodate a

minimum of 0.1 percent slope on the return line and any elevation losses through the flow splitting device. The underdrain

(usually 1.25- to 2.0-inch PVC, class 200 [minimum]) is covered with washed, durable gravel to provide a porous

medium through which the filtrate can flow to the underdrain system. The gravel should be sized to prevent the filter

medium from mixing into the gravel, or a layer of 1/4- to 3/8-inch-diameter washed pea gravel should be placed over the

washed underdrain gravel before the filter medium is added.

The filter basin can be a lined excavation or fabricated tank. For single-home systems, prefabricated concrete tanks are

commonly used. Many single-home filters and most large filters are constructed within lined excavations. Typical liner

materials are polyvinyl chloride and polypropylene. A liner thickness of 30 mil can withstand reasonable construction

activities yet be relatively easy to work with. A sand layer should be placed below the liner to protect it from being

punctured if the floor and walls of the excavation are stony. The walls of the excavation should be brought above the final

grade to prevent entry of surface water.

Filters can be covered or buried. It is often necessary to provide a cover for the filter surface because the surface of a fine

medium (e.g., sand) exposed to sunlight can be fouled with algae. Also, there may be concerns about odors, cold weather

impacts, precipitation, leaf and debris accumulation, and snowmelt. In addition, the cover must provide ample fresh air

venting. Reaeration of the filter medium primarily occurs from the filter surface. The lower 20 percent of the medium’s

depth maintains a high moisture content. At the bottom, the medium is near or at saturation, which is a barrier to air flow

and venting from the underdrain system. The gravel surrounding the distribution piping must be vented to the surface to

provide a fresh air flow. ISF filters open to the surface are built with roofs or removable covers or are merely shaded.

Roofs provide cold weather protection and shed precipitation, debris, and snowmelt that would otherwise enter the system.

Performance

Treatment field performance of single-pass intermittent sand filters is presented in table 2. Typical effluent concentrations

for these single-family wastewater treatment systems are less than 5 mg/L and less than 10 mg/L for BOD and TSS,

respectively. Effluent is nearly completely nitrified but some variability can be expected in nitrogen removal capability.

Controlled studies generally find typical nitrogen removals of 18 to 33 percent with an ISF. Fecal coliform removal ranges

Figure 2. ISF constructed in a mound with direct subsurface infiltration
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from 2 to 4 logs (99 to 99.99 percent). ISF fecal coliform removal is a function of hydraulic loading, with reduced

removals as the loading rate increases above 1 gpm/ft2 (Emerick et al., 1997). Effluent suspended solids from sand filters

are typically low. The media retains the solids. Most organic solids are digested by the media over time.

Management needs

Construction of ISF units usually involves excavation, forming/framing, liner placement with supporting sand layers, and

plumbing. ISF units should never be placed in surface depressions without thoroughly sealing against prolonged inundation and drainage configurations that prevent stormwater entry. In all cases, units must be watertight with sealed entries

and exits for piping. Filter fabric should not be used at any location through which the filtrate would flow. Media delivered to the site should be tested against design-sizing specifications. Excess (3 percent or greater) fines are one of the

greatest concerns of the construction inspector.

The operation and maintenance requirements of packed bed filters are few and simple. As with all treatment systems, flow

monitoring should be conducted to identify excessive flows and check dose volumes and dosing rates. If the flows are

excessive, the source of the flows should be identified and corrective measures taken. Reduced dose volumes or dosing

rates suggest that the distribution network is plugged or the pump is not performing properly. The distribution network

should be flushed annually (or more often, as necessary) using the manual flushing device. Also, the dosing pump should

be recalibrated at least annually.

The filter surface should not pond if the filter is designed properly and the wastewater characteristics do not change

significantly. If standby cells are not available for regular resting and the surface is not covered with pea gravel, the

surface can be raked to break up any material clogging the filter surface. Reducing the dose volume and increasing the

dosing frequency may help to increase the reaeration potential and reduce clogging of the media. If the ponding problem

persists, however, removal of the top layer or complete replacement of the media may be necessary. Before replacing the

media, monitor wastewater flows and concentrations to determine if they are the cause of the problem. Problem sources

should be identified and addressed before repairs are effected. Premature clogging is often traceable to excess TSS and

BOD loading or to fines in the media. Where the problem develops naturally over time and standby cells are available,

resting may be used to supplement the raking and/or surface skimming steps.

Free-access ISFs should be checked regularly (at least every 3 to 4 months), to prevent surface problems. Periodic raking

and resting is recommended to maintain percolation and prevent ponding. Scraping off the top layer (e.g., 1 inch) of sand

helps to prevent clogging. Intervals between scraping vary from a minimum of 3 months up to greater than 1 year.

Removed surface layers need not be replaced until the total filter depth falls below 18 inches. If new filter material is not

Table 2. Single-pass intermittent sand filter performance

a

 Sand media: es = 0.25-0.65 mm; uc = 3-4. Design hydraulic loadings = 1.2 gpd/ft2 based on 150 gpd/bedroom. Actual flows not measured. b

 Sand media: es = 0.4 mm; uc = 2.5. Average loadings = 0.4 gpd/ft2 / 0.42 lb BOD/1,000 ft2. Doses per day = 3.3. c

 Sand media: es = 0.14-0.30 mm; uc = 1.5-4.0. Average loadings = 0.33 gpd/ft2 / 0.6-1.27 lb BOD/1000 ft2 per day. d

 Sand media: not reported; uc = 3-4. Design hydraulic loadings = 1. gpd/ft2. Daily flows not reported.
TFS-58

readily available, it may be cost-effective to clean and reuse the old filter material. Resting is considered the best rehabilitation approach due to possible clogging contributions from raking/scraping.

ISFs have low energy requirements compared with other systems offering comparable effluent quality. Free-access ISFs

using pumped dosing would require approximately 0.3 to 0.4 kWh/day.

Risk management issues

ISF filters are simple in design and relatively passive to operate because the fixed-film process is very stable and few

mechanical components are used. High flow variations after equalization in a septic tank are not a problem because the

residual peaks and valleys are absorbed in the pressurization tank or in the last compartment of the preceding septic tank.

Although ISFs have biological properties, the impact of toxic loading shocks are not well documented.

Free-access ISFs are often installed with removable covers to regulate temperatures in cold climates and to reduce odors.

Space of 12 to 24 inches (30 to 61 cm) should be allotted between the sand surface and the installed cover (EPA, 1980).

Odors from free-access filters treating septic tank effluent may warrant installation away from dwellings, especially if

spray nozzles are used in distribution.

Power outages will impact ISF systems if these systems are uniformly dosed with pumps. During the power outage, all

wastewater generated will accumulate in that dosing facility and septic tank, increasing the potential for odors.

Costs

Filter media is the most expensive component in ISF construction. Typically, filter media can be installed for $10 to $15

per square foot, depending primarily on the type of media and the contractor’s experience with ISF construction. Operation/maintenance costs include electricity for pumping/dosing, and 3 to 6 hours of semiskilled management visits per year

cost about $150 to $200. The electricity is about $10 to $20 of that total.

References

Anderson, D.L., R.L. Siegrist, and R.J. Otis. 1985. Technology Assessment of Intermittent Sand Filters. U.S.

Environmental Protection Agency, Office of Research and Development and Office of Water, Washington, DC.

Bauer, D.H., E.T. Conrad, and D.G. Sherman. 1979. Evaluations of Existing and Potential Technologies for Onsite

Wastewater Treatment and Disposal. EPA/600/S2-81-178. U.S. Environmental Protection Agency, Office of Research

and Development, Cincinnati, OH.

Boller, M., A. Schwager, J. Eugster, and V. Mottier. 1993. Dynamic Behavior of Intermittent Buried Filters. In Small

Wastewater Treatment Plants, ed., H. Odegaard, TAPIR, Trondheim, Norway.

Cagle, W.A., and L.A. Johnson. 1994. On-site intermittent sand filter systems: a regulatory/scientific approach to their

study in Placer County, California. In Proceedings of the Seventh Onsite Wastewater Treatment Symposium, American

Society of Agricultural Engineers, St. Joseph, MI.

Darby, J., G. Tchobanoglous, M. Asri Nor, and D. Maciolek. 1996. Small Flows Journal 2(31): 3-15.

Effert, D., J. Morand, and M. Cashell. 1985. Field performance of three onsite effluent polishing units. In Proceedings of

Fourth Onsite Wastewater Treatment Symposium, American Society of Agricultural Engineers, St. Joseph, MI.

Emerick, R.W., R.M. Test, G. Tchobanoglkous, and J. Darby. 1997. Small Flows Journal 3(1):12-22.

National Small Flows Clearinghouse. 1998. Intermittent Sand Filters. NSFC Fact Sheet for U.S. Environmenetal

Protection Agency, Office of Water, Washington, DC.

Orenco Systems, Inc. 1993. Cost Estimating for STEP Systems and Sand Filters. Orenco Systems, Inc., Roseburg, OR.
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Rhode Island Department of Environmental Management (DEM). 2000. Sand Filter Guidance Document. Department of

Environmental Management, Providence, RI.

Ronayne, M.P., R.C. Paeth, and S.A. Wilson. 1982. Oregon On-site Experimental Systems Program. Final report to U.S.

Environmental Protection Agency, Office of Research and Development, Cincinnati, OH.

Sievers, D.M. 1998. Pressurized intermittent sand filter with shallow disposal field for a single residue in Boone County,

MO. In Proceedings of the Eighth On-site Wastewater Treatment Symposium. American Society of Agricultural

Engineers, St. Joseph, MI.
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 11

Recirculating Sand/Media Filters

Description

Recirculating filters using sand, gravel, or other media provide advanced secondary treatment of settled wastewater or

septic tank effluent. They consist of a lined (e.g., impervious PVC liner on sand bedding) excavation or structure filled

with uniform washed sand that is placed over an underdrain system (see figure 1). The wastewater is dosed onto the

surface of the sand through a distribution network and allowed to percolate through the sand to the underdrain system.

The underdrain system collects and recycles the filter effluent to the recirculation tank for further processing or discharge.

Recirculating sand filters (RSFs) are aerobic, fixed-film bioreactors. Other treatment mechanisms that occur in sand filters

include physical processes, such as straining and sedimentation, that remove suspended solids within the pores of the

media. Also, chemical sorption of pollutants onto media surfaces plays a finite role in the removal of some chemical (e.g.,

phosphorus) constituents. Bioslimes from the growth of microorganisms develop as films on the sand particle surfaces.

The microorganisms in the slimes absorb soluble and colloidal waste materials in the wastewater as it percolates over the

sand surfaces. The absorbed materials are incorporated into a new cell mass or degraded under aerobic conditions to

carbon dioxide and water.

Most biochemical treatment occurs within approximately 6 inches of the filter surface. As the wastewater percolates

through this layer, suspended solids and carbonaceous biochemical oxygen demand (BOD) are removed. Most suspended

solids are strained out at the filter surface. The BOD is nearly completely removed if the wastewater retention time in the

sand media is sufficiently long for the microorganisms to absorb waste constituents. With depleting carbonaceous BOD in

Figure 1. Typical recirculating sand filter system
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the percolating wastewater, nitrifying microorganisms are able to thrive deeper in the surface layer, where nitrification will

readily occur.

Chemical adsorption can occur throughout the media bed. Adsorption sites in the media are usually limited, however. The

capacity of the media to retain ions depends on the target constituent, the pH, and the mineralogy of the media. Phosphorus is one element of concern that can be removed from wastewater in this manner, but the number of available adsorption

sites is limited by the characteristics of the media.

The basic components of recirculating filters include a recirculation/dosing tank, pump and controls, distribution network,

filter bed with an underdrain system, and a return line. The return line or the underdrain must split the flow to recycle a

portion of the filtrate to the recirculation/dosing tank. A small volume of wastewater and filtrate is dosed to the filter

surface on a timed cycle 1 to 3 times per hour. Recirculation ratios are typically between 3:1 and 5:1. In the recirculation

tank, the returned aerobic filtrate mixes with the anaerobic septic tank effluent before being reapplied to the filter.

Recirculating filters must use a coarser media than single-pass filters because recirculation requires higher hydraulic

loadings. Both coarse sand and fine gravel are used as filter media. Because of the high hydraulic conductivities of the

coarse media, filtrate recirculation is used to provide the wastewater residence times in the media necessary to meet the

treatment requirements. Based on forward flow, daily hydraulic loadings are typically about 3 gpd/ft2

 (2 to 5 gpd/ft2) when

the filter media is coarse sand. Therefore, the corresponding combined daily filter hydraulic loading, including the recirculated flow, may be 6 to 25 gpd/ft2

. Where gravel is used as the media, the daily hydraulic loadings are increased to as much

as 10 to 15 gpd/ft2 with a combined daily loading of 30 to 75 gpd/ft2. BOD and TSS removals are generally the same as

those achieved by single-pass filters. Nearly complete ammonia removal by nitrification is also achieved. In addition, the

mixing of the return filtrate anaerobic septic tank effluent removes approximately 50 percent of the total nitrogen. However, because of the greater hydraulic loadings and coarser media, fecal coliform removal is somewhat less than in singlepass filters.

Recirculating filters offer advantages over single-pass filters. Greater control of performance is possible because recirculation ratios can be changed to optimize treatment. The filter can be smaller because of the higher hydraulic loading. Recirculation also reduces odors because the influent wastewater (septic tank effluent) is diluted with return filtrate that is low

in BOD and high in dissolved oxygen.

Many types of media are used in packed-bed filters. Washed, graded sand was the most common, but pea gravel has

generally replaced it in recent times. Other granular media used include crushed glass, garnet, anthracite, plastic, expanded

clay, expanded shale, open-cell foam, extruded polystyrene, and bottom ash from coal-fired power plants. Coarse-fiber

synthetic textile materials are also used. These materials are generally restricted to proprietary units. Contact the system

manufacturers for application and design using these materials.

Other modifications to the basic RSF design include the type of distribution system, the location and design of the recirculation tank, the means of flow splitting the filtrate between discharge and return flows, and enhancements to improve

nitrogen removal. The last is addressed in Technology Fact Sheet 9 on nitrogen removal.

Applications

Recirculating sand filters can be used for a broad range of applications, including single-family residences, large commercial establishments, and small communities. They are frequently used to pretreat wastewater prior to subsurface infiltration

on sites where soil has insufficient unsaturated depth above ground water or bedrock to achieve adequate treatment. They

are also used to meet water quality requirements before direct discharge to a surface water. RSFs are primarily used to

treat domestic wastewater, but they have also been used successfully in treatment trains to treat wastewaters high in

organic materials such as those from restaurants and supermarkets. Single-pass filters are most frequently used for smaller

applications and at sites where nitrogen removal is not required. Recirculating filters are used for both large and small

flows and are frequently used where nitrogen removal is necessary. RSFs frequently replace aerobic package plants in

many parts of the country because of their high reliability and lower O/M requirements.
TFS-63

Design

Packed-bed filter design starts with the selected media. The media characteristics determine the necessary filter area, dose

volumes, and dosing frequency. Availability of media for a specific application should be determined before completing the

detailed design. Typical specifications, mass loadings, and depths for sand and gravel media are presented in chapter 4.

The sand or gravel selected should be durable with rounded grains. Only washed material should be used. Fine particles

passing the U.S. No. 200 sieve (<0.074 mm) should be limited to less than 3 percent by weight. Other granular media are

bottom ash, expanded clay, expanded shale, and crushed glass. These media should perform similarly to sand and gravel

for similar effective sizes, uniformity, and grain shape. Newer commercial media such as textile materials have had limited

testing, but should be expected to perform as well as the above types.

Traditionally, media filters have

been designed based on hydraulic

loadings. However, since they are

primarily aerobic biological

treatment units, it is more appropriate that they be designed based on

organic loadings. Unfortunately,

insufficient data exist to establish

well-defined organic loading rates.

Experience suggests that BOD5

loadings on sand media should not

exceed about 5 lb/1000 ft2

 per day

(0.024 kg/m2 per day) where the

effective size is approximately 1.0

mm and the dosing rate is at least

12 times per day. Higher loadings

have been measured in short-term

studies, but designers are cautioned

about exceeding this loading rate

until quality-assured data confirm

these higher levels. The BOD5

loading should decrease with

decreasing effective size of the

sand. Because of the larger pore

size and greater permeability, gravel

filters can be loaded more heavily.

BOD5

 loadings of 20 lb/1000 ft2 per

day (0.10 kg/m2

 per day) have been

consistently successful, but again

higher loadings have been measured. Some often-quoted design

specifications for RSFs are given in

table 1.

The RSF dose volume depends on the recirculation ratio, dosing frequency, and distribution network:

Dose Volume = Design Flow (gpd) x (Recirculation Ratio + 1) ÷ Number of Doses/Day

Small dose volumes are preferred because the flow through the porous media will occur under unsaturated conditions with

higher moisture tensions. Better wastewater media contact and longer residence times occur under these conditions.

Smaller dose volumes are achieved by increasing the number of doses per day.

a

 1 gpd/ft2 = 4 cm/day = 0.04 m3 / m2 per day (forward flow only). b

 1 lb BOD/1,000 ft2 per day = 0.00455 kg/m2 per day.

Table 1. Typical design specifications for individual home recirculating sand filters
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The recirculation ratio increases the hydraulic loading without increasing the organic loading. For example, a 4:1 recirculation ratio results in a hydraulic loading of five times the design flow (1 part forward flow to 4 parts recycled flow). The

increased hydraulic loading reduces the residence time in the filter so that recirculation is necessary to achieve the desired

treatment. Typical recirculation ratios range from 3:1 to 5:1. As the permeability of the media increases, the recirculation

ratio may need to increase to achieve the same level of treatment.

Media characteristics can limit the number of doses possible. Media reaeration must occur between doses. As the effective

size of the media decreases, the time for drainage and reaeration of the media increases. For single pass filters, typical

dosing frequencies are once per hour (24 times/day) or less. Recirculating sand filters dose 2 to 3 times per hour (48 to 72

times/day).

Distribution network requirements will also limit the number of doses possible. To achieve uniform distribution over the

filter surface, minimum dose volumes are necessary and can vary with the distribution method selected. Therefore, if the

dose volume dictated by the distribution network design is too high, the network should be redesigned. Since the dose

volume is a critical operating parameter, the method of distribution and the distribution system design should be considered carefully.

Distribution methods used include rigid pipe pressure networks with orifices or spray nozzles, and drip emitters. Rigid

pipe pressure networks are the most commonly used method. Orifices with orifice shields, facing upward, minimize hole

blockage by stones. Since the minimum dose volume required to achieve uniform distribution is five times the pipe

volume, large multihome filters are usually divided into multiple cells. Drip emitter distribution is being used increasingly

because the minimum dose volumes are much less than the rigid pipe network volumes.

Recirculation tanks are a component of most recirculation filter systems. These tanks consist of a tank, recirculation pump

and controls, and a return filter water flow splitting device. The flow splitting device may or may not be an integral part

of the recirculation tank. Recirculation tanks store return filtrate, mix the filtrate with the septic tank effluent, and store

peak influent flows. The tanks are designed to either remain full or be pumped down during periods of low wastewater

flows. Since doses to the recirculating filter are of a constant volume and occur at timed intervals, the water level in the

tank will rise and fall in response to septic tank effluent flow, return filtrate flow, and filter dosing.

In tanks designed to remain full, all filtrate is returned to the recirculation tank to refill the tank after each dosing event.

When the tank reaches its normal full level, the remaining return filtrate is discharged out of the system as effluent. This

design is best suited where treatment performance must be maintained continuously. For single-family home systems, the

recirculation tank is typically sized to be equal to 1.5 times the design peak daily flow.

When the filtrate flow is continuously split between the return (to the recirculation tank) and the discharge, the liquid

volume in the recirculation tank will vary depending on wastewater flows. During low flow periods the tank can be

pumped down to the point that the low-water pump off switch is activated. This method leaves less return filtrate available

to mix with the influent flow. While simple, this method of flow splitting can impair treatment performance because

minimum recirculation ratios cannot be maintained. This is less of a disadvantage, however, for large, more continuous

flows typical in small communities or large cluster systems.

The recirculation pump and controls are designed to dose a constant volume of mixed filtrate and septic tank effluent flow

onto the filter on a timed cycle. The pump must be sized to provide the necessary dosing rate at the operating discharge

head required by the distribution system. Pump operation is controlled by timers that can be set for pump time on and

pump time off. A redundant pump-off float switch is installed in the recirculation tank below the minimum dose volume

level. A high water alarm is also installed to provide notice of high water caused by pump failure, loss of pump calibration, or excessive influent flows.
TFS-65

Recirculation tank sizing

In many types of commercial systems, daily flow variations can be extreme. In such systems, the recycle ratios necessary

to achieve the desired treatment may not be maintained unless the recirculation tank is sized properly. During prolonged

periods of high influent flows, the recirculation ratio can be reduced to the point that treatment performance is not maintained

unless the recirculation tank is sized to provide a sufficient reservoir of recycled filtrate to mix with the influent during the

high-flow periods.

To size the tank appropriately for the application, assess the water balance for the recirculation tank using the following

procedure:

1. Select the dosing frequency based on the wastewater strength and selected media characteristics.

2. Calculate the dose volume based on the average daily flow:

Vdose = [(recycle ratio + 1) x Qave. daily] ÷ (doses/day)

Qdose = Vdose ÷ (dose period)

Where V and Q are the flow volume and flow rate, respectively.

3. Adjust the dose volume if the calculated volume is less than the required minimum dose volume for the distribution

network.

4. Estimate the volumes and duration of influent peak flows that are expected to occur from the establishment.

5. Calculate the necessary recirculation tank “working” volume by performing a water balance around the recirculation tank

for the peak flow period with the greatest average flow rate during that peak period.

Inputs = Qinf.x T + Qrecycle x T = Qinf .x T + (Qdose – Qeff) x T = Vinf. + Vrecycle

Outputs = Vdose x (T ÷ dose cycle time)

Where T is the peak flow period duration.

If the inputs are greater than the outputs, then Qeff = Qdose and the peaks are stored in the available freeboard space of the

recirculation tank. If the inputs are less than the outputs, then Qeff. = Qinf.

To provide the necessary recycle ratio, sufficient filtrate must be available to mix with the influent septic tank effluent. The

filtrate is provided by the return filtrate flow and the filtrate already in the recirculation tank.

Recycle ratio x Qinf. x T < Qrecycle x T + minimum tank working volume

Where minimum tank working volume = Recycle ratio x (Qinf. – Qrecycle) x T

6. Calculate the necessary freeboard volume for storage of peak flows when the influent volume is greater than the dosing

volume during the peak flow period.

Freeboard volume = Qinf. x T + Qrecycle x T – Qdose x T

= Qinf. x T(Qdose – Qeff.) – Qdose x T

7. Calculate the minimum total recirculation volume.

Total tank volume = minimum tank working volume + freeboard volume

(Adapted from Ayres Associates, 1998.)
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Several flow splitting devices may be used. The most common are

ball float valves and proportional splitters. The ball float valve is used

where the recirculation tank is designed to remain full. The valve is

connected to the return filtrate line inside the recirculation tank (see

figure 2). The return line runs through the tank. The ball float valve is

open when the water level is below the normally full level. When the

tank fills from either the return filtrate or the influent flow, the ball

float rises to close the valve, and the remaining filtrate is discharged

from the system. The proportional splitters continuously divide the

flow between return filtrate and the filtrate effluent (see figure 3).

Another type of splitter consists of a sump in which two pipes are

stubbed into the bottom with their ends capped. In the crowns of each

capped line, a series of equal-sized, pluggable holes are drilled. The

return filtrate floods the sump, and the flow is split in proportion to

the relative number of holes left open in each perforated capped pipe.

Another type of splitter divides flow inside the filter. The filter floor is raised so that it slopes in opposite directions. The

raised point is located so that the ratio of the floor areas on either side is in proportion to the desired recirculation ratio.

Each side has its own underdrain. One side drains back to the recirculation tank, the other side drains to discharge. This

method has the disadvantage that adjustments to the recirculation ratio cannot easily be made.

Most RSFs are constructed aboveground and with an open filter surface; however, in cold climates, they can be placed in

the ground to prevent freezing.

Placing a cover over an RSF is

recommended to reduce odors and

to provide insulation in cold

climates, although no freezing

was observed in an open RSF in

Canada using coarse gravel media.

Covers must provide ample fresh

air venting, because reaeration of

the filter media occurs primarily

from the filter surface.

The filter basin can be a lined

excavation or fabricated tank. For

single-home systems, prefabricated concrete tanks are commonly used. Many single-home filters and most large filters are constructed within lined excavations. Typical liner

materials are polyvinyl chloride and polypropylene. A liner thickness of 30 mil can withstand reasonable construction

activities yet be relatively easy to work with. A sand layer should be placed below the liner to protect it from puncturing if

the floor and walls of the excavation are stony. The excavation walls should be brought above the final grade to prevent

entry of surface water. It is often necessary to cover the filter surface to reduce the effects of algae fouling, odors, cold

weather impacts, precipitation, and snow melt. The cover must provide ample fresh air venting, however. Reaeration of

the filter media primarily occurs from the filter surface.

The underdrain system is placed on the floor of the tank or lined excavation (figure 4). Ends of the underdrains should be

brought to the surface of the filter and fitted with cleanouts that can be used to clean the underdrains of biofilms if

necessary. The underdrain outlet is cut in the basin wall such that the drain invert is at the floor elevation and the filter can

be completely drained. The underdrain outlet invert elevation must be sufficiently above the recirculation tank inlet to

accommodate a minimum of 0.1 percent slope on the return line and any elevation losses through the flow splitting device.

The underdrain is covered with washed, durable gravel to provide a porous medium through which the filtrate can flow to

Figure 2. Flow splitter operated by a float ball valve

Figure 3. Splitter basin
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the underdrain system. The gravel should

be sized to prevent the filter media from

mixing into the gravel, or a layer of 1/4-

to 3/8-inch-diameter gravel should be

placed over the underdrain gravel before

the filter media is added.

Performance

RSF systems are extremely effective and

reliable in removing BOD, TSS, and

contaminants that associate with the

particulate fraction of the incoming septic tank effluent. Some typical performance data are provided in table 2.

Normally, BOD and TSS effluent concentrations are less than 10 mg/L when RSF systems are treating residential wastewater. Nitrification tends to be complete, except in severely cold conditions. Natural denitrification in the recirculating

tank results in 40 to 60 percent removal of total nitrogen (TN). Fecal coliform removal is normally 2 to 3 logs (99 to 99.9

percent). Phosphorus removal drops off from high percentages to about 20 to 30 percent after the exchange capacity of the

media becomes exhausted. Some media and media mixes have very high iron and/or aluminum content that extends the

initial period of high phosphorus removal. (See Enhanced Nutrient Removal—Phosphorus, Technology Fact Sheet 8.)

Figure 4. Typical underdrain detail.

Table 2. Recirculating filter performance

a

Single-family home filters. Sand media: es = 0.3 mm; uc = 4.0. Average loadings = 0.9 gpd/ft2 (forward flow) / 1.13 lb BOD/1,000 ft2 –day. Recirculation ratio

= 3:1. Dosed 4-6 times per hour. Open surface. b

Single-family home filters. Sand media: es = 1 mm; uc = <2.5. Design hydraulic loadings = 3.54 gpd//ft2 (forward flow). Actual flow not measured.

Recirculation ratio = 3:1. Doses per day = 24. c

Single-family home filters. Sand media: es = 1.2 mm; uc = 2.0. Maximum hydraulic loading (forward flow)= 3.1 gpd/ft2. Recirculation ratio = 3:1-4:1. Doses

per day = 48. d

Single-family home filters. Gravel media: es = 4.0 mm; uc = <2/5. Design hydraulic loading (forward flow)= 23.4 gpd/ft2. Recirculation ratio = 5:1. Doses per

day = 48. Open surface, winter operation. e

Restaurant (grease and oil inf./eff. = 119/<1 mg/L, respectively). Gravel media: pea gravel (3/8 in. dia.) Design hydraulic loading (forward flow) = 15 gpd/ft2.

Recirculation ratio = 3:1- 5:1. Doses per day = 72. Open surface, seasonal operation. f

Small community treating average 15,000 gpd of septic tank effluent. Sand media: es = 1.5 mm; uc = 4.5. Design hydraulic loading (forward flow) = 2.74

gpd/ft2. Recirculation ratio = 1:1-4:1. Open surface, winter operation.

Filter Sand

4" slotted PVC

Underdrain

1⁄2" to 3⁄4" rock 2"

6"

 

 

 


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Management needs

As with all treatment systems, the RSF should be constructed carefully according to design specifications using corrosionresistant materials. Every truckload of media delivered to the site should be tested for compliance with the specifications.

All tanks and lined basins, including the entry and exit plumbing locations, must be watertight.

Inspection and operation/maintenance (O/M) needs are primarily related to inspection and calibration of the recirculation

pump and controls. For sand media units, frequent removal of vegetation and scraping of the surface are required. Regular

maintenance tasks include periodic checks on the pressure head at the end of the distribution system, draining of the

accumulated solids from lines, and occasional brushing of the lines (at least once per year), with bottle brushes attached to

a plumber’s snake.

The recirculation tank should be checked for sludge accumulation on each visit and pumped as necessary (usually one to

three times per year).

Risk management issues

RSFs are extremely reliable treatment devices and are quite resistant to flow variations. Toxic shocks are detrimental to

RSF treatment performance because of the resistance of biofilms to upset and the extended period of contact between

biofilms and wastewater.

Gravel RSFs (or RGFs) are likely viable throughout the United States when proper precautions (e.g., covering, insulation)

are taken. These systems perform best in warmer climates, but they increase opportunities for odor problems. In general,

gravel RSF systems are far less prone to odor production than ISFs. Increased recycle ratios should help minimize such

problems. However, power outages will stop the process from treating the wastewater, and prolonged outages would be

likely to generate some odors.

Costs

Construction costs for recirculating sand filters are driven by treatment media costs, the recirculating tank and pump/

control system costs, and containment costs. Total costs are therefore site specific, but tend to vary from $8,000 to

$11,000. Low-cost alternative media can reduce this figure significantly.

Power costs for pumping at 3 to 4 kWh/day are in the range of $90 to $120 per year, and management costs for monthly

visits/inspections by semiskilled personnel typically cost $150 to $200 annually.

References

Anderson, D.L., R.L. Siegrist, and R.J. Otis. 1985. Technology Assessment of Intermittent Sand Filters. U.S.

Environmental Protection Agency, Office of Research and Development, and Office of Water, Publication,

Washington, DC.

Ayres Associates. 1997. Florida Keys Wastewater Nutrient Reduction Systems Demo Project: Second Quarter Report.

Florida Department of Health, Tallahassee, FL.

Ayres Associates. 1998. Unpublished data from Wisconsin.

Bruen, M.G., and R.J. Piluk. 1994. Performance and Costs of Onsite Recirculating Sand Filters. In Proceedings of the

Seventh On-site Wastewater Treatment Symposium. American Society of Agricultural Engineers, St. Joseph, MI.

Kerri, K.D., and J. Brady. 1997. Small Wastewater System Operation and Maintenance: Vol. 1. California State

University, Sacramento, CA.
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Louden, T.L., D.B. Thompson, L. Fay, and L.E. Reese. 1985. Cold-Climate Performance of Recirculating Sand Filters.

In Proceedings of the Fourth On-site Wastewater Treatment Symposium. American Society of Agricultural Engineers,

St. Joseph, MI.

National Small Flows Clearinghouse. 1998. Recirculating Sand Filters. U.S. Environmental Protection Agency, Office of

Water, Washington, DC.

Orenco Systems, Inc. 1993. Cost Estimating for STEP Systems and Sand Filters. Orenco Systems, Inc., Roseburg, OR.

Owen, J.E., and K.L. Bobb. 1994. Winter Operation and Performance of a Recirculating Sand Filter. In Proceedings of the

WEFTEC 67th Annual Conference. Water Environment Federation, Alexandria, VA.

Piluk, R.J., and E.C. Peters. 1994. Small Recirculating Sand Filters for Individuals Homes. In Proceedings of the Seventh

On-site Wastewater Treatment Symposium. American Society of Agricultural Engineers, Joseph, MI.

Rhode Island Department of Environmental Management. 2000. Sand Filter Guidance Document. Rhode Island

Department of Environmental Management, Providence, RI.

Roy, C., and J.P. Dube. 1994. A Recirculating Gravel Filter for Cold Climates. In Proceedings of the Seventh On-site

Wastewater Systems Symposium. American Society of Agricultural Engineers, St. Joseph, MI.
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 12

Land Treatment Systems

Description

Land (surface) treatment systems (figures 1 and 2) are permitted in some states, but are not widely used because of their

large land area requirements exacerbated by code-required setbacks. For example, a spray irrigation system requires about

four times the area of an individual home lagoon. When these systems are used, large buffer areas and fencing may be

required to ensure minimal human exposure. Also, given the nature of these systems, all requirements include disinfection

and significant pretreatment before application. In wet and cold areas, an additional basin for storage or a larger dosing

Figure 2. Conceptual schematic of rapid infiltration system

Figure 1. Conceptual schematic of spray irrigation system
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tank is necessary to eliminate possible runoff from the application area. The most used variation of these systems is the

spray irrigation system (figure 3).

Spray irrigation systems distribute wastewater evenly on a vegetated plot for final treatment and discharge. Spray irrigation can be useful in areas where conventional onsite wastewater systems are unsuitable due to low soil permeability,

shallow water depth table or impermeable layer, or complex site topography. Spray irrigation is not often used for residential onsite systems because of its large areal demands, the need to discontinue spraying during extended periods of cold

weather, and the high potential for human contact with the wastewater during spraying. Spray irrigation systems are

among the most land-intensive disposal systems. Drifting aerosols from spray heads can be a nuisance and must be monitored for impact on nearby land use and potential human contact. Buffer zones for residential systems must often be as

large as, or even larger than, the spray field itself to minimize problems.

In a spray irrigation system, pretreatment of the wastewater is normally provided by a septic tank (primary clarifier) and

aerobic unit, as well as a sand (media) filter and disinfection unit. Some states do not require the aerobic unit if the filter is

used. The pretreated wastewater in spray irrigation systems is applied at low rates to grassy or wooded areas. Vegetation

and soil microorganisms metabolize most nutrients and organic compounds in the wastewater during percolation through

the first several inches of soil. The cleaned water is then absorbed by deep-rooted vegetation, or it passes through the soil

to the ground water.

Rapid infiltration (RI) is a soil-based treatment method in which pretreated (clarified) wastewater is applied intermittently

to a shallow earthen basin with exposed soil surfaces. It is only used where permeable soils, which generally can accept a

conventional OWTS, are available. Because loading rates are high, most wastewater infiltrates the subsoil with minimal

losses to evaporation. Treatment occurs within the soil before the wastewater reaches the ground water. The RI alternative

is rarely used for onsite wastewater management. It is more widely used as a small-community wastewater treatment

system in the United States and around the world.

The third and last type of land surface treatment is the overland flow (OF) process. In this system, pretreated wastewater is

spread along a contour at the top of a gently sloping site that has minimum permeability. The wastewater then flows down

the slope and is treated by microorganisms attached to vegetation as it travels by sheet flow over very impermeable soils

until it is collected at the bottom of the slope for discharge. This system (figure 4) requires land areas similar to the spray

Figure 3. Typical residential spray irrigation systems

1. House sewer; 2. Septic tank (two required); 3. Aerobic unit; 4. Dosing tank; 5. Sand filter; 6. Cl2 disinfection (or UV); 7. Tank and pump (plus storage); 8. Piping

system; 9. Sprinklers; 10. Application site

Source: Adapted from McIntyre et al., 1994.
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irrigation system. However, surface water discharge requirements (e.g., disinfection) from the OF system must still be

met. Overland flow, like rapid infiltration, is rarely used for onsite wastewater management.

Typical applications

Spray irrigation (SI) is normally considered at site locations that do not permit a conventional SWIS because of relative

impermeability and shallow depths caused by restrictive conditions (e.g., ground water or impermeable bedrock or

fragipan). SI is normally the final step in the treatment sequence as the effluent is reintroduced to the environment. Most

states require advanced treatment and disinfection prior to SI treatment.

Design assumptions

After pretreatment, which at a minimum should be a typical ISF effluent followed by disinfection, the treated wastewater

is conveyed to a holding tank with a pump and controls that deliver it to the sprinkler system. The sprinklers spread the

wastewater over a predetermined area at specific times. In wet climates or frozen soil conditions, an additional holding

(storage) basin or larger dosing tank is required to prevent irrigation during periods when the wastewater would not be

accepted by the soil for treatment and intended environmental incorporation. Regulations for buffer requirements from

Texas, Virginia, and Pennsylvania are incorporated into table 1. Typically, the features listed below and their peripheral

buffer zones are fenced to prevent exposure.

Application rates vary. Texas determines design rates based on evaporation, Virginia bases rates on soil texture, and

Pennsylvania uses a combination of soil depth and slope. From a performance code approach, the application rate should

be based on protecting the receiving surface/ground waters. It should be based on wastewater characteristics, critical

constituent required concentrations (at a monitoring location

where a specific quality standard must be met), and the

characteristics of the site (i.e., features that will mitigate

wastewater contaminants in order to meet the constituent

concentration at the point of use).

In practical terms, all three states require the same pretreatment sequence, which yields SI influent of approximately 5, 5,

25, and 4 mg/L of CBOD, TSS, TN, and TP, respectively, in

addition to a fecal coliform (FC) level of about 10 cfu/100 mL

(if the disinfection step is working properly). Passage through

1 foot of unsaturated soil should for a few years remove most

CBOD, TSS, TP, and FC; therefore, nitrogen will be the

Figure 4. Overland flow system schematic

Table 1. Buffer requirements to various features

Source: North Carolina DEHNR, 1996.
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constituent of most concern. During the growing season, removal should be feasible by crop uptake and, to a lesser degree,

ammonia volatilization.

Therefore, the hydraulic and nitrogen loading rates for a specific site are the primary design parameter. Also, these systems

are rarely considered for permeable soils. The design approach described below is for this set of circumstances.

Spray irrigation systems are designed to treat wastewater and evenly distribute the effluent on a vegetated lot for final

treatment. The application rate is determined by two major factors: hydraulic loading and nutrient loading (usually nitrogen

is the limiting factor). The application rate is designed to meet the capacity of the soil to accept the effluent hydraulically

and subsequently allow it to drain through the soil. The application rate can be varied according to the permeability of the

soil. In Pennsylvania and Virginia, this method results in application rates of 0.6 to 2.5 cm/week. Lower rates can greatly

improve nitrogen removal. The treated wastewater is spread over the required application area through a sprinkler or drip

irrigation system.

Sprinklers are generally low-angle (7 to 13 degrees), large-drop-size nozzles designed to minimize aerosols. Application

areas must be vegetated (with crops not intended for human consumption) and have slopes that preclude runoff to

streams. The type of vegetation determines the nitrogen loading capacity of the site, but the hydraulic capacity depends on

climate and soil characteristics. Additional nitrogen losses may occur through denitrification (only about 25 percent due to

the low BOD:N ratio) and ammonia volatilization (about 10 percent if soil pH is high; less to none if it is acidic).

Spray irrigation of wastewater effluent must be timed to coincide with plant uptake and nutrient use. Temperature factors

in some areas of the country may preclude the use of spray irrigation during certain times of the year. The wastewater

may need to be stored in holding tanks during the coldest period of the year, because plant growth is limited and the

nitrogen in effluent discharged during this time will be mineralized and unavailable for plant uptake.

Some SI systems irrigate only one or two days per week so that the soil can drain and aerate between applications. Others

spray twice during the night or in the early morning to minimize inconvenience to the homeowner and to minimize the

potential for human contact.

The width of the required buffer zone depends on the slope of the site, the average wind direction and velocity, the type of

vegetation, and the types of nearby land uses. For wastewater produced by a single-family home, the minimum recommended SI plot area, including buffer zones, is commonly about 2 acres (0.81 hectares) in Pennsylvania and Virginia.

Performance

Studies that sample both the soil below the spray field and its runoff show that spray irrigation systems work as well as

other methods of managing wastewater. Spray irrigation systems are designed for no degradation; therefore, hydraulic and

nutrient loading rates are based on the type of vegetation used and the hydraulic properties of the soils. If the vegetation

cannot assimilate the amount of nitrogen applied, for example, then nitrogen removal to reduce the nitrogen content of the

effluent prior to spray irrigation may be required. The overall efficiency of a spray irrigation system in removing pollutants

will be a function of the pollutant removal efficiencies of the entire treatment process and plant uptake.

There have been few documented cases of health problems due to the spray irrigation, but use of proper buffer zones is

crucial. One benefit of spray irrigation is savings on potable water because the wastewater is used for irrigation.

Management needs

Construction factors include site preparation and installation of runoff controls, irrigation piping, return systems, and

storage facilities. Since sustained wastewater infiltration is an important component of successful system operation, it is

critical that construction activity be limited on the application site. Where stormwater runoff can be significant, measures

must be taken to prevent excessive erosion, including terracing of steep slopes, contour plowing, no-till farming, establishment of grass border strips, and installation of sediment control basins. Earthworking operations should be conducted in
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such a way as to minimize soil compaction. Soil moisture should generally be low during these operations. High-flotation

tires are recommended for all construction vehicles.

The soil profile must also be managed to maintain infiltration rates by avoiding soil compaction and maintaining soil

chemical balance. Compaction and surface sealing (caused by harvesting equipment and development of fine layers from

multiple wastewater applications) can reduce soil infiltration and increase runoff.

Local regulatory agencies may require ground water monitoring to evaluate system performance. Soil fertility and chemical

balance should be evaluated periodically to determine if soil amendments are necessary. Trace elements may also be

analyzed to monitor possibly toxic accumulations.

Residuals produced by slow-rate land application systems are limited to harvested crops and crop residues that are not for

human consumption. Agricultural crop applications require the most intensive management, while forest application

requires the least management. Management tasks may include soil tillage, planting and harvesting of crops, nutrient

control, pH adjustment, and sodium and salinity control. No special equipment, other than the appropriate agricultural

equipment, is required. Typical pump, controls, and basin requirements prevail for the dosing system.

Virginia’s O/M requirements for onsite spray irrigation systems (not including pretreatment unit processes) include the

following:

• Monthly. Walk over spray area and examine for

– Ponding of effluent

– Bad odors

– Damage to spray heads

– Surfacing liquids

– Vegetation problems

– Surface soil collapse

• Quarterly. Conducted by a qualified, semi-skilled operator

– Proper spray sequence

– Proper pump function

– Proper liquid levels

• Biannually

– Erosion

– Storage unit capacity

• Annually. Effluent sampling by a certified laboratory

– Test water supplied to spray irrigation area for pH, total Kjeldahl nitrogen, fecal coliform bacteria, chlorine, TSS,

and BOD

– Reports of analyses are to be submitted by the laboratory to the local/district health department within 10 days of

the completion of the analyses.

A management contract with an approved operator or operations firm is also required.
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Risk management issues

No crops grown on the SI application area should be consumed by humans. Buffer zones should minimize aerosol exposure. Spray irrigation systems with sufficient storage capacity are essentially unaffected by major flow variations. A water

balance should be conducted to determine the need under the climate conditions, soils, and application rates and patterns of

each rate. Similarly, toxic shock loadings should be largely dissipated in the preceding pretreatment steps, but phytotoxic

compounds may still be a concern at the application site. Spray irrigation cannot function during saturated or frozen

conditions, and the pretreated influent must be stored until proper vegetative uptake (usually nitrogen) conditions return.

Power outages will affect the upstream pretreatment processes rather than the SI system, even though the system must

have power to function.

However, by the time the wastewater effluent is discharged by the sprinklers, the water should be sufficiently treated so as

not to pose health risks. There have been no documented cases of health problems due to the spray irrigation of properly

treated wastewater. However, drifting aerosols from the spray heads should be monitored for impact on nearby land uses.

A benefit of spray irrigation is the use of effluent, instead of potable tap water, to water the landscape.

Costs

Construction costs of SI systems are very high if the generally required pretreatment is included, especially if both an

aerobic unit and a sand filter treating septic tank effluent are included. Such a system could easily cost $20,000 or more.

O/M costs for the SI system alone primarily include labor (15 to 20 hours per year), power (for pumps and other pretreatment needs) and materials (e.g., chlorine, if chosen). O/M costs are estimated at more than $500 per year, given the entire

treatment train suggested by figure 3. If the aerobic treatment unit is not required ahead of the sand filter, and a UV

disinfection unit is used, this cost may reduce to $300 to $400 annually.

References

Crites, R., and G. Tchobanoglous. 1996. Small and Decentralized Wastewater Management Systems. WCB/McGraw-Hill,

San Francisco, CA.

Emery, H.C. 1999. Onsite Spray Irrigation: A Tale of Three Cities. Pumper.

McIntyre, C., C. D’Amico, and J.H. Willenbrock. 1994. Residential Wastewater Treatment and Disposal: On-Site Spray

Irrigation Systems. In Proceedings of the Seventh Onsite Wastewater Treatment Symposium. American Society of

Agricultural Engineers, St. Joseph, MI.

Monnett, G.T., R.B. Reneau, Jr., and C. Hagedorn. 1991. Evaluation of Onsite Spray Irrigation for Disposal on Marginal

Soils. In Proceedings of the 6th Onsite Wastewater Treatment Symposium. American Society of Agricultural Engineers,

St. Joseph, MI.

North Carolina, DEHNR. 1996. On-site Wastewater Management Guidance Manual. Division of Environmental Health.

Raleigh, NC.

Rubin, A.R. 1992. Slow-Rate Spray Irrigation and Drip Disposal Systems for Treatment and Renovation of Domestic

Wastewater from Individual Homes. In Proceedings of the Seventh Northwest Onsite Wastewater Treatment Short

Course. University of Washington, Seattle.

Shuval, H.I., A. Adin, B. Fattal, E. Raisitz, and P. Yekutiel. 1986. Wastewater Irrigation in Developing Countries. World

Bank technical paper no. 51. World Bank, Washington, DC.

U.S. Environmental Protection Agency (USEPA). 1992. Manual: Wastewater Treatment/Disposal for Small Communities.

EPA/625/R-92/005. U.S. Environmental Protection Agency, Office of Research and Development, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1981. Process Design Manual for Land Treatment of Municipal

Wastewater. EPA 625/1-81-013. U.S. Environmental Protection Agency, Office of Research and Development,

Cincinnati, OH.
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Onsite Wastewater Treatment Systems

Technology Fact Sheet 13

Renovation/Restoration of

Subsurface Wastewater Infiltration

Systems (SWIS)

Although an analysis to diagnose problems in OWTSs is provided in chapter 5, this Fact Sheet is included to provide a

special reference to identify alternatives likely to be recommended to renovate and restore SWIS and observed results.

Functions of the subsurface wastewater infiltration system (SWIS)

The subsurface wastewater infiltration system (SWIS) receives the effluent pretreated in the septic tank and purifies it

through biological, physical, and chemical reactions as it passes through the unsaturated soil to the ground water. An

important component of the infiltration system is the biomat, a layer of organic and inorganic material and bacteria that

forms at the interface between the trench and the surrounding soil. The biomat enhances treatment efficiency because it

usually slows down the movement of the effluent, provides the flora and fauna necessary to biologically decompose

wastes, and enhances the physical and chemical removal of very small particles of matter in the wastewater. Permeable

soil textures and structures are required to support these processes.

SWISs are occasionally unable to accept the total daily wastewater load they receive, leading to ponding and eventual

hydraulic failure. This is typically caused by the accumulation of biomass and suspended solids in or near the biomat,

which reduces the soil’s porosity and hydraulic conductivity. If the system fails hydraulically, the first step is usually to

pump the septic tank and clean and replace the effluent screen (also known as a filter).

Restoring the hydraulic function of the infiltration system involves eliminating or reducing the flow restrictions. Various

methods and products have been developed for restoring the infiltration capacity of SWISs. These include resting, additives, hydrogen peroxide, and soil fracturing.

A variety of additives are also marketed to improve the performance of septic tanks or eliminate the need for pumping.

These septic tank additives are discussed in Special Issue Fact Sheet 1.

SWIS restoration alternatives

Periodic resting

Periodic resting is a passive method for restoring the hydraulic capacity of the SWIS. Infiltration surfaces are “rested” by

removing them from service for an extended period of time, typically 6 to 12 months. To remove a portion of the SWIS

from service requires that the SWIS be constructed with multiple cells that have a total hydraulic capacity of 100 to 200

percent of the design flow, or enough suitable reserve SWIS area. Resting may also be used in seasonal facilities that

discharge no wastewater for extended periods of the year. The portion of the SWIS taken offline receives no wastewater

during the resting period, which allows the infiltration surface to drain and dry out. The resulting aerobic biochemical

oxidation of the biomat mass can restore the porosity of the biomat, helping to unclog the system.
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Several studies have shown resting to be an effective method to rejuvenate the hydraulic capacity of soil infiltration

surfaces (Sharpe et al., 1984). Extended periods of resting at regular intervals is effective in preventing excessive soil

clogging and restoring clogged infiltration surfaces. Seventy to eighty percent of the original infiltration capacity of the soil

can be recovered by resting. The rate of restoration is proportional to grain size; that is, sand restores more quickly than

silt and clay.

Some studies have explored the potential for adding earthworms when a malfunctioning SWIS is pumped. Generally, this

approach has not been successful. If the system is basically sound in design, loaded within design limits, and located in

well-drained soils, some improvement in hydraulic function may occur when worms are used, especially if some water

conservation measures are implemented. However, no quantitative data exist to support the concept that worms aid SWIS

restoration.

Additives

In addition to the additives described in Special Issue Fact Sheet 1, there are commercially available compounds that are

apparently benign to the treatment processes in the septic tank and have potential benefit to the SWIS by exchanging with

potentially harmful ions (e.g., sodium), that could destroy existing fine soil structure. Such additives could be useful in

places having high-sodium drinking water or in areas with hard water supplies where ion exchange softeners are used and

the regenerant is not discharged to the SWIS, leaving those soils with an excess of sodium ions. In general, however, the

benefits of SWIS additives are not well documented. Chemical additives that contain strong acids or bases or toxic

chemicals are generally discouraged or banned because of the possible adverse effects these chemical can have on system

components, the soil structure, or ground water quality. Biological additives, on the other hand, may have some small

benefits, but there is little published documentation to support this view. Microbial and enzyme preparations appear to

enhance liquefaction of biodegradable solids in septic tanks. However, the effects of their use on the soil infiltration surface

have not been documented. Studies have shown that biological additives are not directly harmful to traditional onsite

systems, but significant beneficial impacts have not been documented with domestic wastewaters (Clark, 1999).

Hydrogen peroxide

Hydrogen peroxide (H2O2) is a chemical treatment that was once promoted for its ability to treat a clogged SWIS. H2O2, a

strong oxidant, was pumped directly into the absorption trench to restore the hydraulic capacity of the infiltration zone by

oxidizing the biomat and breaking down the crust surrounding it. While early research on the use of hydrogen peroxide to

unclog SWIS in sandy, unstructured soils appeared positive, subsequent testing did not. Controlled field studies found

decreasing infiltration rates for clogged systems treated with H2O2. These reports suggest that hydrogen peroxide mobilizes

fine soil particles during initial treatment in some soils. As the chemical reactions subside, however, these fine particles are

deposited on top of the infiltrative surface, which can result in further clogging. Hydrogen peroxide can produce temporary benefits at a substantial cost, and is not recommended for regular long-term use in unclogging failed drainfields.

Hydrogen peroxide is a strong oxidant that has been shown to be very effective in eliminating biomats in SWIS, but it can

also reduce soil porosity and hydraulic conductivity. The process has been shown to oxidize or “boil” the soil, which

creates a layer of fine particles that are released when the soil peds are destroyed on the infiltrative surface. This dramatically reduces the hydraulic capacity of SWIS.

Pneumatic soil fracturing

Pneumatic soil fracturing is a mechanical treatment used to increase soil porosity by fracturing and lifting the soil surrounding the infiltration surface. A steel probe, inserted below the infiltration surface, is used to inject high-pressure air

into the soil. The air fractures and lifts the ground. As the soil expands, polystyrene beads are discharged into the soil

fractures, thereby holding them open to increase the porosity of the soil after the particles settle. However, any hydraulic

improvements are accompanied by a potential for contamination of underlying aquifers. Insufficient data are available to

recommend use of this concept in any area where sensitive ground water supplies lie in close proximity to the infiltrative

surface.
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Introduced in the early 1990s, pneumatic soil fracturing is a relatively new treatment method. Thus, available performance

data on the method are limited and incomplete. Appropriate applications and expected performance are unknown.

Application

These renovation methods can be applied for either preventive maintenance or rehabilitation after a hydraulic failure has

occurred. Resting and the application of additives are primarily preventive maintenance methods. Standby infiltration cells

to allow resting can be constructed with the original system or additional land can be held for replacement if failure of the

original infiltration system fails. It should be noted that the ability to alternate cells regularly during normal operation is

more effective as a preventive maintenance technique than a method to relieve a failed system. The use of additives and

hydrogen peroxide is generally not recommended.

Users must be aware that when any of these methods are used to correct hydraulic failures, only the symptoms of failure

is treated. The causes of the failure will usually persist. Therefore, the causes of failure should be identified and appropriate corrective action taken to prevent recurrences. Excessive daily flows, inadequate or improperly maintained pretreatment processes (e.g., failure to pump septic tank), and changes in wastewater characteristics because of new ownership

or changes in use are common causes of hydraulic failure. If these failures are not eliminated or accommodated through

appropriate system modifications, the effectiveness of the treatments will be short-lived.

Responsibilities of the homeowner

The key responsibilities of the homeowner in ensuring the best operation of an existing or new septic tank/SWIS system

include the following:

• Using household cleansers in moderation. Excessive use of household cleansers, disinfectants, and other common

products can kill bacteria residing in the septic tank and the soil adsorption field. Used in moderate amounts and

according to label directions, however, cleaners and disinfectants can be flushed into the wastewater system with no

significant impacts. The wastewater stream dilutes the product, and organic material adsorbs it. Slug loading (excessive, instantaneous loadings) of household cleaners can be lethal to septic system bacteria, but normal follow-up usage

usually reestablishes the tank’s bacterial population within a few hours.

• Avoiding disposal of toxic and hazardous materials in the wastewater stream. Many common household products have

toxic properties and should never be poured down the drain. The list includes drugs and antibiotics, solvents, paints,

varnishes, photography chemicals, weed killers, and insecticides. All of these products have the potential to wipe out

septic system bacteria and percolate into ground water supplies.

• Curbing the use of drain cleaners and openers. Products aimed at unclogging indoor wastewater pipes contain strong

acids or alkalis as the active ingredient. Used according to the label directions, they can be effective in removing clogs

of organic matter in indoor drainpipes. Most product labels warn, however, that the product is caustic or corrosive to

pipes and can be hazardous to the user if applied improperly. A controlled study concluded that as little as 1.3 ounces

of a name brand drain cleaner could destroy the bacteria population in a 1,000-gallon septic tank. This amount is

within the general range of normal usage for some people. Bacteria populations in the tank will recover in a few days if

the system inputs return to normal levels.

• Disposing of solids appropriately. Items such as cigarette butts, condoms, sanitary napkins, paper towels, and kitty

litter should never be flushed or washed down the toilet or sink. Septic tanks are not designed as a disposal receptacle

for these wastes. They can clog drainpipes and cause excessive and rapid sludge buildup in the tank.

• Keeping fats, oils, and grease out of kitchen drains. Fats, oils, and grease are natural by-products of cooking meats

and other foods. Grease washed down the drain can stick, accumulate, and in some cases block wastewater drain

pipes and the inlet and outlet structures in septic tanks. Food wastes should be scraped from plates and utensils and

discarded as solid waste.
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• Avoiding the use of a garbage disposal unless the treatment system is designed for one. Homes with garbage disposals

generally have 20 to 28 percent higher biochemical oxygen demand (BOD) and 25 to 40 percent higher suspended

solid loadings to septic tanks than homes without disposals. These significant contributions of organic matter require

special consideration when sizing and installing a septic tank or soil absorption system.

• Conserving water. To function at peak efficiency, the septic tank needs to provide a quiescent environment and

adequate detention time (i.e., more than 24 hours) for the solids and floatable matter to separate from the wastewater.

Limiting water flows and timely repair of leaking fixtures help maintain these conditions and prevent overloading of the

soil adsorption field.

References

Andress, S., and C. Jordan. 1998. Onsite Sewage Systems. Virginia Polytechnic Institute and State University, Civil

Engineering Department, Blacksburg, VA.

Angoli, T. 2000. Hydrogen peroxide not recommended to unclog failed drainfields. Small Flows Quarterly 1(2):42044.

Clark, G.H. 1999. The Effect of Bacterial Additives on Septic Tank Performance. Master’s thesis, North Carolina State

University Department of Soil Science, Raleigh, NC.

Dow, D., and G. Loomis. 1999. Septic Tank Additives. University of Rhode Island Cooperative Extension Service, Onsite

Wastewater Training Center, Kingston, RI.

Gross, M.A. 1987. Assessment of the Effects of Household Chemicals upon Individual Septic Tank Performances.

University of Arkansas, Arkansas Water Resources Research Center, Fayetteville, AR.

Hairston, J.E., G. Speakman, and L. Stribling. 1995. Protecting Water Quality: Understanding Your Septic System and

Water Quality. Alabama Cooperative Extension Publication wq-125.al June 1995. Developed with support from

Auburn University. Auburn University, Auburn, AL.

Hargett, D.L., E.J. Tyler, J.C. Converse, and R.A. Apfel. 1984. Effects of Hydrogen Peroxide as a Chemical Treatment for

Clogged Wastewater Absorption Systems. In Proceedings of the Fourth National Symposium on Individual and Small

Community Sewage Treatment, American Society of Agricultural Engineers, St. Joseph, MI.

Hargett, D.L., E.J. Tyler, and R.L. Siegrist. 1982. Soil Infiltration Capacity as Affected by Septic Tank Effluent Application

Strategies. In Onsite Sewage Treatment, Proceedings of the Third National Symposium on Individual and Small

Community Sewage Treatment. American Society of Agricultural Engineers, St. Joseph, MI.

Jantrania, A., W.A. Sack, and V. Earp. 1994. Evaluations of Additives for Improving Septic Tank Operation Under Stress

Condition. In Proceedings of the International Symposium on Individual and Small Community Sewer Systems.

American Society of Agricultural Engineers, St. Joseph, MI.

Olson, K., D. Gustafson, B. Liukkonnen, and V. Cook. 1977. Septic System Owner’s Guide. University of Minnesota

Extension Services Publication PC-6583-GO. University of Minnesota, College of Agricultural, Food and

Environmental Sciences, St. Paul, MN.

Rupp, G. 1996. Questions and Answers about Septic System Additives. Montana State University Extension Service.

Bozeman, MT.

Scow, K.M. 1994. The Efficacy and Environmental Impact of Biological Additives to Septic Systems. University of

California, Berkeley, CA.

Sharpe, W.E., C.A. Cole, and D.D. Fritton. 1984. Restoration of failing onsite wastewater systems using water

conservation. Journal of the Water Pollution Control Federation 56(7):855-866.

Simons, A.P., and F.R. Magdoff. 1979. Disposal of septic tank effluent in mound and sand filter trench systems on a clay

soil. Journal of Environmental Quality 8:469-473.
TFS-81

Thomas, R.E., et al. 1996. Soil chemical changes and infiltration rate reduction under sewage spreading. In Proceedings of

the Annual Meeting of the Soil Science Society of America, pp. 641-646. Madison, WI.

U.S. Environmental Protection Agency. 1998. A Project to Renovate Failing Gravity Septic Systems with Earthworms.

Section 319 project report. EPA 40-08/98/07/01. U.S. Environmental Protection Agency, Washington, DC.

Virginia Polytechnic Institute and State University. 1996. Septic System Maintenance. Produced by the Water Quality

Program Committee. VTU pub. no. 440-400. Virginia Polytechnic Institute and State University, Blacksburg, VA.
TFS-82 (Click Here to Return to Bookmarks Page) 
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Description

Because of the presence of significant numbers and types of bacteria, enzymes, yeasts, and other fungi and microorganisms

in typical residential and commercial wastewaters, the use of septic system additives containing these or any other ingredients is not recommended. The benefits of consumer products sold as septic system cleaners, degraders, decomposers,

deodorizers, organic digesters, or enhancers are not significant or have not been demonstrated conclusively, depending on

the product. Some of these products can actually interfere with treatment processes, affect biological decomposition of

wastes, contribute to system clogging, and contaminate ground water. The septic tank/soil absorption field system is the

most commonly used onsite wastewater treatment system in the United States. It is relatively low in cost, has no moving

parts, and requires little maintenance.

Septic tanks have a number of important functions, including:

• Remove oils, grease and settleable solids. The septic tank is designed to provide quiescent conditions over a sufficient

time period to allow settleable solids to sink to the bottom of the tank and floatable solids, oils, and grease to rise to the

surface. The result is a middle layer of partially clarified effluent that exits the tank to the soil absorption field.

• Store settleable and floatable material. Tanks are generously sized according to projected wastewater flow and

composition to accumulate sludge and scum at the bottom and top of the tank, respectively. Tanks require pumping at

infrequent intervals (e.g., 1 to 7 years), depending on sludge and scum accumulation rates.

• Digest/decompose organic matter. In an anaerobic environment, facultative and anaerobic bacteria can reduce retained

organic molecules to soluble compounds and gases, including H2

, CO2, NH3, H2S, and CH4. This digestion can significantly reduce sludge volume in warm climates.

Types of additives and effects on treatment processes

There are three general types of commonly marketed septic system additives:

• Inorganic compounds, usually strong acids or alkalis, are promoted for their ability to open clogged drains. Product

ingredients (e.g., sulfuric acid, lye) are similar to those used in popular commercial drain cleaners. These products can

adversely affect biological decomposition processes in the treatment system and cause structural damage to pipes,

septic tanks, and other treatment system components. Hydrogen peroxide, once promoted as an infiltration field

reconditioner, has been found to actually degrade soil structure and compromise long-term viability of soil treatment

potential. Its use to unclog failed infiltration fields is no longer recommended.

• Organic solvents, often chlorinated hydrocarbons (e.g., methylene chloride, trichloroethylene) commonly used as

degreasers and marketed for their ability to break down oils and grease. Organic solvents represent significant risks to

ground water and wastewater treatment processes. These products can destroy resident populations of decomposer

and other helpful microorganisms in the treatment system. Use of products containing organic solvents in onsite

treatment systems is banned in many states. Introduction of organic solvents into onsite systems located in states that

ban the use of these products may trigger liability issues if ground water becomes contaminated.

Onsite Wastewater Treatment Systems

Special Issues Fact Sheet 1

Septic Tank Additives
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• Biological additives, like bacteria and extracellular enzymes mixed with surfactants or nutrient solutions, which

mirror but do not appear to significantly enhance normal biological decomposition processes in the septic tank. Some

biological additives have been found to degrade or dissipate septic tank scum and sludge. However, whether this

relatively minor benefit is derived without compromising long-term viability of the soil infiltration system has not

been demonstrated conclusively. Some studies suggest that material degraded by additives in the tank contributes to

increased loadings of BOD, TSS, and other contaminants in the otherwise clarified septic tank effluent.

Other products containing formaldehyde, paraformaldehyde, quaternary ammonia, and zinc sulfate are advertised to

control septic odors by killing bacteria. This objective, however, runs counter to the purpose and function of septic tanks

(promoting anaerobic bacterial growth). If odor is a problem, the source should be investigated because sewage may be

surfacing, a line might have ruptured, or another system problem might be present.

Another variety of consumer products is marketed for their ability to remove phosphorus from wastewater. These products are targeted at watershed residents who are experiencing eutrophication problems in nearby lakes and streams.

Phosphorus is an essential nutrient for aquatic plant growth and limiting its input to inland surface waters can help curtail

nuisance algae blooms. Aluminum (as alum, sodium aluminate, aluminum chloride, and activated alumna), ferric iron (as

ferric chloride and ferric sulfate), ferrous iron (as ferrous sulfate and ferrous chloride), and calcium (as lime) have been

proven to be effective in stripping phosphorus from effluent and settling it to the bottom of the tank. An important side

effect of this form of treatment, however, can be the destruction of the microbial population in the septic tank due to loss

of buffering capacity and a subsequent drop in pH. Treatment processes can be severely compromised under this scenario.

Finally, baking soda and other flocculants are marketed as products that lower the concentration of suspended solids in

septic tank effluent. Theoretically, flocculation and settling of suspended solids would result in cleaner effluent discharges

to the subsurface wastewater infiltration system. However, research has not conclusively demonstrated significant success

in this regard.

References

Andress, S.; Jordan, C. 1998. Onsite Sewage Systems. Virginia Polytechnic Institute and State University, Civil

Engineering Department, Blacksburg, VA.

Angoli, T. 2000. Hydrogen peroxide not recommended to unclog failed drainfields. Small Flows Quarterly Vol. 1 No. 2, p.

42-44.

Clark, G.H. 1999. The Effect of Bacterial Additives on Septic Tank Performance. Master’s thesis, North Carolina State

University, Department of Soil Science, Raleigh, NC.

Dow, D., and G. Loomis. 1999. Septic Tank Additives. University of Rhode Island Cooperative Extension Service Onsite

Wastewater Training Center, Kingston, RI.

Hairston, J.E., G. Speakman, and L. Stribling. 1995. Protecting Water Quality: Understanding Your Septic System and

Water Quality. Alabama Cooperative Extension Publication wq-125.al, June 1995. Developed with support from

Auburn University, Auburn, AL.

Olson, K., D. Gustafson; B. Liukkonen; and V. Cook. 1977. Septic System Owner’s Guide. University of Minnesota

Extension Services Publication PC-6583-GO. University of Minnesota, College of Agricultural, Food, and

Environmental Sciences, St.Paul, MN.

Rupp, G. 1996. Questions and Answers About Septic System Additives. Montana State University Extension Service,

Bozeman, MT.

Virginia Polytechnic Institute and State University (Virginia Tech). 1996. Septic System Maintenance. VTU publication

no. 440-400, October 1996. Water Quality Program Committee, Virginia Polytechnic Institute and State University,

Blacksburg, VA.
(Click Here to Return to Bookmarks Page) SIFS-3

Onsite Wastewater Treatment Systems

Special Issues Fact Sheet 2

High-Organic-Strength Wastewaters

(Including Garbage Grinders)

Description

Because many onsite treatment alternatives are sensitive to organic loading rate, high-strength wastewaters may require

additional treatment steps to ultimately meet environmental discharge or reuse goals. Among the individual home options

that increase the organic strength of the wastewater (see chapter 3) are water conservation and use of garbage grinders

(disposals). Commercial wastewater may also be high in organic concentration and, thus, organic loading. The database on

such wastewaters is extremely limited for use in design of OWTSs.

The major concern caused by high organic loadings in the pretreated wastewater is higher organic loadings (e.g., BOD) to

the infiltrative surface of the SWIS, which could result in clogging. A certain degree of clogging at the interface of

infiltration trenches and the surrounding soil is expected and helps the soil absorption field function properly. The clogging

layer, or biomat, which forms at this interface, is composed of organic material, trapped colloidal matter, bacteria, and

microorganisms and their by-products. The biomat may slow the infiltrative capacity of the SWIS, but it increases effluent

treatment time under unsaturated aerobic conditions (in the vadose zone below the trenches).

Physical clogging occurs when solid material such as grit, organic material, and grease is carried in the effluent beyond the

septic tank to the soil adsorption field and deposited on the biomat. Biological clogging generally occurs with excessive

organic loading to the biomat, which results in excess microbial growth that restricts the passage of effluent into the soil.

Slimes, sugars, ferrous sulfide, and the precipitation of metals such as iron and manganese are additional clogging byproducts. Chemical clogging can occur in clayey soils when high concentrations of sodium ions exchange with calcium

and magnesium ions in the clay. The soil loses its structure and becomes tighter and more impervious.

Garbage disposals

Garbage disposals, which have become a standard appliance in many residential kitchens in the United States, contribute

excessive organic loadings to the infiltrative field and other system components. Usually installed under the kitchen sink,

disposals are basically motorized grinders designed to shred food scraps, vegetable peelings and cuttings, bones, and other

food wastes to allow them to flow through drain pipes and into the wastewater treatment system. Disposing of food waste

in this manner eliminates the nuisance of an odor of food wastes decaying in a trashcan by moving this waste to the

wastewater stream. Many states accommodate these appliances by prescribing additional septic tank volume, service

requirements, or other stipulations (e.g., septic tank effluent filter, multiple tanks, larger infiltration field) that address

higher BOD and TSS loadings.

Table 1 contains reported information that illustrates that in-sink garbage disposal units increase septic tank loadings of

BOD by 20 to 65 percent, suspended solids by 40 to 90 percent, and fats, oils, and grease by 70 to 150 percent. For any

septic system, the installation of a disposal causes a more rapid buildup of the scum and sludge layers in the septic tank

and an increased risk of clogging in the soil adsorption field due to higher concentrations of suspended solids in the

effluent. Also, it means that septic tank volumes should be increased or tanks should be pumped more frequently.
SIFS-4

Eliminating the use of garbage disposals will significantly reduce the amount of grease, suspended solids, and BOD in

wastewater (see table 1). Elimination of garbage disposal use reduces the rate of sludge and scum accumulation in the

septic tank, thus reducing the frequency of required pumping. All of these can improve wastewater system performance.

For system owners who choose to use garbage grinders, manufacturers recommend grinding wastes with a moderate

flow of cold water. No research data representing claims of enhanced performance of garbage grinders equipped with

septic system additive injectors are available. Additives are not required nor recommended for onsite system operation, and

some might actually interfere with treatment, damage the drainfield, or contaminate ground water below the drainfield.

(See Special Issues Fact Sheet 1.)

The most common unsewered commercial sources that exhibit high organic strength are restaurants, although a variety of

commercial sources produce such wastewaters. These include other facilities with food service capability and dairy

product/processing plants. The preprocessing required to remove the source of excessive organic strength is a function of

(1) the fractionation of the organic content (settleable, supra-colloidal, colloidal, or soluble); (2) the site characteristics;

and (3) the final steps in OWTS processing and the environmental introduction method.

Typical Applications

Additional pretreatment is typically required before discharge to a SWIS or surface water. There are some proprietary

aerobic units that are designed to accept high organic loads, and greatly increase the potential for odors and, where

concrete structures are employed, corrosion. Therefore, odor protection becomes a major issue for the designer in these

situations. These units are usually a combination of suspended growth/fixed growth or enhanced Continuous-Flow,

Suspended Growth Aerobic Systems (CFSGAS; see Technology Fact Sheet 1). Alternatively, anaerobic upflow filters

(UAFs) and other anaerobic proprietary and nonproprietary systems can also “thin” excess organics to permit normal

loading to these final processing steps.

The Safe Drinking Water Act (SDWA) underground injection systems (UIC) Title V Rule, which is discussed in chapter 1,

is designed to eliminate some of these problem wastewater sources of potential ground water contamination (e.g., auto

repair facilities) from further consideration for SWIS disposal.

Design

For domestic systems the additional organic and oil/grease concentrations resulting from use of a garbage grinder usually

does not in itself cause the wastewater to require additional processing as described above, but the designer should at least

calculate any potential design changes that might be required by the increased strength. For example, for a sandy soil, the

Table 1. Increase in pollutant loading caused by addition of garbage disposal

Sources: Hazeltine, 1951; Rawn, 1951; Univ. of Wisconsin, 1978; USEPA, 1992.
SIFS-5

bottom area hydraulic loading rate could be crosschecked against the limiting organic loading rate limits cited in table 4-2.

Most state codes require a septic tank size increase to account for the additional volume of sludge and scum accumulating

in a septic tank but offer no advice as to any increasing field size.

For restaurants, facilities with food preparation, and other producers of high-organic wastewaters, the designer must

evaluate alternative pretreatment schemes that can reduce the excess organics (and sometimes other constituents) to levels

that allow subsequent processes to function normally and achieve surface water effluent discharge or reuse standards, if

applicable.

An analysis of organic waste sources and waste characteristics (particulate/soluble fractions) is required to determine the

best pretreatment approach. On the latter issue, if the majority was coming from a highly concentrated, low-volume source

in the facility, a holding tank/hauling solution may be most cost-effective choice. The fraction that contains the majority

of the excess contaminants might be readily removable by a specific process (e.g., soluble and biodegradable (aerobic unit)

versus supracolloidal and removable by flocculation/sedimentation (vegetated submerged bed or anaerobic upflow filter).

Performance

The performance of these pretreatment devices is discussed in other fact sheets. Influent concentrations which still exceed

normal loading rates can be accommodated by increasing the size or other key basis of computing loading rate or by

investigating and implementating pollution prevention measures to reduce the source concentration of the constituent of

concern (e.g., BOD).

The reliability of anaerobic processes is highly temperature-dependent, thus requiring heating in northern climates. However short-term anaerobic upflow filters and vegetated submerged beds are less sensitive because of their primary reliance

on physical processes. Aerobic treatment processes are also temperature-sensitive, but less so than anaerobic processes.

There is little documented, quality-assured information on the performance of small alternative systems that treat highorganic strength wastewater. However, well-managed aerobic units, upflow filters, and vegetated submerged beds are

known to perform reliably.

Management needs

Management needs are the same as those noted in the unit process fact sheets.

Risk management issues

Depending on the sequence of processes chosen, the impacts of flow variation, toxic shocks, extreme temperatures, and

power outages may cause significant variations from expected treatment performance. However, high-strength wastewaters greatly increase the potential for odors and, where concrete structures are employed, corrosion. Therefore, protection

from odor becomes a major issue for the designer in these situations.

Costs

Costs of treatment trains for high-organic-strength wastewaters can be estimated from the costs of the unit process

components.

References

Andress, S., and C. Jordan. 1998. Onsite Sewage Systems. Virginia Polytechnic Institute and State University, Department

of Civil Engineering, Blacksburg, VA.

Hazeltine, T.R. 1951. Addition of garbage to sewage. Water & Sewage Works, pp. 151-154. Annual compilation, 1951.
SIFS-6 (Click Here to Return to Bookmarks Page) 

Jensen, P.D., and R.L. Siegrist. 1991. Integrated Loading Rate Determination for Wastewater Infiltration System Sizing.

In Proceedings of Sixth Onsite Wastewater Treatment Symposium. American Society of Agricultural Engineers, St.

Joseph, MI.

Mancl, K.M. 1998. Septic Tank Maintenance. Ohio State University Extension publication AEX-740-98. Ohio State

University, Food, Agricultural and Biological Engineering, Columbus, OH

Rawn, A.M. 1951. Some effects of home garbage grinding upon domestic sewage. American City, March, pp.110-111.

Siegrist, R.L. 1987. Hydraulic Loading Rates for Soil Absorption Systems Based on Wastewater Quality. In Proceedings

of the Fifth Onsite Wastewater Treatment Symposium. American Society of Agricultural Engineers, St. Joseph, MI.

Siegrist, R.L., D.L. Anderson, and J.C. Convene. 1984. Commercial Wastewater Onsite Treatment Symposium. American

Society of Agricultural Engineers, St. Joseph, MI.

Stuth, W.L. 1992. Treating Commercial High-Strength Waste. In Proceedings of Seventh Northwest On-Site Wastewater

Treatment Short Course. University of Washington, Seattle, WA.

Tyler, E.J., and J.C. Converse. 1994. Soil Acceptance of Onsite Wastewater as Affected by Soil Morphology and

Wastewater Quality. In Proceedings of Seventh Onsite Wastewater Treatment Symposium. American Society of

Agricultural Engineers, St. Joseph, MI.

University of Wisconsin. 1978. Management of Small Waste Flows. USEPA 600/2-78-73. September, 1978. U.S.

Environmental Protection Agency, Office of Research and Development, Cincinnati, OH.
(Click Here to Return to Bookmarks Page) SIFS-7

Onsite Wastewater Treatment Systems

Special Issues Fact Sheet 3

Water Softeners

Description

Home water softeners, which periodically generate a backwash that is high in sodium, magnesium, and calcium concentrations, can affect wastewater treatment processes and the composition and structure of the infiltration field biomat and the

underlying soil. However, attempts to predict whether impacts will occur and to estimate their severity are difficult and

often inconclusive.

Water softeners remove “hardness” (dissolved calcium and magnesium) through ion exchange processes. Incoming hard

water passes through a tank of containing high-capacity ion exchange resin beads supersaturated with sodium. The

calcium and magnesium ions in the water attach to the resin beads, replacing the sodium, which is released into the water.

The softened water is then distributed for use throughout the house.

Over time, the ion exchange resin beads become saturated with calcium and magnesium ions. When this occurs, the tank

must be recharged by flushing with a salt brine solution. Sodium ions reclaim their position on the resin beads, and the

calcium and magnesium ions are released into the backwash water. The backwash water then exits the tank and is discharged to the wastewater treatment system. The number of times the tank is recharged and the amount of wastewater

generated depends on a number of factors, including the hardness of the water, the amount of water used, the size of the

water softener, and the capacity of the resins to remove calcium and magnesium.

The wastewater generated during the recharge phase of the water softening process mixes with other household wastewaters, enters the septic tank, and eventually moves to the soil adsorption field. Studies conducted by soil scientists at the

University of Wisconsin and the National Sanitation Foundation conclude that the wastewater effluent generated from

properly operating and maintained water softeners will not harm onsite systems that are designed, operated, and maintained

appropriately. Specifically, the studies conclude the following:

• High concentrations of calcium and manganese in the softener backwash water have no deleterious effect on the

biological functions occurring in the septic tank and may, in some cases, be helpful.

• The additional volume of wastewater generated (typically about 50 gallons per recharge cycle) is added slowly to the

wastewater stream and does not cause any hydraulic overload problems.

• Soil structure in the soil absorption field is positively affected by the calcium and mangnesium ions in water softener

effluent (Corey et al., 1977).

Regarding the last conclusion, some people have the misconception that the salt brine that enters the ion exchange tank

also exits the tank as wastewater. In fact, the influent with its high concentration of sodium ions is very different than the

effluent, which has a high concentration of calcium and magnesium ions. Consequently, the potential for chemical clogging of clayey soil by sodium ions is reduced. The calcium and magnesium input may even help improve soil percolation.

Risk management issues

The human health impacts of ingesting softened water are increasingly discussed in addition to the traditional benefits of

reduced use of surfactants and plumbing repair requirements. The choice of the homeowner to soften or not to soften will
SIFS-8 (Click Here to Return to Bookmarks Page) 

factor into all arguments. Also, the preceding descriptions are predicated on whole-house-supply softening. Today pointof-use devices designed for use with specific features in the house make the traditional advantages and disadvantages less

clear.

References

Andress, S., and C. Jordan. 1998. Onsite Sewage Systems. Virginia Polytechnic Institute and State University, Civil

Engineering Department, Blacksburg, VA.

Corey, R.B., E.S. Tyler, and M.U. Olotu. 1997. Effects of Water Softner Use on the Permeability of Septic Tank Seepage

Fields. In Proceedings of Second National Home Sewage Treatment Symposium. Pub. no. 5-77. American Society of

Agricultural Engineers, St. Joseph, MI.

Mancl, K.M. 1998. Septic Tank Maintenance. Ohio State University Extension publication AEX-740-98. Ohio State

University, Food, Agricultural and Biological Engineering. Columbus, OH.

University of Wisconsin. 1978. Management of Small Waste Flows. EPA-600/2-78-173. U.S. Environmental Protection

Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA). 1992. Manual: Wastewater Treatment/Disposal for Small Communities.

EPA 625/R-92/005. U.S. Environmental Protection Agency, Cincinnati, OH.
(Click Here to Return to Bookmarks Page) SIFS-9

Onsite Wastewater Treatment Systems

Special Issues Fact Sheet 4

Holding Tanks and Hauling Systems

Description

A holding tank or vault receives wastewater from a home or commercial establishment and stores it until it is pumped out

and hauled to a receiving/processing facility. Although similar to septic tanks, vaults have no outlet piping and must be

watertight. The volume can range from 1,000 gallons to 4,000 gallons or more. The vault should be equipped with an

audible and visible high-water alarm, which alters the resident to the need for pumping.

Different sizes of vaults and tank trucks can be used; water conservation can reduce costs considerably by reducing the

frequency of pumping. A vault can be equipped with a standpipe and a quick disconnect to which the pumping truck can

be directly connected for efficient (minimal spillage) emptying of the vault.

Holding tanks can be used for the entire wastewater flow in cases where conventional and typical alternative systems are

not feasible. They are often used this way for seasonal homes in sensitive environmental settings. Holding tanks can also

be used to collect only a part of the wastewater flow. Usually, they are used to collect the greywater when non-watercarriage toilets are employed in sensitive areas. This option permits a significant reduction (usually one-third or more) in

the number of tank pumpings as compared to the whole wastewater collection option. Another holding tank option is to

collect only the blackwater fraction of the wastewater while the graywater is treated in an OWTS. This option is most

popular in estuarine areas where significant nitrogen removal is required because the blackwater may contain from 70 to

90 percent of the total nitrogen load. In this case the reduction in pumping frequency from the whole wastewater option

would be about two-thirds.

Over and above these combinations a program to reduce water use can be overlaid. The critical contribution of such a

program (see chapter 3) is to reduce the daily volume of wastewater (blackwater, graywater, or combined) produced and

the required frequency of holding tank pumping. Some onsite wastewater recycling can be added to this program in arid

regions where gravity feed and belowground watering of nonconsumable vegetation can be accomplished. However, such a

program must meet all local public health requirements.

Applications

Pump and haul collection is best used when soil absorption fields do not work (for example, where bedrock or ground

water levels are near the ground surface) and there is no sewer system. Typical applications are second homes, where

annual occupancy may be only a few days to a few months; where a nuisance or public health hazard must be abated;

where an isolated building has no running water; in temporary structures or gathering places; or where nutrients must be

excluded from ground water to protect environmentally sensitive areas. Pump and haul collection may also simply be the

least expensive alternative in some places.

Pump and haul systems are viable only under a wastewater authority that guarantees service. Pump and haul collection can

became prohibitively expensive when homes are occupied all the time or where the distance from the treatment plant to

the home is more than a few miles.
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Management needs

Holding tanks should be used only where a proper management program is in place. Construction requirement are essentially the same as for a septic tank in that the onsite testing for tank leakage is vital to a successful design and the alarm

system must be dredged for proper functioning before acceptance.

In addition to timely pumping, operation and maintenance requirements should include checking the alarm function,

cleaning the activation floats, and comparing volume used vs. volume accumulated in the tank. The skill requirements at

the site are minimal and can be estimated as approximately 1 hour per pumping. There are normally no energy requirements; the residuals are the tank contents, and confined-space entry safety requirements must be followed if tank entry is

required.

Risk management issues

Holding tanks are not subject to upset by flow variation, toxic loads or power outages. They should be insulated and

possibly heat-treated in extremely cold climates. If properly vented through the building sewer, they should not exhibit

odor problems, but use in hot climates may require an increase in pumping frequency or a regular addition of lime for

mitigation. There is a release of objectionable odors during tank pumping, which can cause some discomfort to residents.

Costs

More recent cost estimates for holding tank-hauling wastewater disposal indicate that tank installation is about $1 per

gallon of capacity (up to 5,000 gallons) while the alarm system is about $400.

Tank pumping is generally in range of 10 to 30 cents per gallon, to which labor, travel and equipment amortization may

be added (or these costs may be included in a flat fee). Travel costs will dominate if the round-trip distance to the holding

tank, to the disposal site, and back to home base exceeds 50 miles. The permit costs to discharge at an appointed sit

(treatment plant, land spreading site, or independent treatment facility) is also escalated, multiple pumping from a yearround house can become extremely expensive.

References

Anderson, C.D. 1986 Trucked Collection Systems Experience in the Northwest Territories. In Proceedings of Appropriate

Wastewater Management Technologies for Rural Areas Under Adverse Conditions. Technical University of Nova

Scotia, Halifax, NS.

Burrows, R., and N. Bouwes. The Cost of Holding Tanks for Domestic Wastewater. Small Scale Waste Management

Report. University of Wisconsin, Madison, WI.

Dix, S.P. Case Study Number 4: Crystal Lakes, Colorado. National Small Wastewater Flows Clearinghouse, West Virginia

University, Morgantown, WV.

Mahoney, W.D., ed.-in-chief. 1989. Means Site Work Cost Data. R.S. Means Co., Kingston, MA.

Mahoney, W.D., ed.-in-chief. 1990. Means Site Work Cost Data. R.S. Means Co., Kingston, MA.

Manci, K. No date. Wastewater Treatment Alternatives...Holding Tanks. The Pennsylvania State University, College of

Agriculture, Cooperative Extension Service, University Park, PA.

Mooers, J.D., and D.H. Waller. 1996. Onsite Wastewater Research Program: Phase III. TUNS/Centre for Water Research

Study, Halifax, NS.

National Association of Waste Transporters (NAWT). 1998. Introduction to Proper Onsite Sewage Treatment. National

Association of Waste Transporters, Scandia, MN.
SIFS-11

U.S. Environmental Protection Agency (USEPA). 1984. Final Generic Environmental Impact Statement: Wastewater

Management in Rural Lake Areas. U.S. Environmental Protection Agency, Region 5, Chicago, IL.

Waller D.H., and A.R. Townshend. 1987. Appropriate Wastewater Management Technologies for Rural Areas Under

Adverse Conditions. Special Publication, Technical University of Nova Scotia, Halifax, NS.
SIFS-12 
USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) 5-1

Chapter 5: Treatment System Selection

5.1 Introduction

Selecting the appropriate system type, size, and

location at the site depends on the wastewater flow

and composition information discussed in chapter 3, site- and landscape-level assessments outlined in chapter 3 and in this chapter, performance

requirements as noted in chapter 3, and the array of

available technology options reviewed in chapter 4.

Key to selecting, sizing, and siting the system are

identifying the desired level of performance and

ensuring that the effluent quality at the performance boundaries meets the expected performance

requirements.

5.2 Design conditions and system

selection

An appropriate onsite wastewater treatment system

concept for a given receiver site—proposed

location of the system, regional geologic and

hydrologic features, and downgradient soils used

for treatment—depends on the prevailing design

conditions. Designers must consider and evaluate

the design conditions carefully before selecting a

system concept. Design conditions include the

characteristics of the wastewater to be treated,

regulatory requirements, and the characteristics of

the receiver site (figure 5-1). With sufficient

knowledge of these factors, the designer can

develop an effective preliminary design concept.

This chapter focuses on general guidance for

evaluation of the receiver site, identification of the

site’s design boundaries and requirements, and

selection of suitable designs to meet the perforChapter 5:

Treatment System Selection

mance requirements. This chapter also provides

guidance for evaluating and rehabilitating systems

that are not meeting their performance requirements.

5.3 Matching design conditions to

system performance

Design conditions include wastewater characteristics;

system owner preferences for siting, operation and

maintenance, and cost; regulatory requirements

prescribed by the permitting agency’s rules; and the

receiver site’s capability to treat or otherwise

assimilate the waste discharge. Each of these must

be evaluated in light of the others before an appropriate system design concept can be developed.

5.3.1 Wastewater source considerations

Wastewater source considerations include projections of wastewater flow, wastewater composition,

and owner requirements. Chapter 3 provides

guidance for estimating flow and waste strength

characteristics. The owner’s needs, capabilities, and

expectations might be explicit or implied. The first

consideration is the owner’s use of the property

(present and projected), which informs analyses of

the character and volume of the wastewater generated. The footprint and location of existing or

planned buildings, paved areas, swimming pools,

and other structures or uses will limit the area

available for the onsite system. Second, the owner’s

concern for the system’s visual impact or odor

5.1 Introduction

5.2 Design conditions and system selection

5.3 Matching design conditions to system performance

5.4 Design boundaries and boundary loadings

5.5 Evaluating the receiving environment

5.6 Mapping the site

5.7 Developing the initial system design

5.8 Rehabilitating and upgrading existing systems
Chapter 5: Treatment System Selection

5-2 USEPA Onsite Wastewater Treatment Systems Manual

Figure 5-1. Preliminary design steps and considerations.
USEPA Onsite Wastewater Treatment Systems Manual 5-3

Chapter 5: Treatment System Selection

potential might restrict the range of alternatives

available to the designer. Third, the owner’s ability

and willingness to perform operation and maintenance tasks could limit the range of treatment

alternatives. Finally, costs are a critical concern for

the owner. Capital (construction) costs and recurring (operation and maintenance) costs should be

estimated, and total costs over time should be

calculated if cost comparisons between alternative

systems are necessary. The owner should have both

the ability and willingness to pay construction and

operation and maintenance costs if the system is to

perform satisfactorily.

5.3.2 Regulatory requirements

Designs must comply with the rules and regulations

of the permitting entity. Onsite wastewater systems

are regulated by a variety of agencies in the United

States. At the state level, rules may be enacted as

public health codes, nuisance codes, environmental

protection codes, or building codes. In most (but

not all) states, the regulatory authority for onsite

single-family residential or small cluster systems is

delegated to counties or other local jurisdictions.

The state might enact a uniform code requirement

that all local jurisdictions must enforce equally, or

the state might have a minimum code that local

jurisdictions may adopt directly or revise to be

stricter. In a few states, general guidance rather

than prescriptive requirements is provided to local

jurisdictions. In such cases, the local jurisdictions

may enact more or less strict regulations or choose

not to adopt any specific onsite system ordinance.

Traditionally, state and local rules have been

prescriptive codes that require specific system

designs for a set of specific site criteria. Such rules

typically require that treated wastewater discharged

to the soil be maintained below the surface of the

ground, though a few states and local jurisdictions

do allow discharges to surface waters under their

National Pollutant Discharge Elimination System

(NPDES) permitting programs, as authorized by the

federal Clean Water Act. If applications are proposed outside the prescriptive rules, the agency

usually requires special approvals or variances

before a permit can be issued. Circumstances that

require special action (approvals, variances) and

administrative processes for approving those

actions are usually specified in state or local codes.

5.3.3 Receiver site suitability

The physical characteristics of the site (the location

of the proposed system, regional geologic and

hydrologic features, and the soils to be used in the

treatment process) determine the performance

requirements and treatment needs. A careful and

thorough site evaluation is necessary to assess the

capacity of the site to treat and assimilate effluent

discharges. Treatment requirements for a proposed

system are based on the performance boundary

requirements established by rule and the natural

design boundaries identified through the site

evaluation.

5.4 Design boundaries and

boundary loadings

Wastewater system design must focus on the

critical design boundaries: between system components, system/soil interfaces, soil layer and property boundaries, or other places where design

conditions abruptly change (see figures 5-2 and

5-3). System failures occur at design boundaries

because they are sensitive to hydraulic and mass

pollutant loadings. Exceeding the mass loading

limit of a sensitive design boundary usually results

in system failure. Therefore, all critical design

boundaries must be identified and the mass loadings to each carefully considered to properly select

the upstream performance and design requirements

needed to prevent system failure (Otis, 1999).

The approach discussed in this chapter is based on

characterizing the assimilative capacity of the

receiving environment (ground water, surface

water) and establishing onsite system performance

requirements that protect human health and ecological resources. Desired system performance, as

measured at the final discharge point (after treatment in the soil matrix or other treatment train

components), provides a starting point for considering performance requirements for each preceding

system component at each design boundary (e.g.,

septic tank-SWIS interface, biomat at the infiltrative surface, surface of the saturated zone).

Through this approach, system designers can

determine treatment or performance requirements

for each component of the treatment train by

assessing whether each proposed component can

meet performance requirements (acceptable mass

loading limits) at each subsequent design boundary.
Chapter 5: Treatment System Selection

5-4 USEPA Onsite Wastewater Treatment Systems Manual

Determining the critical design boundaries of the

physical environment is the primary objective of

the site evaluation (see section 5.5). Design boundaries are physical planes or points, or they may be

defined by rule. More than one design boundary

can be expected in every system, but not all of the

identified boundaries are likely to control design.

The most obvious design boundaries are those to

which performance requirements are applied

(figure 5-2). These are defined boundaries that

might or might not coincide with a physical

boundary. For a ground water discharge, the design

boundary might be the water table surface, the

property line, or a drinking water well. For surface

water discharges the performance boundary is

typically designated at the outfall to the receiving

waters, where permit limits on effluent contaminants are applied. Physical boundaries are particularly significant for conventional wastewater

treatment systems that discharge to ground water or

to the atmosphere. Soil infiltrative surfaces,

hydraulically restrictive soil horizons, or zones of

saturation are often the critical design boundaries

for ground water discharging systems.

The site evaluation must be sufficiently thorough to

identify all potential design boundaries that might

affect system design. Usually, the critical design

boundaries are obvious for surface water discharging and evaporation systems. Design boundaries for

subsurface wastewater infiltration systems, however, are more difficult to identify because they

occur in the soil profile and there might be more

than one critical design boundary.

5.4.1 Subsurface infiltration system

design boundaries and loadings

Subsurface wastewater infiltration systems (SWISs)

have traditionally been used to treat and discharge

effluent from residences, commercial buildings,

and other facilities not connected to centralized

sewage treatment plants. These systems accept and

treat wastewater discharged from one or more

septic tanks in below-grade perforated piping,

which is usually installed in moderately shallow

trenches 1.5 to 3.0 feet deep on a bed of crushed

rock 0.5 to 1.5 inches in diameter. Leaching

chambers, leach beds, and other SWIS technologies

have also been approved for use in some states.

Both the trench bottoms and sidewalls provide

infiltrative surfaces for development of the biomat

(see chapter 3) and percolation of treated wastewater to the surrounding soil matrix.

The soil functions as a biological, physical, and

chemical treatment medium for the wastewater, as

well as a porous medium to disperse the wastewater in

the receiving environment as it percolates to the

ground water. Therefore, the site evaluation must

determine the capacity of the soil to hydraulically

accept and treat the expected daily mass loadings of

wastewater. Site and soil characteristics must provide

adequate drainage of the saturated zone to maintain

the necessary unsaturated depth below the infiltrative

surface, allow oxygenation of aerobic biota in the

biomat and reaeration of the subsoil, and prevent

effluent surfacing at downgradient locations.

Traditional site evaluation and design procedures

consider only the infiltrative surface of the SWIS as a

design boundary (figure 5-3). Hydraulic loading

rates to this boundary are usually estimated from

percolation tests and/or soil profile analyses. The

recommended daily hydraulic loading rates typically

assume septic tank effluent is to be applied to the soil

(through the SWIS biomat, across the trench bottom/

sidewall soil interface, and into the surrounding soil).

The estimated daily wastewater volume is divided by

the applicable hydraulic loading rate to calculate the

needed infiltration surface area. This method of

design has endured since Henry Ryon first proposed

Design Boundary

Source: Ayres Associates, 2000.

Figure 5-2. Performance (design) boundaries associated with

onsite treatment systems
USEPA Onsite Wastewater Treatment Systems Manual 5-5

Chapter 5: Treatment System Selection

the percolation test and its empirical relationship to

infiltration system size nearly 100 years ago

(Fredrick, 1948). Although this method of design

has been reasonably successful, hydraulic and

treatment failures still occur because focusing on

the infiltrative surface overlooks other important

design boundaries. Identifying those critical

boundaries and assessing their impacts on SWIS

design will substantially reduce the number and

frequency of failures.

Usually there is more than one critical design

boundary for a SWIS. Zones where free water or

saturated soil conditions are expected to occur

above or below unsaturated zones identify performance boundary layers (Otis, 2001). In SWISs,

these include

• The infiltrative surfaces where the wastewater

first contacts the soil.

• Secondary infiltration surfaces that cause

percolating wastewater to perch above an

unsaturated zone created by changes in soil

texture, structure, consistency, or bulk density.

• The ground water table surface, which the

percolating wastewater must enter without

excessive ground water mounding or degradation of ground water quality.

Figure 5-3. Subsurface wastewater infiltration system design/performance boundaries.

Source: Adapted from Ayres Associates, 1993.

Backfill Natural

Geotextile

Distribution pipe

Design boundary

Infiltration zone and biomat

Vadose zone (unsaturated)

"Perched" saturated zone

Design boundary*

Restrictive horizon*

Capillary

Design boundary

Groundwater surface

Saturated

* If present
Chapter 5: Treatment System Selection

5-6 USEPA Onsite Wastewater Treatment Systems Manual

advantages. For example, denitrification is aided

when saturation results in anaerobic conditions in

interstices in the normally unsaturated zones.

Perched or otherwise layered boundaries require

careful characterization, analysis, and assessment of

system operation to determine how they will affect

the movement of effluent plumes from the SWIS.

The water table surface is where treatment is usually

expected to be complete, that is, where pollutant

loadings, with proper mixing and dispersion, should

not create concentrations in excess of water quality

standards. System designers should seek to ensure

that hydraulic loadings from the system(s) to the

ground water will not exceed the aquifer’s capacity

to drain water from the site. If a SWIS is to perform

properly, the mass loadings to the critical design

boundaries must be carefully considered and incorporated into the design of the system. The types of

mass loadings that should be considered in SWIS

design are presented in table 5-1.

The various design boundaries are affected differently by different types of mass loadings (table 5-2).

The infiltrative surface is a critical design and

performance boundary in all SWISs since free

water enters the soil and changes to water under

tension (at pressures less than atmospheric) in the

unsaturated zone. Many wastewater quality transformations occur at this boundary. For example,

biochemical activity usually causes a hydraulically

restrictive biomat to form at the infiltrative surface.

Failure to consider the infiltrative surface in system

design and to accommodate the changes that occur

there can lead to hydraulic or treatment failure.

Other surfaces that are often critical design boundaries include those associated with hydraulically

restrictive zones below the infiltrative surface that

can cause water to perch. If hydraulic loadings are

too great for these boundaries, surface seepage

might occur at downslope locations as effluent

slides along the perched boundary. Also, the

saturated zone could mound to encroach on the

unsaturated zone to the extent that sufficient

reaeration of the soil does not occur, which can

result in severe soil clogging. If hydraulic problems

do not occur, these conditions offer some treatment

Table 5-1. Types of mass loadings to subsurface wastewater infiltration systems.

Source: Otis, 2001.
USEPA Onsite Wastewater Treatment Systems Manual 5-7

Chapter 5: Treatment System Selection

Table 5-2. Potential impacts of mass loadings on soil design boundaries

The infiltrative surface is the primary design

boundary. At this boundary, the partially treated

wastewater must pass through the biomat, enter the

soil pores, and percolate into unsaturated soil. The

wastewater cannot be applied at rates faster than

the soil can accept it, nor can the soil be overloaded

with solids or organic matter to the point where

soil pores become clogged with solids or an overly

thick development of the biomass. Because solids

are usually removed through settling processes in

the septic tank, the critical design loadings at this

boundary are the daily and instantaneous hydraulic

loading rates and the organic loading rate. System

design requires that daily hydraulic and instantaneous/peak loadings be estimated carefully so that

the total hydraulic load can be applied as uniformly

as feasible over the entire day to maximize the

infiltration capacity of the soil. Uniform dosing

and resting maximizes the reaeration potential of

the soil and meets the oxygen demand of the

applied wastewater loading more efficiently. The

organic loading rate is an important consideration

if the available area for the SWIS is small. In

moderately permeable or more permeable soils,

lower organic loading rates can increase infiltration

rates into the soil and may allow reductions in the

size of the infiltrative surface. Organic loadings to

slowly permeable, fine-textured soils are of lesser

concern because percolation rates through the

biomat created by the organic loading are usually

greater than the infiltration rate into the soil.

Preventing effluent backup (hydraulic failure) by

increasing the size of the SWIS and implementing

water conservation measures are important considerations in these situations.

Secondary design boundaries are usually hydraulically restrictive horizons that inhibit vertical

percolation through the soil (figure 5-2). Water can

perch above these boundaries, and the perching can

affect performance in two significant ways. If the

perched water encroaches into the unsaturated zone,

treatment capacity of the soil is reduced and

reaeration of the soil below the infiltrative surface

might be impeded. Depending on the degree of

impedance, anoxic or anaerobic conditions can

develop, resulting in excessive clogging of the

infiltrative surface. Also, water will move laterally

on top of the boundary, and partially treated

wastewater might seep from the exposed boundaries of the restrictive soil strata downslope and out

onto the ground surface. Therefore, the contour

(linear) loading along the boundary surface contour

must be low enough to prevent water from mounding
Chapter 5: Treatment System Selection

5-8 USEPA Onsite Wastewater Treatment Systems Manual

above the boundary to the point that inadequately

treated wastewater seeps to the surface and creates

a nuisance and possible risk to human health.

Organic loadings at these secondary boundaries are

seldom an issue because most organic matter is

typically removed as the wastewater passes through

the infiltrative surface boundary layer.

Hydraulic and wastewater constituent loadings are

the critical design loadings at the water table

boundary. Low aquifer transmissivity creates

ground water mounding (figure 5-4), which can

encroach on the infiltrative surface if the daily

hydraulic loading is too high. Mounding can affect

treatment and percolation adversely by inhibiting

soil reaeration and reducing moisture potential. A

further potential consequence is undesirable surface

seepage that can occur downslope. Constituent

loadings must be considered where protection of

potable water supply wells is a concern. Typical

wastewater constituents of human health concern

include pathogenic microbes and nitrates (see

chapter 3). Water resource pollutants of concern

include nitrogen in coastal areas, phosphorus near

inland waters, and toxic organics and certain metals

in all areas. If the wastewater constituent loadings

are too high at the water table boundary, pretreatment before application to the infiltrative surface

might be necessary.

5.4.2 Surface water discharging

system design boundaries and

loadings

Surface water discharging systems typically consist

of a treatment plant (aeration/activated sludge/sand

filter “package” system with disinfection) discharging to an outfall (pipe discharge) to a surface water.

The important design boundaries for these systems

are the inlet to the treatment plant and the outfall to

the surface water. The discharge permit and the

performance history of the treatment process

typically establish the limits of mass loading that

can be handled at both the inlet to and the outlet

from the treatment process. The loadings are often

expressed in terms of daily maximum flow and

pollutant concentrations (table 5-3). The effluent

limits and wastewater characteristics establish the

extent of treatment (performance requirements)

needed before final discharge.

5.4.3 Atmospheric discharging system

design boundaries and loadings

Evapotranspiration systems are the most commonly

used atmospheric discharging systems. They can

take several forms, but the primary design boundFigure 5-4. Effluent mounding effect above the saturated zone

Source: Adapted from NSFC diagram.
USEPA Onsite Wastewater Treatment Systems Manual 5-9

Chapter 5: Treatment System Selection

ary is the evaporative surface. Water (effluent)

flowing through the treatment system and site

hydrology must be considered in the design. Water

balance calculations in the system control design

(table 5-4). These loadings are determined by the

ambient climatic conditions expected. Procedures

for estimating these loadings are provided in

chapter 4 (Evapotranspiration Fact Sheet).

5.5 Evaluating the receiving

environment

Evaluation of the wastewater receiver site is a

critical step in system selection and design. The

objective of the evaluation is to determine the

capacity of the site to accept, disperse, and safely

and effectively assimilate the wastewater discharge.

The evaluation should

• Determine feasible receiving environments

(ground water, surface water, or atmosphere)

• Identify suitable receiver sites

• Identify significant design boundaries associated with the receiver sites

• Estimate design boundary mass loading limitations

Considering the importance of site evaluation with

respect to system design, it is imperative that site

evaluators have appropriate training to assess

receiver sites and select the proper treatment train,

size, and physical placement at the site. This

section does not provide basic information on soil

science but rather suggests methods and procedures

that are standardized or otherwise proven for the

practice of site evaluation. It also identifies specific

steps or information that is crucial in the decisionmaking process for the site evaluator.

5.5.1 Role and qualifications of the site

evaluator

The role of the site evaluator is to identify, interpret,

and document site conditions for use in subsurface

wastewater treatment system selection, design, and

installation. The information collected should be

presented in a manner that is scientifically accurate

and spatially correct. Documentation should use

standardized nomenclature to provide geophysical

information so that the information can be used by

other site evaluators, designers, regulators, and

contractors.

The site evaluator needs considerable knowledge

and a variety of skills. A substantial knowledge of

soils, soil morphology, and geology is essential

because most onsite systems use the soil as the final

treatment and dispersal medium. Many states no

longer accept the percolation test as the primary

Table 5-3. Types of mass loadings for point discharges to surface waters

Table 5-4. Types of mass loadings for evapotranspiration systems
Chapter 5: Treatment System Selection

5-10 USEPA Onsite Wastewater Treatment Systems Manual

North Carolina guidelines f th Carolina guidelines for OWTS site evaluations aluationsaluations

The Division of Environmental Health of the North Carolina Department of Environment, Health, and Natural

Resources uses a 10-point guide for conducting site evaluations. The ten guidelines can be grouped into the

following components:

Collecting information before the site visit

Assessing the site and soil at the location

Recording site evaluation data for system design

Relaying the information to the system designer and the applicant.

1. Know the rules and know how to collect the needed information. Applicable codes for sewage treatment and

dispersal systems are usually established by the local agency.

2. Determine the wastewater flow rate and characteristics. Information on wastewater quantity and quality is used

to determine the initial size and type of the onsite system to be installed at a particular site.

3. Review preliminary site information. Existing published information will help the evaluator understand the types

of soils and their properties and distribution on the landscape.

4. Understand the septic system design options. Site evaluators must understand how onsite systems function in

order to assess trade-offs in design options.

5. View the onsite system as part of the soil system and the hydrologic cycle. Typically, onsite systems serving

single-family homes do not add enough water to the site to substantially change the site’s hydrology, except in

areas of high densities of onsite systems.

6. Predict wastewater flow through the soil and the underlying materials. The soil morphological evaluation and

landscape evaluation are important in predicting flow paths and rates of wastewater movement through the soil

and underlying materials.

7. Determine if additional information is needed from the site. Site and soil conditions and the type of onsite

system being considered determine whether additional evaluation is required. Some additional evaluations that

may be required are ground water mounding analysis, drainage analysis, hydrogeologic testing, contour

(linear) loading rate evaluation, and hydraulic conductivity measurements.

8. Assess the treatment potential of the site. The treatment potential of the site depends on the degree of soil

aeration and the rate of flow of the wastewater through the soil.

9. Evaluate the site’s environmental and public health sensitivity. Installing onsite systems in close proximity to

community wells, near shellfish waters, in sole-source aquifer areas, or other sensitive areas may raise

concerns regarding environmental and public health issues.

10. Provide the system designer with soil/site descriptions and your recommendations. Based on the information

gathered about the facility and the actual site and soil evaluation, the evaluator can suggest loading rates,

highlight site and design considerations, and point out special concerns in designing the onsite system.

Source: North Carolina DEHNR, 1996.

suitability criterion. A significant number of

permitting agencies now require a detailed soil

profile description and evaluation performed by

professional soil scientists or certified site evaluators.

In addition to a thorough knowledge of soil

science, the site evaluator should have a basic

understanding of chemistry, wastewater treatment,

and water movement in the soil environment, as

well as knowledge of onsite system operation and

construction. The evaluator should also have basic

skills in surveying to create site contour maps and

site plans that include temporary benchmarks,

horizontal and vertical locations of site features,

and investigation, sample, or test locations. A

general knowledge of hydrology, biology, and

botany is helpful. Finally, good oral and written

communication skills are necessary to convey site

information to others who will make important

decisions regarding the best use of the site.
USEPA Onsite Wastewater Treatment Systems Manual 5-11

Chapter 5: Treatment System Selection

5.5.2 Phases of a site evaluation

Site evaluations typically proceed in three phases: a

preliminary review of documented site information, a

reconnaissance of potential sites, and a detailed

evaluation of the most promising site or sites. The

scale and detail of the evaluation depend on the

quantity and strength of the wastewater to be treated,

the nature of local soils and the hydrogeologic setting,

the sensitivity of the local environment, and the

availability of suitable sites. Using a phased approach

(table 5-5) helps to focus the site evaluation effort on

only the most promising sites for subsurface systems.

5.5.3 Preliminary review

The preliminary review is performed before any

fieldwork. It is based on information available

from the owner or local agencies or on general

resource information. The objectives of the preliminary review are to identify potential receiver

sites, determine the most feasible receiving environments, identify potential design boundaries, and

develop a relative suitability ranking. Preliminary

screening of sites is an important aspect of the site

evaluator’s role. More than one receiving environment might be feasible and available for use.

Focusing the effort on the most promising receiving

environments and receiver sites allows the evaluator to reasonably and methodically eliminate the

least suitable sites early in the site evaluation

process. For example, basic knowledge of the local

climate might eliminate evaporation or evapotranspiration as a potential receiving environment immediately. Also, the applicable local codes often prohibit

point discharges to surface waters from small systems.

Knowledge of local conditions and regulations is

essential during the screening process. Resource

materials and information to be reviewed may

include, but are not limited to, the following:

• Property information. This information should

include owner contact information, site legal

description or address, plat map or boundary

survey, description of existing site improvements (e.g., existing onsite wastewater systems,

underground tanks, utility lines), previous and

proposed uses, surrounding land use and

zoning, and other available and relevant data.

• Detailed soil survey. Detailed soil surveys are

published by the U.S. Department of

Agriculture’s Natural Resources Conservation

Service (NRCS), formerly the Soil Conservation Service (SCS). Detailed soil surveys

provide soil profile descriptions, identify soil

limitations, estimate saturated soil conductivities

and permeability values, describe typical

landscape position and soil formation factors,

and provide various other soil-related information. Soil surveys are typically based on deductive projections of soil units based on topographical or landscape position and should be

regarded as general in nature. Because the

accuracy of soil survey maps decreases as

assessments move from the landscape scale to

the site scale, soil survey data should be supplemented with detailed soil sampling at the site

(table 5-5). Individual surveys are performed on

a county basis and are available for most

counties in the continental United States,

Alaska, Hawaii, and the U.S. territories. They

are available from county extension offices or

Table 5-5. Site characterization and assessment activities for

SWIS applications
Chapter 5: Treatment System Selection

5-12 USEPA Onsite Wastewater Treatment Systems Manual

the local NRCS office. Information on available

detailed soil surveys and mapping status can be

obtained from the National Soil Survey Center

through its web site at http://

www.statlab.iastate.edu/soils/nssc/. The NRCS

publication Fieldbook for Describing and

Sampling Soils is an excellent manual for use in

site evaluation. It is available at http://

www.statlab.iastate.edu/soils/nssc/field_gd/

field_gd.pdf.

• Quadrangle maps. Quadrangle maps provide

general topographic information about a site

and surrounding landscape. These maps are

developed and maintained by the U.S. Geological

Survey (USGS) and provide nationwide coverage

typically at a scale of 1 inch = 2000 feet, with

either a 10- or 20-foot contour interval. At this

scale, the maps provide information related to

land use, public improvements (e.g., roadways),

USGS benchmarks, landscape position and slope,

vegetated areas, wetlands, surface drainage

patterns, and watersheds. More information

about USGS mapping resources can be found at

http://mapping.usgs.gov/mac/findmaps.html.

Quadrangle maps also are available through

proprietary software packages.

• Wetland maps. Specialized maps that identify

existing, farmed, and former wetlands are

available in many states from natural resource

or environmental agencies. These maps identify

wetland and fringe areas to be avoided for

wastewater infiltration areas. On-line and

published wetland maps for many parts of the

United States are available from the U.S. Fish

and Wildlife Service’s National Wetlands

Inventory Center at http://www.nwi.fws.gov/.

• Aerial photographs. If available, aerial photographs can provide information regarding past

and existing land use, drainage and vegetation

patterns, surface water resources, and approximate location of property boundaries. They are

especially useful for remote sites or those with

limited or difficult access. Aerial photographs

may be available from a variety of sources, such

as county or regional planning, property

valuation, and agricultural agencies.

• Geology and basin maps. Geology and basin maps

are especially useful for providing general information regarding bedrock formations and depths,

ground water aquifers and depths, flow direction and velocities, ambient water quality,

surface water quality, stream flow, and seasonal

fluctuations. If available, these maps can be

obtained from USGS at http://

www.nationalatlas.gov/

or Terra Server at http://

www.terraserver.microsoft.com.

• Water resource and health agency information.

Permit and other files, state/regional water

agency staff, and local health department

sanitarians or inspectors can provide valuable

information regarding local onsite system

designs, applications, and performance. Regulatory agencies are beginning to establish Total

Maximum Daily Loads (TMDLs) for critical

wastewater constituents within regional drainage basins under federal and state clean water

laws. TMDLs establish pollutant “budgets” to

ensure that receiving waters can safely assimilate loads of incoming contaminants, including

those associated with an onsite system (e.g.,

bacteria, nutrients). If the site lies in the recharge area of a water resource listed as impaired (not meeting its designated use) because

of bacteria or nutrient contamination, site

evaluators need to be aware of all applicable

loading limits to ground water or surface water

in the vicinity of the site under review.

• Local installer/maintenance firms. Helpful

information often can be obtained from interviews with system installation and maintenance

service providers. Their experience with other

sites in the vicinity, existing technology performance, and general knowledge of soils and

other factors can inform both the site evaluation

and the selection of appropriate treatment

system components.

• Climate. Temperature, precipitation, and pan

evaporation data can be obtained from the

National Oceanic and Atmospheric Administration (NOAA) at http://www.nic.noaa.gov. This

information is necessary if evapotranspiration

systems are being considered. The evaluator

must realize, however, that the data from the

nearest weather station might not accurately

represent the climate at the site being evaluated.
USEPA Onsite Wastewater Treatment Systems Manual 5-13

Chapter 5: Treatment System Selection

5.5.4 Reconnaissance survey

The objectives of the reconnaissance survey are to

obtain preliminary site data that can be used to

determine the appropriate receiving environment,

screen potential receiver sites, and further focus the

detailed survey to follow. A reconnaissance survey

typically includes visual surveys of each potential

site, preliminary soils investigation using hand

borings, and potential system layouts. Information

gathered from the preliminary review, soil sampling

tools, and other materials should be on hand during

the reconnaissance survey.

The site reconnaissance begins with a site walkover

to observe and identify existing conditions, select

areas to perform soil borings, or view potential

routes for piping or outfall structures. The site

evaluator should have an estimate of the total area

needed for the receiver site based on the projected

design flow and anticipated soil characteristics. It is

advisable to complete the site walkover with the

owner and local regulatory staff if possible,

particularly with larger projects. Selection of an

area for soil investigation is based on the owner’s

requirements (desired location, vegetation preservation, and general site aesthetics), regulatory

requirements (setbacks, slope, and prior land use),

and the site evaluator’s knowledge and experience

(landscape position, local soil formation factors,

and geologic conditions). Visual inspections are

used to note general features that might affect site

suitability or system layout and design. General

features that should be noted include the following:

• Landscape position. Landscape position and

landform determine surface and subsurface

drainage patterns that can affect treatment and

infiltration system location. Landscape features

that retain or concentrate subsurface flows, such

as swales, depressions, or floodplains, should be

avoided. Preferred landscape positions are

convex slopes, flat areas with deep, permeable

soils, and other sites that promote wastewater

infiltration and dispersion through unsaturated

soils (figures 5-5 and 5-6).

• Topography. Long, planar slopes or plateaus

provide greater flexibility in design than ridges,

knolls, or other mounded or steeply sloping

sites. This is an important consideration in

gravity-flow treatment systems, collection

piping for cluster systems, treatment unit sites,

and potential routes for point discharge outfalls.

• Vegetation. Existing vegetation type and size

provide information regarding soil depth and

internal soil drainage, which are important

considerations in the subsurface wastewater

infiltration system layout.

• Natural and cultural features. Surface waters,

wetlands, areas of potential flooding, rock

outcrops, wells, roads, buildings, buried utilities,

underground storage tanks, property lines, and

other features should be noted because they will

affect the suitability of the receiver site.

A good approach to selecting locations for soil

investigations is to focus on landscape position. The

underlying bedrock often controls landscapes,

which are modified by a variety of natural forces.

The site evaluator should investigate landscape

positions during the reconnaissance phase to

identify potential receiver sites (figures 5-5, 5-6

and 5-7; table 5-6). Ridgelines are narrow areas

that typically have limited soil depth but often a

good potential for surface/subsurface drainage.

Shoulderslopes and backslopes are convex slopes

where erosion is common. These areas often have

good drainage, but the soil mantle is typically thin

and exposed bedrock outcrops are common.

Sideslopes are often steep and erosion is active.

Footslopes and depressions are concave areas of

soil accumulation; however, depressions usually

have poor drainage. The deeper, better-drained

Figure 5-5. General considerations for locating a SWIS on a

sloping site

Source: Purdue University, 1990.
Chapter 5: Treatment System Selection

5-14 USEPA Onsite Wastewater Treatment Systems Manual

soils are found on ridgelines, lower sideslopes, and

footslopes. Bottomlands might have deeper soils

but might also have poor subsurface drainage.

The visual survey might eliminate candidate

receiver sites from further consideration. Preliminary soil borings should be examined on the

remaining potential sites unless subsurface wastewater infiltration as a treatment or dispersal option

has been ruled out for other reasons. Shallow

borings, typically to a depth of at least 5 feet (1.7

meters), should be made with a soil probe or hand

auger to observe the texture, structure, horizon

thickness, moisture content, color, bulk density, and

spatial variability of the soil. Excavated test pits are

not typically required during this phase because of

the expense and damage to noncommitted sites.

Enough borings must be made to adequately

characterize site conditions and identify design

boundaries. To account for grade variations,

separation distances, piping routes, management

considerations, and contingencies, an area sufficient

to provide approximately 200 percent of the

estimated treatment area needed should be investigated. A boring density of one hole per half-acre

may be adequate to accomplish the objectives of

this phase. On sites where no reasonable number of

soil borings is adequate to characterize the continuity of the soils, consideration should be given to

abandoning the site as a potential receiver site.

Onsite treatment with a point discharge (permitted

under the National Pollutant Discharge Elimination

System) requires evaluation of the potential

receiving water and an outfall location. The

feasibility of a point discharge is determined by

federal and state rules and local codes, if enacted

by the local jurisdiction. Where the impacts and

location of the discharge are considered acceptable

by the regulating agency, effluent concentration

limits will be stipulated and an NPDES permit will

be required.

The final step of the reconnaissance survey is to

make a preliminary layout of the proposed system

on each remaining candidate site based on assessed

site characteristics and projected wastewater flows.

This step is necessary to determine whether the site

has sufficient area and to identify where detailed

soils investigations should be concentrated. In

practice, this step becomes integrated into the field

reconnaissance process so the conceptual design

Figure 5-6. Landscape position features

(see table 5-6 for siting potential)

Source: NRCS, 1998.

Table 5-6. SWIS siting potential vs. landscape position features
USEPA Onsite Wastewater Treatment Systems Manual 5-15

Chapter 5: Treatment System Selection

unfolds progressively as it is adapted to the growing body of site and soil information.

5.5.5 Detailed evaluation

The objective of the detailed evaluation is to

evaluate and document site conditions and characteristics in sufficient detail to allow interpretation

and use by others in designing, siting, and installing the system. Because detailed investigations can

be costly, they should not be performed unless the

preliminary and reconnaissance evaluations

indicate a high probability that the site is suitable.

Detailed site evaluations should attempt to identify

critical site characteristics and design boundaries

that affect site suitability and system design. At a

minimum, the detailed investigation should include

soil profile descriptions and topographic mapping.

(See figure 5-8, Site Evaluation/Site Plan Checklist.) Several backhoe pits, deep soil borings, soil

permeability measurements, ground water characterizations, and pilot infiltration testing processes

may be necessary for large subsurface infiltration

systems. For evapotranspiration systems, field

measurements of pan evaporation rates or other

parameters, as appropriate, might be necessary.

This information should be presented with an

accurate site plan.

The detailed evaluation should address surface

features such as topography, drainage, vegetation,

site improvements, property boundaries, and other

significant features identified during the reconnaissance survey. Subsurface features to be addressed

include soil characteristics, depth to bedrock and

ground water, subsurface drainage, presence of

rock in the subsoil, and identification of hydraulic

and treatment boundaries. Information must be

conveyed using standardized nomenclature for soil

descriptions and hydrological conditions. Testing

procedures must follow accepted protocol and

standards. Forms or formats and evaluation processes specified by regulatory or management

agencies must also be used (for a state example see

http://www.deh.enr.state.nc.us/oww/LOSWW/

soil_form.pdf).

5.5.6 Describing the soil profile

Descriptions and documentation of soil profiles

provide invaluable information for designing onsite

systems that use soil as the final wastewater

treatment and dispersal medium. Detailed soil

characterizations are provided through observation,

description, and documentation of exposed soil

profiles within backhoe-excavated test pits.

Profiles can be described using a hand auger or

drill probe for any single-home SWISs site in

known soil and hydrogeology. However, backhoeexcavated test pits should be used wherever large

SWISs or difficult single-home sites are proposed

because of the quality of information gained. The

grinding action or compression forces from soil

borings taken with a hand auger or drill probe limit

the information obtained for some soil characteristics, especially structure, consistency, and soil

horizon relationships. Depending on project size, it

might be necessary to supplement soil evaluation

test pits with deep borings to provide more detail

regarding soil substratum, ground water, and

bedrock conditions. Table 5-7 summarizes the

processes and procedures discussed below.

It might not be possible to identify all design

boundaries, such as the permanent water table

Figure 5-7. Conventional system layout with SWIS replacement

area
Chapter 5: Treatment System Selection

5-16 USEPA Onsite Wastewater Treatment Systems Manual

Figure 5-8. Site evaluation/site plan checklist
USEPA Onsite Wastewater Treatment Systems Manual 5-17

Chapter 5: Treatment System Selection

Figure 5-8. Site evaluation/site plan checklist (cont.)
Chapter 5: Treatment System Selection

5-18 USEPA Onsite Wastewater Treatment Systems Manual

Table 5-7. Practices to characterize subsurface conditions through test pit inspection

surface or bedrock, if they are beyond shallow

exploration depths (5 to 8 feet). However, it is

imperative to identify and characterize secondary

design boundaries that occur within the range of

subsurface investigation. Soil characteristics

should be described using USDA NRCS nomenclature and assessed by using standardized field soil

evaluation procedures as identified in the Field

Book for Describing and Sampling Soils

(Shoeneberger et al., 1998), which is available on

the Internet at http://www.statlab.iastate.edu/

soils/nssc/field_gd/field_gd.pdf.

Another source for the description of soils in the field

is American Society for Testing and Materials (ASTM)

Standard D 5921-96, Standard Practice for Subsurface Site Characterization of Test Pits for On-Site

Septic Systems (ASTM, 1996), which is summarized in

table 5-7. The primary ASTM soil characterization

reference is Standard Practice for Classification of

Soils for Engineering Purposes (Unified Soil Classification System), ASTM D 2487-00. The ASTM and
USEPA Onsite Wastewater Treatment Systems Manual 5-19

Chapter 5: Treatment System Selection

NRCS soil classification systems have many similarities; both describe and categorize soils according to

silt, clay, and sand composition and relative plasticity.

However, the NRCS guide cited above is a field guide

and is based on soil characterization procedures that

can be conducted through tactile and visual techniques in the field (e.g., the feel of a soil sample,

visual identification of the presence and color of

concretions and mottles) with minimal equipment.

The ASTM approach requires laboratory analysis of

soil particle size (with a series of sieves), plasticity,

and organic content (ASTM , 2000) and is more

commonly used in the engineering profession. Both

approaches meet the technical requirements for

conducting the site evaluation process described in

this section.

Based on the proposed design flow, an area equal to

approximately 200 percent of the estimated required treatment area should be investigated. Test

pits should be spaced in a manner that provides a

reasonable degree of confidence that conditions are

similar between pits. For small cluster systems,

three to five test pits may be sufficient if located

around the periphery and in the center of the

proposed infiltration area. Large projects require

more test pits. Test pit spacing can be adjusted

based on landscape position and observed conditions. Hand auger borings or soil probes may be

used to confirm conditions between or at peripheral

test pit locations. Soil profiles should be observed

and documented under similar conditions of light

and moisture content. Features that should be noted

include the following:

• Soil depth. Test pits should be excavated to a

safe depth to describe soil conditions, typically

4 feet below the proposed infiltrative surface. A

vertical wall exposed to the sunlight is best for

examination. The wall should be picked with a

shovel or knife to provide an undisturbed

profile for evaluation and description. Horizon

thickness should be measured and the soil

properties described for each horizon.

Restrictive horizons that may be significant

secondary design boundaries must be noted. The

depths of each horizon should be measured to

develop a relationship with conditions in other test

pits. Soil below the floor of the backhoe pit can be

investigated by using hand augers in the excavated

pit bottom or by using deep boring equipment.

Key soil properties that describe a soil profile

are horizons, texture, structure, color, and

redoximorphic features (soft masses, nodules, or

concretions of iron or manganese oxides often

linked to saturated conditions). Other properties

include moisture content, porosity, rupture

resistance (resistance to applied stress), penetration resistance, roots, clay mineralogy, boundaries, and coatings. Attention to the listed key

soil properties will provide the most value in

determining water movement in soil.

• Horizons. A soil horizon is a layer of soil that

exhibits similar properties and is generally

denoted based on texture and color. Soil horizons

result from natural soil-forming processes and

human practices. Horizons are designated as

master horizons and layers with subordinate

distinctions. All key soil properties and associated properties that are relevant to water

movement and wastewater treatment should be

described. Particular attention should be given

to horizons with strong textural contrast, stratified

materials, and redoximorphic indicators that

suggest a restriction to vertical water movement.

Certain soil conditions that create a design

boundary can occur within a soil horizon or

layer. These include horizons with low permeFigure 5-9. Soil textural triangle

Source: USDA, 1951.
Chapter 5: Treatment System Selection

5-20 USEPA Onsite Wastewater Treatment Systems Manual

ability that perch water, indurated or massive

horizons, or substrata of dense glacial till.

• Texture. Soil texture is defined as the percentage

by weight of separates (sand, silt, and clay) that

make up the physical composition of a given

sample. It is one indicator of a soil’s ability to

transmit water. The textural triangle (figure 5-9)

is used to identify soil textures based on percentage of separates (Schoeneberger et al., 1998).

The texture of soil profiles is typically identified

in the field through hand texturing. The

evaluator’s skill and experience play an important role in the accuracy of field texturing.

Several field guides, typically in the form of

flow charts, are available to assist the evaluator

in learning this skill and to assist with identifying the texture of soils that occur at or near

texture boundaries. (ASTM, 1997)

• Structure. Structure is more important than

texture for determining water movement in soils.

Soil structure is the aggregation of soil particles

into larger units called peds. The more common

types of structure are granular, angular blocky,

subangular blocky, and platy (figure 5-10).

Structureless soils include single-grain soils

(e.g., sand) and massive soils (e.g., hardpan).

The grade, size, shape, and orientation of soil

peds influence water movement in the soil

profile. This is especially true in fine-textured

soils. Smaller peds create more inter-pedal

fractures, which provide more flow paths for

percolating water. Grade, which defines the

distinctness of peds, is important for establishing a soil loading rate for wastewater dispersal.

A soil with a “strong” grade of structure has

clearly defined fractures or voids between the

peds for the transmittance of water. The interpedal fractures and voids in a soil with a “weak”

grade are less distinct and offer more resistance

to water flow. Soils with a strong grade can

accept higher hydraulic loadings than soils with

a weak grade. Platy and massive soils restrict

the vertical movement of water.

• Color. Color is an obvious property of soil that

is easily discernible. It is an excellent indicator

of the soil’s aeration status and moisture regime.

Soil colors are described using the Munsell

color system, which divides colors into three

elements—hue, value, and chroma (Munsell,

1994). Hue relates to the quality of color, value

indicates the degree of lightness or darkness,

and chroma is the purity of the spectral color.

Munsell soil color books are commercially

available and are universally accepted as the

standard for identifying soil color. The dominant

or matrix color is determined for each horizon,

and secondary colors are determined for

redoximorphic features, ped coatings, mineral

concretions, and other distinctive soil features.

Dark colors generally indicate higher organic

content, high-chroma colors usually suggest

highly oxygenated soils or high iron content,

and low-chroma soils imply reduced conditions

often associated with saturation. The site

evaluator must be aware that colors can be

modified by temperature, mineralogy, vegetation,

ped coatings, and position in the soil profile.

• Redoximorphic features. Redoximorphic

features are used to identify aquic moisture

regimes in soils. An aquic moisture regime

occurs when the soil is saturated with water

during long periods, an indicator of possible

restrictive horizons, seasonal high water tables,

or perched water tables. The presence of

redoximorphic features suggests that the

surrounding soil is periodically or continuously

saturated. This condition is important to identify

because saturated soils prevent reaeration of the

vadose zone below infiltration systems and

reduce the hydraulic gradients necessary for

Figure 5-10. Types of soil structure

Source: USDA, 1951.
USEPA Onsite Wastewater Treatment Systems Manual 5-21

Chapter 5: Treatment System Selection

adequate drainage. Saturated conditions can lead

to surfacing of wastewater or failure due to

significant decreases in soil percolation rates.

Redoximorphic features include iron nodules

and mottles that form in seasonally saturated

soils by the reduction, translocation, and

oxidation of iron and manganese oxides

(Vespaskas, 1996). Redoximorphic features

have replaced mottles and low-chroma colors in

the USDA NRCS soil taxonomy because mottles

include carbonate accumulations and organic

stains that are not related to saturation and

reduction. It is important to note that

redoximorphic features are largely the result of

biochemical activity and therefore do not occur

in soils with low amounts of organic carbon,

high pH (more than 7 standard pH units), low

soil temperatures, or low amounts of iron, or

where the ground water is aerated. Vespraskas

(1996) provides an excellent guide to the

identification of redoximorphic features and

their interpretation. As noted, the NRCS online

guide to redoximorphic and other soil properties

at http://www.statlab.iastate.edu/soils/nssc/

field_gd/field_gd.pdf addresses key identification and characterization procedures for

redoximorphic and other soil features.

• Soil consistence. Soil consistence in the general

sense refers to attributes of soil as expressed in

degree of cohesion and adhesion, or in resistance to deformation or rupture. Consistence

includes the resistance of soil material to

rupture; the resistance to penetration; the

plasticity, toughness, or stickiness of puddled

soil material; and the manner in which the soil

material behaves when subjected to compression. Consistence is highly dependent on the

soil-water state. The general classifications of

soil consistence are loose, friable, firm, and

extremely firm. Soils classified as firm and

extremely firm tend to block subsurface wastewater flows. These soils can become cemented

when dry and can exhibit considerable plasticity

when wet. Soils that exhibit extremely firm

consistence are not recommended for conventional infiltration systems.

• Restrictive horizons. Soil properties like penetration resistance, rooting depth, and clay

mineralogy are important indicators of soil

porosity and hydraulic conductivity. Penetration

resistance is often correlated with the soil’s bulk

density. The greater the penetration resistance,

the more compacted and less permeable the soil

is likely to be. Rooting depth is another measure

of bulk density and also soil wetness. Clay

mineralogies such as montmorillonite, which

expand when wetted, reduce soil permeability

and hydraulic conductivity significantly. A

discussion of these properties and their description can also be found in the USDA Soil Survey

Manual (USDA, 1993) and the USDA NRCS

Field Book for Describing and Sampling Soils

(Schoeneberger et al., 1998).

• Other soil properties. Other soil properties that

affect nutrient removal are organic content and

phosphorus adsorption potential. Organic

content can provide a carbon source (from

decaying organic matter in the uppermost soil

horizons) that will aid denitrification of nitrified

effluent (nitrate) in anoxic regions of the SWIS.

Phosphorus can be effectively removed from

wastewater effluent by soil through adsorption

and precipitation reactions (see chapter 3). Soil

mineralogy and pH affect the soil’s capacity to

retain phosphorus. Adsorption isotherm tests

provide a conservative measure of the potential

phosphorus retention capacity.

• Characterization of unconsolidated material.

Geologists define unconsolidated material as the

material occurring between the earth’s surface

and the underlying bedrock. Soil forms in this

parent material from the actions of wind, water,

or alluvial or glacial deposition. Soil scientists

refer to the soil portion of unconsolidated

material as the solum and the parent material as

the substratum. Typically, site evaluators expose

the solum and the upper portion of the substratum. Knowledge of the type of parent material

and noted restrictions or boundary conditions is

important to the designer, particularly for large

wastewater infiltration systems. Often, if the

substratum is deep, normal test pit depth will be

insufficient and deep borings may be necessary.

5.5.7 Estimating infiltration rate and

hydraulic conductivity

Knowledge of the soil’s capacity to accept and

transmit water is critical for design. The infiltration

rate is the rate at which water is accepted by the

soil. Hydraulic conductivity is the rate at which
Chapter 5: Treatment System Selection

5-22 USEPA Onsite Wastewater Treatment Systems Manual

water is transmitted through the soil. As wastewater

is applied to the soil, the infiltration rate typically

declines well below the saturated hydraulic conductivity of the soil. This occurs because the

biodegradable materials and nutrients in the

wastewater stimulate microbiological activity that

produces new biomass (see chapter 3). The biomass

produced and the suspended solids in the wastewater create a biomat that can fill many of the soil

pores and close their entrances to water flow. The

flow resistance created by the biomat can reduce

the infiltration rate to several orders of magnitude

less than the soil’s saturated hydraulic conductivity. The magnitude of the resistance created by the

biomat is a function of the BOD and suspended

solids in the applied wastewater and the initial

hydraulic conductivity of the soil.

Estimating the design infiltration rate is difficult.

Historically, the percolation test has been used to

estimate the infiltration rate. The percolation test

was developed to provide an estimate of the soil’s

saturated hydraulic conductivity. Based on experience with operating subsurface infiltration systems,

an empirical factor was applied to the percolation

test result to provide a design infiltration rate. This

method of estimating the design infiltration rate has

many flaws, and many programs that regulate onsite

systems have abandoned it in favor of detailed soil

profile descriptions. Soil texture and structure have

been found to correlate better with the infiltration

rate of domestic septic tank effluent (Converse and

Tyler, 1994). For other applied effluent qualities

such as secondary effluent, the correlation with

texture and structure is less well known.

Information on the hydraulic conductivity of the

soil below the infiltrative surface is necessary for

ground water mounding analysis and estimation of

the maximum hydraulic loading rate for the infiltration area. There are both field and laboratory

methods for estimating saturated hydraulic conductivity. Field tests include flooding basin, single- or

double-ring infiltrometer, and air entry permeameter. These and other field test procedures are

described elsewhere (ASTM, 1997; Black, 1965;

USEPA, 1981; 1984). Laboratory methods are less

accurate because they are performed on small soil

samples that are disturbed from their natural state

when they are taken. Of the laboratory tests, the

concentric ring permeameter (Hill and King, 1982)

and the cube method (Bouma and Dekker, 1981) are

the most useful techniques. The American Society

for Testing and Materials posts permeameter

information on its Internet site at http://

www.astm.org (see ASTM Store, ASTM Standards).

5.5.8 Characterizing the ground water

table

Where ground water is present within 5 feet below

small infiltration systems and 10 to 15 feet below

large systems, the hydraulic response of the water

table to prolonged loading should be evaluated.

The ground water can be adversely affected by

treated wastewater and under certain conditions can

influence system performance. This information is

valuable for understanding potential system

impacts on ground water and how the system

design can mitigate these impacts.

The depth, seasonal fluctuation, direction of flow,

transmissivity, and, where possible, thickness of the

water table should be estimated. With shallow, thin

water tables, depth, thickness, and seasonal fluctuations can be determined through soil test pit

examination. However, deeper water tables require

the use of deep borings and possible installation of

piezometers or monitoring wells. At least three

piezometers, installed in a triangular pattern, are

necessary to determine ground water gradient and

direction of flow, which might be different from

surface water flow direction. Estimating the

saturated hydraulic conductivity of the aquifer

materials is necessary to determine ground water

travel velocity. Slug tests or pumping tests can be

performed in one or more existing or new wells

screened in the shallow water table to estimate the

hydraulic conductivity of the aquifer (Bouwer,

1978; Bouwer and Rice, 1976; Cherry and Freeze,

1979). In some cases, it may be possible to estimate

the saturated hydraulic conductivity from a particle

size analysis of aquifer materials collected from the

test pit, if the material is accessible (Bouwer, 1978;

Cherry and Freeze, 1979). Pumping tests may also

be used to determine the effective porosity or

specific yield of the saturated zone.

Ground water mounding beneath an infiltration

system can reduce both treatment and the hydraulic

efficiency of the system. Ground water mounding

occurs when the rate of water percolating vertically

into the saturated zone exceeds the rate of ground

water drainage from the site (figure 5-4). Mounding
USEPA Onsite Wastewater Treatment Systems Manual 5-23

Chapter 5: Treatment System Selection

is more likely to occur where the receiver site is

relatively flat, the hydraulic conductivity of the

saturated zone is low, or the saturated zone is thin.

With continuous application, the water mounds

beneath the infiltrative surface and reduces the

vertical depth of the vadose zone. Reaeration of the

soil, treatment efficiency, and the infiltration

system’s hydraulic capacity are all reduced when

significant mounding occurs. A mounding analysis

should be completed to determine site limits and

acceptable design boundary loadings (linear

hydraulic loading) for sites where the water table is

shallow or the soil mantle is thin, or for any large

infiltration system.

Both analytical and numerical ground water

mounding models are available. Because of the

large number of data points necessary for numerical modeling, analytical models are the most

commonly used. Analytical models have been

developed for various hydrogeologic conditions

(Brock, 1976; Finnemore and Hantzshe, 1983;

Hantush, 1967; Kahn et al., 1976). Also, commercial computer software is available to estimate

mounding potential. The assumptions used in each

model must be compared to the specific site

conditions found to select the most appropriate

model. For examples of model selection and model

computations, see EPA’s process design manual

(USEPA, 1981, 1984). A USEPA Office of Ground

Water and Drinking Water annotated bibliography

of ground water and well field characterization

modeling studies can be found on the Internet at

http://www.epa.gov/ogwdw000/swp/wellhead/

dewell.html#analytical. USGS has available a

number of software packages, which are posted at

http://water.usgs.gov/software/

ground_water.html. For links to software suppliers

or general information, visit the National Ground

Water Association web site at http://

www.ngwa.org/.

5.5.9 Assessments for point source and

evapotranspiration discharges

Sites proposed for point discharges to surface

waters require a permit from the National Pollutant

Discharge Elimination System (see http://

www.epa.gov/owm/npdes.htm) and a suitable

location for an outfall to a receiving water body.

Considerations for locating an outfall structure

include NPDES regulatory requirements, outfall

structure siting, routing from the treatment facility,

construction logistics and expense, and aesthetics.

Regulatory requirements generally address acceptable entry points to receiving waters and hydraulic

and pollutant loadings. The state regulatory agency

typically sets effluent limits based on the water

resource classification, stream flow, and assimilative capacity of the receiving water. Assimilative

capacities take into account the entire drainage

basin or watershed of nearby receiving waters to

ensure that pollutant levels do not exceed water

quality criteria. (See table 3-21 for applicable

Drinking Water Standards; USEPA Drinking Water

Standards are posted at http://www.epa.gov/

ogwdw000/creg.html.) In the case of state-listed

impaired streams (those listed under section 303(d)

of the Clean Water Act), discharges must consider

pollutant loads established or proposed under the

Total Maximum Daily Load provisions of the Clean

Water Act. Piping from the treatment facility needs

to consider gravity or forcemain, route, existing

utilities, and other obstacles to be avoided.

Evapotranspiration (ET) systems treat and discharge wastewater by evaporation from the soil or

water surface or by plant transpiration. These

systems are climate-sensitive and require large land

areas. ET systems function best in arid climates

where there is large annual net evaporation and

active vegetative growth year-round. In the United

States this generally means only the southwestern

states, where humidity is low, rainfall is minimal,

and temperatures are warm enough to permit active

plant growth during the winter season (figure 5-11).

Although the macroclimate of an area might be

acceptable for the use of ET systems, evaluation of

the microclimate is often required because it can

significantly influence system performance. In

addition to temperature, precipitation, and pan

evaporation data, exposure position and prevalent

wind direction should be considered as part of the

evaluation process. Southern exposures in the

northern hemisphere provides greater solar radiation. Exposure to wind provides greater drying of

the soil and plant surfaces. Surface drainage

patterns should also be assessed. Well-drained sites

have a lower ambient humidity to enhance evaporation than poorly drained sites.
Chapter 5: Treatment System Selection

5-24 USEPA Onsite Wastewater Treatment Systems Manual

5.6 Mapping the site

At the completion of the site evaluation, a site map

or sketch should be prepared to show physical

features, locations of soil pits and borings, topography or slopes, and suitable receiver sites. If a map

or aerial photograph was used, field measurements

and locations can be noted directly on it. Otherwise

it will be necessary to take measurements and

sketch the site. The level of effort for developing a

good site map should be commensurate with the

results of the site evaluation and whether the site

map is being completed for a preliminary or

detailed site evaluation.

In addition to the features of the site under consideration, the site map should show adjacent lands

and land uses that could affect treatment system

layout, construction, and system performance.

Maps with a 1- or 2-foot contour interval are

preferred.

5.7 Developing the initial system

design

Developing a concept for the initial system design

is based on integration of projected wastewater

volume, flow, and composition information; the

controlling design boundaries of the selected

receiving environment; the performance requirements for the chosen receiving environment; and

the needs and desires of the owner (figure 5-12).

The site evaluation identifies the critical design

boundaries and the maximum mass loadings they

can accept. This knowledge, together with the

performance requirements promulgated by the

regulating authority for the receiving environment,

establishes the design boundary loadings. Once the

boundary loadings are established, treatment trains

that will meet the performance requirements can be

assembled.

Figure 5-11. Potential evaporation versus mean annual precipitation
USEPA Onsite Wastewater Treatment Systems Manual 5-25

Chapter 5: Treatment System Selection

Figure 5-12. Development of the onsite wastewater system design concept
Chapter 5: Treatment System Selection

5-26 USEPA Onsite Wastewater Treatment Systems Manual

Assembling SWIS treatment tr ling SWIS treatment trains for a site with shallow,

slowly permeable soils over bedrock

Site description

A single-family residence is proposed for a lot with shallow, finely textured, slowly permeable soil over

creviced bedrock. The depth of soil is 2 feet. The slope of the lot is moderate and is controlled by bedrock.

Ground water is more than 5 feet below the bedrock surface.

Design boundaries

Three obvious design boundaries that will affect the SWIS design are present on this site: the infiltrative

surface, the bedrock surface, and the water table. The site evaluation determined that no hydraulically

restrictive horizon is present in the soil profile above the bedrock.

Performance requirements

The regulatory agency requires that the wastewater discharge remain below ground surface at all times, that

the ground water contain no detectable fecal coliforms, and that the nitrate concentrations of the ground water

be less than 10 mg-N/L at the property boundary. In this case study, wastewater modification (reducing mass

pollutant loads or implementing water conservation measures; see

chapter 3) was not considered.

Design boundary mass loadings

Infiltr Infiltrative surface: Referring to table 5-2, the mass loadings that might affect the infiltrative surface are the

daily, instantaneous, and organic mass loadings. The selected hydraulic and instantaneous (dose volume per

square foot) loading rates must be appropriate for the characteristics of the soil to prevent surface seepage.

Assuming domestic septic tank effluent is discharged to the infiltrative surface and that the surface is placed

in the natural soil, the organic mass loading is accounted for in the commonly used daily hydraulic loading

rates. Typical hydraulic loading rates for domestic septic tank effluent control design. Reducing the organic

concentration through pretreatment will have little impact because the resistance of the biomat created by the

organic content is typically less than the resistance to flow through the fine-textured soil.

Bedrock boundar k boundark boundary:The bedrock boundary is a secondary design boundary where a zone of saturation will

form as the wastewater percolates through the soil. This boundary is affected by the daily and linear hydraulic

loadings (table 5-2). If these hydraulic loadings exceed the rate at which the water is able to drain laterally

from the site or percolate to the water table through the bedrock crevices, the saturated zone thickness will

increase and could encroach on the infiltrative surface, reducing its treatment and hydraulic capacity. Because

the site is sloping, the linear, rather than the daily, hydraulic loading will control design.

Water tab ater tabater table boundar le boundary:The wastewater percolate will enter the ground water through the bedrock crevices.

The daily and linear hydraulic loading and constituent loadings are the mass loadings that can affect this

boundary (table 5-2). Because of the depth of the water table below the bedrock surface and the porous nature

of the creviced bedrock, the daily and linear hydraulic loadings are not of concern. However, nitrate-nitrogen

and fecal coliforms are critical design loadings because of the water quality requirements. Table 5-2

summarizes the critical design boundary mass loadings that will affect design.

Assembling feasible treatment train alternatives

Because control of the wastewater is lost after it is applied to the soil, the bedrock and water table boundary

loading requirements must be satisfied through appropriate design considerations at or before the infiltrative

boundary. Therefore, the secondary and water table boundary loadings must be considered first.

Constituent loading limits at the ground water boundary will control treatment requirements. Although the

performance boundary (the point at which performance requirements are measured) may be at the property

boundary, mixing and dilution in the ground water cannot be certain because the bedrock crevices can act as

direct conduits for transporting undiluted wastewater percolate. Therefore, it would be prudent to ensure these

pollutants are removed before they can leach to the ground water. Research has demonstrated that soils

similar to those present at the site (fine-textured, slowly permeable soils) can effectively remove the fecal
USEPA Onsite Wastewater Treatment Systems Manual 5-27

Chapter 5: Treatment System Selection

coliforms if the wastewater percolates through an unsaturated zone of 2 to 3 feet (Florida HRS, 1993).

Because the soil at the site extends to only a 2-foot depth, the infiltrative surface would need to be elevated 1

foot above the ground surface in a mound or at-grade system. Alternatively, disinfection prior to soil application

could be used. Nitrate is not effectively removed by unsaturated, aerated soil; therefore, pretreatment for

nitrogen removal is required.

Maintaining the linear loading at the bedrock surface below the maximum acceptable rate determines the

orientation and geometry of the infiltrative surface. The infiltrative surface will need to be oriented parallel to

the bedrock surface contour. Its geometry needs to be long and narrow, with a width no greater than the

maximum acceptable linear loading (gpd/ft) divided by the design hydraulic loading on the infiltrative surface

(gpd/ft2). Note: If a mound is used on this site, an additional design boundary is created at the mound fill/

natural soil interface. The daily hydraulic loading will affect this secondary design boundary.

If the perched saturated zone above the bedrock is expected to rise and fall with infiltrative surface loadings,

the instantaneous loading to the infiltrative surface should be controlled through timed dosing to maximize the

site’s hydraulic capacity. Failure to control instantaneous loads could lead to transmission of partially treated

wastewater through bedrock crevices, driven by the higher hydraulic head created during periods of peak

system use. Applying the wastewater through a dosing regime will maximize retention time in the soil while

ensuring cyclical flooding of the infiltration trenches, creating optimum conditions for denitrifying bacteria to

accomplish nitrogen removal. The daily and instantaneous hydraulic loadings to the infiltrative surface are

dependent on the characteristics of the soil or fill material in which the SWIS is placed.

Alternative Pretreatment Dosing Infiltration

1 Nitrogen removal Timed dosing Mound with pressure distribution

2 Nitrogen removal with disinfection Timed dosing In-ground trenches with pressure distribution

From this boundary loading analysis, potential treatment train alternatives can be assembled. Table 4-1 and

the fact sheets in chapter 4 should be used to select appropriate system components.

Alternative 1 elevates the infiltrative surface in a mound of suitable sand fill. With at least a foot of fill and the

unsaturated 2 feet of natural soil below, fecal coliform removal will be nearly complete. The mound would be

designed as long and narrow, oriented parallel to the bedrock surface contours (equivalent to the land surface

contours since the slope is bedrock-controlled) to control the linear loading on the interface between the sand

fill and natural soil or at the bedrock surface. The infiltrative surface would be time-dosed through a pressure

or drip distribution network to distribute the wastewater onto the surface uniformly in time and space.

Alternative 2 places the infiltrative surface in the natural soil. With this design, there would be an insufficient

depth of unsaturated soil to remove the fecal coliforms. Therefore, disinfection of the treated wastewater prior

to application to the soil would be necessary. The trenches would be oriented parallel to the bedrock surface

contours (equivalent to the land surface contours since the slope is bedrock-controlled) to control the linear

loading on the bedrock surface. If multiple trenches are used, the total daily volume of treated wastewater

applied per linear foot of trench parallel to the slope of the bedrock surface would be no greater than the

design linear loading for the site. Loadings to the infiltrative surface would be time-dosed through a pressure or

drip distribution network to distribute the wastewater uniformly in time and space.

Note that for the alternatives listed, multiple options exist for each of the system’s components (see table 4-1).

Source: Otis, 2001.
Chapter 5: Treatment System Selection

5-28 USEPA Onsite Wastewater Treatment Systems Manual

Subsurface wastewater infiltration system design in a restricted area

Often, the available area with soils suitable for subsurface infiltration of wastewater is limited. Because local

authorities usually do not permit point discharges to surface waters, subsurface infiltration usually is the only

option for wastewater treatment. However, a SWIS can perform as required only if the daily wastewater flow is

less than the site’s hydraulic capacity.

The hydraulic capacity of the site is determined by the subsurface drainage capacity of the site. The drainage

capacity is defined by the soil profile and the daily hydraulic or linear mass loading to secondary or ground

water boundary surfaces. In some cases, however, the infiltration rate of the wastewater into the soil at the

infiltrative boundary is more limiting. Therefore, it is important to distinguish between the two boundaries if use

of the site is to be maximized. Where hydraulic loadings to secondary boundaries are the principal control

feature, the only option is to limit the amount of water applied to the secondary boundaries. This can be

accomplished through the following:

Orientation, geometr ientation, geometry, and controlled dosing of the infiltr , and controlled dosing of the infiltrative surface

The infiltrative surface should be oriented parallel to and extended as much as possible along the surface

contour of the secondary boundary. Southern, eastern, and western exposures may provide better

evaporation than north-facing slopes. The daily hydraulic loading rate onto the total downslope projection of

“stacked” infiltration surfaces (multiple, evenly spaced SWIS trenches placed on the contour on sloping

terrain) should be limited to the maximum linear loading of the secondary boundary. Timed dosing to the

infiltrative surfaces should be used to apply wastewater uniformly over the full length of the infiltrative

surfaces to minimize the depth of soil saturation over the secondary boundary. Note that the presence of

other SWIS-based treatment systems above or below the site should be considered in load calculations

and design concept development.

Installation of water-conserving plumbing fixtures in the building served

The total daily volume of wastewater generated can be significantly reduced by installation of waterconserving fixtures such as low-volume flush toilets and low-flow showerheads (see chapter 3). Also,

wastewater inputs from tub spas and automatic regenerating water softeners should be eliminated.

Maximizing the evapotranspiration potential of the infiltration system

Where the growing season is long or use of the property is limited to the summer months,

evapotranspiration can help to reduce the total hydraulic loading to the secondary boundary. The infiltrative

surfaces should be shallow and located in open, grassed areas with southern exposures (in the Northern

Hemisphere).

If the infiltr If the infiltration capacity at the soil’ ation capacity at the soil’s infiltr s infiltrs infiltrative surface is the limiting condition, measures to

increase infiltr increase infiltration can be taken. These measures include the f These measures include the following:

Reducing the mass loadings of soil clogging constituents on the infiltrative surface

The mass loadings to the infiltrative surface can be reduced either by increasing the infiltrative surface

area to reduce the mass constituent loading per unit of area or by removing the soil-clogging constituents

before soil application. Where the suitable area for the SWIS is limited, increasing the infiltrative surface

area might not be possible.

Controlled dosing of the infiltrative surface

Timed dosing and alternate “resting” of infiltrative surfaces allow organic materials that might clog the soil

surface to oxidize, helping to rejuvenate infiltrative capacity. Using multiple timed doses throughout the day

with intervals between doses to allow air diffusion maximizes the reaeration potential of the subsoil (Otis,

1997). Dual infiltration systems that can be alternately loaded allow for annual resting of the infiltrative

surfaces to oxidize the biomat. On small lots dual systems are often not feasible because of space

limitations.

Source: Otis, 2001.
USEPA Onsite Wastewater Treatment Systems Manual 5-29

Chapter 5: Treatment System Selection

5.7.1 Identifying appropriate treatment

trains

Multiple treatment trains (system designs) are often

feasible for a particular receiver site and expected

wastewater flow. More than one receiving environment may be suitable for a treated discharge. For

example, subsurface infiltration or a point discharge to surface water might be feasible. Multiple

sites on a property might be suitable as a receiver

site. In addition, more than one treatment train

might meet established or proposed performance

requirements. Each of these alternatives must be

considered to select the most appropriate system for

a given application.

Evaluation of the feasible alternatives is a continuous activity throughout the preliminary design

process. It is beneficial to eliminate as many

potential options as possible early in the preliminary design process so that time can be spent on the

most probable alternatives. Typically, receiving

environments are the first to be eliminated. For

example, in temperate climates atmospheric

discharges are rarely feasible because there is

insufficient net evaporation to evaporate the

wastewater. Surface water discharges usually can

be eliminated as well because often they are not

permitted by the local regulatory agency. Where

such discharges are permitted, subsurface infiltration is usually less costly if the site meets the

regulatory agency’s requirements because monitoring costs for compliance with point discharge

permit requirements can be substantial.

At the completion of the site evaluation, the

receiving environment has been tentatively selected

(see section 5.5). For each potential receiver site,

the design boundaries have been identified.

Integrating information on physical limitations and

established or proposed performance requirements

helps to define the maximum mass loadings to the

design boundaries (see section 5.3). Defining and

characterizing the controlling design boundaries

and their maximum acceptable mass loadings,

estimating the characteristics of the wastewater to

be treated, and evaluating the site conditions

inform the development of a feasible set of potential treatment trains. Treatment train assembly is

usually straightforward for surface water discharges because the effluent concentration limits at

the outfall control design. With soil-based systems

such as SWISs, however, treatment train selection

is more complex because multiple design boundaries can be involved.

Because direct control of SWIS performance is lost

once the partially treated wastewater enters the soil

at the infiltrative surface, management of the

loadings to any secondary design boundaries and

water table boundaries must be accomplished

indirectly through appropriate adaptations at the

primary infiltrative surface. For hydraulic loadings,

control can be achieved by changing the geometry

or size of the infiltrative surface or the dosing

volume, frequency, and pattern. For organic or

constituent loadings, control is achieved either by

pretreating the wastewater before it is applied to

the infiltrative surface or by increasing the size of

the infiltrative surface.

5.7.2 Treatment train selection

Where multiple treatment trains are feasible and

technically equivalent, each must be evaluated with

respect to aesthetics, operation and maintenance

requirements, cost, and reliability before selection

of the final design concept.

5.7.3 Aesthetic considerations

Aesthetics are an intangible factor that must be

addressed with the owner, users, adjacent property

owners, and regulators. They include considerations such as system location preferences, appearance, disruption during construction, equipment

and alarm noise, and odor potential. It is important

that these and possibly other aesthetics issues be

discussed with the appropriate parties before

selecting the design concept to be used. If the

expectations of the concerned parties are not met,

their dissatisfaction with the system could affect its

use and care.

5.7.4 Operation and maintenance

requirements

Specific and appropriate operation and maintenance tasks and schedules are essential if a wastewater system is to perform properly over its

intended service life. Important considerations

include
Chapter 5: Treatment System Selection

5-30 USEPA Onsite Wastewater Treatment Systems Manual

• Types of maintenance functions that must be

performed

• Frequency of routine maintenance

• Time and skills required to perform routine

maintenance

• Availability of operation and maintenance

service providers with appropriate skills

• Availability of factory service and replacement

parts

Traditional onsite systems are passive in design,

requiring little operator attention or skill. Unskilled

owners can usually access maintenance services or

be trained to perform basic maintenance tasks.

Septage removal usually requires professional

services, but these are readily available in most

areas. More complex wastewater systems, however,

require elevated levels of operator attention and

skill. The designer must weigh the availability of

operator services in the locale of the proposed

system against the consequences of inadequate

operation and maintenance before recommending a

more complex system. The availability of factory

service is also an important consideration. Where

operation and maintenance services are not locally

available and the use of alternative systems that

have fewer operation and maintenance requirements is not an option, the prospective system

owner should be advised fully before proceeding.

5.7.5 Costs

Costs of the feasible alternatives should be arrayed

based on the total cost of each alternative. Total

costs include both the capital costs incurred in

planning, designing, and constructing the system

and the long-term costs associated with maintaining

the system over its design life (20 to 30 years in

most cases; see table 5-8). This method of cost

analysis is an equitable method of comparing

alternatives with higher capital costs but lower

annual operating costs to other alternatives with

lower capital costs but higher annual operating

costs. Often, owners are deceived by systems with

lower capital costs. These systems might have much

higher annual operating costs, a shorter design life,

and possibly higher replacement costs, resulting in

much higher total costs. Systems with higher capital

costs might have lower total costs because the

recurring operation and maintenance costs are less.

Choosing between alternatives with varying total

cost options is a financing decision. In some cases,

capital budgets are tighter than operating budgets.

Therefore, this is a decision the prospective owner

must make based on available financing options.

Table 5-8 is an example of such a comparative

analysis.

The USEPA Office of Wastewater Management

posts financing information for onsite wastewater

treatment systems or other decentralized systems

(cluster systems not connected to a wastewater

treatment plant) on the Internet at http://

www.epa.gov/owm/decent/funding.htm. Links are

available at that site to financing programs supported by a variety of federal, state, and other

public and private organizations.

5.7.6 Reliability

The reliability of the proposed system and the risks

to the owner, the public, and the environment if

malfunctions or failures occur must be considered.

Potential risks include public health and environmental risks, property damage, personal injury,

medical expenses, fines, and penalties. Where these

or other potential risks are significant, contingency

plans should be developed to manage the risks.

Contingencies include storage, pump and haul

(holding tank), redundant components, reserve

capacity, and designation of areas for repair or

replacement components (e.g., replacement leach

field). These come at additional cost, so their

benefit must be weighed against the potential risks.

5.7.7 Conceptual design

After evaluating the feasible options, the preliminary treatment train components can be selected. At

this point in the development of the design, the unit

processes to be used and their sequence are defined. A preliminary layout should be prepared to

confirm that the system will fit on the available site.

Sufficient detail should be available to prepare a

preliminary cost estimate if needed. It is recommended that the conceptual system design and

preliminary layout be submitted to the regulatory

agency for conditional acceptance of the chosen

system. Final design can proceed upon acceptance

by the owner and regulatory agency.
USEPA Onsite Wastewater Treatment Systems Manual 5-31

Chapter 5: Treatment System Selection

Table 5-8. Example of a total cost summary worksheet to compare alternativesa.

aCosts displayed are not typical for all states. Costs in other states are significantly higher.
Chapter 5: Treatment System Selection

5-32 USEPA Onsite Wastewater Treatment Systems Manual

5.8 Rehabilitating and upgrading

existing systems

Onsite wastewater treatment systems can fail to

meet the established performance requirements.

When this occurs, corrective actions are necessary.

Successful rehabilitation requires knowledge of the

performance requirements, a sound diagnostic

procedure, and appropriate selection of corrective

actions.

5.8.1 Defining system failure

Failure occurs when performance requirements are

not met (see table 5-9). Under traditional prescriptive rules, onsite wastewater systems must comply

with specific siting and design requirements,

maintain the discharged wastewater below ground

surface, and not cause backup in fixtures. Typically, failures are declared when wastewater is

observed on the ground surface or is backing up in

the household plumbing. However, systems also

may be declared as failed if they do not comply

with the prescriptive design rules. Thus, except for

hydraulic failures, systems can be declared failed

based on their design, but rarely based on treatment

performance to date.

When failure is strictly a code compliance issue

rather than a performance issue, enforcing corrective actions can be problematic because corrective

actions for code-based compliance might not

reduce (and might even elevate) the potential risk

to human health or the environment. Also, code

compliance failures can be much more difficult to

correct because site or wastewater characteristics

might prevent compliance with the prescriptive

requirements. In such instances, variances to the

rule requirements are needed to remove the

noncompliant condition. Performance codes, on the

other hand, define failures based on performance

requirements consisting of specific and measurable

criteria. Usually, treatment options are feasible to

achieve compliance, though costs can be a significant impediment.

5.8.2 Failure diagnosis

Wastewater system failures occur at the design

boundaries when the acceptable boundary loadings

are exceeded. Prescribing an effective corrective

measure requires that the failure boundary and the

unsuitable boundary loading be correctly identified.

The manifestations of boundary failures can be

similar in appearance despite different locations or

causes of failure. For example, the primary infiltrative surface might fail to accept the daily wastewater load, causing the discharged wastewater to seep

onto the ground surface. The cause of failure might

be that the daily hydraulic capacity of the infiltrative surface was exceeded, the instantaneous

hydraulic loading (dose volume) was too great, or

the organic load was too high. In other instances,

the linear loading on a site might be exceeded,

causing a saturated zone above a secondary restrictive horizon to rise and encroach on the infiltrative

surface (effluent mounding). The potential gradient

across this surface is reduced in this situation, and

the reaeration of the subsoil is inhibited. As a result

of the reduced gradient and increased clogging, the

infiltrative surface can no longer accept the daily

Table 5-9. Common onsite wastewater treatment system failures
USEPA Onsite Wastewater Treatment Systems Manual 5-33

Chapter 5: Treatment System Selection

loading and allows wastewater to back up in the

trenches and possibly to surface. Though the causes

of failure in these two instances are different, the

symptoms are similar. Thus, it is important that a

systematic approach to failure diagnosis be used.

Failures occur for a reason. The reason for failure

should be determined before corrective actions are

implemented; if not, failures can recur. The

diagnostic procedure should be comprehensive, but

based on deductive reasoning to avoid excessive

testing and data gathering (figure 5-13). Another

example of a failure diagnosis, Failure Analysis

Chart for Troubleshooting Septic Systems (FACTS)

is provided in Adams et al., 1998.

In addition to specific design boundary failures,

failures can be caused by system age. Tanks and

pipes buried in the ground begin to deteriorate after

20 or more years of use and may require repair or

replacement. In addition, the treatment capabilities

of soils below infiltration fields that have been in

use for several decades might not be adequate for

continued use. Years of treatment use can cause the

interstitial spaces between soil particles to become

filled with contaminants (e.g., TSS, precipitates,

biomass). Soil structure can also be affected after

many years of use. Finally, changes in design and

construction practices in the past 25 years have led

to marked improvements in system performance

and treatment capacity. These issues make consideration of system age a vital component of the

overall failure investigation.

5.8.3 Initial data gathering

When a failure is reported, relevant information

regarding the system should be gathered.

• Visual observation. A visual observation of the

failure should be made to confirm the information provided. Also, the owner should be

interviewed regarding the owner’s observations,

use of the building, and other relevant information. Each of the system components should be

inspected and mechanical components (e.g.,

float switches, flow diverters) tested.

• Past operation and maintenance practices.

Assessing operation and maintenance actions

taken over the past 3 to 5 years can often aid in

detecting relatively simple problems. Perhaps

the tank has not been pumped, the tank filter (if

used) has not been cleaned, the electrical supply

Figure 5-13. Onsite wastewater failure diagnosis and

correction procedure
Chapter 5: Treatment System Selection

5-34 USEPA Onsite Wastewater Treatment Systems Manual

to the pumps has not been checked, or the

switches have not been examined.

• System layout and boundary design loadings.

The system layout can be obtained from the

design drawings or from a site survey. From the

layout, the design boundary loadings should be

determined or estimated based on the original

design flow.

• Soil test reports. Soil test reports should be

obtained. If none are available, soil auger

testing between the trenches or just outside the

SWIS perimeter might be necessary to provide a

simple description of the soil profile to determine whether any significant secondary design

boundaries might be present.

• Age of system. If the system age is less than 2

years, it is likely the design boundary loadings

were in error or improper construction techniques (e.g., operation of heavy equipment on

SWIS area, installation during wet conditions)

that significantly altered the soil characteristics

were used. If the age of the system is greater

than 2 years, it is likely that the design conditions changed. Changed conditions could

include changes in the building’s use, increased

wastewater flows, infiltration and inflow into the

system, surface runoff over the system, improper maintenance, compaction of SWIS soils

by vehicle traffic, and others.

• Description of failure symptoms. The symptoms

of failure are important. Historically, reported

failures have usually been hydraulic in nature

and tended to be manifested by surface seepage.

Information on the location and frequency of

the surface seepage helps to determine the

specific design boundary at which the failure

occurred and possible causes of the failure. For

example, surface seepage above the infiltration

system suggests that the infiltrative surface is

overloaded, either hydraulically or organically.

Seepage downslope from the system suggests

that a secondary design boundary exists and is

overloaded hydraulically. If the failure is

seasonal, wet weather conditions are likely to be

the cause; that is, clear water is infiltrating into

the system or causing inadequate subsurface

drainage.

• Daily flow estimates. Estimates of daily wastewater flows derived from water meter data or

other sources are needed to compare the design

loadings with actual loadings. In the absence of

data, water use should be estimated (see chapter

3) with the caveat that such estimates are seldom

accurate. Where practical, water meters should

be read or installed as soon as the failure is

reported so that metered data can be collected.

Initially, daily flow estimates might need to

suffice for the purposes of failure analysis.

Leaking plumbing fixtures, such as improperly

seated toilet tank flapper valves, should be

investigated.

5.8.4 Determining the cause of failure

From the gathered data, hypotheses of potential

causes of failure should be formulated. Formulating hypotheses is an important step in diagnosing

the problem because the hypotheses can be tested to

provide a systematic and efficient analysis of

possible causes of failure (see case study). Testing

can take many forms (see table 5-10 as an example

of a local approach) depending on the hypotheses

to be tested. It may include soil profile descriptions, soil hydraulic conductivity testing, wastewater characterization, equipment testing and monitoring, and other tests.

5.8.5 Designing corrective actions

If the design boundary failure can be identified and

its cause identified, selecting an appropriate

corrective action is straightforward. Table 5-11 can

be used to select the appropriate corrective action

for a given boundary failure. This table presents

classes of corrective actions and the impacts they

can be expected to have on boundary mass loadings. Several options typically exist for each class

of corrective action. Specific actions will be

determined by the particular needs of the system

and site.

The failure diagnosis and correction procedure

outlined in figure 5-13 provides a summary of

activities required to identify and characterize the

cause of failure. As noted in the previous discussion, data collection, failure cause determination,

and testing of hypotheses (e.g., as in the case study

above) provides key information needed to develop

corrective actions. Failures at design boundaries

(e.g., exceeding mass pollutant or hydraulic load

limits) can be rectified by changing boundary
USEPA Onsite Wastewater Treatment Systems Manual 5-35

Chapter 5: Treatment System Selection

Table 5-10. General OWTS inspection and failure detection processa

Table 5-11. Response of corrective actions on SWIS boundary mass loadings.

Source: Otis, 2001.
Chapter 5: Treatment System Selection

5-36 USEPA Onsite Wastewater Treatment Systems Manual

Failure hypothesis testing at a system serving a highway rest area

A wastewater system serving a highway rest area used a drip distribution system for final treatment and

dispersal of the wastewater. After the first summer of use, water was observed above the dispersal system.

The original soil test results indicated that the soils were deep, loamy sands with no apparent secondary

boundaries. The system design appeared to use appropriate loadings on the infiltrative surfaces.

A visual inspection and interviews with the maintenance staff at the rest area provided important clues:

✔ The site of the dispersal system had been significantly regraded after the soil testing had been

completed. Up to 5 feet of material had been removed from the site.

✔ The system was a replacement for another system that had also failed. The existing septic tanks were used

in the new system.

✔ Water use was metered and recorded daily.

✔ The rest area had a sanitary dump station that discharged into the wastewater system. The dump station

received very heavy use on weekends during the summer. This load was not accounted for in the metering

data.

From these clues, several hypotheses were formulated for testing.

a. Water discharges to the system exceed the hydraulic and constituent design loadings.

This hypothesis can be tested by estimating daily wastewater discharges. The recorded water meter data

provide an accurate estimate of water use at the rest area. The metered data would need to be corrected for

turf irrigation at the rest area. Turf irrigation can be estimated from staff interviews of irrigation schedules.

Unaccounted water from the sanitary dump station must be estimated. Counting the number of vehicles

using the dump station and assuming an average volume of wastewater discharged per vehicle would

provide a reasonable estimate. Because of the strength of the dump station wastewater, wastewater

samples at the septic tank outlets should be taken to determine organic loadings.

Another issue that might need to be considered is load inputs from disinfectants or other chemicals used in

holding tanks that are discharged into the dump station. Significant concentrations of these chemicals could

affect biological processes in the tank and infiltrative zone.

b. Infiltration/inflow of clear water into the system or into the SWIS is excessive.

Only the septic tanks were left in place during the reconstruction of the existing system. All new

components were leak tested during construction. It can be assumed that the new portion of the system

does not leak if inspection records exist and can be verified. The existing septic tanks could be expected to

be the source of any inflow or infiltration. Infiltration of surface runoff from the area over the septic tanks,

revealed by the existence of saturated soils around the tanks, could result in significant infloinfiltration

contributions. If there is evidence that such conditions exist, the septic tanks should be pumped and tested

for leakage. Runoff of storm water onto the SWIS surface could also cause ponding and might require

regrading of the surrounding site or a diversion to route runoff elsewhere

c. The actual soil characteristics at the receiver site are different from the soil test results.

The characteristics of the soils after regrading might be different from those reported by the original soil

tests because of the depth of soil removed. Also, the regrading operations might have compacted the

subsoil, creating a secondary design boundary that was not anticipated. Soil tests could be performed to

determine if the existing profile below the dispersal system is different in texture, structure, and bulk density

from that reported earlier. Also, the source of the surface seepage should be investigated. If the seepage

occurs immediately above a dripperline but the soil is not saturated between the lines, the infiltrative surface

surrounding the dripperline is hydraulically or organically overloaded. If the soil between the lines is

saturated, a secondary boundary that is hydraulically overloaded probably exists. If such a boundary is

present, the soil below the boundary would be unsaturated.

By developing these hypotheses, determination of the failure can be systematic and efficient. The most probable

hypothesis can be tested first, or appropriate tests for all the hypotheses formulated can be performed at one time

for later evaluation.

Source: Otis, 2001.
USEPA Onsite Wastewater Treatment Systems Manual 5-37

Chapter 5: Treatment System Selection

loadings to accommodate the hydraulic or mass

pollutant assimilative capacities at the design

boundary. Loading adjustments may require

lowering water usage through water conservation

measures, eliminating clear water inputs, or

separating graywater; increasing the area of the

infiltrative surface; or diverting precipitation and/

or shallow ground water from the SWIS with berms

or curtain drains.

Approaches for lowering mass pollutant loads

include improving pretreatment by upgrading the

existing system and/or adding treatment units,

improving user habits (e.g., removing food,

kitchen, or dishwashing wastes from the wastewater

stream), reducing or eliminating inputs of cleaners

or other strong chemical products, and reducing

solid waste in the wastewater stream (e.g., ground

garbage from garbage disposals). If measures to

correct failures within the existing receiver site are

not possible, corrective actions may involve

changing the receiver site or changing the receiver

site conditions. These options include adoption of

different treatment technologies, physical alteration

of the receiver site, and installation of a new

infiltration system, thereby resting the existing

system for future alternate dosing.

Attention to established performance requirements

and the design boundaries where they are measured

helps to ensure that corrective actions meet the

overall goals of the management entity and protect

human health and the environment. Implementation

of corrective actions should follow the same

processes and procedures outlined in the preceding

sections for new or replacement OWTSs.

References

Adams, A., M.T. Hoover, W. Arrington, and G.

Young. 1998. FACTS: Failure Analysis Chart for

Troubleshooting Septic Systems. In Onsite

wastewater Treatment: Proceedings of the Eighth

National Symposium on Individual and Small

Community Sewage Systems. American Society

of Agricultural Engineers, St. Joseph, MI.

American Society for Testing and Materials

(ASTM). 1996a. Standard Practice for Surface

Site Characterization for Onsite Septic Systems.

ASTM Practice D5879-95 el. American

Society for Testing and Materials, West

Conshohocken, PA.

American Society for Testing and Materials

(ASTM). 1996b Standard Practice for

Subsurface Site Characterization of Test Pits

for Onsite Septic Systems. ASTM Practice

D5921-96 el. American Society for Testing and

Materials, West Conshohocken, PA.

American Society for Testing and Materials

(ASTM). 1997. Standards Related to On-Site

Septic Systems. ASTM publication code 03-

418197-38. American Society for Testing and

Materials, West Conshohocken, PA.

American Society for Testing and Materials

(ASTM). 2000. Standard Practice for

Classification of Soils for Engineering

Purposes (Unified Soil Classification System).

ASTM D 2487-00. American Society for

Testing and Materials, West Conshohocken,

PA.

Ayres Associates. 1993. Onsite Sewage Disposal

System Research in Florida—An Evaluation of

Current OSDS Practices in Florida. Report to

the Department of Health and Rehabilitative

Services. Ayres and Associates, Tallahassee,

FL.

Black, C.A., ed. 1965. Methods of Soil Analysis.

Part 1: Physical and Microbiological

Properties, Including Statistical Measurement

and Sampling. American Society of Agronomy,

Madison, WI.

Bouma, J., and L.W. Dekker. 1981. A method of

measuring the vertical and horizontal hydraulic

saturated conductivity of clay soils with

macropores. Soil Science Society of America

Journal 45:662.

Bouwer, H. 1978. Groundwater Hydrology.

McGraw-Hill Book Company, New York, NY.

Bouwer, H., and R.C. Rice. 1976. A slug test for

determining hydraulic conductivity of

unconfined aquifers with completely or

partially penetrating wells. Water Resources

Research 12:423-428.

Brock, R.P. 1976. Dupuit-Forcheimer and potential

theories for recharge from basins. Water

Resources Research 12:909.
Chapter 5: Treatment System Selection

5-38 USEPA Onsite Wastewater Treatment Systems Manual

Finnemore, E.J., and N.N. Hantzshe. 1983.

Ground-water mounding due to onsite sewage

disposal. Journal of the American Society of

Civil Engineers Irrigation and Drainage

Division 109:999.

Florida Department of Health and Rehabilitative

Services (Florida HRS). 1993. Onsite Sewage

Disposal System Reseach in Florida.

Tallahassee, FL.

Freeze, R.A., and J.A. Cherry. 1979. Groundwater.

Prentice-Hall, Englewoods Cliffs, NJ.

Fredrick, J.C. 1948. Solving disposal problems in

unsewered areas. Sewage Works Engineering

19(6):292-293, 320.

Glegg, G.L. 1971. The Design of Design.

Cambridge University Press, London, England.

Hantush, M.S. 1967. Growth and decay of ground

water mounds in response to uniform

percolation. Water Resources Research 3:227.

Hantzsche, N.N. 1995. Data Management System

for On-Site Wastewater Inspection Program at

The Sea Ranch, California. In Proceedings of

the Sixth International Symposium on

Individual and Small Community Sewage

Systems. American Society of Agricultural

Engineers, St. Joseph, MI.

Hill, R.L., and L.D. King. 1982. A permeameter

which eliminates boundary flow errors in

saturated hydraulic conductivity measurements.

Soil Science Society of America Journal

46:877.

Hoover, M.T. 1997. A Framework for Site

Evaluation, Design, and Engineering of Onsite Technologies Within a Management

Context. Marine Studies Consortium, Waquoit

Bay National Esturarine Research Reserve, and

ad hoc Task Force for Decentralized

Wastewater Management. Marine Studies

Consortium, Chestnut Hill, MA.

Kahn, M.Y., D. Kirkham, and R.L. Handy. 1976.

Shapes of steady state perched groundwater

mounds. Water Resources Research 12:429.

Munsell. 1994. Munsell Soil Color Charts.

GretagMacbeth LLC, New Windsor, NY.

North Carolina Department of Environment,

Health, and Natural Resources (North Carolina

DEHNR). 1996. On-Site Wastewater

Management: Guidance Manual. North

Carolina Department of Environment, Health,

and Natural Resources, Division of

Environmental Health, On-Site Wastewater

Section, Raleigh, NC.

National Small Flows Clearinghouse. 2000.

National Environmental Service Center. West

Virginia University, Morgantown, WV.

Natural Resources Conservation Service (NRCS).

1998. Field Book for Describing and Sampling

Soils. Version 1.1. U.S. Department of

Agriculture, Natural Resources Conservation

Service, National Soil Survey Center, Lincoln,

NE. <http://www.statlab.iastate.edu/soils/nssc/

field gd/fieldgd.pdf>.

Otis, R.J. 1997. Considering Reaeration. In Ninth

Northwest On-Site Wastewater Treatment Short

Course and Equipment Short Course,

University of Washington, Seattle.

Otis, R.J. 1999. Designing on the Boundaries: A

Strategy for Design of Onsite Treatment

Systems. In Proceedings of the Eighth Annual

NOWRA Conference and Exhibit. National

Onsite Wastewater Recycling Association,

Northbrook, IL.

Otis, R.J. 2001. Boundary Design: A Strategy for

SWIS Designed and Rehabilitation. In Onsite

Wastewater Treatment Proceedings of the Ninth

National Symposium on Individual and Small

Community Sewage Systems. ASAE, St. Joseph,

MI.

Powell, G.M. 1990. Why Do Septic Systems Fail?

Kansas State University Cooperative Extension

Service, Manhattan, KS. <http://

hermes.ecn.purdue.edu:8001/cgi/

convertwq?5862>. Accessed August 6, 2000.

Purdue University. 1990. Steps in Constructing a

Mound (Bed-Type) Septic System. Cooperative

Extension Service, Purdue University, West

Lafayette, IN. <http://www.agcom.purdue.edu/

AgCom/Pubs/ID/ID-163.html>.

Schoeneberger, P.J., D.A. Wysocki, E.C. Benham,

and W.D. Broderson. 1998. Field Book for

Describing and Sampling Soils. U.S.

Department of Agriculture, Natural Resources
USEPA Onsite Wastewater Treatment Systems Manual 5-39

Chapter 5: Treatment System Selection

Conservation Service, National Soil Survey

Center, Lincoln, NE.

Sprehe, T.G. 1997. Onsite Wastewater Management

Practices in the Upper Patuxent Watershed.

Submitted to the Washington Suburban

Sanitary Commission, Laurel, MD, by George,

Miles & Buhr, Hunt Valley, MD.

Tyler, E.J., and J.C. Converse. 1994. Soil

Acceptance of Onsite Wastewater as Affected

by Soil Morphology and Wastewater Quality.

In On-Site Wastewater Treatment: Proceedings

of the Seventh International Symposium on

Individual and Small Community Sewage

Systems, ed. E. Collins. American Society of

Agricultural Engineers, St. Josephs, MI.

U.S. Department of Agriculture (USDA), Soil

Survey Staff. 1993. Soil Survey Manual.

USDA handbook no. 18. U.S. Government

Printing Office, Washington, DC.

U.S. Department of Agriculture (USDA), Soil

Survey Staff. 1951. Soil Survey Manual.

USDA handbook no. 18. U.S. Government

Printing Office, Washington, DC.

U.S. Environmental Protection Agency (USEPA).

1984. Land Treatment of Municipal

Wastewaters Process Design Manual—

Supplement on Rapid Infiltration and Overland

Flow. EPA/625/1-81-013a. U.S. Environmental

Protection Agency, Cincinnati, OH.

U.S. Environmental Protection Agency (USEPA).

1981. Land Treatment of Municipal

Wastewaters Process Design Manual. EPA/625/

1-81-013. U.S. Environmental Protection

Agency, Cincinnati, OH.

Vespaskas, M.J. 1996. Redoximorphic Features for

Identifying Aquic Conditions. Technical

bulletin 301. North Carolina Agricultural

Research Service, North Carolina State

University, Raleigh, NC.
Chapter 5: Treatment System Selection

5-40 USEPA Onsite Wastewater Treatment Systems Manual
USEPA Onsite Wastewater Treatment Systems Manual (Click Here to Return to Bookmarks Page) Glossary-1

Glossary

Glossary

Absorption: The process by which one substance is

taken into and included within another substance,

such as the absorption of water by soil or nutrients

by plants.

Activated sludge process: A biological wastewater

treatment process in which biologically active

sludge is agitated and aerated with incoming

wastewater. The activated sludge is subsequently

separated from the treated wastewater (mixed

liquor) by sedimentation, and most of it is returned

to the process. The rest is wasted as needed.

Adsorption: The increased concentration of

molecules or ions at a surface, including exchangeable cations and anions on soil particles. The

adherence of a dissolved solid to the surface of a

solid.

Aerobic: Having molecular oxygen as a part of the

environment, or growing or occurring only in the

presence of molecular oxygen, as in “aerobic

organisms.”

Aerobic treatment unit (ATU): A mechanical

onsite treatment unit that provides secondary

wastewater treatment by mixing air (oxygen) and

aerobic and facultative microbes with the wastewater. ATUs typically use a suspended growth treatment process (similar to activated sludge extended

aeration) or a fixed film treatment process (similar

to trickling filter).

Alternative onsite wastewater treatment system:

An onsite treatment system that includes components different from those used in a conventional

septic tank and drain field system. An alternative

system is used to achieve acceptable treatment and

dispersal/discharge of wastewater where conventional systems may not be capable of meeting

established performance requirements to protect

public health and water resources. (e.g., at sites

where high ground water, low-permeability soils,

shallow soils, or other conditions limit the infiltration and dispersal of wastewater or where additional treatment is needed to protect ground water

or surface water quality). Components that might be

used in alternative systems include sand filters,

aerobic treatment units, disinfection devices, and

alternative SWISs such as mounds, gravelless

trenches, and pressure and drip distribution.

Anaerobic: Characterized by the absence of

molecular oxygen, or growing in the absence of

molecular oxygen (as in “anaerobic bacteria”).

Anaerobic upflow filter: A high-specific-surface

anaerobic reactor filled with a solid media through

which wastewater flows; used to pretreat highstrength wastewater or to denitrify nitrified wastewater.

Biochemical oxygen demand (BOD): A commonly

used gross measurement of the concentration of

biodegradable organic impurities in wastewater.

The amount of oxygen, expressed in milligrams per

liter (mg/L), required by bacteria while stabilizing,

digesting, or treating organic matter under aerobic

conditions is determined by the availability of

material in the wastewater to be used as biological

food and the amount of oxygen used by the microorganisms during oxidation.

Biomat: The layer of biological growth and

inorganic residue that develops at the wastewatersoil interface and extends up to about 1 inch into

the soil matrix. The biomat controls the rate at

which pretreated wastewater moves through the

infiltrative surface/zone for coarse- to mediumtextured soils. This growth may not control fluxes

through fine clay soils, which are more restrictive

to wastewater flows than the biomat.

Blackwater: Liquid and solid human body waste

and the carriage waters generated through toilet

usage.

Centralized wastewater treatment system: A

wastewater collection and treatment system that

consists of collection sewers and a centralized

treatment facility. Centralized systems are used to

collect and treat wastewater from entire communities.

Chemical oxygen demand (COD): A measure of

oxygen use equivalent to the portion of organic

matter that is susceptible to oxidation by a strong

chemical oxidizing agent.
Glossary

Glossary-2 USEPA Onsite Wastewater Treatment Systems Manual

Chlorine residual: The total amount of chlorine

(combined and free available chlorine) remaining in

water, sewage, or industrial wastes at the end of a

specified contact period following disinfection.

Clarifiers: Settling tanks that typically remove

settleable solids by gravity.

Class V injection well: A shallow well used to

place a variety of fluids at shallow depths below the

land surface, including a domestic onsite wastewater

treatment system serving more than 20 people.

USEPA permits these wells to inject wastes below

the ground surface provided they meet certain

requirements and do not endanger underground

sources of drinking water.

Clay: A textural class of soils consisting of particles

less than 0.002 millimeters in diameter.

Cluster system: A wastewater collection and

treatment system under some form of common

ownership and management that provides treatment

and dispersal/discharge of wastewater from two or

more homes or buildings but less than an entire

community.

Coliform bacteria: A group of bacteria predominantly inhabiting the intestines of humans or other

warm-blooded animals, but also occasionally found

elsewhere. Used as an indicator of human fecal

contamination.

Colloids: The solids fraction that is described as the

finely divided suspended matter that will not settle

by gravity and is too large to be considered dissolved matter.

Compliance boundary: A performance boundary

with enforceable performance limits (through an

operating permit).

Consistence: Attribute of soil expressed in degree

of cohesion and adhesion, or in resistance to

deformation or rupture. Consistence includes the

resistance of soil material to rupture; resistance to

penetration; the plasticity, toughness, or stickiness

of puddled soil material; and the manner in which

the soil material behaves when subjected to compression. General classifications of soil consistence

include loose, friable, firm, and extremely firm.

Constructed wetland: An aquatic treatment system

consisting of one or more lined or unlined basins,

some or all of which may be filled with a treatment

medium and wastewater undergoing some combination of physical, chemical, and/or biological

treatment and evaporation and evapotranspiration

by means of macrophytes planted in the treatment

medium.

Construction permit: A permit issued or authorized by the regulatory authority that allows the

installation of a wastewater treatment system in

accordance with approved plans and applicable

codes.

Continuous-flow, suspended-growth aerobic

system: A typical activated sludge process.

Conventional onsite system: A wastewater treatment system consisting of a septic tank and subsurface wastewater infiltration system.

Decentralized system: Onsite and/or cluster

wastewater systems used to treat and disperse or

discharge small volumes of wastewater, generally

from dwellings and businesses that are located

relatively close together. Decentralized systems in a

particular management area or jurisdiction are

managed by a common management entity.

Denitrification: The biochemical reduction of

nitrate or nitrite to gaseous molecular nitrogen or an

oxide of nitrogen.

Digestion: The biological decomposition of organic

matter in sludge, resulting in partial gasification,

liquefaction, and mineralization.

Disinfection: The process of destroying pathogenic

and other microorganisms in wastewater, typically

through application of chlorine compounds, ultraviolet light, iodine, ozone, and the like.

Dissolved oxygen (DO): The oxygen dissolved in

water, wastewater, or other liquid, usually expressed

in milligrams per liter (mg/L), parts per million

(ppm), or percent of saturation.

Dissolved solids: The fraction of solids dissolved in

water.

Drain field: Shallow, covered, excavation made in

unsaturated soil into which pretreated wastewater is

discharged through distribution piping for application onto soil infiltration surfaces through porous

media or manufactured (gravelless) components
USEPA Onsite Wastewater Treatment Systems Manual Glossary-3

Glossary

placed in the excavations. The soil accepts, treats,

and disperses wastewater as it percolates through

the soil, ultimately discharging to groundwater.

Effluent: Sewage, water, or other liquid, partially

or completely treated or in its natural state, flowing

out of a septic tank, subsurface wastewater infiltration system, aerobic treatment unit, or other

treatment system or system component.

Effluent filter (also called an effluent screen): A

removable, cleanable device inserted into the outlet

piping of the septic tank designed to trap excessive

solids due to tank upsets that would otherwise be

transported to the subsurface wastewater infiltration

system or other downstream treatment components.

Effluent screen: See Effluent filter.

Engineered design: An onsite or cluster system

that is designed to meet specific performance

requirements for a particular site as certified by a

licensed professional engineer or other qualified

and licensed or certified person.

Environmental sensitivity: The relative susceptibility to adverse impacts of a water resource or

other environments that may receive wastewater

discharges.

Eutrophic: A term applied to water that has a

concentration of nutrients optimal, or nearly so, for

plant or animal growth. In general, nitrogen and

phosphorus compounds contribute to eutrophic

conditions in coastal and inland fresh waters,

respectively.

Evapotranspiration: The combined loss of water

from a given area and during a specified period of

time by evaporation from the soil or water surface

and by transpiration from plants.

Fixed-film wastewater treatment system: A

biological wastewater treatment process that

employs a medium such as rock, plastic, wood, or

other natural or synthetic solid material that will

support biomass on its surface. Fixed-film systems

include those in which the medium is held in place

and is stationary relative to fluid flow (tricking

filter), those in which the medium is in motion

relative to the wastewater (e.g., rotating biological

disk), and dual process systems that include both

fixed and suspended biomass together or in a

series.

Graywater: Wastewater drained from sinks, tubs,

showers, dishwashers, clothes washers, and other

non-toilet sources.

Hydraulic conductivity: As applied to soils, the

ability of the soil to transmit water in liquid form

through pores.

Laminar: Used to describe flat, sheet-like ground

water flows that migrate laterally along the upper

surface of a confining layer of soil or rock.

Management entity: An entity similar to a

responsible management entity, but managing a

limited set of management activities (e.g.,

homeowners’ association, contracted provider of

management services).

Management services: Planning, design, permitting, inspection, construction/installation, operation, maintenance, monitoring, enforcement, and

other services required to ensure that the wastewater treatment performance requirements established

by the regulatory authority are achieved. Management services should be provided by properly

trained personnel and tracked by means of a

comprehensive management information system.

Mineralization: The conversion of an element

from an organic form to an inorganic state as a

result of microbial decomposition.

Mottling: Spots or blotches of different colors or

shades of color interspersed with the dominant soil

color caused in part by exposure to alternating

unsaturated and saturated conditions.

Nitrification: The biochemical oxidation of

ammonium to nitrate.

Nonconventional onsite wastewater treatment

system: System using technologies or combinations

of technologies that are used where conventional

onsite treatment systems cannot meet established

performance or prescriptive requirements because

of limiting site conditions. Also referred to as

Alternative onsite wastewater treatment systems.

Onsite wastewater treatment system (OWTS): A

system relying on natural processes and/or mechanical components that is used to collect, treat,

and disperse/discharge wastewater from single

dwellings or buildings.
Glossary

Glossary-4 USEPA Onsite Wastewater Treatment Systems Manual

Operating permit: A renewable and revocable

permit to operate and maintain an onsite or cluster

treatment system in compliance with specific

operational or performance requirements.

Organic nitrogen: Nitrogen combined in organic

molecules such as proteins and amino acids.

Organic soil: A soil that contains a high percentage

(more than 15 to 20 percent) of organic matter

throughout the soil column.

Package plant: Term commonly used to describe

an aerobic treatment unit serving multiple dwellings or an educational, health care, or other large

facility.

Particle size: The effective diameter of a particle,

usually measured by sedimentation or sieving.

Particle-size distribution: The amounts of the

various soil size fractions in a soil sample, usually

expressed as weight percentage.

Pathogenic: Causing disease; commonly applied to

microorganisms that cause infectious diseases.

Ped: A unit of soil structure such as an aggregate,

crumb, prism, block, or granule, formed by natural

processes.

Perched water table: The permanent or temporary

water table of a discontinuous saturated zone in a soil.

Percolation: The flow or trickling of a liquid

downward through a contact or filtering medium.

Performance-based management program: A

program designed to preserve and protect human

health and environmental resources by focusing on

the achievement of specific, measurable performance requirements based on site assessments.

Performance boundaries: The point at which a

wastewater treatment performance requirement

corresponding to the desired level of treatment at

that point in the treatment sequence is applied.

Performance boundaries can be designated at the

discharge point of the pretreatment system (e.g.,

septic tank, package plant discharge to surface

waters), at physical boundaries in the receiving

environment (impermeable strata, ground water

table), at a point of use (ground water well), or at a

property boundary.

Performance requirement: Any requirement

established by the regulatory authority to ensure

future compliance with the public health and

environmental goals of the community. Performance requirements can be expressed as numeric

limits (e.g., pollutant concentrations, mass loads,

wet weather flows, structural strength) or narrative

descriptions of desired performance, such as no

visible leaks or no odors.

Permeability: The ability of a porous medium such

as soil to transmit fluids or gases.

pH: A term used to describe the hydrogen ion

activity of a system.

Physical boundaries: Points in the flow of wastewater through the treatment system where treatment

processes change. A physical boundary can be at

the intersection of unit processes or between

saturated and unsaturated soil zones. A physical

boundary may also be a performance boundary if

so designated by the regulatory authority.

Plastic soil: A soil capable of being molded or

deformed continuously and permanently by

relatively moderate pressure.

Platy structure: Laminated or flaky soil aggregate

developed predominantly along the horizontal axes.

Prescriptive-based management program:

Program that applies predetermined requirements

such as site characteristics, design standards, and

separation distances to permit or otherwise allow

the operation of onsite wastewater treatment

systems. This type of program requires that proposed

sites meet preset specifications that are perceived

to protect public health and the environment.

Prescriptive requirements: Standards or specifications for design, siting, and other procedures and

practices for onsite or cluster system applications.

Proposed deviations from the specified criteria,

procedures, or practices require formal approval by

the regulatory authority.

Pretreatment system: Any technology or combination of technologies that precedes discharge to a

subsurface wastewater infiltration system or other

final treatment unit or process before final dissemination into the receiving environment.
USEPA Onsite Wastewater Treatment Systems Manual Glossary-5

Glossary

Regulatory authority (RA): The level of government that establishes and enforces codes related to

the permitting, design, placement, installation,

operation, maintenance, monitoring, and performance of onsite wastewater treatment systems.

Residuals: The solids generated and retained

during the treatment of domestic sewage in treatment system components, including sludge, scum,

and pumpings from grease traps, septic tanks,

aerobic treatment units, and other components of an

onsite or cluster system.

Responsible management entity (RME): An entity

responsible for managing a comprehensive set of

activities delegated by the regulatory authority; a

legal entity that has the managerial, financial, and

technical capacity to ensure the long-term, costeffective operation of onsite and/or cluster water

treatment systems in accordance with applicable

regulations and performance requirements (e.g., a

wastewater utility or wastewater management

district).

Sand filter: A packed-bed filter of sand or other

granular materials used to provide advanced

secondary treatment of settled wastewater or septic

tank effluent. Sand/media filters consist of a lined

(e.g., impervious PVC liner on sand bedding)

excavation or structure filled with uniform washed

sand that is placed over an underdrain system. The

wastewater is dosed onto the surface of the sand

through a distribution network and allowed to

percolate through the sand to the underdrain

system, which collects the filter effluent for further

processing or discharge.

Septage: The liquid, solid, and semisolid material

that results from wastewater pretreatment in a

septic tank, which must be pumped, hauled, treated,

and disposed of properly (i.e., in accordance with

40 CFR Part 503).

Septic tank: A buried, preferably watertight tank

designed and constructed to receive and partially

treat raw wastewater. The tank separates and retains

settleable and floatable solids suspended in the raw

wastewater. Settleable solids settle to the bottom to

form a sludge layer. Grease and other light materials float to the top to form a scum layer. The

removed solids are stored in the tank, where they

undergo liquefaction in which organic solids are

partially broken down into dissolved fatty acids

and gases. Gases generated during liquefaction of

the solids are normally vented through the

building’s plumbing stack vent.

Sequencing batch reactor: A sequential suspended-growth (activated sludge) process in which

all major steps occur in the same tank in sequential

order. Sequencing batch reactors include intermittent-flow batch reactors and continuous-flow

systems.

Settleable solids: Matter in wastewater that will not

stay in suspension during a designated settling

period.

Silt: A textural class of soils consisting of particles

between 0.05 and 0.002 millimeters in diameter.

Soil horizon: A layer of soil or soil material

approximately parallel to the land surface and

different from adjacent layers in physical, chemical, and biological properties or characteristics

such as color, structure, texture, consistence, and

pH.

Soil map: A map showing the distribution of soil

types or other soil mapping units in relation to the

prominent physical and cultural features of the

earth’s surface.

Soil morphology: The physical constitution,

particularly the structural properties, of a soil

profile as exhibited by the kinds, thickness, and

arrangement of the horizons in the profile and by

the texture, structure, consistence, and porosity of

each horizon.

Soil structure: The combination or arrangement of

individual soil particles into definable aggregates,

or peds, which are characterized and classified on

the basis of size, shape, and degree of distinctness.

Soil survey: The systematic examination, description, classification, and mapping of soils in an area.

Soil texture: The relative proportions of the various

soil separates (e.g., silt, clay, sand) in a soil.

Soil water: A general term emphasizing the

physical rather than the chemical properties and

behavior of the soil solution.

Subsoil: In general, that part of the soil below the

depth of plowing.
Glossary

Glossary-6 USEPA Onsite Wastewater Treatment Systems Manual

Subsurface wastewater infiltration system

(SWIS): An underground system for dispersing and

further treating pretreated wastewater. The SWIS

includes the distribution piping/units, any media

installed around or below the distribution components, the biomat at the wastewater-soil interface,

and the unsaturated soil below.

Topsoil: The layer of soil moved in agricultural

cultivation.

Total Kjeldahl nitrogen (TKN): An analytical

method for determining total organic nitrogen and

ammonia.

Treatment system: Any technology or combination

of technologies (treatment trains or unit processes)

that discharges treated wastewater to surface

waters, ground water, or the atmosphere.

Unsaturated flow: Movement of water in a soil that

is not filled to capacity with water.

Vegetated submerged bed: A constructed wetland

wastewater treatment unit characterized by anaerobic horizontal subsurface flow through a fixed-film

medium that has a growth of macrophytes on the

surface.

Water quality-based performance requirement: A

specific, measurable, and enforceable standard that

establishes limits for pollutant concentrations or

mass loads in treated wastewater discharged to

ground water or surface waters.

Water quality criteria: A set of enforceable

requirements under the Clean Water Act that

establish measurable limits for specific pollutants

based on the designated use(s) of the receiving

water body. Water quality criteria can be expressed

as numeric limits (e.g., pollutant concentrations or

mass loads) or narrative descriptions of desired

conditions (e.g., no visible scum, sludge, sheens, or

odors).

Water quality standards: A set of enforceable

requirements under the Clean Water Act that

include classification of receiving waters in

accordance with their federal or state designated

use(s), use-based water quality criteria that establish measurable limits for specific pollutants, and

antidegradation provisions to ensure that water

quality is maintained or improved.

Water table: The level in saturated soil at which

the hydraulic pressure is zero.
USEPA Onsite Wastewater Treatment Systems Manual Resources-1

Resources

Resources

USEPA Office Web Sites:

• Home Page - http://www.epa.gov/

• Office of Water - http://www.epa.gov/ow/

• Office of Wastewater Management - http://

www.epa.gov/owm/

• Office of Science and Technology - http://

www.epa.gov/waterscience/

• Onsite/Decentralized Treatment Site – http://

www.epa.gov/owm/decent

• Environmental Technology Verification - http://

www.epa.gov/etv/

• Nonpoint Source Pollution - http://www.epa.gov/

owow/nps/

• Source Water Protection - http://www.epa.gov/

safewater/protect.html

• Surf Your Watershed - http://www.epa.gov/surf/

• Total Maximum Daily Load Program (TMDL) -

http://www.epa.gov/owow/tmdl/

• Underground Injection Control Program (UIC) -

http://www.epa.gov/safewater/uic.html

USEPA Documents:

• Guidelines for Management of Onsite/Decentralized Wastewater Systems - http://www.epa.gov/

owm/decent/downloads/guidelines.pdf

• Constructed Wetlands for Wastewater Treatment

and Wildlife Habitat - http://www.epa.gov/owow/

wetlands/construc/content.html

• Onsite Wastewater Treatment and Disposal

Systems (1980) - http://www.epa.gov/cgi-bin/

claritgw?op-Display&document=clserv:epacinn:5276;rank=3&template=epa

• Response to Congress on the Use of Decentralized Wastewater Treatment Systems - http://

www.epa.gov/owm/decent/response/index.htm

• Small Community Wastewater Systems - http://

www.epa.gov/oia/tips/scwsint.htm

• Wastewater Treatment publications (OWM) -

http://www.epa.gov/owm/secttre.htm

Other Links of Interest:

• American Society of Agricultural Engineers -

http://asae.org/

• American Water Works Association - http://

www.awwa.org/

• American Society of Civil Engineers - http://

www.asce.org/

• Association of State and Interstate Water Pollution Control Administrators - http://

www.asiwpca.org/

• Clean Water Network - http://www.cwn.org/

• Conservation Technology Information Center -

http://www.ctic.purdue.edu/CTIC/CTIC.html

• Consortium of Institutions for Onsite/Decentralized Wastewater Management - http://

www.dal.ca/%7Ecwrs/cdwt/

• Council of State Governments - http://

www.statesnews.org/

• Environmental Council of the States - http://

www.sso.org/ecos/

• Groundwater Foundation - http://

www.groundwater.org/

• National Association of Counties (NACo) - http://

www.naco.org/

• National Association of County and City Health

Officials (NACCHO) - http://www.naccho.org/

• National Association of Home Builders - http://

www.nahb.com/

• National Association of Regional Councils -

http://www.narc.org/

• National Association of Towns and Townships

(NATaT) - http://natat.org/natat/Default.htm
Resources

Resources-2 USEPA Onsite Wastewater Treatment Systems Manual

• National Environmental Health Association

(NEHA) - http://www.neha.org/

• National Environmental Training Center for

Small Communities (NETCSC) http://

www.nesc.wvu.edu

• National Center for Small Communities - http://

natat.org/ncsc/Default.htm

• National Onsite Wastewater Recycling Association (NOWRA) - http://nowra.org/

• National Small Flows Clearinghouse (NSFC) -

http://www.nesc.wvu.edu

• New England Interstate Water Pollution Control

Commission (NEIWPCC) - http://

www.neiwpcc.org

• National Sanitation Foundation (NSF) - http://

www.nsf.org

• Rural Community Assistance Program (RCAP) -

http://rcap.org

• USDA-Rural Utilities Service (RUS) - http://

www.usda.gov/rus/

• USEPA Onsite/Decentralized Wastewater

Toolbox - http://www.epa.gov/owm/decent/

tool_right.htm

Fact Sheets:

• Onsite/Decentralized Treatment Technologies

Fact Sheets - http://www.epa.gov/owm/decent/

tech_right.htm

• Barnstable County, Massachusetts, Department

of Health and Environment - http://

www.capecod.net/alternativeseptic/

• City of Austin, Texas - http://www.ci.austin.tx.us/

wri/fact.htm

• The Municipal Technologies Branch of EPA -

http://www.epa.gov/owmitnet/mtbfact.htm

• National Small Flows Clearinghouse - http://

www.estd.wvu.edu/nsfc/NSFC_ETI.html

• University of Rhode Island Onsite Wastewater

Training Center - http://www.edc.uri.edu/

homeasyst/page11.htm

• University of Minnesota Extension Septic

System Owner’s Guide - http:/

www.extension.umn.edu/distribution/

naturalresources/DD6583.html