EPA/625/R-92/005
September 1992
Manual
Wastewater Treatment/Disposal
for Small Communities
U.S. Environmental Protection Agency
Office of Research and Development
Center for Environmental Research Information
Cincinnati, OH
Office of Water
Office of Wastewater Enforcement and Compliance
Washington, DC
Printed on Recycled Paper
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Notice
This document has been reviewed in accordance with the U.S. Environmental protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
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Contents
Page
CHAPTER 1—INTRODUCTION
1.1 Background 1
1.2 Types of Small Community Systems 2
1.3 Use of Small Community Systems > 2
1.4 Importance of Proper Planning/Management 2
1.5 Organization and Use of the Manual 2
1.6 General References 3
CHAPTER 2—PLANNING AND MANAGEMENT OF A SMALL COMMUNITY WASTEWATER PROJECT
2.1 Introduction 5
2.2 Problem Recognition and Mobilization 5
2.3 Planning 5
2.4 Design 15
2.5 Construction 18
2.6 Startup 21
2.7 Operation 22
2.8 References 23
CHAPTER 3—SITE EVALUATION AND CONSTRUCTION CONSIDERATIONS FOR
LAND APPLICATION SYSTEMS
3.1 Introduction 25
3.2 Treatment and Disposal of Wastewater in Soil 25
3.3 Approach to Site Evaluation , 28
3.4 Site Identification 30
3.5 Site Reconnaissance 31
3.6 Detailed Site Investigations 32
3.7 Construction Considerations 35
3.8 References 36
CHAPTER 4—WASTEWATER CHARACTERISTICS
4.1 Introduction 39
4.2 Residential Wastewater Characteristics 39
4.3 Nonresidential Wastewater Characteristics 41
4.4 Predicting Wastewater Characteristics 44
4.5 Water Conservation and Wastewater Flow Reduction 45
4.6 Pollutant Mass Reduction 51
4.7 Onsite Containment Holding Tanks > 52
4.8 Reliability 52
4.9 Impacts on Soil-Based Treatment and Disposal Practices 53
4.10 References 55
CHAPTER 5—TECHNOLOGY OPTIONS
5.1 Constructed Wetlands 57
5.2 Rapid Infiltration 59
in
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Contents (cont.)
Page
5.3 Stabilization Ponds 62
5.4 Overland Flow 66
5.5 Slow Sand Filtration 69
5.6 Slow Rate Land Application 72
5.7 Subsurface Infiltration 75
5.8 Pressure Sewers 79
5.9 Small Diameter Gravity Sewers 82
5.10 Vacuum Sewers 84
5.11 Mechanical Systems for Wastewater Treatment 88
5.12 Extended-Aeration Activated Sludge 88
5.13 Trickling Rlter and Modifications 91
5.14 Oxidation Ditch 94
5.15 Sequencing Batch Reactors 97
5.16 Sludge Handling Alternatives 100
5.17 Septage Handling Alternatives 104
5.18 References 108
IV
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Figure
List of Figures
Page
2-1 Typical Management Organization for Construction of a Small Community Wastewater Project 18
3-1 Hydraulic Conductivity of Various Soils versus Soil Moisture Tension 26
3-2 Fluid Transport Zones through Soil below Land Application Systems 27
3-3 Example of Stratigraphic Cross Section Constructed from Soil-Boring Log Data 35
4-1 Frequency Distribution for Average Daily Residential Water Use/Waste Flows 39
4-2 Peak Discharge versus Fixture Units Present 44
4-3 Strategy for Predicting Wastewater Characteristics 45
4-4 Selected Strategies for Management of Segregated Human Wastes 52
4-5 Flow Reduction Effects on Pollutant Concentrations 54
5-1 Capital Costs for Wetland Systems 59
5-2 Schematic of a Rapid Infiltration Facility 60
5-3 Schematic of an Overland Flow System 67
5-4 Schematics of Slow Sand Filters 69
5-5 Schematic of a Slow Rate Land Application 72
5-6 Schematics of Subsurface Wastewater Infiltration Systems (SWISs) 76
5-7 Major Components of a Vacuum Sewer System 87
5-8 Schematic of an Extended-Aeration Process 88
5-9 Schematics of Trickling Filter-Solids Contact Processes 92
5-10 Schematic of an Oxidation Ditch Process 95
5-11 Schematic and Stages for a Sequencing Batch Reactor Process 98
5-12 Sand Drying Bed Details ' 101
5-13 Basic Septage Management Options 105
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List of Tables
Table
2-1 Aspects of Wastewater Treatment Management Typically Regulated by the Government
3-1 Design and Treatment Performance Comparisons for Land
Application Systems for Domestic Wastewater
3-2 Typical BOD Loading Rates for Land Application Systems for Treatment of Municipal Wastewater.
3-3 Comparison of Trace Elements in Wastewaters to Recommended Limits for Irrigation Water
3-4 Typical Soil Textures Suitable for Land Application Systems ....
4-1 Summary of Average Daily Residential Wastewater Flows
4-2 Typical Residential Water Use by Activity
4-3 Characteristics of Typical Residential Wastewater
4-4 Pollutant Contributions of Major Residential Wastewater Fractions
4-5 Pollutant Concentrations of Major Residential Wastewater Fractions
4-6 Typical Wastewater Flows from Commercial Sources
4-7 Typical Wastewater Flows from Institutional Sources
4-8 Typical Wastewater Flows from Recreational Sources
4-9 Fixture Units per Fixture
4-10 Selected Wastewater Row Reduction Methods
4-11 Wastewater Flow Reduction—^Water-Carriage Toilets and Systems
4-12 Wastewater Flow Reduction—Nonwater-Carriage Toilets
4-13 Wastewater Flow Reduction—Showering Devices and Systems
4-14 Wastewater Flow Reduction—Miscellaneous Devices and Systems
4-15 Wastewater Flow Reduction—Wastewater Recycle and Reuse Systems
4-16 Additional Considerations in the Design, Installation, and Operation of Holding Tanks
4-17 Potential Impacts of Some Wastewater Modification on Disposal Practices
5-1 Typical Rapid Infiltration System Performance
5-2 Suggested Hydraulic Loading Cycles for Rapid Infiltration Systems
5-3 Design Criteria—Slow Sand Filters
5-4 Typical Slow Rate Land Application Treatment Performance
5-5 Typical Subsurface Wastewater Infiltration System Treatment Performance
5-6 Typical Site Criteria for a Large SWIS
5-7 Typical Hydraulic Loading Rates on Horizontal Soil Infiltrative Surfaces Treating
Domestic Septic Tank Effluent
5-8 Average Installed Unit Costs for Pressure Sewer Mains and Appurtenances
5-9 Average Unit Costs for Grinder Pump Services and Appurtenances
5-10 Average Unit Costs for STEP Services and Appurtenances
5-11 Distribution of Causes for Call-out Maintenance on Selected GP
and STEP Pressure Sewer Projects
5-12 Main Line Design Parameters
5-13 Guidelines for Determining Line Slopes
5-14 Governing Distances for Slopes Between Lifts
5-15 Maximum Flow for Various Pipe Sizes
5-16 Maximum Number of Homes Served for Various Pipe Sizes
5-17 Average Installed Cost for Vacuum Station (Mid-1990)
5-18 Summary of Design Criteria for Extended-Aeration Process
5-19 Typical Component Sizing for Extended-Aeration Plants
5-20 O&M Requirements of Extended-Aeration Facilities
5-21 Summary of Available Design Criteria for the TFSC Process
5-22 Typical Component Sizing of TFSC Plants
5-23 O&M Requirements of TFSC Facilities
5-24 Summary of Design Criteria for the Oxidation Ditch Process
5-25 Typical Component Sizing for Oxidation Ditch Plants
5-26 O&M Requirements of Oxidation Ditch Facilities
5-27 Summary of Available Design Criteria for SBR Process
5-28 O&M Requirements of SBR Facilities
Page
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VI
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Tables (cont.)
Page
5-29 Summary of Information on Sludge Dewatering Beds 101
5-30 Summary of Information on Aerobic Digestion of Sludge 102
5-31 Summary of Information on Lime Stabilizing of Sludge 103
5-32 Summary of Information on Land Application of Sludge 103
5-33 Suggested Design Values for Septage Characteristics 104
5-34 Summary of Information on Co-treatment of Septage at a Sewage Treatment Plant 106
5-35 Factors to be Considered in Evaluating Cotreatment of Septage 106
5-36 Summary of Information on Lime Stabilization of Septage 107
5-37 Summary of Information on Septage Lagoons 108
VII
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Acknowledgments
The preparation and review of this Handbook was undertaken by many individuals. Contract administration was
provided by the U.S. Environmental Protection Agency's (EPA) Center for Environmental Research Information
(CERI).
Primary Authors:
Robert P. G. Bowker - Bowker & Associates, Inc., Portland, ME
George Frigon - Dames and Moore, Annapolis, MD
James F. Kreissl - USEPA-CERI, Cincinnati, Ohio
Richard J. Otis - Ayres/Associates, Inc., Madison, Wl
Contributing Authors/Reviewers:
Steven Berkowitz - N. C. Dept. of Environment, Health, & Natural Resources, Raleigh, NC
Terry Bounds - Orenco Systems, Inc., Roseburg, OR
Peter A. Ciotoli - Weston Environmental Consultants, Washington DC
Brian J. Cooper - Ontario Ministry of Environment, Toronto, Canada
Fred J. Crates - Consultant, Findlay, OH
Rfck Dedman - Boals, Brown, & Dedman, Inc., Munford, TN
Stephen P, Dix - National Small Flows Clearinghouse, Morgantown, WV
Alan M. Dunn - Indiana Board of Health, Indianapolis, IN
David Effert - Virginia Dept. of Health, Richmond, VA
James Gidley - West Virginia University, Morgantown, WV
Ian Gunn - University of Auckland, Auckland, New Zealand
Michael T. Hoover - Agricultural Extension Service, Raleigh, NC
Anish Jantrania - National Small Flows Clearinghouse, Morgantown, WV
David Lenning - Thurston County Public Health and Social Services Dept., Olympa, WA
Randy May - Connecticut Dept. of Envir. Protection, Hartford, CN
David A. Pask - National Small Flows Clearinghouse, Morgantown, WV
Diane G. Perley - NY Dept. of Environmental Conservation, Albany, NY
Thomas Peterson - Bioreclamation Assessment Group, Ft. Collins, CO
Sherwood C. Reed - EEC Envir. Engineering Consultants, Norwich, VT
William A. Sack - West Virginia University, Morgantown, WV
Robert L. Siegrist - Oak Ridge National Laboratory, Oak Ridge, TN
Steven J. Steinbeck - NC Dept. of Human Resources, Raleigh, NC
Allan Townshend - Consultant, Gloucester, Ontario, Canada
E. Jerry Tyler - University of Wisconsin, Madison, Wl
Robert C. Ward - Colorado State University, Ft. Collins, CO
Samuel R. Weibel - deceased, Cincinnati, OH
John T. Winneberger - Consultant, Sante Fe, NM
Kenneth C. Wiswall - B H Environmental, Inc., Annapolis, MD
Kevin M. Sherman - Florida Dept. of Health, Tallahassee, FL
VIII
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Peer Reviewers:
Ronald Frey - Arizona Dept. of Environmental Quality, Phoenix, AZ
Charles Pycha - U.S. EPA-Region V, Chicago, IL
Charles P. Vanderlyn - U.S. EPA-OWEC, Washington, DC
Alfred T. Wallace - University of Idaho, Moscow, ID
Technical Direction:
Randy P. Revetta - U.S. EPA-CERI, Cincinnati, Ohio
IX
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CHAPTER 1
Introduction
1.1 Background
Over the past decade, changes in federal policies have
forced states to play a larger role in financing and admin-
istering public works programs and compelled local gov-
ernments to do more for themselves. A 1990 report by
the U.S. Congress, Office of Technology Assessment
identified several national wastewater treatment prob-
lems common to small communities:
• Absence of economies of scale and low per capita
incomes
• Low level of technical expertise of many operating
personnel
• Limited access to existing advanced technologies
Many small communities are without access to the engi-
neering expertise that would enable them to resolve the
technical problems related to assessing needs, evaluat-
ing technologies, siting facilities, and deciding on action
plans to meet regulations. Furthermore, small, low-
income communities have few alternatives to raising
user fees substantially to cover operating and mainte-
nance costs and to pay debt service.
In many cases, traditional wastewater treatment strate-
gies have been shown to be inappropriate for the physi-
cal and economic characteristics of the small community.
In the past, when public sewer systems were not avail-
able, the only practical alternative was to install individual
onsite wastewater systems that used traditional septic
tank-soil absorption treatment. While these individual
systems still represent a viable wastewater management
option for many small communities, not all situations or
community planning strategies are suited for this type of
disposal system. The current trends in wastewater treat-
ment technology and the adoption of innovative manage-
ment strategies have provided new alternatives and
options for small communities. When carefully evaluated
against actual community needs and available re-
sources, these alternatives can result in a final selection
and implementation of a wastewater management sys-
tem that is responsive to the needs of each community
by providing a balanced approach to cost allocation and
operational responsibilities.
The 1977 Clean Water Act (CWA) and subsequent
amendments provided the first federal recognition that
costs are a major problem in the national program to ad-
dress water pollution. This is especially true for estab-
lished small communities where failing onsite systems
and growing rural population densities necessitate the
development of wastewater management programs to
protect public health. Conventional options of providing
gravity sewers and activated sludge treatment are often
excessively expensive and require significant manage-
ment costs that lead to unacceptably high burdens for
small communities. As a result, the emerging focus for
small community systems has shifted to small-scale sys-
tems that are designed to fit the specific needs of the
community, rather than to provide a standard solution for
all situations. Thus small communities have found that
development of specialized wastewater systems calls for
a well thought-out strategy in the early stages of problem
definition and planning, the generation and evaluation of
options pertaining to system selection and costs, and, fi-
nally, the selection of a management approach that
meets the existing and future needs of all the identified
small community user groups. The improved levels of treat-
ment and long-term economic savings of well-designed
small community systems provide engineers and plan-
ners with increasing confidence for reevaluating waste-
water management strategies for existing and developing
communities and give hope to local officials who formerly
despaired when considering the cost of complying with
environmental regulations.
Historically, although much of the initiative for financing
small community systems had been assumed by the
Federal Construction Grants Program, which provided up
to 85 percent of the funding for construction of wastewa-
ter systems, most small communities were unable to ob-
tain funding due to the reliance on priority lists related to
population. Incentives for development of wastewater
systems were provided primarily through the allocation of
state "set-aside" funds for implementation of small com-
munity wastewater system construction grants. However,
for many communities, the impact of the amendments to
the CWA, the changes in the construction grant program
instituted after 1984, and the implementation of the State
Revolving Fund Program have restricted the availability
of grant assistance and increased the local share of pro-
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j'ect costs. These changes have placed increased pres-
sure on small communities to reduce project costs. This
is usually accomplished through greater use of easy-to-
manage, low-cost technologies and implementation of ef-
fective management strategies with appropriate planning
and operational functions.
1.2 Types of Small Community Systems
Wastewater treatment alternatives for small communities
can be broadly defined under three category groupings
that represent the basic approaches to wastewater con-
veyance, treatment, and/or disposal.
• Natural Systems—that utilize soil as a treatment and
disposal medium, including land application, con-
structed wetlands, and subsurface infiltration. Some
sludge and septage handling systems, such as sand
drying beds, land spreading, and lagoons, are in-
cluded.
• Alternative Collection Systems—that use lightweight
plastic pipe buried at shallow depths, with fewer pipe
joints and less-complex access structures when com-
pared to conventional gravity sewers. These include
pressure, vacuum, and small-diameter gravity sewer
systems.
• Mechanical Systems—that utilize a combination of
biological and physical processes, employ tanks,
pumps, blowers, rotating mechanisms, and/or other
mechanical components as part of the overall system.
These include suspended growth, fixed growth, and
combinations of the two. This category also includes
some sludge and septage management alternatives,
such as digestion, dewatering, and composting sys-
tems and appropriate disposal information.
1.3 Use of Small Community Systems
The appropriate selection and use of the various small
community wastewater management system alternatives
will depend on both the physical characteristics of the site
and the configuration of the service community. There
are many technical alternatives from which small commu-
nities may choose in deciding how to collect and treat
wastewater. Each of the various types of wastewater sys-
tems will be affected by the requirements of the service
community and wastewater treatment system objectives.
Water conservation systems can significantly reduce the
amount of wastewater generated and thus affect the fea-
sibility and cost of different wastewater alternatives. Re-
ducing water use can also lower the day-to-day operating
costs for treatment chemicals and other utilities, such as
the drinking water supply system. In some cases, effec-
tive water conservation programs can permit the use of
smaller, less-expensive treatment facilities.
As a preliminary screening tool for system evaluation, it is
often advantageous to consider under which circum-
stances systems are appropriate and eliminate other
treatment strategies on the basis of inability to meet com-
munity or physical requirements for effective wastewater
treatment. This can be accomplished simply by develop-
ing a generalized list of advantages and disadvantages
for each alternative to assist in the preliminary decision-
making process and to facilitate understanding of appli-
cations of treatment alternatives. To a degree, this
manual assists in this process by listing only technolo-
gies that are useful in small community wastewater sys-
tems; it generally avoids discussion of technologies that
are rarely appropriate for those applications.
The appropriateness of a given technical alternative will
depend largely on the physical site constraints and the
management capabilities of the community. Knowing
when and where to apply a particular technology will also
involve consideration of the regulatory requirements for
each collection and treatment system and the capability
of the community to support a management program tai-
lored to the needs of the affected service groups.
1.4 Importance of Proper
Planning/Management
Many issues and considerations affect the management
approach for small community systems. These consid-
erations and discussions of the planning and manage-
ment alternatives for wastewater systems are presented
in detail in Chapter 2. There are, however, several over-
riding concerns that will play a role in the evaluation of
the management options and the selection of a techni-
cally sound treatment alternative. These include careful
evaluation of all feasible technical and management al-
ternatives against the concerns of the community, treat-
ment objectives, economic capability, and management
entities and regulatory environments that provide the
enabling legislation to conduct the required functions and
activities of the system management. The underlying
theme that must be addressed here is to integrate the
selection criteria with all the relevant components that af-
fect an efficiently functioning wastewater management
program. The management approach must not assume
that innovative treatment alternatives are necessarily
more environmentally sound or easier to operate, nor
that a centralized system will necessarily prove easier to
regulate and manage. The resolution of these issues is
extremely site-specific and community-specific, and can
be addressed only through comprehensive evaluation of
all components of the small community wastewater man-
agement program.
1.5 Organization and Use of the Manual
This manual is designed to guide planners and designers
through the required steps for developing well-conceived
small community wastewater management systems and
to highlight the specific characteristics of the system de-
sign and management functions that lead to the success-
ful implementation of the selected system. The focus of
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the manual is to present information about small-scale
wastewater treatment systems that are appropriate for
the growing suburban and rural fringe-treatment areas
that characterize the environmental needs of small com-
munities. For the purpose of this manual, small commu-
nities generally refers to rural communities of fewer than
3,500 people; however, the population of small communi-
ties referred to can be as high as 10,000.
One of the major goals of this manual is to highlight the
importance of planning and management considerations
in choosing the appropriate system. This information is
presented so that planners, consultants, local elected of-
ficials, or other organizational groups charged with pro-
viding wastewater management can better evaluate a
broad range of appropriate options for small communities
and understand the mechanisms for available manage-
ment alternatives. This information is included to assist
those charged with implementation of small community
wastewater management systems in understanding the
basic working principles and practical limitations of the
various alternatives. It also is intended to facilitate the
planning and the final selection process.
The manual is organized into five chapters. The succes-
sive chapters span the technical considerations, the con-
cept and planning stages of alternative wastewater
systems evaluation, and the implementation of compre-
hensive wastewater management programs. Chapter 2
discusses the planning and implementation of small
community systems, placing special emphasis on roles
and responsibilities and the regulatory approvals re-
quired to initiate wastewater programs. Site suitability
and evaluation are discussed in Chapter 3, linking the
characteristics of the physical and chemical conditions of
site soils and hydrogeology to treatment processes. The
specific characteristics of wastewater (e.g., flow, quality)
and the capability of the wastewater treatment designs in
addressing these characteristics are presented in Chap-
ter 4. Chapter 5 comprises a technical presentation of
various alternatives for wastewater treatment and residu-
als management systems and provides a discussion of
the applications, factors affecting performance, features
of system design and construction, and the operation
and maintenance requirements.
The information presented herein is intended as techni-
cal guidance reflective of sound professional practice.
Before any system is designed and constructed, local
and state authorities should be contacted to determine
the local design requirements for a particular system.
1.6 General References
Many technical manuals and guidelines, as well as spe-
cific guidance from state and federal programs, are avail-
able and should be consulted to assist in the evaluation
and selection of a wastewater management system that
is suited to the needs of a particular community. Several
useful resources are listed below; other more specific
guidance documents are provided in the reference sec-
tions within chapters.
When an NTIS number is cited in a reference, that refer-
ence is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
• 1990 Preliminary draft strategy for municipal wastewa-
ter treatment, Office of Water and Waste Manage-
ment, Washington, DC, January 1981.
• A strategy for small community alternative wastewater
systems, Office of Waste Program Operations, Wash-
ington, DC, December 1980.
• Planning wastewater management facilities for small
community, EPA Office of Research and Develop-
ment, EPA/600/8-80-030, NTIS No. PB91-111064,
August 1980.
• Design manual; on-site wastewater treatment and dis-
posal systems, EPA Office of Research and Develop-
ment, EPA/625/1-80-012, NTIS No. PB83-219907,
October 1980.
• Management of on-site and small community waste-
water systems, EPA/600/8-82-009, NTIS No. PB82-
2C0829, EPA Office of Research and Development,
July 1982.
• Reference handbook on small-scale wastewater tech-
nology, US Department of Housing and Urban Devel-
opment, Office of Policy Development and Research,
Washington, DC, April 1985.
• Rebuilding the foundations: state and local public
works financing and management, OTA-SET-447,
Washington, DC, Government Printing Office, March
1990.
• Handbook of septage treatment and disposal, EPA
Office of Research and Development, EPA/625/6-84-
009, Cincinnati, OH, October 1984.
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CHAPTER 2
Planning and Management of a Small Community Wastewater Project
2.1 Introduction
The planning and management activities of a wastewater
treatment project are so closely intertwined as to make
them at times inseparable and indistinguishable. The
purpose of this chapter is to acquaint the reader with the
planning and management work necessary to complete
the six steps in the typical wastewater treatment project
and to discuss approaches that may be particularly use-
ful in small communities. The six distinct phases or steps
in a wastewater project are:
• Problem recognition and mobilization
• Planning
• Design
• Construction
• Startup
• Operation
2.2 Problem Recognition and Mobilization
The driving force behind a wastewater treatment project
is usually one or several of the following:
• A public or private sector individual or group recog-
nizes the need for action and rallies a significant seg-
ment (not necessarily a majority) of the community to
support the need for a project.
• The courts resolve an action to abate pollution and or-
der remedial measures.
• State and/or federal agencies enforce pollution laws
and/or order remedial measures.
The individual, group, judge, or agency responsible for
creating the political or legal pressure that gets the pro-
ject off the ground and who supports it through its evolu-
tion can be called the project's champion. The need for a
champion is apparent when one considers the potential
impediments to any wastewater project, which include
the following:
• Not all community members contribute to or share the
problem. Those that don't may oppose the project.
• Several community members may see the absence of
wastewater collection or treatment facilities as a method
of growth control.
• The cost of the project and the resultant annual fees
may draw substantial opposition.
• The locations available for a treatment site may draw
neighborhood opposition.
• The point of discharge and the perceived negative ef-
fects downstream may draw opposition from within
and outside of the service area.
There is no formal management and planning scenario to
follow in this phase of the project. Judicial and adminis-
trative rulings evolve out of other processes, and private
sector efforts are directed toward raising community
awareness of the problem and developing a public outcry
for action. Nonetheless, the energy or the authority of the
project champion is key for driving the project through
this phase. The momentum generated and then sus-
tained by the champion through the design phase in
most cases makes the difference between a successful
and unsuccessful project.
2.3 Planning
The formal planning and management of a project begins
at the point where a committee is formed or a governing
body is authorized to study the problem, explore options,
and estimate costs. Management actually precedes plan-
ning in the sense that a formal plan to allocate resources—
personnel, technical expertise, time, and funds—and a
mechanism for measuring progress is required prior to
proceeding.
The objective of the planning phase is to generate a rec-
ommendation to the community that covers the type of
wastewater facilities required, with cost estimates for de-
sign, construction, and operation; a plan for managing
the design, construction, startup, and operation of the fa-
cilities; and a plan for financing facilities development
and operation. If a comprehensive facility recommenda-
tion cannot be generated, then alternative plans and cost
estimates can be offered; but no more than three alterna-
tives should be put forth. The choice of the number of al-
ternatives is arbitrary but the reason for limiting the
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choices is that each option possesses a myriad of im-
pacts for community members to consider. More than a
very few choices will lead to confusion.
At the beginning of the planning phase, three key man-
agement actions must be carried out:
1. Appoint a person or, more typically, a group to carry
out the planning effort.
2. Establish a deadline acceptable to the group and
community for submitting recommendations.
3. Establish a preliminary planning budget.
Once formed, the group assumes management respon-
sibility and establishes intermediate tasks, which may be
in the following sequence:
1. Assemble or determine the whereabouts of all docu-
mentation pertaining to the reasons for the project
and the general area to be served.
2. Familiarize all group members with the wastewater
management problem and the reasons for the project.
3. Determine the need for consultant services.
4. Solicit and engage consultant services, if needed.
5. Arrange for public information/feedback.
6. Document the regulations and laws pertinent to the
project.
7. Identify and evaluate technical alternatives.
8. Compare the costs of the most suitable technical al-
ternatives.
9. Prepare a financing plan.
10. Prepare draft recommendation and a report for public
and regulatory agency comment.
11. Prepare the final recommendation and report.
With the goals, schedule, staffing, and budget estab-
lished, the process of bringing about the small commu-
nity wastewater treatment system can commence.
Accepting as a given the major goal of the planning proc-
ess—the development of a recommended wastewater
collection and treatment plan for the community—and
leaving the appointment of the planning group's mem-
bers to local judgment, the remaining discussion in this
chapter focuses on the 11 intermediate tasks.
2.3.1 Assembly of Documentation
The gathering of pertinent documentation concerning the
wastewater treatment project, which can be performed
by the planning group or by a consultant, is the founda-
tion of any successful planning effort. The information
collected must formally document the community's per-
ception of the problem, how the project was conceived,
and who was involved. Supplemental information should
include:
• U.S. Geological Survey quadrangles, tax assessment
records, flood plain data, and any other maps of the
planning area
• Geological studies of the planning area (especially
those concerning local soils and those that include
aerial photography)
• Government (state and local) studies of sanitary con-
ditions in the planning area
• Local climatological reports and data
• Studies of local surface-water and ground-water qual-
ity and quantity
• Reports on the history and archeology of the area
• State, county, and local planning and development
studies of the area
• Any other data or information that could have an im-
pact on the project
All such information is necessary to assist in document-
ing the existence and extent of wastewater management
problems, to evaluate alternative solutions, and to iden-
tify essential information that is lacking. The data will be
used to establish the layout of sewers and the location of
treatment facilities; to identify areas of historical impor-
tance and flood plains that should be avoided; to charac-
terize the soil and hydrogeological features of the area to
determine potential sites for land-based systems (i.e.,
cluster systems); to establish the proximity of future
roads and subdivisions; to plan for treatment and dis-
charge systems; to identify streams with the greatest flow
and the most appropriate stream class designation; to
make preliminary evaluations of the cost and feasibility of
acquiring sewer easements; and most important, to es-
tablish the project's justification. Legal counsel should be
involved in gathering documentation on the enviromental
impacts of each alternative treatment method considered
so that complete findings can be reported to state and
federal regulatory agencies.
2.3.2 Familiarization of Planning Group
Members with the Nature of the
Wastewater Management Problem
and the Reasons for the Project
Although the subject of wastewater problems and poten-
tial solutions can be very technical, the typical small
community wastewater planner is not expected to have
had special training. A general understanding of the
problems and principles involved is all that is necessary.
This is easily acquired if the group receives assistance
from a regulatory expert or a consultant. Unassisted
groups will require training or will have to be composed
of technically competent individuals.
Familiarization begins during the data assembly process
wherein all members are expected to read the various
documents and reports. Technical reports may require
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interpretation by knowledgeable governmental officials or
consultants. Planning groups with consultant assistance
will usually receive a summation of the existing data in
nontechnical terms. Ultimately it is important that all
group members have a firm understanding of existing
conditions to facilitate their review of the implications and
impacts of remedial alternatives.
2.3.3 Determ ination of the Need for a
Consultant and the Extent of Services
Not all wastewater projects require consultants during
the planning stages. Common determinants of the need
for a consultant are as follows:
• Project size (i.e., the larger the project the more likely
that consultant services will be necessary)
• Project complexity (e.g., does the problem definition or
solution require a hydrogeologist and/or do the only
technically feasible treatment methods require some
type of treatment plant, etc.)
• Project schedule (e.g., consultants working full time on
a project are more efficient than part-time volunteers)
• Availability of volunteer experts as committee members
• Capability of existing management agency, if available
It is difficult to establish rules of thumb regarding the ne-
cessity for professional consulting services during the
planning phase. Advice on the matter can be secured
from state, local, and county regulatory agencies that
have an interest in the project. Communities that can af-
ford to hire consultants will usually benefit from the assis-
tance. However, planning committees must not abdicate
their responsibilities to a consultant. The group is ulti-
mately responsible for making critical decisions concern-
ing the project, and a consultant should serve only to
facilitate the work of the group.
Use of a consultant during the planning stages eliminates
potential liability for the performance of the recom-
mended plan. More specifically, if an unassisted nonpro-
fessional group develops a recommended plan that is
subsequently used as the basis for a system design and
the system doesn't work, or it costs substantially more
than anticipated, who is responsible? The use of consult-
ants for final planning and design may provide protection
for the community against the possibility of charges of
substantial system nonperformance, failure, or excessive
cost of the preliminary plan.
2.3.4 Soliciting and Engaging Consultant
Services
Of all the intermediate tasks in the planning process, re-
taining a consultant is the most difficult for small commu-
nities to master, primarily because project group
members will often lack experience in contracting for pro-
fessional services. The process is fraught with pitfalls.
For instance, it can be difficult to define the work to be
done so that all proposals can be evaluated using a com-
mon measure. And it may not be easy to identify the
firms with the most experience in technologies pertinent
to a community's wastewater problem.
There is no quick resolution to such issues. The task re-
quires diligent effort by all members of the planning
group or consultant selection subcommittee. Where fed-
eral grants or loans are involved, federal procurement
regulations should be followed closely. Similarly, special
requirements may apply where state funds are used. In
general, five steps should be followed:
1. Prepare a request for proposals (RFP)
2. Advertise or distribute the RFP to selected firms
(e.g., firms can be selected by issuing a request for
qualifications (RFQ))
3. Collect and review proposals to create a short list of
firms to be considered
4. Interview selected consultants from the "short list"
5. Choose a consultant
Establishing a price for consultant services can be ap-
proached in several ways. The fee may be a required
component of the proposal, a separate simultaneous
submittal, or an issue negotiated following consultant se-
lection, or it may be established by a combination of
these methods. Since the legal wording of a contract for
services will vary in form from state to state and locality
to locality, the discussion below concerns the technical
selection process, with limited reference to the fees and
contract language.
2.3.4.1 The Request for Proposals
A request for proposals (RFP) generally begins with a
brief statement of the problem affecting the community
and the events that led the community to initiate the
wastewater planning process. The statement of the prob-
lem and background is followed by a description of the
work that would be performed by the consultant.
Drawing up an RFP requires the planning group to have
become as knowledgeable as possible about community
wastewater problems. The description of work also re-
quires some knowledge of the possible courses of action.
Information can be acquired by consulting regulatory
agencies, studying the responses to a request for qualifi-
cations from interested engineers, interviewing officials
from nearby communities, and conducting library research.
In general the RFP should request that the consultant:
1. Search out and review existing data on the wastewa-
ter project area and the problem
2. Define the problem using available data and perform
additional analyses as necessary
3. Identify alternative technical solutions and prepare
cost estimates
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4. Evaluate and compare the various technical and
management alternatives and costs, and prepare
draft recommendations
5. Conduct a public hearing to solicit community, regu-
latory agency, and planning group comments on the
draft proposals
6. Prepare a final report and recommendation incorpo-
rating the comments received
The statement concerning work to be performed should
also include instructions regarding interactions between
the consultant and planning group. For instance, regular
meetings should be scheduled for the consultant to re-
port on progress and findings. A formal report should be
made to the committee upon completion of each of the
tasks listed above. These early progress reports, which
can be incorporated in whole or in part into the final text,
will give the planning committee opportunities to provide
input, and allow it to perform a quality assurance review
of the work prior to the final document preparation. The
RFP should also request or establish a schedule for the
work. The remainder of the RFP should contain a descrip-
tion of other required submrttal contents, such as informa-
tion on insurance, liability, and contractual requirements.
The form of the RFP, as well as insurance, liability, and
other legal issues, should be reviewed by the community
lawyer before the solicitation is issued. Such a review
should be relatively brief and inexpensive if the section of
the solicitation on the scope of the work to be performed
is well thought out.
2.3.4.2 Proposal Evaluations
In general, responses to requests for professional serv-
ices use the following as a table of contents:
Description of the problem (from the RFP and re-
search)
The proposer's approach to solving the problem
Detailed scope of services to be provided
Schedule of work
Consulting firm's documented experience with simi-
lar projects
Proposed staffing for the project, with resumes
Staff organization for the project
Categories of staff to be assigned to the project and
the estimated time each will expend on the project
(e.g., accounting, engineering, hydrogeology)
Proposed fee or fee schedule (optional)
In describing an approach to the problem, the consultant
should be allowed considerable latitude. In reviewing and
comparing approaches to the project, the reviewers
should look specifically for clues to the consultants' un-
derstanding of the project needs. Often consultants have
their marketing staff produce promotional statements
about the company and staff's abilities. The reviewers,
however, should look for specific language indicating that
the consultant has made an effort to understand the
community's wastewater problems, and determine
whether the suggested approach seems reasonable and
demonstrates the consultant's competence. The thor-
oughness of understanding and completeness of the
suggested approach should be reflected in the proposed
scope of services, which is a detailed presentation of the
consultant's plan of action.
When comparing the scope of services section of pro-
posals, reviewers should consider the level of effort pro-
posed for various stages of the work. Any emphases
proposed should be noted to see if plans correspond to
the project group's expectations. .
The proposed scope of services section, as well as the
approach section, should also be compared with the sec-
tion describing the firm's experience with similar projects.
Does the proposing firm have experience to support its
statements in the approach and scope-of-services sec-
tions? Do the resumes of proposed team members re-
flect the firm's experience with similar projects? It is
necessary with proposals from multilocation firms to de-
termine the work location of proposed team members.
Are they available to perform the work, and how will their
location impact the ability to participate as required?
Modern communication technologies reduce, but do not
eliminate, the need for all team members to be located at
sites convenient to the project. Finally, the proposed
schedule should be evaluated against the planning
group's deadlines and the need to study the various por-
tions of the completed work prior to taking action.
2.3.4.3 Consultant Interviews
The selection committee generally chooses several of
the best proposals to follow up on with further considera-
tion and consultant interviews. Interviews are carefully
scheduled affairs to help differentiate the best proposal
from the others. All interviewees are given the same
amount of time. The interview itself usually begins with a
presentation by the consultant during which he or she is
asked to embellish and expand on the written proposal
contents. The time allowed for presentations should be
relayed to consultants with the short list announcement.
Following the consultant's presentation, the selection
committee asks questions of the consultant to help it dis-
tinguish the one best suited to the committee's needs.
Following the interview process a consultant is selected.
If a fee quote was requested with the proposal and the
fee is acceptable the process can proceed directly to
contract execution. If a price quotation was not required
by the solicitation, price negotiation and contract execu-
tion follow the technical selection process.
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2.3.5 Arrange for Public Involvement and
Feedback
In the past the EPA grants program called this aspect of
the planning process public participation. Because the
primary responsibility of the individual or group desig-
nated to manage the project's development involves
planning, the public involvement process consists mainly
of providing information and getting feedback. That is,
the public advises the planners of its knowledge about
the wastewater problem and provides commentary or
feedback about various proposals as they are developed
and finalized.
It is important to involve the public during the planning
process in order to improve the chance a majority of peo-
ple in the community will accept the planning group's final
recommendations. A plan to encourage public involvement
must be prepared in the early stages of the planning
process and managed throughout. The wastewater plan-
ners must establish lines of communication with the pub-
lic. Relying on the local newspapers and radio stations
and flyers is not enough. The committee must provide
forums that will draw out the public and encourage
comment.
Successful public involvement programs include the
following:
1. Scheduled public meetings that mark important pro-
ject milestones and provide an opportunity for mak-
ing informational presentations by the planning
committee or its consultants and for soliciting public
comment. A typical schedule includes advertised
meetings at the following points:
- At project kickoff
- At completion of draft recommendations
- At completion of final plan recommendations
- At design-phase kickoff
- At completion of design
- At construction contract award
- At any and all major changes in the projects direction
of the project
2. Public outreach to draw diverse individuals or groups
into the planning process
3. Regular news releases or flyers describing the work
and progress of the committees
Public involvement must be managed. Since the plan-
ning committee is charged with developing a recom-
mended plan for wastewater management, various
community groups both for and against the project will try
to ensure that their concerns will be addressed fully and
that their viewpoints will be given prominence. Meetings
must be conducted in such a manner that all factions are
given a hearing. Arrangements for day-to-day contacts
with the local citizenry must be made such that the public
is heard and comments acknowledged. Special interest
groups require additional attention to ensure that mem-
bers are given every opportunity to express their con-
cerns and that their concerns are thoroughly addressed.
Special interest groups with a stake in the project that do
not become actively engaged in the planning process
dialogue should be approached directly and encouraged
to make concerns known. The degree of difficulty and
level of effort associated with this task will depend on the
level of controversy generated by the project. A consult-
ant is often assigned the responsibility of planning public
involvement activities and managing the program.
2.3.6 Document the Regulations and Laws
Pertinent to the Project
The documentation of pertinent laws and regulations is
usually the responsibility of the consultant, when one has
been retained. The information that is required early on
in the planning effort is important for determining the
technical feasibility, a schedule, and the estimated cost
of the project. This task includes the collection of all per-
mit applications required and regulatory agency guidance
documents. A listing of subjects and issues likely to fall
under regulatory control are listed in Table 2-1.
The list in Table 2-1 is not meant to be all-inclusive and
should only be used as a guide to get the regulation-
gathering process started. The regulation of wastewater
treatment varies from state to state and locale to locale.
Planners should inquire about the existence of regula-
tions at all levels of government.
Using information on applicable regulations, a flow dia-
gram can be constructed illustrating the steps that need
to be taken to carry out the project. A schedule for the
completion of the various tasks can be prepared based
upon the requirements for the various approval applica-
tions and the need to perform studies and engineering
assessments as supporting documentation. Accurate
completion of this subtask is important for establishing
design requirements for the next stage of the project.
2.3.7 Identify and Evaluate Technical
Alternatives
With the first six tasks completed or near completion, the
actual planning of the project can begin. The initial array
of options available to the small community is more inclu-
sive than it is for the larger cities. However, a realistic list
of technologies that can be operated by small communi-
ties is somewhat shorter and includes onsite systems,
cluster approaches, and community-wide options. Even
with this more select variety of available options, small
communities can expend valuable time and money pur-
suing and evaluating inappropriate alternatives. Some
communities have dealt with this problem by only consid-
ering only few options with which they are familiar. While
this approach may produce a solution, it may not be one
that is best suited for the long-term interests of the
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Table 2-1. Aspects of Wastewater Treatment Management
Typically Regulated by the Government
Issue
Usual Level of
Government
National Pollutant Discharge
Elimination System permit
Individual septic system regulations
Ground-water discharge standards
Receiving stream classification and
discharge restrictions
Wetland regulation (sewers and
treatment plant siting may impact
wetlands)
Historical preservation (many areas
require historical and archeological
evaluations of proposed public works
sites)
Highway construction requirements
(the regulation or the prohibition of
road excavation can have enormous
impacts on the feasibility and cost of
sewers. Driveway curb cuts for
treatment facility locations is a lesser
problem)
Flood control (construction and siting
requirements in or adjacent to flood
plains can affect feasibility and cost of
projects)
Erosion and sedimentation control
regulations
Navigable waterway construction (the
Army Corps of Engineers regulates all
construction within the waters of the
United States)
Planning and zoning impacts
Procurement standards
federal, state
state, local
state, local
state
federal, state
state, local
state, local
federal, state, local
state, local
federal
local
federal, state, local
Note: Local level may include county, township, or parish.
community. Suggested criteria are presented below to
assist planning groups to narrow down the list of options.
2.3.7.1 Small Community Systems Must Be
Simple to Operate
Ease of operation is the single most important criterion
for small community wastewater treatment systems. Nu-
merous projects have failed to perform and have cost the
communities they serve large sums expended to correct
problems because planners have overlooked this re-
quirement. This criteria is particularly important because
persons with the training required to operate the more
complex or delicately balanced systems are not readily
available in the general population. Moreover, once
trained, small community wastewater system operators
are difficult to retain at rural and suburban pay scales.
Small communities that are situated close to larger cities
and towns may be able to employ on a part-time basis
the operators of larger, more complex facilities. In addi-
tion, service companies with suitable expertise are more
likely to be available to communities located in a metro-
politan area.
The simplest of all systems is the individual on site septic
system, and its simplicity is a reason for its attractive-
ness as a wastewater management option. Persons with
the skills necessary for onsite system maintenance are
generally available even for the most innovative of the
modern systems.
Large subsurface, sand filter, and wetland-based sys-
tems are examples of larger systems that are simple to
operate. Members from the local farming, electrical con-
tracting, plumbing contracting, and excavating contract-
ing communities collectively have the skills necessary to
construct and operate these systems.
Biological and physical/chemical systems require more
than a knowledge of the systems' electronics and me-
chanics to achieve and maintain optimum performance
of all treatment elements. If systems based on these
principles are to be considered, then equal consideration
must be given to the availability and affordability of the
required operations staff.
2.3.7.2 Small Community Systems Must Be
Reliable
All discharging systems and large subsurface systems
must operate within performance standards that are es-
tablished by regulation and are specified in the operating
permit. Violations of these standards may expose the
community to fines or legal action. Metropolitan opera-
tions with seven-day-week, round-the-clock staffing are
monitored constantly to detect operating aberrations that
could lead to a permit violation. In contrast, small com-
munity systems tend to be monitored on an intermittent
basis and therefore must be designed to be resistant to
upsets and to provide days or weeks of advanced warn-
ing when a problem is developing.
The type of warning of impending performance deficien-
cies provided by a system is somewhat related to the
system's complexity. Onsite systems that are properly
monitored can provide advanced notice of elevated con-
taminant levels in ground water or of absorption system
failure months or years in advance. For instance, sand-
filter ponding, which indicates potential problems, occurs
weeks or months before critical conditions occur. Wetland
systems provide days of advanced notice of performance
failure. Biological and physical/chemical systems provide
advanced notice of only several hours, at the most, of po-
tential problems.
Reliability is also an important consideration in the selec-
tion of alternative wastewater collection systems. Many
of the early designs for alternative sewer systems failed
to provide a level of reliability equal to the reliability of
more conventional systems. To a degree, the early prob-
lems were due to a general unfamiliarity with the weaker
10
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links in these systems. Problems also resulted from ef-
forts to shave costs. Also, a consensus has yet to be
reached among the states on corrosion-resistance stand-
ards for electrical components in septic tank effluent
pump systems. Although the atmosphere in these sys-
tems is extremely corrosive, vulnerable metal compo-
nents are still in use. The use of such materials in
pressure and small diameter gravity sewers can result in
premature failure of system components and necessitate
difficult repairs. Reliability in terms of corrosion protec-
tion, structural durability, ease and access of mainte-
nance are therefore crucial concerns.
2.3.7.3 Small Community Systems Should
Be Economical to Construct and
Operate
For years onsite systems have been proposed as a low-
cost, reliable, and simple method of treating wastewater
problems. Experience has shown that the lower cost is
dependent on the perspective of the evaluator. In a com-
munity of 50 homes, for instance, the total capital cost to
replace 10 or even 20 onsite systems is likely to be lower
than to provide collection and treatment facilities for all
50. However, on an individual basis there may be no dif-
ference in capital cost. That is, the homeowner contract-
ing for an onsite repair or a replacement may incur as
much capital cost as he or she would have incurred were
a central system constructed. On the other hand, onsite
systems have shown themselves to have lower O&M
costs than community-wide systems with central collec-
tion and treatment facilities.
Sand filter and wetland' systems have also been pro-
moted as low-capital-cost treatment options. Unfortu-
nately the cost estimates for these systems are a vestige
of the light-duty designs used during their technological
development. The design standards for working versions
of these systems are those used for public works that re-
quire rugged and durable facilities intended for years of
trouble-free service. They must be equipped with liners
to separate them from the surrounding environment and
with dozens of performance, operation, and flow moni-
tors. These added requirements cause these systems to
cost as much if not more to construct than complex bio-
logical or physical/chemical systems; however, O&M
costs are generally low because of the simplicity of op-
eration and reliability of these systems.
Many of the more complex biological- or physical/chemical-
based systems are available to small communities as
complete prepackaged units. Prefabrication renders
these systems very cost-competitive with those of lesser
complexity. The O&M costs for these systems are gener-
ally higher than the lower technology systems because
they require daily monitoring to check the balance and
performance of the various subsystems. The personnel
performing these tasks are required to be more highly
trained than operators of simpler systems, thus salaries
for personnel with the required skills are generally higher.
Collection systems should not be left out of any discus-
sion of system capital costs. In unsewered areas, they
can account for up to 75 percent of the total system's
capital cost, especially if development density is low and
the length of sewer per user is high.
Their attractiveness has been largely attributed to their
eligibility for supplemental funding as alternative collec-
tion systems under the EPA grant program. They remain
attractive in the post-grant era because they offer poten-
tial cost savings in that they require shallower excava-
tions for pipes, smaller pumps, and simpler pump
stations than more conventional systems. The lower cost
of construction because of shallow excavations can pro-
vide savings of up to 50 percent depending on the soil,
rock, and ground-water conditions in the sewered area.
Savings are the result of reduced requirements for shor-
ing, dewatering, and blasting. The small diameter of
these sewers may permit their installation using emerg-
ing methods such as plowing, trenching, and directional
drilling that provide even greater potential construction
cost savings. There are few direct comparisons regard-
ing the maintenance cost differential between conven-
tional and small diameter gravity sewers (SGD).
However, available data indicate that O&M costs can
vary from less to more than,conventional sewer technol-
ogy depending on local conditions (e.g., O&M costs fora
single conventional pump station can equal those of an
SGD system.)
Thus, simplicity of operation, reliability, construction and
operating costs are the considerations to weigh when se-
lecting potential wastewater management alternatives. A
preliminary screening of alternatives will produce a short
list of options deserving of in-depth consideration. The
planning group consultant will generally perform this task
and report to the committee. Unassisted planners can
seek the assistance of state and federal regulators.
Planning groups should be prepared for potential prob-
lems when turning to consultants or regulators for advice.
Very few advisors have had experience with all techno-
logical approaches. As a result, they are prone to recom-
mend familiar processes while questioning the cost and
performance of processes with which they are less famil-
iar. No method guarantees an open-minded assessment
by all evaluators. The planners must be critical in their
consideration of recommendations and representations
to ensure a fair evaluation. The goal of the alternative re-
view is to produce a list, in order of preference, of the tech-
nically appropriate wastewater management systems.
2.3.8 Compare the Costs and
Environmental Impact Assessments
of the Most Suitable Technical
Alternatives
Making system cost comparisons requires that concep-
tual designs be prepared for all viable alternatives. The
level of detail must be sufficient to permit preparation of
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both a cost estimate and an environmental impact as-
sessment (EIA) for the construction as well as the opera-
tion of each system. Cost estimates and ElAs at this
stage are best prepared in concert with public works
staff, consulting engineers, or other experienced profes-
sionals. The ElAs will also require input from state and
federal environmental management staffs familiar with
stream designations and existing environmental regula-
tions that may restrict the discharge location or even the
use of certain alternatives.
The conventional way of reporting and comparing costs
is in terms of the project's present worth. This approach
involves calculating in today's dollars the value of the
project's engineering cost, construction cost, financing
cost, operating cost, and salvage value for a given plan-
ning period (e.g., 20 years). Planners are also advised to
consider the actual construction cost, engineering costs,
and annual operating cost separately. The ranking of the
various options by present-worth cost is likely to gener-
ate a list different from the one generated by a technical
evaluation.
The objective of the cost comparison is to generate a
preliminary financing plan with projected user costs to
connect to the system, to amortize long-term debt, and to
pay for yearly O&M. The financing plan will be of great
interest to the potential users of the system and will have
a significant effect on the public acceptability of the plan-
ners' recommendations.
2.3.9 Prepare a Financing Plan
2.3.9.1 Overview of Financial Management
Sound financial management is crucial to the efficient op-
eration of wastewater systems. It involves estimating ex-
penses and needed revenues, keeping comprehensive
records, and planning for the future. It is not enough for
small community wastewater systems to break even.
Successful systems charge enough so that funds can be
set aside for the system's future needs, such as equip-
ment replacement, line repairs, and emergencies.
Poor financial management of wastewater systems can
lead to:
• Deferred maintenance or repair
• Inability to replace key system components
• Inability to operate the system so that it performs as
designed
• Eventual operational or financial crises
• Violations of wastewater regulations
The components of financial management include:
• General utility planning (anticipating capital and oper-
ating needs)
• Rnancial planning (meeting capital and operating
needs)
• Budgeting
• Cost recovery
• Accounting and information services
Several of these components are discussed in more de-
tail below.
Financial Planning. Financial planning covers two areas
of concern for the small community wastewater system:
capital improvements and operational expenses. The
costs of capital improvements include not only the costs
of materials and construction, but also legal, engineering,
and certain administrative costs, as well as other costs,
such as land acquisition.
The second area of concern of financial planning for
small communities is financing operational expenses,
which are the annual expenses associated with the op-
eration of the facility. These include:
• Operations, maintenance, and most administrative
costs
• Annual debt service expenses
• Financial reserves
The goal of financing operational expenses is to make
the wastewater system self-supporting. Reliable sources
of income should be found that will match existing and
projected expenses for operation, maintenance, equip-
ment replacement, and loan repayment costs.
Budgeting. Most utility budgets are prepared on an an-
nual basis. The budget should reflect the major activities
of the utility, including detailed information regarding the
revenues and expenses of the facility.
Typical Revenue Accounts
• User service charges
• Hookup/impact fees
• Taxes/assessments
• Interest earnings
Typical Expense Accounts
• Administration
• Wages
• Benefits
• Electricity
• Chemicals
• Fuel and utilities
• Parts
• Equipment replacement fund
• Principal and interest payments
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The budgetary needs of each utility are unique. The in-
formation contained in the budget should be repre-
sentative of the information contained in the general
utility plan and the financial plan. Financial statements
should be prepared on a monthly basis and compared to
the budget. The budget should be revised if the projected
costs and revenues are significantly different from the ac-
tual costs and revenues.
Cost Recovery. In order to maintain the facility as a self-
supporting entity, the operating revenues must be suffi-
cient to meet all system costs. The following steps are
necessary to develop a cost-recovery system:
• Identify all system costs
• Compile system data as a basis for allocating costs
among users
• Classify system users, individually or in groups, to al-
locate costs in proportion to their use
• Estimate annual revenue needs
• Establish rates to meet annual revenue needs
• Develop billing procedures according to use, and for
collection of user charges
A healthy system will have a positive cash flow as shown on
the annual income statement. To stay financially healthy, a
wastewater system must:
• Identify and recover all expenses
• Institute and maintain an effective financial manage-
ment system that includes:
- A utility planning process
- A financial planning process
- Effective annual budgeting
- Effective cost recovery
• Complete annual audits and act upon the recommen-
dations of the auditor
2.3.9.2 Financing Alternatives
Various financing alternatives exist for small communi-
ties, including:
• Community system reserves
• Farmers Home Administration loan/grant programs
• Community development block grants (CDBGs)
• Municipal facilities revolving loan funds (also referred
to as State Revolving Fund loans)
• Bond issues
• Local assessments and local option sales taxes
• Rate increases
When evaluating these financing resources, the impacts
of the financing option upon the system user and future
budgets should be explored. A selection of these financ-
ing mechanisms are discussed below.
User Service Charges. The user charge system is the
central and most important component of a revenue plan,
since it usually accounts for 80 to 90 percent of total
revenues. The user charge system has two components:
setting the user rates and collecting them. The checklist
that follows lists the type of user information that should
be established.
Key revenue criteria for EPA-funded wastewater treat-
ment facilities include:
• Charge each user in proportion to the quantity and
quality of the discharge
• Notify the user of rates annually
• Impose surcharges for industrial or commercial waste-
waters that require additional treatment
• Establish a financial management system to account
for revenues and expenses
User Service Charges Checklist Is This Done at Your
Utility?
Yes
No Unsure
All costs are identified
Costs are allocated proportionately
based on use
Flow characteristics are known for
each customer class
Each customer's use is known or
fairly estimated
Customers are billed in proportion
to use
Billing cycle provides timely
revenues
Established procedures ensure
collection of delinquent bills
Federal Grants and Loans. The Farmers Home Admini-
stration (FmHA) provides loans and grants to rural com-
munities of up to 10,000 people for wastewater treatment
facilities. These loans and grants can be used to build,
repair, improve, or change a facility according to commu-
nity needs. The loans have a maximum term of 40 years
(or a shorter term if specified by state law). Interest rates
may be as low as 5 percent for borrowers that meet spe-
cific criteria (EPA, 1983; EPA, 1984).
FmHA loans are intended for communities that are finan-
cially sound but cannot obtain funds from other sources
at reasonable rates (EPA, 1983). Grants covering up to
75 percent of a project's cost may be made to help the
most financially needy communities reduce the repay-
ment burden on the system's users (EPA, 1984). Eligible
applicants include municipal and county governments,
public service districts and authorities, other nonfederal
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public bodies, Native American tribal organizations, and
broad-based community nonprofit corporations. Priority is
given to local public bodies providing service to commu-
nities of 5,500 or less or to projects meeting certain other
criteria establishing urgent need.
FmHA may participate with other federal or public serv-
ice agencies in jointly funding projects. Community facil-
ity loans are administered by FmHA district offices, but
application information is available at local FmHA offices.
Other federal agencies such as the Economic Develop-
ment Administration and the Department of Housing and
Urban Development provide grants and/or loans to com-
munities for various purposes, including wastewater sys-
tems. County extension agents or regional planning
agencies are good sources of information on such
programs.
State Revolving Fund (SRF) Loan Program. SRFs are
available to towns for purposes such as constructing
wastewater treatment facilities. Loan repayments go di-
rectly back into the fund to be loaned to other communi-
ties (EPA, 1988).
Under the Federal Water Quality Act of 1987, EPA pro-
vides each state with startup money to establish a revolv-
ing loan fund or with money to add to an existing loan
fund for wastewater facilities. As of January 1992 all
states had established SRF programs.
Each state's revolving fund program is slightly different.
Some programs limit assistance to communities with
poor or no credit ratings. Others base their assistance on
such factors as the affordability of the project, public
health benefits, and the potential for economic develop-
ment. Programs also vary according to maximum loan
amount, percentage of total project cost eligible for a
loan, interest rate, and duration of the loan. Some states
simply fund projects on a first-come, first-served basis,
relying exclusively on the community's ability to repay.
Thus the state must be contacted to see how its revolv-
ing loan fund works (EPA, 1988). Each state must estab-
lish legal mechanisms to ensure that the SRF and all
repayments are used in a manner consistent with the
Federal Water Quality Act (EPA, 1989).
There are several advantages and disadvantages asso-
ciated with SRFs. Perhaps the biggest advantage is that,
if implemented properly, the program can become self-
sufficient using loan repayments as a source for future
loans. This reduces future dependence on state funds
and bonds as a funding source. In addition, the use of
loan repayments as a funding source can make a state
program independent of less-reliable revenue streams,
such as federal grants and annual state appropriations
for which potential problems exist. The state interest-rate
ceiling, however, may limit the interest charged to bor-
rowers, potentially limiting levels of future loans. Also,
loan repayments may not flow back in sufficient amounts
to meet program needs in a timely fashion (EPA, 1984).
Bond Issues. Bonds are the most common means of
securing funds for the construction of wastewater treat-
ment facilities. Like a home mortgage, bonds extend pay-
ments for new facilities over a period of 10 to 30 years.
Bond proceeds are primarily used as a source of funds
for bond banks or direct loan programs. They have been
used for leveraging monies in revolving loan funds or
providing grants and interest rate subsidies. Leveraging is
a technique by which bonds are issued to be repaid from
other financing sources in order to provide project funding
sooner than would otherwise be possible (EPA, 1984).
The source of bond payments to investors depends on
the type of bond that has been issued (another term for
bond payments to investors is debt service, which simply
means the interest payments on the bonds and bond
principal as bonds are retired). The three common bond
types are:
• General obligation bonds, which are secured by the
state's full faith and credit and taxing ability; payments
are made directly from the state's general fund.
• Revenue bonds, which are secured by the revenues
generated by project operation. Thus debt service is
paid from project revenues. In the case of traditional
revenue bonds issued by communities to build single
projects, project revenues are made up of user
charges. In the case of a state program that funds
many different projects, project revenues may include
loan repayments from communities and special reve-
nues such as dedicated taxes and EPA capitalization
grants.
• Double-barreled bonds, where debt service payments
may also come from a combination of sources: taxes
and project revenues. In this case, the bonds are se-
cured first by project revenues, then by the full faith
and credit of the state, should project revenues not be
sufficient to repay the bonds. This type of bond is gen-
erally not available to small communities.
Regardless of the bond type used, bonds always work in
combination with another funding source that will be
dedicated to repayment.
General obligation and revenue bonds are traditionally
used by state governments to obtain long-term funds for
the construction of capital facilities such as wastewater
projects because they provide statewide benefits for pre-
sent and future users. Bonds may not be the appropriate
source for program startup or operating funds, however, be-
cause the need for repayment diverts funds from direct finan-
cial assistance and because these costs cannot be recovered
to make future debt service payments (EPA, 1984).
State Bond Banks. Government bonds are backed
either by the general taxing power of the issuing govern-
ment (i.e., general obligation bonds) or by continuing
sources of revenue such as sewer user charges (reve-
nue bonds) (EPA, 1983).
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Most state bond programs act as go-betweens for mu-
nicipalities and the national bond market. These pro-
grams help many small towns that cannot issue bonds,
or if they can, they must pay high interest rates to attract
investors. A state's typically high credit rating allows it to
issue bonds at relatively low interest rates. Bond banks
act in two ways: they may either guarantee local bond
issues or they may actually buy local bond issues.
Not all states have bond banks and those that do have a
variety of alternative arrangements. State offices must be
contacted for additional information about eligibility, how
to apply, and terms.
2.3.10 Prepare Draft Recommendation and
a Report for Public and Regulatory
Agency Comment
Completion of the preceding steps permits the selection
of a wastewater management approach to recommend to
the community. All the previously collected data and
evaluations should be assembled in a document known
as a facility plan that is released for public and regulatory
agency review. Comments from all parties are solicited.
The usual forum for public comment is a public hearing.
Of the several junctures in the wastewater system plan-
ning process where formal public interaction takes place,
the public hearing on the recommended plan of action is
the most critical. The effort put into preparing for this
meeting will be reflected in the ease in which the project
proceeds. Insufficient preparation can create mistrust
and doubt among the public and lead to a lengthy and
potentially costly effort to restore support and confidence.
Aspects of the planning process that should be brought
out in the facility plan include:
• Problem definition
• Thoroughness in the number of alternatives evaluated
• Accuracy in the development of conceptual plans
• Accuracy of cost estimates
• Justification of all charges
• Environmental impacts of the proposed system (e.g.,
benefits and potential problems for all media)
If the plan's foundation is thoroughly established, the rec-
ommendations usually can be modified or changed read-
ily in accordance with feedback from the public and from
agency representatives.
2.3.11 Prepare Final Recommendation and
a Report
Following a well-prepared public hearing on the facility
plan, public and regulatory agency comments are ad-
dressed and a final plan prepared. This is then released
for public review. A meeting of the representatives of the
affected community or the sponsoring community is sub-
sequently held for the express purpose of voting on ac-
ceptance or rejection of the proposed plan. If the public
hearing on the draft plan was well attended and the plan
well prepared, it is likely that public comment at this
meeting will be limited.
2.3.12 Summary
The planning scenario presented is subject to modifica-
tion depending on the state and locality in which the
wastewater project is intended to be built. For instance,
the facility plan may be put to a referendum vote at a
polling station in some communities. Nevertheless, some
expression of community acceptance is necessary to
complete all planning phases. If public participation/liai-
son has been effective, the likelihood of community sup-
port is greatly increased.
Many projects never get beyond a majority vote in oppo-
sition to the plan. If the planning process has imparted
enough momentum to the project, it is likely to get into
the next phase. At this point the energy of the project
champion is often essential.
The major reason for a project stalling after the vote is in-
adequate funding to carry on. This impediment looms larger
now that the federal grant program has been curtailed.
2.4 Design
The first step in the implementation of planning recom-
mendations is the preparation of a formal engineering
design. From this point in the process on, paid consult-
ants and contractors are generally used because of the
expertise required to prepare the technical plans, specifi-
cations, and legal papers required to secure regulatory
approval and for use as construction documents.
The primary work of the community during this phase is
the management of consultant selection, work progress,
and budgets. Although the formal planning phase will
have been completed, during this and subsequent
phases the project group must continue to plan for the
eventual startup and operation of the facility.
The group given responsibility for these activities may be
made up of the same or most of the same people who
participated in the planning effort or may be an entirely
different group depending on the management recom-
mendations developed in the planning phase. The design-
phase manager is expected to complete the following tasks
in achieving the major goal, the production of plans and
specifications:
• Consultant selection
• Budget management
• Schedule management
• Work quality management
These tasks are discussed below. Planning for opera-
tions is a component of the work quality management
task and will be dealt with in that discussion.
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2.4.1 Consultant Selection
The consultant solicitation process for this phase is simi-
lar to the process followed in the planning phase. How-
ever, the request for proposals and the response should
be much more focused. Specific areas on which consult-
ant's proposals will be evaluated are:
Experience
Approach
Scope of
Work
• Schedule
What is the firm's experience with the
type of project and technology being
proposed, and what is the experience
of employees proposed for the work?
This narrative should provide insight
into the depth of a consultant's knowl-
edge of the subject and whether there is
a sharing of the community's philosophy.
This is the consultant's specific work
proposal. Does it answer the RFP and
cover all the known isstfes thoroughly?
Does it suggest activities that may have
been omitted in the RFP?
Does it take into account RFP-imposed
deadlines? Does it allow for community
review at critical points? Does it antici-
pate reasonable times for community
and regulatory review?
• Budget and Is the price considered fair for the
Billing scope of work proposed? How is pro-
gress to be measured and billed?
Other nontechnical requirements such as indemnities, li-
ability insurance, bonds, and guarantees should be es-
tablished with advice of the community's counsel.
Proposal review and final selection are the same as in
the planning phase.
2.4.2 Budget Management
Management of the project's budget is a relatively
straightforward task, although special emphasis should
be put on scrupulous recordkeeping. Recordkeeping to
the specifications of funding agencies is particularly im-
portant.
Assessing the value of the consultant's work and review-
ing bills is an important fiduciary responsibility of the
management team. The manager should be thorough in
evaluating bills and statements of work. He should bear
in mind that the relationship with the consultant is defined
by the contract.
Requests for change orders from the consultant should
be compared with the contract to confirm that the work
was not originally included in the contract.
2.4.3 Schedule Management
Most project milestones will have been established by
the section in the RFP on the scope of work. However,
practical steps can be taken to ensure that deadlines are
met. The most effective action is the scheduling of regu-
lar project meetings for monitoring progress and main-
taining momentum. Such meetings should be held at
least monthly but may need to be held more frequently
on smaller or fast-paced projects. The minutes of these
meetings will serve as a formal record of all parties' ac-
tivities and document discussions and statements,
should a record be needed for use in audits or dispute
negotiations.
2.4.4 Work Quality Management
Although drawing up construction contract documents is
the responsibility of the consultant, the community pro-
ject managers have a responsibility to monitor the quality
and quantity of the work. Monitoring, however, is not eas-
ily accomplished by a group that may have limited tech-
nical expertise and practical experience with wastewater
systems. There are two reviews that can be performed
by the project managers, with the help of consultants, to
ensure that the principal consultant's work meets the in-
tent of the design and additional charges are kept to a
minimum. These reviews are:
• Peer Engineering Review
• Constructability Review
These reviews are generally performed by outside pro-
fessionals, at an added cost to the project. The potential
long-term savings in dollars and the benefits in terms of
peace of mind are more than worth the cost. Potential
consultants for the peer engineering review can be iden-
tified by questioning state and local regulators about the
recognized experts in the technologies proposed for the
project.
The objectives of the peer review are to answer the fol-
lowing questions:
• Is the design capable of performing as intended (i.e.,
consider present vs. design population analyses)?
• Has the engineer omitted required treatment steps?
• Are the design parameters accurate?
• Are there redundant or needless treatment steps or
equipment?
• Does any of the equipment selected lack sufficient du-
rability to serve the community adequately?
• Does the equipment or process require special or
costly maintenance?
• Can the same treatment objectives be met in a less
costly manner?
Even community wastewater management systems em-
ploying unsophisticated technologies can benefit from
peer review. Consider, for example, a proposal to repair
or replace individual septic systems in a community.
Questions the reviewer may address for a simple septic
tank alone are:
• How are watertight pipe connections to be made?
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• How are tank sections to be sealed?
• Is the cleanout access sufficient for proper mainte-
nance?
• Is the chosen tank material the best for the conditions
in the locality? Would plastic, fiberglass, or concrete
be better?
• Are any protective coatings required inside or outside
the tank?
• Are the materials specified for gas baffles, tees, risers,
or covers durable and not easily corroded by septic
sewage?
• Will the tank be susceptible to flotation at any time?
• Are the tank backfill requirements suitable?
• What provisions have been made to prevent settling of
the inlet and outlet pipes.
• Is the tank properly vented?
There are many questions to be asked about the other
septic system components as well. Large or more com-
plex systems have an even greater need for scrutiny.
Such questions may not be obvious to all design consult-
ants. Since money is less available for wastewater sys-
tems for small communities and since lower system
costs are achieved by using packaged systems and lo-
cally available or innovative equipment, financial con-
straints promote the selection of lower cost systems.
There are few validated national design standards for
small package systems or for equipment, such as septic
tanks, commonly used in small community systems.
Many package systems employ patented components for
which performance data are available only from the
manufacturer. Without third-party verification, such data
should be treated with caution.
The septic tank example illustrates the point regarding
design standards. Septic tanks represent one of the old-
est wastewater treatment technologies. They are manu-
factured throughout the country, primarily using concrete.
Most states lack a structural standard for concrete septic
tank design. While most sewer piping used today is plas-
tic, plastic pipe does not bond to concrete and few stand-
ards for watertight tank-to-pipe connections are known.
Other septic tank materials also have incomplete struc-
tural design requirements. Polyethylene and fiberglass
tank designs, for instance, have had numerous prob-
lems, but thus far no state has adopted a structural
standard.
Peer engineering review then is a matter of prudence
and insurance to reduce the risk to the small community
of using less-expensive and untested technologies.
The constructability review may be performed as part of
the peer review task or separately. It is recommended,
however, that separate advice from experienced contrac-
tors be secured for maximum benefit.
The object of a constructability review is to determine if
the system plans or specifications contain requirements
that are impossible or too costly to implement and for
which there are lower-cost or more feasible substitutes.
An example of the kind of issue addressed by a con-
structability review is the appropriateness of chosen sep-
tic tank construction materials. Assume, for instance, that
the bid date and completion date for an onsite system re-
pair or septic tank effluent collection system project re-
quire work through the spring or wet season. Assume
also that the soils in the community have a large percent-
age of silt and clay and that the water table is high during
the wet season. Finally, assume that concrete septic
tanks are specified. The constructability reviewer in this
case might find the following:
• Most yards are likely to be too soft during the wet sea-
son to support standard septic tank delivery trucks.
• The construction methods required to overcome this
problem are likely to be more expensive than the
methods required during drier seasons.
• Construction during the wet season is likely to cause
greater collateral damage to properties, thus increas-
ing the cost of restoration.
The constructability reviewer might recommend a longer
contract period with a planned shutdown to overcome the
problem and associated increased costs, or might rec-
ommend the use of lighter, more easily handled fiber-
glass or polyethylene tanks.
The potential return on the cost of peer and constructa-
bility reviews varies from project to project. In general,
however, such reviews reduce the potential risk and li-
ability that construction bidders associate with projects
involving unfamiliar technologies and conditions. For ex-
ample, consider the bid prices of septic tank effluent sys-
tems when they were first introduced. The specifications
usually required that the private property on which the
tanks and service laterals were placed be restored to
their original condition or better upon completion. Con-
tractors bidding the projects were faced with the dilemma
of entering into a contract with one party (the community)
to perform work on the land of another party (the home-
owner). The contract requirements were vague in stating
that restoration was to equal preexisting conditions; the
quality of preexisting conditions, however, was a subjec-
tive requirement and subjected the contractor's work to
the judgment of a homeowner with whom the contractor
has no contract. Further, the contracts carried no indem-
nification of the contractor from suits resulting from dis-
putes over the adequacy of landscape restoration. There
were other shortcomings in the early contracts, but these
two glaring omissions resulted in inflated bids to cover
the cost of the risk the contractors assumed or felt that
they were assuming. As contractor experience grew with
17
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successive biddings and contracts were more carefully
crafted, the bid prices of septic tank effluent system con-
struction contracts dropped dramatically.
Had these early projects been peer and constructability
reviewed many of the contract and design provisions that
contributed to contractor apprehension would have been
removed.
Concurrent with the management of the design phase, it
is important that the community management group be-
gin planning for the operation of the facility during this
phase. Although the consultant will be expected to pre-
pare an acceptable O&M manual for the system, input
and review by the community is important for the follow-
ing reasons:
• Wastewater facility operating costs will continue to be
incurred for years after construction is completed. The
design and its effect on these costs should be re-
viewed constantly to ensure that what is planned as
an environmental enhancement does not lead to a
chronic fiscal problem.
• Many operating activities can be designed out of facili-
ties. The level of operator attention required affects
operating costs. The community representatives
should have input into this decisionmaking.
• The community must develop an understanding of the
skills and number of individuals required to operate
and maintain the facility in order to prepare accurate
operating cost projections.
Specific areas that the management committee should
concern itself with are:
• Durability of materials, equipment, finishes
• Availability/access to spare parts
• Access to all components
• Safety provisions and requirements
* Energy requirements
• Life expectancy of equipment
• Requirement for special tools
• Potential disasters and effects
• Emergency operations
• Monitoring and controls
Special knowledge is not required to address these is-
sues. Questions relative to these issues and about po-
tential alternatives to the design should be asked, the
cost impact evaluated, and guidance given to the de-
signer so that the constructed facility is appropriate given
the needs, desires, and finances of the community.
2.5 Construction
The construction phase of the project involves more
groups and individuals and more money over a short pe-
riod than all the other phases combined. The participants
are:
• The community representative
• The design consultant
• The construction manager (optional, function may be
carried out by design consultant)
• The contractor(s)
• The subcontractors
• Funding agencies
• Regulatory agencies
Management of this group would be difficult enough were
they all linked contractually. Unfortunately, they are
linked in various ways that could allow for chaos if the
group is not carefully managed. A typical organization
chart is presented in Figure 2-1.
Regulatory
Agency
Community
Representative
Construction
Contractor
Funding
Agency
Construction
Manager
Design
Consultant
Subcontractor
Figure 2-1. Typical Management Organization for Con-
struction of a Small Community Wastewater Project.
The management functions during this phase include:
• Paying the project bills
• Maintaining records of funds expended
• Ensuring that money will be available when needed
• Functioning as a public liaison
• Reviewing and approving change orders
• Making cash-flow projections
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• Monitoring the schedule and status of the work to be
sure that work complies with the funding agencies'
conditions
• Monitoring the work to ensure that the terms and con-
ditions of regulatory approvals are met
• Preparing progress reports as may be required by
funding and regulatory agencies
• Monitoring the quantity and quality of the work
• Resolving disputes as they arise
Even though much of the detailed owner responsibility
may be delegated to a construction manager, the con-
struction period requires substantial effort on the part of
the community representative. The primary demands are
processing payment requests, attending periodic (monthly)
job meetings, reviewing change orders, functioning as a
public liaison, facilitating dispute resolution, and earring out
operational planning.
2.5.1 Functioning as a Public Liaison
Public liaison responsibilities of the community repre-
sentative include conducting a preconstruction commu-
nity meeting and responding to questions from the public
throughout the construction period. The use of all avail-
able local media for providing progress updates and noti-
fication of public hearings is vital to success.
The preconstruction public meeting can often be com-
bined with the design completion meeting. In relative
terms, the preconstruction public meeting is the more im-
portant of the two meetings, especially when a sewer
system is to be constructed or upgraded as part of the
project. Sewer construction brings the public into direct
contact with the construction contractor since it involves
working on public roads. Sewer construction tends to
slow traffic and leaves roads in disrepair for extended pe-
riods. Projects may also dry up wells and interrupt cable-
TV service, electrical power, or water. Construction also
may require that individuals make arrangements with a
contractor for connection to the wastewater collection
system.
Preconstruction public meetings for treatment systems
primarily serve to inform the public of project activity and
to maintain a high level of awareness.
The public meeting should include the following elements:
1. Summation of project history
2. Description of the planned facilities and their ex-
pected performance
3. Description of the construction process, schedule,
and anticipated effects on the community
4. Introduction of project participants, including the
contractor, construction manager, and community
representative
5. Designation of contact representatives for the vari-
ous participants. Designation of the types of com-
plaints and concerns for which each participant is
responsible. Distribution of worksite address and
phone numbers for responsible participant repre-
sentatives and after-hours emergency phone numbers.
The matter of jobsite contacts for the public deserves ad-
ditional comment. Many communities designate one per-
son or entity as the point of contact to which the public is
directed to refer all complaints and inquiries. This is rec-
ommended, if only because it permits the establishment
and maintenance of a tracking system that ensures a re-
sponse to all public contacts.
2.5.2 Change Order Review
It is rare that a project will be executed precisely as
planned or as bid, and project changes always have a
cost associated with them. Therefore the evaluation and
negotiation of change orders are an important manage-
ment function of the community representative. If the de-
velopment of change orders is delegated to a construction
manager, then review of the manager's recommendation
is an important function.
Although a detailed discussion of change order proce-
dures is beyond the scope of this volume, a few major
concepts include the following:
« If a change is unavoidable, don't delay approval.
Some communities consume valuable time trying to
associate the change with a regulatory requirement or
design oversight in order to avoid the cost by transfer-
ring liability. This process only delays the contractor
and can add more cost to the contract than the value
of the change because of the costs of delay. The job
requirements should be addressed quickly. Liability or
other concerns should be addressed separately.
• Get expert help when negotiating costs. Contractors
generally know their actual costs since their business
depends on it. Small communities don't contract for
wastewater systems everyday and may be at a disad-
vantage at the negotiating table. Therefore the com-
munity is advised to employ expert assistance when
valuing changes to the work. A construction manager
usually provides this service. If the manager recom-
mends a price for a change, be sure backup calcula-
tions and an explanation are provided.
• Don't let haggling over the price delay the work. The
community should keep in mind that time is money
and any delay to the project adds cost. When the de-
lay is caused by the owner, the cost is passed on by
the contractor.
Finally, well-designed and specified projects will have
fewer unforeseen problems. If the total value of change
orders exceeds 5 percent of the construction budget, the
planners and designers have not performed well overall.
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2.5.3 Processing Payment Requests
The prompt review and payment of contractor requisi-
tions is the single and simplest action the community
representative can take to ensure a smooth construction
project. Cutoff dates for contractor billings should be co-
ordinated with community finance board meetings to en-
sure quick payment. The community must never transfer
Its cash-flow problems to the contractor by delaying his
payment; for instance, when loan payments, grant pay-
ments, or other funding has not been received. The com-
munity must update cash outflow and inflow plans
regularly and take necessary steps to ensure timely con-
tractor payments.
2.5.4 Attendance at Progress Meetings
The job progress meeting is a formal procedure estab-
lished to bring all project participants together on a regu-
lar basis to discuss all aspects of the work project. It is
the main mechanism by which the community and its
representatives keep up with the details of the project.
The meeting should have a formal agenda and detailed
minutes should be kept. Attendees should include:
• The community representative (the owner)
• The contractor
• The design consultant(s)
• The construction manager
• The funding agency representative (if possible)
• Regulatory agency representatives (if possible)
• Utility representatives
• Traffic control representatives
• Police
• Interested citizens
A typical agenda might include:
Acceptance of previous meeting minutes
Schedule review
1. Describe work completed since previous meeting
2. Compare total work to date against contractor schedu le
3. Compare contractor payments to date against pro-
jected cash flow
4. Agree what action if any is necessary to correct in-
congruities in the proposed versus actual work and
cash-flow schedules
5. Agree on revised schedules and cash-flow projections
6. Describe and discuss in detail activities that will oc-
cur in the next period
Shop drawing review
All projects have a list of detailed equipment descrip-
tions and component configurations that are required
to be submitted and approved before the contractor
proceeds with certain work. The list of submittals and
their approval status is compared to the completed
and proposed work. Problems relative to the ade-
quacy of the contractor's submissions and the timeli-
ness of the review by the design consultant or
construction manager are resolved.
Change order review
The change orders issued, both pending and antici-
pated, are reviewed for their content and cost. Al-
though the monthly meeting is not the proper forum
for negotiation, it can be used to move intractable is-
sues because all the interests involved are generally
represented at the table. Change order review allows
the owner to keep a current total of the cost of pro-
ject extras.
Review of jobsite visitations
All projects have a constant stream of official and un-
official visitors and inspectors. These visitors often
converse with and question the inspectors, foremen,
laborers, and operators on the job. These informal
contacts may result in important findings and direc-
tives being lost or not communicated to the responsi-
ble individuals. This review attempts to overcome
problems caused by lack of communication. One of
the most important sources of comment is com-
plaints or questions from the citizenry.
Review of jobsite problems
This review addresses past and anticipated con-
struction problems and is an advantageous forum for
their resolution because of the array of interests pre-
sent. The discussion can save the owner many dol-
lars otherwise lost to lack of productivity and in
potential change orders. However, even more impor-
tant than the construction-related problems, this seg-
ment of the meeting permits exposure of work
problems associated with friction among the project
participants so that they can be dealt with before
they turn into full-blown disputes.
2.5.5 Facilitating Dispute Resolution
Disputes arise in all construction projects. The rapidity
and skill with which they are addressed have a major im-
pact on the cost and outcome of the project. All construc-
tion contracts have a dispute resolution clause. These
clauses, however, should be reserved for the most egre-
gious disputes, after all efforts to reach a compromise
and conciliation have failed.
Disputes can usually be attributed to some omission, in-
consistency, error, or vagueness in the plans or specifi-
cations for the project. They can also arise from friction
between project participants.
When the community representative becomes aware of a
dispute he or she must take action immediately and per-
20
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severe until a resolution is achieved. While the art of dis-
pute settlement is beyond the scope of this work, there
are a few pertinent points to consider.
1. Put the disputed issues in writing and get the oppos-
ing parties to agree on the points. This single act will
facilitate dispute resolution by allowing the issues to
stand out.
2. Get the disputants to talk with each other to resolve
their differences.
3. Set deadlines for action. Don't let the dispute fester
for a long period.
4. Get actively involved in the negotiations only when
the impasse is judged to be permanent.
5. Take action. In almost all cases the least costly reso-
lution is readily available.
6. Document the actions taken and the reasons for
them.
2.5.6 Operational Planning
Operational planning for small community systems con-
tinues through the construction phase. This effort has
two main components:
1. The assembly of a thorough and usable O&M manual.
2. The preparation of a thorough startup plan.
The technical preparation required for an O&M manual is
usually performed by the design consultant. The con-
struction contractor supplies the O&M manuals for the fa-
cility's individual components, such as valves, flow
meters, and generators. It is the responsibility of the
community representative to review and evaluate these
documents for their accuracy, clarity, and completeness.
Technical assistance is available from regulatory agen-
cies and operations consultants. It should not be left to
the system operations staff to discover O&M manual
shortcomings long after the consultants and contractor
are gone.
Startup planning is an area that is often overlooked. Most
small community systems are designed to serve pre-
viously unsewered areas. As such, flow to the treatment
plant has usually not begun upon completion of the pro-
ject. As the house-by-house connections to the system
are made, flow increases from zero to approach design
levels. This scenario presents two startup problems:
1. How is the finished treatment system to be evaluated
for compliance with design requirements without
flow?
2. Since most treatment systems are designed to oper-
ate within certain flow and contaminant concentration
limits, how will performance—compliance with oper-
ating permits—be achieved with flows and contami-
nant loads outside the design ranges?
Answers to these questions are as varied as the number
of collection and treatment options. The design consult-
ant should be charged with developing a startup plan, ac-
ceptance criteria, and an interim operating plan for the
connection period. Assistance in evaluating the plan is
available from regulatory agencies, other utilities, and op-
erating consultants.
The plan for operational startup must address the issues
of performance and permit compliance during what may
be a protracted period. Physical/biological systems, such
as sand filters, have a relatively short startup period and
a broad operating range. Biological systems using acti-
vated sludge processes have somewhat longer startup
periods; they are dependent upon a supply of wastewa-
ter nutrients to perform properly and have a much nar-
rower operating range. Wetland systems, which are
coming into frequent use in small communities' facilities,
have a very broad operating range but may require a
year or more to develop the necessary plant community.
Sewer systems with small flows may have such long de-
tention times in pump station wetwells that offensive
odors are produced.
The design consultant's plan to address such startup is-
sues might be included in the O&M manual or, since it
may be a one-time process, in a separate document. It is
the community representative's responsibility to be
aware of the requirements for startup and to make the
necessary staffing and resource commitments to carry
out the plan.
In preparation for conditional acceptance and startup, the
community representative should begin to recruit operat-
ing staff early in the construction period. The number of
staff persons and the training requirements should be es-
tablished in a draft of the O&M manual by the design
consultant prior to recruitment.
Once identified, the principal operator should be en-
gaged, placed on the community's payroll, and assigned
to work along with the construction manager and engi-
neers through the end of construction to become familiar
with the physical equipment and the O&M manual. The
earlier the chief operator can be brought onto the project,
the more readily the operator will be able to deal with the
requirements of startup. The need for early engagement
of the operator is dependent on the complexity of the
system and equipment. Each community must determine
its own needs.
2.6 Startup
The construction phase of wastewater management pro-
jects and the startup phase overlap, with acceptance
startup commencing at the announcement that the con-
tractor has completed work and is ready to transfer pos-
session of the facility to the community. At this point all
subcomponents will have been operationally checked.
The acceptance startup will check the operation of all
21
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systems as an integrated unit. Operational startup offi-
cially commences on the day that conditional acceptance
of the facility is established, but actually commences with
the recruitment of competent staff, about midway through
the construction period.
Acceptance startup is the responsibility of the community
representative overseeing construction. Assuming that a
reasonable plan for startup testing has been developed,
this phase of the startup should be a straightforward ex-
ercise. The activity should conclude with the preparation
for the community representative's approval of a startup
and testing report, which compares, on a mechanical per-
formance basis, the requirements of the design report
with the measured performance of the completed facility.
Tank sizes, pump sizes, pump output, blower sizes,
blower output, weir overflow levels are a few of the items
compared. The performance of biological systems is usu-
ally not compared at this time because of the need for
seeding and a steady flow of wastewater to encourage
the ecosystem.
The results of testing should be presented in as simple a
form as possible for the community representative to
evaluate prior to acceptance. All deviations from design
requirements should be explained and an acceptance
recommendation secured from both the design consult-
ant and construction manager. Prior to acceptance, the
community representative should read the document and
understand the reasons for any variation from the design.
The community representative must be aware that accep-
tance of the facility releases much of the financial leverage
that he possesses to ensure compliance with the design re-
quirements—bonds and guarantees notwithstanding.
Operational startup commences on the date of conditional
acceptance of the completed facility. Conditional accep-
tance is the point at which the facility is completed and
ready to be used for its intended purpose with only minor
items (e.g., touchup painting, grass growth) remaining.
Public information and feedback during the startup phase
are usually limited to media announcements about the
startup of the facility or a formal open house. Although not
crucial to the continuance of the project, the open house
provides an appropriate setting to acknowledge the efforts
of the various groups and individuals involved in the project
and to display the fruits of the community's efforts.
2.7 Operation
The operation phase is the long-term continuation of op-
eration after startup. Late in the construction phase or
during startup, the permanent community organization
charged with managing the operation of the facility be-
gins its work.
Management responsibilities include:
• Establishment of annual operating budgets
• Establishment of a preventive maintenance plan
• Establishment of a long-term capital replacement plan
• Monitoring of permit compliance and reporting
• Monitoring of normal O&M
• Monitoring of staff workloads and needs
Planning responsibilities include:
• Regular review and updating of emergency operations
plan
• Preparation and maintenance of capital expansion
plan
Daily operations are carried out by the full- or part-time
operations staff.
The entity responsible for system management should
establish a formal work plan and schedule. Many small,
and even some large, wastewater systems have failed to
perform or survive through their intended life cycle be-
cause of inadequate management attention or the dele-
gation of responsibilities to the operations staff.
It is often difficult for managers to sustain their attention
to well-designed, simple, and reliable systems because
during the early years, after stable operations are estab-
lished, they demand little attention. Management alert-
ness can be maintained by scheduling, at a minimum,
quarterly quality-control and budget reviews. Larger,
more complex systems will require monthly evaluations.
Sample agenda items are listed below.
A. Facility Performance
1. Discharge standards versus discharge performance
2. Establish reasons for noncompliance
3. Establish a corrective-action plan
4. Review trends in facility performance (charts or other
graphics are useful for presenting trends)
a. seasonal
b. long term
c. corrective-action plans
5. Review scheduled maintenance activities
6. Review major unscheduled maintenance activities
B. Staff
1. Actual versus planned labor hours
2. Review training plans
a. operations
b. safety
3. Review licensing status
22
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C. Budget
1. Planned versus actual cash flow
a. maintenance appropriations
b. labor cost
c. capital expenditures
2. Prepare requests for supplemental appropriations
D. Planning
1. Annual budget
2. Capital improvements
3. Staff training
4. System expansion
5. User charge system review (annually)
The operations management group should retain the de-
sign consultant for guidance at least until it is comfortable
with its task. When the EPA grants program was still in
effect, consultants were required to be retained for a
one-year period following startup for the purpose of certi-
fying the successful performance of the project. This ar-
rangement was useful in that it maintained a formal
communications link between the community and the de-
signer that helped resolve operations and management
problems.
In summary, planning for system operations begins dur-
ing the preliminary planning phase of the project,
months, if not years, before operations commence. The
guiding principles are that small community systems
must be simple to operate, reliable, and economical. Ad-
dressing these concerns is then followed by a design ef-
fort during which specific choices are made to implement
the project goals. During construction, an O&M manual is
prepared and operators engaged and trained. Most oper-
ating procedures will have been tested during startup, as
operators are gaining experience. The attention paid and
time devoted to these tasks will greatly enhance the abil-
ity to manage the completed wastewater system.
2.8 References
When an NTIS number is cited in a reference, that refer-
ence is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
EPA. 1992. Small wastewater systems: Alternative sys-
tems for small communities and rural areas. EPA/F-92-001.
EPA. 1989. Environmental Protection Agency. Water and
wastewater managers guide for staying financially
healthy. EPA/430/09-89-004.
EPA. 1988. Environmental Protection Agency. Reference
guide on state financial assistance programs.
EPA/430/09-88-0004.
EPA. 1987. Environmental Protection Agency. It's your
choice: Guide for local officials on small community
wastewater management options. EPA/430/09-87-006.
EPA. 1986. Environmental Protection Agency. Touching
all bases: A financial management handbook for your
wastewater treatment project. EPA/430/09-86-001.
EPA. 1985. Reducing the cost of operating municipal
wastewater treatment facilities. Available from the Na-
tional Small Flows Clearinghouse.
EPA. 1984. Is your proposed wastewater project too
costly? Options for small communities. Available from
the National Small Flows Clearinghouse.
EPA. 1984. User charge guidance manual for publicly-
owned treatment works. EPA/430/9-84-006.
EPA. 1983. Environmental Protection Agency. Managing
small and alternate wastewater system: A planning man-
ual. EPA/430/09-83-008.
EPA. 1979. Environmental Protection Agency. Manage-
ment of small-to-medium-sized wastewater treatment
plants. EPA/430/09-79-013.
23
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CHAPTER 3
Site Evaluation and Construction Considerations for Land Application Systems
3.1 Introduction
3.1.1 Soil as a Treatment and Disposal
Medium
Soil frequently is used to provide both treatment and dis-
posal of wastewaters. It has a large capacity to retain,
transform, and recycle many of the pollutants found in
municipal wastewater. As the wastewater percolates
through the soil to the ground water, physical, chemical,
and biological processes occur to provide a high level of
treatment consistently and reliably. Where properly ap-
plied and operated, soil-based or land application treat-
ment systems have been shown to have relatively low
capital and operating costs. Today several thousand
such systems are being used successfully across the
United States.
Various designs exist for land application systems. They
may apply wastewater either on or below the land sur-
face. Surface application systems include slow rate,
rapid infiltration, and overland flow. Subsurface applica-
tion systems include septic tank systems and subsurface
infiltration systems designed for clusters of homes. The
selection of a design depends on the nature of the
wastewater to be treated, the characteristics of the site,
and regulatory requirements. Except for overland flow,
these designs are "zero discharge" systems, where ulti-
mate disposal of the treated wastewater is to the ground
water, rather than a point discharge to surface water.
Therefore, to protect ground-water quality, the land appli-
cation site must be carefully selected and the system de-
sign appropriately adapted.
3.1.2 Importance of Site Evaluation in
System Performance
Hydraulic and treatment performance of land application
systems are related directly to the soil and site charac-
teristics and the applied wastewater quality. Performance
is measured by the ability of the system to accept and
adequately treat the applied wastewater within the de-
fined system boundaries. The system is commonly de-
fined to include the unsaturated soil below the infiltrative
surface, the permanent ground-water table, and a portion
of the ground water horizontally extending to the property
line. Typically, drinking water standards must be met for
many pollutants before the wastewater/ground-water
mixture leaves the system boundaries.
Site evaluation is the most critical factor in successful
performance of land application systems. It must provide
sufficient information to predict the capacity of the soil to
accept and treat the projected wastewater loading and to
assess how the soil and ground water will respond to that
loading. Design selection, infiltrative surface area, eleva-
tion, geometry, and method of system operation are all
dependent on information gathered during the site evalu-
ation. Most failures of land application systems can be
attributed to inadequate site evaluation and interpretation
of the data collected. It is necessary that the engineer
recognize the importance of a thorough site evaluation
by experienced, qualified professionals.
3.1.3 Importance of Construction
Procedures in System Performance
Soil is a complex physical, chemical, and biological sys-
tem that functions well only when the soil pores remain
relatively open and continuous. Porosity and pore conti-
nuity is essential for the free movement of liquids and
gases within the soil. Construction activities will destroy
many of the pores through compaction, smearing, and
puddling, all of which reduce the hydraulic and treatment
capacity of the soil. Proper construction procedures per-
formed under the appropriate soil moisture conditions will
minimize the damage that can occur.
Soil that has been excessively damaged during construc-
tion cannot be easily repaired. Usually the system must
be redesigned to account for the reduced hydraulic and
treatment capacity of the soil or to avoid use of the dam-
aged soil. Often the site is no longer suitable and must
be abandoned. Therefore, the engineer must consider
construction procedures during design and limit the soil
moisture conditions under which a facility can be built.
3.2 Treatment and Disposal of
Wastewater in Soil
3.2.1 Soil as a Three-Phase System
Soil is a complex, heterogeneous, porous system of par-
ticulate mineral solids and organic matter in which the in-
terfacial surface area is enormously large. It provides a
three-phase treatment system: the solid phase consists
25
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of soil particles; the liquid phase consists of water (con-
taining dissolved substances); and the gaseous phase
consists primarily of soil air (and other gases).
The solid matrix of the soil consists of particles differing
in chemical and mineralogical composition as well as in
size, shape, and orientation. The mutual arrangement of
these particles determines the characteristics of the soil
pores in which water and air are transmitted or retained.
The interaction of these phases in the soil directly im-
pacts the capacity of the soil to accept and treat applied
wastewater.
3.2.2 Water Movement in the Soil System
Water moves in soil through the soil pores in response to
a potential energy gradient. The rate of flow is propor-
tional to the permeability and the potential gradient. Soil
permeability, a measure of the capability of soil to trans-
mit fluids, is determined by the size, shape, and continu-
ity of the pores rather than the porosity of the soil. For
example, clay soil is more porous than sandy soil, but the
sandy soil will transmit much more water under saturated
conditions because "rt has larger, more continuous pores
than the clay soil.
The soil moisture potential consists of several compo-
nents. The primary components of interest in land appli-
cation systems are the gravitational, hydrostatic (positive
pressure), and the matric (negative pressure) potentials.
Water will move in the direction of decreasing potential;
however, its path may not be direct because of the avail-
able flow paths in the porous media.
The gravitational potential is the mass of the water times
the acceleration due to gravity, or the weight of the
water. At any point, it is determined by the elevation of
the point relative to an arbitrary reference level.
The hydrostatic potential is a positive pressure potential
that occurs below the ground-water surface. It is greater
than atmospheric pressure and increases with depth be-
tow the surface due to the weight of water above.
The matric potential is a negative pressure potential cre-
ated by the physical affinity of water for the soil particle
surfaces and capillary pores. Water attempts to wet the
particle surfaces due to adhesive forces between the
water molecule and the surface. Cohesive force between
water molecules pulls other molecules from the bulk liq-
uid—thus a negative pressure, or suction, is created, a
phenomenon referred to as capillarity. The movement of
water stops when the column of water that is being
pulled into a pore is greater than the surface tension at
the air/water surface. Therefore water is pulled further
and held tighter in small pores than large pores, such
that large pores are the first to empty upon draining.
Under saturated soil conditions (ground water), all the
soil pores are filled with water and flow occurs in re-
sponse to the gravitational and hydrostatic potentials,
while under unsaturated soil conditions water movement
occurs in response to the gravitational and matric poten-
tials. The most important difference between saturated
and unsaturated flow is the hydraulic conductivity. Since
all the pores are filled under saturated conditions, flow is
maximal. However, under unsaturated conditions, some of
the pores are air-filled and the conductive cross-sectional
area is reduced. The first pores to empty are the largest
and, therefore, the most conductive. There can be a dra-
matic difference (several orders of magnitude) between
the saturated and unsaturated hydraulic conductivity of
the soil (Figure 3-1).
1000-=
100 —
I 10.
o
O
_o
"5
P 1.0-
JC
0.1 —
245 ="
24.5
, Type I (sand)
§5
2.45
II (sandy loam)
0.24
0.02
III (silt loam)
20 40 60 80 100
Soil Moisture Tension (mbar)
Drying >
Figure 3-1. Hydraulic Conductivity of Various Soils versus
Soil Moisture Tension (EPA, 1980).
In layered soils where the hydraulic conductivities of the
layers differ, water that enters unsaturated soil will move
through alternating unsaturated and saturated soil condi-
tions as it percolates downward. Where a coarse soil
with a higher saturated hydraulic conductivity overlies a
finer textured soil, water will move through the coarse
soil rapidly to the boundary between the two where it will
be slowed to the rate the finer soil can accept. The
coarse soil will saturate above the boundary with the
finer soil, and the rate of water movement through the
soil profile will be controlled by the saturated conductivity
of the finer soil. Similarly, saturated conditions will occur
above the boundary when the finer soil overlies the
coarse soil. However, in this case, the soil suction or ma-
tric potential of the finer soil is greater than the force that
is exerted by the larger pores of the coarse soil. The
26
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water must saturate the finer soil at the boundary before
the matric potential is reduced sufficiently for the water to
continue its downward path. Thus in layered profiles
even a thin layer of soil with a higher or lower conductiv-
ity will impede vertical percolation of water and the rate
of movement through the profile will be controlled by the
soil layer with the lowest saturated hydraulic conductivity.
Land-based systems have failed because seemingly
minute texture changes in the soil profile were ignored or
even overlooked.
3.2.3 Hydraulic Capacity of Soils to Accept
Wastewater
Water transport from land application systems typically
occurs through three zones in the soil: the infiltration
zone, the vadose (unsaturated zone), and the saturated
zone (Figure 3-2). Wastewater enters the soil at the sur-
face of the infiltration zone, a biologically active zone, the
thickness of which will vary with the type of land applica-
tion system used. It acts as a physical, chemical, and
biological filter to remove suspended solids and organics
from the wastewater. The filtered solids accumulate on
the infiltrative surface, within the soil pores, and on the
soil matrix in this zone providing a source of food and nu-
trients for an active biomass. The biomass and metabolic
by-products also accumulate in this zone. Hydraulically,
the infiltration zone is a transitional zone where flow
changes from saturated to unsaturated with a concomi-
tant sharp decline in hydraulic conductivity because of
the blockage and filling of the soil pores by the accumu-
lated solids, biomass, and metabolic by-products.
Below the infiltration zone, the water enters the vadose
zone. In this zone, the water is under a negative pres-
sure potential or matric potential; consequently, flow oc-
curs only in the smaller pores, while larger pores remain
gas-filled. Water transport occurs primarily vertically over
soil particle surfaces and through capillary pores due to
Infiltration
Zone
Vadose
(Unsaturated)
Zone
Ground-Water
Table Surface
Saturated
Zone
—i Capillary
—' Fringe
Figure 3-2. Fluid Transport Zones through Soil below Land
Application Systems.
the gravitational potential, but the matric potential does
cause some dispersive lateral flow.
Below the vadose zone, the water enters the saturated
zone or ground-water table. All the soil pores are filled in
this zone, and flow occurs vertically and/or horizontally in
response to the gravitational and hydrostatic pressure
potentials. It is in this zone that the applied wastewater
ultimately leaves the site. The additional water from the
wastewater treatment system causes a ground-water
mound to form above the ground-water table. This
mound must be taken into account in the hydraulic de-
sign to ensure that a sufficient vadose zone remains for
treatment below the application area.
Soil factors that affect hydraulic performance of the land
application systems include texture, structure, bulk den-
sity, horizonation (stratigraphy, layering), and soil mois-
ture. Texture, structure, and bulk density relate directly to
the size, shape, and continuity of the soil pores. Texture
is defined by the relative proportion of the various sizes
of the soil particulates or soil "separates" (i.e., sand, silt,
and clay), while structure refers to the relative arrange-
ment of the soil particles. Bulk density is the ratio of the
soil mass to its bulk or volume. Finer textured soils such
as silts and clays would be expected to have smaller,
less-continuous pores than sands, but because the parti-
cles can agglomerate to form structural units or peds,
large interpedal pores can exist to enhance permeability.
Soils with higher bulk volumes tend to have lower perme-
abilities because the pore volume is less.
Horizonation (or layering) and soil moisture distribution in
the soil can impact the rate of water movement through
the profile. Soil layers of differing texture, structure,
and/or bulk density can impede vertical percolation be-
cause of their differing hydraulic conductivities. Zones of
higher soil moisture are indicative of reduced conductivity
due to lower matric potentials. High moisture conditions
also limit the movement of air through the soil. Soil aera-
tion is necessary to support aerobic biochemical reac-
tions for rapid degradation of applied wastes.
Factors in the design of land application systems also af-
fect hydraulic performance. These include geometry and
elevation relative to ground surface of the infiltrative sur-
face, uniformity and periodicity of wastewater application,
and the quality of the applied wastewater. Each factor
can affect the aeration status of the soil at and below the
infiltrative surface of the soil due to the oxygen demand
exerted by constituents in the applied wastewater. The
hydraulic capacity of the system decreases if anaerobic
conditions are allowed to occur for extended periods.
3.2.4 Wastewater Treatment Capabilities of
Soil
Physical, chemical, and biological processes occur in the
soil providing effective wastewater treatment through re-
tention, transformation, or destruction of pollutants.
Physical processes include filtration, dispersion, arid dilu-
27
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tion. Volatilization, adsorption, complexation (with or-
ganic residues), precipitation, and photodecomposition
are chemical processes that can occur. Several biologi-
cal processes including biological oxidation (mineraliza-
tion), nitrification, denitriftcation, immobilization, predation,
and plant uptake can take place depending on the envi-
ronmental conditions. These processes occur most effec-
tively when the soil is unsaturated because the
wastewater is forced to percolate over the soil particle
surfaces where many of the treatment phenomena take
place and air is able to diffuse through the soil. In combi-
nation, these processes can produce a treated water of
acceptable quality for discharge to the ground-water ta-
ble under the proper conditions.
Whether these processes occur and their effectiveness
in treatment depends on the physical characteristics of
the soils and the environmental conditions of the soil
through which the wastewater percolates. Studies have
shown that conventional wastewater parameters (e.g.,
biochemical oxygen demand (BODs), suspended solids
(SS), and fecal indicator organisms) are nearly com-
pletely removed where an aerobic, unsaturated zone of
medium-to-fine textured soil, 0.6 to 1.5 m (2 to 5 ft) in
thickness, with neutral pH is maintained below the infil-
trative surface during system operation. Soils with exces-
sive permeability (coarse texture soil or soil with large
continuous pores), low organic content, low pH, low cat-
ion exchange capacity and redox potential, high moisture
content, and low temperature have been shown to have
reduced treatment efficiency.
Other wastewater parameters, such as levels of nitrogen,
phosphorus, heavy metals, toxic organics, and virus, are
removed to varying degrees. Organic or ammonia nitro-
gen is readily and rapidly nitrified biochemically in aero-
bic soil. Because of its high solubility, nitrate leaches to
the ground water. Some biochemical denitrification can
occur in the soil, but without plant uptake, 60 to 90 per-
cent of the nitrate enters the ground water. Under an-
aerobic soil conditions, nitrification will not occur, but the
positively charged ammonium ion is retained in the soil
by adsorption onto the soil particles. The ammonium
may be held until aerobic soil conditions return allowing
nitrification to occur. Phosphorus and metals can be re-
moved through adsorption, ion exchange, and precipita-
tion reactions, but the capacity of the soil to retain these
tons is finite. The capacity varies with the soil mineral-
ogy, organic content, pH, redox potential, and cation ex-
change capacity. The fate and transport of virus is not
well documented. Limited data suggest some types of vi-
rus are able to leach to ground water; however, when
this occurs their infectivity is soon lost. Fine-textured soil,
aerobic subsoils, and high temperatures favor virus de-
struction. Toxic organics appear to be removed in aero-
bic soils, but further study of the fate and transport of
these compounds is needed.
Land treatment system design and operation will also af-
fect the treatment capability of soil. Systems that main-
tain unsaturated, aerobic subsoil conditions provide the
most effective treatment of wastewater. Geometry of the
infiltrative surface, elevation of the surface relative to fi-
nal grade, wastewater mass loading rates, and uniformity
and periodicity of wastewater application influence the
degree of saturation and aeration status of the subsoil.
The loading of oxygen-demanding materials per unit area
must not exceed the rate at which oxygen can be sup-
plied to the subsoil. Where the infiltrative surface is ex-
posed to the atmosphere, mass flow of air will occur
behind the wetting front as the wastewater infiltrates the
soil. Thus the rate of application and the period between
applications are important operating parameters. In sub-
surface systems, however, diffusion from the perimeter
of the system is the primary pathway of oxygen to the
subsoil. Therefore shallow, narrow infiltrative surfaces,
controlled mass loadings, and uniform, periodic applica-
tions will enhance aeration.
The most significant impacts that land application sys-
tems are known to have on the quality of ground water
are increased concentrations of nitrogen (primarily ni-
trate), chlorides, and total dissolved solids; temperature
may also be impacted. Once the treated wastewater en-
ters the ground water, it does not readily mix with the
ground water but remains as a distinct plume for as
much as several hundred feet. Solute concentrations
within the plume can remain above ambient ground-
water concentrations. Dilution of the plume is dependent
on the quantity of natural recharge and travel distance
from the source.
3.2.5 Hydraulic and Treatment
Performance of Land Application
Systems
There are significant differences in design and perform-
ance between the various types of land application sys-
tems because of the differences in application and
operation. A comparison of the systems is presented in
Table 3-1.
3.3 Approach to Site Evaluation
Successful performance of a land application system for
wastewater treatment depends upon the characteristics
of the site on which it is built and how its design and op-
eration are adapted to those characteristics. The design
engineer must understand how the soil, ground water,
vegetation, and other site factors will impact system per-
formance so that design features and operation proce-
dures can be adopted to ensure acceptable performance.
Therefore, careful site evaluation is critical.
The scale and detail of the site evaluation depend on the
quantity and quality of the wastewater and the charac-
teristics of the site to which the wastewater is to be ap-
28
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Table 3-1. Design and Treatment Performance Comparisons for Land Application Systems for Domestic Wastewater
Feature
Site Conditions
Soil texture
Depth to ground water
(m)
Vegetation
Climatic restrictions
Design Loadings
Pretreatment6
Average daily loading
(cm)
Application method
Disposition of
wastewater
Slow Rate
Sandy loams to clay
loams
1.0
Required
Growing seasond
Rapid Infiltration Subsurface Infiltration Overland Flow
Sands, sandy loams
1.0
Optional
None
Sands to clay loamsa Silt loams, clay loams
1.0
Not applicable
None
Not critical0
Required
Growing seasond
Primary sedimentation' Primary sedimentation' Primary sedimentation Primary sedimentation
1.2-1.5
Sprinkler or flooding
Evapotranspiration and
percolation
1.5-10
Flooding
Percolation
0.2-4.0 9
Flooding
Percolation
1.0-6.0
Sprinkler or flooding
Surface runoff and
evapotranspiration
Treatment Performance
BODs (mg/L)
SS (mg/L)
Total nitrogen as N
(mg/L)
Total phosphorus as P
(mg/L)
Toxic organicsj
Fecal conforms
(perl 00 mL)
Virus, log removal
average
Metals (%)
5
5
3-8h
0.1 -0.4
9
< 10
= 3+
High
10
5
10-20i
1 -2
?
<200
= 2
Medium
5
5
25 -351
0.1 -0.5
?
< 10
= 3
High
15
20
5-101
4-5
?
<2000
< 1
Low
aApplies to single or small cluster household systems; larger systems limited to sands and sandy loams (where significant, depth
to top of ground-water mound).
bMinimum separation distance from infiltration surface to highest ground-water mound elevation.
"Critical only if significant percolation occurs.
Application during few weeks before and after growing season.
eMinimum pretreatment requirements.
fWith restricted public access; crops not for direct human consumption.
9Loading based on trench bottom area, not total site area.
hVaries with applied concentration and crop.
Varies with applied concentration.
JData are limited, but good removals (>90%) appear to occur at low application rates in aerobic soils for biodegradable organics,
adsorbed species are removed effectively until the underlying soil column becomes saturated, whereupon removals cease;
volatiles are removed effectively in the unsaturated soil zone if rates are sufficiently low.
plied. Because of the many factors involved, site evalu-
ation for land application systems requires the input of a
multidisciplinary team of qualified engineers, soil scien-
tists, hydrogeologists, and agronomists experienced in
the siting, design, and management of such systems. It
may involve extensive, and in some cases expensive,
site investigations and testing.
To avoid unnecessary effort and expense, the site evalu-
ation should focus on only the most promising sites.
Therefore, the evaluation effort should be phased. A
three-phase approach is suggested.
The first phase includes the identification and screening
of potential sites. Efforts during this phase are limited to
review of available resource materials and the develop-
ment of site screening criteria based on knowledge of the
wastewater source and its characteristics. This phase
concludes with the elimination of sites that do not meet
the established screening criteria.
During phase two, a reconnaissance survey of the poten-
tial sites identified in the first phase is made. A visual sur-
vey and preliminary soil borings are performed to confirm
the potential of each site. This phase also includes a pre-
29
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liminary layout of an appropriate system design to deter-
mine if sufficient suitable area exists to construct the sys-
tem. On the basis of the findings during this phase, the
sites are ranked according to priority for conducting de-
tailed site evaluations.
The final phase is the detailed investigation. It is during
this phase that the critical site characteristics that affect
site suitability and system design and operation are iden-
tified. The investigation typically includes soil profile de-
scriptions, deep soil borings, ground-water assessments
and testing, soil permeability measurements, and topo-
graphic mapping. Only the highest rated site need be in-
vestigated unless the investigation indicates that it
should not be ranked above the others or that it should
be eliminated from consideration. At that point, the next
ranked site should be investigated. This procedure is fol-
towed until a suitable site is selected.
3.4 Site Identification
3.4.1 Wastewater Characteristics
The projected wastewater volume and characteristics im-
pact both site suitability and cost. Site suitability is fre-
quently determined by the estimated hydraulic capacity
of the site to accept the projected wastewater flows.
Where wastewater flow data are not available, per capita
flows of 200 to 400 L/d (50 to 100 gpd) are typically used
to estimate average daily flow. A factor of 1.8 to 2.0 is
often used to estimate the peak daily flows. The parame-
ters used to estimate flows for a particular application de-
pend on the characteristics of the community, the
climate, and the condition of the collection system. If sig-
nificant commercial and/or industrial development exists
in the community, daily wastewater flows should be esti-
mated for each establishment and added to the residen-
tial estimate.
Conventional wastewater parameters such as BOD and
SS seldom limit land application system feasibility or ca-
pacity. (Typical BOD loading rates for municipal waste-
water systems are presented in Table 3-2.) However,
wastewater constituents such as nitrogen, phosphorus,
potassium, chlorides, total dissolved solids, pH, and
trace elements may override hydraulic considerations in
determining land area requirements or selecting a suit-
able land application design.
Table 3-2. Typical BOD Loading Rates for Land Application
Systems for Treatment of Municipal Wastewater3
Technology
Slow rate
Rapid Infiltration
Subsurface
Infiltration
Overland flow
(kg/ha/yr)
370-1,830
8,000-40,000
5,500-22,000
2,000-7,500
(Ib/ac/yr)
2,000-10,000
44,000 -253,000
30,000-121,000
11,000-41,000
Frequently, nitrogen controls system sizing and, hence,
land area requirements. Excessive nitrogen contamina-
tion of potable ground-water aquifers below land applica-
tion systems often is a concern, but this threat can be
controlled by limiting the nitrogen loading or promoting
nitrogen removal through system design and operation.
Nitrate nitrogen concentrations in the ground water at the
project property boundary should not exceed the drinking
water standard of 10 mg-N/L. To meet this limit, the total
nitrogen loading rather than the hydraulic loading may
determine system sizing, potentially increasing land area
requirements. The nitrogen removal potential of the sys-
tem depends on the crop grown, if any, and system man-
agement practices, which vary with the type of system
selected.
In some cases, other wastewater constituents such as
phosphorus or trace elements control system sizing and
design. For example, if wastewater trace element con-
centrations exceed the maximum recommended concen-
trations for irrigation water (Table 3-3), slow-rate land
application systems may not be feasible or may require
special operating conditions. Most land application sys-
tems, however, will be controlled by hydraulic or nitrogen
loadings.
Table 3-3. Comparison of Trace Elements in Wastewaters
to Recommended Limits for Irrigation Water3
Element
Arsenic
Boron
Cadmium
Chromium
Copper
Iron
Lead
Manganese
Mercury
Nickel
Zinc
Untreated
Wastewater"
(ppm)
0.003
0.3-1.8
0.004-0.14
0.02-0.70
0.02-3.36
0.9-3.54
0.05-1.27
0.11 -0.14
0.002 - 0.044
0.002-0.105
0.03 - 8.31
Recommended
Maximum
Concentrations
for Irrigation
Water0 (ppm)
0.1
0.5-2.0
0.01
0.1
0.2
5.0
5.0
0.2
No standard
0.2
2.0
* EPA, 1980 and 1981; WPCF, 1990; Reed, 1988.
aEPA, 1981.
bRange of reported values. See EPA, 1981.
cBased on unlimited irrigation at 1 m/yr (3 ft/yr).
3.4.2 Climate
Local climate may affect system selection and land area
requirements. Precipitation, evapotranspiration potential,
temperature, length of the growing season, wastewater
application cycles and storage requirements, and other
factors will impact land area requirements and the type of
system selected. These factors should be considered as
candidate sites are selected because of the influence
they have on the suitability of a site for land application of
wastewater.
30
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3.4.3 Review of Resource Materials
Many resource materials are readily available to assist in
identifying suitable sites for a land application system.
They should be used to select candidate sites before field
investigations are initiated. The materials include soil sur-
veys, U.S. Geological Survey (USGS) quadrangles, well
logs, geologic maps, hydrologic and ground-water maps,
land-use and zoning maps, and National Oceanographic
and Atmospheric Administration (NOAA) climatic data. In-
terviews with local residents knowledgeable about the area
should also be included in this review.
Soil surveys are available from the local Soil Conserva-
tion Service (SCS) office. They contain a collection of
aerial photographs of the mapping area, usually a county,
on which the distributions of the various soil series are
shown. The scales of the maps usually range from
1:31,680 to 1:15,840. A typical profile description to a
depth of 1.5 m (5 ft) for each soil series is provided. The
descriptions include texture, structure, consistency, color,
rooting, estimated permeability, and thickness of each
horizon. The surveys also provide limited information on
permeability, slopes, landscape position, drainage, erosion
potential, flooding potential, chemical properties, engineer-
ing properties, general suitability for locally grown crops,
and management information. Where published surveys
are not available, field sheets with interpretive tables may
be available through the local county agent. Typical soil
textures and permeabilities suitable for land application
systems that can be used in site identification are pre-
sented in Table 3-4.
USGS quadrangles provide information on topography,
landscape position, rock outcrops, wetlands, regional
drainage patterns, and surface water elevations. These
maps, which are usually drawn to a scale of 1:24,000
(7.5 minute series) or 1:62,000 (15 minute series), can
help identify potential sites based on landscape position,
slope, drainage, and separation from the ground water.
Landscape positions best suited for land application sys-
tems include hilltops, ridge lines, and sideslopes. Depres-
sions and footslopes of hills should be avoided. Slopes
less than 12 percent are generally preferred. Ground-
water gradients and elevations can be inferred from the
elevations shown for surface water bodies.
Well logs are generally available locally and are an ex-
cellent source of information on soil profile and potential
impermeable layers.
Existing land use also is an important factor in selecting
candidate sites. Land use of both the identified parcel
and the surrounding parcels should be determined. Land
use can be tentatively determined from the soil survey
maps, USGS quadrangles, and local zoning maps.
The location of the service area relative to potential treat-
ment sites is an important economic consideration. The
distance and elevation of the treatment site relative to the
service area directly impact wastewater transmission
costs. As a general guideline, sites within 10 to 12 km (6
to 7 mi) and less than 60 m (200 ft) above the elevation
of the pumping station may still be competitive with other
treatment alternatives.
3.4.4 Site Screening
Site screening criteria should be established according to
which potential sites can be identified and ranked. The
key criteria may be broken down into environmental-
physical characteristics, economic and institutional con-
siderations, and other technical and engineering factors.
In developing these criteria, federal, state, and local
regulatory requirements must be considered for any im-
posed limitations such as siting and design requirements,
water quality requirements, and monitoring requirements.
A numerical approach using qualitative factors for the
key selection criteria should be used to rank sites. This
will focus the following site investigation activities on the
sites with the most potential.
Criteria should include distance and elevation from
source, available area, soils, landscape position, slope,
ownership, and land use. The second phase of evalu-
ation activities should be carried out only with the highest
ranked sites.
3.5 Site Reconnaissance
3.5.1 Visual Inspection
Site reconnaissance begins with a visual survey of each
of the sites identified from a review of the resource mate-
rials. The objective of the visual survey is to verify the
site information collected from the resource materials
and to note any general features that may affect site suit-
Table 3-4. Typical Soil Textures Suitable for Land Application Systems3
Soil Parameter
Textura! class*3
Unified soil class0
Technology
Slow Rate
Clay loams to sandy
loams
GM, SM, ML OL, MH,
PT
Rapid Infiltration
Sand to sandy loams
GW, GP, SW, SP
Subsurface Infiltration Overland Flow
Sand and sandy loams Clay and clay loams
GW, GP, SW, SP
GM, GC.SM, SC, CL
OL, CH, OH
aEPA, 1981.
bUSDA-SCS System (SCS, 1951).
cGroup symbols are from ASTM Unified Classification System.
31
-------
ability or system design. It is important to walk the site
noting significant features including topography, land-
scape position, vegetation, land use surrounding the site,
and others that may affect site suitability or system de-
sign and performance.
Site topography impacts system design and surface and
subsurface drainage. Long, planar slopes or large, level
areas allow greater flexibility in system layout than
steeply sloping or hummocky sites. Sloping sites are
usually better drained than large, level sites. Drainage is
an important factor when considering the contribution of
surface runoff to the hydraulic load of the system and the
response of the ground water below the site to system
loading.
Landscape position is an important consideration in site
drainage. Although sites may have favorable topography,
the position of the site relative to the surrounding land
form may impact site suitability. For example, hilltops
and sideslopes can be expected to have good surface
and subsurface drainage, while depressions or
footslopes are more likely to drain poorly. There are
known exceptions to these generalities, such as alluvial
soils in the valleys of arid regions; however, without infor-
mation on the particular locality, these rules of thumb
serve as effective initial planning tools.
The type and extent of vegetation on the site provide an
indication of soil depth and subsurface drainage. It also
may impact site suitability directly. For instance, vegeta-
tion associated with wet soils suggests that subsurface
drainage is not good, and numerous large trees may
make system construction difficult.
Surrounding land use is a necessary consideration. Con-
cerns for the fate of aerosols, odors, and ground-water
drainage from the site must be addressed. Adequate buffer
areas will need to be provided from developed areas.
Other significant features to observe include areas of
flooding, surface water, rock outcrops, wells, roads,
buildings, and buried utilities. Each of these will help to
determine site suitability or system design. Surface water
bodies, for instance, would suggest the need to investi-
gate the potential water-quality and hydrologic impacts of
the system. Rock outcrops would raise concerns about
soil depth. Water supply wells raise concerns about
ground-water flows and potential ground-water-quality
impacts.
3.5.2 Preliminary Soil Borings
Preliminary soil borings should be performed on sites
that appear to be suitable following the visual survey.
The purpose of the borings is to provide information re-
garding suitability and variability of the surface soils and
to identify the presence of any shallow, impermeable lay-
ers. The number and density of these borings should be
made to provide sufficient information regarding the soil
properties and variability necessary to determine whether
more detailed site investigations are warranted. A boring
density of one per 0.2 ha (0.5 ac) is usually adequate to
make this determination. If the soil variability is such that
a greater density of borings is necessary to characterize
the site, consideration should be given to eliminating the
site from further evaluation.
Soil characteristics from the surface to approximately 2 m
(6 ft) below the anticipated infiltrative surface elevation
should be observed. Important characteristics to note in-
clude soil texture, structure, horizon thickness, moisture
content, color, bulk density, and spatial variability.
A hand-held soil probe or auger is sufficient for these
borings unless it is anticipated that the infiltrative surface
of the system will be placed deep within the soil profile. If
a deep system is anticipated, a truck-mounted soil probe
may be necessary. Excavated pits or deep soil borings
typically are not performed during this phase because of
the expense and damage of sites to which no commit-
ment has been made.
3.5.3 Conceptual System Design
While on the site, a conceptual layout of the proposed
land application system should be made based on the
knowledge of the site and projected wastewater flows.
This is done to determine if the site is of sufficient size
and to assess where more detailed site investigation ef-
forts are needed if the site is selected for further consid-
eration.
3.5.4 Site Screening
The information gained from the reconnaissance survey
should be used to revise the site ranking made during
the first phase of the site evaluation. It is important that
no potential sites be eliminated too early in the process;
however, to limit the expense of the site selection proc-
ess, sites must be carefully ranked so that further investi-
gative efforts proceed systematically beginning with the
highest ranked site.
3.6 Detailed Site Investigations
3.6.1 Soil Profile Descriptions
The soil is a critical factor in determining site suitability
and system design. It is the primary determinate of the
hydraulic and treatment capacity of the site. The charac-
teristics of the soil profile must be described in detail to
observe and identify features that may affect the capacity
of the site to accept and treat the applied wastewater.
Any soil layer that may affect vertical percolation of water
should be described even if less than 1 cm in thickness.
The description should include texture, structure, particle
sorting, coarse fragment content, relative density, macro-
porosity, rooting patterns, thickness, transition, and conti-
nuity for each identifiable soil horizon.
The characteristics of the surface soil materials to a
depth of 2 to 5 m (5 to 15 ft) are the most critical for de-
termining site suitability. Backhoe-excavated test pits and
32
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soil borings extending to a depth of 2 to 3 m (6 to 10 ft)
below the proposed infiltrative surface elevation should
be used. Backhoe pits are the most effective for expos-
ing an undisturbed soil profile. However, unless appropri-
ate safety precautions can be taken, hollow-stem-auger
and split-spoon samples may be required.
Locations of test pits and borings should be carefully
planned in advance based on information gained during
the resource materials review and reconnaissance sur-
vey. The number and density of test pits and borings
should be sufficient to enable accurate prediction of soil
characteristics and variability, but not so excessive that
unnecessary damage to the proposed infiltration area re-
sults. Where spatial variability is not great, a test pit den-
sity of one per 0.4 ha (1 ac) is usually adequate. Each
test pit and boring should be carefully located on a plot
plan and the surface elevation measured relative to a
permanent bench mark.
A qualified soil scientist familiar with land application sys-
tems should describe the soil profile morphology. Par-
ticular emphasis should be placed on those features that
affect water movement through the profile. U.S. Depart-
ment of Agriculture Soil Conservation Service nomencla-
ture, rather than the Unified System, is typically used to
describe the soil characteristics in the zone of natural soil
development, typically within 3 m (10 ft) of the ground
surface. The USDA system, as described in the SCS Soil
Survey Manual (1951), provides a more comprehensive
description to aid in understanding morphological details
relevant to water movement.
3.6.2 Characterization of the
Unconsolidated Substratum
The deeper unconsolidated materials below the site must
be examined also. Hydraulically restrictive layers may
exist that would cause "perched" saturated zones to form
under system operation. Because these zones could af-
fect the hydraulic and treatment capacity of the site, they
must be identified.
Hollow stem auger borings are used to examine the
deep unconsolidated materials to depths of 3 to 8 m (10
to 25 ft) or to bedrock, whichever is shallower. The bor-
ings should be made near the center and at the perime-
ters of the proposed infiltration area. A minimum of three
borings should be made across the site. More may be
necessary for large sites or to assess any suspected
ground-water boundary zones that may impact system
operation, but the maximum density of borings usually
need not be more than one per 0.4 ha (1 ac).
Split-spoon samples should be taken at maximum inter-
vals of 1.5 m (5 ft) and described sufficiently to assess
internal drainage. All pertinent details of morphology, li-
thology, and physical characteristics should be noted.
Characteristic descriptions that are of particular impor-
tance are texture, consistence, blow counts, moisture
content, estimated permeability, and the presence of free
water. Borings are typically logged using the Unified Sys-
tem (ASTM) nomenclature.
Each boring should be located accurately on a plot plan
of the site and the surface elevation established relative
to a permanent bench mark.
3.6.3 Characterization of the
Ground-Water Table
If ground water is present within 8 m (25 ft) of the ground
surface, the hydraulic response of the water table to pro-
longed system loading should be evaluated. The primary
concerns are the direction of ground-water flow from the
site, potential development of a ground-water mound be-
low the site that encroaches within the vadose zone
needed for treatment, and water-quality impacts.
Characterization of the ground water to address these
concerns may be accomplished by the installation of
monitoring wells and/or piezometers. A minimum of three
wells or piezometers that penetrate the water table by at
least 1 to 1.5 m (3 to 5 ft) should be installed in the shal-
lowest saturated zone in a triangular pattern across the
site. After allowing the water surface to equilibrate in the
well, water elevation measurements are taken and com-
pared to establish the horizontal gradient of the ground
water. On large sites, more piezometers are usually
needed to allow a better estimate of the gradient. At least
one nested group of three piezometers at different
depths should be installed and monitored to determine
the vertical ground-water gradient. All wells and pie-
zometers should be fitted with a protective casing with a
locking cover to prevent vandalism and foreign sub-
stances from entering the well.
To evaluate the mounding potential, the depth to ground
water, ground-water gradient, hydraulic conductivity, ef-
fective porosity or specific yield, and thickness of the
saturated zone are needed. The hydraulic conductivity
can be determined by performing slug tests or pumping
tests in one or more wells on the site (Bouwer, 1978;
Bouwer and Rice, 1976; Freeze and Cherry, 1979). In
some cases, it may be possible to estimate the saturated
hydraulic conductivity from a particle size analysis of the
aquifer materials (Bouwer, 1978; Freeze and Cherry,
1979). Pumping tests may also be used to determine the
effective porosity or specific yield of the saturated zone.
One well should be installed upgradient of the proposed
system to monitor ambient ground-water quality. The lo-
cation of this well can only be determined after the
ground-water gradient has been established. At a mini-
mum, water quality monitoring should include field meas-
urement of specific conductance, pH, and temperature,.
and laboratory analyses for total dissolved solids, total
organic carbon, nitrogen species, phosphorus, chloride,
and alkalinity. After system construction, this well can
serve as the background monitoring well for assessing
treatment performance.
33
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If the ground water does not occur within 8 m (25 ft) of
the ground surface, it may be appropriate to assume a
"worst case" scenario to avoid the cost of deep well in-
stallation. This can be done by assuming that an imper-
meable barrier exists at that depth for the mounding
analysis. A similar assumption can be made where
ground water is encountered but the bottom of the aqui-
fer cannot be determined. Again, by assuming an imper-
meable barrier at this depth, the worst case scenario is
accepted in determining the thickness of the aquifer and
the mounding analysis.
Once installed, the wells and piezometers should be
carefully located on an accurate plot plan and the ground
and top of pipe elevation measured to a permanent
bench mark.
3.6.4 Measurement of Hydraulic Properties
of the Soil
Knowledge of the capacity of the soil to accept and trans-
mit water is critical to the design of land application sys-
tems. Infiltration rates and hydraulic conductivities should
be measured in the field to provide the data necessary
for determining design hydraulic loadings.
The infiltration rate of soil is defined as the rate at which
water enters the soil from the surface, while the hydraulic
conductivity is defined as the rate at which water moves
through the soil. The initial infiltration rate of a clean, ex-
posed soil surface can equal the saturated hydraulic con-
ductivity of the soil below, but with time, the infiltration
rate typically declines well below the capacity of the soil
to transmit the water. Infiltration rate decline is caused by
deposition of wastewater SS at the infiltrative surface,
growth of active biomass stimulated by the carbon and
nutrient rich wastewater, and changes in the charac-
teristics of the soil. As a result of the decline in the infil-
tration rate, the subsoil cannot maintain saturation, and
unsaturated conditions prevail. The unsaturated conduc-
tivity of soil is one to four orders of magnitude less than
the saturated conductivity. Therefore the saturated hy-
draulic conductivity of the soil cannot be used directly to de-
termine the long-term infiltration rate for design purposes.
There are both field and laboratory methods for estimat-
ing infiltration rates. Reid methods provide better esti-
mates and, therefore, are preferred. They include
flooding basin, single- or double-ring infiltrometer, sprin-
kler infiltrometer, and air-entry permeameter tests. These
and other test procedures are described elsewhere
(Black, 1965a; EPA, 1980; 1981; 1984; ASTM).
Double-ring infiltrometer and basin tests are most com-
monly used for land application system design. Infil-
trometer tests are small-scale tests that measure the
hydraulic conductivity of the soil over an area approxi-
mately 50 cm (1.5 ft) in diameter. Such tests can be eas-
ily used to estimate the saturated hydraulic conductivity
at several locations and soil horizons over the site. Basin
tests are large-scale tests that are typically run over an
area 3 m (10 ft) in diameter. Basin tests provide more ac-
curate measurements of hydraulic conductivity, but re-
quire much larger volumes of water to run. Therefore
they are usually run on only the most restrictive soil hori-
zons in the design area.
Laboratory methods for estimating hydraulic conductivity
are described elsewhere (Black, 1965a; ASTM). Be-
cause only small, disturbed soil samples can be tested,
the results are much less accurate than field tests. Meth-
ods that minimize disturbance of the soil sample and
best reproduce field operational conditions are preferred.
The concentric-ring permeameter (Hill and King, 1982)
and the cube method (Bouma and Dekker, 1981) are the
most useful techniques.
3.6.5 Analysis of Soil Chemical Properties
Soil chemical properties can affect permeability and crop
growth potential. Chemical properties of interest include
pH, cation exchange capacity (CEC), exchangeable so-
dium percentage (ESP), and electrical conductivity (EC).
Soil pH has a significant effect on the solubility of various
compounds, microorganism activity, and ion exchange
capacity. The CEC is a measure of the capacity of the
soil to adsorb exchangeable cations. The sum of the ex-
changeable sodium, potassium, calcium, and magne-
sium expressed as a percentage of the CEC is called
percent-base saturation. There are optimum ranges for
percent-base saturation for various crop and soil type
combinations. Also, high percentages of sodium and po-
tassium will result in significant reductions in permeability
due to soil swelling. ESP should be kept below 15 per-
cent. EC is a diagnostic tool used to determine salinity of
the soil water. Salinity will affect plant growth and the
type of plants that will thrive. Analytical procedures for
measuring soil chemical properties can be found else-
where (Black, 1965b; Jackson, 1958; Walsh and Beaton,
1973).
3.6.6 Topographic Mapping
A topographic survey of the site should be completed in
sufficient detail to produce accurately a topographic map
with 25 to 50 cm (1 to 2 ft) contour intervals. All soil bor-
ings, soil pits, monitoring wells, and piezometers should
be located on this map with horizontal and vertical posi-
tions established relative to a permanent bench mark.
Significant site features such as surface water, rock out-
crops, and large trees that may affect system layout and
design should also be accurately located on this map.
3.6.7 Data Interpretation
Interpretation of the site data collected can be a complex
and difficult task. The interpretation process must estab-
lish the elevation of the infiltrative surface and the ac-
ceptable ranges of wastewater loadings that can be
applied. It must also establish the acceptable boundaries
of the system and its geometry.
34
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1,005-
995 -
985 -
975 -
965 -
955 -
S5°E
A1
TP6
TP4
TP7
TP8
Sandy Loam
Sandy Clay Loam
Gravelly Sand
50 100
200
-1,005
- 995
- 985
- 975
Elev. of
Lake James
- 955
Figure 3-3. Example of Stratigraphic Cross Section Constructed from Soil-Boring Log Data.
Stratigraphic cross sections of the site are useful to de-
termine the most appropriate elevation of the infiltrative
surface. Using data from borings and pits, enough cross
sections should be constructed along various transects
of the site to depict the continuity and interrelationships
of key strata affecting system design and operation (Fig-
ure 3.3).
The interpretation process must take into account the
type of land application design that is proposed for the
site because required site characteristics vary with sys-
tem type. The designer is referred to Chapter 5 for descrip-
tion of the required site conditions for each system type.
Where ground water or a hydraulically restrictive horizon
exists within 8 m (25 ft) of the proposed elevation of the
infiltrative surface, the response of the ground water to
system loading must be analyzed. 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 most com-
monly used. Analytical models have been developed for
various hydrogeologic conditions (Brock, 1976; Fin-
nemore and Hantzshe, 1983; Hantush, 1967; Kahn et al.,
1976). The assumptions used for each model must be
compared to the specific site conditions found to select
the most appropriate model. (For examples of model se-
lection and their computations, see EPA 1981; 1984.)
3.7 Construction Considerations
Methods used to construct land application systems are
critical to the long-term performance of the installations.
Earthmoving operations and general construction traffic
will damage the soil properties that are necessary for fa-
vorable wastewater infiltration and percolation. Careful
planning of system construction can minimize soil dam-
age, maintaining the capacity of the system to accept
and treat the applied wastewater.
Soils are not equally susceptible to construction damage.
The degree of soil compaction, smearing, and puddling
that occurs is a function of the soil texture and moisture
content. Soil with greater than 25 percent by weight of
clay (all soils excepting sands and loamy sands) is the
most susceptible to damage.
To limit damage to the soil, a construction plan should be
developed for each project. The plan should address
type of equipment to be used, methods and sequence of
construction operations, site traffic and materials stock-
piling, site preparation, and soil conditions during con-
struction operations.
35
-------
Only low-toad-bearing equipment should be used in the
design area. High-flotation tires and track-mounted vehi-
cles are preferred. Scrapers or blades should not be
used near the infiltrative surface because the scraping
action can smear the soil. In areas to be filled with sand,
blades may be used to spread the fill material if a mini-
mum of 30 cm (1 ft) of fill is maintained below the treads
of the vehicle.
Construction methods and the sequencing of operations
must be carefully planned in advance to prevent unnec-
essary damage to the soil. Unlike conventional earthwork
practices, soil compaction is to be avoided. The number
of passes required to be made over the design area to
reach the desired grade and elevation should be kept to
a minimum. Once the infiltrative surface is established,
further traffic over the area should be avoided. This re-
quires that the construction of large systems be se-
quenced to limit unnecessary traffic. Testing of the soils
during construction may be included in the construction
specifications as a measure of contractor performance.
Site traffic must be controlled at the outset. Site access
and acceptable traffic lanes and stockpiling areas for
construction machinery and materials delivery should be
delineated on the construction drawings. The site should
be staked accordingly before work commences. Ap-
proaches to the design area should be made from the
upslope side wherever possible. If the approach must be
made from the downslope side, traffic should be kept to
minimum and routed as far from the area as possible.
Brush and tree removal may be required prior to other
earthwork. Grubbing of the site following tree removal
should not be done since it will damage the structure of
the soil and aggravate soil compaction during sub-
sequent operations. If the area is to be filled, the site
should be mowed and raked before chisel plowing to a
depth of 20 to 30 cm (8 to 12 in.) along the slope contour.
The soil conditions during construction should be care-
fully monitored. If the soil is near its plastic limit or if the
soil is frozen within 30 cm (1 ft) of the infiltrative surface
finish elevation, construction should not proceed.
If the soil is damaged to the point that the infiltration rate
is significantly affected, the damaged soil must be re-
moved to expose an undamaged surface. As much as 10
to 15 cm (4 to 6 in.) may have to be removed to restore
the infiltrative capacity of the infiltrative surface. This will
tower the finished elevation and may require adjustments
in the design.
3.8 References
When an NTIS number is cited in a reference, that refer-
ence is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
ASTM. 1988. American Society for Testing and Materi-
als. Test method for infiltration rate of soils in field using
double-ring infiltrometers. C3385. Philadelphia, PA.
ASTM. 1974. American Society for Testing and Materi-
als. Test method for permeability of granular soils (con-
stant head). D2434. Philadelphia, PA.
Black, C.A., ed. 1965a. Methods of soil analysis, part 1:
Physical and mineralogical properties, including statistics
of measurement and sampling. Madison, Wl: American
Society of Agronomy.
Black, C.A., ed. 1965b. Methods of soil analysis, part 2:
Chemical and microbiological properties. Madison, Wl:
American Society of Agronomy.
Bouma, J., and L.W. Dekker. 1981. A method of measur-
ing the vertical and horizontal hydraulic saturated con-
ductivity of clay soils with macropores. Soil Sci. Soc. Am.
J. 45:662.
Bouwer, H. 1978. Groundwater hydrology. New York:
McGraw-Hill Book Co.
Bouwer, H., and R.C. Rice. 1976. A slug test for deter-
mining hydraulic conductivity of unconfined aquifers with
completely or partially penetrating wells. Water Re-
sources Research 12:423-428.
Brock, R.P. 1976. Dupuit-Forcheimer and potential theo-
ries for recharge from basins. Water Resources Re-
search 12:909.
EPA. 1984. Environmental Protection Agency. Process
design manual: Land treatment of municipal wastewa-
ter—Supplement on rapid infiltration and overland flow.
EPA/625/1-81-013a. Cincinnati, OH.
EPA. 1981. Environmental Protection Agency. Process
design manual: Land treatment of municipal wastewater.
EPA/625/1-81-013. Cincinnati, OH.
EPA. 1980. Environmental Protection Agency. Design
manual: Onsite wastewater treatment and disposal sys-
tems. EPA/625/1-80-012. NTIS No. PB83-219907.
Finnemore, E.J., and N.N. Hantzshe. 1983. Ground-water
mounding due to onsite sewage disposal. J. Irrigation &
Drainage Am. Soc. Civ. Eng. 109:199.
Freeze, R.A., and J.A. Cherry. 1979. Groundwater.
Englewood Cliffs, NJ: Prentice-Hall.
Hantush, M.S. 1967. Growth and decay of ground water
mounds in response to uniform percolation. Water Re-
sources Research 3:227.
Hill, R.L., and L.D. King. 1982. A permeameter which
eliminates boundary flow errors in saturated hydraulic
conductivity measurements. Soil Sci. Soc. Am. J. 46:877.
Jackson, M.L. 1958. Soil chemical analysis. Englewood
Cliffs, NJ: Prentice-Hall.
36
-------
Kahn, M.Y., D. Kirkham, and R.L. Handy. 1976. Shapes
of steady state perched groundwater mounds. Water Re-
sources Research 12:429.
SCS. 1951. Soil Conservation Service. Soil survey man-
ual. U.S. Department of Agriculture Handbook 18. Wash-
ington, DC.
Walsh, L.M., and J.D. Beaton eds. 1973. Soil testing and
plant analysis. Madison, Wl: Soil Sci. Soc. Am.
37
-------
-------
CHAPTER 4
Wastewater Characteristics
4.1 Introduction
The effective management of any wastewater flow re-
quires a reasonably accurate knowledge of its charac-
teristics. This is particularly true for flows from rural
residential dwellings, commercial establishments, and
other facilities where individual water-using activities cre-
ate an intermittent flow of wastewater that can vary
widely in volume and degree of pollution. For small com-
munities with collection systems, the possibility of a sig-
nificant infiltration and inflow (I/I) contribution must be
taken into account for any wastewater planning effort.
Detailed characterization data regarding these flows are
necessary not only to facilitate the effective design of
wastewater treatment and disposal systems, but also to
enable the development and application of water conser-
vation and waste load reduction strategies.
For existing developments, characterization of the actual
wastewaters to be encountered can often be carried out.
However, for many existing developments, and for al-
most all new developments, wastewater characteristics
must be predicted. The purpose of this chapter is to pro-
vide a basis for characterizing the wastewater from rural
developments. A detailed discussion of the charac-
teristics of residential wastewaters is presented first, fol-
lowed by a limited discussion of the characteristics of the
wastewaters generated by nonresidential establishments,
including those of a commercial, institutional, and recrea-
tional nature. Finally, a general procedure for predicting
wastewater characteristics for a given residential dwell-
ing or nonresidential establishment is given.
4.2 Residential Wastewater
Characteristics
Residential dwellings exist in a variety of forms, including
single- and multi-family homes, condominiums, apart-
ment houses, and cottages or resort residences. In all
cases, occupancy can occur on a seasonal or year-round
basis. The wastewater discharged from these dwellings
is comprised of a number of individual wastewaters gen-
erated through water-using activities employing a variety
of plumbing fixtures and appliances. The characteristics
of the wastewater can be influenced by several factors.
Primary influences are the characteristics of the plumb-
ing fixtures and appliances present as well as their fre-
quency of use. Additionally, the characteristics of the re-
siding family in terms of the number of resident family
members, age levels, and mobility are important as is the
overall socioeconomic status of the family. The other
characteristics, including seasonal or yearly occupancy,
geographic location, and method of water supply and
wastewater disposal, appear as additional, but lesser, in-
fluences.
4.2.1 Wastewater Flow
4.2.1.1 Average Daily Flow
The average daily wastewater flow from a typical resi-
dential dwelling is approximately 170 Lpcd (45 gpcd) (Table
4-1). While the average in-house daily flow experienced at
one residence compared to that of another can vary con-
siderably, it is typically no greater than 227 Lpcd (60
gpcd) and seldom exceeds 284 Lpcd (75 gpcd) (Figure
4-1). Residential water use in arid regions of the country
can be significantly greater owing to on-lot demands,
which nationally average nearly one-half of the residen-
tial (in-house) demand cited above. These figures are
even higher in the Southwest.
Note: Based on the average daily flow
measured for each of 71 residences studied.
90
f 80
£L
-5!
5 70
.2
I
'
5°
in
rr 40
_>,
'« Of)
Q d0
-------
Table 4-1. Summary of Average Daily Residential Wastewater Flows
Study8
Wastewater Flow
Number of Residences
Duration of Study
(months)
Study Avg. (gpcd)
Range of Individual
Residence Averages
(gpcd)
Linaweaver et al.
Anderson and Watson
Watson et al.
Cohen and Wallman
Laak
Bennett and Lfnstedt
Slegrlst et al.
Otis
Duffy et al.
Weighted Average
22
18
3
8
5
5
11
21
16
-
4
2-12
6
24
0.5
1
12
12
49.0
44.0
53.0
52.0
41.4
44.5
42.6
36.0
42.3
44.0
6-66
18-69
25-65
27.8-101.6
26.3 - 65.4
31.8-82.5
25.4-56.9
8-71
aSee References at end of chapter.
4.2.1.2 Individual Activity Flows
The individual wastewater generating activities within a
residence are the building blocks that serve to produce
the total residential wastewater discharge. The average
characteristics of several major residential water-using
activities are presented in Table 4-2. A water-using activ-
ity that falls under the category of miscellaneous in this
table, but deserves additional comment, is water-softener
backwash/regeneration flows. Water-softener regenera-
tion typically occurs once or twice a week, discharging
about 114 to 333 L (30 to 88 gal)/regeneration cycle (Wel-
ckart, 1976). On a daily per capita basis, water-softener
flows have been shown to average about 19 Lpcd (5 gpcd),
ranging from 8.7 to 59.4 Lpcd (2.3 to 15.7 gpcd) (Siegrist et
al. 1976).
Tabla 4-2. Typical Residential Water Use by Activity3
Activity
Toilet flushing
Gal/use
4.3
4.0-5.0
Bathing
Clotheswashlng
Dishwashing
Garbage grinding
Miscellaneous
Total
21.4
33.5
7.0
2.
24.5
-27.2
37.4
-40.0
8.8
-12.5
2.0
0-2.1
NA
NA
Uses/cap/d
3.5
2.3-4.1
0.32-
0.25-
0.15-
0.4-
0.43
0.50
0.29
0.31
0.35
0.50
0.58
0.75
NA
NA
gpcdb
9.2-
6.3-
7.4-
1.1
0.8
5.7
41.4
16.2
-20.0
9.2
-12.5
10.0
-11.6
3.2
-4.9
1.2
-1.5
6.6
-8.0
45.6
-52.0
'Mean and ranges of results reported in Cohen and Wallman,
1974; Laak, 1975; Bennett and Linstedt, 1975; Siegrist et al.,
1976; and Ligman et al., 1974.
gpcd may not equal gal/use multiplied by uses/cap/d due to
difference in the number of study averages used to compute
the mean and ranges shown.
NA « not applicable
4.2.1.3 Wastewater Flow Variations
The intermittent occurrence of individual wastewater-
generating activities creates large variations in the
wastewater flow rate from a residence.
Minimum and Maximum Daily Flows. The daily wastewa-
ter flow from a specific residential dwelling is typically 10
to 300 percent of the average daily flow at that dwelling,
with the vast majority within 50 and 150 percent of the
average day. At the extreme, however, minimum and
maximum daily flows of 0 and 900 percent of the average
daily flow may be encountered (Anderson and Watson,
1976; Watson et al., 1967; Witt, 1974).
Minimum and Maximum Hourly Flows. Minimum hourly
flows of zero are typical. Maximum hourly flows are more
difficult to quantify accurately. On the basis of typical fix-
ture and appliance usage characteristics, as well as an
analysis of residential water usage demands, maximum
hourly flows of 380 L (100 hr)/gal can occur (Anderson
and Watson, 1967; Jones, 1974). Hourly flows in excess
of this can occur due to plumbing fixture and appliance
misuse or malfunction (e.g., faucet left on or worn toilet
ball cock).
Instantaneous Peak Flows. The peak flow rate from a
residential dwelling is a function of the characteristics of
the fixtures and appliances present and their position in
the overall plumbing system layout. The peak discharge
rate from a given fixture/appliance is typically around 0.3
L/s (5 gpm), with the exception of the tank-type water
closet that discharges a short-duration peak flow of up to
1.6 L/s (25 gpm). The use of several fixtures/appliances
simultaneously can increase the total flow rate over the
rate for isolated fixtures/appliances. However, attenu-
ation occurring in the residential drainage network tends
to decrease the peak flow rates in the sewer exiting the
residence.
Although field data are limited, peak discharge rates from
a single-family dwelling of 0.3 to 0.6 L/s (5 to 10 gpm)
can be expected. For multifamily units, peak rates in ex-
40
-------
cess of these values commonly occur. A crude estimate
of the peak flow in these cases can be obtained using
the fixture-unit method described in Section 4.3.1.2.
4.2.2 Wastewater Quality
4.2.2.1 Average Daily Flows
The characteristics of typical residential wastewater are
outlined in Table 4-3, including daily mass loadings and
pollutant concentrations. The wastewater characterized
is typical of residential dwellings equipped with standard
water-using fixtures and appliances, excluding garbage
disposal(s), that collectively generate approximately 170
Lpcd (45 gpcd).
4.2.2.2 Individual Activity Contributions
Residential water-using activities contribute varying
amounts of pollutants to the total wastewater flow. The
individual activities may be grouped into three major
Table 4-3. Characteristics of Typical Residential Wastewater3
Parameter
Total solids
Volatile solids
SS
VSS
BOD5
OD
Total N
Ammonia
Nitrites and nitrates
Total P
Phosphate
Total coliformsb
Fecal coliformsb
Mass Loading
(gm/cap/d)
115-170
65-85
35-50
25-40
35-50
115-125
6-17
1-3
<1
1-2
0.3-1.5
1,010-1,012
108-1,010
Concentration
(mg/L)
680-1,000
380-500
200 - 290
150-240
200 - 290
680 - 730
35-100
6-18
<1
6 -'12
2-9
aFor typical residential dwellings equipped with standard
water-using fixtures and appliances (excluding garbage
disposals) generating approximately 170 Lpcd (45 gpcd).
Based on the results presented in Laak, 1975; Bennett and
Linstedt, 1975; Siegristet al., 1976; Ligman et al., 1974; and
Jones, 1974.
bConcentrations presented in organisms/L.
wastewater fractions: garbage disposal wastes, toilet
wastes, and sink, basin, and appliance wastewaters. A
summary of the average contribution of several key pol-
lutants in each of these three fractions is presented in
Table 4-4.
With regard to the microbiological characteristics of the
individual waste categories, studies have demonstrated
that the wastewater from sinks, basins, and appliances
can contain significant concentrations of indicator organ-
isms as total and fecal conforms (Olsson et al., 1968;
Hypes et al., 1974; Small Scale Waste Management Pro-
ject, 1978; Brandes, 1978). Traditionally, high concentra-
tions of these organisms have been used to assess the
contamination of a water or wastewater by pathogenic
organisms. One assumes, therefore, that these waste-
waters possess some potential for harboring pathogens.
4.2.2.3 Wastewater Quality Variations
Since individual water-using activities occur intermittently
and contribute varying quantities of pollutants, the
strength of the wastewater generated from a residence
fluctuates with time. Accurate quantification of these fluc-
tuations is impossible. An estimate of the type of fluctua-
tions possible can be derived from the pollutant
concentration information presented in Table 4-5 consid-
ering that the activities included occur intermittently.
4.3 Nonresidential Wastewater
Characteristics
The rural population, as well as the transient population
moving through the rural areas, is served by a wide vari-
ety of isolated commercial establishments and facilities.
For many establishments, the wastewater-generating
sources are sufficiently similar to those in a residential
dwelling that residential wastewater characteristics can
be applied. For other establishments, however, the
wastewater characteristics can be considerably different
from those of a typical residence.
Providing characteristic wastewater loadings for "typical"
nonresidential establishments is a very complex task due
to several factors. First, there is a relatively large number
of diverse establishment categories (e.g., bars, restau-
Table 4-4. Pollutant Contributions of Major Residential Wastewater Fractions3 (gm/cap/d)
Parameter
BODs
SS
N
P
Garbage Disposals
18.0
10.9-30.9
26.5
15.8-43.6
0.6
0.2 - 0.9
0.1
0.1 -0.1
Toilets
16.7
6.9-23.6
27.0
12.5-36.5
8.7
4.1-16.8
1.2
0.6 - 1 .6
Basins, Sinks,
Appliances
28.5
24.5 - 38.8
17.2
10.8-22.6
1.9
1.1-2.0
2.8
2,2 - 3.4
Approximate Total
63.2
70.7
11.2
4.0
aMeans and ranges of results reported in Laak, 1975; Bennett and Linstedt, 1975; Siegrist et al., 1976; Ligman et al., 1974;
Olsson et al., 1968
41
-------
Table 4-5. Pollutant Concentrations of Major Residential Wastewater Fractions3 (mg/L)
Parameter
BODs
SS
N
P
Garbage Disposals
2,380
3,500
79
13
Toilets
280
450
140
20
Basins, Sinks,
Appliances
260
160
17
26
Approximate Total
360
400
63
23
"Based on the average results presented in Table 4-4 and the following wastewaterflows: Garbage disposals—8 Lpcd (2 gpcd);
toilets—61 Lpcd (16 gpcd); basins, sinks, and appliances—110 Lpcd (29 gpcd); total—178 Lpcd (47 gpcd).
rants, drive-in theaters). The inclusion of diverse estab-
lishments within the same category produces a potential
for large variations in waste-generating sources and the
resultant wastewater characteristics. Further, many intan-
gible influences, such as location and popularity, may
produce substantial wastewater variations between oth-
erwise similar establishments. Finally, there is consider-
able difficulty in presenting characterization data in units
of measurement that are easy to apply yet predictively
accurate (e.g., at a restaurant, wastewater flow in vol-
ume/seat is easy to apply to estimate total flow, but is
less accurate than if volume/meal served were used).
In this section, limited characterization data for nonresi-
dential establishments, including commercial estab-
lishments, institutional facilities, and recreational areas,
are presented. These data are meant to serve only as a
guide, and as such should be applied cautiously. Wher-
ever possible, characterization data for the particular es-
tablishment in question, or a similar one in the vicinity,
should be obtained.
4.3.1 Wastewater Flow
4.3.1.1 A verage Daily Flows
Typical daily flows from a variety of commercial, institu-
tional, and recreational establishments are presented in
Tables 4-6 through 4-8.
4.3.1.2 Wastewater Flow Variation
The wastewater flows from nonresidential establishments
are subject to wide fluctuations with time. While difficult to
quantify accurately, an estimate of the magnitude of the
fluctuations, including minimum and maximum flows on
an hourly and daily basis, can be made if consideration is
given to the characteristics of the water-using fixtures
and appliances, and to the operational characteristics of
the establishment (e.g., hours of operation, patronage
fluctuations).
Tablo 4-6. Typical Wastewater Flows from Commercial Sources9
(gpd/unit)
Source
Airport
Automobile service station
Bar
Hotel
Industrial building
(excluding industry and cafeteria)
Laundry (self-service)
Motel
Motel with kitchen
Office
Restaurant"
Rooming house
Shopping center
Store, department
•"Metcaif and Eddy, 1979.
Unit
Passenger
Vehicle served
Employee
Customer
Employee
Guest
Employee
Employee
Machine
Wash
Person
Person
Employee
Meal
Resident
Parking space
Toilet room
Employee
bDoes not Include all of the facility's wastewater streams,
Range
2.1 -4.0
7.9-13.2
9.2-15.8
1 .3 - 5.3
10.6-15.8
39.6 - 58.0
7.9-13.2
7.9-17.2
475-686
47.5 - 52.8
23.8 - 39.6
50.2-58.1
7.9-17.2
9.0-12.0
23.8 - 50.1
0.5-2.1
423-634
7.9-13.2
only those related to customers.
Typical
2.6
10.6
13.2
2.1
13.2
50.1
10.6
14.5
580.0
50.1
31.7
52.8
14.5
10.0
39.6
1.1
528.0
10.6
42
-------
Table 4-7. Typical Wastewater Flows from Institutional Sources'
(gpd/unit)
Source
Hospital, medical
Hospital, mental
Prison
Rest home
School, boarding
School, day:
with cafeteria, gym, showers
with cafeteria only
without cafeteria, gym, showers
Unit
Bed
Employee
Bed
Employee
Inmate
Employee
Resident
Employee
Student
Student
Student
Student
Range
132-251
5.3-15.9
79.3-172
5.3-15.9
79.3-159
5.3-15.9
52.8-119
5.3-15.9
52.8-106
15.9-30.4
10.6-21.1
5.3-17.2
Typical
172.0
10.6
106.0
10.6
119.0
10.6
92.5
10.6
74.0
21.1
15.9
10.6
aMetcalf and Eddy, 1979.
Table 4-8. Typical Wastewater Flows from Recreational Sourcesa
(gpd/unit)
Source
Apartment, resort
Cabin, resort
Cafeteria
Campground (developed)
Cocktail lounge
Coffee shop
Country club
Day camp (no meals)
Dining hall
Dormitory, bunkhouse
Hotel, resort
Laundromat
Store resort
Swimming pool
Theater
Visitor center
Unit
Person
Person
Customer
Employee
Person
Seat
Customer
Employee
Member present
Employee
Person
Meal served
Person
Person
Machine
Customer
Employee
Customer
Employee
Seat
Visitor
Range
52.8 - 74
34.3 - 50.2
1.1 -2.6
7.9-13.2
21.1-39.6
13.2-26.4
4.0 - 7.9
7.9-13.2
66.0-132
10.6-15.9
10.6-15.9
4.0-13.2
19.8-46.2
39.6 - 63.4
476 - 687
1 .3 - 5.3
7.9-13.2
5.3-13.2
7.9-13.2
2.6-4.0
4.0 - 7.9
Typical
58.1
42.3
1.6
10.6
31.7
19.8
5.3
10.6
106.0
13.2
13.2
7.9
39.6
52.8
581.0
2.6
10.6
10.6
10.6
2.6
5.3
aMetcalf and Eddy, 1970.
Peak wastewater flows can be estimated utilizing the
fixture-unit method (Water Pollution Control Fed., 1976;
Uniform Plumbing Code, 1976). As originally developed,
this method was based on the premise that under normal
usage, a given type of fixture had an average flow rate
and duration of use (Hunter, 1940; 1941). One fixture
unit was arbitrarily set equal to a flow rate of 0.5 Us (7.5
gpm), and various fixtures were assigned a certain num-
ber of fixture units based upon their particular charac-
teristics (Table 4-9). On the basis of probability studies,
relationships were developed between peak water use
and the total number of fixture units present (Figure 4-2).
4.3.2 Wastewater Quality
The qualitative characteristics of the wastewaters gener-
ated by nonresidential establishments can vary signifi-
cantly between different types of establishments due to the
extreme variation that can exist in the waste-generating
sources present. Consideration of the waste-generating
sources present at a particular establishment can give a
general idea of the character of the wastewater and serve
to indicate whether the wastewater will contain any prob-
lem constituents, such as high grease levels from a res-
taurant or lint fibers from a laundromat.
43
-------
Table 4-9. Fixture Units per Fixture3
Fixture Type Fixture
Units
Ona bathroom group consisting of 6
tank-operated water closet, lavatory, and
bathtub or shower stall
Bathtub (with or without overhead shower) 2
Bidet 3
Combination sink-and-tray with food-disposal 4
unit
Combination sink-and-tray 3
Dental lavatory 1
Dental unit or cuspidor 1
Dishwasher, domestic 2
Drinking fountain 0.5
Roor drains 1
Kitchen sink, domestic, with food-waste grinder 3
Kitchen sink, domestic 2
Laundry tray (1 or 2 compartments) 2
Lavatory 2
Shower stall, domestic 2
Showers (group) per head 3
Sinks
Surgeon's 3
Flushing rim (with valve) 8
Service (trap standard) 3
Service (P trap) 2
Pot, scullery, etc. 4
Urinal, pedestal, siphon jet, blowout 8
Urinal, wall lip 4
Urinal stall, washout 4
Urinal trough (each 2-ft section) 2
Wash sink (circular or multiple) each set of 2
faucets
Water closet, tank-operated 4
Water closet, valve-operated 8
"Water Pollution Control Fed., 1976.
If the waste-generating sources present at a particular
establishment are similar to those typical of a residential
dwelling, an approximation of the pollutant mass loadings
and concentrations of the wastewater produced may be
derived using the residential wastewater quality data pre-
sented in Tables 4-3 to 4-5. For establishments where
the waste-generating sources appear significantly differ-
ent from those in a residential dwelling, or where more
refined characterization data are desired, a detailed re-
view of the pertinent literature as well as actual wastewa-
ter sampling at the particular or a similar establishment
should be conducted.
450
Note: Curves show probable amount of time indicated peak flow
will be exceeded during a period of concentrated fixture use.
400 - ,—
| 350
LL.
300
_ 250
J> 200
3 150
.£>
S
Q_
100
50
0
System in which flush
valves predominate
System in which flush
tanks predominate
0 2
22
4 6 8 10 12 14 16 18 20
Fixture Units on System (hundreds)
Figure 4-2. Peak Discharge versus Fixture Units Present.
4.4 Predicting Wastewater
Characteristics
4.4.1 General Considerations
4.4.1.1 Parameter Design Units
In characterizing wastewaters, quantitative and qualita-
tive characteristics are often expressed in terms of other
parameters. These parameter design units, as they may
be called, vary considerably depending on the type of es-
tablishment considered. For residential dwellings, daily
flow values and pollutant contributions are expressed on
a per capita basis. Applying per capita data to predict to-
tal residential wastewater characteristics requires that a
second parameter be considered; namely, the number of
persons residing in the residence. Residential occupancy
is typically 1.0 to 1.5 persons/bedroom. Although it pro-
vides for a conservative estimate, the current practice is
to assume that maximum occupancy is two persons per
bedroom.
For nonresidential establishments, wastewater charac-
teristics are expressed in terms of a variety of units. Al-
though per capita units are employed, a physical
characteristic of the establishment such as per seat, per
car stall, or per square foot is more commonly used.
4.4.1.2 Factors of Safety
To account for the potential variability in the wastewater
characteristics at a particular dwelling or establishment,
versus that of the average, conservative predictions or
factors of safety are typically utilized. These factors of
safety can be applied indirectly through choice of the de-
44
-------
sign criteria for wastewater characteristics and the occu-
pancy patterns, as well as directly through an overall fac-
tor. For example, if an average daily flow of 284 Lpcd (75
gpcd) and an occupancy of two persons per bedroom
were selected, the flow prediction for a three-bedroom
home would include a factor of safety of approximately 3
when compared to average conditions (i.e., 170 Lpcd (45
gpcd) and 1 person/bedroom). If a direct factor of safety
were also applied (e.g., 1.25), the total factor of safety
would increase to approximately 3.75.
Great care must be exercised in predicting wastewater
characteristics so as not to accumulate multiple factors of
safety that would yield an extremely conservative estimate.
4.4.2 Strategy for Predicting Wastewater
Characteristics
Predicting wastewater characteristics from rural develop-
ments can be a complex task. Following a logical step-
by-step procedure can help simplify the characterization
process and render the estimated wastewater charac-
teristics more accurate. A flow chart detailing a proce-
dure for predicting wastewater characteristics is
presented in Figure 4-3.
4.5 Water Conservation and
Wastewater Flow Reduction
An extensive array of techniques and devices are avail-
able to reduce the average water use and concomitant
wastewater flows generated by individual water-using ac-
tivities and, in turn, the total effluent from the residence
or establishment. The diversity of present wastewater
flow reduction methods is illustrated in Table 4-10. As
shown, the methods may be divided into three major
groups: elimination of nonfunctional water use; water-
saving devices, fixtures, and appliances; and wastewater
recycle/reuse systems.
4.5.1 Elimination of Nonfunctional Water
Use
Wasteful water-use habits can occur with most water-
using activities. A few illustrative examples include using
a toilet flush to dispose of a cigarette butt, allowing the
water to run while brushing teeth or shaving, or operating
a clotheswasher or dishwasher with only a partial load.
Obviously, the potential for wastewater flow reductions
through elimination of such wasteful use vary tremen-
dously between homes, from minor to significant reduc-
tions, depending on habits.
4.5.1.1 Improved Plumbing and Appliance
Maintenance
Unseen or apparently insignificant leaks from household
fixtures and appliances can waste large volumes of water
and generate similar quantities of wastewater. Most nota-
ble in this regard are leaking toilets and dripping faucets.
For example, a steadily dripping faucet can waste up to
several hundred gallons of water per day.
Determine primary function of facility
and classify it accordingly
(e.g., single-family home, restaurant)
Identify intended application of wastewater
characterization data
I
Identify wastewater characterization
data needed (e.g., Q, BODs)
Determine physical characteristics of facility
• Wastewater-generating fixtures
and appliances
• Parameter design units (e.g., bedrooms,
seating spaces)
•Occupancy or operation patterns (e.g.,
seasonal homes, hours of operation)
Obtain characterization data from literature
• Tables and text of this chapter
• References to this chapter
• Other sources
Gather existing measured characterization
data applicable to facility
• Water meter records
• Holding tank pumpage records
• Other
Evaluate available data
• Select data judged most accurate
• Determine if needed data has
been obtained
Calculate waste load characteristics
(e.g., 45 gpcd x 2 cap/bedroom x
2 bedrooms =180 gpd)
Apply overall factor of saftey as required
by intended application of data
Conduct
characterization
field studies at
facility in question
or a very similar one
Estimate wastewater characteristics
Figure 4-3. Strategy for Predicting Wastewater
Characteristics.
45
-------
Table 4-10. Selected Wastewater Row Reduction Methods
Elimination of Nonfunctional Water Use
• Improved water-use habits
• Improved plumbing and appliance maintenance and
monitoring
• Reduced excessive water supply pressure
Water-Saving Devices, Fixtures, and Appliances
• Toltet
1. Water-carriage toilets
a. Toilet-tank inserts
b. Water-saving toilets (3.5 gal.)
c. Ultra-low flush toilets (ULF) (1.6 gal/flush or less)
- Wash-down flush
- Mechanically assisted
- Pressurized tank
- Compressed air
- Vacuum
- Grinder
2. Nonwater carriage toilets
a. Pit privies
b. Biological toilets
c. Incinerator toilets
d. Oil-carriage toilets
• Bathing devices, fixtures, and appliances
1. Shower flow controls
2. Reduced-flow showerheads
3. On/off showerhead valves
4. Mixing valves
5. Air-assisted, low-flow shower system
• Clotheswashing devices, fixtures, and appliances
1. Front-loading washer
2. Adjustable cycle settings
3. Washwater recycle feature
• Miscellaneous
1. Faucet Inserts
2. Faucet aerators
3. Reduced-flow faucet fixtures
4. Mixing valves
5. Hot water pipe insulation
6. Pressure-reducing valves
7. Hot water recirculation
Wastewater Recycle/Reuse Systems
• Bath/laundry wastewater recycle for toilet flushing
• Toilet wastewater recycle for toilet flushing
• Combined wastewater recycle for toilet flushing
• Combined wastewater recycle for several uses
4.5.1.2 Maintain Nonexcessive Water
Supply Pressure
The water flow rate through sink and basin faucets,
showerheads, and similar fixtures is highly dependent on
the water pressure in the water supply line. For most
residential uses, a pressure of 280 kPa (40 psi) is ade-
quate. Pressure in excess of this can result in unneces-
sary water use and wastewater generation. To illustrate,
the flow rate through a typical faucet opened fully is
about 40 percent higher at a supply pressure of 560 kPa
(80 psi) versus that at 280 kPa (40 psi).
4.5.2 Water-Saving Devices, Fixtures, and
Appliances
The quantity of water traditionally used by a given water-
using fixture or appliance is often considerably greater
than actually needed. Certain tasks may even be accom-
plished without the use of water. As presented in Table
4-2, over 70 percent of a typical residential dwelling's
wastewater flow volume is collectively generated by toilet
flushing, bathing, and Clotheswashing. Thus efforts to ac-
complish major wastewater flow reductions should be di-
rected toward these three activities.
4.5.2.1 Toilet Devices and Systems
Each flush of a conventional water-carriage toilet uses
15-18 L (4-5 gal) of water depending on the model and
water supply pressure. On average, flushing typically
generates approximately 16 L (4.3 gal) of wastewater.
When coupled with 3.5 uses/cap/d, a daily wastewater
flow of approximately 61 Lpcd (16 gpcd) results (Table
4-2). Since the mid-1980s 13L (3.5 gal)/flush (M) has
been more common. Recently, several states have man-
dated that in new construction 6 L (1.6 gal)/flush toilets
be installed. Nonetheless, the data above are reasonable
estimates of demand per toilet flush in the United States.
A variety of devices have been developed for use with a
conventional flush toilet to reduce the volume of water
used in flushing. Additionally, alternatives to the conven-
tional water-carriage toilet are available, certain of which
use little or no water to transport human wastewater
products. Tables 4-11 and 4-12 present a summary of
various toilet devices and systems. Additional details re-
garding the nonwater-carriage toilets may be found else-
where (Kreissl, 1985; Laak, 1975; Enferadi et al., 1986;
Molland, 1984).
4.5.2.2 Bathing Devices and Systems
Although great variation exists in the quantity of waste-
water generated by a bath or shower, typical values in-
clude approximately 95 L (25 gal)/occurrence coupled
with a 0.4 use/cap/d frequency to yield a daily per capita
flow of about 38 L (10 gal) (Table 4-2). The majority of
devices available to reduce bath ing-waste water flow vol-
umes are concentrated around the activity of showering,
with their objective being to reduce the normal 0.25 to
0.63 L/s (4-10 gpm) showering flow rate. Several flow re-
duction devices and systems for showering are charac-
terized in Table 4-13. The amount of total wastewater
flow reduction accomplished with these devices is highly
dependent on individual user habits. Reductions vary
from a negative value to as much as 12 percent of the to-
tal wastewater volume.
46
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Table 4-11. Wastewater Flow Reduction—Water-Carriage Toilets and Systems
Generic Type
Toilets with tank
inserts
Water-saving
toilets
Washdown flush
toilets
Pressurized-tank
toilets
Compressed
air-assisted flush
toilets
Description
Displacement
devices placed into
storage tank of
conventional toilets
to reduce volume
but not height of
stored water
Varieties: Plastic
bottles, flexible
panels, drums or
plastic bags
Variation of
conventional flush
toilet fixture; similar
in appearance and
operation. Redesigned
flushing rim and
priming jet to initiate
siphon flush in
smaller trapway
with less water
Flushing uses only
water, but sub-
stantially less due to
washdown flush
Van'eties: Few
Specially designed
toilet tank to
pressurize air
contained in toilet
tank. Upon flushing,
the compressed air
propels water into
bowl at increased
velocity
Varieties: Few
Similar in
appearance and
user operation to
conventional toilet;
specially designed
to utilized
compressed air to
aid in flushing
Varieties: Few
Application
Considerations
Device must be
compatible with
existing toilet and
not interfere with
flush mechanism
Installation by
owner
Reliability low
O&M
Post-installation and
periodic inspections
to ensure proper
positioning
Water Use Per
Event (gal)
3.3-3.8
Total Flow
Reduction3
(gpcd)(%)
1.8-3.5 4-8
Interchangeable
with conventional
fixture
Essentially the
same as for a
conventional unit
1.0-3.5
3.2-13 6-20
Rough-in for unit Similar to
may be nonstandard conventional toilet
Drain-line-slope
and lateral-run
restrictions
Plumber installation
advisable
Compatible with
most conventional
toilet units
Increased noise
level
Water supply
pressure of 35-120
psi
Interchangeable
with rough-in for
conventional fixture
Requires source of
compressed air;
bottled or air
compressor
If air compressor,
need power source
Increased noise
level
Cleaning possible
0.8-1.6
(but more
frequent)
9.4-12.2 21-27
Similar to
conventional toilet
fixture
2.0-2.5
6.3-8.0 14-18
Periodic
maintenance of
compressed air
source
0.002 kWh per use
0.5-1.5
13.3 30
47
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Table 4-11. Wastewater Flow Reduction—Water-Carriage Toilets and Systems (continued)
Generic Type
Vacuum-assisted
Description
Similar in
Application
Considerations
Application largely
O&M
Periodic
Water Use Per
Event (gal)
0.3
Total Flow
Reduction3
(gpcd)(%)
14 31
Hush toilets
appearance and
user operation to
conventional toilet;
specially designed
fixture is connected
to vacuum system
that assists a small
volume of water in
flushing
Varieties: Few
for multi-unit
installations, e.g.
vacuum sewer
system. Above
floor, rear dis-
charge. Drain pipe
may be horizontal
or inclined
Requires vacuum
source, usually from
vacuum sewer
Increased noise
level
maintenance of
vacuum source
0.002 kWh per use
8Compared to conventional toilet usage (4.3 gal/flush, 3.5 uses/cap/d, and a total daily flow of 45 gpcd).
Table 4-12. Wastewater Flow Reduction—Nonwater-Carriage Toilets
Generic Type*
Pit privy
Biological privy
Biological toilets
Description
Hand-dug hole in the ground
covered with a seat in an
enclosed structure
May be sealed vault rather
than dug hole
Similar to pit privy except
organic matter is added after
each use. When pit is full it is
allowed to compost for a
period of about 12 months
prior to removal
Large units with a separated
decomposition chamber.
Accept toilet wastes and other
organic matter, and over a
long time period partially
stabilize excreta through
biological activity and
evaporation
Varieties: Several
Application Considerations
Requires same site conditions
as for Wastewater disposal
Handles only toilet wastes
Outdoor installation
Odor potential
Can be constructed
independent of site conditions
if sealed vault
Handles only toilet waste and
garbage
May be constructed by user
Outdoor installation
Residuals disposal
Installation requires 6 to 12 in.
diameter roof vent, space
beneath floor for
decomposition chamber,
ventilation system, and heating
Handles toilet waste and some
kitchen waste
Restricted usage capacity
cannot be exceeded
Difficult to retrofit and
expensive
Only units that meet
Scandinavian and NSF testing
standards should be used
O&M
When full, cover with 2 ft of soil
and construct new pit, unless
sealed vault
Addition of organic matter after
each use
Removal and disposal/reuse of
residuals may represent health
hazard
Periodic addition of organic
matter
Removal of product material at
6 to 24 month intervals should
be performed by management
authority
Power use: 0.3-1.2kWh/d
Heat loss through vent
48
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Table 4-12. Wastewater Flow Reduction—Nonwater-Carriage Toilets (continued)
Generic Type3
Incinerator
Description
Small self-contained units that
volatilize the organic com-
ponents of human waste and
evaporate the liquids
Varieties: Several
Application Considerations O&M
Installation requires 4-in.
diameter roof vent
Handles only toilet waste
Power or fuel required
Increased noise level
Residuals disposal
Limited usage rate(frequency)
Only units that meet NSF
testing standards should be
used
Weekly removal of ash
Semiannual cleaning and
adjustment of burning
assembly and/or heating
elements
Power: 1.2 kWh or 0.3 Ib LP
gas/use
aNone of these devices uses any water; therefore, the amount of flow reduction is equal to the amount of conventional toilet use:
16.2 gpcd or 36 percent of normal daily flow (45 gpcd). Significant quantities of pollutants (including N, BODs, SS, P, and
pathogens) are therefore removed from wastewater stream.
Table 4-13. Wastewater Flow Reduction—Showering Devices and Systems
Generic Type3
Shower flow-control inserts
and restrictors
Reduced-flow showerheads
On/off showerhead valve
Mixing valves
Air-assisted, low-flow shower
system
Description
Reduce flow rate by reducing
the diameter of supply line
ahead of shower head
Varieties: Many
Fixtures similar to con-
ventional, except restrict flow
rate
Varieties: Many manu-
facturers, but units similar
Small valve device placed in
the supply line ahead of
showerhead allows shower
flow to be turned on/off without
readjustment of volume or
temperature
Specifically designed valves
maintain constant temperature
of total flow. Faucets may be
operated (on/off) without
temperature adjustment
Specifically designed system
uses compressed air to
atomize water flow and provide
shower sensation
Application Considerations Water Use Rate (gpm)
Compatible with most existing 1.5-3.0
showerheads
Installed by user
Compatible with most
conventional plumbing
Installed by user
Compatible with most
conventional plumbing and
fixtures
Usually installed by plumber
Usually installed by plumber
Compatible with most
conventional plumbing and
fixtures
May be impossible to retrofit
Shower location <50 ft away
from water heater
Requires compressed air
source
Power source required
Maintenance of air compressor
Power: 0.01 kWh/use
1.5-3.0
Unchanged, but duration (and
waste) are reduced
Unchanged, daily duration and
use reduced
0.5
aNo reduction in pollutant mass; slight increase in pollutant concentration.
49
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Table 4-14. Wastewater Flow Reduction—Miscellaneous Devices and Systems
Generic Type*
Faucet Inserts
Faucet aerators
Reduced-flow faucet
fixtures
Mixing valves
Hot-water system
Insulation
Description
Device that inserts into faucet valve or supply line and restricts
flow rate with a fixed or pressure-compensating orifice
Varieties: Many
Devices attached to taucet outlet that entrain air into water flow
Varieties: Many
Similar to conventional unit, but restrict flow rate with a fixed or
pressure-compensating orifice
Varieties: Many
Specifically designed valve units that allow flow and
temperature to be set with a single control
Varieties: Many
Hot-water heater and piping is wrapped with insulation to
reduce heat loss
Varieties: Many
Application Considerations
Compatible with most plumbing
Installation simple
Compatible with most plumbing
Installation simple
Periodic cleaning of aerator screens
Compatible with most plumbing
Installation identical to conventional
Compatible with most plumbing
Installation identical to conventional
May be difficult to wrap entire
hot-water piping system
"No reduction in pollutant mass; insignificant increase in pollutant concentration.
4.5.2.3 Clotheswashing Devices and
Systems
The operation of conventional clotheswashers consumes
varying quantities of water depending on the manufac-
turer and model of the washer and the cycle selected.
For most, water usage is 87 to 201 L (23 to 53 gal)/use.
On the basis of home water-use monitoring, an average
water-use/wastewater flow volume of approximately 140 L
(37 gaiyuse has been identified, with the clotheswasher
contributing about 38 Lpcd (10 gpcd) or 22 percent of the
total daily water-use/wastewater flow (Table 4-2). Practi-
cal methods to reduce these quantities are somewhat
limited. Eliminating wasteful water-use habits, such as
washing with only a partial load, is one method. Front-
loading automatic washers can reduce water used for a
comparable load of clothes by up to 40 percent. In addi-
tion, wastewater flow reductions may be accomplished
through use of a clotheswasher with either adjustable cy-
cle settings for various load sizes or a wash-water recy-
cle feature.
The wash-water recycle feature is included as an op-
tional cycle setting on several commercially made wash-
ers. Selection of the recycle feature when washing
provides for storage of the wash water from the wash cy-
cle in a nearby laundry sink or a reservoir in the bottom
of the machine for subsequent use as the wash water for
the next load. The rinse cycles remain unchanged. Since
the wash cycle accounts for about 45 percent of the total
water use per operation, if the wash water is recycled
once, about 64 L (17 gal) will be saved, if twice, about
129 L (34 gal), and so forth. Actual water savings and
wastewater flow reductions are highly dependent on the
user's cycle selection.
4.5.2.4 Miscellaneous Devices and Systems
There are a number of additional devices, fixtures, and
appliances available to help reduce wastewater flow vol-
umes. These are directed primarily toward reducing the
water flow rate through sink and basin faucets. Table 4-
14 presents a summary of several of these additional
flow reduction devices. Experience with these devices in-
dicates that wastewater volume can be reduced by 4 to 8
Lpcd (1 to 2 gpcd) when used for all sink and basin faucets.
4.5.3 Wastewater Recycle and Reuse
Systems
Wastewater recycle and reuse systems collect and proc-
ess the entire wastewater flow or the fractions produced
by certain activities with storage for subsequent reuse.
The performance requirements of any wastewater recy-
cle system are established by the intended reuse activi-
ties. To simplify the performance requirements, most
recycle systems process only the wastewaters dis-
charged from bathing, laundry, and bathroom sink usage,
and restrict the use of the recycled water to flushing
water-carriage toilets and possibly to lawn irrigation. At
the other extreme, systems are under development that
process the entire wastewater flow and recycle it as a
potable water source.
The flow sheets proposed for residential recycle systems
are numerous and varied, and typically employ various
combinations of the unit processes described elsewhere
(Bennett and Linstedt, 1975; Witt, 1974; Hypes, et al.
50
-------
Table 4-15. Wastewater Flow Reduction—Wastewater Recycle and Reuse Systems
Flow-Sheet
Description
Recycle bath and
laundry for toilet flushing
Recycle portion of total
wastewater stream for
toilet flushing
Recycle toilet
wastewaters for flushing
water-carriage toilets
Application Considerations O&M
Requires separate toilet
supply and drain line
May be difficult to retrofit to
multistory building
Requires separate
wastewater disposal system
for toilet and kitchen sink
wastes
Requires separate toilet
supply line
May be difficult to retrofit to
multistory building
Periodic replenishment of
chemicals, cleaning of
filters and storage tanks
Residuals disposal
Power use
Cleaning/replacement of
filters and other treatment
and storage components
Residuals disposal
Total Flow
Reduction
(gpcd)
16
Requires disposal system for Periodic replenishment of
unused recycle water
Requires separate toilet
plumbing network
Utilizes low-flush toilets
Requires system for
nontoilet wastewaters
May be difficult to retrofit
Application restricted to high
use on multiunit installations
chemicals
Cleaning/replacement of
filters and other treatment
components
Residuals disposal
Power use
16
16
Comments
Foaming is a common
problem
Toilet water has a turbid
apperance
Foaming is a common
problem
Toilet water has a turbid
apperance
Colorant is usually used to
disguise flush-water
characteristics
Significant removal of
pollutants
Limited to high-volume,
commercial installations
Large capital investment
required
1974; Small Scale Waste Management Project, 1978). In
Table 4-15, generic units are characterized according to
their general recycle flow sheet.
4.6 Pollutant Mass Reduction
A second strategy for wastewater modification is directed
toward decreasing the mass of potential pollutants at the
source. This may involve the complete elimination of the
pollutant mass contributed by a given activity or the isola-
tion of the pollutant mass in a concentrated wastewater
stream. Several methods for pollutant mass reduction
are available. A few practical ones are described below.
4.6.1 Improved User Habits
Unnecessary quantities of many pollutants enter the
wastewater stream when materials that could be readily
disposed of in a solid waste form are added to the waste-
water stream. A few examples include flushing dispos-
able diapers or sanitary napkins down the toilet, or using
hot water and detergents to remove quantities of solidi-
fied grease and food debris from pots and pans to enable
their discharge down the sink drain.
4.6.2 Household Product Selection
The use of certain cleansing agents can contribute sig-
nificant quantities of pollutants. In particular, cleaning ac-
tivities, such as clotheswashing and dishwashing, can
account for over 70 percent of the phosphorus in resi-
dential wastewater (Table 4-4). Modern detergents have
reduced this percentage significantly due to lower phos-
phorus content, but very low P content products can re-
duce the total P content of wastewater to mg/L. Further
reductions in nutrients, metals, and toxic organic chemi-
cals are possible through product substitution. Additional
details are available elsewhere (Atkins and Hawley,
1978; Hathaway, 1980).
4.6.3 Elimination of the Garbage-Disposal
Appliance
The use of a garbage disposal contributes substantial
quantities of BODs and SS to the wastewater load (Table
4-4). As a result, it has been shown that the use of a gar-
bage disposal may increase the rate of sludge and scum
accumulation and produce a higher failure rate for con-
ventional disposal systems under otherwise comparable
conditions (Kreissl, 1985). For these reasons, and be-
51
-------
Human Waste
Privy
Compost Toilet
Very Low Volume
Flush Toilet
Closed-Loop
Recycle Toilet
Incinerator
Toilet
Disinfection
Treatment
Holding
Tank
Soil
Amendment
Onsite
Disposal
Disinfection
Land
Disposal
Sewage
Treatment Plant
Refuse
Figure 4-4. Selected Strategies for Management of Segregated Human Wastes.
cause most waste handled by a garbage disposal could
be handled as solid wastes, the elimination of this appli-
ance is advisable.
4.6.4 Segregated Plumbing Systems
Several toilet systems provide segregation and separate
handling of human excreta (often referred to as blackwater)
and, in some cases, garbage wastes. Removal of human
excreta from the wastewater stream serves to eliminate
significant quantities of pollutants, particularly SS, N, and
pathogenic organisms (Table 4-4).
A number of potential strategies for management of seg-
regated human excreta are presented in Figure 4-4. A
discussion of the toilet systems themselves is presented
in the wastewater flow reduction section of this chapter.
Wastewaters generated by fixtures other than toilets are
often referred to collectively as graywater. Charac-
terization studies have demonstrated that typical graywa-
ter contains appreciable quantities of organic matter, SS,
P, and grease in a daily flow volume of 110 Lpcd (29
gpcd) (Siegrist et al., 1976; Hypes et al., 1974) (see Ta-
ble 4-4). Its temperature as it leaves the residence is
about 31 °C (88°F), with a pH slightly on the alkaline side.
The organic materials in graywater appear to degrade at
a rate not significantly different from those in combined
residential wastewater (Hypes et al., 1974). Microbiologi-
cal studies have demonstrated that significant concentra-
tions of indicator organisms as total and fecal conforms
are typically found in graywater (see References gener-
ally). One should assume, therefore, that graywater har-
bors pathogens.
Design allowances should be made only for the reduc-
tions in flow volume, as compared to typical residential
wastewater. Although improved performance has been
demonstrated for graywater-dosed soil absorption sys-
tems (SAS) when compared to combined wastewater
SAS, (Malland, 1984) the variability of local codes dic-
tates that SAS size reduction should not exceed the hy-
draulic reduction. For example:
SAS Area = QGraywater/Qtotal Wastewater.
4.7 Onsite Containment—Holding
Tanks
Wastewaters may be contained onsite using holding
tanks, and then transported offsite for subsequent treat-
ment and disposal. However, this method is expensive
and often aesthetically undesirable and is considered a
"last resort" alternative. In many respects, the design, in-
stallation, and operation of a holding tank is similar to
that for a septic tank. Several additional considerations
do exist, however, as indicated in Table 4-16. A discus-
sion regarding the disposal of pumpage from holding
tanks is presented in Chapter 5.
4.8 Reliability
An important aspect of wastewater modification concerns
the reliability of a given method to yield a projected modi-
fication at a specific dwelling or establishment over the
long term. Reliability is of particular importance when de-
signing an onsite wastewater disposal system based on
modified wastewater characteristics.
52
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Table 4-16. Additional Considerations in the Design,
Installation, and Operation of Holding Tanks
Item
Sizing
Consideration
Liquid holding capacity >7 days wastewater
flow generation. Minimum capacity = 1,000 gal
Discharge There should be no discharge/exfiltration
Alarm system High-water alarm positioned to allow at least
3 days of use after activation
Accessibility Frequent pumping is likely; therefore holding
tank(s) must be readily accessible to pumping
vehicle and employ proper fittings to minimize
exposure
Flotation Large tanks may be subject to severe
flotation forces in high-groundwater areas
when pumped
Cost Frequent pumping and disposal of residuals
result in very high operating costs
Assessing the reliability of wastewater modification meth-
ods is a complex task that includes considerations of a
technological, sociological, economic, and institutional
nature. Major factors affecting reliability include:
• Actual wastewater characteristics prior to modification
compared to the average
• User awareness and influence on method perform-
ance
• Installation
• Method performance
• User circumvention or removal
In most situations, projections of the impact of a waste-
water modification method must be made, assuming the
wastewater characteristics prior to modification are rea-
sonably typical. If the actual wastewater characteristics
deviate significantly from that of the average, a projected
modification may be inaccurate.
The prospective user should be fully aware of the char-
acteristics of a method considered for use prior to its ap-
plication. Users who do not become aware of the
characteristics of a method until after it has been put into
use are more likely to be dissatisfied and attempt to cir-
cumvent or otherwise alter the method and negate the
wastewater modification expected.
In general, passive wastewater modification methods or
devices not significantly affected by user habits tend to
be more reliable than those that are subject to user hab-
its and require a preconceived active role by the users.
For example, a low-flush toilet is a passive device, while
a flow-reducing shower head is an active one. Toilets
that reduce flushing volume yet clean the bowl equally
well are considered passive since additional flushing
would not be required. Reduced shower flows can result
in longer showers, thus negating potential savings, and
are considered active devices.
Installation of any devices or systems should be made by
qualified personnel. In many situations, a post-installation
inspection is recommended to ensure proper functioning
of the device or system.
Method performance is extremely important in assessing
the reliability of the projected modification. Accurate per-
formance data are necessary to estimate the magnitude
of the reduction and to predict the likelihood that the
method will receive long-term user acceptance. Accurate
performance data can only be obtained through field
tests and evaluations. Since many methods and system
components are presently in various stages of develop-
ment, only preliminary or projected operation and per-
formance data may be available. These preliminary or
projected data should be considered cautiously.
The continued employment of a wastewater modification
method can be encouraged through several manage-
ment actions. First, the user(s) should be made fully
aware of the potential consequences should they discon-
tinue employing the modification method (e.g., system
failure, water pollution, rejuvenation costs). Also, the ap-
propriate management authority can approve only those
methods for which characteristics and merits indicate a
potential for long-term user acceptance. Further, installa-
tion of a device or system can be made in such a manner
as to discourage disconnection or replacement. Finally,
periodic inspections by a local inspector within the
framework of a sanitary district or the like may serve to
identify plumbing alterations; corrective orders could
then be issued.
In summary, to help ensure that a projected modification
will actually be realized at a given site, efforts can be ex-
pended to accomplish the following tasks:
• Confirm that the actual wastewater characteristics
prior to modification are typical.
• Make the prospective user(s) of the modification
method fully aware of the characteristics of the
method, including its operation, maintenance, and
costs.
• Determine if the projected performance of a given
method has been confirmed through actual field evalu-
ations by organizations other than those with a pro-
prietary interest.
• Ensure that any device or system is installed properly
by competent personnel.
• Prevent user removal or circumvention of devices,
systems, or methods.
4.9 Impacts on Soil-Based Treatment
and Disposal Practices
4.9.1 Modified Wastewater Characteristics
Reducing the household wastewater flow volume without
reducing the mass of pollutants contributed will increase
53
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the concentration of pollutants in the wastewater stream.
The increase in concentrations will likely be insignificant
for most flow reduction devices with the exception of
those producing flow reductions of 20 percent or more.
The effects of such changes on onsite treatment compo-
nents are not considered sufficient to justify design modifi-
cations for single-family homes. The increase in pollutant
concentrations in any case may be estimated utilizing
4.9.2 Wastewater Treatment and Disposal
Practices
Table 4-17 presents a brief summary of several potential
impacts that wastewater modification may have on soil-
based disposal practices. It must be emphasized that the
benefits derived from wastewater modification are poten-
tially significant. Wastewater modification methods, par-
ticularly wastewater flow reduction, should be considered
an integral part of any onsite wastewater disposal system.
1
£
.o
I
§
I
§.
Note: Assumes pollutant contributions are the same
under the reduced flow volume.
80
60
40
20
I
j_
0 10 20 30 40
Wastewater Flow Reduction
(% of total daily flow)
Figure 4-5. Flow Reduction Effects on Pollutant Concen-
trations.
Table 4-17. Potential Impacts of Some Wastewater Modification on Disposal Practices
Modification Practice
Disposal System Type
Subsurface
Surface discharge
Evapotransplration
Containment
(holding tank)
Potential Impact
May extend service life of functioning system
Reduce contamination of ground water and surface water
Reduce frequency of septic tank pumping
Reduce sizing of infiltrative area
Partially relieve hydraulically overloaded system
Reduce O&M costs
Reduce sizing and initial cost of components
Eliminate need for certain components, (e.g., N removal)
Remedy hydraulically overloaded system
Remedy hydraulically overloaded system
Reduce sizing of ET area
Reduce frequency of pumping
Reduce sizing of containment structure
Flow Reduction
X
X
X
X
X
X
X
X
X
X
X
Pollutant
Reduction
X
X
X
X
X
54
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4.10 References
When an NTIS number is cited in a reference, that refer-
ence is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
Anderson, J.S., and K.S. Watson. 1967. Patterns of
household usage. J. AWWA 59:1228-1237.
Atkins, E.D., and J.R. Hawley. 1978. Sources of metals
and metal levels in municipal wastewater. Environment
Canada/Ontario Ministry of the Environment Report No.
80.
Bendixen, T.W., R.E. Thomas, A.A. McMahan, and J.B.
Coulter. 1961. Effect of food waste grinders on septic
tank systems. Cincinnati, OH: Robert A. Taft Sanitary
Engineering Center.
Bennett, E.R., and E.K. Linstedt. 1975. Individual home
wastewater characterization and treatment. Completion
Report Series No. 66. Fort Collins, CO: Environmental
Resources Center, Colorado State University.
Brandes, M. 1978. Characteristics of effluents from sepa-
rate septic tanks treating grey and black waters from the
same house. J. WPCF 50:2547-2559.
Cohen, S., and H. Wallman. 1974. Demonstration of
waste flow reduction from households. EPA/670/2-74/071,
NTIS No. PB236 904.
Duffy, C.P., et al. 1978. Technical performance of the
Wisconsin mound system for on-site wastewater dis-
posal—An interim evaluation. Presented in preliminary
environmental report for three alternative systems
(mounds) for onsite individual wastewater disposal in
Wisconsin. Wisconsin Department of Health and Social
Services.
Enferadi, K.M., R.C. Cooper, S.C. Goransow, A.W.
Olivieri, J.H. Poorbaught, M. Walker, and B.A. Wilson.
1986. A field investigation of biological toilet systems and
grey water treatment. EPA/600/2-86/069, NTIS No.
PB86-23464.
Environment Canada. 1979. Cold climate utilities delivery
design manual. ETR EPS/3-WP-79-2.
Hathaway, S.W., Sources of toxic compounds in house-
hold wastewater. EPA/600/2-80-128, NTIS No. PB81-
110942.
Hunter, R.B. 1941. Water distribution systems for build-
ings. Building Materials and Structures Report BMS79.
Washington, DC: National Bureau of Standards.
Hunter, R.B. 1940. Method of estimating loads in plumb-
ing systems. Building Materials and Structures Report
BMS65. Washington, DC: National Bureau of Standards.
Hypes, W.D., C.E. Batten, and J. R. Wilkins. 1974. The
chemical, physical and microbiological characteristics of
typical bath and laundry wastewaters. NASA TN D-7566.
Langley Station, VA: Langley Research Center.
Jones, E.E., Jr. 1974. Domestic water use in individual
homes and hydraulic loading of and discharge from sep-
tic tanks. Proceedings of the National Home Sewage Dis-
posal Symposium, Chicago. Am. Soc. Agricultural Eng.,
St. Joseph, Ml.
Kreissl, J.F. 1985. North American and European experi-
ences with biological toilets. Proceedings of IAWPRC 1st
Asian conference on treatment, disposal and manage-
ment of human wastes, Tokyo.
Laak, R. 1975. Relative pollution strengths of undiluted
waste materials discharged in households and the dilu-
tion waters used for each. Manual of grey water treat-
ment practice, part 2, Santa Monica, CA: Monogram
Industries.
Ligman, K., N. Hutzler, and W.C. Boyle. 1974. Household
wastewater characterization. J. Environ. Eng. Div., Am.
Soc. Civil Eng. 150:201-213.
Linaweaver, F.P., Jr., J.C. Geyer, and J.K. Wolff. 1967. A
study of residential water use. Department of Environmental
Studies. Baltimore, MD: Johns Hopkins University.
Maddaus, W.O. 1987. The effectiveness of residential
water conservation measures. J. AWWA 79(3):52.
Metcalf and Eddy, Inc. 1979. Wastewater engineering:
Treatment/disposal/reuse. 2d ed. New York: McGraw-
Hill.
Molland, O. 1984. Testing of biological (composting) toi-
lets and practical experiences in Norway. Proceedings of
the international conference on new technology for
wastewater treatment and sewage in rural and suburb ar-
eas. Helsinki, Finland.
Olsson, E., L. Karlgren, and V. Tullander. 1968. House-
hold wastewater. Stockholm, Sweden: National Swedish
Institute for Building Research.
Otis, R.J. 1978. An alternative public wastewater facility
for a small rural community. Madison, Wl: Small-Scale
Waste Management Project, University of Wisconsin.
Proceedings of National Conference on Water Conserva-
tion and Municipal Wastewater Flow Reduction, Novem-
ber 28 & 29, 1978. Chicago, IL. EPA/430/09-79-015.
Siegrist, R.L. 1978. Management of residential grey
water. Proceedings of the second Pacific Northwest on-
site in wastewater disposal short course. Seattle: Univer-
sity of Washington.
Siegrist, R.L., M. Witt, and W.C. Boyle. 1976. Charac-
teristics of rural household wastewater. J. Env. Eng. Div.,
Am. Soc. Civil Eng. 102:553-548.
Small-Scale Waste Management Project, University of
Wisconsin, Madison. 1978. Management of small waste
flows. EPA/600/2-78-173, NTIS No. PB286-560.
55
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Tyler, E.J., W.C. Boyle, J.C. Converse, R.L. Siegrist, D.L.
Hargett, and M.R. Schoenemann. 1985. Design and
management of subsurface soil absorption systems.
EPA/600/52-85/070, NTIS No. PB85-216570.
Uniform Plumbing Code. 1976. International Association
of Plumbing and Mechanical Officials. Los Angeles, CA.
Wagner, E.G., and J.N. Lanoix. 1958. Excreta disposal
for rural areas and small communities. Monograph 39.
Geneva, Switzerland: World Health Organization.
Water Pollution Control Fed. 1976. Design and construc-
tion of sanitary and storm sewers. Manual of Practice No.
9. Washington, DC.
Watson, K., R.P. Farrell, and J.S. Anderson. 1967. The
contribution from the individual home to the sewer sys-
tem. J. WPCF 39:2039-2054.
Welckart, R.F. 1976. Effects of backwash water and re-
generation wastes from household water conditioning
equipment on private sewage disposal systems. Lom-
bard, IL: Water Quality Association.
Witt, M. 1974. Water use in rural homes. M.S. study. Uni-
versity of Wisconsin-Madison.
56
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CHAPTER 5
Technology Options
The summary of technologies in this chapter is provided
to further aid the process of evaluation and selection of
technologies by identifying both general factors and
those specific to particular locations. Technologies dis-
cussed include soil-based approaches, mechanical sys-
tems, and collection systems. In an effort to be concise,
not all the evaluation factors that are relevant to a tech-
nology or other considerations worthy of discussion have
been included.
To facilitate the evaluation and selection process, the
discussion of each technology is presented as criteria
that should be considered to meet a certain need or ap-
plication.
The criteria are:
• Technology Description—the function and operation
of the technology, with some discussion of typical ex-
pected removal rates and modifications.
• Applicability and Status—the extent of use of the
technology, along with typical flow ranges and com-
patibility with other options
• Advantages/Disadvantages—a relative comparison
of the technology in regard to a variety of factors, in-
cluding power consumption/energy cost, level of op-
erator skill required, potential for odor nuisance,
quality of effluent achievable, and sludge production
• Design Criteria—presentation of typical ranges of
process loading rates, land requirements, and per-
formance
• Capital Cost Sensitivity—consideration of the most
obvious significant costs related to the particular tech-
nology as well as other costs that should be assessed
such as pretreatment or transmission to the treatment site
• O&M Requirements—system complexity and opera-
tor skill requirements; reliability of mechanical equip-
ment and spare parts needed to maintain operation in
case of equipment failure
• Monitoring—frequency of sampling and level of
analysis relative to permit requirements; process con-
trol monitoring relative to complexity and sensitivity of
the system
• Construction Issues—desired level of operation and
other considerations related to this phase of the project
• Residuals—characterization of residuals generated
from the process, as well as volume relative to other
processes; options for removal and disposal of sludges
• Special Considerations—factors not covered by
other criteria such as special chemical requirements
5.1 Constructed Wetlands
5.1.1 Technology Description
Wetlands are lands where the water surface is near the
ground surface for enough of the year to maintain satu-
rated soil conditions and promote related vegetation.
Constructed wetlands are similar systems specifically de-
signed for wastewater treatment.
Constructed wetlands can be considered part of a waste-
water treatment system, while most natural wetlands are
considered receiving waters and are therefore subject to
applicable laws and regulations regarding discharge. The
influent to currently operating constructed wetland sys-
tems ranges from septic tanks to secondary effluents.
One constructed wetland is included as a component in a
system designed to treat septage.
The two different types of constructed wetlands are char-
acterized by the flow path of the water through the sys-
tem. The first, called a free-water surface (FWS) wetland,
includes appropriate emergent aquatic vegetation in a
relatively shallow bed or channel. The surface of the
water in such a system is exposed to the atmosphere as
it flows through the area. The second type, called a sub-
surface flow (SF) wetland, includes a foot or more of perme-
able media—rock, gravel, or course sand—that supports the
root system of the emergent vegetation. The water in the
bed or channel in such a system flows below the surface
of the media.
Both types of constructed wetlands typically include a
barrier to prevent ground-water contamination beneath
the bed or channel; barrier materials range from com-
pacted clay to membrane liners. A variety of wastewater
application methods have been used with constructed
wetlands, and a number of different outlet structures and
57
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methods have been included to control the depth of
water in the system.
5.1.2 Applicability and Status
As of 1991 a total of 143 communities were known to be
using or considering the feasibility of using a constructed
wetland system for wastewater management (Brown and
Reed, 1991). Systems in use include 31 FWS and 26 op-
erating SF wetlands. The range in size is from 2.2 to 880
Us (0.05 to 20 mgd) for FWS systems and from 0.04 to
132 L/s (0.001 to 3 mgd) for SF systems. (Because the in-
formation gathered to date is incomplete, the data that fol-
low may not contain information for all of the wetlands.)
Wastewater treatment using constructed wetlands has
come into wider use only since the late 1980s, although
a few FWS systems were in operation earlier. Although
currently there are slightly more FWS than SF systems in
operation, SF systems are projected to outnumber FWS
systems over time.
The majority of constructed wetland systems (70 percent
of FWS and 90 percent of SF) treat less than 44 Us (1
mgd). Smaller systems tend to be SF configurations
(mean - 13 L/s (0.3 mgd), median = 2 Us (0.04 mgd)).
Larger systems tend to be FWS configurations that range
in size from 2.3 to 880 Us (0.05-20 mgd) (mean = 85 L/s
(1.9 mgd), median « 20 L/s (0.5 mgd)).
5.1.3 Advantages/Disadvantages
Constructed wetlands offer the following advantages for
wastewater management:
• Low construction cost
• Passive system easily managed by small community
with O&M personnel
• Excellent removal of biochemical oxygen demand
(esp. BODs) and suspended solids (SS) from primary
or septic/lmhoff tank effluents
• Generally attractive systems with secondary ecologi-
cal benefits in terms of wildlife habitat enhancement
The following disadvantages should be taken in account
when considering a constructed wetland system:
• Lack of generally agreed-upon design factors, result-
ing in several unproven approaches to design of land-
intensive systems, especially FWS types where up to
4 ha/L/s (450 ac/mgd) have been required
• SF systems remain unproven for other than BODs and
SS removal
5.1.4 Design Criteria
No generally accepted consensus has been established
in the United States regarding design of constructed wet-
land systems. There are several schools of thought
concerning design approaches, ranging from unsubstan-
tiated empirical to semirational models based on very
limited data. Also, no consensus has been reached on
system configuration and other system details such as
aspect ratio, depth of water or media, type of media,
slope of surfaces and bottoms of beds, and inlet and out-
let structures. Moreover, pretreatment varies widely, with
facultative lagoons the most common form of pretreat-
ment. These are used at 41 percent of the FWS and 44
percent of the SF systems. Septic tanks are used at 24
percent of the SF systems. Aerated lagoons, secondary
treatment, and advanced treatment have also been used.
About one-third of the systems in operation use a mixture
of plant species; the other two-thirds use bulrush, cattail,
or reeds alone. One-third of the FWS systems use only
cattails, and 40 percent of the SF systems use only bul-
rush. Canna lilies, arrowhead, duckweed, reed canary
grass, and torpedo grass have also been used in con-
structed wetland systems.
The broad ranges in the following parameters for SF sys-
tems illustrate the lack of consensus regarding system
design: hydraulic loading rates varying from 0.02 to 0.4
m3/m2 x d surface area (0.5 to 10 gpd/sq ft) and 1 to 110
m3/m2 x d vertical facial area perpendicular to flow (25 to
2,700 gpd/sq ft); organic loading rates of 2 to 160 kg
BOD/ha x d (0.2 to 140 Ib/ac x d); gravel sizes of 6 to
130 mm (0.25 to 5 in.); and depths of 30 to 76 cm (1.0 to
2.5 ft). For comparison, the data below presents design
parameters for state-of-the-art systems used in Europe,
where over 500 constructed wetland sites have been es-
tablished.
Areal requirement = 5 m2/PE
1 PE = 56gpd(200L/d)
Performance:
BODrem: 80-90 percent
total Nrem: <30 percent
total Prem: <15 percent
Plants: Common reed (Phragmites australis)
Pretreatment: Primary clarification or septic/lmhoff tanks
Bed slopes: top = 0 percent
bottom = 1 percent toward outlet
Bed depth: average = 0.6 m (2 ft)
at inlet > 0.3m (1 ft)
Containing walls: slope = as vertical as possible
Freeboard: >0.5m
Liner: yes
Media: 3-10 mm (rounded)
Inlet: maximize horizontal distribution
Outlet: variable water level control (20 cm (8 in.) above
rock surface to bed bottom)
Width: determined by following Darcy's equation:
58
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Ac = Qs /kt (dh/ds)
where:
Ac = facial area perpendicular to flow (m2)
Qs = design flow (m3/s)
kf = hydraulic conductivity (with gravel sizes recom-
mended = 3 x IO"3 m/s)
dh/ds = slope of bed bottom (m/m)
Design parameters are similarly broad for FWS systems:
hydraulic loadings are 10 to 40 L/m2 surface area (0.2 to
1.0 gpd/sq ft) for BOD and SS removal (as high as 120
L/m2 x d (3 gpd/sq ft) for nitrification) and organic loadings
are 10 to 20 kg BOD/ha x d (9 to 18 Ib/ac x d) for BOD
and SS removal (as low as 1 kg/ha x d (0.9 Ib/ac/d) for
nitrification). Hydraulic residence time of FWS systems
should be at least 7 days, but as many as 365 days have
been used.
5.1.5 Capital Costs
Capital costs per unit of flow for both types of con-
structed wetlands in the United States are shown in Fig-
ure 5-1. The most sensitive costs are for land, earthwork,
liners, vegetation planting, and, for SF systems, fill media.
800
600
8
400
200
jlLJ
iimilili
-I $3.00/gpd
Designed AWT NBj Sec
Purpose: Removal Treatment
CWType
Free Water Surface
Region VI TVA Other Designed
Source
Subsurface Flow
CW Type
Figure 5-1. Capital Costs for Wetland Systems.
5.1.6 O&M Requirements/Costs
Constructed wetland systems should be inspected at
least weekly. This O&M requirement includes inlet and
outlet inspection and flow recording. Overall experience
has not yet allowed quantification of these costs, but they
are considered negligible in comparison to treatment al-
ternatives. Sidewall maintenance is also required.
5.1.7 Monitoring
With constructed wetlands, vegetation growth should be
monitored and standard sampling and analysis require-
ments of the permit should be fulfilled.
5.1.8 Construction Issues
Considerations concerning the construction of wetlands
for wastewater treatment include:
• Media for SF systems should be rounded and washed
prior to placement
• Vertical sidewalls for SF systems ensure proper vege-
tation at edge of bed
• Use of gabions and large slopes at the inlet simplifies
construction and ensures good horizontal distribution
of flow in SF systems
5.1.9 Residuals
Although not generally practiced, harvesting of an SF
system's vegtetation would enhance removal of nutrients
at the cost of increased O&M and residuals disposal
problems. It is also possible that eventual dislodging may
be required, but quantification of this measure is not yet
available. SF systems should not be immediately pre-
ceded by facultative lagoons since algae will aggravate
any clogging problems.
5.1.10 Special Considerations
The role of plants in a constructed wetland system is at
present undefined. Moreover, the hybrid designs of con-
structed wetlands will evolve such that their present ca-
pabilities will be upgraded. At this time, however, the SF
systems and small FWS systems should not be expected
to remove more than BOD and SS in significant quanti-
ties; however, very large (very lightly loaded) systems
may also accomplish nitrification.
5.2 Rapid Infiltration
5.2.1 Technology Description
Rapid infiltration is a soil-based wastewater treatment
method that typically consists of a series of earthen ba-
sins with exposed soil surfaces designed for a repetitive
cycle of flooding, infiltration/percolation, and drying (Fig-
ure 5-2). The method depends on a relatively high rate of
wastewater infiltration into the soil and percolation
through a vadose or unsaturated soil zone below the infil-
trative surface before recharge to the ground-water table.
System design is based on the capability of soils to pro-
vide acceptable treatment before the percolate reaches
the ground water.
5.2.2 Applicability and Status
Rapid infiltration is a proven technology for year-round
treatment of domestic, municipal, and other organic
wastewaters. Its application is limited primarily by soil
characteristics, ground-water impacts, and land costs. It
is not well suited for inorganic industrial wastewaters.
5.2.3 Advantages
Rapid infiltration is a method of land application providing
very favorable removal of the conventional wastewater
parameters—including ammonia—that is simple to oper-
ate and requires only a minimum of operator intervention.
Moreover, it requires less land area than other land appli-
cation methods and may be operated year round. It is a
59
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Applied
Wastewater
Evaporation
Percolation
(a) Hydraulic Pathway
Flooding Basin
GW Mound
(b) Natural Drainage into Surface Waters
Figure 5-2. Schematic of a Rapid Infiltration Facility (EPA, 1981).
"zero-discharge" method that provides ground-water
recharge rather than requiring an outfall for direct dis-
charge to surface water. Frequently, renovated wastewa-
ter is recovered via wells for reuse in irrigation.
5.2.4 Disadvantages
Use of rapid infiltration is limited by site and soil charac-
teristics that affect the capability of the soil to accept and
treat the applied wastewater. Potential ground-water im-
pacts from nitrate nitrogen are a serious concern and
may also limit its application.
5.2.5 System Design
5.2.5.1 Site Selection
Site selection is based on the treatment capacity of the
soils and the availability of sufficient land area of suitable
topography. The treatment capacity of the soil is depend-
ent primarily on texture, structure, and unsaturated thick-
ness. No soil type provides optimum conditions for the
removal of all wastewater constituents. Fine-texture soil,
such as silt and clay loams, has a relatively low hydraulic
conductivity and is therefore unsuitable for rapid infiltra-
tion. Coarse soil, such as sand, has a higher rate of hy-
draulic conductivity and reaeration to allow higher
hydraulic and organic loading and shorter cycles for ba-
sin resting, which can permit rapid infiltration. However,
coarse soil can be a less-effective physical filter, have a
lower cation exchange capacity, and allow more rapid
percolation of wastewater through the vadose zone
where most treatment occurs. Typically, sites with sand
to sandy loam soils are selected. A summary of typical
treatment performance data is presented in Table 5-1.
60
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Unsaturated depth of soil is another critical site-selection
criterion. A minimum of 1.5 to 2.5 m (5 to 8 ft) of unsatu-
rated soil with relatively uniform hydraulic conductivity is
necessary below the infiltrative surface to provide neces-
sary treatment. Greater unsaturated thicknesses are usu-
ally required in coarse, granular soils to provide longer
residence times. Ground-water mound height analysis
must be performed to determine if the separation dis-
tance can be maintained during system operation. If the
desired separation distance cannot be maintained, the
hydraulic loading can be reduced, basin geometry can be
changed to have a high-aspect ratio in relation to the
ground-water gradient, basin spacing can be increased,
or ground-water drainage can be provided.
5.2.5.2 Hydraulic Loading Rate
The design hydraulic loading rate is determined by soil
characteristics, ground-water mounding potential, treat-
ment requirements, applied wastewater quality, and pre-
cipitation. The maximum hydraulic loading is established
by the hydraulic conductivity of the least permeable soil
horizon in the vadose zone. Typically, 4 percent of the
measured saturated hydraulic conductivity is used as a
preliminary estimate. This rate may be adjusted down-
ward to limit the BOD loading to within 21 to 126 kg/ha/d
(19 to 112 Ib/ac/d) to control infiltrative surface clogging.
Primary treatment is the minimum level of pretreatment
suggested, but where the public has even limited access,
secondary treatment is required. If there is concern about
nitrogen contributions to the ground water below the sys-
tem, nitrogen rather than BOD may control the hydraulic
loading (pattern and total rate) and pretreatment. Poten-
tial for excessive ground-water mounding and other site-
specific issues may require the hydraulic loading to be
reduced further. Typical average hydraulic loading rates
are 1.5 to 35.0 cm/d. To assist in selecting appropriate
hydraulic loading rates, loadings used by systems oper-
ating under similar conditions should be compared (EPA,
1981,1984; Reed & Crites, 1984; Overcash & Pal, 1979;
WPCF, 1990).
Table 5-1. Typical Rapid Infiltration System Performance3
5.2.5.3 Application Patterns
The selected application pattern for basin operation will
determine the number and size of the basins and the
land area required. The pattern is selected to control infil-
trative surface clogging and/or to promote nitrogen re-
moval (Lance et al., 1976; Lance, 1984). Typical ratios of
application periods to drying periods vary from 0.2,
where maximum hydraulic loading is the objective, to 1.0
for maximum nitrogen removal. Suggested loading cy-
cles are presented in Table 5-2.
5.2.6 Design Modifications
Underdrains or extraction wells may be used to lower the
water table to increase the unsaturated depth of soil be-
low the infiltrative surface or to reclaim the renovated
water. Information on the design of underdrain systems
can be found elsewhere (USDA, 1971; U.S. Dept. of Inte-
rior, 1973; Luthin, 1973). Some states require an aquifer
Table 5-2. Suggested Hydraulic Loading Cycles for Rapid
Infiltration
Loading
Cycle
Objective
Maximize
infiltration
rates
Maximize
nitrogen
removal
Maximize
nitrification
Systems'1
Applied
Wastewater
Primary
Secondary
Primary
Secondary
Primary
Secondary
Application
Season
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Summer
Application
Period"
(days)
1-2
1-2
1-3
1-3
1-2
1-2
7-9
9-12
1-2
1-2
1-3
1-3
Drying
Period
(days)
5-7
7-12
4-5
5-10
10-14
12-16
10-15
12-16
5-7
7-12
4-5
5-10
aEPA, 1981.
" A rvrtll/^otlrtrt r-ii-M'ti-ijrJrt -Fj-tt- rvKii^ri <-tfi J i»i£flrmn4> nliAi ild-J L^M I ! wi !+ ** ,*J *j«
1-2 days to prevent excessive clogging. Periods maybe
affected by the rate of ground-water mounding (rise).
Parameter
BOD5
Nitrogen
Phosphorus
Toxic organics
Fecal coliforms
Virus
Loading (kg/ha x d) Removal
45-158
3-37
1-12
NA
NA
NA
86-98%
10-80%
29-99%
Varies with
structure
2-6 logs
2-4 logs
Comments
Lower removals associated with high loading rates on coarse soils
Removal depends on preapplication treatment, BOD:N, climate,
hydraulic loading, and wet/dry cycle
Removal correlates with soil texture, time of service, soil
mineralogy, and travel distance
Favorable removal of volatile and biodegradable organics appears
to occur where the subsoil remains aerobic
Removals correlate with soil texture, travel distance, and resting
time
Limited data suggest favorable removals with low loadings,
fine-texture soil, aerobic subsoil, and high temperatures
aWPCF, 1990; Nilsson, 1991.
NA = not applicable
61
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protection permit, which calls for demonstration of non-
degradation and, in some cases, upgrading of pretreat-
ment and monitoring requirements.
Continuously flooded infiltration basins are a modified
operation method that has been used where high-rate in-
filtration is desired. Treatment performance of such sys-
tems is unfavorable because of the rapid percolation and
the elimination of subsoil reaeration and aerobic conditions.
5.2.7 Capital Cost Sensitivity
Capital costs of rapid infiltration projects are controlled
primarily by land costs and earthwork. Earthwork includes
infiltration surface preparation and dike construction. If the
topography is not favorable for basin construction, site
grading may add a significant cost. Other associated
costs that may be significant are installation of under-
drains, if needed, transmission of wastewater to the
treatment site, and pretreatment.
5.2.8 O&M Requirements
The primary O&M requirements of rapid infiltration sys-
tems include basin cycling, infiltration surface mainte-
nance, winter operations, nitrogen management, and
monitoring. Other O&M tasks include maintenance of
pumps and other equipment and basin dikes. These
tasks are easy to perform and require little specialized
training. No special equipment is necessary except for a
tractor and harrow for infiltration surface maintenance. A
useful description of these tasks is provided by EPA (1984).
Costs of O&M are associated primarily with labor costs.
Other costs include power costs for wastewater pumping
and equipment depreciation. Total labor requirements,
excluding maintenance of the pretreatment works, should
not exceed 10 to 15 hr/wk. O&M costs are in the range of
5 to 10 cents/1,000 gal of wastewater treated.
5.2.9 Construction Issues
Construction activities include infiltration, surface prepa-
ration, dike construction, and installation of basin piping,
inlet structures, and, where needed, basin underdrains.
These construction activities are as important as site
evaluation in achieving successful performance of a
rapid infiltration system. Infiltration surface preparation is
the most critical factor. The surface must be carefully lev-
eled to within 5 cm (2 in.) of the specified finished eleva-
tion in each basin with a minimum of surface
compaction. Once the surface is fine-graded, it should be
ripped and cross-ripped to a depth of 60 cm (2 ft). Con-
struction practices should minimize equipment travel in
the direction of ground-water movement. The dikes be-
tween the basins must prevent leakage from the basins
and provide vehicular access to each basin, and they
must be graded such that erosion of the embankments
does not occur. Proper construction procedures are dis-
cussed elsewhere (EPA, 1984; WPCF, 1990).
5.2.10 Monitoring
Monitoring requirements cover wastewater volumes ap-
plied to each basin, daily wastewater ponding levels in
each basin, basin cycle times, applied wastewater qual-
ity, and ground-water elevations and quality.
5.2.11 Residuals
No residuals are usually produced with rapid infiltration
systems. Occasional surface scrapings and residual al-
gae, for instance, may be buried onsite given the minimal
volume of such residuals.
5.3 Stabilization Ponds
5.3.1 Technology Description
Facultative lagoons, or ponds, are the most frequently
used form of municipal wastewater treatment in the
United States, with over 5,000 systems currently in op-
eration. These lagoons are usually 1.2 to 1.8 m (4 to 6 ft)
in operating depth and are not mechanically mixed or
aerated. The layer of water near the surface is aerobic
due to atmospheric reaeration and algal respiration. The
layer at the bottom of the lagoon is anaerobic and in-
cludes sludge deposits. The intermediate layer, termed
the facultative zone, ranges from aerobic near the top to
anaerobic at the bottom. The layers may be indistinct or
be clearly defined due to temperature-related water-
density variations. Disruptions can occur in the spring
and fall of each year when the surface layer of melted ice
may have a higher density than lower layers. This higher
density water induces vertical movement, mixing the
pond contents and producing objectionable odors due to
the release of anaerobically formed gases during these
unstable periods.
The presence of algae in the near-surface water is es-
sential to several aspects of treatment performance of
facultative ponds. In the presence of sunlight the algal
cells take up carbon dioxide from the water and release
oxygen. On warm, sunny days the oxygen concentration
in the near-surface water can exceed saturation levels. In
addition, because of the temporary carbon dioxide re-
moval, the pH of the near-surface water can exceed 10.
This results in conditions favorable for ammonia removal
via volatilization, and long-detention-time facultative ponds
have been shown to be very effective for ammonia re-
moval. This photosynthetic activity by the algae occurs
on a diurnal basis so that both oxygen and pH levels shift
from a maximum during daylight hours to their minimums
at night.
The oxygen produced by the algae and by surface
reaeration is used by aerobic bacteria to stabilize the or-
ganic material in the upper layer of water. Anaerobic fer-
mentation is the dominant activity in the bottom layer in
the lagoon. Both of these reaction rates are significantly
62
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reduced during the winter and early spring months in
cold climates. Effluent quality may be reduced to the
equivalent of long-detention-time settling when an ice
cover persists on the water surface. For this reason,
many states in the northern United States and in Canada
prohibit discharge from facultative lagoons during the
winter months.
Typical detention times range from 20 to 180 days for
continuously discharging facultative lagoons. Detention
times can approach 200 days in northern climates where
discharge restrictions prevail. Effluent BOD <30/mg/L
can usually be achieved; effluent TSS may range from
<30 mg/L to over 100 mg/L depending on the algal con-
centrations. Many existing facultative lagoons are large,
single-cell systems, with the inlet constructed near the
center of the cell. That configuration can result in under-
mining the design volume of the system. A multiple-cell
system, with at least three cells, is recommended with
appropriate inlet and outlet structures to maximize the
utilization of the design volume.
Although the concept is somewhat land-intensive, par-
ticularly in northern climates, stabilization ponds offer a
reliable, easy-to-operate process that makes it attractive
for small, rural communities where sufficient land is often
available.
5.3.1.1 Aerated Lagoons
Aerated lagoons are smaller and deeper than facultative
lagoons and are designed for biological treatment of
wastewater on a continuous basis. In contrast to stabili-
zation ponds, which obtain oxygen from photosynthesis
and surface reaeration, aerated lagoons typically employ
devices that supply supplemental oxygen to the system.
Aerated lagoons evolved from stabilization ponds when
aeration devices were added to counteract odors arising
from anaerobic conditions. The aeration devices may be
mechanical (i.e., surface aerator) or diffused air systems
using submerged or overhead pipes. Surface aerators
are divided into two types: caged aerators and the more
common turbine and vertical shaft aerators. Diffused air
systems utilized in lagoons typically consist of plastic
pipes supported near the bottom of the lagoon cells with
regularly spaced sparger holes drilled in the tops of the
pipes, although overhead air headers and finer bubble
systems have recenty been applied. Because aerated la-
goons are normally designed to achieve partial mixing
only, aerobic/anaerobic stratification may occur, and
large fractions of incoming solids and the biological sol-
ids produced from waste conversion can settle to the bot-
tom of the lagoon cells.
5.3.1.2 Controlled Discharge
A common operational modification is the controlled-
discharge mode where pond discharge is prohibited dur-
ing the winter months in cold climates and/or during the
peak algal growth periods in all climates. In this ap-
proach, each cell in the system is isolated and then dis-
charged sequentially. A common operational modification to
aerated and facultative lagoons is the controlled-discharge
pond system. Sufficient storage capacity is provided in the
lagoon system to allow wastewater storage during winter
months, peak algal growth periods, or receiving-stream
low-flow periods. In this approach, each cell in the sys-
tem is isolated and then discharged sequentially. In the
Great Lakes states of Michigan, Minnesota, and Wiscon-
sin, as well as in the province of Ontario, many pond sys-
tems are designed to discharge in the spring and/or fall
when water quality effects are minimized. As a secon-
dary benefit, operational costs are lower than for a con-
tinuous discharge lagoon because of reduction in
laboratory monitoring requirements and the need for less
operator control. A similar modification is called a hy-
drograph controlled release lagoon (HCRL) where water
is retained in the pond until flow volume and conditions in
the receiving stream are adequate for discharge, thus
eliminating the need for costly additional treatment.
5.3.1.3 Modifications
A recently developed physical modification to upgrade
the performance of existing one- or two-cell systems in-
volves dividing existing lagoons into multiple cells to im-
prove hydraulic conditions. Another recent patented
development uses a floating plastic grid to prevent the
transport of duckweed (Lemna sp.) plants on the surface
of the final cell(s) in the lagoon system. This duckweed
cover restricts the penetration of light to destroy algae
and improve effluent SS quality. With sufficient detention
time (>30 days) and intensive harvesting, significant nu-
trient removal may be possible. Some systems also have
the capability to draw effluent from several levels and
thereby avoid the high algal concentrations near the
water surface during discharge.
The rock filter, used as an alternative means of removing
algae from lagoon effluents, consists of a submerged
bed of rocks (5 to 20 cm (2 to 8 in.) diameter) through
which the lagoon effluent is passed vertically or horizon-
tally, allowing the algae to be removed in the rock filter.
The basic simplicity of O&M is the key advantage of this
process. The effluent quality achievable and the depend-
ability of long-term operation, however, have not been
well documented yet. Rock filtration following treatment
in single-cell lagoons has been problematic and therefore
is not recommended.
5.3.1.4 Sand Filter
The intermittent sand filter is an outdoor, gravity-actuated,
slow rate filtration system that capitalizes on the avail-
ability of land area. It is a biological and physical
wastewater treatment mechanism consisting of an
underdrained bed of granular material, usually sand. The
filter surface is flooded intermittently with lagoon effluent
at intervals that permit the surface to drain between ap-
plications. It is recommended that the flow be directed to
one filter for 24 hours. That filter is then allowed to drain
63
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and dry for one to two days, and the flow goes to an adja-
cent filter. It is preferable to have three filter beds where
good operation and treatment can be accomplished over a
three-day cycte. (This technology is described in more de-
tail in Section 5.3.4.6.) The system can remove suspended
solids (algae) and BOD as well as convert ammonia to
nitrate-nitrogen.
5.3.2 Applicability and Status
Lagoon (wastewater stabilization pond) technology, the
most common means of wastewater treatment in the
United States, is primarily used in smaller communities.
The concept is well-suited for rural communities and in-
dustries where land is readily available but skilled mainte-
nance is not. Lagoons that discharge to larger receiving
streams have been designated as "equivalent to secon-
dary treatment" by the EPA. However, many existing la-
goons will require upgrading by methods such as those
described above if they cannot meet stream effluent re-
quirements or if they are leaking into the ground water.
5.3.3 Advantages/Disadvantages
The advantages of stabilization ponds include the follow-
ing:
• Minimum operational skills required
• Low capital cost requirements
• Many means of upgrading available, minimizing capi-
tal outlays where lagoons already exist
• Sludge disposal required only at 10- to 20-year intervals
Disadvantages of this technology include:
• Large land area requirements
* Continous flow (traditional) facultative ponds cannot
meet stringent effluent standards during warm sea-
sons without upgrading
• Lagoons can negatively impact ground water if an inade-
quate liner is installed or if an existing liner is damaged
• Most lagoons discharge to smaller, water-quality-limited
streams and may, therefore, require upgrading
• Several upgrading techniques are not fully charac-
terized, making their adoption speculative
5.3.4 Design Criteria
5.3.4.1 Facultative Lagoons
Every system should have a series of at least three cells.
Most states have design criteria that specify the areal or-
ganic loading (Ib/ac/d) and hydraulic residence time.
Typical organic loading for the system is 22 to 67 kg
BOD/ha/d (20 to 60 Ib/ac/d). Overall typical detention
times range from 20 to 180 days or more, depending on
location.
A number of empirical models exist for design of faculta-
tive lagoons. These include first-order plug flow, first-
order complete mix, and models proposed by Gloyna,
Marais, Oswald, and Thirumurthi. None of these have been
shown to be clearly superior to the others. All will provide a
reasonable design if the proper parameters are selected
and if the hydraulic characteristics of the system are known.
Depth is normally 1.5 to 1.8 m (5 to 6 ft), although a
range of 0.9 to 2.4 m (3 to 8 ft) is often suggested. Typi-
cal removals of BODs are 75 to 95 percent, but SS may
vary from negligible to 90 percent. A high degree of toxic
organics and metals are generally removed. Ammonia
conversion (20 to 80 percent) and phosphorus removal
(10 to 50 percent) can be significant in warm, algae-con-
trolled periods. Likewise, fecal coliform reductions are
dependent on the detention time and are quite significant
during these periods.
5.3.4.2 Aerated Lagoons
Every system should have a series of at least three cells
that are lined (or similarly secured) to prevent adverse
ground-water impacts. Many states have design criteria
that specify the design loading, the hydraulic residence
time, and the aeration requirements. Pond depths range
from 1.8 to 6 m (6 to 20 ft), with 3 m (10 ft) being typical
(the shallow depth systems are usually converted facul-
tative lagoons). Detention times range from 3 to 20 days,
with 10 days being typical (shorter detention times use
higher intensity aeration). The design of these systems
for BOD removal is based on first-order, complete-mix ki-
netics. Even though the system is not completely mixed,
a conservative design will result. The model commonly
used is:
1
Ce
Co'
Where:
Ce = effluent BOD
Co = influent BOD
KT = temperature-dependent rate constant 12.51"1 at
20°C)
t = total detention time in system
n = number of equal-size cells in system
Oxygen requirements are typically 30 to 40 hp/Mgal ca-
pacity; they may range as high as 100 hp/Mgal where
complete mixing is desired.
BOD removal can range from 80 to 95 percent. Effluent
total suspended solids (TSS) can range from 20 to 60
mg/L depending on the concentration of algae in the ef-
fluent. Removal of ammonia nitrogen is usually less ef-
fective than in facultative lagoons. Nitrification of
ammonia can occur in underloaded aerated lagoons or if
the system is specifically designed for that purpose.
Phosphorus removal is also less effective than in faculta-
tive lagoons due to the more stable pH and alkalinity
conditions. Phosphorus removals of about 10 to 20 per-
64
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cent might be expected. Removal of conforms and fecal
conforms can be effective, depending on detention time
and temperature. Disinfection will be necessary to meet
effluent limits consistently.
5.3.4.3 Rock Filters
Key considerations for rock filters include:
• Hydraulic loading rates (HLR): 0.3 to 0.9 m3/m3/d (7.5
to 22 gpd/sq ft)
• Submerged inlets
• 7.5 to 15 cm (3 to 6 in.) rock
• SS removal (percent) = 100 - 66H (H=HLR in m3/m3/d)
• At least three cells are required in the system that pre-
cede the rock filter
5.3.4.4 Hydrograph Controlled Release
Lagoon (HCRL) Systems
An HCRL system has three principal components: a
stream-flow monitoring system, a storage cell, and an ef-
fluent discharge system. The stream-flow monitoring sys-
tem measures the flow rate in the stream and transmits
this data to the effluent discharge system. The effluent
discharge system consists of a controller and a dis-
charge structure. The controller operates a discharge de-
vice (e.g., a motor-driven sluice gate) through which
wastewater is discharged from the storage lagoon; how-
ever, these tasks can be manually performed.
Key considerations for HCRLs include:
• Need release model keyed to flow (as measured by
depth) in receiving stream
• Outlets should be capable of drawing from different
depths to ensure best quality
• Storage cells sized by use of water-balance equations
5.3.4.5 Duckweed Systems
The key considerations for duckweed systems include:
• A 30-day retention time with shallow depth and fre-
quent harvesting required (= every 1 to 3 days at peak
season) if nutrient removal sought
• Plan must be in place on fate of harvested plants (i.e.,
processing on site, transportation, and ultimate fate)
• Manageability by local authorities is yet to be estab-
lished (need contract management)
• May need post-aeration to meet effluent dissolved
oxygen (DO) requirements
5.3.4.6 Intermittent Sand Filters
The preferred design (EPA, 1980) for lagoon upgrading
is as follows:
• 3 cells in series, all 76 to 91 cm (30 to 36 in.) deep
• Progressively finer sands (e.g., effective sizes of 0.72
mm, 0.40 mm, and 0.17 mm)
• Effluent quality = 10 mg/L of BODs and SS
• Complete nitrification expected, except under ex-
tremely cold conditions
This configuration is considered to provide the most ef-
fective approach (in terms of achieving the most favor-
able effluent quality attainable) for lagoon upgrading.
5.3.5 Capital Costs
5.3.5.1 Facultative Lagoons
a. Assumptions
1. Design basis:
Areal loading method, 45 kg BOD/ha x d (40 Ib/ac x d)
in warm climates, 22 kg BOD/ha x d (20 Ib/ac x d) in
the primary cells only in cold climates
Water depth 1.5m (5 ft)
Effluent BOD = 30 mg/L
Effluent TSS = 60 mg/L
2. Costs based on November 1990 prices (ENR = 4780)
3. Construction costs include excavation, grading, berm
construction, and inlet and outlet structures. Costs
do not include a lagoon liner, purchase of the land,
or delivery of wastewater to the site.
4. The construction cost equations below are valid for a
preliminary estimate for design flows up to 440 Us
(10mgd), where
C = Costs (in millions of dollars)
Q = Wastewater flow (mgd)
b. Construction Costs
Warm climates: C = 0.750(Q)°-729
Cold climates: C = 1.800(Q)a711
5.3.5.2 Aerated Lagoons
a. Assumptions
1. Design basis:
Influent BOD = 210 mg/L
Effluent BOD = 30 mg/L
Depth = 3m(10ft)
Detention time = 10 days
Floating mechanical aerators at 10 hp/Mgal
2. Costs based on November 1990 prices (ENR = 4780).
3. Construction costs include excavation, grading, berm
construction, and inlet and outlet structures, and
membrane liner. Costs do not include purchase of
the land or delivery of wastewater to the site.
4. The construction cost equations below are valid for a
preliminary estimate for design flows up to 440 Us
(10 mgd), where
C = Costs (millions of dollars)
Q = Wastewater flow (Mgal/a)
65
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a. Construction Costs
0-1
0.310(Q)'
,0.301
,0.412
0.513
,0.732
1-1 Omgd:C = 0.31 0(Q)
5.3.5.3 Rock Filters and Intermittent Sand
Filters
Information on costs is currently insufficient to provide
firm estimates, but construction costs of $1.00/gpd of
capacity should be considered quite conservative.
5.3.5.4 Duckweed and HCRL
Cost information is currently insufficient to serve as a
basis for providing estimates.
5.3.6 O&M Costs
5.3.6.1 Facultative Lagoons (All Climates)
C = 0.016(Q)°-496
5.3.5.2 Aerated Lagoons
0-1 mgd: C « 0.025 (Q) "'"
1-10mgd:C = 0.025(Q)
5.3.6.3 Upgrading Systems
A range of 50 to 75 cents per year per gallons per day of
capacity is a reasonable estimate of O&M costs for inter-
mittent sand filters.
Controlled discharge, HCRL, and duckweed and rock
filters: No valid data.
5.4 Overland Flow
5.4.1 Technology Description
Overland flow is a land application method of wastewater
treatment with a point discharge to a surface water. The
technology consists of a series of uniformly sloped, vege-
tated terraces with a wastewater distribution system lo-
cated at the top of the terrace and a runoff collection
channel at the bottom (Figure 5-3). Facilities for waste-
water storage during wet or freezing weather are also
provided. In overland flow, wastewater is applied inter-
mittently across the top of the terraces and allowed to
sheet flow over the vegetated surface to the runoff col-
lection channel. The system is not designed for soil per-
colation, though some percolation may occur.
Treatment is achieved primarily through sedimentation,
filtration, and biochemical activity as the wastewater
flows through the vegetation on the terraced slope. SS
settle or are filtered from the flow to be degraded or in-
corporated into the soil. Effluent TSS concentrations of
less than 10 mg/L are possible. However, algae removal
is not consistent because many algal cells are buoyant or
motile and resist removal by sedimentation or filtration.
Organics are degraded by biological films attached on
the plant and soil surfaces. BODs removals of 90 percent
are often achieved. Total nitrogen removals of up to 80
percent can be achieved, but will vary with the mass
loading, ambient temperature, and other environmental
factors and are more comrrjonly much lower. The princi-
pal removal mechanism is biological denitrification, but
some plant uptake and ammonium volatilization may oc-
cur. Phosphorus may be partially removed by soil ad-
sorption (in the early operational stage) and plant uptake.
Removal efficiencies as high as 50 percent are possible,
but 20 to 60 percent is common). Overland flow is not ef-
fective in pathogen removal, however. Little is known
about treatment performance for metals and toxic organics
removals. Because treatment is dependent on active
biomass and vegetation, the terraces are operated on
wet/dry cycles and applications are ceased during freezing
periods. Further information on treatment performance is
available elsewhere (WPCF, 1990).
5.4.2 Applicability and Status
Overland flow first gained widespread use in the early
1970s. It was developed for areas where soils are poorly
suited for other land application methods dependent on
ground-water recharge for ultimate disposal of the
treated wastewaten Today, overland flow i$ successfully
used by many small communities, primarily in the south-
ern half of the United States. Its use is limited primarily
by land suitability, climate, and land costs.
5.4.3 Advantages
Overland flow is well suited for wastewater treatment by
rural communities and seasonal industries with organic
wastes. It provides secondary or advanced secondary
treatment, yet is relatively simple and inexpensive to op-
erate. If the vegetative cover can be harvested and sold
(e.g., as forage), overland flow can provide an economic
return from the reuse of water and nutrients. Of the land
application methods of wastewater treatment, overland
flow is the approach least restricted by soil charac-
teristics; however, this method does require a relatively
impermeable soil for conventional operation.
5.4.4 Disadvantages
Overland flow is primarily limited by climate, crop water
tolerances, and land slope. Application is restricted dur-
ing wet weather and can be limited when temperatures re-
main below freezing. Application rates may be restricted by
the type of crop grown. Steeply sloping or flat terrain is
not well suited for this method of treatment. Disinfection
is required in order to meet effluent permit requirements
prior to discharge (unique to land application processes).
5.4.5 Design Criteria
5.4.5.? Site Selection
Topography is the most important site-selection criterion
because of its impact on the costs of earthwork. Terrace
slopes should be between 2 and 8 percent and relatively
uniform, with sufficient length to provide adequate travel
time for treatment. Flatter slopes may result in wastewa-
ter ponding on the terraces, and erosion may be signifi-
cant on steeper slopes. Terrace lengths are typically 30
to 60 m (100 to 200 ft). South-facing slopes are preferred
66
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Wastewater
Application
by Surface
Spray, or
Sprinkler
Methods
Water
Tolerant
Grasses
Terrace
Back
Slope
Limited
Percolation
Overland Flow
Terrace
Terrace
Front
Slope
Sprinkler Circles
Collection
Ditches
Figure 5-3. Schematic of an Overland Flow System.
in cold climates to extend the operating season (EPA,
1984; WPCF, 1990).
Soil type and permeability are of limited importance.
Overland flow was developed for use on soils with low
permeabilities. Therefore, detailed soil evaluation is only
necessary if significant wastewater percolation into the
soil is expected. Where the soil is permeable, it should
meet the same criteria used for slow-rate land application
if ground-water protection is to be provided.
5.4.5.2 Preapplication Treatment
Secondary treatment levels (30 mg/L BODs and 30 mg/L
TSS or greater) can easily be achieved by overland flow
systems with primary pretreatment. Whether higher lev-
els of pretreatment are required depends on local regula-
tions and aesthetics. The most common method of
pretreatment used in the United States is lagoons, which
effectively remove grit and other solids that may damage
or clog the pumps and distribution system. However, in
lagoons with long retention times, such as facultative or
67
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aerated ponds, algae growth can be significant, adversely
affecting effluent quality from overland flow systems. There-
fore, primary treatment or lagoon stabilization with retention
times limited to 1 to 2 days are suggested (WPCF, 1990).
Where climatic restrictions obtain, storage facilities must
also be provided that are sufficient to hold the flow during
extented periods.
5.4.5.3 Hydraulic Loading Rate
The rate of wastewater application is dependent on the
effluent discharge limitations, level of pretreatment,
length and slope of the terrace, and climate. The degree
of treatment provided is directly proportional to the length
of the slope and the ambient temperature, and inversely
proportional to the hydraulic loading rate and terrace
slope. The degree of pretreatment primarily affects nitro-
gen removal. Low BODs concentrations in the applied
wastewater enhance nitrification, while higher BODs
concentrations enhance denitrification. Typical hydraulic
loading rates for primary pretreated wastewater are 0.2 to
0.4nr?/h x m of terrace width (16 to 32 gal/h x ft). For sec-
ondary pretreated wastewater, hydraulic loadings may
reach 0.6 m3/h x d (48 gal/h x t). The design rate selected
for a particular site should take into account the influence of
each of the factors above (EPA, 1981; 1984; WPCF, 1990).
5.4.5.4 Operating Cycle
Overland flow systems have performed best when appli-
cations of wastewater are alternated with drying periods.
Common practice is to toad a terrace for 12 hr followed by
a 12-hr drying period. Lower ratios enhance nitrification.
5.4.5.5 Distribution
Uniform application of wastewater across the width of the
terrace is critical to system performance. Wastewater is
applied at the top of each terrace by sprinklers or weir
overflow methods. Because weir overflow methods re-
quire precise leveling of the device used, they are sug-
gested for small systems only. Sprinklers are the most
commonly used method. They are located either at the
top of the terrace or within the top one-third of the ter-
race. Where sprinklers are used, terrace lengths should
be increased, with 45 m (150 ft) the minimum length. The
stope length should be at least 20 m (65 ft) greater than the
diameter of the sprinkler pattern. Distribution types and their
design are described elsewhere (EPA, 1984; WPCF, 1990).
5.4.5.5 Vegetation Selection
Perennial, water-tolerant grasses are best suited for
overland flow systems. Suitable cool-season grasses in-
clude reed canary grass, fescue, and rye grass. Suitable
warm-season grasses include common and coastal Ber-
muda, Dallis, and Bahia. Local agricultural extension
agents should be consulted for further guidance.
5.4.5.7 Runoff Collection
The runoff collection channel must be designed with suf-
ficient capacity and grade to prevent water from ponding
at the base of the terrace. At a minimum, it should be de-
signed to handle the 10-year, 1-hour rainfall event for the
area. The channel may be lined or unlined, but unlined
ditches require more maintenance to control weed growth.
5.4.5.8 Storage
Sufficient storage is needed to hold wastewater during
periods when applications cannot be made. In cold cli-
mates, the storage requirements are determined by the
number of days the temperature is below freezing. In the
United States, storage requirements can reach 160 days
(EPA, 1984; WPCF, 1990). In climates where freezing is
not a significant factor, sufficient storage should be pro-
vided to allow flexibility in operation and crop harvesting,
typically 2 to 5 days.
5.4.6 Capital Cost Sensitivity
Land and earthwork costs are the most significant capital
costs of overland flow systems. Distribution network con-
struction may also be significant depending on the type used
and the topography. Typical capital costs vary from $1.40
(0.1 mgd) to $14.00 (0.01 mgd) per gallon of dailycapacity.
5.4.7 O&M Requirements
O&M requirements of overland flow systems include har-
vesting of the cover crop, maintenance of the distribution
and collection systems, and pest control. Periodic mowing
is necessary to maintain a healthy growth of grass. An an-
nual minimum of 4 to 6 mowings per year is suggested. Re-
moval of the grass is necessary for high-level nitrogen and
phosphorus control. Mosquitoes and weed control through the
use of pesticides or microbiological agents may be necessary.
Costs of O&M are associated primarily with labor costs.
Almost no special equipment other than the appropriate
agricultural equipment is required. Total labor require-
ments, excluding maintenance of the pretreatment works
and effluent disinfection system, should not exceed 10 to
15 hr/wk. O&M costs typically range from $1.40 to
$2.80/1,000 gal treated wastewater (at 10,000 gal/d) to
$0.35 to $0.70/1,000 gal (at 100,000 gal/d).
5.4.8 Construction Issues
Terrace grading is critical to achieving sheet flow for
successful operation. The slopes should be final graded
with a land plane to within 1.5 cm (0.6 in.) of the pre-
scribed slope. Soil compaction is not a critical factor ex-
cept as it relates to establishment of the vegetative cover
or to prevention of ground-water contamination. The ter-
races should be irrigated after seeding, but wastewater
applications should be limited until the vegetation is well
established; the acclimation period is typically 3 to 4
months in duration.
5.4.9 Monitoring
Monitoring requirements cover volumes, rates, frequen-
cies, and patterns of wastewater applications and influent
and effluent quality monitoring dictated by the permit.
Where significant infiltration of wastewater into the soil
occurs, ground-water monitoring may also be required.
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5.4.10 Residuals
Residuals produced by overland flow systems are limited
to harvested crops.
5.5 Slow Sand Filtration
5.5.1 Technology Description
Slow sand filters are used for both small communities
and individual homes. They consist of one or more beds
of granular material, typically graded sand, 60 to 90 cm
(2 to 3 ft) deep, underlain with collection drains imbedded
in gravel. Pretreated wastewater is intermittently applied
to the surface of the sand bed and allowed to percolate
through the bed where it receives treatment. The filter
media remains unsaturated and is vented to the atmos-
phere, such that an aerobic environment is maintained in
the filter. The percolate is usually collected by the under-
drains, which remove it from the filter for further treat-
ment or disposal.
Three types of slow sand filters are commonly used.
They include buried, open, and recirculating configura-
tions (Figure 5-4). While all three are somewhat similar in
design, they may differ in method of operation, perform-
ance, access, and filter media specifications.
Distribution Box
House
Sewer
Inspection/Disinfection Tank •
(if required)
(a) Buried (Single-Pass) Sand Filter
Vent Pipes
Splash Plate
Pea Gravel
Discharge
Insulated Cover
(if required)
Distribution Pipe
Collection Pipe
Graded Gravel —/ >—
1/4 to 1-1/2 in.
(b) Open (Intermittent) Sand Filter
Perforated or Open Joint
Raw Waste
Sump Pump
Recirculation
Tank
Discharge
(c) Basic Recirculating Sand Filter
Free-Access
Sand Filter
Figure 5-4. Schematics of Slow Sand Filters.
69
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5.5.1.1 Buried Sand Filters
Buried sand filters are single-pass filters constructed be-
low grade in an excavation lined with an impermeable
membrane and covered with backfill material. Distribu-
tion piping is imbedded in gravel or crushed rock placed
on top of the filter media. A geotextile fabric over the top
of the gravel prevents the backfill material from piping
into the filter. To provide venting of the filter, the up-
stream ends of the underdrains are extended vertically
above grade. The distribution piping may also be ex-
tended above grade for venting. These systems have been
used primarily for individual home wastewatertreatment.
5.5.1.2 Open or Intermittent Sand Filters
Open sand filters are single-pass (intermittent) filters ex-
posed to the surface to allow inspection and periodic
maintenance of the infiltrative surface. Wastewater is
usually distributed over the surface by flooding. The fil-
ters are typically exposed, although removable covers
may be used depending on climatic conditions.
5.5.1.3 Recirculating Sand Filters
Recirculating sand filters are open sand filters designed
to recirculate the filtrate. The recirculation tank receives
pretreated wastewater and a portion of the filtrate. The
pump, which is engaged by a submersible pump oper-
ated by a timer, regularly doses the filter with the mixture
of the pretreated wastewater and return filtrate. Modifica-
tions to recirculating filters that provide substantial nitro-
gen removal have been successfully applied (Lamb et
al., 1990; Piluk and Hao, 1989; Sandy et al., 1988).
The mechanism of treatment is a combination of bio-
chemical and physical filtration and chemical adsorption.
Slow sand filters operate as aerobic, fixed-film biological
reactors capable of producing a high-quality effluent con-
sistently. The treatment process is very stable and reli-
able, but dependent on temperature. It is able to accept
variations in hydraulic and organic loading with little ef-
fect on effluent quality.
Performance will vary with the media used, temperature,
hydraulic and organic loadings, and method of operation
(EPA, 1985; Oregon DEQ, 1982). Recirculating sand fil-
ters typically provide a high-quality effluent similar to sin-
gle-pass (open) filters. Typically effluent BODs and SS
concentrations below 10 mg/L are achieved. The effluent
is very low in turbidity. Nearly complete nitrification is
also achieved by all filters except in cold-temperature
conditions. Denitrification has been shown to occur in re-
circulating filters; varying with modifications in design and
operation, removals of 50 percent or more of the applied
nitrogen may be achieved. Phosphorus may be removed
initially through chemical adsorption on the media grains,
but removals decline with time as the adsorption capacity
Is reached. Fecal coliform removal accomplished with re-
circulating filters is less efficient than single-pass filters,
but removal of two to three logs is commonly achieved.
5.5.2 Applicability and Status
Sand filters are a proven method for providing advanced
secondary wastewater treatment. They have been used
for treatment of municipal wastewater since the late
1800s. They are well suited to rural communities, small
clusters of homes, individual residences, and business
establishments. The life cycle costs are lower than for
extended aeration package plants for small community
treatment. Their use is limited by land availability and
capital cost, but they are easy to operate, requiring per-
sonnel with a minimum of skills.
5.5.3 Advantages
Sand filters are moderately inexpensive to construct,
have low energy requirements, and do not require highly
skilled personnel to operate. They produce high-quality
effluents, significantly better than those produced by
extended-aeration package plants or stabilization
lagoons. The treatment process is extremely stable, re-
quiring limited intervention by operating personnel.
Through modular design, treatment capacity can easily
be expanded.
5.5.4 Disadvantages
Sand filters require somewhat more land area than pack-
age plants, but their land requirements are lower than for
lagoons. The amount of head required by the filters typi-
cally exceeds 1 m (3.25 ft), possibly requiring pumping
for effluent disposal where available land relief is insuffi-
cient. Odors from open, single-pass filters treating pri-
mary or septic/lmhoff tank effluent do occur and may
require buffer zones from inhabited areas. Suitable filter
media may not be available locally.
5.5.5 Design Criteria
5.5.5.1 Preapplication Treatment
The minimum of primary treatment of wastewater is re-
quired before application to sand filters. Either septic
tanks or Imhoff tanks are commonly used. Higher levels
of treatment can reduce filter size requirements or pro-
long filter life, but such cost savings must be weighed
against the increased costs of pretreatment. For larger
flows, stabilization ponds may be used (see Section 5.3).
5.5.5.2 Media Characteristics and Depth
Filter media are described by the effective size (Dio or
particle diameter with 10 percent of grains by weight or
smaller) and uniformity coefficient (Deo/Dio) or some
other measure of the grain size range. The smaller the
effective size, the higher the level of treatment, the lower
the hydraulic loading, and the more frequent the need for
maintenence. The lower the uniformity coefficient, the
longer the filter's life span. In practice, effective sizes are
0.10 to 1.5 mm, but the requirements differ for each type.
Uniformity coefficients are typically less than 4.0.
Sand is the most commonly used media, but anthracite,
garnet, mineral tailings, and bottom ash have been used.
Alternative media must be durable and insoluble in
70
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water. The media used should have a total organic con-
tent of less than 1 percent, total acid soluble matter of
less than 3 percent, a hardness greater than 3 on the
Moh's scale, and be generally round in shape.
Media depths commonly used are 0.6 to 0.9 m (2 to 3 ft).
Deeper beds tend to provide more complete treatment
and more constant effluent quality. Greater depths are
suggested where the media have smaller effective sizes,
because the smaller media retains more moisture, which
reduces aeration of the media necessary for effective
biochemical activity.
5.5.5.3 Loading Rates
Hydraulic loading rates vary with the characteristics of
the media, the type of filter design, and the wastewater
strength. Rates are typically 3 to 40 cm/d (0.75 to 10
gpd/sq ft). Buried filters usually have the lowest hy-
draulic loadings, while recirculating filters have the
highest (Table 5-3). Higher rates generally reduce the
effluent quality and increase the frequency of mainte-
nance for a given media and filter design. Organic
loading rates have not been widely used in current
designs.
Table 5-3. Design Criteria—Slow Sand Filters3
Design Factor
Pretreatment
Media specifications
Effective size (mm)
Uniformity coefficient
Depth (m)
Hydraulic loading
(cm/d)
Dosing frequency
Recirculation ratio
Buried
k XimTrvti
0.7-1 .00
<4.0
0.60-0.90
4-6
2-4/d
NA
Open Recirculating
im of ^rvHimr>ntTt'inn
0.40-1.00 1.0-1.50
<4.0 <4.0
0.60-0.90 0.60-0.90
5-10 12-20
(forward
flow)
1-4/d 5-10min/
30 min
NA 3:1-5:1
aEPA, 1985.
NA = not applicable
5.5.5.4 Wastewater Application
The manner in which wastewater is applied to the sur-
face is critical to performance. Doses of wastewater must
be applied uniformly to the filter surface at sufficient inter-
vals for the wastewater to infiltrate completely. This al-
lows ample aeration of the filter media to effect aerobic
treatment. Distribution methods commonly used involve
surface flooding, spray nozzles, and interlaced pipe later-
als (gravity and pressure).
Dosing frequencies are related to the type of filter, me-
dia size, and wastewater strength. Smaller, more fre-
quent doses are preferred for coarse media to
increase residence time in the media. Typical dosing
frequencies for intermittant filters range from 1 to 4
times daily. Recirculating filters are dosed for 5 to 10
minutes every half hour.
Multiple sand filters are suggested to provide standby ca-
pacity and allow filter loading and resting cycles. Typi-
cally, two or four cells are provided.
Dosing tanks are sized for the maximum size dose to be
used. The dosing tank for recirculating filters must be sized
for at least five times the forward-flow dose volume.
5.5.6 Capital Cost Sensitivity
Filter media availability will have the most significant im-
pact on construction costs. Other associated costs that
may be significant relate to land, earthwork, pretreatment,
and transmission of wastewater to the treatment site.
5.5.7 O&M Requirements
Slow sand filters require relatively little operational control
or maintenance. Primary O&M tasks include filter surface
maintenance, dosing equipment servicing, and influent
and effluent monitoring. With continued use, sand filter
surfaces will become clogged with organic biomass and
solids. Once operating infiltration rates fall below the hy-
draulic loading rate, permanent ponding of the filter sur-
face will occur indicating that the filter should be taken
off-line for "resting" and surface tilling or removal. Buried
filters are designed to operate without maintenance for
their design life. Filters exposed to sunlight may develop
algae mats, which may require control by shading the
surface. For community systems, disinfection will be re-
quired prior to discharge, but disinfectant quantity re-
quirements are low due to their high quality.
5.5.8 Construction Issues
Filter media placement and underdrain construction are
the most critical construction issues. The filter media is
usually placed in lifts. Filter performance is best when the
filter media is homogeneous. The surface of the filter
must be graded carefully to ensure uniform treatment.
The underdrains must also be sufficiently sloped to en-
sure adequate drainage.
5.5.9 Monitoring
Monitoring requirements cover wastewater volumes and
quality applied to each filter, dosing frequencies, and sur-
face infiltration rates. All discharge permit requirements
must also be met.
5.5.10 Residuals
Small quantities of residuals are usually produced. In
most intermittent sand filtration facilities, sand must be
periodically removed from the surface as a method of fil-
ter rejuvenation. The sand may be stockpiled onsite and
later washed and reused, or landfilled. Other filters gen-
erally do not generate solids in quantities sufficient to re-
quire utilization of a disposal method, except at the end
of their service lives.
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5.6 Slow Rate Land Application
5.6.1 Technology Description
Slow rate land application is a soil-based wastewater
treatment method designed to apply intermittently
unchlorinated primary or secondary treatment effluent at
a controlled rate to a vegetated soil surface of moderate
to slow permeability (Rgure 5-5). The wastewater is ap-
plied via sprinklers or flooding of furrows. Following appli-
cation, the wastewater infiltrates the land surface and
percolates through the soil profile to the ground-water ta-
ble. A tailwater return system is usually provided to con-
tain and recycle wastewater runoff from the site due to
excessive application or precipitation. It consists of a col-
lection pond, pump, and return pipeline. A storage reser-
voir must also be provided for adverse weather conditions,
crop cultivation and harvesting, and emergencies.
The relatively low application rates on natural vegetated
soil surfaces provide the potential for slow rate systems
to produce the highest treatment levels of the land appli-
cation methods. Wastewater constituents are removed in
the soil matrix by filtration, adsorption, ion exchange, pre-
Applied
Wastewater
Evapotranspiration
Percolation
(a) Application Pathway
GW Mound
(b) Subsurface Pathway
Flgum 5-5. Schematic of a Slow Rate Land Application System.
72
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cipitation, microbial action, and plant uptake (WPCF,
1990). Part of the water is lost to evaporation and plant
transpiration. Organics are removed by soil adsorption
and biochemical oxidation. Nitrogen is removed primarily
by crop uptake, but denitrification can also be significant.
Chemical immobilization and plant uptake are mechanisms
of phosphorus removal. Metals, certain toxic organics, and
pathogens are also effectively removed. Typical treatment
performance data are presented in Table 5-4.
Table 5-4. Typical Slow Rate Land Application
Treatment Performance3
Parameter
BOD5
Nitrogen
Loading
(kg/ha xd)
3-11
Removal Comments
94-99
0.3-2.4 65-95
Phosphorus 0.1-3.0 75-99
Toxic
organics
Pathogens
NA
NA
Varies
with
structure
>99
Percolate typically <1
mg/L
Removal efficiency
depends on crop and
crop management
practices
Plant uptake accounts
for approximately 25
percent
Limited data suggest
effective removals are
provided for volatile
and biodegradable
organics
Nearly complete
removal
aEPA, 1981; WPCF, 1990.
NA = not applicable
Vegetation is an important component that serves to ex-
tract nutrients, control erosion, and maintain soil perme-
ability. It should be selected and managed according to
the site conditions and treatment objectives.
Either cultivated or forested sites are used. In cold cli-
mates, wastewater storage is required during most of the
nongrowing seasons; however, year-round application is
often employed on forested sites.
5.6.2 Applicability and Status
Slow rate land application for wastewater treatment is a
proven technology for municipal and other organic
wastewaters. Used for over one hundred years, it has
evolved from a "disposal" method to one that can be
used to recycle wastewater onto agricultural crops, for-
ests, or park lands. Its use is limited primarily by land
suitability, climate, and land costs.
5.6.3 Advantages
Slow rate land application is well suited for treatment of
wastewater from rural communities and seasonal indus-
tries such as vegetable canning. It can provide an eco-
nomic return from the reuse of water and nutrients for
irrigation of landscaped areas or production of market-
able, commercially processed crops. It also provides
ground-water recharge. Of the various land treatment
methods, slow rate land application is the least limited by
surface slopes.
5.6.4 Disadvantages
Because of the vegetation component, slow rate land ap-
plication is limited by climate and nutrient requirements
of the vegetation. Climate affects the growing season
and will dictate the period of wastewater application and
storage requirements. Crop water tolerances, nutrient
requirements, and nitrogen removal capacity of the
vegetation-soil complex limit the hydraulic loading rate.
Application must be suspended during wet periods or frozen
soil conditions. Because of the limits on the hydraulic
loading rate, the area of land necessary is significantly
larger than for other land application methods.
5.6.5 Design Criteria
5.6.5.1 Site Selection
Important site-selection criteria include soil charac-
teristics, ground-water conditions, and topography. Soil
characteristics are important for wastewater treatment
and crop management. Generally, loamy soils (loamy
sands to clay loams) are best suited for slow rate land
application systems. Finer texture soils (clays) do not
drain well, retaining water for long periods, which makes
crop management more difficult. Coarse texture soils
(sands) can accept higher application rates and do not
retain water, which may be important where crops with
low moisture tolerance are used. In addition to texture,
unsaturated depth of the soil is important. Adequate
depth must be provided for root development and waste-
water treatment. An unsaturated depth greater than 1.0 m
(3.3 ft) to the top of any ground-water mound is usually
necessary. Greater depths may be necessary for deep-
rooted crops. In some cases, subsurface drainage can
be provided where shallow ground water occurs.
Ground-water mound height analysis is usually neces-
sary to determine if the required unsaturated depth can
be maintained during system operation.
Slope, relief, and susceptibility to flooding are important
topographic features to consider. Where crop cultivation
is to be practiced, slopes greater than 15 percent should
be avoided. Uncultivated crops, such as pasture, can be
irrigated on slopes of 15 to 20 percent. Woodlands have
been irrigated successfully with sprinklers on slopes of
up to 40 percent. Site relief, or the differences in surface
elevations across the site, is an important consideration
for wastewater distribution to the points of irrigation and
wastewater ponding in depressions. Flood-prone areas
should be avoided unless the land application system is de-
signed to be an integral part of the flood management plan.
Land area requirements range from 25 to 220 ha (60 to
550 ac) for treatment of 3,780 m3/d (1 mgd). Land for
pretreatment facilities, storage, roads, and buffer zones
is additional.
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5.6.5.2 Crop Selection
Important criteria for crop selection are climate, soil char-
acteristics, wastewater characteristics and application
rate, and available management skill, labor, and equip-
ment. On sites where wastewater treatment rather than
crop water requirements are the objective, the vegetation
selected should have high nitrogen uptake capacity, high
consumptive use or evapotranspiration potential, high
soil-moisture tolerance, low sensitivity to wastewater
constituents, and minimum management requirements.
Crops meeting these requirements include various per-
ennial forage grasses, turf grasses, some tree species,
and some field crops. These include reed canary grass,
tall fescue, Bermuda grass, perennial rye grass, Italian
rye grass, and orchard grass. Mixed hardwoods and
pines are the most common tree crops for systems with
higher application rates. Where the wastewater is for
crop irrigation, the crops listed above may be grown, as
well as other crops such as legumes, cotton, soybeans,
safflower, grains, and some fruit crops including citrus,
apples, and grapes. Crop selection guidance can be ob-
tained from local agricultural extension agents and avail-
able texts (CSWRCB, 1984; EPA, 1981; WPCF, 1990).
5.6.5.3 Preapplication Treatment
The degree of pretreatment required for slow rate land
application is dependent on public health considerations,
nuisance control, soil and crop considerations, nitrogen
concentration, and distribution method. In general, primary
treatment is acceptable for isolated, restricted-access sites
where the crops are not intended for human consumption.
State requirements vary for controlled agriculture irriga-
tion for various crops, for turf irrigation in parks, and on
golf courses. Where nitrogen loading is critical or where
other wastewater constituents can affect plant growth,
additional pretreatment may be required.
5.6.5.4 Hydraulic Loading Rate
The design hydraulic loading rate is controlled by either
the hydraulic conductivity of the soil, nitrogen loading, or
the water requirement of the crop. Local rates of precipi-
tation and evapotranspiration potential of the crop must
also be considered. The maximum daily application rate
must not exceed the soil percolation rate. Typically, the
maximum application rate used is 2 to 4 percent of the
saturated hydraulic conductivity of the least permeable
soil horizon within 2.4 m (8 ft) of the surface. This rate
must be adjusted for precipitation and evapotranspiration
during the application period. Where nitrogen contribu-
tions to the ground water must be controlled, the nitrogen
mass loading will control the hydraulic loading rate. Crop
requirements for water or tolerances to salinity or other
wastewater constituents may also be a controlling factor.
Methods for estimating the design hydraulic loading rate
may be found elsewhere (EPA, 1981; WPCF, 1990).
The application period must also be determined. Applica-
tions cannot be made during very wet periods, during
freezing weather, or during crop harvesting activities.
These periods must be estimated to determine the total
land area requirements.
5.6.5.5 Wastewater Distribution
Common methods of wastewater distribution used in
slow rate land application systems include sprinkling,
flooding, and drip irrigation. Sprinkling is the most com-
mon method because is can be adapted to a wide range
of soil and topographic conditions and is suited to a vari-
ety of crops. Either fixed, portable, center pivot, or trav-
eling gun systems are used. Flooding of furrows or other
bermed areas is used where it is compatible with con-
ventional agricultural irrigation practices. Drip irrigation
requires that the applied wastewater have a high level of
pretreatment and be low in iron, hydrogen sulfide, and to-
tal bacteria to prevent emitter clogging. Therefore, this
method is restricted to systems where the resource value
of the wastewater is high. Guidance for distribution sys-
tem design can be found elsewhere (EPA, 1981; WPCF,
1990; Booher, 1974; Hart, 1975; SCS, 1983; Irrigation
Association, 1983).
5.6.5.6 Storage Requirements
Storage of wastewater must be provided during periods
when applications cannot be made. A water balance for
each month of operation should be calculated to estab-
lish these requirements (EPA, 1981).
5.6.6 Capital Cost Sensitivity
Land, earthwork, distribution system, and storage facili-
ties are the most significant capital costs. The land area
required is dependent on the nature of the irrigated crop,
soil characteristics, and acceptable application periods.
Distribution costs can be affected by the type of system
selected and site topography. In cold, wet climates, stor-
age requirements can be significant. Other costs that
should be considered are underdrain installation, if
needed, transmission of wastewater to the treatment site,
and pretreatment.
5.6.7 O&M Requirements
The primary O&M requirements of slow rate land appli-
cation systems are crop management and servicing of
the distribution tailwater return systems. Agricultural
crops require the most intensive management, while for-
est application requires the least management. Manage-
ment tasks may include soil tillage, planting and
harvesting of crops, nutrient control, pH adjustment, tail-
water return system maintenence, and sodium and salin-
ity control (EPA, 1981). No special equipment other than
the appropriate agricultural equipment is required. Costs
of operation are associated primarily with labor costs.
Other costs include power for wastewater pumping and
equipment depreciation. Total labor requirements, ex-
cluding maintenance of the pretreatment works, are esti-
mated to be approximately 15 to 20 cents/1,000 gal of
applied water.
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5.6.8 Construction Issues
Construction factors include site preparation and installa-
tion of runoff controls, irrigation piping, a tailwater sys-
tem, return systems, and storage facilities. Since
sustained wastewater infiltration is an important compo-
nent of successful system operation, it is critical that con-
struction activity be limited on the application site. Where
storm-water runoff can be significant, measures must be
taken to prevent excessive erosion including terracing of
steep slopes, contour plowing, no-till farming, and the es-
tablishment of grass border strips and installation of sedi-
ment control basins.
5.6.9 Monitoring
Monitoring requirements may cover volumes, rates, fre-
quencies, and patterns of wastewater applications,
ground-water quality and elevations, soil fertility, and
plant tissue. Ground-water monitoring may be required
by the local regulatory agency to evaluate system per-
formance. Soil fertility should be evaluated periodically to
determine if soil amendments are necessary. Trace ele-
ments should also be analyzed to avoid toxic accumula-
tions. Plant tissue analysis is used to determine whether
there are deficient or toxic levels of elements.
5.6.10 Residuals
Residuals produced by slow rate land application sys-
tems are limited to harvested crops and crop residues.
5.7 Subsurface Infiltration
5.7.1 Technology Description
Subsurface wastewater infiltration systems (SWISs) are
subgrade land application systems. The soil infiltration
surfaces are exposed in buried excavations that are gen-
erally filled with porous media. The media maintains the
structure of the excavation, allows the free flow of pre-
treated wastewater over the infiltrative surfaces, and pro-
vides storage of wastewater during peak flows. The
wastewater enters the soil where treatment is provided
by filtration, adsorption, and biochemical reactions. Ulti-
mately, the treated wastewater enters and flows with the
local ground water.
Various SWIS designs have been developed for use de-
pending on the site and soil conditions encountered. The
designs differ primarily in where the infiltrative surface is
placed (Figure 5-6). The surface may be exposed within
the natural soil profile (conventional) or at or above the
surface of the natural soil (at-grade on mound systems).
The elevation of the infiltrative surface is critical because
of the need to provide an adequate depth of unsatu rated
soil between ,the infiltrative surface and a limiting condi-
tion (i.e., bedrock or ground water).
The geometry of the infiltrative surface also varies. Long,
narrow infiltrative surfaces (trenches) are preferred. Wide
infiltrative surfaces (beds) and deep infiltrative surfaces
(pits and deep trenches) do not perform as well, even
though they require less area.
Subsurface infiltration systems are capable of high levels
of treatment for most domestic wastewater pollutants of
concern (Table 5-5). Under suitable site conditions, re-
moval of biodegradable organics, SS, phosphorus, heavy
metals, and virus and fecal indicators is nearly complete.
The fate of toxic organics and metals has not been as
well documented, but limited studies suggest that many
of these constituents do not travel far from the system.
Nitrogen is the most significant wastewater parameter
that is not readily removed by the soil. Nitrate concentra-
tions above the drinking water standard of 10 mg-N/L are
commonly found in ground water below SWISs.
Table 5-5. Typical Subsurface Wastewater Infiltration
System Treatment Performance
Parameter Applied Removal References
Concentration (%)
(mg/L)
BOD5 130-150 90-98 Siegristetal., 1986
Univ. Wis., 1978
Nitrogen 45-55 10-40 Reneau, 1977
Sikora & Corey, 1976
Phosphorus 8-12 85-95 Sikora & Corey, 1976
Tofflemire & Chen,
1977
Fecal NA 99-99.99+ Gerbaetal., 1975
coliforms Univ. Wis., 1978
NA = not applicable
5.7.2 Applicability and Status
Subsurface infiltration systems are well suited for treat-
ment of small wastewater flows. Small SWISs, com-
monly called septic tank systems, are traditionally used
in unsewered areas by individual residences, commercial
establishments, mobile home parks, and campgrounds.
Since the late 1970s, larger SWISs have been increas-
ingly used by clusters of homes and small communities
where wastewater flows are less than 1.1 L/s (25,000
gpd). They are a proven technology, but require specific-
site conditions to be successfully implemented. SWISs
are often preferred over mechanical treatment facilities
because of their consistent performance with few O&M
requirements, lower life cycle costs, and less aesthetic
impact on the community.
5.7.3 Advantages
Because subsurface infiltration systems are buried, they
are ideally suited for decentralized treatment of wastewa-
ter. For rural homes and business establishments, they
are often the only method of wastewater treatment avail-
able. Some communities choose subsurface infiltration
systems to avoid costly sewer construction. Where indi-
vidual lots are not suited for their construction, suitable
remote sites may be used to cluster homes onto a single
SWIS, thereby limiting the extent of sewers. Alterna-
75
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Infiltrath/e
Surfaces
(a) Conventional SWIS
Backfill
Barrier
Material
Porous
Material
t---.-A Jw£8fe>-<&--
zmmmffftwfffc^s
>T-** -?•=' •45V£:^VAf?:Kf ;w:f?£?^/-J:V^;v-;".:':
«•"-**" "••*•''* * "
(b) At-Grade SWIS
Barrier Material
Sand Fill
Topsoil
Distribution
Lateral
Absorption
Area
3sa?lr==P"l^g.' -^sg^sr. _Rgck Strata oxlmpemTeable^Soil Layer S-=
(c) Mound SWIS
Figure 5-6. Schematics of Subsurface Wastewater Infiltration Systems (SWISs).
76
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tively, wastewater from entire communities may be
treated by an SWIS. Because the system is buried, the
land area can be used as green space or park land. In
addition, SWISs provide ground-water recharge.
5.7.4 Disadvantages
Use of SWISs are limited by site and soil conditions. Be-
cause the infiltrative surface is buried, it can be managed
only by taking it out of service every 6 to 12 months to
"rest." This requires that standby cells be constructed
with alternating loading cycles. Therefore larger SWISs
are usually restricted to well-drained sandy soils to re-
duce land area requirements. Because nitrogen is not re-
moved effectively by SWIS, pretreatment to remove
nitrogen may be necessary to prevent nitrate contamina-
tion above the drinking water standards in the underlying
ground water.
5.7.5 Design Criteria
5.7.5.1 Site Selection
Site selection is based on the treatment capacity of the
soils and the availability of sufficient land area of suitable
topography. The treatment capacity of the soil depends
primarily on texture, structure, and unsaturated thick-
ness. No soil type provides optimum conditions for the
removal of all wastewater constituents. Fine-texture soil,
such as silt loams and clay loams, provides particularly
favorable removal of conventional constituents. However,
fine-texture soil has a relatively low hydraulic conductivity
and reaeration rate. Therefore the hydraulic loading and
infiltration basin rest cycle will control system sizing, and
a large land area may be required. Coarse soil, such as
Table 5-6. Typical Site Criteria for a Large SWIS3
sand, has a higher hydraulic conductivity and reaeration
rate to allow higher hydraulic and organic loading and
shorter cycles for basin "resting," which can reduce the
required land area. However, coarse soil can be a less
effective physical filter, have a lower cation exchange ca-
pacity, and allow more rapid percolation of wastewater
through the vadose zone where most treatment occurs.
Thus the treatment objectives may control system de-
sign. For large SWISs serving clusters of homes or com-
munities (Table 5-6), sites with sand to sandy loam soils
are typically selected. Site selection for individual home
SWISs are regulated by local onsite system codes, which
should be consulted before site investigations are made.
Unsaturated depth of soil is another critical site-selection
criterion. A minimum of 1.5 to 2.5 m (5 to 8 ft) of unsatu-
rated soil with relatively uniform hydraulic conductivity is
necessary below the ground surface to provide neces-
sary treatment. For large SWISs, ground-water mound
height analysis must be performed to determine if the
separation distance can be maintained during system op-
eration. If the desired separation distance cannot be
maintained, the hydraulic loading can be reduced. For
large SWISs, the system should be located along a con-
tour perpendicular to the ground-water gradient. Basin
spacing can be increased or ground-water drainage pro-
vided in areas where limiting conditions place restrictions
on conventional designs.
Topography should also be considered in site selection.
Depressions, footslopes, concave slopes, flood plains,
and other areas that exhibit poor surface and subsurface
drainage should be avoided. Mild planar slopes, ridge
lines, and shoulder slopes are best suited for an SWIS.
Characteristic
Site
Landscape position
Topography
Soil Characteristics
Texture
Structure
Mineralogy
Permeability
Bulk density
Drainage
Hydrogeology
Depth to phreatic
surface/bedrock
Transmissivity
Flow boundaries
Typical Application
Ridge lines, hill tops, shoulder slopes
Planar, mildly undulating, slopes
< 12 percent
Sands, sandy loams
Granular, blocky
<20 percent expansive (2:1) clays
Moderate, rapid
Slight to moderate resistance to
penetrometer
Moderately well, well, somewhat poorly
drained
>1.5 m (4.5 ft)
High
Near discharge point
Applications to Avoid
Depressions, footslopes, concave slopes, flood plains
Complex slopes, slopes >18 percent
Clay loams, clays
Platy, prismatic, columnar
>20 percent (2:1) clays
Very rapid, moderately slow to very slow,
hydraulically restrictive horizons
Moderate to strong resistance to penetrometer
Extremely well, very poorly, excessively well drained
<1.5 m (4.5 ft), sole source aquifers
Low
Near no-flow boundary
aSWISs serving individual homes are adaptable to a broader range of soil and site conditions. Local codes should be consulted.
77
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5.7.5.2 Hydraulic Loading Rate
The design hydraulic loading rate is determined by soil
characteristics, ground-water mounding potential, and
applied wastewater quality. Clogging of the infiltrative
surface will occur in response to prolonged wastewater
loading, which will reduce the capacity of the soil to ac-
cept the wastewater. However, if the loading is control-
led, biological activity at the infiltrative surface will
maintain waste accumulations in relative equilibrium so
that reasonable infiltration rates can be sustained.
Selection of the design hydraulic loading rate must con-
sider both soil and system design factors. Typically, de-
sign rates for larger SWISs are based on detailed soil
analyses and experience, rather than measured hydrau-
lic conductivities. Commonly used loading rates are pre-
sented in Table 5-7; however, these rates should be
adjusted up or down depending on the specific site con-
ditions, design concept, and applied wastewater quality
(WPCF, 1990).
5.7.5.3 Wastewater Pretreatment
The minimum of primary treatment is required of waste-
water before application to an SWIS. Septic tanks are
commonly used for smaller sources and Imhoff tanks for
larger ones. Higher levels of treatment can reduce SWIS
size or prolong system life, but this must be weighed
against the increased costs of pretreatment and potential
damage from poor maintenance of the system. Where
pretreatment is desirable, aerobic biological treatment
processes are generally used.
5.7.5.4 Design Concept
SWIS designs should be adapted to the specific site and
soil conditions and wastewater characteristics. The initial
Table 5-7. Typical Hydraulic Loading Rates on Horizontal
Soil Infiltrative Surfaces Treating Domestic Septic Tank
Effluent8
Soil Texture
Gravel, very coarse sand
Coarse to medium sand
Fine sand, loamy sand
Sandy loam, porous loam
Loam, silt loam
(moderate to strong blocky structure)
Clay loam
(moderate to strong blocky structure)
Clay loams and clays with appreciable
shrink-swell potential or weak,
columnar, or prismatic structure
Hydraulic Initiative
Surface Hydraulic
Loading Rate
(cm/d13)
Not recommended
4.0
3.2
2.1
2.5
1.0
Not recommended
"Adjustments to these rates may be necessary for specific
applications to account for other soil factors and design
concepts. See WPCF, 1990.
"Conversion: 1 cm/d = 0.24 gal/ft2 d
design concept should strive to include the following
features:
• Narrow trenches, 0.2 to 1.0 m (0.5 to 3.0 ft) wide, ex-
cavated parallel to surface or ground-water piezometric
surface contours (based on analysis or ground-water
mounding potential) with level bottom surfaces
• Shallow placement of the infiltrative surfaces, less
than or equal to 0.6 m (2 ft) below final grade
• Pretreatment capability to remove organics, sus-
pended solids, grease, oils, etc. to concentrations less
than or equal to typical domestic septic tank effluent
• Uniform dosing of infiltrative surfaces one to four times
daily
• Multiple cells (3 to 4 minimum) to allow annual or
semiannual resting and standby capacity for opera-
tional flexibility
• Devices for monitoring daily wastewater flows, infiltra-
tive surface ponding, and ground-water elevations
Modifications to this design concept are usually required
if site limitations cannot be removed. Guidelines for
adapting SWISs to common site limitations encountered
are presented elsewhere (Converse & Tyler, 1990; Con-
verse et al., 1989; EPA, 1980; Siegrist et al., 1986;
WPCF, 1990).
5.7.6 Capital Cost Sensitivity
Land and earthwork are the most significant capital
costs. Where select fill must be used to bed the primary
infiltrative surface, the cost of transporting the material
also becomes significant. Other costs that should be con-
sidered are pretreatment costs and transmission of the
wastewater to the treatment site.
5.7.7 O&M Requirements
A well-designed SWIS requires limited operator attention.
Management functions primarily involve tracking system
status, testing for solids accumulation, evaluating pump
performance, and monitoring system controls. Monitoring
performance of pretreatment units, mechanical compo-
nents, and wastewater ponding levels above the filtration
surface are essential. If a change in status is noted, op-
erator intervention may be required. Routine servicing of
SWIS may be limited to annual or semiannual alternating
of the infiltration cells.
5.7.8 Construction Issues
A frequent cause of early SWIS failure is poor construc-
tion. If the soil structure is damaged during construction,
the system may not be able to accept the design hydrau-
lic loading rates. To avoid damage, a construction plan
should be developed that addresses the type of con-
struction equipment, construction procedures, site ac-
cess, site preparation, and existing soil conditions (Univ.
Wis., 1978; WPCF, 1990). Compaction of the infiltrative
surface should be avoided by using proper procedures
78
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and low load-bearing construction equipment. After exca-
vating to grade, the infiltrative surface should be carefully
scarified. Construction should not proceed if the soil mois-
ture is near the plastic limit or frozen conditions exist.
5.7.9 Monitoring
Monitoring requirements cover applied wastewater flows
and quality, ponding levels above the infiltrative surfaces,
ground-water elevations below the system, and, if appro-
priate, ground-water quality. Flows should be recorded
daily. Ponding levels should be measured biweekly or
monthly. Water quality measurements should be per-
formed at least quarterly.
5.7.10 Residuals
No residuals are produced from subsurface infiltration
systems. In an individual home SWIS (septic tank sys-
tems), where the septic tank is considered an integral
part of the system, septage is generated within the tank.
Septage should be removed every three to five years.
The septage is typically treated at a municipal treatment
plant or is land spread (EPA, 1984).
5.8 Pressure Sewers
5.8.1 Technology Description
Pressure sewer systems generally use smaller pipe di-
ameters than conventional sewers and are operated with
pumping instead of gravity. In less populated areas they
usually result in lower construction costs relative to con-
ventional sewer systems. Pressure sewers are consider-
ably independent of slope, and systems have been
developed and applied to reduce the high capital cost of
sewer systems that have been designed in accordance
with accepted design parameters, namely slope and ve-
locity. Pressure sewer systems involve a number of pres-
surizing inlet points and an outlet to a treatment facility or to
a downstream gravity sewer, depending on the application.
The two major types of pressure sewer systems are the
grinder pump (GP) system and the septic tank effluent
pump (STEP) system. The major difference between the
two systems is in the onsite equipment and layout. Nei-
ther pressure sewer system alternative requires any
modification of household plumbing.
In both designs household wastes are collected in the
sanitary sewer and conveyed by gravity to the pressuri-
zation facility. The onlot discharge piping arrangement in-
cludes at least one check valve and one gate valve to
permit isolation of each pressurization system from the
main sewer. GPs can be installed in the basement of a
home to provide easier access for maintenance and
greater protection from vandalism.
A GP pressure sewer has a pump at each service con-
nection. The pumps are 0.75 kW (1 hp) or more, require
110V or 220V, and are equipped with a grinding mecha-
nism that macerates the solids. The head provided by
the pumps are typically about 15 to 30 m (50 to 100 ft)
and flow rates vary widely. The pumps discharge into a
pressurized pipe system that terminates at a treatment
plant or a gravity collector. Because the mains are pres-
surized there is no infiltration into them; however, infiltra-
tion and inflow in the house sewers and the pump wells
can occur. In areas where the GP sewer system has re-
placed septic tank and leaching field systems, these may
be retained for emergency overflow, but they should be
separated from the pump well by a gate valve that is
opened only when necessary to accommodate emer-
gency overflow from the GP unit; otherwise, the septic
tank and leaching field can become sources of large vol-
umes of infiltration.
Electrical service is required at each service connection.
The pipe network typically has no closed loops. The
sewer profile often parallels the ground surface profile.
Horizontal alignment can be curvilinear. Plastic pipe is
typically used. Service connection diameters are typically
1 1/4 in. (30 mm). Cleanouts are used to provide access
for flushing. Automatic air release valves are required at
and slightly downstream of summits in the sewer profile.
Because of the small diameters and curvilinear horizontal
and vertical alignment, excavation depths and volumes
are typically much smaller for a GP pressure sewer than
for conventional sewers, sometimes requiring only a
chain trencher.
Centrifugal and positive displacement pumps have been
used in GP systems. Positive displacement pumps have
a discharge nearly independent of head. Although this
may simplify some design problems, it results in some
additional operational ones. The choice is typically up to
the design engineer's preference.
Several dwelling units or other service locations can be
clustered to a single pump well, which should have an in-
creased working volume depending on the total popula-
tion equivalent it serves. Clustered service connections
have often led to disputes over billing and responsibility
for nuisance conditions and service calls. Duplex pump
wells are often used on clustered, commercial, institu-
tional, or other larger services.
A STEP pressure sewer typically has a septice tank and
a pump at each service connection. The pumps dis-
charge septic tank effluent into a pressurized pipe sys-
tem that terminates at a treatment plant or a gravity
sewer. Because the mains are pressurized, there will be
no infiltration into them, but infiltration and inflow into the
house sewers and the septic/interceptor tanks should be
minimized during the construction of onlot facilities. The
volume of these tanks is usually about 3,800 L (1,000
gal). They remove grit, settleable solids, and grease. The
pumps typically are 0.25 to 0.37 kW (1/3 to 1/2 hp) and
require 110 to 120 V. The head provided by the pumps is
typically about 9 to 15 m (30 to 50 ft) and 1 L/s (15 gpm),
but flow rates vary widely. The working volume of the
79
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pump well is typically 150 to 230 L (40 to 60 gal). The
discharge line from the pump is equipped with at least
one check valve and one gate valve. Electrical service is
required at each service connection. The pipe network
can contain closed loops but typically does not. The
sewer profile typically parallels the ground surface pro-
file, and the horizontal alignment can be curvilinear. Plas-
tic pipe is typically used. The service lateral diameter is
typically 11/4 in. (30 mm). Cleanouts are used to provide
access for flushing. Automatic air release valves are re-
quired at and slightly downstream of summits in all pres-
sure sewer profiles. Because of the small diameters,
curvilinear horizontal and vertical alignments, excavation
depths and volumes are typically much smaller for pres-
sure sewers than for conventional sewers, sometimes re-
quiring only a chain trencher.
A service connection at an elevation higher than the hy-
draulic guide line may be served by gravity, avoiding the
need for a pump. The use of a gravity connection in this
situation is advantageous because a pump would be
subject to siphoning and air-binding. Hybrid designs are
common in current practice. Septic tanks with integral
pump vaults are available; they reduce onlot excavation.
Existing septic tanks are not used owing to their propen-
sity to leak and be a source of infiltration and inflow.
5.8.2 Applicability and Status
Pressure sewer systems are most cost-effective where
housing density is low, where the terrain has undulations
with relatively high relief, and where the system outfall
must be at a higher elevation than most or all of the serv-
ice area. They can also be effective where flat terrain is
combined with high ground water or bedrock, making
deep cuts and/or multiple lift stations excessively expen-
sive. They can be cost-effective even in densely popu-
lated areas where the terrain will not accommodate
gravity sewers.
Since pressure systems do not have the large excess ca-
pacity typical of conventional gravity sewers, they must
be designed with a balanced approach keeping future
growth and internal hydraulic performance in mind.
Where pressure sewers are indicated, the choice be-
tween GP and STEP systems depends on two main fac-
tors: First, the costs of onlot facilities will be typically over
75 percent of the total system cost. Thus there will be a
strong incentive to use a system with less expensive on-
lot facilities for a particular project. STEP systems may
allow some gravity service connections, thus lowering
onlot costs. GP systems must have a pump at each serv-
ice connection to grind the solids.
Second, GP systems require a higher velocity because
they carry heavy solids and grease. STEP systems will
better tolerate the low flow conditions that occur in loca-
tions with a highly fluctuating seasonal occupancy and in
locations with slow buildout from a relatively small initial
population to the ultimate design population.
GP units are preferable for use at individual homes that
discharge into a conventional gravity sewer at a higher
elevation, while STEP systems are most compatible with
small-diameter gravity sewers.
5.8.3 Advantages/Disadvantages
Key advantages of pressure sewer systems include:
• Pressure sewer systems are less expensive than con-
ventional gravity sewerage, due to the fact that the
cost of on-property facilities represents a major portion
of the capital cost of the entire system. This can be-
come an economic advantage since on-property com-
ponents are not required until the house is constructed
and then borne directly by the homeowner. Low front-
end investment makes the present-value cost of the
entire system lower than that of conventional gravity
sewerage, especially in new development divisions
where it may take many years before homes are built
on all lots.
• Due to the fact that the wastewater is pumped, gravity
flow is not necessary, and the strict alignment and
slope restrictions for conventional gravity sewers can
be discarded. Network layout does not depend on
ground contours: pipes can be laid in any location,
and extensions can be made in the street right-of-way
at a relatively small cost without damage to existing
structures or to the natural environment.
• Because the pipe size and depth requirements are re-
duced, material and trenching costs are significantly
lower.
• Manholes are eliminated; low-cost clean outs and
valve assemblies are used instead and are spaced
further apart than manholes in a conventional system.
• Infiltration is greatly reduced or may even be elimi-
nated, resulting in reductions in pipe size.
• The user pays for the electricity used to operate the
pump unit. The resulting increase in the user's elec-
tricity bills is small, but this small increase often re-
places large bills for central pumping, which is
eliminated by the pressure system. In other words, the
responsibilities for installation and paying for pumping
are transferred from the authorities to the users; the
authorities are involved in pump and collection mainte-
nance only.
• Final treatment may be substantially reduced, both in
hydraulic and organic loading in the case of STEP sys-
tems. For GP systems hydraulic loadings are reduced.
• More flexibility is allowed in siting final treatment facili-
ties, and this may help reduce long outfall lines. The
location can be chosen to facilitate site work.
Key disadvantages of pressure sewer systems include:
80
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• A high level of institutional involvement is required,
since the pressure system has many mechanical com-
ponents located all over the area served.
• The O&M cost for a pressure system is often higher
than that of a conventional gravity system due to the
high number of pumps in use. However, the existence
of lift stations in a conventional gravity sewer can
quickly reverse this situation.
• Yearly preventive maintenance calls are usually
scheduled for onlot 'components at pressure sewers,
and STEP systems also require pumpout of intercep-
tor tanks at prescribed (3 to 5 yr) intervals.
• Public education is necessary so that the user knows
how to deal with emergencies and how to avoid block-
ages or other emergency maintenance initiators.
• Malodors and corrosion are potential problems since
the wastewater in the collection sewers is usually an-
aerobic. Proper ventilation and odor control must be
provided for in the design, and noncorrosive compo-
nents should be used. Air release valves are often
vented to soil beds to minimize odor problems, and
special discharge and treatment designs are required
to avoid terminal discharge problems.
5.8.4 Design Criteria
A wide variety of design flows has been used. When
positive displacement GP units are used, the design flow
is obtained by multiplying the pump discharge by the
maximum number of pumps expected to be operating si-
multaneously. When centrifugal pumps are used, the
equation used is: Q = 20 + 0.5D, where Q is the flow in
gpm and D is the number of equivalent dwelling units
served. The operation of the system under various as-
sumed conditions should be simulated by computer as a
check on the adequacy of the design. No allowances for
infiltration and inflow should be required. No minimum
velocity is generally used in design, but GP systems
must attain 3-5 fps at least once per day. A Hazen-Wil-
liams coefficient (C) = 130 to 140 is suggested for hy-
draulic analysis. Pressure mains generally use 5 cm (2
in.) or larger PVC pipe (SDR 21), although at least 7.6
cm (3 in.) pipe is preferred owing to the availability of
standard tapping equipment. Rubber-ring joints are pre-
ferred over solvent welding due to the high coefficient of
expansion for PVC pipe. High-density polyethylene
(HOPE) pipe with fused joints is widely used in Canada.
Electrical requirements, especially for GP systems, may
necessitate rewiring and electrical service upgrading of
dwellings served. Pipes are generally buried to at least
the winter frost penetration depth; in far northern sites in-
sulated and heat-traced pipes are generally buried at a
minimal depth. GP and STEP pumps are sized to ac-
commodate the hydraulic grade requirements of the sys-
tem. Discharge points must employ drop inlets to
minimize odors and corrosion. Air release valves are
placed at high points in the sewer and often are vented
to soil beds. GP effluent is generally about twice the
strength of conventional sewer wastewater (e.g., BOD
and TSS of 350 mg/L). STEP effluent is, of course, pre-
treated and has a BODs of 100 to 150 mg/L and SS of 50
to 70 mg/L. Both can be assumed to be anaerobic and
potentially odorous if subjected to turbulence (stripping of
gases such as HaS).
5.8.5 Capital Costs
Pressure sewers are generally more cost-effective than
conventional gravity sewers in rural areas. However,
even though capital cost savings of 90 percent have
been achieved, no universal statement of savings is pos-
sible owing to the site specificity of each system. Recent
evaluations of the actual costs of pressure sewer mains
and appurtenances (essentially the same for GP and
STEP) and items specific to each type of pressure sewer
have yielded the following data: Average installed unit
costs for pressure sewer mains and appurtenances are
presented in Table 5-8. Average unit costs for GP serv-
ices and appurtenances are presented in Table 5-9. Av-
erage unit costs for STEP services and appurtenances
are presented in Table 5-10.
Within Table 5-8 the linear cost of mains can vary by a fac-
tor of 2 to 3, depending on the type of trenching equipment
and local costs of high-quality backfill and pipe.
5.8.6 O&M Requirements/Costs
Energy costs are borne by the homeowner. For GP sys-
tems this cost may vary from about $1.00 to $2.50/month
depending on the horsepower of the unit. For STEP units
the costs are almost always less than $1.00/month.
Preventive system maintenance is normally carried out
annually for each unit with monthy maintenance of other
mechanical components. STEP systems also require pe-
riodic pumping of tanks. Total O&M costs include trou-
bleshooting, inspection of new installations, and
Table 5-8. Average Installed Unit Costs for Pressure
Sewer Mains and Appurtenances (Mid-1991 )a
Item Unit Cost ($)
2 in. mains 7.50/LF
3 in. mains 8.00/LF
4 in. mains 9.00/LF
6 in. mains 11.00/LF
8 in. mains 14.00/LF
Extra for mains in asphalt concrete pavement 5.00/LF
2 in. isolation valves 250/each
3 in. isolation valves 275/each
4 in. isolation valves 350/each
6 in. isolation valves 400/each
8 in. isolation valves 575/each
Automatic air release stations 1,500/each
aEPA, 1991
LF = linear feet
81
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Table 5-9. Average Unit Costs for Grinder Pump Services
and Appurtenances (Mid-1991)
item
2 hp centrifugal GP
List price
Quantity price
Simplex GP package
List price with 30 in. vault
Quantity price with 30 in. vault
Installation
4 in. building sewer
1.25 in. service line
Abandon septic tank
Unit Cost ($)
1,200/each
600/each
4,100/each
1,800/each
500-1,500/each
16/LF
6/LF
400/each
Table 5-10. Average Unit Costs for STEP Services and
Appurtenances (Mid-1991)
Item
Effluent pump list price
Effluent pump quantity price
Simplex factory package list price
Quantity package price w/extemal vault
Quantity package price w/intemal vault
New septic tank
Installation (retrofit of existing tank)
Installation (with new septic tank)
4 in. building sewer
1.25 in. service line
Abandon septic tank
Unit Cost ($)
300-800/each
200-500/each
2,500-3,000/each
700-1,500/each
600-1,200/each
600-1,000/each
600-1,200/each
1,000-1,500/each
14-18/LF
4-8 /LF
300-500/each
LF = linear feet
responses to problems are estimated at $100-
200/yr/un'rt. The breakdown on emergency service calls
on selected GP and STEP sewer projects are presented in
Table 5-11.
Mean time between service calls (MTBSC) data vary
greatly, but values of 4 to 10 yr for both GP and STEP
units are reasonable estimates for quality installations.
5.8.7 Construction Issues
It Is very important for the engineer to ensure that the
building sewer and ail onlot piping and structures are free
Table 5-11. Distribution of Causes for Call-out
Maintenance on Selected GP and STEP Pressure Sewer
Projects
(% of occurrences)
GP Projects STEP Projects
25-40 40-60
20-25 10-30
20-30 20-40
5-15 1-5
5-15 1-10
Category
Electrically related
Pump related
Miscellaneous
Pump vault or tank related
Piping related
of infiltration and inflow (I/I) and sump pump/foundation
drain connections, via proper testing prior to hookup.
Proper bedding of PVC pipe is also particularly impor-
tant, and accurately prepared "as built" plans must be
provided to the system owner, including lot facility plans
showing the onlot component locations. Each section of
pipe and all tanks should be tested prior to covering with
backfill to ensure water-tight conditions. Videotaping be-
fore and after onlot construction will resolve most resto-
ration claims.
5.8.8 Monitoring
Detailed daily records of maintenance and annual sum-
maries should be provided. Also specific records for each
unit should be kept with the lot facility plan in order to
permit maintenance staff to evaluate potential problems
prior to arrival at the site of the emergency call. On larger
flow sources, cycle counters may be useful to track any
trends, just as periodic line-pressure checks can alert the
O&M staff to impending needs.
5.8.9 Residuals
STEP systems incorporate septic (interceptor) tanks, and
therefore need periodic pumping to remove trapped
grease and solids. Usually this is required every 3 to 5 yr,
but some systems attempt to lengthen this pumping fre-
quency by requiring biannual inspections to avoid unnec-
essary pumping. Heavy grease generators, such as
commercial sources, may need to be pumped out annu-
ally or semiannually.
Owing to their tendency to accumulate grease in their
tankage, GP units are often pumped as part of the an-
nual preventive maintenance check.
5.9 Small Diameter Gravity Sewers
5.9.1 Technology Description
A small diameter gravity (SDG) sewer collects effluent
from septic tanks at each service connection and trans-
ports it by gravity to a treatment plant or a gravity sewer.
Such systems are also known as diameter effluent sew-
ers, effluent drains, and small bore sewers. The volume
of the septic tanks in these systems is often 1,000 gal but
varies widely. The tanks remove grit, settleable solids,
and grease, and they attenuate peak flows. Both the
horizontal and vertical alignments of the pipes can be
curvilinear. The pipe network includes no closed loops.
Uphill sections can be used provided there is enough
elevation head upstream to maintain flow in the desired
direction and there is no backflow into any service con-
nection. Minimum diameters are usually 10 cm (4 in.),
but smaller sizes have been used successfully. Plastic
pipe is typically used since it is economical in small sizes
and resists corrosion by the septic wastewater.
Cleanouts are used to provide access for flushing. Man-
holes are used infrequently, usually only at the major
junctions of main lines. Air release risers may be re-
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quired at or slightly downstream of extreme summits in
the sewer profile. Because of the small diameters and
flexible slope and alignment, excavation depths and vol-
umes are typically much smaller than with conventional
sewers, sometimes requiring only a chain trencher.
Two varieties of SDG systems have been used in the
United States: variable grade and minimum grade. Vari-
able grade systems follow surface contours rather
strictly, taking advantage of the flexibility of horizontal
and vertical alignment when there is enough elevation
head to maintain flow in the desired direction and there is
no backflow into any service connection at design flow.
For minimum grade systems, minimum downward slopes
are imposed. Recent designs blend both of these ap-
proaches, allowing variable grade but minimizing the
number of flooded sections.
Individual service connections can be equipped with a
septic tank effluent pump (STEP) unit, creating a hybrid
of the STEP pressure sewer. The use of STEP connec-
tions is advantageous when excavation costs can be re-
duced enough to offset increased onlot costs. Hybrid
designs are common in current practice. Inline lift sta-
tions can also be used if required by the terrain, but their
use may signal a need to reevaluate the cost-effectiveness
of an SDG system vs. other alternative sewer systems. Use
of hybrid systems should be considered before evaluating
high-cost lift stations.
While two-compartment septic tanks may be more effi-
cient at retaining solids, single-compartment tanks have
performed well. Also, several dwelling units or other serv-
ice locations can be clustered to a single septic tank,
which should have an increased volume depending on
the total population equivalent it serves.
Other variations of SDG sewers—known as "simplified
sewer systems," "steep slope sewers," and "flat-grade
sewers"—are in use in Nebraska as well as in South
America, Africa, and Asia. These systems employ
smaller diameter pipes, simplified manhole requirements,
shallower depths, and other cost-saving features in com-
parison to conventional gravity sewers.
5.9.2 Applicability and Status
Approximately 250 SDG sewers have been financed in
the United States by the EPA Construction Grants Pro-
gram. Many more have been financed with private or lo-
cal funding in North America.
SDG sewer systems are likely to be most cost-effective
where the housing density is low, the terrain has undula-
tions of low relief, and the elevation of the system termi-
nus is lower than all or nearly all of the service area.
They can also be effective where the terrain is too flat for
conventional gravity sewers without deep excavation.
SDG sewer systems do not have the large excess ca-
pacity typical of conventional gravity sewers. Therefore
they must be designed with an adequate allowance for
future growth if that is desired. These systems were in-
troduced in the United States in the mid-1970s, but have
been used in Australia since the 1960s.
5.9.3 Advantages/Disadvantages
Key advantages of SDG systems include:
• Construction is faster than for conventional sewerage,
requiring less time to provide service.
• O&M can be carried out by unskilled personnel.
• Any required lift and pumping station can be reduced
in size and simplified owing to the nature of the waste-
water.
• Elimination of manholes helps to further reduce inflow
(further reducing the sizes of pipes), lift/pumping sta-
tions, and final treatment.
• Reduced excavation costs: Trenches for SDG sewer
pipelines are typically narrower and not as deep as in
conventional sewers.
• Reduced material costs: As the name indicates, SDG
sewer pipelines are smaller than conventional sewers,
reducing pipe and filling costs.
• Power requirements are negligible or low.
• Final treatment requirements are not only reduced hy-
draulically but are also scaled down in terms of or-
ganic loading since partial treatment is performed at
the source.
• Reduced depth of mains minimizes additional con-
struction costs due to high ground water or rocky con-
ditions.
While SDG systems have no major disadvantages spe-
cific to temperate climates, some restrictions exist gener-
ally that limit their application:
• SDG sewers are not suitable to serve as combined
sewers, even if pipe size could be increased. Due to
the nature of variable grades and relatively flat slopes,
solids drawn to SDG sewers will block them.
• For the same reason, SDG sewers cannot handle
commercial wastewater having high grit or settleable
solids levels. Restaurants may be hooked up if they
are equipped with effective grease traps. Laundromats
could be a constraining factor for SDG in small com-
munities. There has been no report on the use of SDG
as a commercial wastewater collection option.
• Corrosion has been a problem in some SDG systems
in the United States. Noncorrosive materials must be
incorporated in the design.
• Desludging interceptor tanks and disposing of col-
lected septage are probably the most complex as-
pects of the SDG system. This should be carried out
by the local authorities. Contracting the private sector
83
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to carry out these tasks is an option, provided the
authorities have enforceable power for hygiene control.
• The most common problem has been related to odors.
Many early systems utilized an onlot balancing tank
that promoted stripping of HaS from the interceptor
(septic) tank effluent. Other odor problems have been
due to inadequate house ventilation systems and
mainline manholes or venting structures. In all cases,
appropriate engineering can control these problems.
5.9.4 Design Criteria
Peak flows are based on the formula Q = 20 + 0.5D,
where Q is gpm and D is the number of dwelling units
served by the system. A determination of peak flows is
used for design instead of actual flow data. Each seg-
ment of the sewer is analyzed by the Hazen-Williams or
Manning equations. Roughness coefficients of 120 to
140 (H-W) and 0.013 (M) are common. No minimum
velocity is required and PVC pipe (SDR 35) is commonly
employed for all gravity segments. But stronger pipe
(e.g., SDR 21) may be dictated where several STEP
units feed the system. Also, check valves may be used in
flooded or other sections on service laterals where
backup from the main is possible (surcharging).
All components must be corrosion-resistant and all dis-
charges (e.g., to a conventional gravity interception or
treatment facility) must be made through drop inlets be-
low the liquid level to minimize odors. The system is ven-
tilated through service-connection house vent stacks.
Other atmospheric openings should be directed to soil
beds for odor control, unless they are located away from
the populace.
Interception tanks are generally sized in accordance with
local septic tank codes. STEP units employed for below-
grade services are covered under pressure sewers. It is
incumbent on the engineer to ensure that onlot infiltration
and inflow (I/I) be eliminated through proper testing of
building sewers and pre-installation testing of intercep-
tion tanks.
Mainline cleanouts are generally spaced at 120 to 300 m
(400 to 1,000 ft) apart. Septic (interceptor) tank effluent
is generally assumed to contain 100 to 150 mg/L BODs
and 50 to 75 mg/L SS. Treatment is normally by stabili-
zation pond or by subsurface infiltration.
5.9.5 Capital Costs
The installed costs of the collector mains and laterals
and the interceptor tanks constitute more than 50 percent
of total construction cost. Average unit costs for 12 pro-
jects (adjusted to January 1991) were: 10 cm (4 in.)
mainline, $12.19/ft ($3.71/m); cleanouts, $290 each;
service connections, $9.08/ft ($2.76/m); and 440 L (1,000
gal) interceptor tanks, $1,315. The average cost per con-
nection was $5,353 (adjusted to January 1991).
5.9.6 O&M Requirements/Costs
The major O&M requirement for SDG systems is the
pumping of interceptor tanks, usually at 3 to 5 yr inter-
vals. Other O&M activities include line repairs from exca-
vation damage, supervision of new connections, and
inspection and repair of mechanical components. Most
SDG system users pay $10 to 20/month for manage-
ment, including O&M and administrative costs.
5.9.7 Construction Issues
As with all alternative sewer systems, onlot construction is a
major part of the project. All sites should be videotaped prior
to and after construction to minimize claims from home-
owners. Onlot facility plans should be developed at this
time and used throughout the project by maintenance
personnel. Testing of all piping installed should be per-
formed, and as-built drawings of the installation should
be created and submitted to the local authority. Where
possible, trenching machines should be considered for
substantial (=50 percent) cost savings. Granular backfill
is preferred, as are rubber-ring joints, given the nature of
PVC pipe. Magnetic tape above the installed pipe is con-
sidered extremely cost-effective to prevent future dam-
age. In some cases, the authority has included the
building sewer in the project to ensure I/I control. Ease-
ments are required for O&M tasks and usually are in the
form of a wide strip centered on the service lateral and
interceptor tank.
5.9.8 Monitoring
Some management schemes involve biannual tank in-
spection and pumping only when needed. Most have
merely dictated a pumping schedule (e.g., 3 to 5 yr for
residential users and every year for commercial users).
Otherwise, no monitoring plan is typically established.
5.9.9 Residuals
SDG systems, like STEP systems, require interceptor
tank pumping. Septage treatment and disposal are de-
scribed elsewhere.
5.10 Vacuum Sewers
5.10.1 Technology Description
A vacuum sewer system has three major subsystems:
the central collection station, the collection network, and
the onsite facilities. Vacuum pressure is generated at the
central collection station and is transmitted by the collec-
tion network throughout the area to be served. Wastewa-
ter from conventional plumbing fixtures flows by gravity
to an onsite holding tank. When about 38 to 57 L (10 to
15 gal) of wastewater has been collected, the vacuum in-
terface valve opens for a few (3 to 30) seconds allowing
the wastewater and a volume of air to be sucked through
the service pipe and into the main. The difference be-
tween the atmospheric pressure behind the wastewater
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and the vacuum ahead provides the primary propulsive
force. The fact that both air and wastewater flow simulta-
neously produces high velocities that prevent blockages
under normal operating conditions. Following the valve
closure, the system returns to equilibrium and the waste-
water reaches the central collection tank, which is under
vacuum. When the wastewater in that tank reaches a
certain level, a conventional nonclog wastewater pump
discharges it through a force main to a treatment plant or
gravity interceptor.
The vacuum interface valve is the unique component of a
vacuum sewer system. It operates automatically using
pneumatic controls; thus, the onsite facilities do not use
any electricity. The valve is placed in a valve pit that is
usually buried above the holding tank. Plastic pipe is
used throughout a vacuum sewer system. The gravity
flow house sewer is usually 10 cm (4 in.) pipe. It normally
incorporates an external vent to admit air when the valve
cycles, thus preventing the house plumbing traps from
being sucked dry. Typical service laterals are 7.6 cm (3 in.)
pipe, and mains range from 10 to 25 cm (4 to 10 in.) de-
pending on the flow and layout. Joints are either solvent-
welded or (preferably) vacuum-certified rubber-ring type.
The profile of the collection network makes use of the
limited ability of vacuum propulsion to flow upward in or-
der to avoid excessive excavation. Where the ground
slopes in the flow direction more than 0.2 percent, the
pipe parallels the ground. Otherwise the pipe is laid with
a downward slope of 0.2 percent until the depth becomes
excessive. When this occurs, a lift formed by two 45 de-
gree elbows and a short length of pipe is inserted to gain
elevation. The typical lift raises the pipe by 0.6 m (2 ft) or
less. Division valves are usually placed at main junctions
and at 450 m (1,500 ft) intervals to facilitate troubleshoot-
ing and repairs. Service lines or tributary mains always
join the continuing main from above through a wye
connection.
Several mains may be served by a single collection sta-
tion. Each main is connected directly to the collection
tank through a division valve. Air usually flows from the
collection tank through a vacuum reserve tank to the vac-
uum pumps, which discharge to the atmosphere. Dual
vacuum pumps are provided to improve reliability. Both
liquid ring and sliding vane vacuum pumps have been
used. Automatic controls cycle the vacuum pumps alter-
nately to maintain the vacuum in the desired range, usu-
ally 5.5 to 7.0 m (18 to 23 ft) of water. A backup
diesel-generator set is used to maintain service during
electrical outages. An autodialing telephone alarm is pro-
vided to summon the operator in case of malfunctions.
Detailed design recommendations differ among manu-
facturers and engineers. A single interface valve may
serve several houses, a school, or a small business
area. A few systems use vacuum toilets, which require
about 1 to 2 L (0.25 to 0.5 gal)/flush, contain their own in-
terface valves, and have their own vacuum service lines.
Cleanouts are provided as access to mains.
The vacuum reserve volume may be provided in the col-
lection tank rather than in a separate vacuum reserve
tank. One manufacturer offers an ejector-type vacuum
pump in which wastewater from the collection tank is re-
circulated by a centrifugal pump as the primary fluid in a
multiphase jet pump. Another manufacturer provides a
factory-assembled, skid-mounted central collection sta-
tion that can handle design sewage flows of up to 10 Us
(150 gpm). In addition to residential applications, vacuum
plumbing is used in office buildings, hospitals, factories,
and marinas.
5.10.2 Applicability and Status
On January 1, 1990, there were 42 residential vacuum
sewer systems operating in 12 states, including Alaska
and Florida. These systems served more than 50,000
persons using 100 central collection stations, more than
10,000 vacuum interface valves, and about 160 vacuum
toilets. The first of these systems has operated since
1970. All but three systems use the same brand of inter-
face valves.
Vacuum sewers are most likely to be cost-effective when
excavation costs are high, population densities are mod-
erate to high (but isolated), and the topography is flat to
moderately rolling. They are well suited for combined
sewer separation projects in finite urban areas where
prohibitive costs of construction must be minimized.
Other factors favoring vacuum sewers are the need for
water conservation and the need to minimize the risk of
sewage spills.
5.10.3 Advantages/Disadvantages
Key advantages of vacuum sewers include:
• Low cost of mains
• No electricity required at service locations
• Shallow depth of mains
• Single valve can serve several homes (sources)
• With backup generator, complete independence from
local power outages
• Well suited for dense but isolated housing develop-
ment
Key disadvantages of this type of system include:
» Requires large (>75 generally) numbers of homes for
cost-effectiveness
• Somewhat more mechanically sophisticated than other
alternative systems
• Requires fast response to malfunctions and greater
O&M skill level than other alternative systems
• Slightly more difficult to install main lines than other
alternatives
85
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5,10.4 Design Criteria
Although there are no universally accepted criteria, the
following are widely used:
The maximum capacity of a 7.6 cm (3 in.) interface valve
is 2 Us (30 gpm). The minimum vacuum head needed to
operate an interface valve is 1.5 m (5 ft) of water.
Vacuum sewer design rules have been developed largely
by studying operating systems. Important design pa-
rameters are presented in Tables 5-12 and 5-13.
Table 5-14 shows at what length the 0.2 percent slope
will govern vs. the percentage of pipe diameter for the
slopes between lifts.
The AIRVAC company has developed a table recommend-
ing maximum design flows for each pipe size (Table 5-15).
The maximum number of homes served for various pipe
sizes is presented in Table 5-16.
The sum of frictional and lift losses should not exceed
about 4 m (13 ft) of water. Frictional losses may be esti-
mated using a modified Hazen-Williams formula. The
recommended height of a lift is 30 cm (1 ft) in 10 cm (4 in.)
pipe and 46 to 70 cm (1.5 to 2.3 ft) in larger pipes. The
loss due to a lift is taken as the invert to invert rise less
the internal pipe diameter.
Table 5-12. Main Line Design Parameters
Minimum distance between lifts 20 ft
Minimum distance of 0.2 percent slope prior to 50 ft
a series of lifts
Minimum distance between top of lift and any 6 ft
service lateral
Minimum slope 0.2%
Table 5-13. Guidelines for Determining Line Slopes3
Line Size Use Largest of:
4 in. Mains -0.2%
- Ground slope
-80% of pipe dia. (between lifts only)
Bin. Mains -0.2%
- Ground slope
-40% of pipe dia. (between lifts only)
"Assuming minimum cover at top of slope.
Table 5-14. Governing Distances for Slopes Between Lifts
Pipe Diameter Distance (ft) Governing Factor
(In.)
4 <135
4 >135
>6 <100
>6 >100
Table 5-15. Maximum Flow for Various Pipe Sizes3
80% of pipe diameter
0.2% slope
40% of pipe diameter
0.2% slope
Pipe Diameter (In.)
4
6
8
10
Maximum Flow (gpm)
55
150
305
545
aBMCI, 1989.
Table 5-16. Maximum
Various Pipe Sizes3
Number of Homes Served for
Pipe Diameter (in.)
4
6
8
10
Homes Served
70b
260
570
1,050
aBMCI, 1989.
bThe recommended maximum length of any 4-in run is 2,000
ft, which may limit the amount of homes served to a value
less than 70.
Use dual vacuum pumps; size each to handle airflow at
design conditions. Use dual sewage pumps; size each to
handle design flow. The collection tank volume is at least
three times the working volume. Choose the working vol-
ume that allows a sewage pump to start every 15 min-
utes at design flow. A 1,500 L (400 gal) vacuum reserve
tank is normally used. The vacuum pump run time
should be 1 to 3 minutes.
5.10.5 Environmental Impact
Construction impacts may be much less than conven-
tional sewers because of reduced excavation. Risk of
sewage spills is minimal since pipes are under vacuum.
Aeration of sewage in mains nearly eliminates odor
problems.
5.10.6 Capital Cost Sensitivity
Costs are highly site-specific. The following data are gen-
eralized estimates based on a 1989 telephone survey
concerning 32 out of 42 U.S. vacuum systems, on bid
tabulations and information from manufacturers and de-
sign engineers. All costs are in December 1989 dollars
(ENR Construction Cost Index = 4679). On the basis of
data from 17 systems, the total construction cost of a
vacuum sewer system may range from $7,000 to
$18,000/valve. Note that one valve may serve more than
one house. A more detailed estimate can be based on
the following typical installed unit costs, but wide vari-
ations from these values are to be expected.
3 in. interface valve, pit, cover $2,000-2,300 each
4 in. auxiliary vent 50-60 each
4 in. gravity flow house sewer 5/ft
3 in. vacuum service pipe 7/fl
4 in. vacuum main 8-11/ft
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6 in. vacuum main 11-14/ft
8 in. vacuum main 14-17/ft
10 in. vacuum main 1 9/ft
4 in. division valve 350 each
6 in. division valve 400-500 each
8 in. division valve 550-700 each
10 in. division valve 1,000 each
4 in. cleanout 150 each
6 in. cleanout 180 each
Gauge taps 50 each
Lifts 50 each
Cycle counter 125 each
Table 5-17 gives average installed prices for both custom-
designed stations as well as for package stations. The
prices include the equipment (including the generator for
all stations), station piping, electrical, excavation, site
restoration, and labor.
5.10.7 O&M Requirements/Cost Range
Power consumption varies with design flow, length of
mains, lift, and quality of maintenance. On the basis of
records from six systems, annual power consumption
ranges from 150 to 600 kWh/valve. A value of 200
kWh/valve/yr is recommended for preliminary estimates.
A study of six systems yielded a MTBSC from 1 to 22
years. A planning value of 6 to 8 years is reasonable for
new vacuum valve designs.
Preliminary O&M cost estimates can be derived with the
following formula:
C = (2,430 x NS) + (205 x LR x NS) + (0.5 x LR x NDV)
+ (5.1 x NIV) + (1.2 x LR x NIV) + (500 x NIV x ER)
Where:
C = annual O&M cost in December 1989 dollars
NS = number of central collection stations
LR = labor rate including fringe benefits and overhead
in December 1989$/hr
NDV = number of division valves
NIV = number of vacuum interface valves
ER = electric power rate in December 1989$/kWh
Major components of a vacuum sewer system are pre-
sented in Figure 5-7.
5.10.8 Construction Issues
Vacuum system manufacturers generally include assis-
tance to engineers and communities during the construc-
tion phase of the project. One key difficulty arises due to
the common practice of using two separate phases of in-
Vacuum
Main #2
Vacuum
Main #3
Branch
Line
Vacuum
Main #1
3 in. Vacuum
Service Line
Vacuum Station
Division Valve
Valve Pit
Building Sewer
House
Figure 5-7. Major Components of a Vacuum Sewer System.
Table 5-17. Average Installed Cost for Vacuum Station (Mid-1990)
Item
Package station
Package station
Package station
Custom station
Custom station
Custom station
Number of
Customers ($)
10-25
25-50
50-150
100-300
300-500
>500
Equipment Cost3
($)
50,000
75,000
90,000
120,000
140,000
170,000
Building Cost ($)
25,000
30,000
40,000
50,000
60,000
75,000
Installed Cost ($)
20,000
25,000
30,000
40,000
50,000
75,000
Total Cost ($)
95,000
130,000
160,000
210,000
250,000
320,000
Includes generator.
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stalling onsite services, with subsequent main-line instal-
lation and hookup. Careful coordination of these activities
is necessary. Vacuum testing of all sewer segments
must be performed daily during construction. During con-
struction of a vacuum sewer system there is potentially
less noise impact, fewer problems with fugitive dust, and
less erosion than with conventional sewers because of
smaller equipment requirements.
5.10.9 Monitoring
In addition to operating tasks discussed above, cycle
counter readings and spot checks of vacuum pressure at
various locations in the piping network should be in-
cluded in any monitoring program to anticipate potential
problems.
5.11 Mechanical Systems for
Wastewater Treatment
Mechanical systems utilize a combination of biological
and physical processes for the treatment of wastewater,
employing tanks, pumps, blowers, rotating mechanisms,
and/or other mechanical components as part of the over-
all wastewater treatment system. Mechanical systems
are frequently used for medium to large municipal waste-
water treatment plants, but also have been widely ap-
plied for the treatment of wastewater associated with
small, sewered communities or clusters of residential
housing and commercial establishments. For very low
flows (<2 L/s (<50,000 gpd)), preengineered "package
plants" are the mechanical systems normally used. Efflu-
ent from mechanical systems can either be discharged to
surface water or applied to the land.
Virtually all mechanical systems employ suspended-
growth or attached-growth (fixed-film) biological proc-
esses or a combination of the two. In suspended-growth
systems, microorganisms responsible for the breakdown
of the organic matter are suspended in liquid by mixing.
In attached-growth systems, microorganisms become at-
tached to an inert medium, such as rock or plastic. Oxy-
gen is provided mechanically in suspended-growth
systems and naturally in attached-growth systems. The
biological process is followed by a clarification step to al-
low separation of the biological solids from the treated
wastewater. Suspended-growth treatment approaches
discussed in this section include the sequencing batch
reactor, oxidation ditch, and extended-aeration system.
The trickling filter process is discussed as an example of
an attached-growth system.
Mechanical systems have potential advantages over
some "natural" alternatives in that they can provide a
high-quality effluent, and they are generally very land-ef-
ficient. Disadvantages include the need for close skilled-
operator supervision, high maintenance requirements,
and high power consumption compared to natural waste-
water treatment systems.
5.12 Extended-Aeration Activated
Sludge
5.12.1 Technology Description
The extended-aeration process is a widely used modifi-
cation of the conventional suspended-growth, activated-
sludge process characterized by low loading rates and
long hydraulic and solids retention times. Hydraulic re-
tention times are typically 24 hours, with solids retention
times of 20 to 40 days. Because of the low BOD loading,
the process operates in the endogenous respiration
phase of the microbial growth cycle, resulting in partial
oxidation of biological solids. Some extended-aeration
processes operate in a completely mixed regime, with
the contents of the aeration basin being nearly homoge-
neous. Some technologies are designed to operate in a
plug-flow mode. The high solids retention times associated
with the process promote nitrification. In a well-operated
facility, BOD and SS removals can be expected to range
from 85 to 95 percent. A properly designed and operated
extended-aeration facility can be expected to produce an
effluent with BOD and SS levels less than 30 mg/L 90
percent of the time and less than 20 mg/L 50 percent of
the time. Because of the long aeration times, biodegrad-
able toxic compounds are likely to be removed. A flow
diagram of the process is shown in Figure 5-8.
An extended-aeration process typically consists of coarse
screening or comminution, activated-sludge aeration us-
ing course air diffusers or mechanical aerators, secon-
dary clarification using surface skimming and return
sludge pumping, disinfection often with chlorine storage
and feed facilities, and transport to a contact basin.
Sludge is typically wasted to an aerobic holding or aero-
bic stabilization compartment. Sludge disposal varies
widely. Primary clarification is rarely used.
Screened and/or Complete Mix
Raw Wastowator
^ *• AeranonranK >
Return Sludge
^ Clarifler I ^
Sludge
V
Excess Sludge
1
W
^^
w
H
Chlorination
Digestion
Effluent ^
To Disposal
Aerobic Holding Tank
or Aerobic Digester
»
Figure 5-8. Schematic of an Extended-Aeration Process.
88
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5.12.2 Applicability and Status
Extended-aeration processes are used widely throughout
the United States for municipal wastewater flows less
than 2 L/s (50,000 gpd). Most of these are preengineered
package plants that serve residential subdivisions, small
clusters of homes, commercial or institutional estab-
lishments, or individual homes. Such package plants are
largely preassembled, allowing rapid installation with a
minimum of site preparation. For this reason, package
plants are sometimes used for temporary or emergency
applications. Package plants generally utilize steel tank-
age, but precast concrete is also used. Preengineered
extended-aeration systems are available for flows up to
about 40 L/s (1 mgd). However, the extended-aeration
process has been used to treat wastewater flows of 220
L/s (5 mgd) and greater.
Most package plants discharge to surface waters, al-
though they have been utilized for treatment prior to land
application or even subsurface disposal. However, the
mechanical complexity of such plants is generally not
compatible with low-maintenance, land-based disposal
options.
Extended-aeration processes and package plants are
considered a fully developed technology, having been in
wide use since the 1950s.
5.12.3 Advantages/Disadvantages
Key advantages of the extended-aeration process in-
clude:
• Lowest sludge production of any activated-sludge
process
• High-quality effluent achievable
• Preengineered package plants quickly installed with
minimal site preparation
• Favorable reliability with sufficient operator attention
• Nitrification likely at wastewater temperatures >15°C
• Relatively minimal land requirements
• Relatively low initial cost
• Can handle moderate-shock hydraulic loadings with
minimal problems
Key disadvantages of this type of system include:
• High power consumption and energy cost compared
to land-based or natural systems
• High O&M requirements compared to land-based or
natural systems; skilled operator necessary
• Susceptible to excursions in effluent SS and associ-
ated BOD due to high flow variations and operator in-
attention
• Potential freezing problems in cold climates
• Possibility of poor settleability of mixed liquor sus-
pended solids (MLSS) due to formation of "pinpoint"
floe
• Potential for rising sludge due to dentrification in final
clarifier in warmer months
• Blower noise and sludge handling odor potential
• Preengineered plants may require additional components
or modification to meet specified effluent limitations
5.12.4 Design Criteria
Table 5-18 summarizes the design criteria for the
extended-aeration process. For package plants, compo-
nents such as aeration basins and clarifiers are preengi-
neered, and system selection for domestic wastewater
applications typically is based solely on flow. The size of
the system selected should be conservative to account
for peak flow conditions. Table 5-19 provides typical siz-
ing of unit processes at average design flows of 0.4 to
4.0 L/s (0.01 to 0.1 mgd).
For engineered plants using the extended-aeration proc-
ess, final clarifier design should be conservative to ac-
count for high MLSS and poor settleability of the
biological solids. Since the process lacks primary clarifi-
Table 5-18. Summary of Design Criteria for Extended-
Aeration Process
Parameter Range
Volumetric loading (Ib BODs/d/1,000 cu ft) 8-15
MLSS (mg/L) 2,500-6,000
F/M (Ib BODs/d/lb MLVSS) 0.05-0.15
Aeration detention time (hr) (based on daily 18-36
flow)
Air requirements (scf/lb BODs applied Ib Oa/lb 3,000-4,000
BODs applied) 2.0-2.53
Solids retention time (days) 20-40
Recycle ratio (R) 0.75-1.5
Volatile fraction of MLSS 0.6-0.7
aBased on 1.5 Ib Oa/lb BODs removed + 4.6 Ib O2/lb
removed.
Table 5-19. Typical Component Sizing for Extended-
Aeration Plants
At Average Design
Flow (mgd)
Unit Process
Raw sewage pumping (mgd)
Aeration basin volume (cu ft)
Secondary clarifier area (sq ft)
Chlorinator capacity (Ib/d)
Chlorine contact chamber volume (cu ft)
Drying bed area (sq ft)
Site area (ac)
0.01
0.04
1,330
40
10
120
400
0.5
0.05
0.20
6,690
200
25
560
1,000
0.7
0.10
0.40
13,400
400
50
1,200
2,000
1.0
89
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cation and due to the potential for rising sludge, secon-
dary clarifiers must be equipped with surface skimming
devices to remove grease and floating solids. Surface
aerators are not recommended for extended-aeration
processes in cold climates because of potential for
freezing problems.
Design life of a wastewater treatment plant using the
extended-aeration process can be expected to vary de-
pending on whether it is a preengineered system or one
employing cast-in-place concrete tanks. Steel package
plants may have an effective service life as short as 10
years if the maintenance program is inadequate, but as
long as 20 years with regular and effective maintenance.
Larger, custom-engineered systems may have a design
life of 20 years or more. Mechanical equipment used in
either type is likely to have a service life of 5 to 15 years,
depending on the type and quality of the equipment.
Because of concerns for blower noise and odors associ-
ated with handling of residuals, adequate buffer zones
are necessary between the facility and residential areas.
This distance should be at least 200 ft. In addition, ade-
quate fencing is necessary to restrict access to the site.
Small mechanical plants typically are unattended at
night. For this reason, plants should be designed with an
alarm system to alert emergency personnel, such as a
police or fire department dispatcher.
5.12.5 Capital Cost Sensitivity
Package plants are relatively economical since site
preparation and engineering costs are minimized. Some
units are installed above ground on a concrete slab. In
general, package plant costs are sensitive to flow, with larger
units required to adequately handle peak flow conditions.
Costs of engineered systems or package plants may be
affected by site conditions such as geology, climate, and
surrounding aesthetics. Larger treatment plants may re-
quire buildings to house operations and control centers,
laboratories, and sludge dewatering facilities. In general,
preengineered package systems are less costly than
custom-designed systems employing cast-in-place con-
crete tankage.
5.12.6 O&M Requirements
O&M requirements for an extended-aeration facility are
high because of the need for skilled and regular operator
supervision to ensure performance, and the general me-
chanical complexity of the components. O&M require-
ments are summarized in Table 5-20. An inventory of
spare parts must be maintained in case of equipment
failure, which eventually will occur. Inventories should
include pump packing and seals, bearings, motors,
diffusers, chlorinator components, and flow metering
equipment. In many cases, lack of spare parts and the
long lead times to procure parts and equipment have re-
sulted in long periods of poor performance.
Table 5-20. O&M Requirements of Extended-Aeration
Facilities
Operation
• Effluent quality monitoring required by NPDES permit
• Operation assessment analyses (e.g., MLSS, DO, sludge
blanket, settleability)
• Regular cleaning of screens, weirs, skimmer
mechanisms, diffusers, and other components
• Regular adjustment of sludge return rate and air injection
rate
• Regular wasting of solids
• Sludge dewatering and disposal
• Control of disinfectant dosage
• Administration and recordkeeping
Maintenance
• Blowers or mechanical aerators
• Influent and return sludge pumps
• Electrical equipment and instrumentation
• Mechanical dewatering equipment
• Laboratory equipment such as pH, DO meters, chemicals
• Disinfection system
Many small communities lack the skilled personnel nec-
essary to operate properly and maintain a mechanical fa-
cility of this type. Thus an extended-aeration plant may
not be the system of choice for flows less than 2 L/s
(50,000 gpd). If such a system already exists, the munici-
pality may wish to procure, under contract, the services
of an individual or firm qualified to operate and maintain
the facility, relieving the municipality of this burden.
Energy consumption for an extended-aeration facility is
high relative to other mechanical treatment alternatives,
primarily due to mixing and the high oxygen requirement
associated with the long solids retention time. Energy is
consumed by aeration of the activated sludge basin (as
well as the aerated sludge holding tank or aerobic di-
gester, if applicable), influent pumping (if applicable), return
sludge pumping, mechanical dewatering (if applicable),
building heating, cooling, and lighting, and by operating
other miscellaneous processes such as clarification and
disinfection. However, the aeration/mixing step accounts
for the overwhelming majority of power demand. Electri-
cal energy requirements may be expected to be approxi-
mately 15,000 kWh/yr for a 0.4 L/s (10,000 gpd) facility,
40,000 kWh/yr for a 2 L/s (50,000 gpd) plant, and 60,000
kWh/yr for a 4 L/s (100,000 gpd) facility.
5.12.7 Monitoring
Monitoring consists of sampling and conducting analyses
as required by the National Pollutant Discharge Elimina-
tion System (NPDES) or other permits, and as is neces-
sary to ensure optimum plant performance. NPDES
requirements will vary substantially depending on the
size of the facility and the characteristics of the receiving
waters. NPDES monitoring requirements may include in-
90
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fluent flow, influent BOD and SS, and effluent BOD, SS,
chlorine residual, and nutrients (i.e., ammonia, phospho-
rus). Sampling frequency may range from daily to bi-
monthly. Operational monitoring generally includes MLSS,
aeration basin, dissolved oxygen (DO), pH, settleability, al-
kalinity, effluent chlorine residual, influent flow, return
sludge flow, and sludge waste rate.
5.12.8 Residuals
Residuals generated from an extended-aeration facility
include screenings and waste sludge. Grit is also gener-
ated if the plant is equipped with a separate grit cham-
ber. Grit and screenings must be removed and disposed
of promptly because of their putrescible nature. If no
separate grit chamber is provided, grit accumulates in the
aeration tank, eventually requiring draining and cleaning.
Due to the long solids-retention time, waste sludge pro-
duction is somewhat less than for conventional activated
sludge. Sludge may be wasted to a sludge holding tank
or stabilization process such as an aerobic digester and
removed from the plant as a liquid, or may be sub-
sequently dewatered by sand bed or other technologies
and hauled away as a solid. Options for the handling of
waste sludge from mechanical wastewater treatment sys-
tems are discussed in greater detail later in this chapter.
5.13 Trickling Filter and Modifications
5.13.1 Technology Description
The trickling filter process is an attached-growth process
in which wastewater that has undergone primary clarifi-
cation is distributed periodically over an inert media such
as rock or plastic. Organisms contained in biological slimes
attached to this media are responsible for the breakdown
of organic matter. Periodically, portions of this slime layer
"slough off" the inert media and are removed by gravity,
settling in a secondary clarifier. A typical trickling filter
process consists of screening, grit removal, primary clari-
fication, biological treatment with the trickling filter, sec-
ondary clarification, and disinfection.
Conventional, standard-rate trickling filters were at one
time one of the most popular methods of wastewater
treatment, with many such facilities constructed from
1930 to 1950. The conventional trickling filter process is
simple and easy to operate. The early plants used stones
as the media for biological growth and were designed for
discharge of the secondary sludge or "humus" back to
the primary clarifier for co-settling. The combined pri-
mary and secondary sludge was typically dewatered on
sand drying beds, particularly at the smaller facilities.
The conventional trickling filter process is limited with re-
spect to performance. Many older facilities are unable to
meet secondary treatment standards (30 mg/L BOD; 30
mg/L SS) on a year-round basis. Finely divided panicu-
late matter with poor settling properties, characteristic of
the trickling filter process, often contributes to high SS
levels in the effluent. Attention to the design of the final
clarifier can overcome this problem. In 1984, EPA re-
laxed performance standards for existing trickling filter
plants, allowing discharge of an effluent containing no
more than 45 mg/L SS and 45 mg/L BOD. A conven-
tional existing trickling filter plant would be expected to
produce an effluent with less than 40 mg/L BOD and SS
90 percent of time, and less than 30 mg/L BOD and SS
50 percent of the time.
Several modifications to the conventional standard rate
of trickling have been developed to overcome the per-
formance limitations of the original process. These in-
clude "high-rate" processes and the trickling filter-solids
contact (TFSC) process. High-rate trickling filters typi-
cally employ higher hydraulic and organic loading rates
to the trickling filters and continuous recirculation of efflu-
ent through the filter at 50 to 300 percent of the influent
flow. Many high-rate systems employ plastic media con-
sisting of either random-dump packing or prefabricated
plastic modules that are set in place. Considerable devel-
opment has occurred in the manufacture of such modular
plastic media for trickling filter applications.
The TFSC process is a relatively innovative approach to
the trickling filter process. In this process, discharge from
the trickling filter is "contacted" with secondary return
sludge in an aerated, short-detention-time tank. This al-
lows flocculation and agglomeration of the trickling fil-
ter "fines," improving SS and associated BOD removal
in the final clarifier. In some cases, round, secondary
clarifiers equipped with a central flocculation well are
used to improve flocculation and settleability of the bio-
logical solids. The process is capable of consistently
meeting secondary and some advanced secondary
treatment standards.
Another modification is the coupled trickling filter-activated-
sludge approach, which employs both attached-growth
and suspended-growth biological processes. In this vari-
ation, discharge from the trickling filter enters an acti-
vated-sludge aeration basin for additional carbonaceous
removal and nitrification before final clarification. This
modification may be used where ammonia removal (nitri-
fication) is required. The process is particularly stable
and capable of consistently achieving a high-quality efflu-
ent having BOD and SS levels less than 20 mg/L.
Flow diagrams of the various TFSC processes are
shown in Figure 5-9.
5.13.2 Applicability and Status
Because of the inability to meet secondary treatment
standards consistently, conventional trickling filter facili-
ties are no longer being built. However, many such
plants are still in operation, having been constructed to
treat wastewater from both small and large communi-
ties. Some of these have been abandoned, while oth-
ers have been upgraded to the TFSC process to
improve performance.
91
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Trickling
Filter
Primary
Effluent
Flocculator
Center Well
Aerated Solids
Contact Tank
Treated
Effluent
Waste Sludge
Model
Return Sludge
Trickling
Filter
Primary
Effluent
Waste Sludge •<-
Mode 2
Return Sludge
Aeration Tank
Treated
Effluent
Return Sludge
Trickling
Filter
Primary
Effluent
Aerated Solids
Contact Tank
Liquor
Flocculator
Center Well
Waste Sludge
ModeS
>• Treated
Effluent
Return Sludge
Aeration Tank
Return Sludge
Figure 5-9. Schematics of Trickling Filter-Solids Contact Processes.
High-rate trickling filters are also rarely used today for
secondary treatment because of these same perform-
ance limitations. However they are applicable for use as
"roughing" filters ahead of activated-sludge processes to
reduce the organic loading to the aeration basin. Such
roughing filters are particularly applicable for reduction of
high organic loadings resulting from industrial dis-
charges.
In 1991, it was estimated that over 50 TFSC plants were
in design construction or operation. Eleven plants were
known to be in operation. Although the process was only
recently developed (late 1970s), its relatively widespread
application allows consideration as a fully developed
technology. The TFSC process is clearly appropriate as
a mechanical wastewater system for small, sewered
communities, both for new systems and as a retrofit to
existing trickling filter plants. Features that make TFSC
well suited for such applications include its relative sim-
plicity, consistently favorable performance, and low O&M
requirements compared to other mechanical treatment
alternatives.
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5.13.3 Advantages/Disadvantages
Advantages of the TFSC process include:
• Applicable for new facilities or upgrading existing trick-
ling filter plants
• Capable of consistently achieving very high-quality ef-
fluent (<20 mg/L BOD & SS)
• Relatively simple process
• Low cost and reliable upgrading technique for trickling
filters
• Can be designed to provide nitrification
Key disadvantages of this process include:
• Primary clarification required
• Pumping required to douse trickling filter
• Potential for nuisance odors from primary clarifiers,
trickling filter, sludge handling
• Moderate O&M requirements; skilled operator neces-
sary.
5.13.4 Design Criteria
Available design criteria for the TFSC process are sum-
marized in Table 5-21. Definitive design criteria have not
been established, and as a result, a wide variation exists
in some design parameters. Table 5-22 provides typical
sizing criteria for components of the TFSC process.
There are several design considerations that must be ad-
dressed for a successful operation. It is essential to es-
tablish conditions in the aerated solids' contact tank and
final clarifier that promote favorable flocculation. For this
Table 5-21. Summary of Available Design Criteria for the
TFSC Process
Parameter
Hydraulic loading (mgd/ac)
BOD loading (lb/1 ,000 cu ft/d)
Recirculation ratio
Media depth (ft)
Detention time (hr)
Solids retention time (days)
MLSS (mg/L)
DO (mg/L)
Overflow rate based on total area
(gpd/sqft)
Sidewater depth (ft)
Flocculatorcenterwell (% of total
area)
Trickling Filters
Low rate High rate
1-4 10-40
5-25 30-60
0 0.5-3
6-8 4-6
Solids Contact/
Aeration Tank
Low rate High rate
0.3-1.5 6-12
0.5-2.0 >6
700-3,00 1,500-3,500
1 .5-3.5 2.0-4.0
Secondary Clarifier
300-500
15-18
5-16
0.1
170
0.4
7,150
0.3
0.5
850
2.0
35,750
1.5
1.0
1,700
4.0
71 ,500
3.0
370 1,850 3,700
330 1,750 3,500
50 250 500
1,200 5,600 11,100
1.5 3 5
Table 5-22. Typical Component Sizing for TFSC Plants
At Average Design
Flow (mgd)
Unit Process
Primary clarifier area (sq ft)
Primary effluent pumping (mgd)
Trickling filter volume (cu ft)
Recirculation pumping (mgd)
(for high-rate systems)
Solids contact basin volume (cu ft)
(30-min contact time w/33%
return rate)
Secondary clarifier area (sq ft)
Chlorinator capacity (Ib/d)
Chlorine contact chamber volume
(cuft)
Site area (ac)
reason, fine bubble diffusers are typically used in the sol-
ids contact tank to provide sufficient dissolved oxygen and
gentle mixing without excessive turbulence that might shear
the floe. Channels or pipes leading to the final clarifier
should not impart turbulence to the liquid. Some of the
early designs for the process employed flocculating cen-
ter wells that could be gently mixed using mechanical
paddles or air spargers. However, it was found that hy-
draulic conditions in the center well were such that addi-
tion of mixing devices was not necessary.
Design life for a trickling filter plant is 20 years or more.
However, mechanical equipment used in the plant is
likely to have a service life of 5 to 15 years depending on
the type and quality of the equipment.
Trickling filter plants, because of the presence of a pri-
mary clarifier, accumulation of biomass on the trickling
filter, and the handling of primary sludge, have greater
potential for release of objectionable odor than extended-
aeration facilities. "Filter flies" may also be a localized
nuisance. Buffer distance around the facility should be at
least 60 m (200 ft). Fencing must be provided to restrict
access.
Mechanical plants serving small communities are typi-
cally unattended at night. Plants should be equipped with
an alarm system to alert emergency dispatch personnel.
5.13.5 Capital Cost Sensitivity
In general, new TFSC processes are more costly than
either custom-designed extended-aeration plants or
package plants. This is attributable mainly to the addi-
tional costs associated with primary clarification and
sludge handling. However, use of the TFSC process is
probably the most economical means of upgrading an
existing trickling filter plant to meet secondary standards.
Costs of TFSC plants may be affected by site conditions
such as climate, geology, and surrounding aesthetic con-
93
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ditions. Relatively high costs are likely to be associated
with sludge dewatering and stabilization.
5.13.6 O&M Requirements
O&M requirements for a TFSC facility are relatively high
due to the need for skilled and regular operator supervi-
sion and the general mechanical complexity of the com-
ponents. However, performance is less dependent on
operator skill and supervision than the extended-aeration
process. O&M requirements are summarized in Table 5-
23. An inventory of spare parts must be maintained in
case of equipment failure, which eventually will occur. In-
ventories should include pump packaging and seals,
bearings, motors, diffusers, chlorinator components, and
flow metering equipment. In many cases, lack of spare
parts and the long lead times to procure parts and equip-
ment have resulted in long periods of poor performance.
Table 5-23. O&M Requirements of TFSC Facilities
Operations
• Effluent quality monitoring required by NPDES permit
• Operation assessment analyses (e.g., MLSS, DO, sludge
blanket, settleability)
• Regular cleaning of screens, weirs, filter media, skimmer
mechanisms, diffusers and other components
• Regular adjustment of trickling filter recirculation rate,
sludge return rate and air injection rate
• Regular wasting of solids
• Sludge dewatering, stabilization, and disposal
• Control of disinfectant dosage
• Administration and recordkeeping
Maintenance
• Blowers
• Primary effluent and return sludge pumps
• Trickling filter distribution system
• Electrical equipment and instrumentation
* Mechanical dewatering equipment
• Laboratory equipment such as pH, DO meters, chemicals
• Disinfection system
Many small communities lack the skilled personnel nec-
essary to operate properly and maintain a mechanical fa-
cility of this type. Thus the TFSC process is unlikely to be
the system of choice for flows less than 2 L/s (50,000
gpd). If such a system already exists, the municipality
may wish to procure, under contract, the services of an
individual or firm qualified to operate and maintain the fa-
cility, relieving the municipality of this burden.
Energy consumption for a TFSC facility is higher than a
trickling filter plant, but less than an activated sludge fa-
cility due to the low power requirement to supply oxygen
to the contact tank. This may increase substantially if ni-
trification is required. Energy is consumed by trickling fil-
ter influent and recirculation pumping, aeration of the
solids contact or activated-sludge basin, return sludge
pumping, mechanical dewatering (if applicable), sludge
stabilization, the heating, cooling, and lighting of the
building housing the system, and other miscellaneous
processes such as clarification and disinfection. Pumping
energy may be estimated using the equation:
kWh/yr = 1900 x (flow, mgd) x (discharge head, ft)
5.13.7 Monitoring
Monitoring consists of sampling and conducting analyses
as required by NPDES or other permits, and as is neces-
sary to ensure optimum plant performance. NPDES re-
quirements will vary substantially depending on the size
of the facility and the characteristics of the receiving wa-
ters. NPDES monitoring requirements may include influ-
ent flow, influent BOD and SS, and effluent BOD, SS,
chlorine residual, and nutrients (i.e., ammonia, phospho-
rus). Sampling frequency may range from daily to bi-
monthly. Operational monitoring generally includes
MLSS, aeration basin DO, pH, settleability, alkalinity, ef-
fluent chlorine residual, influent flow, trickling filter recir-
culation flow, return sludge flow, and sludge waste rate.
5.13.8 Residuals
Residuals generated from a typical TFSC system include
grit and both primary and secondary sludge. Grit and
screenings must be removed and disposed of promptly
because of their putrescible nature.
Secondary sludge may be blended with primary sludge in
the primary tank or pumped to a stabilization process
such as an aerobic or anaerobic digester. Sludge may be
removed from the plant as a liquid or may be dewatered
by sand bed or mechanical devices and hauled away as
a solid. Options for the handling of waste sludge from
mechanical wastewater treatment systems are discussed
in greater detail later in Section 5.16.
5.14 Oxidation Ditch
5.14.1 Technology Description
The oxidation ditch process is a closed-loop variation of
the extended-aeration activated-sludge process. As with
extended aeration, the process is characterized by hy-
draulic retention times of 18 to 30 hours, and solids re-
tention times of 10 to 33 days. The process is highly
stable and reliable, and is suitable for relatively small
wastewater flows associated with small communi-
ties or subdivisions.
An oxidation ditch process typically consists of coarse
screening, grit removal, one or more closed-loop aerated
channels, secondary clarification, and disinfection. Pri-
mary clarification is rarely used. A typical means of aera-
tion is the use of horizontally mounted rotating brush,
cage, or disc aerators, which operate at 60 to 110 rpm
and provide transfer of oxygen as well as impart velocity
to the wastewater. Another common aeration system as-
sociated with a proprietary oxidation ditch process is a
vertical turbine, nonsparged aerator mounted at 180 de-
94
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gree bends in the channel near the dividing wall. Several
novel aeration and mixing devices have also been used
with varying degrees of success in regard to oxygen trans-
fer and maintaining solids in suspension. A schematic of
the oxidation ditch process is provided in Figure 5-10.
Screened and
Degritted Raw
Wastewater
r
f Dividing Strip
ill
Aeration.
Rotor
Return Sludge
Excess Sludge
Figure 5-10. Schematic of an Oxidation Ditch Process.
Oxidation ditch loops are generally oval in shape. Some
larger plants have employed "folded loop" or horseshoe
configurations to make efficient use of space. For small
facilities, a single channel, 1.2 to 1.8 m (4 to 6 ft) deep
with 45 degree sloping sidewalls, is typically used. This
configuration allows several options for construction.
Often, cast-in-place reinforced concrete is used to form
the ditch. However, gunite and asphalt lined ditches have
been successfully constructed. Lining is necessary to
prevent erosion from the moving wastewater. Experience
has shown the cost of gunite lining to be higher than rein-
forced concrete for this application. Deeper ditches
(>3m) (but smaller in area) with common vertical, con-
crete walls are also used for larger facilities.
In the 1980s, a number of "in channel" clarifier designs
were marketed for oxidation ditch plants. Here a clarifica-
tion device is placed within the channel to provide a qui-
escent settling zone, with solids returned back to the
ditch through ports or slots at the bottom of the clarifier.
These systems eliminate the need for separate final clari-
fiers and return sludge pumping; however, there are
trade-offs: lack of the ability to waste a thickened sludge
from a separate clarifier (requiring wasting of a relatively
dilute mixed liquor); lack of operational flexibility in
adjusting sludge return rates; and the need to increase
mixing to overcome additional headloss through the ditch
resulting from the restriction of flow around the in-
channel clarifier.
The oxidation ditch is capable of reliably producing a
very high-quality effluent. A1978 study by EPA that com-
pared the performance of 29 oxidation ditch plants with
competing biological processes demonstrated that the
oxidation ditch systems outperformed conventional acti-
vated sludge, package plants, trickling filters, and rotat-
ing biological contactors. Average annual effluent quality
was less than 15 mg/L BOD and SS. A 20-20 effluent
was achieved 85 to 90 percent of the time and a 30-30
effluent was achieved 95 percent of the time.
Oxidation ditch plants are capable of substantial nitrogen
removal, with 40 to 80 percent nitrogen removal achiev-
able. This favorable performance results from the devel-
opment of zones of low dissolved oxygen or anoxic
conditions between aerators where denitrification can
take place. Achieving high nitrogen removal efficiency in-
volves careful placement of aerators and adjustment of
variables such as aerator speed, immersion depth, and
on/off cycling frequency.
5.14.2 Applicability and Status
The oxidation ditch was developed in the Netherlands,
with the first plant constructed in 1953 for a community of
400 people. Because of its high performance and reliabil-
ity it now is used widely throughout the United States. It
is well suited for wastewater flows in excess of 0.4 L/s
(10,000 gpd). In 1980, over 650 oxidation ditch plants
were in use in the United States and Canada. Where a
mechanical system is desired that will provide excellent
performance, high reliability, and relatively minor opera-
tor attention, the oxidation ditch should be considered.
5.14.3 Advantages/Disadvantages
Key advantages of the oxidation ditch process include:
• Low sludge production
• Excellent performance
• High reliability
• Nitrogen removal likely
• Relatively low initial cost
• Can be designed for biological phosphorus and nitro-
gen removal
Key disadvantages of this process include:
• Protection from aerator freezing problems necessary
in cold climates
• Relatively high maintenance requirements for aerators
• Potential for rising sludge due to denitrification in final
clarifier
• Requires good operator skills and routine monitoring
5.14.4 Design Criteria
Table 5-24 summarizes the design criteria for the oxida-
tion ditch process. These criteria would apply to a con-
ventional ditch (extended aeration) designed for
carbonaceous BOD removal, without specific considera-
tions for nutrient (i.e., nitrogen, phosphorus) removal.
Typical sizing of components of an oxidation ditch plant
is summarized in Table 5-25.
Manufacturers of aeration equipment for oxidation
ditches have developed design criteria and suggested
plant layouts to meet a wide range of performance and
site requirements. Some ditches have been designed as
a single reactor that operates as both aeration basin and
clarifier. The most common oxidation ditch configuration
95
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Table 5-24. Summary of Design Criteria for the Oxidation
Ditch Process
Parameter
Volumetric loading (Ib BODs/lb/1,000 cu ft)
MLSS (mg/L)
F/M (Ib BODs/d/lb MLVSS)
Aeration detention time (hr)
(based on average daily flow)
Solids retention time (days)
Recycle ratio (R)
Volatile fraction of MLSS
Channel velocity (fps)
Oxygen transfer efficiency for horizontal
aerators (Ib Oa/hp-hr/LF)
Range
10-15
2,100-6,300
0.034-0.10
18-30
10-33
0.75-1.5
0.6-0.7
>1.00
3-5
Table 5-25. Typical Component Sizing for Oxidation Ditch
Plants
Unit Process
Aeration basin volume (cu ft)
Aerator, installed (hp)
Secondary clarifier area (sq ft)
Return sludge pumping (mgd)
Chtorinator capacity (Ib/d)
Chlorine contact chamber volume
(cuft)
Site area (ac)
At Average Design
Flows (mqd)
0.1 0.5 1.0
13,400 66,400 134,000
7.5 40 60
400 2,000 4,000
0.2 1.0 1.5
50 250 500
1,200 6,000 11,000
1.0
2.5
in the United States is a continuous-flow, single-channel
ditch followed by a final clarifier. Another arrangement
comprises multiple concentric channels connected by
submerged ports. Such plants allow flexibility in operating
mode. For example, some four-channel plants are de-
signed to allow operation in the extended-aeration mode,
conventional activated sludge-aerobic digester mode, or
contact stabilization mode simply by adjustment of gates
and valves.
The procedure for design of an oxidation ditch plant is
approximately the same as for an extended-aeration
activated-sludge plant. Some low-alkalinity waste-
waters may require pH adjustment when subjected to
long detention times in the aeration basin. Calculation of
oxygen requirements should assume that nitrification will
occur. Design life of an oxidation ditch plant is 20 years
or more, however, mechanical equipment is likely to
have a service life of 5 to 15 years depending on the type
and quality of the equipment.
5.14.5 Capital Cost Sensitivity
Oxidation ditch plants are competitive with other acti-
vated sludge processes in the range of 4 to 440 L/s (0.1
to 10.0 mgd) and appear to be particularly cost-effective
in the larger size ranges. However, local factors and spe-
cific process design can have a major impact on relative
construction costs.
Ditch configuration can affect construction costs. For
small plants, the shallow ditch with 45 degree sloped
sidewalls appears most cost-effective. For plants over 44
L/s (1 mgd), the deep channel with straight sidewalls is
cost effective. Ditch construction techniques can also af-
fect capital costs, as discussed elsewhere. As with other
mechanical plants, capital costs are influenced by site,
geological, and local aesthetic conditions.
5.14.6 O&M Requirements
O&M requirements for the oxidation ditch process are
summarized in Table 5-26. O&M costs are high relative
to natural systems for wastewater treatment. Successful
performance requires skilled operators and regular in-
spection, monitoring, and maintenance.
Table 5-26. O&M Requirements of Oxidation Ditch Facilities
Operations
• Effluent quality monitoring required by NPDES permit
• Operation assessment analyses (e.g., MLSS, DO, sludge
blanket, settleability)
• Regular cleaning of screens, weirs, skimmer
mechanisms, tank walls, and other components
• Regular adjustment of sludge return rate and oxygenation
rate
• Regular wasting of solids
• Sludge dewatering and disposal
• Control of disinfectant dosage
• Administration and recordkeeping
Maintenance
• Mechanical aerators
• Influent and return sludge pumps
• Electrical equipment and instrumentation
• Mechanical dewatering equipment
• Laboratory equipment such as pH, DO meters, chemicals
• Disinfection system
In oxidation ditch plants, aerators and aerator drives ac-
count for a major portion of mechanical problems. In an
EPA study of oxidation ditch plants, many plants experi-
enced the following aerator-related problems every 2 to 5
years/unit:
• Loss of "teeth" from brush-type aerators
• Bearing problems in gear drives, line shafts, and aera-
tor shafts
• Failure of flexible couplings between line shafts
• Failure of gear reducer and gear reducer shaft seals
• Corrosion of bearings, couplings, and drive units due
to continuous wetting from aerator spray
In cold climates, spray from aerators will freeze on equip-
ment, structures, and walkways. Aerators should be cov-
96
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ered in moderately cold areas, and also heated in very
cold areas.
Many small communities lack the skilled personnel nec-
essary to operate properly and maintain a mechanical fa-
cility of this type. Thus the oxidation ditch process is
unlikely to be the system of choice for flows less than 2
Us (50,000 gpd). If such a system already exists, the mu-
nicipality may wish to procure, under contract, the services
of an individual or firm qualified to operate and maintain the
facility, relieving the municipality of this burden.
Energy requirements for an oxidation ditch facility include
drives for aerators and pumps, sludge dewatering (if ap-
plicable), heating, lighting, and miscellaneous uses. An-
nual electrical energy requirements may be expected to
be approximately 72,000 kWh/yr for a 4 Us (0.1 mgd) fa-
cility, 280,000 kWh/yr for a 22 Us (0.5 mgd) plant, and
500,000 kWh/yr for a 44 Us (1 mgd) facility.
5.14.7 Monitoring
Monitoring consists of sampling and conducting analyses
as required by NPDES or other permits, and as is neces-
sary to ensure optimum plant performance. NPDES re-
quirements will vary substantially depending on the size
of the facility and the characteristics of the receiving wa-
ters. NPDES monitoring requirements may include influ-
ent flow, influent BOD and SS, and effluent BOD, SS,
chlorine residual, and nutrients (i.e., ammonia, phospho-
rus). Sampling frequency may range from daily to bi-
monthly. Operational monitoring generally includes
MLSS, aeration basin DO, pH, settleability, alkalinity, ef-
fluent chlorine residual, influent flow, return sludge flow,
sludge waste rate, and dewatered sludge solids content.
5.14.8 Residuals
Residuals generated from an oxidation ditch facility
include screenings, grit, and waste sludge. Grit and
screenings must be removed and disposed of promptly
because of their putrescible nature. If no separate grit
chamber is provided, grit accumulates in the oxidation
ditch, eventually requiring draining and cleaning.
Due to the long solids retention time, waste sludge pro-
duction is the same as for an extended-aeration system
and somewhat less than for conventional activated
sludge. Sludge may be wasted to a sludge holding tank
or stabilization process such as an aerobic digester and
removed from the plant as a liquid, or may be dewatered
by sand bed or other devices and hauled away as a
solid. Options for the handling of waste sludge from me-
chanical wastewater treatment systems are discussed in
greater detail later in this chapter.
5.15 Sequencing Batch Reactors
5.15.1 Technology Description
The sequencing batch reactor (SBR) process is a form of
the activated-sludge process in which aeration, sedimen-
tation, and decant functions are combined in a single re-
actor. Most municipal SBRs consist of two or more paral-
lel tanks. The process employs a five-stage cycle: fill, re-
act, settle, draw, and idle. During the fill stage,
wastewater enters the tank and mixes with the settled
biological solids remaining from the previous cycle. The
tank is mixed during the fill stage and may be aerated.
During the react stage, the wastewater and mixed liquor
are subjected to aeration, causing oxidation of organic
matter. Aeration and mixing are stopped during the settle
stage, allowing solids to settle. Clarified supernatant is
decanted during the draw stage. After decanting, solids
are removed from the bottom of the tank during the idle
stage. By this stage the first reactor will have finished its
cycle and the other reactor(s) will continue filling. When
the second reactor completes its fill stage, it begins its
react cycle and influent wastewater is directed to the first
reactor.
Modifications to the process include systems that con-
tinually accept influent flow, and oxidation ditch systems
that function as SBRs without the use of external clarifiers.
A typical SBR process consists of screening, grit re-
moval, SBR cycling, and disinfection. The disinfection
system must be designed so that it is either large enough
to accommodate the high periodic flow during decanting
or preceded by flow equalization. Primary clarification is
not commonly practiced. A schematic diagram that in-
cludes the five stages of an SBR cycle is provided in Fig-
ure 5-11.
Critical components of an SBR system include the aera-
tion/mixing system, the decant system, and the control
system. Aeration/mixing systems include jet aeration,
fine and coarse bubble diffusers, and mechanical turbine
aerators. Of these, the jet aeration system appears to be
most common. It has the advantage of being able to mix
independently of aeration. This may be important if bio-
logical nutrient removal is desired.
A well-designed and reliable decant system is critical to
ensuring a high-quality effluent by avoiding discharge of
floating solids or MLSS. At least five different proprietary
designs are available through vendors of SBR systems.
Development of SBR technology in the United States
has progressed rapidly since 1980, resulting in many im-
provements to decanter designs. Decanters include me-
chanically activated surface skimmers, floating decanters
with hydraulically activated valves, floating pipes and ori-
fices equipped with flapper valves operated by air pres-
sure, and fixed-position siphons.
Process control is provided by a programmable logic
controller (PLC), which along with associated software is
provided by the manufacturer. Programs are typically de-
veloped and modified by the vendor to suit the intended
application. PLC hardware is generally reliable. Con-
struction is modular, and troubleshooting and module re-
placement are straightforward.
97
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Bar Screen
Raw Wastewate
^
Chlorinatlon
SBR Tank No. 1
SBR Tank No. 2
Stream Discharge
Waste Sludge to
Sludge Handling System
Influent
r^
1
r-
1
r-
_
L_
r
ji
] Fill
JEj
_J Aeration
.jq
IJ
1 Settle
7]
[ Effluent 4— L
1
r
i
Discharge
- t]
1 ^ Waste
Figure 5-11. Schematic and Stages for a Sequencing Batch Reactor Process.
Sludge wasting is accomplished by wasting either aer-
ated MLSS or settled solids. Often, submersible pumps
are used to waste solids.
SBRs are capable of producing a high-quality effluent.
Data collected by EPA from 13 secondary plants using
SBR technology showed effluent average BOD levels of
3.3 to 21.0 mg/L, average SS levels of 3 to 25 mg/L, and
average NHs-N levels of 0.3 to 12.0 mg/L. These data
are average values representing 7 to 12 months of oper-
ating data.
SBRs are also capable of both biological nitrogen and
phosphorus removal, which is accomplished by proper
reactor sizing and selection of stage lengths and aeration
times to achieve the desired distribution of aerobic, an-
oxic, and anaerobic conditions. Scant data on full-scale
operations are available to document the effectiveness
for nitrogen and phosphorus removal, yet phosphorus re-
moval appears quite easily achievable.
5.15.2 Applicability and Status
The SBR process has widespread application where me-
chanical treatment of small wastewater flows is desired.
Because it provides batch treatment of wastewater, it is
ideally suited for the wide variations in flow rates that are
typically associated with small communities. Operation in
the "fill and draw" mode prevents the "washout" of bio-
logical solids that often occurs with extended aeration
systems subjected to wide flow variations.
Another advantage of SBR systems for small communi-
ties is that they require less operator attention than most
mechanical treatment alternatives, yet in most cases
they provide a very high-quality effluent. With rare excep-
tion, operators of SBR systems are enthusiastic about
the technology, preferring it to conventional activated-
sludge systems.
Although most municipal applications of SBR technology
have been online only since 1980, the concept is well es-
tablished. Many full-scale fill-and-draw systems were op-
erated between 1914 and 1920, but most were later
converted to the continuous-flow mode. The Pasveer
ditch was a variation implemented during the early 1960s
that employed continuous feeding of wastewater, with in-
termittent settle and discharge. The system was intro-
duced in Denmark, and 250 plants were in operation by
1980. Similar systems were constructed in Australia be-
ginning in 1965, all employing continuous-feed and inter-
mittent discharge processes.
The first application of SBR technology in the United
States was in 1980. Since that time, SBR systems have
become more widespread. In 1989, approximately 150
SBR plants were in design or operation; at least 90 facili-
ties were known to be operating. Nearly half of these had
design flows less than 11 Us (0.25 mgd), and a third had
flows less than 4 Us (0.1 mgd).
5.15.3 Advantages/Disadvantages
Key advantages of the SBR process include:
• Simple, reliable
• Ideally suited for wide flow variations
• Capable of very high and consistent effluent quality
due to quiescent batch settling
• Requires less operator attention than most other me-
chanical systems
• High operational flexibility allowing capability for nutri-
ent removal, filamentous growth control
98
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The disadvantages of this process include:
• Some problems reported with decant systems
• Improvements to hardware continue to be made as
the technology develops
• Requires skilled operator and regulator inspection and
maintenance
5.15.4 Design Criteria
Available design criteria for the SBR process are sum-
marized in Table 5-27. Vendors of SBR equipment typi-
cally provide substantial technical assistance in the
design of the particular SBR system. Some vendors offer
preengineered SBR package plants. No standard proce-
dures have been developed for sizing an SBR system.
Table 5-27. Summary of Available Design Criteria for SBR
Process
Parameter
Total tank volume
Number of tanks
Tank depth (ft)
F/M
SRT (days)
Aeration system
Cycle times (hr)
Range
0.5-2.0 times average daily flow
Typically 2 or more
10-20
0.04-0.2
20-40
Sized to deliver sufficient oxygen during
aerated fill and react stage; 02
requirements similar to conventional
activated sludge (with nitrification as
required)
4-12 (typical)
5.15.5 Capital Cost Sensitivity
SBR systems are likely to be extremely cost-competitive
with other mechanical wastewater treatment systems
over a wide range of flows. Unfortunately, scant historical
data have been compiled comparing the capital cost of
SBR systems with other competing mechanical systems.
Clearly, the lack of need for an external secondary clari-
fier and return sludge pumping system offers potential
savings in construction costs. In addition, primary clarifi-
cation is normally not employed. Design considerations
regarding the disinfection system (see Section 5.15.1),
however, have the potential to offset some of the inher-
ent advantages of SBR systems.
As with other mechanical systems, capital costs may be
affected by site conditions such as climate, geology, and
surrounding aesthetics. Costs for sludge stabilization and
dewatering may account for a significant percentage of
construction costs.
5.15.6 O&M Requirements
O&M requirements for an SBR system are expected to
be among the lowest of any mechanical wastewater
treatment plant. Experience with operating SBR proc-
esses has shown excellent reliability and relatively low
maintenance. As with other mechanical systems, how-
ever, a more skilled operator is necessary. Sludge stabili-
zation and dewatering are likely to constitute a substan-
tial portion of the operating and maintenance costs. O&M
requirements of an SBR plant are summarized in Table 5-28.
Table 5-28. O&M Requirements of SBR Facilities
Operations
• Effluent quality monitoring required by NPDES permit
• Operation analyses (e.g., MLSS, DO, settleability)
• Cleaning of screens, weirs, decant mechanisms,
diff users, and other components
• Adjustment of cycle times as required to optimize
performance
• Regular wasting of solids
• Sludge dewatering and disposal
• Control of disinfectant dosage
• Administration and recordkeeping
Maintenance
• Blowers, mechanical aerators, or mixing pumps
• Waste sludge pumps
• Electrical equipment, including programmable logic
controller
• Mechanical dewatering equipment
• Laboratory equipment such as pH, DO meters, chemicals
• Disinfection system
Energy consumption for an SBR facility is expected to be
similar to that for an extended-aeration or oxidation ditch
plant, but slightly greater than for a TFSC facility. This
variability in energy requirements for the various systems
is primarily due to the differences in energy required to
transfer oxygen into the wastewater. Energy is con-
sumed by aeration, mechanical sludge dewatering, pre-
liminary treatment, heating, cooling, and lighting of the
building housing the system, and other miscellaneous
processes.
5.15.7 Monitoring
Monitoring consists of sampling and conducting analyses
as required by NPDES or other permits, and as is neces-
sary to ensure optimum plant performance. NPDES re-
quirements will vary substantially depending on the size
of the facility and the characteristics of the receiving wa-
ters. NPDES monitoring requirements may include influ-
ent flow, influent BOD and SS, and effluent BOD, SS,
chlorine residual, and nutrients (i.e., ammonia, phospho-
rus). Sampling frequency may range from daily to bi-
monthly. Operational monitoring generally includes
MLSS, aeration basin DO during react stage, pH, settle-
ability, alkalinity, effluent chlorine residual, influent flow,
and sludge waste rate.
5.15.8 Residuals
Residuals generated from an SBR facility include screen-
ings and waste secondary sludge. Grit is also generated
if the plant is equipped with a separate grit chamber. Grit
99
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and screenings must be removed and disposed of
promptly because of their putrescible nature.
Due to the long solids retention time associated with
many SBR facilities and the lack of primary clarification,
waste sludge production is relatively low. Sludge may be
wasted to a sludge holding tank or stabilization process,
such as an aerobic digester, and removed from the plant
as a liquid, or may be dewatered by sand bed or other
devices and hauled away as a solid. Options for the han-
dling of waste sludge from mechanical wastewater treat-
ment systems are discussed in greater detail later in this
chapter.
5.16 Sludge Handling Alternatives
5.16.1 Introduction
Treatment and disposal of sludge generated from the
mechanical treatment of wastewater is a major problem
facing small communities. Sludge handling can account
for 50 percent of the cost of operating a wastewater
treatment plant. Many sanitary landfills refuse or are re-
luctant to accept sewage sludge, and suitable areas for
land spreading of stabilized sludge are becoming difficult
to find in some urbanized areas of the country.
Regulations promulgated by EPA are quite specific with
regard to the degree of stabilization required before
sludge can be applied to the land. At a minimum, agricul-
tural use of sludge requires the sludge to be pretreated
by aerobic digestion, air drying, anaerobic digestion, or
lime stabilization. Additional sludge stabilization can be
accomplished by utilizing composting, heat drying, heat
treatment, thermophilic aerobic digestion, dry lime stabili-
zation, and other processes described by the EPA.
Sludge handling approaches that have the widest appli-
cation in small communities are as follows:
Holding Tank mm
Rewatering
Aerobic or
Anaerobic
Digestion
Land
Application
Land
'Application
Hauling Larger POTW
Liming or digestion followed by land application is gener-
ally the most simple and economical means of handling
and disposing of sewage sludge. It does require, how-
ever, the availability of land for application within a rea-
sonable proximity of the sewage treatment plant. A
significant number of communities employ landfill dis-
posal instead. Virtually all landfills now require sludge to
be dewatered to a minimum solids content of 20 to 25
percent.
5.16.2 Sludge Holding Tank
The first component of many small community sludge
management schemes is a holding tank. The holding
tank should be conservatively sized to hold sludge during
periods of inclement weather, when dewatering capacity
is not available, or should any other problem arise; a
volumetric capacity sufficient to accommodate 30 days of
sludge wasting is recommended. Most sludge holding
tanks are aerated to prevent septic conditions and en-
train solids in suspension when required. Diffusers
should be of the coarse-bubble type with the capability to
be removed for cleaning. They should provide a
minimum of 0.4 L of air per m3 of tank volume(s) (25
cfm/1,000 cu ft). Depending on the application, holding
tanks should be designed to allow periodic sludge set-
tling and decant of supernatant, increasing solids content
and reducing the volume for dewatering and/or disposal.
Even if septic conditions are prevented with aeration,
odor emissions may be a problem. Simple offgas biofil-
ters composed of soil, compost, or a combination of such
materials can provide an economical means of control-
ling such emissions.
5.16.3 Dewatering Beds
Dewatering beds remove sludge moisture by drainage
and by evaporation. Underdrainage is collected and re-
turned to the plant headworks. Drying beds typically con-
sist of 15 to 25 cm (6 to 10 in.) of sand placed over 20 to
46 cm (8 to 18 in.) of gravel or stone. Effective size of the
sand is 0.3 to 1.2 mm, and uniformity coefficient is less
than 5. Gravel is generally graded from 1/8 to 1 in. Un-
derdrains are vitrified clay with open joints or perforated
PVC pipe with a minimum diameter of 10 cm (4 in.) and
minimum slope of 1 to 2 percent. A cross section of a
typical sand drying bed is shown in Figure 5-12.
Sludge is preferably applied to the beds in 20 to 25 cm (8
to 10 in.) lifts. Drying beds are partitioned into sections 3
to 6 m (10 to 20 ft.) wide and 6 to 30 m (20 to 100 ft.)
long. Partitions may be constructed using pressure-
treated wooden planks, earthen dikes, or concrete walls.
Concrete foundations are required around the drying
area if it is to be covered (in colder climates).
Feed piping may be ductile iron or PVC. It should be
sized to prevent clogging yet maintain adequate cleans-
ing velocities. Provision should be made for flushing and
draining the line. Diversion boxes or valves are used to
feed the desired bed. Splash plates are usually em-
ployed in order to minimize erosion of the sand and im-
prove sludge distribution.
After the sludge has dried to a minimum solids content of
20 to 25 percent, it is removed from the bed. For small op-
erations this is typically done manually, although small
front-end loaders are preferable. Sand that may be re-
moved in the process will eventually require replacement.
Modern beds are designed with longitudinal concrete strips
allowing access by a truck or front-end loader.
For cold or wet climates, glass enclosures can be con-
structed to improve drying during inclement weather.
Ventilation is necessary for moisture control.
100
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Sludge
Liner
Figure 5-12. Sand Drying Bed Details.
Modifications to traditional sand drying beds include use
of wedgewire or plastic block bottoms, paved drying
beds, vacuum-assisted drying beds, and special mixing
devices for areas of intense sunlight. All but the paved
bed require chemical (usually polymer) conditioning prior
to sludge application, thus limiting these particular modifi-
cations to systems operated by skilled O&M staffs. All of
the alternative systems can be cleaned by mechanical
means, substantially reducing labor costs associated
with dewatered sludge removal. Field experience indi-
cates that the use of totally and partially paved drying
beds result in shorter drying time than with conventional
drying beds. Experience with polymer treatment followed
by drying on plastic- or wedgewire-bottom beds has been
limited in the United States, but indications are promis-
ing. Final solids vary from about 12 percent with immedi-
ate removal, to higher percentages with continued
drying.
Vacuum-assisted sludge drying (VASD) beds are a rela-
tively recent innovation developed in the mid-1970s. This
technology consists of an array of specially made porous
blocks or plates with sealed joints. After the polymer-
treated sludge has drained supernate, a pump applies a
vacuum to the bottom, pulling additional moisture from
polymer-conditioned, gravity-drained sludge until it cracks.
The major advantage of these alternative systems is
shorter drying times. The normal VASD cycle time for di-
gested sludges is considerably less than 24 hours. Final
solids are generally 13 to 15 percent, unless dried further
before disposal.
Table 5-29 provides a summary of information on sludge
dewatering beds.
Table 5-29. Summary of Information on Sludge Dewatering
Beds
Applicability and
status
Advantages
Disadvantages
Design criteria
Capital cost
sensitivity
O&M
Monitoring
Special
considerations
Simple, widely-used, effective system
Minimal skill and operator attention
required; low construction costs;
effectiveness, variable and subject to site
conditions
Sludge removal is labor intensive;
covering may be required in cold or wet
climates; odor and vector potential; land
requirements are considerable
Open beds: 2.0 - 2.5 sq ft/capita
10-28lb/sqft/yr
Closed beds: 60 - 75 percent of open
bed area
12-40lb/sqft/yr
Capital cost may be substantially
increased if covering is required
Sludge removal is labor intensive, but
overall operating costs are low compared
to mechanical dewatering. Energy costs
are minimal; operation is simple
As required by disposal or end-use permit;
operational monitoring consists of cake
solids content measurement
Odors, flies may be a problem, particularly
if the sludge is not well stabilized
5.16.4 Sludge Conditioning
5.16.4.1 Aerobic Digestion
Aerobic digestion of sewage sludge is commonly used
by small communities as a means of stabilization prior to
land application. If application sites are in reasonably
close proximity to the source of sludge, direct application
of liquid sludge is clearly the most economical approach.
If not, dewatering via sand drying beds may be used to
reduce the volume of sludge to be hauled away.
Aerobic digestion as practiced by small communities is
typically performed in an open tank. Continuous introduc-
tion of air allows biological oxidation of organic matter
under aerobic conditions. Aerobic digestion results in a
reduction in biodegradable volatile solids, improved de-
waterability, and odor reduction. Oxygen may be sup-
plied by mechanical aerators or diffusers.
Solids retention times (SRTs) in aerobic digesters may
vary from 30 to 60 days depending on the ambient tem-
peratures of the site and the sludge. Minimum retention
times are likely to be dictated by state regulations that
101
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specify the degree of stabilization necessary prior to agri-
cultural utilization. EPA guidelines indicate 60 days at
158C (59°F) to 40 days at 20°C (68°F), with a volatile sol-
ids reduction of at least 38 percent. Actual operating ex-
perience would suggest that 40 percent VSS reduction
can be achieved at lower SRTs and that the minimum
SRT to achieve such reduction is dependent on the type
of sludge. For example, extended aeration and oxidation
ditch sludges typically contain no primary sludge and
have already undergone partial stabilization from endo-
genous respiration.
For small operations, single-tank digesters are com-
monly used. Periodically, aeration is stopped and solids
are allowed to settle for 6 to 12 hours. Clarified super-
natant is then decanted and returned to the plant. This
procedure maximizes the solids capacity of the digester,
and also allows a thickened sludge to be removed,
thereby reducing the volume to be hauled away. Gener-
ally, supernatant quality from an aerobic digester is fa-
vorable, and impacts on the plant from supernatant
decant are minimal.
A recent development from Germany is the autothermal
thermophilic aerobic digestion process (ATAD). It is just
now being introduced into North America. European
studies indicate a superior stabilization with greater cost-
effectiveness: greater destruction of organisms and re-
duction of VSS at a price competitive with conventional
aerobic digestion (EPA, 1990).
Table 5-30 provides a summary of information on the
aerobic digestion process.
5.16.4.2 Anaerobic Digestion
Anaerobic digestion is nearly as common among existing
small U.S. community systems as aerobic digestion. Like
the latter, subsequent processing may be a function of
proximity of the disposal site. When dewatering is called
for, sand beds are the most frequent choice. However,
the use of anaerobic digesters for new treatment facilities
is becoming far less popular now that designers are rec-
ognizing the value of operational simplicity in new facilities.
When used, modern anaerobic digesters are generally
designed as two-stage systems, to maximize the stabili-
zation process and gas production. The recovery and
use of this gas (primarily methane) is considered the
most attractive feature of anaerobic digesters, but the
complexity of the recovery and use process makes their
feasibility marginal for wastewater systems with capaci-
ties in the range of 100,000 to 1 million gpd and unlikely
in smaller systems, unless regional energy recovery fa-
cilities are in place. In these cases, the sludge from the
wastewater treatment plant becomes an additional energy-
producing raw material that must merely be transferred
from the treatment facility to the energy recovery facility
along with the responsibility for disposal.
Table 5-30. Summary of Information on Aerobic Digestion
of Sludge
Applicability and Simple, widely used process for sludge
status stabilization
Advantages Minimal operator attention; low odor
potential; simple process; resistant to
upset; no chemical required
Disadvantages Very high power consumption; aeration
system requires high maintenance; larger
volume tank required for cold climate
application; foaming potential
Design criteria SRT: 40 days at 20°C
60daysat15°C
VSS loading: 0.1-0.4 Ib/cuft/d
Diffused air req'd: 20-60 cfm/1,000 cu ft
Mech. aeration: 0.75-1.25 HP/1,000 cu ft
Minimum DO: 1-2 mg/L
Capital cost Detention times, digester volumes, and
sensitivity construction costs increase with
decreasing sludge temperatures
Operation and Relatively little operator attention for
maintenance process except for monitoring and
decant/sludge withdrawal operations; high
maintenance for mechanical or diffused
aeration system; high power requirements
for oxygen transfer
Monitoring As required by end-use permit;
operational monitoring consists of DO,
VSSin, VSSout, temperature
5.16.4.3 Lime Stabilization
The addition of lime to sledges in sufficient quantities to
raise the pH above 12 for 30 minutes has been employed
by small municipalities in Europe and in the United States
for many years. Lime treatment has several advantages
over biological conditioning including low capital cost,
ease of use, and improved pathogen reduction. It also
serves as an ideal temporary or interruptable supplemen-
tal process for periodically overloaded existing digester
systems. The chief disadvantages of lime stabilization
are the inability of the process to reduce volatile solids (or
sludge mass) and the large increase in additional inert
solids for dewatering and disposal. Also, subsequent loss
of the advantages of alkaline status upon storage must
be accounted for in the subsequent sludge processing
steps (e.g., odors, slower dewatering rates). Design crite-
ria and other information on lime stabilization in small com-
munity treatment plant sludges is provided in Table 5-31.
In recent years, much has been made of new commercial
processes for the alkaline treatment of waste sludge
"cake." Most of these processes have been applied to
larger municipal facilities (>5 mgd capacity). It is doubtful
that these systems will have a significant impact on small
facilities unless they contribute to a regional sludge man-
102
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Table 5-31. Summary of Information on Lime
Stabilization of Sludge
Applicability and
status
Advantages
Disadvantages
Design criteria
Capital cost
sensitivity
O&M
Monitoring
Simple process
Low capital cost; favorable pathogen
destruction; on/off capability; low
maintenance requirements.
Large increase in sludge solids; no
destruction of volatile SS; loss of benefits
after storage > 2 wk.
Dosage = attainment of pH >12 for 30
minutes after mixing; mixing by aeration
(preferred) or mechanical means;
high-calcium lime yields faster reaction
time (hydrated lime preferred)
Lime dosage to meet pH/time
requirements, storage time, and mixing
requirements directly affect operating
costs. Higher calcium content of lime
reduces cost. Capital costs of
< $150,000 can be anticipated for STPs
of mgd.
Lime costs of < than $50/day; total O&M
costs are primarily in range of $10 to
20/metric tone of solids, the great
majority being lime and labor;
maintenance of lime feeding and mixing
equipment is greatest demand
pH is the primary parameter for
monitoring
agement facility. In this case the responsibility for such
processing would be transferred to the regional facility.
5.16.5 Land Application of Sludge
Application of stabilized sludge to the land is commonly
practiced by small, rural communities. It is a simple proc-
ess and provides substantial benefit to agricultural and
marginal lands given the value of sewage sludge as a
soil conditioner and source of organic matter and nutrients.
Sludge can be applied as a liquid or as a dewatered
cake. Many small municipalities favor applying sludge as
a liquid since it eliminates the dewatering step and stabi-
lized sludge can be taken directly from'the plant to the
land. Liquid sludge can be applied by tank truck, subsur-
face injection, or spray irrigation. Dewatered sludge can
be applied by a manure spreader and incorporated into
the soil by disking or plowing.
Using the same tank truck both to haul and apply sludge
is a simple and economical approach, but it can cause
compaction and rutting of the soil. To overcome this
problem, spray irrigation is recommended. Equipment
used for spray irrigation of sludge is simple and reliable.
Another approach is to use specially designed applicator
trucks with flotation tires, but such equipment is costly for
small communities to purchase and maintain unless they
offset costs by receiving sludges from neighboring com-
munities.
One of the major drawbacks of land application is the
need to store .sludge when weather or soil conditions do
not permit application. Sludge is not applied when the
soil is wet or frozen. In certain areas of the country, this
restriction requires the capability to store sludge for long
periods of time.
The rate of sludge application to land may be governed
by nutrient loadings or loadings of trace elements such
as cadmium. In general, application rates depend on
sludge composition, soil/hydrogeological characteristics,
climate, vegetation, and cropping practices. Annual appli-
cation rates have varied from 1 to over 220 t/ha (0.5 to
100 tons/ac) per year. For most small municipalities
treating sewage of domestic origin, sludge metals levels
are generally low, and application rates are often based
on supporting the nitrogen needs of the crop. Typically
900 dry kg (1 ton) of sludge provides 27 kg (60 Ib) of ni-
trogen (50 percent of which is available), 18 kg (40 Ib) of
phosphate, and 2 kg (5 Ib) of potash.
Table 5-32 provides a summary of information on land
application of sludge.
Table 5-32. Summary of Information on Land Application
of Sludge
Applicability and
status
Advantages
Simple, widely used process of sludge
disposal for small communities
Simple; low cost; utilizes sludge as soil
conditioner, source of organic matter and
nutrients
Disadvantages Storage requirements may be
considerable to accommodate sludge
generation during wet, frozen soil
conditions; odor potential; local
opposition possible; close monitoring
required
Design criteria Dependent on sludge composition, soil
characteristics, vegetation, cropping
practices
Capital cost Costs may increase substantially if large
sensitivity storage volumes required; cost of
specialized application vehicles high,
service life is approximately 10 yr
O&M Costs sensitive to distances between
sludge source and application points;
maintenance of vehicles and application
equipment required; monitoring costs
may be high
Monitoring Monitoring requirements dictated by state
regulations; requirements increase with
size of operation; monitoring required for
sludge soil, ground water, crops
103
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5.17 Septage Handling Alternatives
5.17.1 Introduction
Providing adequate treatment and disposal alternatives
for septic tank pumpings (septage) is a major challenge
for small communities. Raw septage is a highly putre-
scible material that may contain high levels of grit,
grease, and debris. It is difficult to dewater, and the char-
acteristics are highly variable. In addition, daily septage
generation rates are unpredictable, with substantial sea-
sonal variations occurring due to changes in soil and
ground-water conditions. In the past, septage has been
commonly applied to the land in a raw state. However,
many states are now implementing more stringent regu-
lations requiring stabilization of septage prior to land ap-
plication in order to minimize the potential for disease
transmission, vector attraction, and odor emissions. In
some areas, high densities of septic systems, lack of
available land, and lack of available sewage treatment
plant capacity have justified the construction of high-cost,
independent facilities dedicated solely to the treatment of
septage.
5.17.2 Septage Characteristics
Septage characteristics are highly variable depending on
such factors as tank size and design, user habits, pump-
ing frequency, climate and seasonal weather conditions,
and presence of appliances such as garbage grinders,
washing machines, and water softeners. Table 5-33 pro-
vides recommended design values for various charac-
terization parameters.
Ratios of peak monthly to average monthly septage vol-
ume that can be expected at a treatment site range from
1.5 to 2.5. Ratio of peak daily to average daily volume
can be expected to range from 3.0 to 5.0. Annual sep-
tage volumes can be estimated from historical records, if
available; by assuming a generation rate of 230 to 380
L/cap/yr (60 to 100 gal/cap/yr); or by assuming a typical
tank volume of 3,800 L (1,000 gal) and a pumpout fre-
quency of every three to five years.
5.17.3 Overview of Septage Handling
Options
Rgure 5-13 provides an overview of septage handling
options available to a small community. The most com-
mon approach is land application; co-treatment at a sew-
age treatment plant is also widely employed. For small
communities, independent facilities are1 usually limited to
simple, low-cost approaches such as stabilization la-
goons or lime stabilization systems.
5.17.4 Land Application
Land application of septage employs the same tech-
niques used for land application of liquid sewage sludge
discussed earlier in this chapter. Where stabilization of
liquid septage is required prior to land application, lime
stabilization should be considered, since it is one of the
Table 5-33. Suggested
Characteristics81
Parameter
TS
TVS
TSS
VSS
COD
BODs
TKN
NH4-N
Total P
Alkalinity
Grease
PH
LAS
Heavy metals
Zinc
Copper
Lead
Cadmium
Mercury
Toxic organics
Methyl alcohol
Isopropyl alcohol
Acetone
Methyl ethyl ketone
Tuluene
Design Values for Septage
Value (mg/L)b
40,000
25,000
15,000
10,000
20,000
7,000
15,000
700
150
1,000
8,000
6.0
150
10
5
1
0.1
0.005
16
14
11
4
0.2
aEPA1984a;1991.
b Except for pH
TS = total solids
TVS = total volatile solids
TSS = total suspended solids
VSS = volatile suspended solids
COD=chemical oxygen demand
BOD = biochemical oxygen demand
TKN = total kjeldahl nitogen
NH4-N = ammonia nitogen
P = phophate
LAS = linear alkyl sulphonate
most simple and economical techniques to meet applica-
ble stabilization criteria. Lime stabilization involves addi-
tion and mixing of sufficient lime to achieve a pH of 12 for
at least 30 minutes. To achieve pH of 12, lime dosages
can be expected to be 1,500 to 3,500 mg/L as Ca(OH)2,
which is hydrated lime. (Lime stabilization is discussed in
more detail earlier in this chapter.)
5.17.5 Co-treatment at a Sewage Treatment
Plant
Many septage haulers dispose of septage at wastewater
treatment plants. This is an acceptable alternative if the
plant has sufficient capacity and is equipped to handle
the material.
The most common approach is to add the septage to the
headworks of the plant. In many plants, this allows the
septage to undergo screening and grit removal, which is
104
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Land Spreading
Trench/Landfill Burial
Subsurface Incotporalion
Addition to Liquid Stream
Addition to Sludge Stream
Addition to Both Streams
Stabilization Lagoon
Composting
Conventional Biological Treatment
Aerobic Digestion
Anaerobic Digestion
Lime Stabilization
Figure 5-13. Basic Septage Management Options.
important to remove debris and grit that can foul equip-
ment or cause excessive wear on pump components.
Larger plants (> 88 Us (2 mgd)) often allow haulers to
discharge directly to an influent channel or manhole im-
mediately upstream of the headworks. However, for
small plants, this can result in a substantial "shock load"
to the biological system, particularly if the plant is not
equipped with a primary clarifier. Primary clarifiers allow
removal of many of the organic septage solids, damping
the organic loading to the aeration basin. For small
plants lacking primary clarifiers or for larger facilities re-
ceiving high volumes of septage, a septage hold-
ing/equalization tank is strongly recommended. The tank
should be sized to handle twice the peak daily volume
and should be equipped with an air mixing system to
maintain solids in suspension. A solids handling pump is
then used to transfer septage to the headworks at a pre-
determined rate. A timer and level control system can be
used to establish these feed patterns. Because odor
emissions from a septage holding tank are a major con-
cern, provisions should be made for a biofilter or other
simple odor-control device.
Details of a septage receiving pad may be found else-
where (EPA, 1984; Rezek and Cooper, 1980). However,
provision should be made for a trash rack and washdown
facilities. It is important that a log be maintained of all
septage deliveries; at a minimum, entries should include
hauler's name, time of delivery, volume, and source of
septage. Also, a sample of each load received at the fa-
cility should be taken.
Septage may also be treated as a sludge, and handled
as part of the solids management scheme at the sewage
treatment plant. This can be accomplished effectively by
adding septage upstream of a primary clarifier; however,
for small extended-aeration plants or oxidation ditches
without primary clarification, handling septage in the sol-
ids handling train deserves consideration. Septage may
be added to the stabilization process, such as an aerobic
digester, or to the sludge holding tank prior to dewater-
ing. Separate screening and grit removal is recom-
mended for either approach, however. Septage is not
easily dewatered, and blending with treatment plant
sludge prior to dewatering may have major adverse
impacts on the dewatering unit's performance. For this
reason, addition of septage to the stabilization process
is the preferred method of handling this waste in the
solids train.
Table 5-34 provides a summary of information on sep-
tage handling at a wastewater treatment plant. Table 5-
35 lists specific factors to be evaluated for co-treatment
of septage and watewater.
5.17.6 Independent Septage Treatment
Facilities
Independent facilities for the treatment of septage may
be warranted if land is not available for land spreading,
or if adequate treatment plant capacity is not available
within a reasonable proximity (16 to 32 km (10 to 20 mi))
to the sources of septage. Independent facilities can vary
in scope from simple lime stabilization systems to com-
plex mechanical septage treatment plants comprising
multiple physical and biological processes.
Independent septage treatment facilities using technol-
ogy compatible with the capabilities of a small commu-
nity would most likely utilize aerobic digestion, lime
stabilization, or lagoons.
5.17.6.1 Aerobic Digestion
Aerobic digestion of septage is very similar to aerobic di-
gestion of sewage sludge (see discussion of the aerobic
digestion process, earlier in this chapter).
An independent septage treatment facility using the aero-
bic digestion process should employ screening and grit
removal as part of a preliminary treatment scheme. Re-
siduals from these processes require regular disposal.
Although the aerobic digestion process is a relatively
simple means of stabilization, there are several important
concerns related to its use at an independent septage
treatment facility. First, power costs are likely to be quite
high to accomplish the transfer of oxygen. Second, su-
pernatant decanted from the digester must be disposed
of properly. Supernatant may require additional treatment
prior to introduction to a subsurface disposal system as
well as storage prior to use in an irrigation system. Vari-
ous options for an independent aerobic digestion facility
are shown in Figure 5.14.
5.17.6.2 Lime Stabilization
Lime stabilization is likely to be among the most cost-
effective options for stabilization of septage to meet land
application criteria and/or conditioning of septage prior to
105
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Table 5-34. Summary of Information on Co-treatment of
Seplage at a Sewage Treatment Plant
Applicability and
status
Advantages
Disadvantages
Design criteria
Capital cost
sensitivity
O&M
Monitoring
Commonly used technique for septage
disposal
Sewage treatment plants generally
equipped to handle such waste;
compatible with sewage for biological
treatment; maintains waste treatment
operations at one location
Some plants may have insufficient
capacity to handle large volumes;
increases aeration, scum, and sludge
handling requirements; requires
adequate preliminary treatment systems;
may increase odor emissions
Screening and grit removal should be
provided; organic loading capacity of
plant must be evaluated; primary
clarification desirable to minimize impact
on biological system; separate receiving
station and equalization recommended
Additional capital cost may be only for
receiving station, equalization/holding
tank, odor control system
Additional O&M costs likely for increased
aeration, sludge and scum handling
requirements, grit and screenings
disposal, cleanup
Septage volumes, characteristics, normal
wastewater plant monitoring
dewatering. The process is simple and requires a mini-
mum of operator skill and attention.
The process involves addition of sufficient lime or other
alkaline material to raise the pH to 12 for a period of 30
minutes. This destroys pathogenic organisms, improves
dewaterability, and reduces objectionable odors. Lime
can be added in the form of quicklime, CaO, or Ca(OH)2.
Table 5-35. Factors To Be Considered in Evaluating Co-
treatment of Septage
Design Factors
• Types of unit processes at wastewater treatment plant
• Design capacity (hydraulic and organic) of the plant
• Proposed location of septage input
• Volume of septage (average, peak)
• Mode of addition (slug or continuous loading)
• Ratio of existing loadings to design loadings (hydraulic
and organic)
Potential Impacts
• Increased organic loading to biological process
• Increased solids loading to sludge handling processes
• Increased hydraulic loading
• Scum buildup on primary clarifier surfaces
• Foaming problems in aeration basins
• Increased odor emissions from headworks, primary
clarifiers
• Potential upsets due to toxic substances present in
nondomestic septage
• Potential to exceed effluent limitations for BOD, TSS,
nutrients
In addition, other alkaline materials such as lime kiln dust
or cement kiln dust can be used to elevate the pH.
Lime stabilization of septage is typically conducted on a
batch basis using a single- or two-tank system. For small
operations, lime is added manually to the tank containing
screened, degritted septage. The tank contents are then
mixed; air mixing appears to be the most effective means
of mixing lime with liquid septage. The pH of the liquid is
monitored several times during the lime addition process.
When a pH of 12 is reached, the contents are maintained
at that pH for a period of 30 minutes. At this pH, release
of ammonia (NHs) can be expected.
Septage
'-H ,
1
nings Grit
i
k
Filtrat
Aerobic
Digester
1
1
1
Supernatant ^
Treatment/
Storage
— ^ Surface Water
— ^- Irrigation
r
Dewatering
~l
Land Application
of Dewatered Sludge
Land Application
of Liquid Sludge
Figure 5-14. Alternatives for an Independent Aerobic Digestion Facility for Septage Treatment.
106
-------
The stabilized sludge may be applied to the land as a liq-
uid or dewatered first, using sand drying beds. If dewa-
tered, treatment and disposal of the filtrate must be
considered.
Information on lime stabilization of septage is summa-
rized in Table 5-36.
Table 5-36. Summary of Information on Lime Stabilization
of Septage
Applicability Simple, fully developed technology with
and status broad application for small communities
Advantages Simple batch processes; minimal operator
skills; economical; meets EPA pathogen-
reduction criteria; reduces objectionable
odors; improves dewatering
Disadvantages No organic destruction; increases mass of
solids to be handled; mechanical lime feed
systems, if used, require high degree of
operator attention; dusty operation;
potential ammonia releases
Design criteria Screening, grit removal recommended. Air
mix tank, batch process
pH = 12.0 for 30 minutes
Dosage: 1,500-3,500 mg/L as Ca(OH)2
O&M Generally low O&M for small batch
operations; larger, mechanized operations
may have high O&M requirements;
relatively low power consumption
Construction If dewatering is part of treatment scheme,
issues facilities must be constructed to
treat/dispose of filtrate
Monitoring As required by disposal permit; operational
monitoring consists of pH measurement;
solids content of cake, if applicable
5.17.6.3 Lagoons
Lagoons are commonly used for the treatment of sep-
tage, particularly in the northeastern United States. Prop-
erly designed and sited, lagoons provide consistent
performance and are easy to operate. The simplest sys-
tem consists of two earthen basins in a series. Raw sep-
tage is discharged into the primary lagoon. Partially
clarified effluent from the first cell is discharged into a
second lagoon that also acts as a percolation pond. Liq-
uid is disposed of through a combination of infiltration
and evaporation. Figure 5-15 shows various configura-
tions for septage lagoons, including those that discharge
to the ground water using percolation ponds or infiltration
basins and those that produce an effluent that is sub-
jected to further treatment and/or is applied to the land.
Depending upon State regulations and subsurface geo-
logical and ground-water conditions, lining of the lagoons
may be required, and the type that discharges to ground
water may be prohibited. Concrete, asphalt, or clay liners
are recommended over membrane liners due to the po-
tential for damage to membrane liners during removal of
accumulated solids. Lagoons are often constructed
above grade with earthen embankments to minimize ex-
cavation costs.
Although performance data are limited, existing data sug-
gest that septage be treated in two lagoons in a series
prior to treatment in infiltration beds and percolation
ponds, and before land application or further treatment
and surface discharge. One of the major operating con-
siderations with septage lagoons is accumulation of sol-
ids. Lagoons may require periodical dredging, and
percolation ponds may require draining and cleaning to
restore infiltrative capacity. For these reasons, use of two
parallel lagoon systems is recommended.
To minimize odor emissions during the discharge of sep-
tage from hauler trucks, the material should be dis-
One-Cell Lagoon with Percolation Pond (Controlled Discharge)
Septage
Two-Cell Lagoon with Percolation Pond (Controlled Discharge)
Septage
' Recircuiating
Sand Filter
Discharge
' Spray Irrigation
' Overland Flow
Two-Cell Lagoon with Surface Discharge or
Land Application (Controlled Discharge)
Two-Cell Lagoon with Percolation Pond (Continuous Discharge)
Septage
> Recircuiating
Sand Filter
Discharge
1 Spray Irrigation
1 Overland Flow
Two-Cell Lagoon with Surface Discharge or
Land Application (Continuous Discharge)
Figure 5-15. Septage Lagoon Variations.
107
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charged into a concrete chamber with a tight-fitting man-
hole or hatch. Septage then flows by gravity into the first
cell of the lagoon, below the water surface to avoid turbu-
lence. Because of the large exposed surface area, odor
emissions may be a problem. Large buffer zones (300 ft
(90 m)) are recommended. Addition of lime to maintain a
pH 7 may help reduce objectionable odors.
A summary of information on septage lagoons is pro-
vided in Table 5-37.
5.18 References
When an NTIS number is cited in a reference, that refer-
ence is available from:
National Technical Information Service
5285 Port Royal Road
Springfield,VA22161
(703) 487-4650
Argaman, Y. 1988. Continuously fed intermittently de-
canted activated sludge: A rational basis for design.
Water Res. 22:303.
Arora, M.L., E.F. Barth, and M.B. Umphres. 1985. Tech-
nology evaluation of sequencing batch reactors. WPCF
57:867.
ASAE. 1980. Design and operation of farm irrigation sys-
tems. M.E. Jensen ed. Monograph No. 3. St. Joseph, Ml.
American Society of Agricultural Engineers.
BMCI. 1989. AirVac vacuum sewage system design man-
ual. Rochester, IN.
Booher, L.J. 1974. Surface irrigation. FAO Agricultural
Development Paper No. 94. United Nations. Rome, Italy:
Food and Agricultural Organization.
Borrelli, J. 1984. Overland flow treatment of domestic
wastewater in northern climates. EPA/600/2-84-161. En-
vironmental Protection Agency. Ada, OK. R.S. Kerr Envi-
ronmental Research Laboratory.
Carlson, C.A., et al. 1974. Overland flow treatment of
wastewater. Misc Paper Y-74-3. U.S. Army Corps of En-
gineers. Vicksburg, MS. Waterways Experiment Station.
Condren, A.J. 1978. Pilot-scale evaluations of septage
treatment alternatives. EPA/600/2-78-164, NTIS No.
PB28-8415/AS.
Condren, A.J., A.T. Wallace, I.A. Cooper, and J. F.
Kreissl. 1987. Design, operational and cost considera-
tions for vacuum-assisted sludge dewatering bed sys-
tems. J. WPCF, 59(4):228-234.
Converse, J.C., and E.J. Tyler. 1990. Wisconsin mound
soil absorption system siting, design and construction
manual. Madison, Wl: University of Wisconsin Small
Scale Waste Management Project.
Converse, J.C., E.J. Tyler, and J.O. Peterson. 1989. Wis-
consin at-grade soil absorption system manual: Siting-
Table 5-37. Summary of Information on Septage Lagoons
Applicability Suitable for septage disposal where
geological conditions, buffer areas are
adequate
Advantages Relatively low construction cost; consistent
performance; simple operation
Disadvantages Potential for ground water contamination
and odor problems; large land area
required; ground-water monitoring
essential; eventually, dredging required to
remove accumulated solids
Design criteria Minimum 2 lagoons in series followed by
percolation beds, land application,
treatment/discharge.
Two parallel systems recommended
Percolation bed loading: 1 gpd/sqft
Minimum ground-water separation
distance: 4 ft (to top of ground-water mound)
Detention time (min.): 20 days in lagoons
Lagoon depth (min.): 3 ft
Lining: as required by state: concrete,
asphalt, clay
pH control: 6.8 - 7.2 using lime
Capital cost Cost increase with lagoon lining
sensitivity requirements and need for further
treatment prior to surface discharge
O&M Does not require skilled operator;
dredging/cleaning operations are labor
intensive; ground-water monitoring
necessary; lime addition may reduce odor
emmisions
Monitoring Ground water monitoring required; effluent
monitoring may be necessary depending
on final disposal; minimal operational
monitoring
Residuals Lagoon dredgings; scrapings from
infiltration basins
design-construction. Madison, Wl: University of Wiscon-
sin Small Scale Waste Management Project.
Cooper, P.F. 1992. The use of reed bed systems to treat
domestic sewage. IAWPRC Orlando Wetlands Confer-
ence Proceedings (in press).
Cooper, R.F., and B.C. Findlater, eds. 1990. Constructed
wetlands in water pollution control. Proceedings of the In-
ternational Conference on the Use of Constructed Wet-
lands in Water Pollution Control. Oxford, England:
Pergamon Press.
CSWRCB, 1984. California State Water Resources Con-
trol Board. Irrigation with reclaimed municipal wastewa-
ter—A guidance manual. Report No. 84-1 wr.
Sacramento, CA.
Deeny, K., J.A. Heidman, and W.W. Schuk. 1991. Imple-
mentation of sequencing batch technologies in the
United States. Paper presented at the 64th annual con-
108
-------
ference of the Water Pollution Control Federation,
Toronto (October).
EPA. 1991. Environmental Protection Agency. Manual of
alternative wastewater collection systems. Cincinnati,
OH. EPA/625/1-91/024.
EPA. 1991. Environmental Protection Agency. Supple-
mental manual on the development and implementation
of local discharge limitations under the pretreatment pro-
gram. Washington, DC. Office of Water Publication No.
EN-336.
EPA. 1990. Environmental Protection Agency. Environ-
mental regulation and technology: Autothermal thermo-
philic aerobic digestion of municipal wastewater sludge.
Cincinnati, OH. EPA/625/10-90/007.
EPA. 1988. Environmental Protection Agency. Design
manual: Constructed wetlands and aquatic plant sys-
tems. Cincinnati, OH. EPA/625/1-88/022.
EPA. 1987. Environmental Protection Agency. Report on
the use of wetlands for municipal wastewater treatment
and disposal. Washington, DC. EPA/430/09-88-005.
EPA. 1986. Environmental Protection Agency. Trickling
filter/solids contact process: Full scale studies. Cincin-
nati, OH. EPA/600/S2-86/046.
EPA. 1985. Environmental Protection Agency. Technol-
ogy assessment of intermittent sand filters. Cincinnati,
OH.
EPA. 1984a. Environmental Protection Agency. Hand-
book: Septage treatment and disposal. Cincinnati, OH.
EPA/625/6-84-009.
EPA. 1984b. Environmental. Protection Agency. Process
design manual: Land treatment of municipal wastewater-
Supplement on rapid infiltration and overland flow. Cin-
cinnati, OH. EPA/625/1-81-013a.
EPA. 1983. Environmental Protection Agency. Design
manual: Municipal wastewater stabilization ponds.
EPA/625/1-83-015. NTIS No. PB88-184023.
EPA. 1983. Environmental Protection Agency. Process
design manual for land application of municipal sludge.
Cincinnati, OH. EPA/625/1-83-016.
EPA. 1982. Environmental Protection Agency. Process
design manual for dewatering municipal wastewater
sludges. Cincinnati, OH. EPA/625/1-82-014.
EPA. 1981. Environmental Protection Agency. Process
design manual: Land treatment of municipal wastewater.
Cincinnati, OH. EPA/625/1-81-013.
EPA. 1980. Environmental Protection Agency. Wastewa-
ter stabilization lagoon intermittent sand filter systems.
Cincinnati, OH. EPA/600/2-80-032.
EPA. 1980. Environmental Protection Agency. Design
manual: Onsite wastewater treatment and disposal sys-
tems. EPA/625/1-80-012. NTIS No. PB83-219907.
EPA. 1980. Environmental Protection Agency. Innovative
and alternative technology assessment manual. Wash-
ington, DC. EPA/430/9-78-009.
EPA. 1979. Environmental Protection Agency. Process
design manual for land application of municipal sludge.
Cincinnati, OH. EPA/625/1-83-016.
EPA. 1979. Environmental Protection Agency. Process
design manual for sludge treatment and disposal.
EPA/625/1-79-011. NTIS No. PB80-200546.
EPA. 1978. Environmental Protection Agency. The cou-
pled trickling filter-activated sludge process: Design and
performance. EPA/600/2-78-116. NTIS No. PB-287555.
EPA. 1978. Environmental Protection Agency. Innovative
and alternative technology assessment manual. MCD-
53. Washington, DC. EPA/430/9-78-009.
EPA. 1977. Operator manual: Stabilization ponds. Wash-
ington, DC. EPA/430/09-77-012.
EPA. 1977. Environmental Protection Agency. Process
design manual—Wastewater treatment facilities for sew-
ered small communities. EPA/625/1-77-009. NTIS No.
PB-299711.
EPA. 1977. Environmental Protection Agency. Package
treatment plants operations manual. Washington, DC.
EPA/430/9-77-005.
Ettlich, W.F. 1978. A comparison of oxidation ditch plants
to competing processes for secondary and advanced
treatment of municipal wastes. EPA/600/2-78-051. NTIS
No. PB-281380.
Feige, W.A., E.T. Oppelt, and J.F. Kreissl. An alternative
septage treatment method: Lime stabilization/sand bed
dewatering. EPA/600/2-75-036. NTIS No. PB24-
5816/4BE.
Gerba, C.P., C. Wallis, and J.L. Melnick. 1975. Fate of
wastewater bacteria and viruses in soil. J. Irrigation and
Drainage, ASCE 101:157-175.
Hall, D.H., et al. 1979. Municipal wastewater treatment
by the overland flow method of land application.
EPA/600/2-79-178. NTIS No. PB80-102908.
Hart, W.E. 1975. Irrigation system design. Department of
Agricultural Engineering. Ft. Collins, CO: Colorado State
University.
Irrigation Association. 1983. Irrigation. C. Pair, ed. Fifth
edition. Silver Spring, MD.
Irvine, R.L. 1985. Technology assessment of sequencing
batch reactors. Cincinnati, OH. EPA 600/2-85-007
Irvine, R.L., et al. 1983. Municipal application of se-
quencing batch treatment. J. WPCF 55:484.
Lamb, B.E., A.J. Gold, G.W. Loomis, and C.G. McKeil.
1990. Nitrogen removal for on-site sewage disposal—A
recirculation sand filter/rock tank design. Trans. ASAE
33(2):525-531.
109
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Lance, J.C. 1984. Land disposal of sewage effluents and
residue in Groundwater pollution microbiology. G. Bitton
& C.P. Gerba eds. New York, NY: John Wiley & Sons.
Lance, J.C., F.D. Whisler, and B.C. Rice. 1976. Maximiz-
ing den'rtriffcation during soil infiltration of sewage water.
J. Environmental Quality 5:102.
Luthin, J.N. 1973. Drainage engineering. Huntington, NY:
R.E. Krieger Publishing Co.
Martel, C.J., et al. 1980. Rational design of overland flow
systems. Proceedings of the National Environmental En-
gineering Conference. W. Saukin, ed. New York, NY.
ASCE.
Matasci, R.N., D.L. Clark, J.A. Heidman, D.S. Parker, B.
Patrick, and D. Richards. 1988. Trickling filter/solids con-
tact performance with rock filters and high organic load-
ings. J.WPCF 60(1).
Matasci, R.N., C. Kaemper, and J.A. Heidman. 1986.
Full-scale studies of the trickling filter/solids contact proc-
ess. J. WPCF 58(11).
Nielson, J.S., and M.D. Thompson. 1988. Operating ex-
periences at a large continuously fed, intermittently de-
canted, activated sludge plant. J. WPCF 60:199.
Nilsson, P. 1990. Infiltration of wastewater. Report No.
1002. Lund, Sweden: Lund University
Norris, D.P., et al. 1982. Production of high quality trick-
ling filter effluent filtration. J. WPCF 54:1087.
Oregon DEQ. 1982. Department of Environmental Qual-
ity. Oregon on-s'rte experimental systems program. Final
report. Salem, OR.
Otis, R.J. 1985. Soil clogging: Mechanisms and control,
/n On-site wastewater treatment. Pub. 07-85. St. Joseph,
Ml: ASAE.
Otis, R.J. 1981. Design of pressure distribution networks
for septic tank systems. J. Environmental Engineering
Division, ASCE. 108(EE1):123-140.
Overcash, M.R., and D. Pal. 1979. Design of land treat-
ment systems for industrial wastes—theory and practice.
Ann Arbor, Ml: Ann Arbor Science.
Piluk, R.J., and O.J. Hao. 1989. Evaluation of on-site
waste disposal system for nitrogen removal. J. Environ-
mental Engineering Div., ASCE. 115:725.
Reed, S.C. 1991. Constructed wetlands for wastewater
treatment. BioCycle32(1):44-49.
Reed, S.C., and R.W. Crites. 1984. Handbook of land
treatment systems for industrial and municipal wastes.
Park Ridge, NJ: Noyes Publications.
Reed, S.C., E.J. Middtebrooks, and R.W. Crites. 1988.
Natural systems for waste management and treatment.
New York, NY: McGraw-Hill Book Co.
Reneau, R.B., Jr. 1977. Changes in inorganic nitroge-
nous compounds from septic tank effluent in a soil with a
fluctuating water table. J. Environmental Quality 6:173-
178.
Rezek, J.W., and I.A. Cooper. 1980. Septage manage-
ment. EPA/600/8-80-032, NTIS No. PB81-142481.
Sandy, AT., W.A. Sack, and S.P. Dix. 1988. Enhanced
nitrogen removal using a modified recirculating sand filter
(RSF2), in On-site wastewater treatment. Proceedings
5th National Symposium on Individual and Small Com-
munity Sewage Systems. St. Joseph, Ml: ASAE.
SCS. 1983. Soil Conservation Service. Irrigation. Depart-
ment of Agriculture. Washington, DC.
SCS. 1971. Soil Conservation Service. Drainage of agri-
cultural land. National Engineering Handbook, Section
16. Washington, DC.
SCS. 1970. Soil Conservation Service. Irrigation water
requirements. Technical Report No. 21. Department of
Agriculture. Washington, DC.
Siegrist, R.L., D.L. Anderson, and D.L. Hargett. 1986.
Large soil absorption systems for wastewaters from mul-
tiple-home developments. EPA/600/2-86-023. NTIS No.
PB86-164084.
Sikora, L.J., and R.B. Corey. 1976. Fate of nitrogen and
phosphorus in soils under septic tank waste disposal
fields. Transactions, ASAE 19:866.
Tofflemire, T.J., and M. Chen. 1977. Phosphate removal
by sands and soils. Ground Water 15:377.
U.S. Dept. of Interior. 1978. Drainage Manual. Washing-
ton, DC: Bureau of Reclamation.
U.S. Dept. of Interior. 1973. Drainage manual. Washing-
ton, DC: Bureau of Reclamation.
University of Wisconsin. 1978. Management of small
wastewater flows. Environmental Protection Agency.
EPA/600/2-78/173. NTIS No. PB-286560.
USDA. 1971. U.S. Department of Agriculture. Drainage
of agricultural land. National Engineering Handbook,
Section 16. Washington, DC: Soil Conservation Service.
WEF. 1992. Water Environment Federation. Wastewater
treatment plant design. Manual of practice no. 8. Alexan-
dria, VA.
WPCF, 1990. Water Pollution Control Federation. Natu-
ral systems for wastewater treatment. Manual of practice
FD-16. Alexandria, VA.
WPCF. 1986. Alternative sewer systems. Water Pollution
Control Federation Manual of Practice FD-12. Alexan-
dria, VA.
WPCF. 1985. Water Pollution Control Federation. Opera-
tion of extended aeration package plants. Manual of
practice OM-7. Alexandria, VA.
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