US EPA Wastewater Treatment Manual Septic Systems Design Guide: US EPA Onsite Wastewater Treatment Systems Manual - online copy

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Online version of the U.S. EPA Wastewater Manual with supplemental text, annotations, citations, references.

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Onsite Wastewater Treatment
Systems Manual
February 2002

Office of Water
Office of Research and Development
U.S. Environmental Protection Agency


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.


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

Table of Contents

List of Figures 
List of Tables 

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

Chapter 2. Management of onsite wastewater treatment systems  
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

Chapter 3. Establishing treatment system performance requirements
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
3.9 Monitoring system operation and performance

Chapter 4. Treatment processes and systems 
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

Onsite wastewater treatment systems technology fact sheets

1 Continuous-Flow, Suspended-Growth Aerobic Systems (CFSGAS) - local
2 Fixed-film processes - local
3 Sequencing batch reactor systems - local
4 Effluent disinfection processes - local
5 Vegetated submerged beds and other high-specific-surface anaerobic reactors - local
6 Evapotranspiration and evapotranspiration/infiltration - local
7 Stabilization ponds, FWS constructed wetlands, and other aquatic systems
8 Enhanced nutrient removal--phosphorus
9 Enhanced nutrient removal--nitrogen
10 Intermittent sand/media filters  - local - intermittent sand bed septic systems
11 Recirculating sand/media filters  - recirculating sand bed septic systems
12 Land treatment systems
13 Renovation/restoration of subsurface wastewater infiltration systems (SWIS)

Onsite wastewater treatment systems special issues fact sheets

1 Septic tank additives  - local, expanded text and information by InspectAPedia
2 High-organic-strength wastewaters (including garbage grinders ) - local expanded text by InspectAPedia
3 Water softeners local, expanded text and information by InspectAPedia
4 Holding tanks and hauling systems local, expanded text and information by InspectAPedia

Chapter 5. Treatment system selection
5.1 Factors for selecting appropriate system design and size 
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 



Figure 1-1. Conventional onsite wastewater treatment system
Figure 1-2. Typical single-compartment septic tank
Figure 1-3. Onsite treatment system distribution in the United States
Figure 1-4. Fate of water discharged to onsite wastewater treatment systems
Figure 1-5. The watershed approach planning and management cycle
Figure 1-6. Large-capacity septic tanks and other subsurface discharges
Figure 2-1. Onsite wastewater management overlay zones example
Figure 2-2. Process for developing onsite wastewater management
Figure 3-1. Distribution of mean household daily per capita indoor water use
Figure 3-2. Indoor water use percentage, including leakage, for 1,188 data logged homes
Figure 3-3. Daily indoor water use pattern for single-family residence
Figure 3-4. Peak wastewater flows for single-family home
Figure 3-5. Average hourly distribution of total unfiltered BOD5
Figure 3-6. Typical graywater reuse approach
Figure 3-7. Strategy for estimating wastewater flow and composition
Figure 3-8. Plume movement through the soil to the saturated zone 
Figure 3-9. An example of effluent plume movement
Figure 3-10. Soil treatment zones
Figure 3-11. Zinc sorption by clay as a function of pH
Figure 3-12. Example of compliance boundaries for onsite wastewater treatment systems
Figure 3-13. Input and output components of the MANAGE assessment method 
Figure 3-14. Probability of environmental impact decision tree
Figure 4-1. Conventional subsurface wastewater infiltration system
Figure 4-2. Lateral view of conventional SWIS-based system
Figure 4-3. Subsurface infiltration system design versus depth to a limiting condition
Figure 4-4. Raising the infiltration surface with a typical mound system
Figure 4-5. Schematic of curtain drain construction
Figure 4-6. Capacity chart for subsurface drains
Figure 4-7. Pathway of subsoil reaeration
Figure 4-8. Distribution box with adjustable weir outlets
Figure 4-9. Serial relief line distribution network and installation detail
Figure 4-10. Drop box distribution network
Figure 4-11. Various gravelless systems
Figure 4-12. Placement of leaching chambers in typical application
Figure 4-13. Typical pressurized distribution system layout 
Figure 4-14. Pressure manifold detail
Figure 4-15. Horizontal design for pressure distribution
Figure 4-16. Rigid pipe pressure distribution networks with flushing cleanouts
Figure 4-17. Pressure manifold and flexible drip lines prior to trench filling 
Figure 4-18. Emitter discharge rates versus in-line pressure
Figure 4-19. Dripline layout on a site with trees
Figure 4-20. Pumping tank (generic)
Figure 4-21. Profile of a single-compartment septic tank with outlet screen
Figure 4-22. Two-compartment tank with effluent screen and surface risers 
Figure 4-23. Examples of septic tank effluent screens/filters
Figure 4-24. Tongue and groove joint and sealer
Figure 4-25. Underdrain system detail for sand filters
Figure 4-26. Schematics of the two most common types of sand media filters
Figure 5-1. Preliminary design steps and considerations 
Figure 5-2. Performance (design) boundaries associated with onsite treatment systems
Figure 5-3. Subsurface wastewater infiltration system design/performance boundaries
Figure 5-4. Effluent mounding effect above the saturated zone
Figure 5-5. General considerations for locating a SWIS on a sloping site
Figure 5-6. Landscape position features (see table 5-6 for siting potential) 
Figure 5-7. Conventional system layout with SWIS replacement area
Figure 5-8. Site evaluation/site plan checklist 
Figure 5-9. Soil textural triangle
Figure 5-10. Types of soil structure
Figure 5-11. Potential evaporation versus mean annual precipitation
Figure 5-12. Development of the onsite wastewater system design concept
Figure 5-13. Onsite wastewater failure diagnosis and correction procedure


Table 1-1. Typical pollutants of concern from onsite wastewater treatment systems
Table 1-2. Census of housing tables: sewage disposal, 1990
Table 1-3. Estimated onsite treatment system failure rates in surveyed states
Table 2-1. Organizational approaches for managing onsite systems
Table 2-2. Survey of state certification and licensing programs
Table 2-3. Components of an onsite system regulatory program
Table 2-4. Compliance assurance approaches
Table 2-5. Example of functional responsibilities matrix
Table 2-6. Funding options
Table 2-7. Advantages and disadvantages of various funding sources 
Table 3-1. Summary of average daily residential wastewater flows
Table 3-2. Comparison of daily per capita indoor water use for 12 study sites
Table 3-3. Residential water use by fixture or appliance
Table 3-4. Typical wastewater flow rates from commercial sources
Table 3-5. Typical wastewater flow rates from institutional sources
Table 3-6. Typical wastewater flow rates from recreational facilities
Table 3-7. Constituent mass loadings and concentrations
Table 3-8. Residential wastewater pollutant contributions by source
Table 3-9. Wastewater flow reduction methods
Table 3-10. Flow rates and flush volumes before and after U.S. Energy Policy Act
Table 3-11. Wastewater flow reduction: water-carriage toilets and systems
Table 3-12. Wastewater flow reduction: non-water-carriage toilets
Table 3-13. Wastewater flow reduction: showering devices and systems
Table 3-14. Wastewater flow reduction: miscellaneous devices and systems
Table 3-15. Reduction in pollutant loading achieved by eliminating garbage disposals
Table 3-16. Typical wastewater pollutants of concern
Table 3-17. Examples of soil infiltration system performance
Table 3-18. Case study: septic tank effluent and soil water quality
Table 3-19. Wastewater constituents of concern and representative concentrations
Table 3-20. Waterborne pathogens found in human waste and associated diseases
Table 3-21. Typical pathogen survival times at 20 to 30 oC
Table 3-22. MCLs for selected organic chemicals in drinking water
Table 3-23. Case study: concentration of metals in septic tank effluent
Table 3-24. MCLs for selected inorganic chemicals in drinking water
Table 3-25. Treatment performance requirements for New Shoreham, Rhode Island
Table 3-26. Resource listing, value ranking, and wastewater management schematic
Table 3-27. Proposed onsite system performance standards in various control zonesr
Table 3-28. Treatment performance standards in various control zones
Table 3-29. Nitrogen loading values used in the Buttermilk Bay assessment
Table 3-30. Typical laboratory costs for water quality analysis 
Table 4-1. Commonly used treatment processes and optional treatment methods
Table 4-2. Characteristics of typical SWIS applications
Table 4-3. Suggested hydraulic and organic loading rates for sizing infiltration surfaces
Table 4-4. Geometry, orientation, and configuration considerations for SWISs
Table 4-5. Distribution methods and applications
Table 4-6. Dosing methods and devices
Table 4-7. Pressure manifold sizing
Table 4-8. Contingency options for SWIS malfunctions
Table 4-9. Operation, maintenance, and monitoring activities
Table 4-10. Characteristics of domestic septic tank effluent
Table 4-11. Average septic tank effluent concentrations for selected parameters
Table 4-12. Average septic tank effluent concentrations from various commercial establishments
Table 4-13. Septic tank capacities for one- and two-family dwellings
Table 4-14. Watertightness testing procedure/criteria for precast concrete tanks
Table 4-15. Chemical and physical characteristics of domestic septage
Table 4-16. Single pass and recirculating filter performance
Table 5-1. Types of mass loadings to subsurface wastewater infiltration systems
Table 5-2. Potential impacts of mass loadings on soil design boundaries
Table 5-3. Types of mass loadings for point discharges to surface waters
Table 5-4. Types of mass loadings for evapotranspiration systems
Table 5-5. Site characterization and assessment activities for SWIS applications
Table 5-6. SWIS siting potential vs. landscape position features
Table 5-7. Practices to characterize subsurface conditions through test pit inspection
Table 5-8. Example of a total cost summary worksheet to compare alternatives
Table 5-9. Common onsite wastewater treatment system failures
Table 5-10. General OWTS inspection and failure detection process
Table 5-11. Response of corrective actions on SWIS boundary mass loadings


This update of the 1980 Design Manual: Onsite Wastewater Treatment and Disposal Systems (see was developed to provide supplemental and new 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 BarryTonning of Tetra Tech, Inc. Other persons who made significant contributions to the manual includeRobert 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 ofEnvironmental 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 Paskand Tricia Angoli of the National Small Flows Clearinghouse; James Davenport of the NationalAssociation 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; JamesJacobsen 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 Rutledgeof 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; RichardCarr of the World Health Organization; Ralph Benson of the Clermont County, Ohio, G eneral HealthDistrict; Rich Piluk of the Anne Arundel, Maryland, county government; Jerry Nonogawa of theHawaii 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 OnsiteWastewater Recycling Association; Anish Jantrania, private consultant; Larry Stephens of StephensConsultants; 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, andMichael 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 TetraTech edited the manual. The cover was produced by the National Small Flows Clearinghouse.

Review Team Members for the Onsite Wastewater Treatment Systems Manual

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 Jantrania, 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


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 e ffluent 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 controlle d 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 hyd rologic 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 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 courages 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 ecologica l resources.


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 envi ronmental 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, long-term, 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 consumption 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-capac ity 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 developing 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.

Overview of the Onsite Wastewater Treatment Systems Manual

Chapter 1, Background and use of onsite wastewater treatment systems Review of the history and current use of onsite treatment systems, introduction of management concepts, and brief discussion of alternative technologies.
Chapter 2, Management and regulation of onsite wastewater treatment systems Discussion of methods to plan, institutionalize, and manage OWTS programs, including both prescriptive and performance-based approaches. If prescriptive-based management programs are used, parts of this chapter will not apply because the basic functions of prescriptive-based management are more simplified.
Chapter 3, Establishing treatment system performance requirements Discussion of methods for estimating wastewater flow and composition, identifying pollutants of concern and their transport and fate in the environment, establishing performance requirements, and estimating watershed-scale impacts.
Chapter 4, Treatment processes and systems Identification of conventional and alternative OWTS technologies, pollutant removal effectiveness, design parameters, operation and maintenance requirements, costs, and special issues.
Chapter 5, Treatment system selection Discussion of strategies for establishing site-specific performance requirements and performance boundaries based on wastewater flow and composition and site characteristics, selection of treatment alternatives, and analysis of system failure and repair or replacement alternatives.
Glossary Definitions of terms used in the manual.
Resources Selected reference documents and internet resources.



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