EPA/600/R-98/006
Storage/Sedimentation Facilities for Control of
Storm and Combined Sewer Overflows
Design Manual
by
W. Michael Stallard. William G. Smith,
Ronald W. Crites, and John A. Lager
Metcalf & Eddy. Inc.
Palo Alto, California 94303
Contract No. 68-03-2877
Project Officer
Richard Field
Urban Watershed Management Branch
National Risk Management Research Laboratory
Edison, New Jersey 08837
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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NOTICE
The U.S. Environmental Protection Agency (EPA) through its Office of Research
and Development funded and managed the preparation of this document under
Contract 68-03-2877 with Metcalf & Eddy, Inc. The draft report was prepared
in the time frame of September 1979 to October 1981 and was revised in 1997
for this publication. Although the report was prepared many years ago, it was
not published at that time. It is being released currently in order to
provide information to communities in support of their wet-weather flow
management efforts. Despite the report's revision, some of its content may no
longer be entirely current. The document has been subjected to the Agency's
peer and administrative review, and it has been approved for publication as an
EPA document. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with
protecting the Nation's land. air. and water resources. Under a mandate of
national environmental laws, the Agency strives to formulate and implement
actions leading to a compatible balance between human activities and the
ability of natural systems to support and nurture life. To meet this mandate.
EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary
to manage our ecological resources wisely, understand how pollutants affect
our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for reducing risks
from threats to human health and the environment. The focus of the
Laboratory's research program is on methods for the prevention and control of
pollution to air, land, water, and subsurface resources; protection of water
quality in public water systems; remediation of contaminated sites and ground
water; and prevention and control of indoor air pollution. The goal of this
research effort is to catalyze development and implementation of innovative,
cost-effective environmental technologies; develop scientific and engineering
information needed by EPA to support regulatory and policy decisions; and
provide technical support and information transfer to ensure effective
implementation of environmental regulations and strategies.
This publication has been produced as part of the Laboratory's strategic
long-term research plan. It is published and made available by EPA's Office
of Research and Development to assist the user community and to link
researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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ABSTRACT
This report describes applications of storage facilities in wet-weather flow
management and presents step-by-step procedures for the analysis and design of
storage-treatment facilities. Retention, detention, and sedimentation storage
are classified and described. International as well as national state-of-the-
art projects are discussed.
Retention storage facilities capture and dispose of stormwater runoff through
infiltration, percolation, and evaporation. Detention storage is temporary
storage for stormwater runoff or combined sewer overflow. Stored flows are
subsequently returned to the sewerage system at a reduced rate of flow when
downstream capacity is available, or the flows are discharged to the receiving
water with or without further treatment. Sedimentation storage alters the
wastewater stream by gravity separation. The stormwater runoff and combined
sewer overflow must be characterized to estimate the efficiency of any
sedimentation basin.
sign methodology of the storage and/or sedimentation facility
is report includes: identifying functional requirements;
The detailed desi
presented in this
identifying site constraints; establishing basis of design; selecting storage
and/or treatment option; and conducting a cost analysis.
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CONTENTS
Section Page
1 INTRODUCTION AND USERS' GUIDE
Stormwater, Urbanization, and Control 1-1
Purpose of the Manual 1-5
Organization 1-5
Users' Guide 1-6
Section 1 - Introduction and Users's Guide 1-6
Section 2 - Urban Stormwater: An Overview 1-6
Section 3 - Terminology and Classification
of Storage/Sedimentation Facil i ties 1-7
Section 4 - System Planning, Design
Procedures, and Integration 1-7
Section 5 - Design of Retention Storage Facilities 1-7
Section 6 - Design of Detention Facilities 1-8
Section 7 - Design of Storage/Sedimentation Facilities 1-8
Section 8 - International Perspective 1-8
Appendixes 1-8
Spec i al Acknowl edgments 1-8
2 URBAN STORMWATER: AN OVERVIEW
Urban Stormwater Poll ution 2-1
Representative Concentrations 2-3
Impacts of Urbanization 2-5
Receiving Water Assessments 2-11
Urban Stormwater Control 2-12
References 2-14
3 TERMINOLOGY AND CLASSIFICATION OF STORAGE
AND/OR SEDIMENTATION FACILITIES
Terminology 3-1
Cl assi f i cati on 3-3
Main Function Principles 3-8
Location in the Sewerage Network 3-9
Technical Configuration 3-11
References 3-14
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CONTENTS (Continued)
4 SYSTEM PLANNING, DESIGN PROCEDURES, AND INTEGRATION
System PI anning 4-2
Conditions for PI anning 4-2
Establ i shment of Goal s 4-3
Planning Methodology 4-4
Cost Optimization Methodology 4-8
Storage Vol time Determination Methods 4-10
Effect of Storage and/or Sedimentation 4-11
The Integration Process 4-12
Identify Existing System an Needs 4-12
Establ i sh System Needs 4-12
Identify Applicable Control Alternatives 4-12
Determine Control Method Compatibility 4-15
Design Procedure for Combined Sewer Systems 4-16
Problem Identification 4-16
Data Needs 4-16
Determine Pollution Load -4-22
Identify Pollutant Removal Objectives 4-23
Control Optimization 4-23
Pollutant Budget Analysis 4-24
Operating Strategy for Design 4-25
Instrumentation and Control Strategy for Operation 4-26
Design Procedure for Separate Storm Sewer Systems 4-27
Problem Identification 4-27
Data Needs 4-27
Determine Pol 1 ution Load 4-28
Identify Flood Control and Pollutant Removal Objectives 4-28
Control Optimization 4-29
Pol 1 utant Budget Analysis 4-30
Operati ng Strategy for Desi gn 4-30
Retrofitting of Existing Flood Control Facilities 4-30
Integration Process Examples 4-32
Storage and/or Sedimentation Basin Integration 4-32
Fl ood Control Retro f i t 4-34
Retention and Attenuation Facility Integration 4-36
References 4-38
5 DESIGN OF RETENTION STORAGE FACILITIES
Desi gn Consi derations 5-1
Size 5-2
Locati on 5-5
Design Procedure 5-6
Step 1 - Quantify Functional Requirements 5-6
Step 2 - Identify Required Waste Load and Flow Reduction 5-7
Step 3 - Determine Preliminary Basin Sizing 5-7
Step 4 - Identify Feasible Pond Sites 5-10
Step 5 - Investigate Most Promising Sites 5-12
Step 6 - Establish Basin Sizes 5-12
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CONTENTS (Continued)
Step 7 - Design Solids Removal Technique and Facilities 5-14
Step 8 - Determine Pond Configuration 5-16
Performance 5-16
Operati ons 5-21
Co Sts 5-25
References 5-27
6 DESIGN OF DETENTION FACILITIES
Introduction 6-1
Onsite Detention 6-3
In-System Detention Storge 6-3
Design Considerations 6-4
Tri b utary Area 6-4
Storage Area and Volume 6-5
Structural Considerations 6-5
Responsibility to the Owner 6-5
SI ope and Si ze of Sewers 6-6
Peak Flows 6-6
Control s 6-6
Resuspension of Sediment 6-6
Desi gn Procedure/Exampl es 6-7
Step 1 - Identify Functional Requirements 6-7
Step 2 - Identify Site Constraints 6-8
Step 3 - Establish Basis of Design 6-9
Step 4 - Select Storage Option(s) and Locations 6-11
Step 5 - Estimate Costs and Cost Sensitivities 6-12
Step 6 - Complete Design 6-13
Operation and Maintenance Considerations 6-13
Onsite 6-13
In-System 6-13
Costs 6-14
References 6-14
7 DESIGN OF SEDIMENTATION FACILITIES
Design Considerations 7-1
Storage 7-1
Treatment Efficiency 7-5
Disinfection 7-9
Site Constraints 7-11
Design Procedure/Example 7-12
Step 1 - Identify Functional Requirements 7-12
Step 2 - Identify Site Constraints 7-13
Step 3 - Establish Basis of Design 7-14
Step 4 - Select Main Treatment Geometry 7-17
Step 5 - Identify and Select Preteatment Components 7-22
Step 6 - Detail Auxiliary Systems 7-22
Step 7 - Estimate Costs and Cost Sensitivities 7-23
Step 8 - Complete Design 7-24
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CONTENTS (Concluded)
Operation and Maintenance Considerations ' 7-24
Costs 7-25
Capital Cost Breakdown - Illustrative Examples 7-25
Operation and Maintenance Costs 7-26
References 7-28
8 INTERNATIONAL PERSPECTIVE
Introduction 8-1
Storage/Sedimentatin Practices Manual 8-2
Fl ow Control Devices 8-3
Flow Regulator 8-3
Hydrobrake 8-4
Wirbel drossel 8-6
Fl ow Val ve 8-8
Innovative Technology Appl ications 8-9
Fl ow Bal ance System 8-9
Sel f-Cl eaning Storage/Sedimentation Basin 8-9
References 8-12
Appendix
A POLLUTANT CHARACTERIZATION AND ESTIMATION OF REMOVAL
B ASSESSMENT METHODS
C INFILTRATION MEASUREMENT TECHNIQUES
VI 1 1
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FIGURES
1 Response of watershed to precipitation under different conditions 1-2
2 Common elements of a combined sewer system 1-4
3 Hyetographs and hydrographs for watershed under predevelopment and
postdevelopment conditions 2-2
4 Combined sewer overflow quality versus time for selected cities 2-10
5 Hydrograph of a watershed showing effects of storage 2-13
6 Schematic classification of storage and/or sedimentation facilities...3-4
7 Classification scheme based on type of sewer system 3-5
8 Classification of storage and/or sedimentation facilities based
on main function of principle 3-7
9 Source control design methodol ogy 4-5
10 In-system control design methodology 4-6
11 Downstream control design methodology 4-7
12 Determination of optimal combination of storage and/or
treatment al ternatives 4-9
13 Storm magnitude versus frequency 4-17
14 Storm intensity versus frequency 4-18
15 Storm duration versus frequency 4-18
16 Schematic showing relation of total head, pressure head,
and gravitation head for saturation flow 5-3
17 Routing curves for a typical reservoir 5-8
18 Brune's trap efficiency curve 5-9
IX
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FIGURES (Continued)
19 Soil permeabilty versus ranges of application rates ...5-11
20 In! et structure/forebay 5-15
21 Effect of impoundment on storm runoff in The Woodlands, Texas 5-17
22 Effect of vegetation on soil infiltration raes 5-23
23 Floating material trapped by log boom 5-24
24 Cost of dry pond construction 5-26
25 Storage pond construction costs ....5-26
26 Representative process schematics 7-2
27 Typical TSS removal efficiencies by sedimentation 7-8
28 Experienced TSS removal efficiency variations 7-8
29 Pollution and volumetric retention versus storage tank
vol ume for wet- and dry-years 7-10
30 Unit removal efficiencies for combined sewer overflow
detention tanks 7-10
31 Flushing water system concept 7-23
32 Flow regulator 8-3
33 Comparison of discharge curves for unrestricted pipe and
pipe with flow regulator 8-4
34 Schematic of Hydrobrake 8-5
35 Schematic of flow patterns during Hydrobrake operation 8-5
36 Discharge curve comparison for Hydrobrake and
short pi pe of same diameter 8-6
37 Schematic of flow pattern in a Wirbeldrossel 8-7
38 Discharge curves for Wirbeldrossel and circular
outlet of the same size ...8-7
39 Diagram of a flow valve 8-8
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FIGURES (Concluded)
40 Typical discharge curve for fl-ow valve 8-8
41 Schematic of pontoon tank system at Lake Tehormingen, Sweden 8-10
42 Self-cleaning storage/sedimentation basin 8-11
A-l Flows into, through, and out of a storage/tretament unit A-5
A-2 Camp's sediment trap efficiency curves A-14
A-3 Limiting cases in sediment trap efficiency A-15
B-l Assessment model categories B-3
C-l Cylinder infiltrometer in use C-4
C-2 Variability of infil trometer test results on
rel atively homogenous si te C-6
C-3 Number of tests required for 90% confidence that the calculated
mean is within stated percent of the true mean C-8
C-4 Number of tests required for 95% confidence that the calculated
mean is within stated percent of the true mean C-8
XI
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TABLES
1 Average Pollutant Concentrations in Stormwater Runoff 2-3
2 Average Pollutant Concentrations in Combined Sewer Overflows 2-4
3 Comparison of Stormwater Discharges to Other Pollutant Sources 2-4
4 Priority Pollutants Measured in Urban Stormwater Systems -
Liquid Fraction 2-6
5 Priority Pollutants Measured in Urban Stormwater Systems -
Sediment Fraction 2-8
6 Surface Runoff Loadings to Bay, Pre- and Post-Development 2-11
7 Classification of Storage and/or Sedimentation Facilities 3-6
8 Storage and/or Sedimentation Control Method Versus
Flow or Quality Approach 4-13
9 Storage and/or Sedimentation Control Method Versus
Physical and Effectiveness Limitations 4-14
10 Trace Organics in Groundwater Donwstream of Spreading Basins
of Whittier Narrows, California 5-19
11 Reported Isolations of Virus Beneath Land Treatment Sites 5-20
12 Factors that Affect the Survival of Enteric Bacteria
and Viruses in Soil 5-20
13 Removal Mechanisms of Trace Elements in Soil 5-22
14 Some Insecticdes Used for Lagoon Insect Control 5-25
15 Detention Facilities in Use in the United States and Canada 6-2
16 Objectives in Requiring Detention 6-2
17 Depth of Flooding Allowed on Streets... 6-10
18 Pilot Plant Performance on Richmond-Sunset Stormwater Flows 7-9
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TABLES (Concluded)
19 Common Removal Efficiencies Associated with Primary
Sedimentation of Sanitary Wastewater 7-9
20 Performance of the Red Run CSO Sedimentation/Transport Basin 7-12
21 Performance of the Hancock Street Sedimentation, Saginaw 7-17
22 Performance of the Spring Creek Auxiliary Water
Pol 1 ution Control PI ant 7-18
23 Net Benefits Approximation of Spring Creek Facilities 7-19
24 Exampl e Capi tal Cost Breakdowns 7-26
A-l Preservation of Wastewater Samples A-3
A-2 Routing Data for Hypothetical Reservoir A-6
B-l Characteristics of Assessment Models, By Level,
In Order of Increasing Complexity B-4
B-2 Model Selection Criteria B-5
C-l Comparison of Infiltration Measurement Using Standard
USPHS Percolation Test and Double-Cylinder Infiltrometer C-2
XT
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Section 1
INTRODUCTION AND USERS' GUIDE
As municipal wastewater treatment is upgraded in accordance with the Federal
Clean Water Act, urban stormwater runoff and combined sewer overflows (CSO)
are emerging as significant sources of surface water pollution in the United
States. A 1975 survey of 56 public agencies located throughout the United
States revealed that "...control [of] stormwater pollution from sources other
than erosion to make significant improvements in existing wet-weather quality
of streams, lakes, etc." ranked second only to flood control as a stormwater
management goal [1].
Temporary storage of runoff, a widely used method of flood control, is gaining
acceptance in the United States as a cost-effective means of reducing the
pollutant load of stormwater runoff. Existing flood control facilities may be
retrofitted and new flood control facilities designed to enhance pollution
removal. Similarly, the basic reason for incorporating storage into CSO/urban
runoff control systems is to provide flow equalization so the overall cost of
the storage-treatment system can be optimized. Flow equalization at the
treatment plant and combined storage and/or sedimentation effects have been
demonstrated as cost-effective measures to reduce pollution from CSO systems.
The purpose of this design manual is to summarize applications of storage
facilities in stormwater management and to present step-by-step procedures for
analysis and design of stormwater and CSO storage and/or sedimentation
treatment facilities. The manual is directed toward the technical designer
and planner and primary emphasis is placed on the cost-effective reduction of
total pollutants discharged.
STORMWATER, URBANIZATION, AND CONTROL
Urban stormwater runoff is not a new problem. Among the earliest examples of
public works are urban drainage systems. The early sewers were designed to
minimize flooding by rapidly transporting converging runoff through developed
low-lying areas. The stormwater was discharged to natural drainage channels
or streams with little regard to downstream effects.
More recently, it has been recognized that urban areas have significant
impacts on stormwater runoff quantity and quality and on the attainment of
downstream receiving water quality objectives. The response of a watershed to
precipitation for undeveloped conditions and for urbanized conditions with and
without stormwater controls is illustrated in Figure 1.
1-1
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LAND USE
HYDROLOBIC RESPONSE WATER QUALITY RESPONSE
(a)
UNDEVELOPED
LAND
DEVELOPED LAND
WITHOUT STORAGE
CONTROLS
DEVELOPED LAND
WITH STORAGE
CONTROLS
UNDEVELOPED
HYDROGRAPH
ATTENUATED
HYDROGRAPH
UNDEVELOPED
POLLUTANT LOAD
DEVELOPED
POLLUTANT LOAD
DEVELOPED
POLLUTANT LOAD
LOAD NOT
CAPTURED
OVERALL
WATERSHED
RESPONSE
POLLUTANT LOAD
CAPTURED
Figure 1. Response of watershed to precipitation under different conditions [2J.
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In the figure, hydrographs show the rate of flow of runoff (y-axis) at a given
point versus time (x-axis). Under natural conditions, rain falling on the
ground surface may do one of ythree things: it may be intercepted and held on
vegetation, roots, and the ground as the surfaces are wetted; it may infiltrate
through the ground surface and percolate downward to become part of the
groundwater; or it may collect on the surface in depressions or move across the
surface as runoff. The paved areas and buildings that characterize an urban
environment prevent or retard infiltration. Urban areas usually have much less
vegetation so that interception is reduced and heavily trafficked ground
surfaces compact and become less pervious.
Precipitation, falling through the air and flowing over the ground surface,
captures, dissolves, and suspends a portion of the material contacted and
carries these "pollutants" along, usually to a receiving water. Densely
populated areas are characterized by the discharge of waste materials to the
.air and ground surface as well as to water bodies. Runoff from urban areas is
contaminated by this waste material. For example, it may contain three to four
times the concentration of suspended material as is typically found in raw
domestic wastewater as well as significant quantities of toxic and oxygen
demanding substances, nutrients, pesticides, salts, and bacteria.
In addition to the pollution content of urban runoff itself, the runoff in many
older cities of the United States is combined with municipal wastewater in the
sewer system. Sewers were originally constructed for stormwater conveyance.
When human and industrial wastes came to be recognized as urban problems in the
mid and late 19th century, in many cities these wastes were introduced into the
storm sewer system making it a combined sewer system. When overflows of
combined sewers occur, a mixture of runoff and raw municipal wastewater is
spilled. A typical combined sewer system is illustrated in Figure 2. In 1995,
the EPA estimated the CSO abatement costs for the 1100 communities served by
combined sewer systems to be over $40 billion. [3a]
The cost of controlling stormwater pollution can be substantial. The American
Public Works Association's 199Z report, Nationwide Costs to Implement BMPs [3b]
identified possible capital costs of up to $407 billion and operation and
maintenance costs of $542 billion/year to meet water quality standards for
stormwater discharges. While legislative oversight reviews and reassessments
may reduce or defer elements of the program, the development and application of
cost-effective control technologies remains an essential national goal.
The major problem in controlling stormwater runoff is its variability.
Rainfall and runoff occur as intermittent and random events, varying widely in
volume and rate of flow during single events and from one event to the next.
Transport and treatment facilities for excess wet-weather flow control, sized
to handle some medium storm size, frequently are idle during dry periods and
overflow during large storms. Secondary (dual) use of facilities during
nonstorm periods may improve their cost effectiveness and should be evaluated
on an integrated total system basis.
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INCLUDING
DRAINAGE )\ \
'^n^'-^-fer^
SANITARMNTERCEPTOR
TO iATER POLLUTION
CONTROL PLANTS FOR
TREATMENT
COMBINED SEWER OVERFLOW
MIXTURE OF MUNICIPAL
WASTENATER AND STORMWATER
DISCHARGING INTO THE
RECEIVING IATERS
Figure 2. Common elements of a combined sewer system [4],
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Temporary storage of excess runoff can be an effective and economical method
of controlling stormwater flooding and pollution. Storage capacity can reduce
flowrates and enhance pollutant removals. Excess runoff stored during large
storms or during more intense rainfall periods can be released slowly when
capacity in the drainage and treatment system or stream channel is
available. Because the peak rate of flow is much less, smaller capacities are
needed to transport and to treat the same quantity of flow, and overflows or
flooding occur less frequently. In some CSO systems in Europe, selective
storage timed to peak pollutant loads has been effectively implemented with
resultant high pollutant capture.
When the storage capacity is exceeded, storage basins may provide treatment to
the overflow by sedimentation. The ability of flowing water to carry heavier
solid materials is directly related to the velocity of flow. In traveling
through a storage facility, the stormwater slows, so that the heavier solids
are deposited. By carefully designing inlet and outlet structures to maximize
the sedimentation effect, and providing some method of removing and disposing
of captured solids, significant water quality benefits can be achieved.
PURPOSE OF THE MANUAL
Over the past decade, there has been a large commitment by the U.S.
Environmental Protection Agency (USEPA) to identify pollution sources other
than municipal wastewater discharges and to develop viable methods for their
control. A large amount of information on stormwater runoff and,
particularly, CSO characteristics and treatabilities, has been developed.
Much less information is available on the effect of stormwater runoff on
receiving water impacts. The purpose of the design manual is to summarize the
existing information for stormwater and CSO storage and/or sedimentation
control facilities, including examples of foreign practice, and to suggest
step-by-step analysis and design procedures for their application.
ORGANIZATION
For ease of reference and consistency, certain terminology and classifications
of control systems have been adopted for this manual (see Section 3). Basic
categories are: (1) the type of collection system, and (2) the general
placement of the storage and/or sedimentation facility or practice within the
collection network. The user should recognize that some devices, theories, or
practices will fall in more than one category. In these cases, the detailed
discussion is presented in the category representing the most common use and
cross referenced under other categories. For example, sedimentation theory is
described in connection with downstream controls (where treatment impacts are
paramount) and infiltration/percolation is described under source controls
(where maximum system sizing economies are likely to be achieved).
Fundamentals on storage volume requirements, common to all applications, are
introduced under "system planning" and elaborated upon where applicable under
the specific control description.
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Typically in stormwater management systems, problems and remedial costs
increase as one moves downstream in the collection network. Also, there tends
to be a direct impact on the design of downstream facilities as a consequence
of actions or lack of actions in terms of upstream controls, whereas the
reverse is not the case. The order of presentation of the design procedures
and the design criteria starts with facilities for combined sewer systems and
then proceeds to separate storm sewer systems. Facilities for detention,
sedimentation, and retention are presented in that order.
USERS' GUIDE
This manual is organized to present, in a logical sequence, specific
application methods and design procedures for storage and/or sedimentation
control of stormwater pollution. It is, however, a design manual, and
sequential reading of the material contained herein is not necessary. As an
aid to ready use of the manual, the following description of section contents
is provided.
Section 1 - Introduction and Users' Guide
Introduction. The problems of flooding, groundwater loss, and water pollution
that may result from urban stormwater and CSO are briefly introduced. The
importance of stormwater as an urban pollution source is emphasized. The
potential of storage and/or sedimentation as a control method is discussed.
Purpose of the Manual. The specific aims of the design manual are given.
Organization. The classification scheme upon which the manual is structured
is described.
Users' Guide. A brief summary of the purpose, content, and organization of
each of the chapters is presented as a quick reference for the potential user
of the manual.
Section 2 - Urban Stormwater: An Overview
Urban Stormwater Pollution. The sources and representative characteristics of
urban stormwater pollution are described; the concept of "first flush" is
introduced; and problem intensification through urban development is
illustrated by example. The approach to receiving water protection and
restoration as practiced in the United States is described and the importance
and limitations in evaluating urban stormwater based impacts are discussed.
Urban Stormwater Control
The use
and flow
of storage facilities for retention,
infiltration/percolation, and flow attenuation is outlined briefly and
potential resulting water quality benefits are discussed. Optional
supplemental or alternative controls to storage are described and the need
an integrated approach is stressed.
for
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Section 3 - Terminology and Classification of Storage/Sedimentation Facilities
Terminology. The most commonly used terms are defined.
Classification. The classifications of storage and/or sedimentation
facilities are discussed based on (1) major function, (2) location within the
sewerage network, and (3) technical configuration.
Section 4 - System Planning, Design Procedures, and Integration
Planning. Planning concepts, methodologies, and tools common to all storage
and/or sedimentation applications are introduced. The need for goal setting
and realistic appraisals as forerunners to design are stressed.
Design Procedures. Design procedures for both combined sewer systems and
separate storm sewer systems are described. The topics covered include
problem identification, data needs, determination of pollutant loads, identi-
fication of pollutant removal objectives, control optimization, pollutant
budget analysis, and operating strategy for design.
Integration. The role of storage/sedimentation facilities in an integrated
stormwater management plan is discussed. The concurrent growth of stormwater
control systems and urban areas is examined. Retrofit of existing flood
control and drainage facilities to maximize pollution control is discussed.
Examples of urban stormwater control are described to illustrate the several
points.
Section 5 - Design of Retention Storage Facilities
Design Considerations. The principal factors affecting design are the size
and locations of the facilities. Factors that influence facilities size are
described and include volume, surface area, soil permeability, and infil-
tration rates. Considerations related to the location and siting of the
facilities are also introduced.
Design Procedures. A step-by-step procedure is described for the planning and
design of retention facilities. The procedure includes basin sizing require-
ments such as flow routing and pollutant removals, facilities site evaluation,
final design factors, and solids control and removal facilities.
Performance. The removal efficiencies and treatment effectiveness of reten-
tion ponds are discussed. Constituents of concern are described and include
organic compounds, bacteria, and viruses.
Operations. Operational problems with stormwater retention facilities are
highlighted along with solutions to mitigate these problems. Special
attention is given to the control and removal of solids and floatables.
Costs. Cost curves are presented for estimating the construction costs of
retention and storage ponds.
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Section 6 - Design of Detention Facilities
Design considerations are discussed for onsite detention and for in-system
detention storage. Onsite detention is exemplified by rooftops, parking lots
and streets, drainage swales, and detention reservoirs. In-system detention
includes available storage in underground conduits and offline storage. A
step-by-step design approach is presented for both onsite and in-system
detention. Operation and maintenance considerations are also discussed.
Section 7 - Design of Storage/Sedimentation Facilities.
Discussions similar to Sections 5 and 6 are presented for the design of
storage/sedimentation facilities. Representative process schematics are
presented for planned or constructed facilities in the United States. Data
are given on actual and expected removal rates for several facilities. A
step-by-step design procedure is provided as well as preliminary cost data.
Section 8 - International Perspective.
A technology review of several international projects is presented. The
review includes flow control devices developed in Sweden, Denmark, and
Germany; a flow balancing system developed and applied in Sweden; and an
innovative self-cleaning storage/sedimentation basin used in Zurich,
Switzerland.
Appendixes
Appendix A - Pollutant Characterization and Estimation of Removal. The
characterization of pollutants based on sample collection and analysis,
pollutants to be analyzed, particle size determination, and pollutant distri-
bution versus particle size and specific gravity are presented. A suggested
analytical method for flow routing and pollutant routing is described.
Appendix B - Assessment Methods. Several desk top and computer hydro!ogic
and stormwater pollution analysis methods are listed.
Appendix C - Infiltration Measurement Techniques. Soil infiltration rate
testing procedures for use in investigating stormwater retention sites are
presented.
SPECIAL ACKNOWLEDGMENTS
It has been stated earlier that recognition of urban stormwater runoff as a
significant pollutant source and the development and application of counter-
measures are receiving increasing attention from administrators, planners, and
designers. This is particularly true in the area of storage and/or sedimen-
tation controls where, concurrent with the development of this manual, three
major projects, related in subject matter, have been initiated or completed.
Through the efforts of USEPA's Storm and Combined Sewer Section, and in
particular Mr. Richard Field, Chief and Project Officer for this study, drafts
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of portions of these documents have been made available to us. With the
authors' permission, pertinent comments, data, and concepts have been used and
are so referenced in this manual.
Because the time available to scan and incorporate highlights from this
material has been very brief, interested readers are encouraged to seek out and
review the following documents:
Stahre, P. Flow Equalization in Sewer Systems. Report No. ISBN 91
540-3455 for the State Council for Building Research. Stockholm,
Sweden. March 1981.
Technical Wastewater Union E.V. (West Germany). Guidelines for trie
Sizing and Design of Stormwater Discharges in Combined Sewers.
Working Instruction A-128. Draft. July 1977.
American Public Works Association. Urban Stormwater Management -
Special Report No. 49. 1981.
REFERENCES
1. American Public Works Association. Urban Stormwater Management - Special
Report No. 49. 1981.
2. Armandes, C., and P. Bedient. Stormwater Detention in Developing
Watersheds. Journal of Environmental Engineering Division, ASCE. April
1980.
3a. U.S. Environmental Protection Agency, Office of Wastewater Management.
Combined Sewer Overflows -- Guidance for Long-Term Control Plan. USEPA
Report No. EPA/832-B-95-002. September 1995.
3b. American Public Works Association. Nationwide Costs to Implement BMPs.
1992.
4. Lager, J. A. et al. Urban Stormwater Management and Technology: Update
and Users' Guide. USEPA Report No. EPA/600/8-77-014. NTIS No. PB 275
654. September 1977.
1-9
-------
Section 2
URBAN STORMWATER: AN OVERVIEW
Stormwater control and disposal have been recognized as serious urban problems
for many centuries. Traditionally, the major goal of Stormwater management
has been to reduce the incidence and severity of flooding. Other goals
include control of soil erosion and sedimentation, and protection and
enhancement of Stormwater as a groundwater recharge source. Increasingly,
Stormwater also is being recognized as a significant source of urban water
pollution.
Hydrology is the study of the occurrence, distribution, movement, and
properties of the water on the earth. Since there are numerous texts
available that discuss hydrology in detail, a review of the hydrologic
fundamentals will not be presented here. If the reader needs additional
background in this area, a typical text is that of W. Viessman _et__a]_. [1].
However, in brief summary, the effects of urbanization on the way in which
Stormwater runoff is generated are to increase the rainfall excess portion of
the hyetograph and to increase the peak of the hydrograph. Hydrographs and
hyetographs for the same rain event on a watershed under both developed and
undeveloped conditions are shown in Figure 3. As can be seen, the time to
peak decreases and the runoff volume increases with urbanization.
The pollutants often found in urban runoff and CSOs are described and their
importance assessed. Commonly applied Stormwater control methods to minimize
adverse impacts are introduced.
URBAN STORMWATER POLLUTION
As precipitation falls on and travels through the urban environment, it
contacts and is contaminated by pollutants. Falling through the air,
precipitation dissolves (e.g., "acid rain") and collects pollutants such as
smog, dust, particulate matter, vapors, gases, etc. While typically the
airborne contaminant pickup is minor [3], it may logically be assumed to be
directly related to common air pollution indicies and thus is intensified
downwind of stack and vehicular emissions.
Additional pollutants are dislodged and may be dissolved or suspended in the
runoff as raindrops fall on the ground, structures, plants, chemical
stockpiles, and littered surfaces. Thus, density of development, soil types,
slopes and credibility, and basic neighborhood cleanliness are logical further
indicators of potential pollution. A diminishing source of available source
pollutants due to washoff may account for reduced pollution loads from
sequential storms or sequential periods within a single storm.
2-1
-------
HYETOGRAPH LEGEND
mm/ti
PREOEVELOPMENT
RAINFALL
HYETOGRAPH
POSTDEVELOPHENT
RAINFALL
HYETOGRAPH
LOSSES
RUNOFF
1 ,200
1 , 000
800
600
400
200
POSTDEVELOPMENT
HYDROGRAPH
PREDEVELOPMENT
HYDROGRAPH
.0
2.0 3.0 4.0 5.0
6.0
THE, HOURS
Figure 3. Hyetographs and hydrographs for watershed under
predevelopment and postdevelopment conditions(adapted from [2]),
2-2
-------
Finally, the collection system itself (whether open channels or closed
conduits, separate or combined sewer systems) provides for additional
contaminant contact opportunity from which the pollutant gain may be
proportional to prestorm solids deposition (source material) and flow
velocities and turbulance (resuspension ability). In combined sewer systems,
obviously the sanitary and industrial wastewater is a major factor. In terms
of oxygen demanding substances, University of Florida investigators [4] found
"national average" CSO concentrations to be approximately four times the
equivalent separate stormwater discharges. Similarly, bacterial contamination
from untreated CSO discharges is typically two orders of magnitude higher than
separate stormwater, although both are far above limits considered safe for
body contact recreational use.
Representative Concentrations
Urban stormwater, whether conveyed in separate sanitary and storm sewer
systems or combined sewer systems, essentially has all of the pollutants found
in sanitary wastewaters. The concentration levels, however, vary markedly
from location to location, storm to storm, and within a single storm.
Representative mean values based on end-of-the-pipe samples from separate
storm sewer and combined sewer systems are shown in Tables 1 and 2,
respectively. The averages of these mean values are compared to
representative uncontaminated background receiving water levels and "typical"
raw sanitary wastewater in Table 3. The reader is cautioned that these values
represent normalized values from a broad mix of observations and that site
specific monitoring and sampling is essential for detailed goal setting and
design.
Table 1. AVERAGE POLLUTANT CONCENTRATIONS IN STORMWATER RUNOFF [5]
mg/L Unless Otherwise Noted
City
Atlanta,
Georgia
Des Moines ,
Iowa
Durham,
North Carolina
Knoxville,
Tennessee
Oklahoma City,
Oklahoma
Tulsa,
Oklahoma
Santa Clara,
California
Pullach,
Germany
Average (not
weighted)
Range
TSS
287
419
1,223
440
147
367
284
158
415
147-1,223
VSS
__
104
122
--
70
53
88
53-122
BOD
9
56
7
22
12
20
11
20
7-56
COD
48
170
98
116
86
147
125
113
48-170
Kjeldahl
nitrogen
0.57
2.09
0.96
1.9
2.08
0.85
--
1.41
0.57-2.09
Total
nitrogen
0.82
3.19
2.5
3.22
--
5.8
3.11
0.82-5.8
Phosphorus
0.33
0.56
0.82
0.63
1.00
0.23
0.62
0.33-1.00
Ortho-
phosphate
0.15
-
0.30
1.00
0.38
0.46
0.15-1.00
Lead
0.15
--
0.46
0.17
0.24
0.75
--
0.35
0.15-0.75
Fecal
col i forms3
6,300
230
20,300
40,000
420
--
13,500
230-40,000
a. Organisms/100 ml.
2-3
-------
Table 2. AVERAGE POLLUTANT CONCENTRATIONS IN
COMBINED SEWER OVERFLOWS [5]
mg/L Unless Otherwise Noted
Location
Des Moines,
Iowa
Hi Iwaukee,
Wisconsin
New York City,
New York
Newton Creek
Spring Creek
Poissy,
France0
Racine,
Wisconsin
Rochester,
New York
Average
(not weighted)
Range
TSS
413
321
306
347
751
551
273
370
273-551
VSS
117
109
182
387
154
--
140
109-182
BOD
64
59
222
111
279
158
65
115
59-222
COD
264
481
358
1,005
--
367
264-481
Kjeldahl Total
nitrogen nitrogen
4.3
4.9 6.3
16.6
43
__
2.6
3.8 9.1
2.6-4.9 4.3-16.6
P04-P
1.86
1.23
4~.l*>
17"
2.78
--
1.95
1.23-2.78
Fecal
OP04-P Lead conforms3
1.31
0.86
0.60
..
0.92 - 201,000
0.88 0.14 1,140,000
1.00 0.37 670,000
0.86-1.31 0.14-0.60 201,000-1,140,000
a. Organisms/100 ml.
b. Total P (not included in average).
c. Not Included in average because of high strength of municipal wastewater when compared to the United States.
Table 3. COMPARISON OF STORMWATER DISCHARGES
TO OTHER POLLUTANT SOURCES [5]
mg/L Unless Otherwise Noted
Kjeldahl Total Total Fecal
TSS VSS BQD5 COD nitrogen nitrogen P04-P OP04-P Lead coliformsa
Background 5-100 -- 0.5-3 20 0.05-O.Sb 0.01-0.2° <0.01 <0.1 <1
levels [6]
Stormwater 415 90 20 115 1.4 3.10 0.6 0.4 0.35 13,500
runoff
Combined 370 140 115 367 3.8 9.10 1.9 1.0 0.37 670,000
sewer
overflow
Sanitary 200 150 200 500 40 40 10 0.05- 0.17 750,000
wastewater 1.27
a. Organisms/100 mL.
b. N03 as N.
c. Total phosphorus as P.
Note: Background levels are the nonpoint pollution loads that can arise from land that has
not been disturbed by man's activities.
For Tables 1-3: TSS - Total Suspended Solids; VSS - Volatile Suspended Solids;
BOD - Biochemical Oxygen Demand; COD - Chemical Oxygen Demand
2-4
-------
Concern over toxicity and trace substances in wastewater led USEPA to establish
an extensive list of priority pollutants. As an aid to future investigators,
laboratory results from nationally collected grab samples processed by USEPA's
Region II laboratory are reported in Tables 4 and 5. The data represent the
liquid fraction and sediment fraction analyses, respectively. The
characteristically high copper, lead, and zinc values for highway runoff are
apparent as well as a predominance of petroleum-derived hydrocarbons in all
samp!es.
In combined systems and to a lesser extent in separate storm drains,'many
investigators report a characteristic pattern of high pollutant concentrations
early in the period of storm runoff followed by diminishing concentrations as
the storm progresses and presumably as the source of readily suspendable
material and litter is washed away. Data that have been normalized to
accentuate this "first flush" effect are shown in Figure 4.
Three factors appear to be of major significance in the degree to which a first
flush is observed: (1) the quantity and location of readily suspended or
resuspended material; (2) the intensity of the runoff (since suspension and
resuspension are a function of flow velocity); and (3) the size of and time of
travel, in the collection system. If the collection system is large, a
downstream observer will see a stormwater mixture of runoff components having
vastly different travel times, thus dampening and extending concentration
peaks. Similarly, the dependency on high intensity runoff may place the first
flush anywhere in the storm according to the rainfall pattern, providing a
similar scouring opportunity has not yet occurred.
Impacts of Urbanization
The tendency of urban development to intensify stormwater pollution was
introduced earlier. The following example illustrates probable impacts on a
macroscale.
Under a contract for the Association of Bay Area Governments [8], simplified
mathematical model techniques coupled with a local monitoring program were used
to address regional stormwater pollution over the nine county 13,000 km2 (5,000
mi2) area. Comparisons of annual pollutant loadings to San Francisco Bay under
pre- and post-development conditions are shown in Table 6. Predevelopment
loadings were simulated by setting runoff and pollution unit parameters to
monitored present open-space values. (For example, in 1981, 87% of the total
land area remained in a nominally undeveloped-open-space condition.)
2-5
-------
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Diethyl phthalate
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Bis(2-ethylhexyl) phth
Butylbenzyl phthalate
Di-n-octyl phthalate
2-6
-------
Table 4 (Concluded)
Combined
Parameter-t Site/sample
Phenols
Phenol
Pentachl orophenol
Phenol ics
2, 4-Dimethyl phenol
2, 4, 6-Trichl orophenol
Nitroso compounds
N-Nitrosodiphenylamine
Pesticides
4,4'-DDE
Metals
Antimony
Arsenic
Beryl! ium
Cadmi urn
Chromium
Copper
Lead
Mercury
Nickel
Selenium
Silver
Thallium
Zinc
Notes :
1. A - Boston, Massachusetts;
key-»- A
18.0
--
--
<60.0
1-0.60
<4.0
<3.0
<10.0
110.0
i/IOO.O
<0.2
^20.0
<0.70
<10.0
<0.40
210.0
B
--
<20.0
<1.0
<1.0
<3.0
<7.0
<2.0
<30.0
<0.2
<9.0
<0.8
<3.0
<0.4
20.0
B - Clatskanie,
wastewater
C
0.32
--
--
<20.0
1-3.0
<1.0
1-2.0
i-lO.O
40.0
1-30.0
0.25
<9.0
<10.0
<3.0
<10.0
i-lOO.O
Oregon; C
D
--
--
<20.0
<0.4
<1.0
<3.0
<7.0
<2.0
<30.0
<0.20
<9.0
i/I.O
<3.0
<0.40
i-lO.O
E
0.70
0.90
--
--
<40.0
<0.20
<3.0
<2.0
<7.0
14.0
i/IOO.O
<0.2
<9.0
1/1.0
<6.0
<0.2
65.0
F!
0.30
0.90
0.50
0.30
--
<20.0
1-5.0
<2.0
<4.0
<9.0
1-40.0
107.0
0.45
<40.0
<3.0
<8.0
<2.0
140.0
- Rochester, New York;
F2 G!
0.70 0.34
0.90
0. 70
--
-
--
<20.0 <20.0
1-4.0 <2.0
<2.0 <1.0
iJ.O 1,7.0
<9.0 1.20. 0
<40.0 24.0
113 360.0
0.30 <0.20
<40.0 <9.0
<3.0 <10.0
<8.0 <3.0
<2.0 <10.0
i,90.0 330.0
0 - St. Helens,
Stormwater runoff
G2
0.85
--
--
--
<20.0
<2.0
<1,0
1,5.0
i-lO.O
11.0
310.0
0.56
<9.0
<10.0
1-5.0
<10.0
-x-90.0
Oregon;
Hi
__
--
--
<20.0
15.0
<1.0
1-5.0
1-20.0
47.0
1-80.0
<0.2
<30.0
<4.0
<7.0
<4.0
170.0
H2
0.40
--
-
--
<20.0
1-3.0
<1.0
1-3.0
36.0
49.0
400.0
<0.2
<30.0
<4.0
<7.0
<4.0
260.0
I
1.90
--
0.80
--
<20.0
i-I.O
<3.0
9.8
43.0
280.0
2600.0
<0.20
1-40.0
«0.80
<2.0
<0.40
780.0
E - Austin, Texas; FI ,
J K
_-
--
--
<60.0
1-2.0
<4.0
<3.0
<10.0
1-20.0
1-70.0
0.67
<10.0
1-0.80
<10.0
<0.40
120.0
F2 - San Jose,
L
--
--
<20.0
<0.4
<1.0
<3.0
<7.0
<2.0
<30.0
0.30
<9.0
<0.8
<3.0
<0.40
23.0
California
(Coyote Creek): G-|, G2 - Orlando, Florida,(Lake Eola); HI, H2 - Trenton, New Jersey (Lawrence Shopping Center); I - Milwaukee, Wisconsin (Interstate
94); J - New Roxbury, Massachusetts; K - Rochester, New York; L - St. Helens, Oregon.
Samples were predominantly grab samples, collected by different observers in 1979-80, and processed on a time available basis, at the USEPA,
Region II Surveillance and Analysis Division Laboratory, in Edison, New Jersey, under the supervision of the USEPA Project Officer, Richard
Field, Chief, Storm and Combined Sewer Section, Municipal Environmental Research Laboratory - Cincinnati in Edison, New Jersey [7].
-------
ro
i
oo
Table 5. PRIORITY POLLUTANT MEASURED IN URBAN STORMWATER SYSTEMS - SEDIMENT FRACTION
Concentrations in yg/kg (except as noted)
Parameter^ Site/sample
Aroma tics
Benzene
Chlorobenzene
1 ,3-D1chlorobenzene
1 ,4-Dichlorobenzene
Ethyl benzene
Tol uene
1 ,2-Dichlorobenzene
Polynuclear aromatic
hydrocarbons
1 ,2-Benzanthracene
3,4-Benzofl uoranthene
11,1 2-Benzof 1 uoranthene
Benzo(a)pyrene
Ideno(l,2,3-cd)pyrene
1 ,12-Benzoperylene
Acenaphthene
Acenaphthylene
Anthracene
Chrysene
Fl uoranthene
Fl uorene
Naphthalene
Phenanthrene
Pyrene
1 ,2r5,6-Dibenzanthracene
key* A
__
,
17.0
5,900.0
2,300.0
110,000.0
56,000.0
56,000.0
75,000.0
40,000.0
26,000.0
11,000.0
140,000,0
110,000,0
110,000.0
11,000.0
4,900.0
140,000.0
81,000.0
--
B
310.0
310.0
20.0
910.0
1,100.0
530.0
530.0
^2,800.0
720.0
1,100.0
1,000.0
..
720.0
800.0
--
Combi ned
C
2.0
--
42.0
4,100.0
2,300.0
2,300.0
25,000.0
740.0
810.0
270.0
7,000.0
4,100.0
4,200.0
300.0
3,000.0
~
sewers
D
..
560.0
560.0
8.60
32,000.0
1,600.0
870.0
870.0
<\4, 500.0
--
930.0
1,600.0
1,400.0
930.0
1,200.0
El
__
0.60
73.0
73.0
1.10
94.0
19,000.0
13,000.0
13,000.0
20,000.0
3,900.0
5,800.0
440.0
1,100.0
10,000.0
19,000.0
15,000.0
1,100.0
170.0
10,000.0
13,000.0
E2
2,000.0
1,100.0
1,300.0
150.0
16,000.0
12,000.0
12,000.0
12,000.0
4,600.0
4,200.0
2,400.0
20,000.0
16,000.0
18,000.0
2,200.0
900.0
20,000.0
13,000.0
Storm
F
__
12.0
100.0
2,400.0
1,200.0
1,200.0
1,200.0
260.0
560.0
110.0
1,100.0
2,400.0
2,000.0
87.0
47.0
1,100.0
1,600.0
--
drains or outlets
G H
__
..
__
0.20
1,100.0
1,400.0
1,400.0
600.0
.-
375.0 430.0
1,500.0
1,200.0 2,300.0
210.0
375.0 2,000.0
1,200.0 2,600.0
--
Halogenated hydrocarbons
Tetrachloroethylene
Methyl chloride
Methylene chloride
Chloroform
Hexachloroethane
1,2-Dichloropropane
Dlchlorobromomethane
THchlorofluorome thane
1,2-Trans-dichloroethylene
1,1,1-Trichloroethane
Trichloroethylene
Phthalates
Diethyl phthalate
Di-n-butyl phthalate
Bis(2-ethylhexyl) phthalate
Butylbenzyl phthalate
Di-n-octyl phthalate
16.0
1,500.0
0.20
59,000.0
1,700.0
0.20
1.70
5,600.0
1.30
2,000.0
7,700.0
2,300.0
0.70
1,300.0
2,600.0
190.0
--
300.0
1,100.0
450.0
0.60
850.0
3,800.0
320.0
160.0
360.0
43,000.0
160,000.0
840.0
40,000.0
--
860.0
1,800.0
--
2,500.0
59,000.0
1,200.0
370.0
130.0
-------
Table 5 (Concluded)
Combined sewers
Storm drains or outlets
Parameter* Site/sample key-»
ro
i
Phenols
Phenol
Pentachlorophenol
Phenolics
2,4-Dimethylphenol
2,4,6-Trichlorophenol
1.300.0
140.0 1-60.0
32.0
Nitroso compounds
N-Nitrosodiphenylamine
Pesticides
4,4'-DDE
Metals, mg/kg
Antimony
Arsenic
Beryll ium
Cadmium
Chromium
Copper
Lead
Mercury
Nickel
Selenium
Silver
Thallium
Zinc
<2.
1.
<1.
1-2.
32.
920.
0
70
0
0
0
0
560.0
0.
18.
<0.
<0.
i-O.
700.
63
0 '
02
40
01
0
<3.0
2.80
<0.20
2.30
26.0
200.0
160.0
0.78
13.0
0.20
<0.50
<0.20
910.0
3.30
4.80
i-O.lO
<0.06
28.0
140.0
370.0
2.10
15.0
0.06
2.10
<0.20
380.0
1-8.
3,
<0.
2.
25.
1,000.
360.
0.
23.
0.
8.
<0.
0
.50
.2
.80
.0
.0
0
.74
,0
.12
90
20
710.0
12.0
3.30
i-O.lO
0.50
42.0
83.0
410.0
0.32
19.0
0.27
6.10
<0.20
120.0
1-6.0
2.90
1-0.30
5.5
110.0
1,000.0
37.0
13.0
67.0
<0.02
i-l.O
<0.20
300.0
180.
16.
2
<0
.0
.0
.30
.20
1-0.80
14.
17.
1,300.
0.
5.
2.
<0.
<0.
.0
.0
.0
.26
.90
.80
.50
.20
160.0
--
<3.0
2.80
<0.20
i-l.O
8.10
7.0
24.0
2.39
1-3.0
1.40
<0.5
<0.20
37.0
Notes:
1. A - Boston, Massachusetts; B - Clatskanie, Oregon; C - Rochester, New York; 0 - St. Helens, Oregon; E-J , Eg - Syracuse, New
York; F - Austin, Texas; G - San Jose, California (Coyote Creek); H - Trenton, New Jersey (Lawrence Shopping Center).
2. Samples were predominantly grab samples, collected by different observers in 1979-80, and processed on a time available
basis, at the USEPA, Region II Surveillance and Analysis Division Laboratory, in Edison, New Jersey, under the supervision
of the USEPA Project Officer, Richard Field, Chief, Storm and Combined Sewer Section, Municipal Environmental Research
Laboratory - Cincinnati in Edison, New Jersey [7].
-------
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2-10
-------
Table 6. SURFACE RUNOFF LOADINGS TO BAY,
PRE- AND POST-DEVELOPMENT [8]
Average annual
loading, 1000s lba
Parameter 1800 1975 Increase
Total
Total
Total
Total
BOD
SS
nitrogen
phosphorus
3
790
1
,190
,091
,193
89
18
765
3
,590
,982
,677
428
5.8:
0.97:
3.1:
4.8:
:1
: 1
:1
:1
a. Based on 1969-1970 and 1970-1971 water
year rainfall data.
Ib x 0.4536 = kg
The following observations were made:
It can be seen that organic loadings have probably increased about 6-
fold and nutrients about 4- to 5-fold while total solids have
remained relatively unchanged. Thus it appears that the Bay, as in
most shallow embankments, has always been turbid and no actions by
man will likely reverse this condition. With respect to dissolved
oxygen, the combined actions of Bay filling and increased organics
discharged have, no doubt, worsened conditions in backwaters and
areas of poor circulation. While nutrient loadings have grown
substantially, their impact when compared to point discharged remains
small. [8]
In terms of BOD loadings, it was further observed that while the nine county
average change was an increase of 5.8:1, the range between counties was
greatvarying from 124:1 for the fully developed and combined sewered San
Francisco to 1.7:1 for the predominantly rural and agricultural Sonoma County.
Receiving Water Assessments
The approach to receiving water protection and restoration in the United
States over the past decade has entailed three principal activities: (1) a
definition of beneficial uses and evaluation of the receiving waters
assimilative capacity with respect to preserving or restoring these uses;
(2) a systemized waste load allocation between competing point and nonpoint
discharges within the limits of this calculated assimilative capacity; and
(3) a national permitting program for dischargers to provide a vehicle for
enforcement and compliance monitoring. The process was simplified by adoption
of the 1972 Clean Water Act which set an effluent-based standard of secondary
treatment for all publicly owned treatment works. In the case of separate
stormwater and CSO treatment facilities, latitude was provided to permit a
case-by-case approach at the discretion of USEPA Regional Administrators.
2-11
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Field and Turkeltaub [9] in an overview of receiving water impacts of
stormwater and CSO discharges conclude that "under certain conditions storm
runoff can govern the quality of receiving waters regardless of the level of
dry-weather flow treatment provided." Examples cited include Milwaukee.
Wisconsin; Syracuse, New York; Seattle, Washington; Orlando, Florida; San Jose,
California; and New York City, New York. Principal pollutants and impacts
include bacterial contamination, toxic substances, sediment loads, and oxygen
demand and depletion.
Heaney, et al. [10] summarized data from 248 urbanized areas in an attempt to
quantify receiving water impacts from urban stormwater. The present data base
was found to be poor; numerous definitions of "problems" are being used; and
"...relatively little substantive data to document impacts have been
collected." Impacts found were most noticeable in small streams, but impacts
were difficult to isolate from other sources such as municipal and industrial
wastes. Accidental or deliberate discharges from point sources under wet-
weather conditions were sometimes the primary cause of wet-weather impacts.
URBAN STORMWATER CONTROL
Control of urban stormwater has been practiced for a number of centuries. The
cloaca maxima, a storm drain constructed for the ancient Roman Forum, is still
in use today. The primary goal of urban stormwater control has been to reduce
the incidence and severity of flooding. In addition to direct conveyance
facilities, the approach to flood control has often been to attenuate, or slow,
the flow of runoff so that the peak rate of flow is reduced and the urban
runoff hydrograph is made to more closely approximate the predevelopment
hydrograph. The urban hydrograph from Figure 3 is reproduced in Figure 5,
along with a storage facility attenuated hydrograph for the same watershed and
storm.
Flow attenuation may be accomplished by increasing the permeability of the
sides and bottom of unlined channels and basins, increasing the roughness of
the flow surfaces, or by constructing storage facilities in which the runoff of
CSO is temporarily stored and released slowly. Where earthen storage
facilities take advantage of the permeability of the sides and bottom, the
2-12
-------
runoff volumes are reduced. The design of storage facilities where soil
permeability is a major factor is described in Section 5. The design of
detention storage facilities is described in Section 6. Sedimentation
facility design is discussed in Section 7.
N.
1 ,200
1,000
800
600
400
200
Figure 5. Hydrograph of a watershed showing
effects of storage.
Water quality benefits may be realized from storage options in many ways,
principally;
t Increased effectiveness of existing treatment facilities through flow
equalization
Increased interception of pollutant-laden runoff flows with
subsequent diversion to treatment
Reduced erosion and scour by flowrate control
« Groundwater recharge through increased infiltration/percolation
opportunity
Treatment by sedimentation during storage
2-13
-------
Treatment by biological stabilization in cases of extended, several
days to several weeks, storage
Reduced wastewater collection/treatment system overloads and bypasses
when utilized,in an integrated manner
Storage application should be approached in a systemized manner, such as
through the classification system recommended in Section 3. Virtually every
urban stormwater control will use storage to some degree; however storage by
itself may not offer the most cost-effective solution.
Optimal supplemental controls to storage include best management practices,
high-rate unit processes, and innovative systems approaches that optimize
treatment and controls on a total systems basis. Extensive discussion of
these options as applied to separate urban stormwater and CSO systems is
available in the literature [5, 11, 12].
Best management practices (nonstructural and low structurally intensive
alternatives) offer considerable promise as the first line of action to
control urban runoff pollution. By treating the problem at its source, or
through appropriate legislation curtailing its opportunity to develop,
multiple benefits can be derived. These include lower cost, earlier results,
erosion/flood control benefits, and an improved and cleaner neighborhood
environment.
Physical treatment alternatives are primarily applied for solids removal from
waste streams, and are of particular importance to CSO treatment for removal
of settleable and suspended solids and floatable material. Physical treatment
systems have a demonstrated capability to handle high and variable influent
concentrations and flowrates and operate independently of other treatment
facilities, with the exception of treatment and disposal of the sludge/solids
residuals. The principal disadvantage relates to those periods of time when
equipment sits idle during periods of dry weather. When implemented on a
dual-use basis as either pretreatment or effluent polishing of conventional
treatment plant flows, reduced capital investments may be realized.
The size and complexity of urban runoff management programs are such that
there is a need for an integrated approach to their solution. The solution is
most often a combination of various best management practices and unit process
applications. Demonstrated implementation progress to date is predominately
in the areas of CSO control, excess flow treatment from heavily infiltrated
sanitary systems, and best management practices applications in new
communities.
REFERENCES
1. Viessman, W., et al. Introduction to Hydrology. Intext
Educational Publishers. 1972.
2-14
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2. Wanielista, M. Stormwater Management Quantity and Quality. Ann Arbor
Science. 1978.
3. Chambers, G. M. and C. H. Tottle. Evaluation of Stormwater Impoundments
in Winnipeg. Report SCAT-1 Environmental Protection Service, Environment
Canada. April 1980.
4. University of Florida and The American Public Works Association.
Nationwide Evaluation of Combined Sewer Overflows and Urban Stormwater
Discharges, Volume II: Cost Assessment and Impacts. Report for the U.S.
Environmental Protection Agency, EPA-600/2-77-064b. NTIS No. PB 266
005. March 1977.
5. Lager, J. A., et al. Urban Stormwater Management and Technology: Update &
Users' Guide. USEPA Report No. EPA-600/2-77-014. NTIS No. PB 275 654.
September 1977.
6. McElroy, A. D., et al. Loading Functions for Assessment of Water
Pollution From Nonpoint Sources. USEPA Report No. EPA-600/2-76-151. NTIS
No. PB 253 325. May 1976.
7. Analyses by USEPA Region II Surveillence and Analysis Division, Water
Quality Laboratories, Edison, New Jersey.
8. Metcalf & Eddy, Inc. Surface Runoff Modeling. Report to the Association
of Bay Area Governments, San Francisco Bay Region. April 1978.
9. Field, R. and R. Turkeltaub. Urban Runoff Receiving Water Impacts:
Program Overview. The Journal of the Environmental Engineering Division
ASCE. February 1981.
10. Heaney, J. P., et al. Nationwide Assessment of Receiving Water Impacts
from Urban Stormwater Pollution: Volume 1. Summary. USEPA Report No.
EPA-600/S2-81-025. NTIS No. PB 81-161 812. March 1981.
11. Lager, J. A. and W. G. Smith. Urban Stormwater Management and
Technology: An Assessment. USEPA Report No. 670/2-74-040. NTIS No. PB
240 687. December 1974.
12. Lynard, W.G., et al. Urban Stormwater Management and Technology: Case
Histories. USEPA Report No. EPA-600/8-80-035. NTIS No. PB 81-107153.
August 1980.
2-15
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Section 3
TERMINOLOGY AND CLASSIFICATION
OF STORAGE AND/OR SEDIMENTATION FACILITIES
The term "Storage and/or Sedimentation Facility" is used here as a collective
term for various arrangements for balancing, detaining, and storing the flow of
stomwater (with or without any sedimentation) in combined sewer systems or the
storm drainage portion of separate sewer systems.
For ease of reference and consistency, certain terminology and classifications
of control systems have been adopted for use herein. The basic categories for
classification of storage and/or sedimentation facilities are:
1. The main function principles that are applicable to both separate
sanitary sewer and storm drainage systems and to combined sewer
systems:
Retention
Detention
Sedimentation
2. The general placement of the storage and/or sedimentation facility
or practice within the collection network:
Source controls (including onsite and upstream facilities)
In-system controls (storage developed within the transport
network by regulators, flow restrictors, etc.; and storage
developed in offline basins located along the transport
network)
Downstream (end-of-pipe) controls
3. The technical configuration of the various facilities based on
location, topography, and local conditions.
TERMINOLOGY
Various storage and/or sedimentation systems have been described in the
technical literature in this field. To avoid any unnecessary confusion or
3-1
-------
misunderstandings, the terms most commonly used in this manual are defined as
follows:
Combined SewerA sewer designed to receive both stormwater runoff and
municipal sewage (sanitary and industrial wastewater).
Combined Sewer OverflowThe untreated discharge of a combined sewer system
that generally occurs when the dry-weather flow treatment or interceptor
capacity is exceeded.
Combined Sewer SystemA system wherein storm drainage is commingled and
conveyed with municipal sewage.
Detention StorageThe form of storage where stormwater runoff or CSO are
stored temporarily. Stored flows are subsequently returned to the sewerage
system at a reduced rate of flow when downstream capacity is available, or the
flows are discharged to the receiving water with or without further treatment.
Downstream Control OptionsOnline or offline storage and/or sedimentation
facility located immediately upstream of the receiving water or final
treatment facility.
Downstream StorageA storage facility designed to serve as a primary
sedimentation treatment device and/or as a flow equalization device
immediately upstream of a treatment works or receiving water.
Dry PondsPonds which are normally dry and fill in response to storm
conditions. The usable storage volume may include, where measureable, pore
space in the basin walls and floor which are inundated rapidly in the filling
process.
Inlet Control--Detention storage where stormwater is temporarily stored on
impervious surfaces before it enters the sewer system.
Inline StorageIn-system control storage facilities, either in series or
parallel with the collection system, which are filled and emptied only by
gravity.
In-System Control sOptions wherein basins, tunnels, or caves in the
collection network or temporary excess pipe capacity in the collection network
are used for storage through the use of regulator devices (including both
static and mechanical regulators) and pumping facilities.
Off!ine StorageIn-system control storage facilities where pumping is
required to convey flow to the storage facility or to return the stored water
to the sewer system. An advantage to offline storage is the easier
flexibility to select when the stored flow is to be returned to the sewer for
treatment or discharge.
Onsite StorageStorage of stormwater in natural ditches, open ponds or
basins, rooftops, parking lots, or recreational facilities (athletic fields,
tennis courts, etc.) before the stormwater reaches a sewer network.
3-2
-------
Retention StorageStorage where stormwater runoff is captured and disposed of
through infiltration, percolation, and evaporation. An emergency overflow
structure must be included to prevent structural damage to the facility by
occasional overflows.
Sanitary Sewer--A sewer that carries liquid and water-carried wastes from
residences7 commercial buildings, industrial plants, and institutions,
together with relatively low quantities of groundwater, stormwater, and .
surface waters that are not admitted intentionally.
Separate Sewer SystemA system wherein stormwater runoff is conveyed
independent of sanitary and industrial wastewater. In reality, most separate
stormwater systems contain at least a few cross-connections for relief of
overloaded sanitary sewers.
Source Control Options Facilities and practices, which initiate corrective
action close to the origin of the stormwater runoff, to mitigate stormwater or
CSO adverse impacts.
Storm SewerA sewer that carries stormwater runoff, street wash and other
washwaters, or drainage, but excludes domestic sewage and industrial wastes.
Stormwater RunoffHater resulting from precipitation which runs off freely
from the surface, or is captured by storm sewer, combined sewer, or to a
limited degree sanitary sewer facilities.
Uet Ponds--Ponds which are normally partly filled and in which storage is
attained by a change in water surface elevation.
CLASSIFICATION
The classification or systemization of storage and/or treatment facilities can
be based on the technical configuration (i.e., area available, topography,
adjacent land use, local conditions, etc.); location within the sewerage
network (i.e., prior to flow entering a pipe network, within the pipe network,
or just upstream of a treatment plant or the receiving water); or function
(i.e., retention, detention, or sedimentation).
The classification scheme and basic terminology used in this manual and shown
in Figure 6 are based on location within the sewer network. However, the
relationship of the classification and application of the various control
options to separate storm sewer and combined sewer systems is shown in
Figure 1. In current practice, source control retention and detention storage
applies to separate storm sewer systems and to combined sewer systems upstream
of the introduction of sanitary or industrial wastewater. To date, in-system
and downstream controls have been applied mainly on combined sewer systems.
An example of an in-system or downstream control on a storm sewer would be a
basin, either inline or offline, used for flood control. Such a basin can be
designed to take advantage of the sedimentation that occurs in a storage
facility to improve the quality of the discharged flow while controlling the
discharge rate.
3-3
-------
RAINFALL
SOURCE
CONTROL
IN-3YSTEM
CONTROL
DOWNSTREAM
CONTROL
INLINE
STORAGE
ft£WER NETWORK
PIPE PACKAGE
LINED BASIN
CAVE
TUNNEL
OFFLINE
STORAGE
PIPE PACKAGE
LINED BASIN
CAVE
TUNNEL
DOWNSTREAM
STORAGE
LINED BASIN
OPEN POND
CAVE
TUNNEL
TREATMENT PLANT
WETLAND
t
HJLET
CONTROL
ROOFTOP STORAGE
PARKING AREA
PLAZA
h-^-
*
ONSITE
STORAGE
DITCH
DRY POND
WET POND
LINED BASIN
1
t
RETENTION
STORAGE
INFILTRATION
PERCOLATION
POROUS PAVEMENT
WET POND
DRY POND
(END OF PIPE OR AT
TREATMENT PLANT)
RECEIVING WATER
Figure 6. Schematic classification of storage
and/or sedimentation facilities.
3-4
-------
SOURCE
CONTROL
OPTIONS
(INCLUDES
ONSITE
MEASURES)
IN-SYSTEM
CONTROL
OPTIONS
DOWNSTREAM
CONTROL
OPTIONS
STORM SEWER
SYSTEMS
COMBINED
SEWER
SYSTEMS
RETENTION STORAGE
WET PONDS
DRY PONDS
OTHER
DETENTION STORAGE
SYSTEM INLET CONTROLS
SWALES. WET-,
DRY-, LINED PONDS
UPSTREAM STORAGE/
8ED. BASINS
OTHER
INFREQUENT
APPLICATIONS**
DETENTION STORAGE
FLOOD BASIN
MANAGEMENT FOR
QUANTITY AND
QUALITY
ENHANCEMENT
OPTIONS SAME
AS FOR
BTORM SEWER
8Y8TEMS
INLINE AND OFFLINE STORAGE
CONTROLLED STORAGE
IN PIPE NETWORK
TUNNEL 8TORAGE
OTHER
DOWNSTREAM DETENTION
8TORAGE/8ED. BASINS
INLINE
OFFLINE
OTHER
"DESIGNED FOR LIMITED FLOW CAPTURE VERSUS ESSENTIALLY FULL FLOW
CAPTURE FOR RETENTION STORAGE ALTERNATIVES.
**WHERE NECESSITATED BY WATER QUALITY CONSIDERATIONS.TREATMENT
LAS IN C80 OPTIONS) SHOULD BE EVALUATED (I.E.. INTERCEPTION AND
DIVERSION OF DRY-WEATHER SPILLS TO TREATMENT).
Figure 7. Classification scheme based on type of sewer system.
3-5
-------
The relationship of the main grouping, technical configuration, and main
function principle is shown in Table 7. A schematic of the classification of
storage and/or sedimentation facilities according to their basic function of
principle is shown in Figure 8.
Table 7. CLASSIFICATION OF STORAGE
AND/OR SEDIMENTATION FACILITIES
Main group
Source control
In-system control
Downstream control
Technical
configuration
Rooftop
Parking lot
Plaza
Ditch
Dry pond
Wet pond
Lined basin
Porous pavement
Sewer network
Pipe package
Lined basin
Cave
Tunnel
Lined basin
Open pond
Cave
Tunnel
Treatment plant
Main
Retention
.
-
-
X
X
X
-
X
_
-
-
-
-
_
-
-
-
-
function
Detention
X
X
X
X
X
X
X
-
X
X
X
X
X
X
X
X
X
X
principle
Sedimentation
.
-
-
-
X
X
X
-
_
-
-
-
-
X
X
X
X
X
There are three main function principles associated with storage and/or
sedimentation facilities: (1) retention storage, (2) detention storage, and
(3) sedimentation. Any storage and/or sedimentation facility can be designed
primarily to meet any one of the main function principles (retention and
sedimentation or detention and sedimentation).
The main groupings of storage and/or sedimentation facilities by location
within the sewerage network are: (1) source control, (2) in-system control,
and (3) downstream control. Within these main groupings a variety of
technical configurations can be accommodated.
Many of the technical configurations can be used to satisfy one or more main
function principles or main groupings. The technical configurations include:
Percolation basins
Drainage swales
Dry wel Is
Trenches
Porous pavement
Blue-green storage
Rooftop ponding
Parking lots
3-6
-------
Inlet
Inlet
« 4 . ' « *
>« I * I «
* * * *
» . » »
1 . <
t I t » I '
Percolation
a) Retention basin
Overflow
Overflow
Outlet
b) Detention basin
Inlet
Outlet
Sludge removal
c) Sedimentation basin
Figure 8. Classification of storage and/or sedimentation facilities
based on main function of principle (adapted from [1]).
3-7
-------
Pedestrian plazas and malls
Dry ponds
Wet ponds
Check dams
In-pipe storage in existing sewers
Pipe bundles
Concrete basins
Tunnels and caverns
Main Function Principles
Retention Storage. Retention is the storage of stormwater runoff for complete
removal from the surface drainage and discharge system. The primary intent in
the use of retention storage is to allow the stormwater to evaporate and/or to
infiltrate and percolate into the ground.
Percolation of stormwater to the groundwater offers a number of benefits in
addition to controlling stormwater flows. The groundwater is recharged
helping to reduce or prevent ground subsidence; lowering the water table
increases the overburden pressure of the soil located between the original and
the lowered water table causing compression of the soil mass. This is
particularly important in areas where the groundwater basins are being
overdrawn and increased urbanization is reducing normal infiltration.
Percolation of stormwater can be used as water supply recharge in areas where
ground subsidence due to overdraft pumping is not a problem. In addition,
percolation through a soil column has been shown to be very effective in
removing bacteria, oxygen demanding material, and suspended material from
wastewater.
Detention Storage. Detention is the storage of stormwater runoff for delaying
and controlling the release rate to attenuate peak flows in the surface
drainage and discharge system. Detention storage can be accomplished through
the use of system inlet controls, online and offline ponds, or onsite
storage.
Onsite detention refers to the storage of stormwater runoff at or very near
the site of its origin, and its subsequent discharge at a predetermined
release rate. The intent of onsite detention is to utilize existing or
proposed impervious areas or structures to store and control runoff. Typical
examples of such onsite storage include rooftops, plazas, parking lots and
streets, underground structures, ponds, and multipurpose detention reservoirs.
Sedimentation. Sedimentation, as used here, refers to those facilities whose
primary purpose is to separate suspended particles from water by gravitational
settling. Sedimentation basins may also be referred to as sedimentation
tanks, settling basins, or settling tanks.
The objective of treatment by sedimentation is to remove readily settleable
solids and floating material and thus to reduce the suspended solids
content. When applied to storm sewer discharges and combined sewer overflows,
sedimentation is used to provide a moderate detention period to remove a
3-8
-------
portion of the organic solids and a substantial portion of the inorganic
solids that otherwise would be discharged directly to the receiving water.
This can prevent the formation of offensive sludge banks. Sedimentation
basins have also been used to provide sufficient detention periods for
effective chlorination of such overflows [2].
Dual Purpose Detention-Sedimentation. Detention facilities provide flow
and/or flood control by retaining, buffering, and attenuating flows; this also
provides some level of pollution control by detaining flow long enough for
sedimentation or gravity settling to occur. This dual purpose, detention and
sedimentation, should be considered carefully during evaluation and design.
Through appropriate selection of the design parameters for a facility, the
dual detention and sedimentation functions can be optimized. Retrofitting of
existing detention facilities can improve their sedimentation efficiency.
Whenever possible, facilities should be designed to optimize pollutant removal
as well as flow control.
Location in the Sewerage Network
Source Control. Source control methods are used near the source of the
stormwater runoff. Source control refers to facilities and practices, used to
mitigate stormwater or CSO adverse impacts, which initiate corrective action
close to the origin of the stormwater runoff. Inlet control, onsite detention
storage, and retention storage are the main categories of source control
options.
Inlet control refers to methods used to regulate the flow at the inlet to the
sewer system. The detention volume is created by controlling the outflow from
specially prepared areas such as horizontal roofs, parking lots, industrial
grounds, or other impervious areas.
Cases where the detention takes place at or near the source of the runoff but
prior to its entry into the sewer system are referred to as onsite storage.
Local handling and controlled release typify onsite storage. Generally,
onsite storage is used for detaining the runoff from one or more pieces of
real estate. The runoff usually has been transported only a short distance
before it reaches the detention facility. Typical onsite storage facilities
may be in the form of a diked area, ditch, pond, lined basin, or underground
vault with a basin as the inlet.
Retention storage, utilizing the ability of the soil to store or transport
water, is another form of source control. With this type of facility, the
stormwater is allowed to infiltrate into the soil and percolate down to the
groundwater table. In areas unsuitable for infiltration, the stormwater can
be directed into specially built storage excavations in a permeable stratum
below the ground surface. From these, the water can then percolate into the
soil. Retention storage facilities may include percolation basins, drainage
swales, dry wells, trenches, porous pavement, and ponds.
In-System Control. In-system storage includes controlled storage in the pipe
or drainage channel network and/or in tank or tunnel storage. Detention of
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stormwater runoff or combined sewer overflows may be accomplished by two
methods of in-system control: (1) inline storage, and (2) offline storage.
Inline Storage. Inline storage is the use of excess capacity ordinarily
found in a sewer system and/or artificial storage facilities (basins,
channels, tunnels, etc.,) to store stormwater runoff or combined sewer
overflows. The term, as used here, denotes that the facilities are emptied by
means of gravity. A characteristic function in the case of inline storage is
that the detained flow interacts directly with the flow which is being
transported further in the system. Inline storage is a particularly
attractive option in large cities, especially those with large, flat drainage
channels or combined sewers for CSO and urban stormwater runoff.
Inline storage is provided by damming, gating, or otherwise restricting flow
passage to create additional storage by backing up the water in upstream
conduits, channels, tanks or basins. Although simple regulators can be used
to generate inline storage, some inline facilities can require a sophisticated
monitoring and control system to maximize the storage effectiveness. Although
inline storage has usually been applied to combined sewer systems, it is
apparent that it also can be used for separate storm sewer systems. The
objective of inline storage, when applied to combined sewer systems, has been
to maximize the volume of flow directed to a downstream treatment plant while
minimizing the overflow frequency, volume, and associated pollutant load to
the receiving water.
Inline storage systems have been developed and implemented with varying
degress of success, for example, in Seattle [3], Minneapolis-St. Paul [4],
Detroit [5], New York City [6], and Switzerland [7]. In the case of New York
City, most of the storage is drained by gravity while the remainder is
dewatered by pumping.
Offline Storage. Offline storage refers to those cases where the sewer
system, through some form of overflow or pumping arrangement, releases flow in
excess of some predetermined rate to special storage facilities. Such storage
may be in a mined labyrinth, lined or unlined tunnel, cavern, or basin. In
this case, the storage facilities are either filled by pumping to storage and
emptied by gravity or filled by gravity and emptied by pumping the stored flow
back to the sewer system after the storm. In offline storage, the detained
flow is temporarily withheld from all further transportation in the pipe
network. Often, there is no interaction between the stored water and water
which is transported within the system. The objective of offline storage is
similar to that of inline storage except that, in many cases, only the
initial, heavily polluted flow is captured.
Offline storage facilities have been developed and successfully implemented in
Boston [8, 9], Chippewa Falls [10], Saginaw [11, 12], Sacramento [13],
Rochester [14], Chicago [15], Milwaukee [16], and San Francisco [17].
Downstream Control. Conceptually, a downstream storage and/or sedimentation
facility differs from other detention facilities only in its location in the
sewerage systemimmediately upstream of the receiving water and/or treatment
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plant. However, many of its design features are dictated by its function
(interception of tributary flows), performance expectations, and environmental
setting. Downstream control facilities may be in operation at all times (no
bypass option) or in operation intermittently (used at the operator's
option). The general application of downstream control facilities is on
combined sewer systems, so the volume of wastewater receiving treatment at the
dry-weather plant can be maximized. These facilities are usually dewatered
during and after storm events as allowed by the available capacity in the
sewers, interceptors, and treatment plants. If the storage and/or
sedimentation facilities discharge directly to confined waters, the facilities
normally include disinfection capabilities.
Downstream storage and/or sedimentation facilities on separate storm drainage
systems can be used for flow control and/or sediment and fleatables removal.
Such a facility can be used to contain first flush runoff from an urban area
to minimize the pollutant discharge from additional development. The stored
runoff can be discharged slowly to the sanitary sewer system for treatment
before discharge to the receiving water. This approach allows the pollutant
mass discharged to be held constant or reduced even though the pollutant mass
in the runoff may be increased due to the additional urban development. The
flow discharged from such a facility, after the storage has been filled,
usually does not require disinfection since domestic sewage is excluded from
separate storm drainage systems.
Downstream control facilities include lined basins, open ponds, tunnels,
caverns, and buried tanks.
Technical Configuration
The technical design of retention, detention, or sedimentation facilities is
strongly affected by local conditions. The required volume can be created in
a number of ways. There are three principal places to provide the volume:
(1) above ground, (2) at ground level, and (3) underground. The following are
brief descriptions of many of the most common methods for providing the
required volume.
Percolation Basins. Stormwater runoff is conveyed to a suitable open area,
which may or may not be covered by vegetation, where the water is allowed to
percolate into the ground. In urban areas, the percolation basins may be
specially built excavations below the ground surface where the soil is
permeable.
Drainage SwaTes. Drainage swales adjacent to roadways without curbs or in
residential areas can be planned to retain runoff. Such swales must be away
from houses and side yards so that swampy areas do not develop.
Dry Wells. Dry wells can provide a means of storage as well as significant
discharge or dissipation potential in permeable soils. Dry wells should be
deep enough so that possible seepage downhill does not create a problem.
Provision should be included to minimize siltation and clogging of the
permeable soil strata to avoid significant impairment of infiltration
capacity [18].
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Trenches. Perforated drain pipe or open graded rock fill with the use of
filter cloth in a lateral trench can be used for below ground disposition of
stormwater runoff. Silt and debris must be trapped before the water enters
the trench to prevent clogging of the soil strata.
Porous Pavement. Porous type asphaltic pavement can be used for streets or
parking lots where the subgrade has sufficient infiltration and percolation
capacity. In cases where the underlying soil does not meet the infiltration
and percolation capacity requirements, perforated pipes can be used to collect
the stormwater from beneath the subgrade and convey it to another location
suitable for infiltration and percolation. Porous pavement is usually used in
climates where the ground is not subject to freezing. However, with special
design considerations, porous pavement has been used in cold climate locations
such as Rochester, New York.
Lakelet System. A series of small water bodies, arranged in series, which
provide the necessary storage capacity, can be used to provide sediment
control also. Flow introduced into the initial lakelet then flows serially
into the remaining lakelets; in effect, acting as a series of storage
reservoirs. Sedimentation takes place in each lakelet. Flow through such a
lakelet system is usually by gravity with either a gravity discharge or pumped
discharge from the final lakelet. A lakelet system can be used on either
separate storm drainage or on CSO (as in Mount Clemens, Michigan [12]).
Blue-Green Storage. Stormwater storage in urban drainageways traversing
roadways utilizes the roadway embankments as dams and control structures. The
structures generally pass small flows unimpeded while ponding occurs when the
flow exceeds the pass-through rate [18].
Rooftop Ponding. Horizontal roof surfaces can be used to detain stormwater
flow. Such roofs are common for industrial, commercial, and apartment
buildings. Building codes in many parts of the country specify that roofs be
designed to support snow loads or other live loads. The detention is
controlled by a simple drain ring set around the roof drains. As stormwater
begins to pond on the roof, flow is controlled by orifices or slits in the
ring; extreme flows overflow the ring to prevent structural damage to the
roof.
Parking Lots and Streets (Major-Minor Flooding). Stormwater can be detained
on paved parking lots by shallow basins or swales. The parking lot should be
graded to create multiple storage areas like saucers. At each low point, a
catch basin or inlet is used to control the outflow. The outflow control can
be accomplished either by restricting the size of the outlet pipe or by using
a special cover with drilled holes. The arrangement of the ponding areas
within the parking lot should be planned so that pedestrians are
inconvenienced as little as possible.
In what can be termed major-minor flooding, the curbline along streets and
roads can be used to store stormwater. This storage can be developed by
restricting the rate of entry of flow into catchbasins and inlets at the
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curbline. The stormwater then ponds along the curbline and extends out into
the street. The desired depth of the water at the curbline and the spread of
the water on the pavement are often criteria for spacing the inlets. It is
often possible to allow a greater depth of storage along rural roads than
.along urban streets. This method of allowing minor flooding within the
drainage area can be used rather than inline storage or added treatment
capacity to prevent major flooding downstream in the sewer system or
receiving water.
Pedestrian Plazas and Malls. In heavily urbanized areas, effective stormwater
detention can be incorporated into the design of pedestrian plazas, malls, and
other similar type developments. The ponding requirement can be accomplished
using shallow depths. Outlet control devices must be checked and maintained
frequently to avoid flooding problems and to avoid public inconvenience.
Dry Ponds. Permanent ponds are frequently used for surface storage of
stormwater runoff and combined sewer overflows. Dry ponds are small to large
depressions, constructed by usual excavation and embankment procedures, that
provide for controlled release of impounded water but do not retain water
between storms. They can be made to fit well into small developments because
they can be designed and installed as small structures.
Athletic fields can be incorporated into permanent dry ponds used for separate
storm drainage storage. Soccer or football fields, baseball diamonds, and
tennis courts can be made part of a dry pond. The pond can be used as a
recreation area for the surrounding community when it is not in active use for
stormwater storage.
Wet Ponds. Wet ponds are similar to dry ponds but with additional temporary
storage above the normal pool elevation and with provision for controlled
release. They are effective in reducing stormwater runoff and channel erosion
and have the added advantages of providing water recreation opportunities and
of increasing local land value [19].
Check Dams. Check dams or other streambed structures intended to impede and
to pool runoff in open channels may be used for areas that contribute to high
levels of stormwater suspended solids concentrations. Not only are the
adverse impacts of runoff mitigated by the reduced flowrates and lengthened
time of flow concentration, but erosion and sediment control can be
improved [18].
In-Pipe Storage in Existing Sewers. Because storm sewers and combined sewers
are designed to convey maximum flows occurring, say, once in 5 years (50 to
100 times the average dry-weather flow), during most storms there is consid-
erable unused volume within the conduits. This unused volume can be used to
detain flow so the peak discharge rate is reduced or so that additional flow
is directed to treatment facilities. Various types of regulators can be used
to develop this in-pipe storage volume.
Pipe Bundles. The term "pipe bundle" refers to detention facilities that are
designed in the form of a number of parallel pipelines with large diameters.
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The pipe bundle detention facility is usually connected in series to the sewer
network, which means that all of the flow passes through the facility. As a
special case, the detention can take place in a single oversize conduit.
Concrete Basins. Concrete basins probably are the most flexible and the most
commonly used types of installations. The technical configuration of such
basins can be tailored to fit conditions at a given site. Concrete basins can
be connected both in series and in parallel with the sewer network. The
basins can be above ground, at ground level, or below ground as needed to fit
the site. Outflow control devices can include pipes or orifices, weirs,
proprietary devices, or pumps as required.
Tunnels and Caverns. Tunnels and caverns can be used as transport, detention*
or sedimentation facilities for stormwater and combined sewage. Tunnels may
be either inline or offline facilities while caverns (mined labyrinths) are
usually offline facilities. Tunnels may be lined or unlined depending on
their location with respect to the groundwater table and the geological
conditions. Depending on the depth of the tunnel, construction may be by
cut-and-cover methods for shallow tunnels or by underground heading for deep
tunnels.
REFERENCES
1. Stahre, Peter. Flodesutjamning i Avloppsnat (Flow Balancing in Waste
Water Nets). Byggforskningsradet (Construction Research Council). 1981.
2. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. Second Edition. McGraw-Hill Book Company. New York. 1979.
3. Leiser, C.P. Computer Management of a Combined Sewer System. USEPA
Report No.EPA-670/2-74-022. NTIS No. PB 235 717. July 1974.
4. Metropolitan Sewer Board - St. Paul, Minnesota. Dispatching System for
Control of Combined Sewer Losses. USEPA Report No. 11020FAQ03/71. NTIS
No. PB 203 678. March 1971.
5. Watt, T.R., et al. Sewerage System Monitoring and Remote Control. USEPA
Report No. EPA-670/2-75-020. NTIS No. PB 242 107. May 1975.
6. Feuerstein, D.L. and W.O. Maddaus. Wastewater Management Program, Jamaica
Bay, New York; Volume I: Summary Report. USEPA Report No. EPA-600/2-76-
222a. NTIS No. PB 260 887. September 1976.
7. Koral, J. and C. Saatci. Rain Overflow and Rain Detention Basins. 2nd
Edition. Zurich, Switzerland. 1976.
8. Commonwealth of Massachusetts, Metropolitan District Commission. Cottage
Farm Combined Sewer Detention and Chlorination Station, Cambridge,
Massachusetts. USEPA Report No. EPA-600/2-77-046. NTIS No. PB 263 292.
November 1976.
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9. Environmental Assessment Statement for Charles River Marginal Conduit
Project in the Cities of Boston and Cambridge, Massachusetts.
Commonwealth of Massachusetts, Metropolitan District Commission.
September 1974.
10. Liebenow, W.R. and J. K. Bieging. Storage and Treatment of Combined Sewer
Overflows. USEPA Report No. EPA-R2-72-070. NTIS No. PB 214 106.
October 1972.
11. Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan, on
Preliminary Design of the Hancock Street Combined Sewage Overflow Storage
and Treatment Facility. March 16, 1973.
12. Lynard, W.G., et al. Urban Stormwater Management and Technology: Case
Histories. USEPA Report No. EPA-600/8-80-035. NTIS No. PB 81-107153.
August 1980.
13. Sacramento Area Consultants, Sacramento, California. Contract Documents
for Pioneer Reservoir Sump 1 Modifications. Contract No. 1108, Sacramento
Regional County Sanitation District. September 1977.
14. Drehwing, F.J., et al. Combined Sewer Overflow Abatement Program,
Rochester, New York. Volume 1. USEPA Report No. EPA-600/2-79-031a.
NTIS No. PB 81 219602. February 1979.
15. Lager, J.A., et al. Urban Stormwater Management and Technology: Update
and Users' Guide. USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
16. City of Milwaukee, Wisconsin, and Consoer, Townsend and Associates.
Detention Tank for Combined Sewer Overflow, Milwaukee, Wisconsin,
Demonstration Project. USEPA Report No. EPA-600/2-75-071. NTIS No.
PB 250 427. December 1975.
17. Metcalf & Eddy, Inc. City and County of San Francisco Southwest Water
Pollution Control Plant Project. Final Project Report. February 1980.
18. American Public Works Association. Urban Stormwater Management - Special
Report No. 49. 1981.
19. Mason, John M. Jr. On-site Stormwater Detention: An Overview. Public
Works, Vol. 113, No. 1. January 1982.
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Section 4
SYSTEM PLANNING, DESIGN PROCEDURES, AND INTEGRATION
The multivariable and complex nature of stormwater management assessments make
systematic approaches essential. Toward this end it is necessary that system
planning be undertaken to ensure that the solution to one problem will not
create other problems in the future. The system planning should include an
upgrading plan or master plan for the entire system. Usually, the most
economical and effective stormwater management system consists of a
combination of facilities and techniques integrated into an overall pollution
control plan. Storage and/or sedimentation facilities are or should be the
backbone of such an integrated plan. They provide inexpensive, effective, and
flexible stormwater control that can be constructed singly, in series, or in
combination with more advanced techniques as needed. The use of storage
and/or sedimentation facilities can be optimized with respect to dual use, as
a tradeoff with treatment facilities, multiuse (aesthetic, erosion control,
recreation, irrigation, etc.), and augmentation of existing combined sewer
overflow treatment facilities.
Of course, stormwater is usually only one of several pollution sources in an
urban area. The compatability of various stormwater control techniques with
other pollution control facilities and of pollution control with flood control
facilities must be considered when developing a unified pollution control
plan. The advantages of an integrated control plan include lower overall
price and system flexibility. Integration of several small control facilities
into a pollution control plan for a developed area is often easier and less
costly to construct than a single, large facility. Sites for smaller control
facilities are more easily found. Also, additional facilities needed for
stormwater control from adjacent developing areas can be integrated into the
plan as development occurs.
A multiunit control system allows the level and type of control to be matched
to the catchment. For instance, storage and/or sedimentation basins may be
used to capture first flushes in areas with combined sewers, or chemically
assisted sedimentation may be needed for control of toxic runoff from an
industrial area. A multiunit control system retains its flexibility.
Additional units or control techniques may be added if the runoff
characteristics or available assimilative capacities change. Existing flood
control facilities also may be retrofitted to enhance pollution removal in
integrated systems. An integrated systems approach allows the addition of new
facilities in adjacent areas as the areas are developed, as well as the
incorporation of new or improved types of pollution control facilities.
4-1
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An integrated system approach also allows staging of the implementation or
construction of the pollution control facilities. For example, the least
expensive (but cost-effective) option, storage and/or sedimentation, can be
implemented before going on to a more costly option in a phased program. The
use of a source control option is most often less expensive than downstream
control, while also helping to alleviate problems in the pipe or drainage
channel system downstream. Dual use of an existing pipe network or drainage
channel system with bleeding of the stored volume into an existing treatment
facility could be implemented prior to or in place of offline storage or
downstream control. The development of downstream storage and/or
sedimentation options (either with or without utilizing existing treatment
facilities) could be the next option. The use of such an integrated system
approach can be applied to both separate storm drainage and combined sewer
systems.
In this section, it is assumed that the master planning has been completed and
that a decision has been reached to include a storage and/or sedimentation
facility as part of that plan. Planning concepts, methodologies, and tools
common to all storage and/or sedimentation applications are introduced in this
section. The need for goal setting and realistic appraisals of options as
forerunners to design are stressed.
The role of storage and/or sedimentation in an integrated stormwater
management plan is also discussed in this section. The concurrent growth of
stormwater control systems and urban areas is examined. Retrofit of existing
flood control and drainage facilities to maximize pollution control is
discussed. Examples of urban stormwater control are described to illustrate
the several points.
SYSTEM PLANNING
Once the master plan or upgrading plan for the system has been completed,
detailed planning for a storage and/or sedimentation facility can begin.
Conditions for Planning
Before beginning the detailed planning of a storage and/or sedimentation
facility, the general planning conditions that prevail for the facility must
be identified. Among the questions to be answered are the following:
Is storage and/or sedimentation the best solution for dealing with
the problems involved?
Within what geographic region .will the storage and/or sedimentation
facility be located?
What type of wastewater will enter the facility?
t What is the goal of the planned facility [1]?
4-2
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Until the planning conditions are identified and storage and/or sedimentation*
is concluded to be the most suitable solution, detailed planning for any
storage and/or sedimentation facilities should not begin.
Establishment of Goals
Storage and/or sedimentation can be accomplished for different lengths of
time, all the way from minimal noticeable detention effect to complete
retention of the flow. For planning of such facilities to be meaningful, it
is a necessity that the goal(s) for the facility be established.
The goal or goals established are usually based on the overall effect desired
for the facility. For example, this can be done in one or more of the
following ways:
The facility should not be overloaded more than a certain number of
times per year.
The volume of wastewater spilled during one year should not exceed a
given volume.
t The volume of wastewater spilled on each individual occasion should
not exceed a given volume.
The annual mass emission for a selected pollutant should not exceed a
stated value.
The number of violations of a water quality standard or attainment of
receiving water beneficial uses should not exceed a stated value.
t A specified minimum detention time for a particular stormwater runoff
rate from a given storm shall not be exceeded.
When the goal or goals have been selected, the storage and/or sedimentation
facility can be given the dimensions required to meet the goal. However, the
goal established often cannot be used directly in volume determination but
must first be transformed into dimensional-design criteria. Examples of such
dimensional-design criteria include:
The facility should be able to contain the flow caused by rain with a
certain given statistical recurrence time.
The facility should be able to contain the flow caused by a certain
selected combination of real rains.
The facility should be able to handle a certain specified flow
situation [1].
To design storage and/or sedimentation facilities on a performance oriented
basis generally requires extensive calculations. To check whether the
4-3
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performance criterion is satisfied, it is usually necessary to perform
continuous simulations of the facility for long periods of actually measured
rain. Models available for this continuous planning are discussed later in
this section.
Planning Methodology
When storage and/or sedimentation is included as a suitable solution to the
established goals, detailed planning of the storage and/or sedimentation
facility begins. The use of a defined planning methodology to guide the
process ensures that all tasks in the process will be included. Typical
design methodologies for source control options, in-system control options,
and downstream control options are shown in Figures 9, 10, and 11,
respectively.
The basic planning methodology includes the following steps:
Identify functional requirements
Identify site constraints
Establish basis of design
Select storage and/or treatment option
Estimate costs and cost sensitivities
Check that facilities satisfy objectives
Refine and complete or modify and repeat
The major items to be identified in this procedure are:
The technical configuration of the storage and/or sedimentation
facility(ies)
The exact location(s) of the installation(s)
The storage volume(s) required
t The cost of the facility(ies)
Many of these factors are dependent upon one another. To these must be added
external factors such as design considerations (flexibility, reliability,
management consideration, land requirements, etc.) and environmental
assessments (environmental impact, public health effects, social impact,
economic impact, etc.). All reasonable alternative solutions should be
developed and analyzed as part of the planning process. As stated by
Poertner:
Specific plans are usually oriented only toward various portions of the
drainage network; they should always address important relationships to
regional land development and resources management. As a study
progresses, the periodic need to investigate data and refine the planning
work will usually become evident. Thus, the planning process is
iterative [2].
4-4
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IDENTIFY FUNCTIONAL
REQUIREMENTS
i. CONTROL OF RUNOFF
RATE
8. REDUCTION OF
RUNOFF VOLUME
IDENTIFY SITE
CONSTRAINTS
i. IRE*
8. HYDRAULIC
C. ENVIRONMENTAL
0. STRUCTURAL.
ESTABLISH BASIS
OF DESIGN
I. DESIGN SIORI
B. INFLOI R»TE
C. GlfTFLOl RATE
D. STORAGE »OLU«E
SELECT DITENTION OPTIM(S)
I. tPEftATIONAL CIMCCFT
I. INLET/OUTLET IflRIS
C. AREA/DEPTH
D. CLEANING ACCESS
SHIFT TO ALTERNATE
SITE OR METHOD
FACILITIES
SATISFY
OBJECTIVES?
ESTIMATE COSTS AND
COST SENSITIVITIES
A, CAPITAL
B. OPERATION AND
MAINTENANCE
Figure 9, Source control design methodology.
-------
IDENTIFY FUNCTIONAL
REQUIREMENTS
A. OVERFLOW FREQUEKCr
REDUCTION
B. OVERFLOW VOLUME
DEDUCTION
C. OVERFLOW QUALITY
IMPROVEMENT
IDENTIFY SITE
CONSTRAINTS
A. IREI
B. SYSTEM HYDRAULICS
C. SYSTEM CONFIGURATION
D. ENVIRONMENTAL
i. STRUCTURAL
ESTABLISH BASIS
OF DESIGN
». FLOW CHARACTERISTICS
6. STORAGE CAPACITY
C. CONTROLS
D. SEDIMENT RESUSPENS I ON
FACILITIES
SATISFY
OBJECTIVES?
ESTIMATE COSTS AND
COST SENSITIVITIES
A. CAPITAL
B. OPERATION AND
MAINTENANCE
C. VALUE ENGINEERING
SHIFT TO ALTERNATE
SITE OR METHOD
SELECT STORAGE
LOCATIONS
A. OPERATIONAL CONCEPT
B. CONTROL METHODS
C. ACCESS
Figure 10. In-system control design methodology.
-------
IDENTIFY FUNCTIONAL
REQUIREMENTS
t. VOLUMETRIC CAPACITY
I. FLOATABLE/SETTLEHBLE
lEHOm EFFICIENCY
C. DISINFECTION
DETAIL AUXILIARY
SYSTEMS
t. SLUDGE PROCESSING
I. FLUSHING
t. DISINFECTION
D. tIR HANDLING
E. ENERGY AND CONTROL
ESTIMATE COSTS AND
tOST SENSITIVITIES
A. CAPITAL
I. OPERATION AND
MAINTENANCE
C. VALUE
ENGINEERING
IDENTIFY SITE
CONSTRAINTS
A. AREA
I. HYDRAULIC
C. ENVIRONMENTAL
D. STRUCTURAL
ESTABLISH BASIS
OF DESIGN
A. INFLUENT
CHARACTERISTICS
D. DESISN LOADING
DATES
C. PERFORMANCE
ESTIMATES
IDENTIFY AND SELECT
PRETREATMENT/POST-
TREATMENT COMPONENTS
T. COARSE SCREENING
B. FINE SCREEHING
C. CRIT REMOVAL
D. FLOW MEASUREMENT
SELECT MAIN
TREATMENT GEOMETRY
A. OPERATIONAL CONCEPT
B. COMPARTMENTALIZATION
C. INLET/OUTLET HORKS
C. SLUDGE/SCUM REMOVAL
SYSTEM
FACILITIES
SATISFY
OBJECTIVES?
SHIFT TO ALTERNATE
SITE OR PROCESS
Figure 11. Downstream control design methodology.
4-7
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Because the planning process is iterative (as shown in Figures 9, 10, and 11),
numerous alternative storage and/or sedimentation locations and/or facility
sizes may need evaluation. The next step is to select the required storage
and/or sedimentation volume.
Cost Optimization Methodology
The methodology used to evaluate the optimum cost of storage and/or
sedimentation facilities depends on the purpose of the facility: flow control
only, or a combination of flow control and pollutant reduction. In the case
of a facility for flow control only, a mass-diagram method similar to that for
water supply reservoir sizing should be used. Facilities for flow control and
pollutant reduction should use a production theory approach as described by
Heaney, jt_jl_. [3].
Mass-Diagram Method. A graph of the cumulative runoff and treatment plotted
against time is known as a mass-diagram or flow-mass curve. It is the
integral curve of the hydrograph which expresses the area under the hydrograph
from one time to another. A mass diagram permits a simple graphical
inspection of the entire runoff record or any portion of it for determination
of either (1) the reservoir capacity required to produce a specified treatment
rate (outflow), or (2) the treatment rate which can be expected for a given
reservoir capacity. More detailed information is presented by Linsley and
Franzini [4] or Chow [5].
Given the series of storage values for the period of record, a statistical
analysis of the arrayed storage values can be prepared on a probability
plot. A plot of storage capacity with respect to the cost associated with
development of that storage capacity should be prepared also. The design
storage capacity can then be selected based on some reasonable return
frequency and the cost associated with that capacity.
In addition, to meet discharge quality limitations that may be set by
regulatory agencies, the storage and/or sedimentation facility may have to be
augmented by added treatment facilities. The facilities required can range
from the limits of maximizing the storage volume so that no new treatment
capacity is required, all the way to providing sufficient additional treatment
capacity so that no storage is required. However, the optimum from a cost
standpoint usually falls somewhere between these two limits where storage is
combined with additional treatment. The optimum combination occurs where the
sum of the cost of the required storage and treatment facilities needed to
meet the discharge limitations is a minimum.
Production Theory Method. Using the economic principles of production theory,
a series of computations can provide an optimized total annual cost for
combinations of storage and treatment providing various levels of runoff
and/or pollutant control [3]. A graphical representation of this methodology
is given in Figure 12. For different combinations of treatment rate and
storage capacity (expressed as the depth of runoff contained over the entire
drainage area), the isoquant curves in Figure 12 represent equal degrees of
4-8
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90% -*
- LEVEL OF CONTROL:
PERCENT RUNOFF TREATED
TREATMENT RATE, in./h
in. x 2.54 = cm
Figure 12. Determination of optimal combination
of storage and/or treatment alternatives.
4-9
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treatment (can be expressed in terms of percent of the runoff treated, percent-
of pollutants removed, or number of overflows per year). Isocost lines
represent storage and/or treatment combinations which may be implemented at
the same total cost. The point of tangency between an isoquant curve and an
isocost line represents the most economical combination for a given degree of
treatment. The optimum combination for any degree of treatment can be found
from the "expansion path" through all tangent points.
Storage Volume Determination Methods
A common problem found in the analysis of stormwater management options is to
determine how various possible combinations of storage and/or sedimentation
capacity will affect runoff control. For example, in a combined sewer system
it may be desirable to determine how the effectiveness of an existing
treatment plant may be improved by providing storage ahead of the plant. In a
currently undersized storm sewer system, it might be desirable to know how the
frequency of flooding might be changed by providing various small amounts of
storage on the watershed or in the pipes themselves by regulating devices. In
a new system, it may be desirable to know the combination of storage and pipe
or treatment capacity that will provide the desired level of runoff and
pollution control at minimum cost.
The dimensions of such storage and/or sedimentation facilities are determined
by the type of facility involved. Depending upon the type of facility and
size of the area, a number of approach methodologies are available for
determining the required volume. Listed in order of the easiest and simplest
for a small area to the most complex for a large area with a complex sewer
system, the methodologies are:
Desktop hand computations
Statistical analysis of rainfall and flow data
Simple, continuous simulation of stormwater systems
Detailed, continuous or single event simulation of stormwater systems
The application of each of these approaches to storage and/or sedimentation
volume requirement determination depends upon the size and complexity of the
drainage area and/or sewerage system. For small drainage areas with
uncomplicated sewer systems, the use of desktop, hand computational methods
such as the unit hydrograph approach, reservoir routing, or SWMM Level I [6]
for determining the effect of storage on the runoff is usually sufficient.
The desktop analysis approach can be used for larger areas when limited
rainfall or runoff data availability prevents more detailed analysis.
Whenever possible, local rainfall, streamflow, or sewer flow records should be
used to verify the desktop analysis or as input for more detailed analysis
methods. Information on the unit hydrograph and reservoir routing approaches
is available in most hydrology texts [7, 8].
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In recent years, mathematical techniques have been developed to reduce the
amount of computer work necessary for analysis of stormwater management
systems. Methods using statistical characterization of runoff, overflow, and
pollution events for preliminary sizing of storage and/or sedimentation
facilities for small or simple systems have been developed by DiTorio,
Hydroscience, Driscoll jet__al_., and Howard et al. [9, 10, 11, 12]. These
methods develop the required statistical information from historical rainfall
data and present the levels of control estimated for various storage and/or
treatment combinations.
More complex systems can be modeled using EPAMAC, STORM, or SWMM-Version III
[13, 14, 15]. These are continuous simulation models. STORM and SWMM-Version
III can also be used for single-event storms simply by using a shorter time
step. The new Storage/Treatment Block of SWMM-Version III can be run with
most runoff simulators (e.g., EPAMAC, STORM, and SWMM-Version III). This
model allows the user to input the required relationship between storage
volume and outflow rate similar to that for a reservoir routing problem. This
approach, permitting the user to best approximate the desired functional
relationship, simplifies the model and allows a simulation of a wider range of
reservoir geometries and operating policies. Pollutants are characterized by
their magnitude (i.e., mass flow and concentration and, if desired, by
particle size and specific gravity distributions). Describing pollutants by
their particle size and specific gravity distribution is especially
appropriate where small or large particles dominate or where several storage
and/or treatment units are operated in series. Also, if several units are
operated in series, the first units will remove a certain range of particle
sizes, thus affecting the performance of downstream units. This model,
coupled with site specific settleability and solids characterization
information, can be used to make a thorough evaluation and design of retention
or detention facilities.
Dynamic wave models, such as EXTRAN, may be needed to design inline storage
systems where surcharging and other hydraulic effects are important [16].
This is probably the most sophisticated channel/pipe flow routing model
available in the public domain.
Information on numerous runoff models can be found in recent EPA reports
[17, 18], These include ILLUDAS (Illinois Urban Drainage Area Simulator),
USGS's DR3M, and SOGREAH's CAREDAS.
Thus, the size of the tributary area and the complexity of the sewer network
combine to determine the detail required for sizing and designing the storage
and/or sedimentation facility required.
Effect of Storage and/or Sedimentation
To evaluate the different storage and/or sedimentation alternatives, the
effect achieved by each alternative must be compared. The term effect can in
principle be thought of as the degree of fulfillment of the goal or goals
established for the facility. The evaluation of which technical solution is
most advantageous is usually arrived at through a cost-effectiveness
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comparison. Thus, the best apparent alternative should be the facility that
both meets the technical goal(s) established and has the least cost.
THE INTEGRATION PROCESS
The process of integrat-ing stormwater control into a pollution control system
involves initial planning where existing facilities are identified and goals
are determined. Additional steps then involve selecting control methods that
are both applicable and compatible to the existing facilities and established
goals. Each of the steps is briefly described.
Identify Existing System and Needs
The first step in the integration process is the identification of existing
system components and function. This information is the basis of plan
development and determination of control methods that are most appropriate for
the system.
The watershed characteristics and existing facility components are examined.
Information on the land uses of the watershed(s) and acreage of each is used
to evaluate the potential for development and to estimate the runoff quality
and quantity.
Identification of existing facility location, function, and capacity is
required. The compatibility of existing facilities with proposed control
methods must be determined. The potential for retrofits with the existing
facilities, and whether the existing facilities are in suitable condition must
be established. Separate storm and sanitary or combined sewer line location,
treatment facility types, and capacities will strongly influence the location
and points of interconnection for new facilities.
Establish System Needs
After identifying the existing system and conditions, the next step is to
establish additional flood and pollution control needs. Plans should
encompass the entire urban wastewater control system including both dry- and
wet-weather facilities, and both developed and undeveloped areas. Planning
over a whole system enables the use of all resources available. By examining
the potential from undeveloped areas, the system can be planned to expand and
accommodate future needs much more easily.
Identify Applicable Control Alternatives
The engineer or planner, at this point in the integration process, should be
familiar with the existing system and the goals of stormwater management.
Alternative control methods should be examined, on the basis of the three
factors: (1) physical limitations, (2) effectiveness, and (3) institutional
limitations.
Physical limitations may exclude some storage and/or sedimentation methods.
Steep slopes or extensive development may preclude the use of retention
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ponds. In general, older developed areas impose more constraints on the
number of options available because the sewer network(s), and other
underground utilities and structures are already in place. The same may also
be true in developing areas, but usually to a lesser extent. Land
availability will also limit the control method options. Limited land
availability tends to preclude open ponds, but storage and/or sedimentation
basins would be applicable because the structure could be built to physically
support a second use above it. A list of the more common control methods and
the physical situations favoring their selection are shown in Table 8.
Table 8. STORAGE AND/OR SEDIMENTATION CONTROL METHOD
VERSUS FLOW OR QUALITY APPROACH
Flow attenuation Quality improvement
In-system storage
Rooftop ponds
Plaza ponds
Parking lot ponds
Storage/sedimentation basins
Dry detention/retention
Wet detention/retention
Major-minor flooding
Note: Flow attenuation
ponds
ponds
can be
X
X
X
X
X
X
X
X
considered to
X
-
-
-
X
X
X
-
provide some quality
improvement through flow reduction, flow redistribution, and
pollutant load redistribution.
The control methods also vary as to their effectiveness in reducing pollutant
concentrations and mitigating storm volumes. Retention ponds for separate
storm drainage are highly effective for both flow attenuation and pollutant
removal, since the flow is totally contained. Thus, neither the flow nor the
pollutant load is transmitted downstream as long as the available volume of
the pond is not exceeded. Flow in excess of the pond capacity is discharged
through the overflow structure (see Figure 8). Thus, a retention pond
provides both flow control and pollutant reduction for all flows in excess of
pond capacity by acting as a storage and sedimentation facility during the
overflow period.
Source control may be used to attenuate storm flow for both separate storm
drainage and combined sewer systems. It is somewhat less effective than
retention ponds for pollution control; the pollutants remain available for
later resuspension and transport unless they are physically removed from the
source control facility. In some cases, source control provides only flow
attenuation since the total volume (including the pollutants) is eventually
discharged to the sewer system. This redistributes the pollutant load
(reducing shock loads) and increases the volume that is directed to treatment.
In-system and downstream controls offer both flow and pollutant mass discharge
rate attenuation for the storm flows during a storm event. In separate storm
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drainage systems, both the total flow volume and pollutant mass discharged
remain undiminished since the stored volume is usually discharged to the
receiving water just as before storage was implemented. Shock loads on the
receiving water are reduced through redistribution of the pollutant load. If
the stored stormwater is bled back to the sanitary sewer for treatment, the
flow volume and pollutant load reaching the receiving water are reduced.
In combined sewer systems, in-system and downstream controls provide
attenuation and reduction for both flow and pollutant loads. During a storm
event, the storage and/or sedimentation facilities provide flow and pollutant
mass discharge rate attentuation. Following a storm, the flow and pollutants
remaining in storage are released to the sewer or interceptor for transport to
the treatment plant for processing. The pollutants remaining in storage at
the end of a storm event are usually more than just those associated with the
flow volume retained due to the sedimentation that occurs during storage.
Thus, the pollutant load reduction resulting from storage during the storm
event may be much greater than the flow reduction for that same event.
The effectiveness of the various storage and/or sedimentation control methods
is shown in Table 9.
Table 9. STORAGE AND/OR SEDIMENTATION CONTROL METHOD
VERSUS PHYSICAL AND EFFECTIVENESS LIMITATIONS
Effectiveness
Physical/environmental
Flow
Quality
In-system storage
Rooftop ponds
Plaza ponds
Parking lot ponds
Storage/
sedimentation basins
Dry retention ponds
Wet retention ponds
Dry detention ponds
Wet detention ponds
Extra capacity must be
present inline or offline
Flat roof structures
Land area for development
Public inconvenience
Land use conflicts
Large space requirement,
flat terrain
Large space requirement
Large space requirement,
flat terrain
Large space requirement,
flat terrain
Proportional to
capacity available
Yes for peaking flows
Yes for peaking flows
Yes for peaking flows
Potentially high
depending on mode of
operation
Yes, 100%
Yes, 100%
Yes for peaking flows
Yes for peaking flows
Some, for combined
sewer systems*
Some, for combined
sewer systems
Some, for combined
sewer systems
Some, if street sweeping
program in effect
Yes--up to 60% SS
re'moval; other parameters
vary
Yes, 100%
Yes, 100%
Yes up to 60% SS
removal; other parameters
vary
Yes--up to 60% SS
removal; other parameters
vary
*Some quality improvement for separate storm sewer systems where the stored storm runoff is bled
back into the sanitary sewer system for conveyance to a treatment facility.
The third factor determining applicability involves institutional
limitations. The means of implementation, operation, maintenance, and
financial support are crucial to the applicability of a management system.
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The institutional authority must exist to ensure the implementation of the
system and continued system operation.
Funding for implementation, operation, and maintenance must be secured. An
institutional organization must exist to collect funds, impose fees, or make
other necessary financial arrangements. If the cost is to be borne by
developers, then the legal and enforcement authority must exist to confirm
that the control methods are applied.
Determine Control Method Compatibility
Once the applicable control methods are identified, the final step in the
integration process is to assess the process compatibility. Process questions
include (1) which control methods have compatible treatment methods, (2) will
capacity of existing treatment be exceeded due to installation of control
method, and (3) is the method flexible for use in other treatment process
trains.
Control method compatibility with treatment processes is an important
consideration of the integration process. A chemical treatment process that
could upset a biological process should not be used ahead of the latter.
Also, pretreatment requirements for the functioning of some processes must be
incorporated into a system and could be part of stormwater treatment
facilities.
Flexibility of dry- and wet-weather facilities in either combined sewer or
separate storm drainage systems can take advantage of the wet-weather
facilities as a pretreatment process for dry-weather flows, or as a standby in
case of dry-weather facility failure. Wet-weather facilities might also be
used as an effluent polishing step during dry weather.
Another important compatibility consideration is the effect of sludge
generation by the stormwater management control methods. From a pollution
control standpoint, the sludge resulting from the use of storage and/or
sedimentation facilities should be removed, whenever possible, from basins,
ponds, or pipe networks where it first settles to prevent its resuspension and
transport downstream. This would reduce the solids mass load downstream on
either additional storage and/or sedimentation facilities or the receiving
water. From a practical standpoint, this may create a massive logistical
problem of providing access, sludge removal equipment, and transportation for
the sludge removed. In most cases, it is most economical to resuspend the
settled matter and discharge it to the sanitary or combined sewer for
transport to the dry-weather treatment plant where sludge collection,
processing, and disposal equipment already exist providing that there is
sufficient capacity to handle the additional suspended solids. Onsite removal
of sludge is usually practiced only at large, lined open basins where easy
access for sludge removal equipment is provided. An integrated facility must
include transportation and processing of the solids (either onsite or offsite)
in the overall plan.
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DESIGN PROCEDURE FOR COMBINED SEWER SYSTEMS
The main steps to be followed in the design procedure for combined sewer
systems are (1) problem identification, (2) data needs, (3) determination of
the pollution load, (4) identification of the pollutant removal objectives,
(5) control optimization, (6) pollutant budget analysis, (7)' operating
strategy for design, and (8) instrumentation and control strategy for
operation. During the design procedure, specific consideration must be given
to sludge/residuals removal and disposal from the facilities that provide
storage, sedimentation, or both.
Problem Identification
The investigation of stormwater discharges is concerned with two different
types of polluted flowsseparate stormwater runoff from storm sewers or
drainage channels, and combined sewer overflows from sewers containing both
storm runoff and sanitary sewage. However, the problems associated with these
discharges can be identified as (1) quantity of flow, and (2) quality of
flow. Before the design procedure can proceed, it is necessary to identify
the problem as either flow related, quality related, or both.
Flow. Flow problems are usually identified with flooding and flood damage.
This can be streams overtopping their banks, runoff exceeding the capacity of
surface drainage channels or combined sewer inlets, flooding of basements, or
flooding of streets and surface areas due to surcharging of combined sewers.
Quality. Overflows from combined sewers can produce serious pollution of
local waterways and receiving water bodies. The surcharged sewers often spill
their contents into streets, highway underpasses, and basements of
buildings. This results in flooding, pollution, health threats,
inconvenience, and economic losses associated therewith. The magnitude of the
potential effects resulting from CSOs was presented earlier in Table 3.
Data Needs
Comprehensive master planning is required to achieve the goals and objectives
of urban combined sewer overflow control. It is most important to gather
sufficient data so the number of iterations required for the planning and
design of facilities is minimized. Information relating to historic,
existing, and future land use; basic hydrology (including rainfall, runoff,
vegetation, soils, and infiltration); discharge capacities of existing
facilities; impacts on adjacent properties; evaluation of the existing
problems; and details of existing master plans for the area are needed.
Alternatives can be developed only after analyzing the collected data.
Rainfall. The design of combined sewer systems is usually based on
precipitation events having a statistical frequency of occurrence. In the
past, statistical rainfall intensity-duration-frequency relationships were
used to size the sewers and storage facilities. This approach resulted in
sizing for peak flows but did not account for the effects of short intervals
between storms or uneven area! rainfall during storms. The use of intensity-
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duration-frequency relationships should be avoided except for initial rough-
cut estimates.
The sizing of combined sewers, storage facilities, and sedimentation
facilities should be based on a continuous historical or synthesized rainfall
record that is typical of any long-term rainfall record. The actual
historical records selected may be based on the hourly intensity, storm
duration, total rainfall for the storm, or any combination of these.
For example, rainfall characterization analyses run on historical hourly
rainfall records for San Francisco periods of 70 years (full historical
record), 4 years (October 1971 through April 1975), and 4 months (October 1972
through January 1973) resulted in remarkably consistent results [19]. These
results are shown in Figures 13, 14, and 15, for
and duration versus frequency, respectively. In
correlated well with the full 70 years of record,
month record permitted the preliminary screening
storm magnitude, intensity,
all cases, the 4-month record
Thus, the use of the 4-
of a large number of
alternatives (storage volume and treatment plant capacity) while using only a
modest input of project funds, time, and labor. These shorter periods were
judiciously selected after detailed review of the full 70 years of record.
The purpose of these selections was to minimize the computer time and cost
associated with evaluating the alternatives for storage and treatment capacity
combinations.
10. o
INDIVIDUAL STORMS
IN 4-MO SUBSET
70-YR RECORD
(MAY-SEP ONLY)
0.1 0.2
1 10
OCCURRENCES PER YEAR
100
Figure 13. Storm magnitude versus frequency.
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1 .0
II
0.01
0. 1
70-YR RECORD (ALL)
INDIVIDUAL STORMS
IN 4-MO SUBSET
70- YR RECORD
(MAY-SEP ONLY)
1 10
OCCURRENCES PER YEAR
100
Figure 14. Storm intensity versus frequency.
100
it °io.o
1 . 0
INDIVIDUAL STORMS
IN 4-MO SUBSET
0. 1
1 10
OCCURRENCES PER YEAR
100
Figure 15. Storm duration versus frequency.
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Rainfall data are available from the National Weather Service (NWS).
Additional local rainfall data may be available from municipal sources, local
flood control or water districts, or utilities.
Flow Records. To accurately assess either the flow or the quality problem (or
both), if it is necessary to know what problem or problems were caused by the
flow. Use of historical flow records, when matched with the appropriate
rainfall data, can show what problems are caused, when they occur, and where
they occur (i.e., surcharging of sewers, flooding of basements, overflowing at
pumping stations, etc.).
Dry-weather flow records are needed to determine the diurnal flow and quality
variations. The stormwater carrying capacity of the combined sewer at any time
can be found by subtracting the dry-weather flow from the maximum capacity of
the sewer. The pollutant mass load in the dry-weather flow at any time must be
added to the pollutant load in any stormwater present to determine the total
mass load or the concentration of the total flow at that time.
Wet-weather flow records are needed to determine the runoff coefficients for
the tributary area, the response of the sewer system to various rainfalls, and
the effect of the storm flow on pollutant loads (i.e., first flush phenomenon).
It is important to know the flow response to various rainfall events so that
the effects on the sewer system of a design storm or a particular series of
historical storms can be predicted. In most cases, the wet-weather flow data
available cover only a small portion of the time for which historical rainfall
data are available.
Drainage Area Characteristics. The drainage area characteristics influence the
volume of flow and the rate of runoff from the tributary area. Pertinent
physical characteristics of drainage areas that affect both the volume and rate
of stormwater runoff include topography, land use, population density, geology
and soils, and size.
The topography can affect the rainfall patterns (i.e., additional rainfall due
to orographic lifting) as well as the rate of runoff. The runoff rate is
usually increased as the slope of the ground increases.
The distribution and types of land use within the drainage area can greatly
affect the runoff. Usually, as the population density increases, so does the
percentage of imperviousness in the drainage area. The percentage of
imperviousness can be affected by the type of land use also (i.e., parks and
recreational, streets and highways, industrial, commercial, and residential).
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While the size of the drainage area alone does not determine the runoff volume
and rate, it can have a great effect on them. The runoff volume, on a unit
area basis, is determined by the other physical characteristics of the area
for any given rainfall event. However, the total runoff volume is determined
by the unit area runoff and the size of the area. The rate of runoff is also
affected by the size of the area. Usually there is some attenuation of the
runoff rate as the drainage area increases caused by attenuation of the runoff
flow by travel time and by areal distribution of the rainfall. In large
areas, the rainfall may not be uniform over the entire area (i.e.,
thunderstorm cells), thus producing a relative reduction in rainfall intensity
for the whole area.
The geology and soils affect the runoff also. The depth, porosity, and type
of material determine the rainfall storage capacity before surface runoff
occurs. The antecedent conditions (i.e., time since last rainfall, soil
saturation, etc.) also affect the runoff rate and volume. The type of soil
has a great effect on the amount of suspended solids that is contained in the
runoff. Cohesionless soils usually contribute more to the suspended solids
load in the runoff than do cohesive soils.
Suspended Solids Characterization. In addition to those solids normally found
in sanitary sewage, combined sewer overflows contain solids washed into the
sewer system from urban roadways and land areas. High flowrates in the sewers
during storm events resuspend solids deposited in the lines, adding additional
suspended solids (generally grit and sand) to the solids load. A
characterization of the suspended solids, including floatables, in the flow is
necessary to determine or estimate the sediment/fleatables removal resulting
from storage or sedimentation facilities in the system or proposed for the
system.
Stormwater from different locations generally has extremely varying
properties. Among the factors that are of significance are:
The pollutant content of the combined sewage
The proportion of easily settling pollutants
The particle size distribution in the combined sewage
The particle volume distribution in the combined sewage
The density of the combined sewage particles
With regard to sedimentation of combined sewage, it is mainly the content of
suspended material in the water which is of interest. Other pollutants (such
as BOD, heavy metals, etc.) that may be bound with the suspended material that
is settleable can be removed during storage or sedimentation. Therefore, it
is important to identify those fractions of the pollutants bound to the
settleable solids so that the effectiveness of sedimentation on overall
pollutant removal can be estimated. The remainder of the pollutants will be
included in the supernatant. The content of suspended material in the
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combined sewage should be at least 75 to 100 mg/L for sedimentation to be
useful; sedimentation is of virtually no value when the suspended solids
concentration is below 20 mg/L [1].
The heavier suspended material can represent a quite significant part of the
total pollution content of combined sewage. The time required for settling-
out the coarsest pollutants ranges up to just a few minutes. The limiting
value for the separation of the coarser solids in the combined sewage should
be set at 5 minutes. This will include particles in the sand range and
larger.
The particle size distribution in different stormwaters has a significance on
the sedimentation properties of the suspended solids. For combined sewage
that has first undergone a coarse separation as described above, the majority
of the remaining suspended solids particles are usually found in the 5 to
75 jim range. The number of particles is largest in the 10 to 25 jjm interval.
The number of particles larger than 40 jum is often smaller than 10 per mL [lj.
To estimate how large a quantity of the pollutants will be separated out by
sedimentation, the density of the particles must be known. While data in the
literature are sparse and contradictory, typical values that have been
reported are in the range of 68.6 to 81.1 Ib/ft3 (1100 to 1300 kg/m3) [20].
The suspended solids should be characterized by both particle size
distribution and density. The particle size distribution can be determined
from a sieve analysis of the suspended solids. The specific weight of the
particles must also be determined. A settling column can be used to determine
the settling characteristics of the suspended solids. The procedure and
equipment for settling column tests are described by Dalrymple, etal., and
Pisano, et al. [21, 22]. ;
Collection System. The configuration of the collection system will determine
where storage or sedimentation facilities can be located. The size and slope
of the pipes have an effect on the use of the pipes for inline storage. The
diameter of the pipe will limit the volume per linear foot of pipe that can be
stored; the slope of the pipe will determine the length of pipe available to
provide storage without exceeding any surcharge limitations at the control
point. The slope also determines the flow velocity which affects the shear
forces on solids particles. This affects the suspension or resuspension of
the particles.
The overall length of pipes and the pipe density within an area also determine
the potential volume that can be stored from any control point within the
system. The longer the pipes are and the higher the number of pipes per unit
area, the more volume is available for storage at any given pipe slope.
The configuration of the sewer system affects the flowrate and time of travel
within the system. For a given unit area, the flowrate increases and the
travel time decreases as the number of branches increase providing the sewer
slope remains the same.
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The type and location of overflow structures or flow regulators or both also
affect the inline storage capacity of a collection system. Any storage
downstream of an overflow structure will be limited by the depth to which flowi
can be stored before the level rises to the elevation of the overflow. Inline
storage downstream of a regulator must be limited so that the backwater from
the storage does not affect the operation of the regulator.
Existing pumping stations can be used to control inline storage upstream of
the pumping station. The storage can be effected by controlling the number of
pumps operating at any given time. The flow can be stored until the hydraulic
grade line upstream reaches a predetermined level (overflow elevation,
basement level, maximum surcharge on a sluice gate, etc.) that will not cause
aesthetic or economic problems. If a pumping station is used for storage
control, the station pumping capacity must be sufficient to pump any flow
required to prevent problems upstream.
Determine Pollution Load
Most analyses of pollutant concentration measure the total quantity but do not
distinguish between soluble and particulate fractions. Sedimentation
computations are based on the particulate or settleable fraction (this should
also include floatables since they are removed by skimming as part of the
sedimentation process). However, overall removal efficiency is expressed in
terms of total quantities of pollutant, which is the most relevant way to
express results for control decisions and forms the basis for reporting
observed results to be used for comparison with computations.
Therefore, it is necessary to determine the fraction of the total
concentration or load which is settleable. This is most conveniently done by
determining the fraction of pollutant associated with each of several ranges
of particle settling velocities and combining the results to obtain the
overall removal. It is also important to know the soluble fraction of
pollutant for reasons that are discussed below.
The effectiveness of any storage or sedimentation facility cannot be estimated
unless the pollution loads entering the facility are known. This requires
knowledge of the various pollutants in the combined sewage as well as the time
variation of the mass loading of these pollutants. The pollutants of most
common interest are BOD and suspended solids. However, other pollutants of
interest usually include one or more of the following: volatile suspended
solids; COD; various forms of nitrogen; phosphorus, both total and
orthophosphate; heavy metals, particularly lead; and total and fecal
coliforms.
Some of these pollutants are most frequently adsorbed onto the suspended
solids rather than being in the dissolved form. Thus, it is necessary to
determine whether the pollutants of interest in any particular study are
associated with the suspended solids, dissolved, or both. If the pollutants
of interest are adsorbed onto the suspended solids, it is necessary to
determine the fractions associated with the settleable solids and the
colloidal solids. This information is needed to determine whether any removal
will be affected by use of storage or sedimentation facilities.
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The determination of the pollutant load can best be accomplished through a
site-specific sampling program. The sampling program should be developed to
characterize the quality of both the dry-weather flow and the combined sewage
during wet weather for the predesign of an abatement program. The pollutant
values are a combination of the sanitary sewage pollutant concentrations and
the stormwater runoff pollutant concentrations. Site specific concentrations
that result from this mixture depend on the quality of the two base flows and
their proportional mix.
Identify Pollutant Removal Objectives
Once the pollutant loads are known, the pollutant removal objectives can be
identified. The removal objectives should be selected in conjunction with the
problem identified, quantity of flow or quality of flow or both, and the
pollutant loadings.
If the prime objective is storage (i.e., flood control, flowrate control,
etc.), the facility should be designed primarily for storage. However, since
there is sedimentation associated with any storage facility, the sedimentation
cannot be ignored. A means for removing or resuspending the settled solids
must be incorporated into the design. Otherwise, the solids would continue to
build up over time and reduce the effectiveness of the storage.
If the major objective is the removal of a pollutant that is primarily
associated with the settleable solids, the facility should be designed as a
sedimentation facility to maximize the removal of the settleable solids. If
the prime objective is the removal of a pollutant that is dissolved or
associated with the colloidal solids that do not settle, the facility should
be designed to maximize the storage volume. In both cases, any settled solids
can be removed at the storage or sedimentation facility location or they can
be conveyed to a treatment plant for removal. The volume of flow remaining in
the storage or sedimentation facility after a storm event should be conveyed
to a treatment plant for treatment before discharge.
Control Optimization
The objective of any combined sewer overflow control system should be to
maximize the effectiveness of the control system by producing the maximum
desired effect for the funds available. The desired effect can be the
reduction of a single parameter (i.e., overflow frequency, overflow volume,
pollutant mass, etc.) or the reduction of any combination of parameters. In
any case, the desired effect must be identified and selected prior to
optimization of the control strategy.
The steps involved in the optimization of the control strategy are:
Select the desired effect(s)
t Identify location(s) and capacity(ies) for storage or sedimentation
or both
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Identify location(s) and capacity(ies) for treatment facilities
Identify the costs associated with each storage, sedimentation, and
treatment facility
Evaluate the effect of changes in the number and size of the various
facilities upon the cost of the control strategy
Identify Location(s) for Storage. The maximum size and the location of each
existing or proposed storage facility must be determined. For any given site,
there is a maximum storage volume that can be physically accommodated.
However, any smaller storage or sedimentation facility can be accommodated.
The storage or sedimentation or both can be developed through source control,
in-system facilities, downstream facilities, or any combination of these
facilities.
Identify Treatment Location(s). The maximum size and location of each
existing or potential treatment facility must be identified. The size for any
treatment facility can vary downward from the identified maximum. Treatment
can be provided at existing plants, expanded existing plants, satellite
plants, or any combination of these facilities.
Cost Analysis. The cost identified for the storage, sedimentation, and
treatment facilities should include both capital and operation and maintenance
costs. A range of costs for each facility should be determined based on cost
per unit of storage volume, per unit of pollutant removed, per unit of
overflow volume reduced, or per unit of treatment capacity as is appropriate
for the particular facility.
Optimization. The effect of the variation of size or capacity of the
individual facility on the entire system cost should be determined. The
combination of storage, sedimentation, and treatment facilities that provides
the greatest effectiveness for the acceptable cost can be identified as
described previously in this section under Cost Optimization Methodology.
Pollutant Budget Analysis
To determine the overall effectiveness of a storage and/or sedimentation
facility, it is necessary not only to determine the fraction of any specific
pollutant that is retainied in the facility, or removed as a result of the
facility, but also to determine the timing of the removal. In other words, it
is necessary to know when and where the removal occurs. A pollutant budget
analysis is simply a means of keeping track of the mass of pollutant that is
associated with the residuals (sediment) and with the supernatant (overflow or
discharge). It is necessary to know not only that the pollutant mass inflow
is equal to the sum of the pollutant mass in the residual and the supernatant,
but also the time relationship among masses.
Residuals. The sediment remaining in a storage or sedimentation facility
depends on the particle size and density distribution as well as the particle
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volume distribution. It is important to know the time relationship of the
residuals accumulation to determine whether it is necessary to continue the
sedimentation operation or whether the flow can be passed on downstream
untreated. It is possible for the sediment distribution throughout the storm
to be such that most of the readily settleable material is included during thie
early part of the storm. Therefore, the later portion of the flow may be
passed on untreated without serious effect on the downstream receiving water.
Specific pollutants of interest may be associated with a specific particle
size/density or settling velocity range. The effect of sedimentation on this
particle range must be determined so the efficacy of sedimentation for removal
of the pollutants can be estimated.
Supernatant. The supernatant (treated discharge or overflow) will contain a
portion of the pollutant also. The effectiveness of storage or sedimentation
on pollutant removal is determined by the ratio of the mass of the pollutant
removed to the total mass of the pollutant included in the flow.
Thus, the pollutant budget analysis is a time-related mass balance for the
pollutant or pollutants of interest.
Operating Strategy for Design
At this point, it is necessary to determine an operating strategy for the
combined sewer overflow control facilities. The alternatives for the physical
layout of the facilities must be selected (both locations and sizes), the
pollutant budget analyzed and refined, and an optimization analysis
performed. This is necessary to identify the apparent best alternative.
Layout Alternatives. The location of alternative storage or sedimentation or
combination facilities must be selected so that a manageable number of
alternatives can be evaluated. The facilities should be evaluated based on
modular sizes to facilitate the evaluation process. The facilities can be
individual facility locations or combinations of locations. The entire
combined sewer collection system and treatment facilities should be included
in the analysis.
Pollutant Budget Analysis Refinement. The pollutant budget must be reanalyzed
for the most attractive alternatives to determine whether they meet the
desired goal for pollutant removal. This is necessary to assist in the
optimization evaluation.
Optimization. The costs associated with each facility in the combined sewer
overflow control system are used to determine the apparent best alternative.
The optimization should determine the alternative that provides the greatest
desired benefit for the least cost as long as that cost is less than the
economic limit established for the project. It may be necessary to run
through an iteration process to determine the optimum project.
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Instrumentation and Control Strategy for Operation
A means of controlling the storage or sedimentation facility(s) must also be
identified. This should include identification of the instruments for control
as well as a control strategy. A variety of instruments and control
strategies are available. These can include monitoring opacity, use of radar
to monitor rainfall, establishment of a rule curve for operation, or remote
control from another location.
Opacity. The opacity of the flow in a combined sewer can be used to control
flow into or out of a storage or sedimentation facility. For example, as the
opacity of the sewer flow increased as a result of increased suspended solids
concentrations due to stormwater, the flow can be directed into a storage
facility. Later, as the opacity decreases when the stormflow becomes more
dilute, the flow into the storage facility can be stopped and the flow
directed on downstream.
Opacity can be used in sedimentation facilities to control sediment removal
rates or chemical additions. Chemicals can be added to improve the sediment
removal.
Radar. Radar is presently used to monitor the location and intensity of
rainfall during storms. Radar may be used to help determine the best strategy
for sequencing operation of storage or sedimentation facilities on a real-time
basis. Radar can be used to provide decision-making information for
controlling the drawdown of storage or sedimentation facilities prior to the
arrival of a storm or between a series of storms. Radar is not presently used
as the sole control for the operation of any specific combined sewer overflow
or separate storm sewer storage or sedimentation facility or facilities.
Remote Control. Remote control of combined sewer overflow facilities is
common now. It is not necessary to staff each facility as long as enough data
for decision making are transmitted from the facility site to the manned
control location. The operation of the storage or sedimentation facility can
be monitored or controlled from the remote location. A typical example of
this is the combined sewer overflow regulator control at Seattle. A series of
regulators located throughout the city are controlled from a central location.
Rule Curve. Control facilities can be operated on the basis of a rule curve
established from previous experience. A typical example of this is the use of
a programmed process controller. A series of control functions are triggered
based on a previously established time sequence or liquid level. In this
case, the operation of the facility is the same each time it is activated. A
rule curve is used most often for remote facilities.
The cost of a combined sewer overflow control or treatment facility (storage,
sedimentation, or both) is affected by the cost of the instrumentation and the
control strategy selected. This cost must be incorporated into the
optimization process.
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DESIGN PROCEDURE FOR SEPARATE STORM SEWER SYSTEMS
The main steps to be followed in the design procedure for separate storm sewer
systems are (1) problem identification, (2) data needs, (3) determination of
the pollution ,load, (4) identification of the flood control and pollutant
removal objectives, (5) control optimization, (6) pollutant budget analysis,
and (7) operating strategy for design.
Problem Identification
As with combined sewer systems, the problems associated with separate storm
sewer systems fall into two categories: (1) quantity of flow, and (2) quality
of the flow. It is also possible that the problem is related to a combination
of the two categories.
Quantity. Flow problems are usually identified with flooding and flood
damage. Increased urban development upstream of existing storm sewers may
contribute increased stormwater flows that exceed the capacity of the existing
sewers. This is usually the result of increasing the impermeable area within
the watershed. Flooding of streets, public areas, and buildings may result
from the increased flow. Receiving streams may flood also as a result of the
increased stormwater runoff.
Quality. Urban development can introduce many additional pollutants to the
stormwater. Examples of the pollutants may include waste engine oil dumped
into catchbasins, paints and solvents, additional suspended solids, and
floatables (bottles, cans, styrofoam containers, etc.). These pollutants may
create problems within the storm sewer system by settling and creating
restrictions or they can create problems in the receiving water.
Data Needs
Comprehensive master planning is required to achieve the goals and objectives
of urban stormwater control. The data needed for the comprehensive planning
include land use, basic hydrology (including rainfall, runoff, vegetation,
soils, and infiltration rates), storm sewer configuration and capacities,
impacts on adjacent properties, evaluation of existing problems, and details
of any existing master plans. Alternatives can be developed only after
analysis of the collected data.
Rainfall. Storm sewers are typically designed based on precipitation events
having a statistical frequency of occurrence. This is the same as for
combined sewers. The information needed is the same as that described
previously for combined sewers in this section.
Flow Records. As with combined sewers, both dry-weather and wet-weather flow
information is needed. However, the primary emphasis is on dry-weather
flow. Dry-weather flow records will identify the base flow from infiltration
and inflows (basement sump discharges, cooling water, surface drainage, etc.)
that limits the capacity of the storm sewer for conveyance of the storm
flows. It is also necessary to determine the capacity available in sanitary
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sewers if stored flows are to be bled into the sanitary sewer for conveyance
to an existing treatment plant.
Drainage Area Characteristics. The characteristics previously identified for
combined sewer systems also apply to separate storm sewer systems. These
include topography, land use, geology and soils, and size.
Suspended Sol ids Characterization. The need for suspended solids
characterization of the stormwater in separate storm sewers is similar to that
for combined sewer overflows. The procedure and application should be the
same as described previously for combined sewage.
Collection System. The configuration of the collection system will determine
where storage or sedimentation facilities can be located. In-system storage
is best accommodated where pipes are large and relatively flat. Inline and
offline storage can be located throughout the collection system.
Discharge and regulator locations can be used to control flow to storage or
sedimentation facilities. Additional controls may be required to direct flow
into or discharge flow from facilities at these locations.
Existing pumping stations can be used to effect storage within existing storm
sewers by adjusting the pump operation controls. Pumping stations can be used
to direct selected flow to a storage facility or direct all flow to a
sedimentation facility. Discharge from a storage facility can be controlled
by a pumping station also.
Determine Pollution Load
The need to determine the pollution load for separate storm sewer systems is
the same as for combined sewer systems as described previously.
Since the particle size distribution and the particle volume distribution do
not remain constant during the runoff process, it is necessary to determine
the typical change of these factors with time. This is true not only for
suspended solids but also for any other pollutants of interest that are
associated with the suspended solids. The change of pollutant load with time
will determine whether storage of a certain volume of runoff or sedimentation
for the entire runoff will be most effective in removing the desired
pollutants. If several pollutants are of concern, a combination of storage
and sedimentation may be necessary depending on the pollutant load variation
with time.
Identify Flood Control and Pollutant Removal Objectives
At this point, it is necessary to identify the flood control or pollutant
removal objectives or both to be used for design. Storage or sedimentation
facilities can be used not only for pollutant removal but also for flood
control.
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Flood Control Only. If flood control is the only objective, storage would be
used rather than sedimentation. The storage could be onsite, in-system,
inline, offline, downstream, or any combination thereof. The facilities would
be designed strictly for flood control and the pollutant removal that occurs
when storage is used would be simply accepted.
Dual Purpose. When the pollutant loads are known, the decision can be made to
incorporate pollutant removal as a stragegy into the control facilities. In
this case, the design of the facilities is such that both flow control and
sedimentation are included. However, the two are not necessarily weighted
equally. The overriding design concern may be flood control, but with the
maximum sedimentation possible consistent with the flood control need.
If a dual need is identified, it is also possible to retrofit existing storage
of flood control facilities to improve pollutant removal. Innovative designs
may be necessary to effect improved pollutant removal in existing flood
control facilities.
New facilities can be designed while maximizing both the flood control and
pollutant removal objectives.
Control Optimization
The objective of any separate storm sewer control system should be to maximize
the effectiveness of the control system by producing the greatest desired
effect for the funds available. The desired effect is the objective(s)
identified in the previous section. The steps involved are essentially the
same as those described in the Combined Sewer section.
Identify Sources of Storage. The storage can be centralized or dispersed
throughout the separate storm sewer system. It can be onsite before it ever
enters the sewer system, in-system as online or offline storage, or downstream
just before discharge to the receiving water.
Identify Treatment Locations. Treatment locations of the pollutants removed
in storage or sedimentation facilities must be identified. The sediment
removed from the storm flow can be discharged to the sanitary sewer for
conveyance to existing dry-weather water pollution control plants for
treatment and disposal. Another option is to have a dual-use treatment plant
that handles both dry- and wet-weather flow. Such dual-use facilities are
usually new treatment facilities or expansions of existing dry-weather
treatment facilities.
Cost Analysis. The cost analysis procedure is the same as described for the
combined sewer overflow control.
Optimization. The effect on the cost and pollutant removals of the variations
in the size and location of the various facilities should be determined. For
the funds available, the locations and sizes of the required facilities is
optimized to provide the greatest effectiveness.
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Pollutant Budget Analysis
To determine the overall effectiveness of the flood control and sediment
removal facilities, it is necessary to perform a pollutant budget analysis. A.
mass balance of the residuals (solids) in storage during both dry and wet
periods is needed. This is necessary so that the pollutant loadings on both
the treatment facilities and the receiving water can be determined. The
treatment facilities must be able to handle the solids during an after-the-
storm event. Those solids include any deposited by dry-weather flow in the
storm sewer as well as those from the storm flows.
If an objective is to minimize the pollutant loading on the receiving water,
any sediment removal facilities must not have accumulations of dry-weather
pollutants that could mix with the storm flow to produce a greater pollutant
concentration in the outflow from the storage or sedimentation facility than
would have occurred if the stormwater were allowed to pass untreated.
The pollutant budget analysis may act as a modifier to the flood control
design if pollutant removal is a major objective. In other words, the design
of the facility for pollutant removal may take precedent over the flood
control design criteria.
Operating Strategy for Design
It is necessary to determine an operating strategy for the design of the
separate storm sewer control facilities. As with the combined sewer overflow
control facilities, the alternatives for the physical layout of the facilities
must be selected (both locations and sizes), the pollutant budget analyzed and
refined, and an optimization analysis performed. If necessary, this may be an
iterative process to determine the apparent best alternative for
implementation.
RETROFITTING OF EXISTING FLOOD CONTROL FACILITIES
Temporary storage is one of the most commonly applied flood control
techniques. Many existing flood control detention facilities may be modified
to enhance pollution control of the stormwater as well.
The most common flood control facilities in the United States are wet and dry
ponds [23]. In many cases, the pond is not designed to maximize the potential
for pollutant removal while acting as a flood control facility. However,
often it is possible to retrofit such facilities so that they serve as both
flood control and pollutant removal facilities. Toward that end it is
important that the detention time be maximized; horizontal velocity gradients,
vertical velocity gradients, and turbulence near the inlet and outlet be
minimized; and access be provided for removal of sediment and floatable
debris. Pollutant removal in a flood control pond or facility is most often
accomplished by sedimentation; however, biological action and adsorption in
the soil resulting from percolation and infiltration can also remove
pollutants.
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To promote effective sedimentation in a flood control facility, the following
design features should be incorporated [2]:
1. Lengthrwidth ratio greater than 2:1 where the length is the straight
line distance between inflow and outflow.
2. Wedge shape with the inlet at the apex of narrow end.
3. Minimum length between inlet and outlet of 3 ft (1 m) for each acre
of watershed.
4. A surface withdrawal system to minimize resuspension of deposited
materials.
5. Minimum permanent pool depth of 3 ft (1 m) for wet ponds.
6. Provision for complete drainage.
For dry ponds, a swale should be provided in the pond bottom so that any dry-
weather flow can be transported directly to the outlet. The outlet should be
at the invert of the swale and be designed to pass the dry-weather flow
unrestricted.
Several modifications are possible to increase the pollutant removal
efficiency of existing or planned flood control facilities. Gravel or cement
may be added to the pond walls and bottom to prevent erosion by stabilizing
the soil. Baffles or other energy dissipation devices installed near the pond
inlet may distribute flow and reduce turbulence caused by the velocity of the
flow entering the pond. The pollutant removal performance of wet ponds can be
ensured or improved by keeping the length:width ratio greater than 3:1.
Installation of vertical baffles, similar to a fence that extends from the
pond bottom to the high water level, can provide a series of channels that
guide the flow on a route that promotes plug flow (first in, first out)
through the pond. Such channels should have a width:depth ratio of from 1:1
to 2:1. This helps prevent short circuiting of the flow and excessive
suspended solids escape through the outlet caused by insufficient detention
time. Concrete lining of the area near the inlet where the heavy sediment
settles makes removal of that sediment much easier.
Since biological activity, straining, and adsorption take place as flow
infiltrates and percolates through the soil underlying a pond, increasing the
infiltration capacity will improve the pollutant removal efficiency.
Installation of underdrains in dry ponds is one method of increasing
infiltration capacity. The pollutant removal performance of wet ponds may be
increased by raising the level of the outlet pipe. The permanent pond size
will be increased and sedimentation performance should improve. Of course,
some flood control capacity will be lost.
Some pollutants, particularly nutrients, are utilized by plants growing in the
pond. If the plants are allowed to die and decompose, the nutrients are
released back to the pond. Harvesting and proper disposal of such vegetation
will remove those pollutants from the stormwater control system [24].
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INTEGRATION PROCESS EXAMPLES
The general principles presented in the integration section can be practically
explained by use of illustrations. In the first example, a storage and/or
sedimentation basin is to be placed in a' developed urban area to mitigate
flooding and stormwater pollution impacts. The process of choosing a location
for the basin and examination of the applicability and compatibility of this
control method are examined.
The second example involves the retrofitting of a flood control facility to
improve the pollutant removal effectiveness and improve the quality of the
stormwater discharged. The third example describes the integration of a
retention and attenuation facility into a developing area.
Storage and/or Sedimentation Basin Integration (Flood and Pollution Control)
Assumptions. The siting of a storage and/or sedimentation basin involves
three factors: existing facility interface, land use compatibility, and space
availability. The preliminary phases of integration planning and data
gathering had identified a storage and/or sedimentation facility as the most
appropriate control method for accommodating the stormwater from a portion of
the city's separate storm sewers.
The goals of stormwater management are: (1) to eliminate flooding along a
river, and (2) to reduce the pollutants in overflows to a river. The storage
and/or sedimentation facility was selected as a control method because storage
volume is required to prevent flooding in the urbanized portion of the city
and sedimentation also afforded a pollutant reduction mechanism to improve
overflow quality.
For the basin to interface with existing facilities, it must be located near
existing sewers. It should also be sufficiently close to the treatment plant
for minimizing sludge transmission distance and upstream of the area with
storm sewers subject to flooding during high runoff periods. The areas
meeting these criteria are in an area of the city where the land uses are
commercial and light to medium industry. Within the area are a few unimproved
lots used for parking and some older warehouses where renovation is planned.
The installation of a storage and/or sedimentation basin in the area would be
compatible with existing land uses.
Due to limited space availability, the locations for the basin are narrowed to
two potential sites: an existing parking lot and a warehouse scheduled for
demolition and renovation. The basin, if located at the parking site, could
be constructed to allow parking above it, thus permitting dual use of the
site. Alternatively, the warehouse site is proposed as a recreational
facility for workers in the area, which also offers a dual use potential.
Functional Compatibility. In initial planning stages, the functional
compatibility between the basin and existing facilities is briefly considered
only in selecting a general area to search for compatible locations. During
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the integration process, the flow scheme, impacts, and operation and
maintenance aspects must be investigated in more detail for functional
compatibility.
The proposed flow scheme is to have the basin offline and connected to the
storm sewer by regulators. The primary function of the basin is to prevent
flooding. When flow in the sewer reaches maximum stage without flooding, the
basin is to be brought online to intercept and hold the stormwater excess.
When the peak flow subsides, stored water will then be drained back into the
sewer.
The impacts of the storage/sedimentation basin in the storm sewer system are
to reduce peak flows and remove solids. Should wet-weather treatment become
necessary in the future, the basin will help to distribute the flow volume
over a longer period enabling a smaller maximum design capacity for the
treatment plant while maximizing the volume of the flow receiving treatment.
The operation of the basin will be triggered automatically by flow depth
sensors in the sewer. Sludge is discharged to the sanitary sewer following
the storm. Drainage of the basin under normal operation is to the storm
sewer. Maintenance and cleanup operations will occur after each storm.
An alternate drainage path for the facility is through the dry-weather
treatment facility. Current design of the basin has the dry-weather drainage
path option, as well as permitting a potential reverse flow path from the dry-
weather facility to the basin. The pipe, to allow dry-weather flow diversion
to the basin, is to be installed at the same time the basin sludge discharge
line is being constructed.
Process Compatibility. The two flow schemes for the basin have different
impacts on the treatment facilities. Flexibility, capacity, and quality
impacts are discussed. The flexibility of the system is high. The basin is
designed to operate as a peak flow detention facility with the added ability
of being able to route flow to the dry-weather treatment facility. For dry-
weather plant failure or emergency conditions, the basin could provide standby
storage for subsequent return to the dry-weather plant for treatment.
Additional flexibility is obtained by using the connection from the dry-
weather plant to the offline storage for temporary storage of dry-weather
flows during peak periods. When the treatment plant reaches its design
hydraulic capacity, the useful function of the plant is extended by storage of
peak dry-weather volume and then processing it during low flow periods. This
flexibility can only be achieved during non-storm periods. In all of the
operation options, odor control measures must be implemented; therefore, the
dry-weather processing option in this case does not impose added design cost
due to odor control.
The impacts of the basin on treatment facilities are both positive and
negative. The positive impact includes postponing capacity expansion of the
dry-weather plant due to mitigating peak flow demands. The negative impact
includes increased maintenance and operation of the sludge processing
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facilities. If the sludge process requires degritted sludge, then
arrangements for degritting the wet-weather sludge must be provided. Options
include installation of grit removal equipment at the basin or introduction o~f
the sludge ahead of grit removal equipment in the dry-weather treatment
facility. Since the stormwater flows are seasonal and sporadic, the demands
which stormwater processing imposes on treatment facilities are highly
variable. Thus, the treatment plant should be able to handle the additional
loads since they will not drastically increase the average annual load on the
plant.
The development of treatment facilities for the city is still undergoing
analysis. To meet flow and quality requirements, the city will have to
construct treatment facilities. Available options are expansion of the dry-
weather plant to accommodate some stormwater treatment, or construction of new
separate facilities for wet-weather processing. The storage and/or
sedimentation basin is an integral part of either option. The functioning of
the basin in regard to dry-weather facilities was previously discussed. The
basin will operate in a similar fashion with wet-weather facilities. The
basin's primary function is to prevent flooding and, by storing flow, permit a
reduced peak design volume for the downstream treatment plant. Sludge would
be transferred to expanded sludge facilities at the dry-weather site.
The improvement in quality of the discharge to the river will consist
primarily of reduced sediment loadings. The load of heavy metals and other
pollutants associated with sediment will also be reduced in the discharge.
The number of overflows from the area served by the basin will be reduced,
with a corresponding increase in discharge quality due to processing through
the treatment plant.
Flood Control Retrofit
Assumptions. To alleviate the flooding in a portion of the city, a flood
control basin was constructed on city property adjacent to the river. The
embankment around the basin and along the river was raised to prevent
flooding. Stormwater is discharged to the river from the basin by both
pumping and gravity. Flap gates were installed on the gravity discharge pipes
to prevent river water from entering the basin. When the water level is high
in the river, stormwater is discharged from the basin by pumping. When the
water level in the river subsides, stored runoff flows to the river by
gravity.
With the increased concern and regulations regarding stormwater pollutant
discharges, the city has determined it would be cost effective to retrofit the
existing flood control basin to improve the quality of the stormwater
discharge into the river.
Facility Modifications. The modifications needed include physical and
operational changes. The current mode of operation is to drain runoff into
the river as quickly as possible. When gravity flow into the river is
possible, no buildup of runoff volume occurs in the basin. When pumping is
required, the volume in the basin is allowed to accumulate sufficiently to
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prevent frequent, short-period operation of the pumps. Most of the volume,
however, is kept in reserve for sudden peak runoff occurrences.
The operational changes required involve detaining a volume of stormwater
sufficiently for sedimentation to occur. During low river level, gravity flow
would fill, the basin, overtop a weir (or other restraining device), and
continue by gravity flow to the river. During the pumping phase, effluent
from the sedimentation basin would flow to the pump sump. For peak flush
periods, a partial flow bypass from the entry of the facility to the sump
would prevent velocities in the sedimentation basin from getting high enough
to resuspend the sediment. However, a final decision on the inclusion of a
partial flow bypass should not be made until a pollution budget analysis has
been completed. The pollution budget analysis will help to determine whether
the most efficient removal at high flows occurs when part of the flow is
bypassed or when all of the high flow is passed through the basin.
The physical modifications to permit the above operation mode include
installation of stilling basin, weir (or other restraining device), sediment
removal equipment, compartmentalization of the basin, and peak flush
conveyance channel.
To create the sedimentation basin, a weir or a stop log type of flow restraint
is installed. The flow then is subject to detention time in the basin. The
flow then proceeds from the basin to the pump sump.
By compartmentalizing the basin, only a portion of the basin is used for
sedimentation while the remainder is available to handle the peak flow without
sedimentation. This prevents sudden large flows from resuspending material in
the sedimentation basin. An increase in the pumping capacity is necessary to
offset the loss in standby volume in the sedimentation portion of the basin.
To keep the basin operating as desired, sediment must be removed on a routine
basis. The frequency of the required sediment removal operations is dependent
upon the volume and configuration of the basin, the frequency of storm events,
and the suspended solids loads associated with the storm events. For small
basins, sediment may have to be removed following each storm event or after
only a few events. Sediment may be allowed to accumulate for several years
between cleanings in very large basins.
Installation of sediment removal equipment in small basins that must be
cleaned frequently may be required as part of the retrofitting of the
facility. For large basins where long periods between cleanings are
acceptable, modifications may include provision for access for cleaning and
maintenance work.
Functional Compatibility. The flood control facility, as retrofitted, is
compatible with stormwater management goals. Impacts of the retrofit of the
facility on flow handling are minor. The decrease of the storage volume is
offset by increased pumping capacity so a higher volume of stormwater will be
pumped at the design storm peak flow, but there is no practical change in the
ability of the system to protect the residential area from flooding.
4-35
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Operation and maintenance of the facility is altered due to increased cleanup
activities after storms and higher maintenance requirements. The facility is
self-actuating so the beginning of a storm event does not necessitate rapid
mobilization of staff.
Process Compatibility. The quality impacts of retrofitting the flood control
facility are to reduce the amount of sediment and associated pollutants
released during storm events. Except for the largest storms, most of the
storm flow undergoes primary treatment. During bypass conditions, only part
of the flow is subject to primary treatment. If the first flush phenomenon
applies to the city's stormwater system, the retrofit flood control facility
will detain the first flush volumes for treatment, enabling a greater percent
removal efficiency on an average annual basis.
Retention and Attenuation Facility Integration
Assumptions. An existing cattle ranch has been rezoned for single family
residential use with a local commercial business district. A small creek
channel, with a pond for watering cattle, crosses the property and enters the
storm sewer in a presently developed area of the city. The creek is dry
except during the spring. Storm and sanitary sewers will be extended into the
developing area. The city has an ordinance requiring the peak runoff rate
from any future development not exceed the peak rate from the undeveloped
site.
The potential retention and attenuation control methods for this site are
numerous. Rooftop storage, parking lot ponding, and low structural need
retention or detention basins offer the highest potential for being optimum
stormwater management control techniques. The stormwater control measures can
be easily integrated as the site plans are being developed.
The adaptability of the control methods for implementing storage or
sedimentation or both and retention is an advantage. As new development in
the area continues, the most appropriate control method can be chosen for each
subsequent site without conflicting with other areas. For an expanding
system, the net effect is that each expanding area adds facilities as
required, which prevents negative impacts on downstream facilities.
Control methods need to be selected with compatibility in mind. Rooftop
storage, plaza ponding, and parking lot ponding are all compatible with
commercial business land use. Retention or detention ponds can be compatible
control methods with residential land uses. Other facilities requiring more
structural components for storage or sedimentation or both also can be
compatible. Low structural need control facilities can be blended with the
natural surroundings.
The low structural need facilities often require larger land areas. In the
new development, there is sufficient land for retention ponds. The options
are open to have dry ponds or wet ponds. The cattle watering pond, having
been in existence for many years, has developed natural vegetation for pond
4-36
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areas. This portion of the site could be converted to a wet pond with
additional storage volume requirements being provided by dry pond storage.
Functional Compatibility. After analysis of the site to optimize the
integration of stormwater control methods, the following combination of
controls was selected. For the commercial business area controls include:
(1) rooftop storage and controlled rate of release, and (2) parking lot
storage with drains to a small structural basin to aid in petroleum
hydrocarbon and sediment removal and to add additional required volume. The
additional volume was required because, without becoming a nuisance or hazard,
parking lot storage was not sufficient to handle the business district runoff
volume. The designed rooftop and parking lot storage is expected to be
sufficient to detain the peak flows and prevent exceeding the storm sewer
capacity.
The residential section has both a wet and dry pond to regulate storm flows.
The wet pond was established at the cattle watering pond. A dike surrounding
the pond was built up sufficiently to provide for storage above the normal
level of the pond. The wet pond serves as a retention basin for most
storms. The large storm runoffs exceeding the capacity of the wet pond
continue on to the dry pond. The dry pond also was established as a
recreational field during periods without rain. Drainage swales throughout
the development have stepped barriers to add to the detention storage volume
available.
The required maintenance procedures can be distributed over a period of time
and would not, therefore, represent a labor intensive period at any point
during the year. Channels must be kept free of debris. The rooftop units
must be checked periodically. The parking lot area should be kept swept with
normal street cleaning procedures. The dry pond must be maintained in grass,
free from excess vegetative growth. Plant growth around the edges of the wet
pond also must be controlled.
Additional maintenance around the wet pond is required after storms to remove
accumulated floatable materials. Dredging or scraping the bottom of the pond
periodically will prevent loss of recreational use due to sedimentation. None
of the control methods conflict with the operation of the storm sewer.
Process Compatibility. Retention and attenuation facilities are compatible
with any of the other treatment processes. Since the control measures in the
new development are primarily flow control measures, little impact of
treatment processes will be observed. The only quality effects will be from
sweeping of the parking lot to reduce the quantity of pollutants in the runoff
and the wet retention pond that will contain pollutants from part of the
residential section. The chosen control methods, being independent of the
other stormwater management controls, do not impose constraints on downstream
facilities and therefore allow for maximum flexibility.
Treatment capacity will not be negatively impacted, since the new development
is designed to have the same net runoff effect as the unimproved property. A
slight decrease in pollutants is expected, causing a reduction in pollutant
4-37
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loading. The accumulated sediment removed from the wet and dry ponds must be
disposed of at appropriate landfill sites.
The quality impacts of this stormwater management system are to slightly
reduce pollutant loadings in the stormwater runoff reaching the river.
Groundwater impacts of retention facilities must also be examined. Stormwater
runoff from developed areas may not contain high sediment loads but additional
pollutants such as petroleum hydrocarbons, heavy metals, and other toxic
compounds are higher than from the undeveloped area. This should be taken
into consideration when percolation is used for disposal of stormwater.
REFERENCES
1. Stahre, Peter. Flodesutjamning i Avloppsnat (Flow Balancing in Waste
Water Nets). Byggforskningsradet (Swedish Council for Building
Research). 1981.
2. American Public Works Association. Urban Stormwater Management - Special
Report No. 49. 1981.
3. Heaney, J.P., et al. Nationwide Evaluation of Combined Sewer Overflows
and Urban Stormwater Discharges: Volume II. Cost Assessment and
Impacts. USEPA Report No. EPA-600/2-77-064b. NTIS No. PB 266 005.
March 1977.
4. Linsley, Ray K. and Joseph B. Franzini. Water Resources Engineering, 2nd
Edition. McGraw-Hill Book Company. 1972.
5. Chow, Ven Te, Editor-in-Chief. Handbook of Applied Hydrology. McGraw-
Hill Book Company. 1964.
6. Heaney, J.P., et al. Storm Water Management Model: Level I - Preliminary
Screening Procedure's. USEPA Report No. EPA-600/2-76-275. NTIS No. PB 259
916. October 1976.
7. American Society of Civil Engineers. Hydrology Handbook, Manual of
Engineering Practice No. 28. 1949.
8. Linsley, R.K. Jr., et al. Hydrology for Engineers. McGraw-Hill Book Co.,
Inc. 1958.
9. DiTorio, D.M. Statistical Analysis of Urban Runoff Treatment Devices.
EPA National Conference on 208 Planning and Implementation, Washington,
D.C. March 1977.
10. Hydroscience, Inc. Procedures for Assessment of Urban Pollutant Sources
and Loadings; Chapter 3 in Areawide Assessment Procedures Manual. USEPA
Report No. EPA-60/9-76-014. July 1976.
11. Driscoll, E.D., et al. A Statistical Method for Assessment of Urban
Stormwater: Loads - Impacts - Controls. USEPA Report No. EPA-440/3-79-
023. NTIS No. PB-299 185. May 1979.
4-38
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12. Howard, C.D.D., et al. Storm and Combined Sewer Storage Treatment Theory
Compared to Computer Simulation. USEPA Report No. EPA-600/2-91-125. NTIS
No. PB81-222341. July 1981.
13. Smith, W.6. and M.E. Strickfaden. EPA Macroscopic Planning Model (EPAMAC)
for Stormwater and Combined Sewer Overflow Control: Application Guide and
Users' Manual. USEPA Report No. EPA-600/2-83-086. NTIS No. PB 83-259
689. November 1983.
14. Hydrologic Engineering Center, Corps of Engineers. Urban Storm Water
Runoff: STORM. Generalized Computer Program 723-58-L2520. May 1975.
15. Huber, W.C., et al. Storm Water Management Model User's Manual, Version
III. EPA Report No. (in press). Project No. CR-805664.
16. Huber, W.C., et al. Interim Documentation, November 1977 Release of EPA
SWMM. Draft Report to EPA. November 1977.
17. Basta, D.J. and B.T. Bower, Editors. Analyzing Natural Systems - Analysis
for Residuals for Environmental Quality Management. Johns Hopkins
Press. Baltimore, Maryland. June 1982.
18. Brandstetter, Albin. Assessment of Mathematical Models for Storm and
Combined Sewer Management. USEPA Report No. EPA-600/2-76-175a. NTIS No.
PB 259 597. August 1976.
19. Metcalf & Eddy, Inc. City and County of San Francisco Southwest Water
Pollution Control Plant Project. Final Project Report. February 1980.
20. Bondurant, J.A., et al. Some Aspects of Sedimentation Pond Design.
National Symposium on Urban Hydrology and Sediment Control. Kentucky
University. 1975.
21. Dalrymple, Robert J., et al. Physical and Settling Characteristics of
Particulates in Storm and Sanitary Wastewaters. USEPA Report No. EPA-
670/2-75-001. NTIS No. PB 242 001. April 1975.
22. Pisano, W.C. et al. Dry-Weather Deposition and Flushing for Combined
Sewer Overflow Pollution Control. USEPA Report No. EPA-600/2-79-133.
NTIS No. PB 80-118 524. 1979.
23. American Public Works Association. Survey of Stormwater Detention
Practices in the United States and Canada. Unpublished Report.
24. Akeley, R. Retention Basins for Control of Urban Stormwater Quality.
Proceedings: National Conference on Urban Erasion and Sediment Control.
EPA-905/9-80-002. 1980.
4-39
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SECTION 5
DESIGN OF RETENTION STORAGE FACILITIES
Stormwater retention is the storage of excess runoff for complete removal from
the surface drainage and discharge system. Stormwater retention facilities
may take a variety of forms. For instance, in Orlando, Florida, perforated
aluminum culverts have recently been installed below street level in the
downtown area to store and percolate ground water [1]. This section describes
design procedures and operation considerations for the most common retention
storage facility type--the pond. Stormwater retention ponds may be divided
into two general categories: dry ponds and wet ponds. Dry ponds are earthen
basins that are wet only during and immediately after runoff events. Excess
flows are directed to them and allowed to percolate to the groundwater. Wet
ponds are permanent ponds in which Stormwater is stored by varying the level
of the pond. Stormwater also percolates to the groundwater from wet ponds.
Early in 1980, the American Public Works Association conducted a survey of
Stormwater storage practices in the United States and Canada [2]. Of the
12,683 facilities reported, almost 50% were dry ponds. An additional 2,382
facilities were wet ponds. Not all, however, operate as retention basins.
Percolation of Stormwater to the groundwater offers a number of benefits in
addition to controlling Stormwater flows. The groundwater is recharged. A
total of 1,513 facilities were reported in use for groundwater recharge. This
is particularly important in areas where the groundwater basins are being
overdrawn and increased urbanization is reducing normal infiltration. In
addition, percolation through a soil column has been shown to be very
effective in removing bacteria, oxygen demanding material, and suspended
material from a wastewater. However, dissolved toxic materials may pass
through to the groundwater.
DESIGN CONSIDERATIONS
Runoff storage and percolation are the primary ways in which dry ponds reduce
pollutant loadings to receiving waters. As with other Stormwater storage
facilities, ponds allow sedimentation removal of suspended materials during
overflows. Additional pollutant removal in wet ponds may also result from
biological oxidation of suspended and dissolved organic material in the
runoff. Soil characteristics and permeabilities play an important role in
design and operation of these facilities. In addition, ponds frequently are
designed to serve multiple purposes, usually for flood control and
recreational facilities. Other purposes also include aesthetics as well as,
less frequently, Stormwater pollution control facilities. Dry ponds may serve
5-1
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as playgrounds or athletic fields when not in use for stormwater control while
wet ponds are often also recreational lakes. Other uses (storage reservoir,
drainage control,, and improved local aesthetics) may have as great or greater
impact on size, location, and configuration decisions as pollution control.
This section includes a discussion of design considerations and factors for
both types of ponds.
Size
Size requirements include not only volumetric capacity but also both surface
and soil interface area requirements, as well. The pond configuration depends
on:
The runoff storage volume needed.
t The surface area and weir length required to assure adequate settling
during sedimentation operation.
The surface area needed for adequate transfer of oxygen into the pond
water to allow aerobic decomposition of organic pollutants.
The soil-water interface area needed for adequate percolation of
stored runoff between storm events.
t The area needed to serve whatever dual uses the basin may have.
The ideal pollution control design is a balance of storage capacity and
sedimentation removal that will yield the necessary wasteload reduction for
the lowest cost. Sizing determinations, based on these factors, are the same
as for detention storage/sedimentation basins discussed in Section 7.
The storage volume required is also a function of soil characteristics of the
pond site, particularly soil permeability, subsurface geologic conditions,
stormwater pollutant makeup, and the antecedent dry-weather period between
runoff events.
Permeability is a term used to describe the ease with which liquids and gases
pass through soil. In general, water moves through soils or porous media in
accordance with Darcy's law (Figure 16):
q = k dH/dl (5-1)
where q = the flux (rate of flow of water per unit cross-sectioned
area), in./h (cm/h)
k = the permeability, in./h ( cm/h)
dH/dl = the toal head (hydraulic) gradient, ft/ft (m/m)
5-2
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REFERENCE DA TUX LEVEL
V
AH - K -H
SATURATED FLO!
Figure 16. Schematic showing relationship of total
head (H), pressure head (h), and gravitation head (Z) for
saturation flow [3].
The total head (H) is the sum of the soil water pressure head (h), and the head
due to gravity (z), or H = h + z. The hydraulic gradient is the change in
total head (dH) over the path length (dl). The permeability is defined as the
proportionality constant, k.
Soil permeability is determined to a large extent by soil texture with coarse
materials generally having higher permeabilities. In some cases, the soil
structure may be of equal importance. A well-structured clay with good
stability, for example, can have a greater permeability than a much coarser
soil. The ionic nature of the soilwater (capillary or contact water that
remains in the soil after groundwater has drained by gravity) and type of
vegetation may also affect permeability by reducing the effective pore size in
the soil.
Percolation, the movement of water through the soil, is a distinctly different
property from infiltration. The infiltration rate of a soil is defined as the
rate at which water enters the soil from the surface. When the soil profile
is saturated, the infiltration rate is equal to the effective saturated
permeability of the soil profile. When the soil profile is relatively dry,
the infiltration rate is higher because water is entering large pores and
cracks. When water is applied, large pores fill and clay particles swell,
reducing the infiltration rate to a near steady state value.
As with permeability, infiltration rates are affected by the ionic composition
of the soilwater and by the type of vegetation. Of course, any tillage of the
soil surface will affect infiltration. Factors that tend to reduce
infiltration rates include clogging by solids in the applied water, gradation
5-3
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of fine soil particles, clogging due to biological growth, and gases produced
by soil microbes.
Field measurements of soil infiltration rates and permeabilities are an
essential part of the design of retention/percolation basins. Generally, the
steady state infiltration rate serves as the basis of selection of the design
hydraulic loading rate. The rapidity with which water moves through a soil
profile will be determined by the least permeable layer in the profile.
However, subsurface permeabilities must also be considered.
There are many available techniques for measuring infiltration, including
flooding basins, sprinkler infiltrometers, cylinder infiltrometers, and
lysimeters. The preferred technique is one that approximates the actual
method of water application. In the case of dry ponds, this would be flooding
basins or cylinder infiltrometers. It is strongly recommended that hydraulic
tests of any type be conducted with actual stormwater when possible. The
infiltration rate of a particular site will tend to decrease with time as
stormwater is applied as a result of continuous inundation which excludes
oxygen from the soil, thus engendering the growth of anaerobic bacteria on the
organic matter deposited on or contained in the soil with resultant clogging
of the soil system [4], This clogging generally occurs only at the surface,
and the infiltration rate may be returned to nearly its original value by
scarifying the surface or permitting it to drain and to reestablish aerobic
conditions [4]. Selection of a design infiltration rate must take into
account this clogging [3].
Particularly for wet ponds, the biodegradable organic content of the runoff
may also influence pond sizing. Biological stabilization of organic materials
is accomplished with the use of dissolved oxygen, if available. If the
dissolved oxygen is depleted, anaerobic decomposition occurs, producing
odorous gases and discoloring the water. Oxygen is dissolved into the water
at the air-water interface or from oxygen released by algae in the water. The
rate at which oxygen is used is a function of the organic loading and of the
water temperature. The rate at which oxygen is dissolved depends on the
magnitude of the oxygen deficit and the surface turbulence. Therefore,
organic loading per unit surface area may be an important consideration. The
acceptable loading rate depends on alternative pond uses. Domestic wastewater
treatment ponds are usually loaded at 15 to 35 Ib BODg/acre'd (17 to
40 kg/ha*d) [5]. It is recommended that stormwater retention ponds not be
loaded at higher than 5 to 10 Ib BODc/acre-d (6 to 11 kg/ha-d). An active
biomass to provide BOD reduction in domestic wastewater treatment ponds can be
maintained since the flow and pollutant loadings remain relatively constant;
the intermittent flow and variable pollutant loads reaching stormwater
retention ponds are not conducive to maintaining a stable active biomass.
Organic loading usually is not a problem for ponds that control stormwater
runoff. Anaerobic conditions may result in ponds with small surface areas
used to control combined sewer overflows.
To avoid the generation of malodorous gases or the development of nuisance
insect populations, it is recommended that dry ponds should be designed to
5-4
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allow complete percolation of the full pond retained flow in not more than 7
days for stormwater runoff, or 3 days for combined sewer overflows.
In most cases, flood control is a dual purpose of stormwater retention/
percolation ponds, and the flood control and hydrograph attenuation needs
usually determine the storage volume required. The 1980 APWA survey revealed
that for detention facilities, the most frequently cited basis for flood
control storage sizing is the 100 year rainstorm, followed by the 10 year and
the 25 year storm, in that order [2]. Design of ponds specifically for
control of flooding is not within the scope of this manual, but is adequately
discussed in the literature.
Location
The suitability of various sites within a drainage area for pond facilities
depends on (1) the availability of the site, (2) compatability of surrounding
land uses with a stormwater retention facility use and other dual use
functions, (3) the area required, (4) the soil characteristics, and (5) the
location of the site with respect to tributary catchment size and to other
sewer or drainage facilities.
Evaporation may also be a factor in the disposition of water from retention/
percolation ponds. For dry ponds, its effect is slight, since such ponds are
usually designed to empty within 7 days, and evaporative losses over such a
short period are small. Evaporation may be a consideration in maintaining the
permanent pool in a wet pond, particularly in areas subject to seasonal
rainfall. In the western United States, for instance, a retention/percolation
pond may operate as a wet pond during the winter rainy season and be dry
during the summer.
The first consideration in identifying potential locations for a stormwater
control facility is the availability of a given site. For new development
areas where urbanization is yet to occur, this is not usually a problem. For
installations in established areas, site availability can be an important
factor. Ideal locations may already be occupied by buildings or highways.
Even if the agency developing the control facility has powers of eminent
domain, exercise of such powers should be a last resort. In any case, fair
market value of an already developed site may make it prohibitively
expensive. Any site which has already been developed should be considered for
redevelopment as a stormwater control facility only after all undeveloped
sites have been identified, evaluated, and rejected.
Another important consideration for retention basins is the compatability of
land uses in the surrounding area. For facilities in or near residential or
commercial areas, more intense operation and maintenance efforts are required
to prevent nuisance conditions from occurring. Less stringent operation and
maintenance efforts may be required for retention facilities in more remote or
less visible locations.
Obviously, the size of the facility needed and the site soil characteristics
play very important roles. Preliminary screening of sites may be accomplished
5-5
-------
based on information from soil maps. Final designs must be based on field
testing of soil permeabilities.
First flushes, which often occur in both storm sewer discharges and combined
sewer overflows, usually are of a shorter duration for small tributary
catchments than for larger catchments. Thus, since the volume associated with
the first flush from a small catchment would be proportionately less than that
from a large catchment, several small retention basins may be more appropriate
than a single large basin. If first flush control is the established goal,
retention/percolation basins should be located accordingly. In addition,
rapid percolation of a stored volume of stormwater runoff, so that storage
volume is available in time for the next storm, usually requires a large soil-
water interface area. If the catchment is large, the necessary pond area will
also be large.
Location of the site with respect to the drainage and/or sewer system is
another factor. Ideally, locations should be selected to minimize transport
drainage conduits/channels from the existing drainage facilities, and also to
allow discharge of basin overflows with minimum of outfall piping
construction.
DESIGN PROCEDURE
The following section consists of a step-by-step procedure for design of
retention ponds. The first two steps are typically carried out at a planning
stage, and are discussed only briefly. The approach used in the design of
retention facilities should make use of existing experience, known concepts,
and developing theories. An integrated design procedure must be used to
insure that the desired functions of the pond (sediment removal, infiltration
and percolation, flood control, or flow reduction) are compatible with the
types of flow reaching the pond (stormwater runoff or combined sewer overflow)
and any other multi-use aspects (recreation, aesthetics, etc.). In actual
practice, retention ponds are very seldom used for combined sewer overflows
because the organic solids tend to seal the pond bottom and reduce the
infiltration capacity.
Step 1 - Quantify Functional Requirements
Using an accepted hydrologic analysis method, determine storm distribution
patterns for the urban area being analyzed. This should include a statistical
distribution of storm volumes, storm intensities, and storm durations and
frequencies. Historical rainfall data are available from a variety of
sources. The length of the rainfall time-step used to calculate the runoff
depends on the size of the watershed being analyzed, the length and intensity
of the storm, and the degree of accuracy desired. Typically, analysis of a
small watershed may require a 5- to 10-minute interval while a large watershed
may only require a 1-hr interval. Hourly rainfall data for many locations in
the Uni.ted States is available from the National Weather Service. Shorter
interval rainfall data are often available from drainage, sewerage, flood
control, or other special districts.
5-6
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Runoff is related to rainfall occurrence. Many methods are available; often,
a regional flood control agency will specify the runoff calculation method to
be used. For a first-cut analysis, a single event hydrograph may be
sufficient. However, for the final design a long-term hydrological evaluation
should be used and analyzed in the same fashion as a long term streamflow
record.
Next, pollutant characteristics should be determined. Actual field data
specific to the catchment being considered or to a nearby and similar
catchment should be collected. Included should be pollutant types and
concentrations. Particle size distributions, specific gravities, and/or
settleabil ities or settling velocities of suspended solids and associated
pollutants of concern should be determined. Information on settleability
characteristics is discussed in Section 7.
If a significant first flush of pollutants or suspended solids is evident, the
time variation of the mass loading rate can be an important consideration in
the design of the retention facilities. If most of the pollutant mass load
occurs during the early part of the storm event, it is important that that
portion of the flow be retained while later, more dilute portions of the flow
are allowed to pass on downstream. Significant solids depositions could also
affect operation and maintenance of the retention basins and need to be
considered in planning and designing the basins.
Step 2 - Identify Required Waste Load and Flow Reduction
In some cases, regulatory agencies may specify the level of flood control
required; the level of pollution control may also be specified but only in a
few cases. If an allowable pollutant load is not specified, the expected
impacts of the stormwater on the established beneficial uses of the receiving
water may be used to calculate the pollutant waste load reductions required.
Step 3 - Determine Preliminary Basin Sizing
Since a dual purpose of most stormwater retention ponds is flood control,
preliminary determination of storage volume needs is often based on flood
control requirements. Flood control requirements are usually expressed as
control of runoff peak flow from a given design storm to some specified rate,
often the predevelopment rate. The effect of a storage pond on runoff peak
flows is estimated by a flow routing procedure.
Based on the flood control aspects alone, the retention basin can be treated
as a simple reservoir. Flow routing for a reservoir requires that three
relationships for the reservoir be known: (1) an inflow hydrograph, (2) a
depth-storage relationship, and (3) a depth-discharge relationship. Routing
is the solution of the storage equation which is an expression of continuity:
T-TT=AS/At (5-2)
5-7
-------
where I = inflow rate
0 = outflow rate
S = storage
t = time
Using subscripts 1 and 2 to represent the beginning end of the period,
respectively,
0
S2 -
(5-3)
2 2
Equation (5-3) may be transformed to
°1 72S2\+°2
VT
t2 -
\~t
(5-4)
where t = time (routing period)
Solution of equation (5-4) requires a routing curve showing 2S/t + 0 vs. 0.
All terms on the left-hand side of the equation are known and a value of 2So/t
+ 02 can be computed. The corresponding value of 02 can be determined from
the routing curve. The computation is then repeated for subsequent routing
periods.
The depth-discharge relationship can be a composite made up of the
relationships for multiple outlets. An example of the routing curves for
typical reservoir is shown in Figure 17.
140
ICO 200 300 400 500 600 700 800 900 1000
Sloroge in sfd, discharge and ( 1- 0 \ in cfs
Figure 17. Routing curves for a typical reservoir.
5-8
-------
Alternative flood routing procedures, either hand computation, computer based
simulations, or graphical methods, may also be used.
The effectiveness of the pond in removing heavier sediments (soil particles)
can be estimated using the curve shown in Figure 18, based on Brune's work
[6]. Estimating removal efficiencies for lighter materials, such as organic
solids, is not so easy, since the effects of eddies and currents within the
basin are more pronounced for such particles.
UJ
> BO
O
*
Q 60
UJ
o.
a.
< 40
2 20
Ul
1
0 0
Ul
v) 10
' ' ~
Coarse sediment
(Sa
/
110.
7
,
/
A
f
^
k.
/
^,:
/
s
'/
1 ^*
1 1
/
/
A
-
,. .:
y
/
^'f.
*?-
= =1
-3 1 0 ' 2
'-^'
X
::= M
P
,*
*r
-*-
^>
EDIAN
5NDED
V
M
CURVE FOR NORM
RESERVOIRS
Illl 1
' Fine sed i m
.... ENV
PON
ent (Claj
II
ELOPE CURVES FOR
DEO RESERVOIRS
Mill
1 II
)
NORMAL
1 II
III
-
O"1 1 O1
CAPACITY-INFLOW RATIO
OR
POND VOLUME
AVERAGE ANNUAL INFLOW
Figure 18. Brune's trap efficiency curves [6].
A more rigorous method for estimating pollutant removals is the utilization of
a mathematical model for either single event or continuous simulation. The
two methods used in the Storage/Treatment block of SWIWI-Version III are
examples [7]. In the first case where pollutants are characterized only by
mass flow and concentration, the removal may be simulated as a function of
detention time, influent concentration, inflow rate, removal fraction of
another pollutant, incoming concentration of another pollutant, or any
combination of the above. The selection of the equation variable is left to
the user. In the other case, pollutants are characterized by their particle
size and specific gravity distribution, (or apparent specific gravity
distribution depending on the amount of air or oil in the interticies of the
particles), then are removed by particle settling. The removal is determined
by summing the effects of several ranges passing through the unit.
Application of these methods provides improved estimates of the removal
efficiency of the basins. However, if the actual specific gravity and
particle size distributions are not available or the concentration of
5-9
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pollutants in the flow is uncertain, the use of the latter method is not
warranted. This is particularly true in the case of combined sewer overflows
due to the generally higher percentage of organic solids when compared to
stormwater discharges (see Section 2).
,The required surface area for oxygen transfer should be based on a surface
loading of 5 to 10 Ib BOD5/acre-d (6 to 11 kg/ha-d).
Step 2 and Step 3 are iterative steps. The costs of mass load reduction must
be compared with the beneficial uses being protected.
Step 4 - Identify Feasible Pond Sites
Topographic and land use maps of the area may be used to locate potential
sites. Land use plans should be reviewed to make sure that conflicts of land
use will not occur in the future.
Removal of pollutants from water percolating through a layer of soil is a
complex process, and the efficiencies of removal may vary from pollutant to
pollutant. Percolation through soil is very effective in removing BODg,
bacteria, and suspended material. The removal of these pollutants from
percolated stormwater is usually complete. Other pollutants, such as some
dissolved heavy metals or salts, may be carried into the groundwater or
transported via the groundwater to resurface downgradient. The possibilities
of groundwater contamination or groundwater transport should be considered
when selecting a retention/percolation pond. See the performance section of
this chapter for a discussion of treatment mechanisms and efficiencies.
Once the pollutant removal that can be accomplished by sedimentation is
determined in Step 3, the additional pollutant removal that can be
accomplished by percolation must be determined so that feasible pond sites can
be identified. A time step approach using the hydrograph from Step 1 and the
associated pollutant loads from Step 2 should be used.
The volume of stormwater that can be percolated is dependent on the volume of
water in storage at the end of each time step and the infiltration/percolation
rate. Infiltration/percolation also takes place during the storm event when-
ever there is water in the pond. The volume percolated during each time step
is equal to the percolation rate times the length of the time step. But in no
case can the percolated volume exceed the stored volume at the beginning of
the time step.
The pollutant load removed is equal to the pollutant concentration in the
percolated volume multiplied by the percolated volume.
For wet ponds, even though the percolation is continuous the pollutant load
varies between dry-and wet-weather values. Also, evaporation should be
considered when sizing wet ponds.
Preliminary percolation area sizing of the pond may be performed using the
nomograph shown in Figure 19 and soil permeability ranges obtained from Soil
Conservation Service soil maps.
5-10
-------
1 , 000
400
- 200
- 1 00
PROBABLE RANGE OF
LONG-TERM
.INFILTRATION-
\
a.
v>
40
20
10
4.0
2.0
1 .0
0.4
0.2
0.1
0.02 OJM tl 0.« 2.0 6.0 tOjO MuD
PERMEABILITY RATES OF MOST RESTRICTIVE LAYER IN SOIL PROFILE, in./h
PERMEABILITY? SOIL CONSERVATION SERVICE DESCR I PT 1 VE TERMS
VERY SLOW
< 0.06
SLOW
0. 06-0. 20
MODERATE-
LY SLOW
0. 20-0.60
MODERATE
0. 60-2. 0
MODERATE-
LY RAPID
2.0-6.0
RA PI D
6.0-20.0
VERY RAPID'
> 20.0
* MEASURED WITH CLEAR WATER
1 i n. = 2. 54 cm
Figure 19. Soil permeability versus ranges of
application rates [3].
5-11
-------
The design infiltration rate should be 10 percent of the initial soil
permeability value to take into account the decrease in infiltration that will
result from surface clogging by suspended solids. The design infiltration
rate shown in Figure 19 should be used only for preliminary sizing purposes.
Actual field test results should be used for final design of the pond. The
required soil/water interface area may be calculated by Equation 5-5.
SV (5-5)
A = DI x ET
where A = the soil/water interface area required
SV = the calculated storage volume
01 = the design infiltration rate
ET = the emptying time
The storage volume required is that volume calculated in Step 3. The
permeability may be obtained from soil maps of the site and by reading the
chart at the bottom of Figure 19.
The emptying time, ET, is the time required to completely percolate the
storage volume. Ideally, the entire storage volume should be available at the
beginning of each runoff event. However, storm events are random, and the
interstorm time will vary. In addition, alternative uses will require that
the pond be dry some percentage of time. For purposes of performing a
preliminary estimate of bottom area, ET may be estimated for dry ponds by
Equation 5-6.
ET = (1 - N/100) x Avg antecedent dry period (5-6)
where Avg antecedent dry period = the average time between the
end of one storm and the beginning of the next storm where the
runoff volume is equal to or greater than storage volume.
N = the percentage of time during the year that alternative uses
require that the pond be dry, %.
For wet ponds, ET is equivalent to average interstorm time. Equation 5-6 may
be adjusted to account for seasonal rainfall patterns by calculating average
interstorm time and N for the most critical season. For dry ponds, ET should
not exceed 7 days for stormwater runoff; for combined sewer overflows, ET
should not exceed 3 days.
As many feasible sites as possible should be identified. Sites should then be
ranked, based on apparent acceptability. Some subjective judgment must be
used since many difficult to quantify factors must be considered, such as land
use compatibilities and possible alternative facility uses.
Step 5 - Investigate Most Promising Sites
Beginning with the highest ranked site, soil borings and infiltration and
permeability tests of the site should be accomplished. The two preferred
5-12
-------
methods of infiltration testing are flood basins and ring infiltrometers.
Each method is discussed in Appendix C of this manual. Infiltration testing,
particularly using ring infiltrometers, should be conducted on the most
restrictive soil layer underlying the site within the potential elevation
range of the excavated pond bottom.
Again, the measured infiltration rate can be expected to decrease with time as
stormwater solids clog the soil surface. The infiltration rate used in design
should be 10% of the measured infiltration rate at the soil surface but should
not exceed the permeability of the most restrictive soil layer.
Step 6 - Establish Basin Sizes
The next step is to determine a final basin size based on the measured
infiltration rate and on a readjusted basin emptying time and percolation
area.
The storage volume calculated in Step 3 and the percolation area calculated in
Step 4 assumed that, after each runoff event, adequate time for the basin to
empty would be available before the next runoff event. In practice, however,
events are random and all storage volume may not be available at the beginning
of the next storm. Therefore, the basin volume and percolation area must be
adjusted to account for this.
The maximum emptying time, ET, and the design infiltration rate, DI, (10% of
the measured rate) determine the maximum allowable depth of ponding, d:
d = ET x DI (5-7)
For any given storage volume, a direct relationship between available storage
volume, ASV, and interstorm period, t, exists:
ASV = (A x DI x t) (5-8)
where A = area of pond bottom
ASV may not exceed the storage volume for dry ponds.
From Step 1, the probability distribution of interstorm periods is known. The
interstorm period is related directly to available storage volume by Equation
5-8. Therefore, the probability of occurrence for various available storage
volumes is known. Multiplying the probability that the storm will be exceeded
by the probability of having the corresponding storage volume available will
yield a probability of overflow for each ASV.
By assuming an acceptable level of containment, say 80%, one may obtain the
runoff volume controlled by the basin size considered. By performing this
calculation for several basin sizes, one may obtain a basin size runoff volume
curve for the assumed level of containment.
5-13
-------
The design basin size can be selected, based on the percentage of runoff to be
contained and the associated waste load reduction (Step 2).
The basin sizing process involves a number of repetitive calculations and can
be carried out on a computer.
Note that this process assumes that the storage basin is always full at the
end of the preceding storm event. This is a conservative assumption.
The need for a large pond bottom area for infiltration and percolation, due to
the existence of a low permeability subsurface soil layer, can sometimes be
reduced by installation of underdrains to collect and discharge percolated
stormwater.
Step 7 - Design Solids Removal Technique and Facilities
The handling and removal of captured solids within the retention/percolation
ponds present the biggest problems in facility operation. In addition, during
overflow conditions and sedimentation, short-circuiting and resuspension of
previously settled solids may occur, reducing overall basin removal
efficiency. Very important aspects of the dry pond design are inlet and
outlet structures and solids removal methods and/or devices.
Inlet structures should be designed to provide even distribution of flow
across the head of the basin. Devices include overflow weirs, multiple pipe
inlets, and hydraulic energy dissipation devices such as stilling walls.
Prescreening of stormwater flows and combined sewer overflows is often
necessary to reduce the cleanup required if large quantities of paper and
other gross solids or floatable materials are present. Compartmentalization
of the pond can localize the cleanup of fleatables.
As the stormwater flow enters the pond from a channel or conduit, the velocity
quickly drops. Since the ability of flowing water to transport heavy solids
is directly related to the velocity, a large quantity of the suspended
material settles out of the stormwater in the first few feet of the basin. If
not removed, a bank of solids may develop, emitting odors as biodegradable
materials are anaerobically decomposed. Many flood control basins are
constructed with forebays to confine the solids for easier removal. This
capture and removal of solids can help extend the period between scarifying of
the dry pond bottom or dredging of the wet pond and reduce operational costs.
The forebay may be designed to act as a detention storage/sedimentation basin
during small runoff events, with return of the captured flow and solids to the
sewers for treatment. A possible inlet/forebay structure is illustrated in
Figure 20. Alternatively, a detention storage/sedimentation basin may be used
as a pretreatment facility in front of a dry pond.
The width of the forebay should be based on expected changes in flow velocity
and settleability of the stormwater, similar to the design approach for
storage/sedimentation basins (Section 7).
5-14
-------
0
PLAN
If/FII/Efflt&f/f \
INIET PIPE
STILLING f«LL
DRY POND
FBSEMY
//f&fff
PROFILE
SECTION
Figure 20. Inlet structure/forebay,
5-15
-------
An alternative to the forebay or pretreatment detention basin is to construct
the pond in a triangular shape, with inflow at the apex and the overflow along
the base. In this way, the drop in velocity is gradual along the length of
the basin and the deposited solids are more evenly spread over the basin
bottom. The use of baffles or compartments within the pond can improve
hydraulic distribution and minimize temperature gradients and salinity
stratifications (particularly for combined sewer overflows).
The velocity of the basin flow together with the settling velocity of the
suspended particles play predominant roles in the sediment trapping
performance of a pond during overflow conditions. The velocity of basin flow
depends upon the basin outflow rate. Outflow rates are usually determined by
the outflow structure configuration. Commonly used pond outlet forms include
overflow weirs, sluice gates, orifices, and spillways. Hydraulic texts should
be consulted for the equations to be used in calculating flowrate of the
structure selected. The selection of the overflow and its design are usually
based on flood discharge requirements.
Step 8 - Determine Pond Configuration
Economical earth construction methods usually dictate that square or
rectangular configurations be used with the length not greater than three
times the width [8]. Side slopes should be shallow enough (1:3 or less) to
allow mowing or other maintenance of any vegetative cover. Many times,
however, the final configuration of the retention facility is determined not
only by the necessary storage volume and infiltration/ percolation area but
also by any alternative uses (recreation, aesthetic, etc.) or physical
constraints of the selected site. The method selected for sediment removal or
pond bottom maintenance can effect the configuration also.
The overall objective is to provide the best effective retention and pollutant
removal facility consistent with the constraints imposed by the site
configuration and topography in addition to any other desired uses.
PERFORMANCE
The efficiency of retention ponds in reducing stormwater pollutant loadings
depends heavily on the underlying soil as a treatment medium. Soils have been
found to be very effective in removing a broad range of pollutants from
wastewater (including suspended material, phosphorus, some metals, bacteria
and viruses) and the soil also provides a medium for stabilization of oxygen-
demanding materials. The effect of wet pond retention on suspended solids for
an impoundment at Woodlands, Texas, is shown in Figure 21. The mechanisms of
removal include settling, filtering (straining), biological activity,
coagulation, adsorption, and chemical reaction. Percolation of wastewater may
result in degradation of the groundwater; therefore, it is important to have
an understanding of the removal processes at work in soils and removal
efficiencies that might be anticipated.
5-16
-------
2800 p 14°
d
z
UJ
0.
2400
2000
1600
1200
800
400
0
2800 r
2400
2000
1600
1200
800
400
0 U
120
100
- 80
60
40
20
0
LAKE INFLOW
SU8PENDED
SOLIDS
CONCENTRATION
4 6 8 10 12 14
TIME FROM START OF STORM, h
18 20
LAKE OUTFLOW
SUSPENDED
SOLIDS
CONCENTRATION
DISCHARGE
4 6 8 10 12 14 16
TIME FROM START OF STORM, h
1 8 20
Figure 21. Effect of impoundment on storm runoff
in The Woodlands, Texas [9].
5-17
-------
Very few investigations of removal efficiencies and treatment performance of
soil in percolating stormwater have been conducted. The following discussion
is based on observations of land treatment systems for rapid infiltration of
municipal wastewater treatment effluent.
Filtration in the soil profile effectively eliminates suspended solids from
percolating wastewater. This filtration occurs almost exclusively on the
surface. Removed particles tend to fill soil void spaces, further improving
the removal by straining. Once retained in the soil profile, biodegradable
solids undergo decomposition. Nondegradable and slowly degradable solids,
however, tend to gradually build up within the soil, causing clogging and
decreasing the infiltration rate. Aerobic decomposition of retained
degradable solids and clearing of surface soil pores is enhanced by scarifying
or plowing the soil surface.
The ultimate fate of an organic compound in the soil environment depends
largely on its ability to be metabolized by soil microorganisms. Micro-
organisms growing on the soil particles quickly contact and stabilize
degradable organic compounds as the wastewater trickles through the soil. If
the soil is unsaturated, oxygen will circulate through the soil pores and the
stabilization will be aerobic. If the soil is saturated for some period of
time, available oxygen may be used up and the process may become anaerobic.
It is important to note than many organic compounds are not susceptible to
microbial degradation. Additionally, a variety of organic compounds may be
unavailable for microbial or enzymatic decomposition because of environmental
factors such as pH, organic matter, moisture content, temperature, aeration,
and cation exchange capacity. Other removal mechanisms, in addition to
biological stabilization at work on organic molecules in soils, include
volatilization, sorption, and chemical degradation. Unless removed by these
mechanisms, wastewater organics move through the soil by mass-transport and
dispersion and into the groundwater. Concentrations of trace organics in
groundwater downstream from spreading basins at Whittier Narrows, California,
that receive stormwater, reclaimed wastewater, and surface water are presented
in Table 10.
Bacteria, viruses, and parasites present in stormwater discharges and combined
sewer overflows may pose a threat to human health due to waterborne disease
transmission. Percolation of wastewater through soil can effectively
eliminate pathogenic microorganisms. Filtering at the soil surface and at
intergrain contacts, and sedimentation and sorption by soil particles are the
major removal mechanisms for bacteria. Bouwer and Chaney [10] stated that
fecal coliform bacteria are generally removed after 2 to 3 in. (5 to 7.5 cm)
of travel in soils. Coarse soils and high rates of application may make
100 ft (30 m) or more of travel necessary for complete removal [11].
5-18
-------
Table 10. TRACE ORGANICS IN GROUNDWATER DOWNSTREAM OF
SPREADING BASINS OF WHITTIER NARROWS, CALIFORNIA [12]
Concentrations in pg/L
Wells
Un chlorinated
Target compound
Vinyl Chloride
Methylene Chloride
1 ,1-dichloroe thane
Chloroform
1 ,2-dichloroe thane
Carbon tetrachloride
Bro modi chlorome thane
Tri chl oroethylene
Dibromochl orome thane
1 ,1,2-trichloroethane
Benzene
Bromoform
Toluene
Chlorobenzene
1 ,4-dichlorobenzene
1 ,2-di Chlorobenzene
Tetrachl oroethylene
6-V-W
<1
2.2
<0.1
2.6
<0.2*
._
0.2
2.3
<0.1
<0.2
0.9
0.4
0.2
<0.1
0.7
<0.1
0,8
8-V-W
<]
39.2
<0.1
7.2
<0.2
0.2
<0.2
1.0
<0.1
<0.2
<0. 1
<0. 1
<0.1
<0. 1
<0.2
<0.1
1.2
11-V-W
<1
1.6
<0.1
1.8
<0.2
0.6
0.2
1.6
0.4
<0.2
<0.2
0.4
<0.1
<0.1
<0.2
<0.1
1.1
15-V-W
<1
3.8
<0. 1
1.0
<0.2
<0. 1
0.2
1,2
0.6
<0.2
<0.2
<0. 1
<0.1
<0.1
<0.2
<0. 1
0.4
Chlorinated
16-V-W 10-V-W
<1
1,6
<0. 1
0.4
<0.2
<0.2
3.6
<0. 1
<0.1
<0.2
<0. 1
1.2
<0.1
<0.1
0.2
1.2
0.5
<,
1.6
0.8
7.2
<0.2
<0.2
<0. 1
<0.2
>50
<0.2
<0.2
>40
<0.1
<0. 1
<0.2
<0. 1
0.4
12-V-W
-------
Table 11. REPORTED ISOLATIONS OF VIRUS BENEATH
LAND TREATMENT SITES [13]
Distance of virus
migration, ft
Site location
St. Petersburg,
Florida
Cypress Dome,
Florida
Fort Devens,
Massachusetts
Vine! and,
New Jersey
East Meadows,
New York
Holbrook,
New York
Vertical
20
10
60
55
37
20
Horizontal
--
23
600
820
10
150
Table 12. FACTORS THAT AFFECT THE SURVIVAL OF ENTERIC
BACTERIA AND VIRUSES IN SOIL [13]
Factor
Remarks
pH
Antagonism
from soil
microflora
Moisture
content
Temperature
Sunlight
Organic
Bacteria Shorter survival in add soils (pH 3 to 5)
than in neutral and alkaline soils
Viruses Insufficient data
Bacteria Increased survival time In sterile soil
Viruses Insufficient data
Bacteria
and
viruses
Bacteria
and
viruses
Bacteria
and
viruses
Bacteria
and
vi ruses
Longer survival 1n moist soils and during
periods of high rainfall
Longer survival at low (winter) temperatures
Shorter survival at the soil surface
Longer survival (regrowth of some types of
bacteria when sufficient amounts of organic
matter are present)
5-20
-------
Organic nitrogen compounds in wastewaters applied to soil are quickly oxidized
to the nitrate form under aerobic conditions. Under anaerobic conditions,
nitrates may be denitrified and nitrogen removed as a gas.
Denitrification does require anaerobic conditions and the presence of an
available carbon source. Land application systems may be operated to maximize
denitrification; however, the unpredictability of stormwater makes such
operation difficult if not impossible. Therefore, nitrogen removal by
percolating stormwater is likely to be poor.
In contrast to nitrogen, the behavior of stormwater applied phosphorus is
controlled primarily by chemical, rather than biological reactions. Soluble
orthophosphate can be chemically adsorbed onto soil surfaces or directly
precipitated. In the adsorption process, orthophosphates react with iron,
aluminum, or calcium ions exposed on soil surfaces. Over time, reactions
occur that use adsorbed orthophosphate to form phosphate minerals with low
solubilities. This, coupled with the creating of new sorption sites, by
alternating drying and wetting, regenerates new sites for adsorption.
Phosphorus removals can range from 70 to 99%.
In addition to toxic organic pollutant removals discussed above, soils have
been effective in reducing the concentrations of trace elements in percolating
water over limited periods of time. However, the long-term ability to remove
metals is questioned, as soil sorption sites are thought to become saturated.
Removal mechanisms for several trace elements are shown in Table 13.
OPERATIONS
As with most stormwater pollutant control facilities, the major operational
problems with ponds center around handling of captured solids. Inlet and
outlet structures also can present operational problems. Other operational
concerns for dry ponds include maintenance of vegetative cover through
alternating wetting and drying periods, control of insects, and maximizing
availability of the pond for alternative uses.
The problem of solids deposition in intermittently used flood control basins
has been recognized for some time. Often, such facilities are constructed
with forebays in an attempt to concentrate the solids for easier removal. Dry
ponds may be constructed in series with a detention storage/sedimentation
facility ahead of the pond. During small volume runoff events, the heaviest
solids are captured in the detention facility and returned to the sewers for
treatment. The settled overflow is allowed to infiltrate and percolate from
the ponds. For ponds in which stormwater control is the exclusive use,
frequent discing of the pond bottom will aid aerobic decomposition of biode-
gradable solids and disperse the captured solids through a thicker layer of
the soil. This will reduce surface clogging. No matter what the alternative
use of the dry ponds, discing or scarifying of the bottom surface must be
practiced periodically to maintain infiltration capacity.
5-21
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Table 13. REMOVAL MECHANISMS OF TRACE ELEMENTS IN SOIL [13]
Principal forms In soil
Trace elements
Solution
Principal removal mechanism
Ag
As
(silver)
(arsenic)
Ag+
ASO.-3
*t
Precipitation
Strong association
soil
with clay
fraction of
Ba (barium)
Cd (cadmium)
Co (cobalt)
Cr (chromium)
Cu (copper)
F (fluorine)
Fe (iron)
Hg (mercury)
Mn (manganese) Kn+2
N1
Pb
(nickel]
(lead)
Se (selenium)
Zn (zinc)
Ba+2
Cd+2, complexes, chelates
Co+2, Co*3
Cr+3, Cr+6, Cr20g-z, Cr04'2
i p i
Cu , Cu(OH) , anionic chelates
Fe+2, Fe+3, polymeric forms
Hg°, HgS, HgCl,-, HgCl4-2,
rH~Hn+, Hg+2
-2
Se03-2, Se04
Zn+2, complexes, chelates
Precipitation, sorption into metal
oxides and hydroxides
Ion exchange, sorption, precipitation
Surface sorption, surface complex ion
formation, lattice penetration, ion
exchange, chelation, precipitation
Sorption, precipitation, 1on exchange
Surface sorption, surface complex 1on
formation, ion exchange, chelation
Sorption, precipitation
surface sorption, surface complex 1on
Volatilization, sorption, chemical and
microbial degradation
Surface sorption, surface complex ion
formation, ion exchange, chelation,
precipitation
Surface sorption, ion exchange, chelation
Surface sorption, ion exchange, chelation,
precipitation
Ferric oxide-ferric selenite complexation
Surface sorption, surface complex ion
formation, lattice penetration, ion
exchange, chelation, precipitation
Vegetative cover on dry ponds serves a variety of purposes. The most obvious
is to make the pond more aesthetically acceptable and to allow alternative
uses such as athletic fields. In addition, vegetation can supply some removal
of pollutants, particularly nitrogen, by plant uptake and vegetation helps to
maintain high infiltration rates. The effects of vegetation on infiltration
rates are illustrated in Figure 22. The vegetation prevents reduction of the
infiltration capacity through compaction of the soil surface. Successfully
used vegetation includes fescue, perennial rye, and bermudagrass.
5-22
-------
.c
X
c
LLJ
t
oe
z
0
-------
and odors), and may provide conditions suitable to insect breeding. Floating
materials are also unsightly. Floating materials may be controlled by
prescreening and/or installation of a floating boom (see Figure 23) near the
pond inlet. The boom must be cleaned after each runoff event.
Figure 23. Floating material trapped by log boom.
Erosion of the pond banks may result from wave action particularly if the pond
is large and exposed to winds. Erosion problems are primarily the result of
neglect of surveillance and maintenance. The maintenance of proper grass
cover and riprap minimizes the problem.
Aquatic plants have both desirable and undesirable effects in ponds. The
weeds may exert a significant oxygen demand as they decompose. In addition,
they are often unsightly. They provide a suitable habitat for insect
breeding. Woody plants and trees tend to weaken dikes by their root growth.
On the other hand, marsh treatment systems rely on aquatic plants to uptake
nutrients from wastewater and to promote settling by enhancing quiescent
5-24
-------
conditions. Maintenance of 2 ft (0.6 m) of water depth will discourage growth
of weeds. Weeds growing from the pond bottom should be pulled and removed,
rather than sprayed or cut, since the decaying material may exert a
significant oxygen demand.
Control of algae growth is another important operational consideration. Algae
blooms, sudden and extreme growth usually of blue-green algae, tend to die-off
with a rapidity equal to that of the growth. The dead algae then furnish an
extremely large and sudden oxygen demand, frequently producing anaerobic
conditions and odor problems. Algae blooms may be controlled by the use of
chemicals or certain algaecides approved by the USEPA.
Insects commonly found near retention ponds include mosquitoes, midges,
beetles, and dragonflies. Insect generation occurs in sheltered areas or
quiescent portions of the pond where there may be substantial growth of rooted
plants or layers of scum. The basic measures for insect control are control
of weeds and scum and prevention of stagnant conditions. Insecticides also
may be used, as shown in Table 14. If insecticides are used, 1 or 2 days of
contact time is usually required. Prolonged contact periods will lessen the
dose required for equal effect.
Table 14. SOME INSECTICIDES USED FOR
LAGOON INSECT CONTROL [7]
Insect
Culex
Mosquitoes
Midges
"Shrimp- like"
Insects;
«lgal
predators
Insecticide
Dursban
Naled
Fenthlon
Abate
Diesel oil
Malathlon
Abate
BHC
Fenthlon
(Baytex)
Abate
Sursban
D1brom-8
Application rate
1 mg/L
1 mg/L
1 mg/L
1 mg/L
6 to 8 gal /acre
21 sprayed around edge
21 sprayed around edge
Dust, 31 ganma Isomer
As directed on package
As directed on package
As directed on package
As directed on package
COSTS
The costs of constructing dry or wet retention/percolation ponds may be
estimated using the curves shown in Figures 24 and 25. The cost curves are
based on construction costs for rapid infiltration basins and storage ponds
for disposal of domestic wastewater. Land costs are not included. These
costs should be used for preliminary estimates only. Actual costs depend on
the initial conditions of the site and the dual uses of the pond, particularly
recreation uses. Operation and maintenance costs for either type of basin are
highly site specific and depend on stormwater pollutant concentrations,
frequency of runoff events, and dual facility uses.
5-25
-------
7,000
1 , 000
100
10
X
7>
10 100
FIELD AREA. ACRES
1 , 000
Figure 24. Cost of dry pond construction, ENR 4000 [14]
0.80
0.70
"_ 0.60
X
*
£ 0.50
e
u
z
e
^ 0.40
«
S3
CC
»-
69
I 0.30
^-
= 0.20
0.10
STORAGE PONDS REQUIRING SUBSTANTIAL EXCAVATION,
EMABANKMENT, AND SCILLIAY »OR«
STORAGE PONDS CREATED FROM EXISTING
IETLANDS AND NATURAL LOI AREAS
-0 500 1,000 1.500 2,000 2.500 3,000 3,500 4,000
TOTAL STORAGE CAPACITY, 1.000 ft3
Figure 25. Storage pond construction costs, ENR 4000 [9].
5-26
-------
REFERENCES
1. Wanielista, M.P., et al. An Example of Urban Watershed Management for
Improving Lake Water Quality. Presented at International Symposium on
Inland Waters and Lake Restoration. Portland, Maine. September 8-12,
1980. .
2. American Public Works Association. Survey of Stormwater Detention
Practice in the United States and Canada. Unpublished report.
3. USEPA, U.S. Army COE, and U.S. Department of Agriculture. Process Design
Manual for Land Treatment of Municipal Wastewater. USEPA Report No. EPA
625/1-77-008. October 1977.
4. McGauhey, P.M. and R.B. Krone. Soil Mantle as a Wastewater Treatment
System: Final Report. SERL Report No. 67-11, University of California,
Berkeley. December 1967.
5. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. Second Edition. McGraw-Hill Book Company. New York. 1979.
6. Chen, C. Design of Sediment Retention Basins. Proceedings National
Symposium on Urban Hydrology and Sediment Control. Lexington, Ky. July
28-31, 1975.
7. WPCF. Operation of Wastewater Treatment Plants: A Manual of Practice.
(MOP 11). 1968.
8. WPCF. Wastewater Treatment Plant Design: A Manual of Practice (MOP 8).
1977.
9. Lynard, W., et al. Urban Stormwater Management and Technology: Case
Histories, USEPA Report No. EPA 600/8-80-035. NTIS No. PB 81-107153.
August 1980.
10. Bouwer, H. and R. L. Chaney. "Land Treatment of Wastewater." Advances in
Agronomy. Vol. 26. Academic Press, San Francisco, 1974.
11. Gerba, C.P. and J.C. Lance. "Pathogen Removal from Wastewater During
Groundwater Recharge." Proceedings of Symposium on Wastewater Reuse for
Groundwater Recharge. Pomona, CA. September 1979.
12. Garrison, W.E., et al. A Study on the Health Aspects of Groundwater
Recharge in Southern California. CSD of Los Angeles County, Whittier,
CA. September 1979.
13. Metcalf & Eddy-L.D. King. Chi no Basin Water Reclamation Study-Trace
Organics Demonstration Project Work Plan. December 1979.
14. Reed, S. et al. Cost of Land Treatment Systems. EPA 430/9-75-003.
Revised September 1979.
5-27
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Section 6
DESIGN OF DETENTION FACILITIES
INTRODUCTION
Many communities throughout North America have stormwater runoff and flooding
problems according to a recent APWA survey [1]. For example, nearly half of
the respondents stated that disposal of stormwater runoff is a problem, with
basement flooding a serious problem experienced in more than half the
communities. Some of the problems identified include soil erosion;
sedimentation; flooding of commercial and industrial property, places of human
habitation, streets, intersections, and highway underpasses; bridge and street
washouts; recurring basement backups from surcharged sanitary sewers,
attributable to illicit roof and foundation drain connections, and from
combined sewers; inflow and infiltration of stormwater into sanitary sewers;
wastewater treatment plant bypassing and overflows of stormwater and wastes
from combined sewers. The consequences of these problems include loss of
human life and damage to real and personal property; health hazards; delays of
emergency vehicles and workers reaching places of employment; cleanup demands
on municipalities and citizens; adverse effects on the aesthetics of natural
areas and urban environments; personal inconvenience; pollution threats to
groundwater supplies; disruption of ecological balances; disturbance of
wildlife habitats; loss of animal life; and economic losses associated with
the problems identified above [2].
To alleviate these problems, planning for stormwater detention is often part
of the overall stormwater management plan. For example, of the drainage
master plans reported, more than half included the development of detention
facilities. Two hundred nineteen public agencies reported having detention
facilities; the number and type are listed in Table 15. A total of 12,683
facilities were reported, an average of 58 per community. Nearly 40% of those
communities without detention facilities said that facilities are being built,
are in the planning stage, or have been considered and are a priority item for
the near future.
Objectives reported by the public agencies for establishing detention
facilities are given in Table 16 [1]. Reducing the cost of drainage systems
and reducing pollution from stormwater are two of the top seven objectives on
the list.
Stormwater detention storage delays excess runoff and attenuates peak flows in
the surface drainage system. This storage, because of sedimentation during
detention, can also be considered a treatment process.
6-1
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Table 15. DETENTION FACILITIES IN USE IN
THE UNITED STATES AND CANADA [1]
Total In use
Type of facility
Dry basin
Parking lot
Pond
Rooftop storage
Underground tank
Oversized sewer
Underground tunnel
Other
Total
No.
6.053
3,134
2.382
694
160
135
9
116
12.683
X
47
24
18
5
1
1
0
0
.8
.7
.8
.5
.3
.0
.1
.9
Private
No.
4.913
2.982
1.199
644
142
83
8
64
10.035
%
81
95
50
93
89
61
89
55
79
Public
No.
1.140
152
1.183
SO
18
52
1
52
2.648
X
19'
5
50
7
11
39
11
45
21
Table 16. OBJECTIVES IN REQUIRING DETENTION [1]
Objective Rank8
Reduce downstream flooding 100
Reduce cost of drainage systems 71
Reduce onslte flooding 70
Reduce soil erosion 66
Capture silt 64
Improve onslte drainage 63
Reduce pollution from stormwater 56
Improve aesthetics 53
Enhance recreational opportunities 51
Replenish groundwater 42
Supplement domestic water supply 36
Capture water for Irrigation 35
Other 22
a. In order of importance using 100 as
"most important."
The sizing of detention facilities also requires consideration of additional
parameters such as design storms, site constraints, and outflow rates.
Whenever possible, the "design storm" should consist of a continuous
historical or synthesized rainfall record that is typical of any long-term
rainfall record. The use of statistical rainfall intensity-duration-frequency
relationships should be avoided except for an initial rough-cut estimate since
this approach does not account for the effects of short intervals between
storms. Actual historical rainfall records selected may be based on the
hourly intensity, storm duration, total rainfall for the storm, or any
combination of these. Site constraints to be considered for detention storage
6-2
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facilities include tributary area, topography, local land use, and area
available for the structure or basin. The outflow rate from the detention
facility may be based on the capacity (size and slope) of the drainage channel
or conduit downstream, the capacity of a treatment facility downstream, or a
regulatory limitation. An example of a regulatory limitation might be that
the rate of runoff from a developed piece of property be no more than that
before the property was developed.
On site Detention
The concept of onsite detention was presented in Section 3. Typical examples
of onsite storage include rooftops, plazas, parking lots and streets, drainage
swales, blue-green storage, check dams, underground structures, and
multipurpose detention reservoirs.
Many municipalities, having faced the results of increased stormwater runoff
volumes and rates from urban development, are now enacting ordinances
requiring developers to limit the rate of stormwater runoff from developed
areas. An example of such an ordinance is that enacted by the Metropolitan
Sanitary District of Greater Chicago (MSDGC).
The MSDGC ordinance limits the peak rate of runoff from newly developed areas
to that which would occur on the land in its undeveloped state as a result of
the 3-year frequency rain storm. Any amount exceeding that must be stored for
gradual release. Detention facilities must be designed to handle the 100-year
storm without flooding.
In-System Detention Storage
The concept of in-system detention storage was presented in Section 3.
Typical examples of in-system detention include inline storage (the use of
available volume in trunk sewers, interceptors, and tunnels to store
stormwater or combined sewage) and offline storage (open or covered basins,
caverns, mined labryinths, and lined or unlined tunnels).
Overflows from combined sewer systems without stormwater detention controls
generally occur whenever the rainfall intensity after the time of concen-
tration for the tributary area exceeds 0.02 to 0.03 in./h (0.05 to 0.07
cm/h). This occurs because the peak treatment rate at plants serving combined
sewer systems is usually about 1.5 times the dry-weather flow. Because
combined sewers are designed to carry maximum flows occurring, say, once in 5
years (50 to 100 times the average dry-weather flow), during most storms there.
will be considerable unused volume within the major conduits.
Inline storage is provided by restricting the flow with static or dynamic
regulators. The installation of regulator devices can create significant
inline storage. Static regulators, such as the Hydrobrake, Steinscruv,
bulkheads with orifices, weirs, etc., can be used to generate inline storage
without controls.
6-3
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Dynamic regulators, such as sluice gates, Fabridams, etc., usually require
sophisticated monitoring and control systems. Dynamic regulators that operate
based on interceptor capacity can reduce overflows without sophisticated
monitoring and control systems.
Large inline storage installations such as Seattle's CATAD system or
Minneapolis-St. Paul's use computers to control multiple dynamic regulators
[3, 4]. These systems include (1) remote flow and level sensors; (2) signal
transmission; (3) display and data logging; (4) centralized control
capability; and (5) in the case of fully automated control, a computer program
capable of making decisions and executing control options.
Offline storage as applied in Chicago [5], New York City [6], and Milwaukee
[7], for example, does not require such sophisticated controls. These systems
generally use pumping stations for controlling the discharge. Thus,
sophisticated computer control of the offline storage is not required.
Hybrid storage, such as that used in San Francisco [8], incorporates inline
storage tunnels (in effect, greatly oversized transport conduits) with a pump
station at the end for discharge control. The utilization of the storage is
controlled by the water level settings for the pump controls at the pump
station.
DESIGN CONSIDERATIONS
Urban stormwater runoff and combined sewer overflows can be controlled through
implementation of storage as a means of source control (onsite detention) or
in-system control. Functionally, the application of onsite detention differs
little from in-system storage other than the location where the storage
occurs. However, while onsite detention is used primarily to minimize the
cost of constructing new storm sewers to serve a developing area, in-system
storage is generally used to decrease the frequency and volume of overflows
from combined sewer systems. Offline storage can be used to selectively
capture and direct to the treatment plant a portion of the stormflow (i.e., a
first flush contaminant load).
Factors to be considered in the design of onsite storage facilities are
(1) tributary area, (2) storage area and volume, (3) structural integrity, and
(4) responsibility of the owner. Factors to be considered in the design of
in-system storage facilities are (1) size and slope of sewers, (2) peak
flowrates, (3) controls, and (4) resuspension of sediment.
Tributary Area
The size of the tributary area determines the volume of water from a given
storm that will have to pass the discharge point. The runoff volume is a
function of the amount of rainfall, the extent and nature of the topography
and land cover, and the size of the tributary area.
In the cases of rooftop and parking lot storage, the tributary area is usually
the actual rooftop or parking lot surface area (i.e., no additional tributary
6-4
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area). However, the tributary area for plaza, underground structure, and
multipurpose detention facilities usually includes additional area surrounding
the facility itself.
Storage Area and Volume
The area and volume available for detention storage depend on the topography
of the particular site. In the case of rooftops, the area available is fixed
by the geometry of the building. The volume is limited by the rooftop area
and the depth of water that can be supported without endangering the
structure.
For parking lots, plazas, and multipurpose detention reservoirs, the storage
area is less well defined since all or only portions of the area may be used
depending on the nature of the site or the desires of the builder. There is
usually more latitude available for increasing the detention volume for these
facilities by adjusting depth to which the water is allowed to pond.
Underground storage structures may include concrete, fiber glass, or metal
tanks and pipe bundles for storage. The storage volume depends on the surface
area and depth of the tank or on the diameter and length of pipe bundle. The
desired storage volume can be accommodated by varying the dimensions as
necessary. In most cases, the depth of such structures is limited by the
topography and the location of the sewer to which the structure must
discharge.
Structural Considerations
Each of these facilities requires somewhat different structural
considerations. Rooftops are limited by the building code design load and the
need to prevent leaks into the structure below. The pavement and base for
parking lots must be carefully designed and constructed. The structural
integrity of the pavement can be jeopardized and the service life drastically
reduced because of the presence of water in the pavement base. Plazas,
underground structures, and multipurpose detention reservoirs must be designed
to allow access for sediment and debris removal. Outlet structures must also
be designed to fit the specific application.
Responsibility of the Owner
Even when the designer has provided for maintenance of a detention facility,
it is not always easy to determine who is responsible for performing the
maintenance. For rooftop detention or for a basin or tank serving an
apartment complex or a commercial complex, the ownership and the maintenance
responsibility is easily determined. However, for the other types of
facilities, this may not be true. A basin may be owned by a number of
individuals as part of their house lots. The homeowners, individually, are
not a legal entity that can be forced to maintain such a structure. To
prevent this, a developer can donate the detention facility to the
municipality, which must then provide the maintenance.
6-5
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Slope and Size of Sewers
To make the most effective use of the unused volume in combined sewers, the
conduits used for inline storage should be as large as possible and have
relatively flat grades. The flatter the grade, the farther upstream the
backwater effect created by the flow control device will extend. Also, the
larger the diameter of the conduit, the more storage volume is available per
unit length.
Frequently, inline storage is implemented at existing overflow or diversion
structures because the conduits are large (and usually relatively flat) at
such locations. However, inline storage can be implemented anywhere in the
system it is feasible and an appropriate control device can be installed.
Peak Flows
Usually, it is necessary to allow overflow structures to pass the design flow
unrestricted when required to prevent surcharging of the sewer system and
ponding of runoff on streets. Thus, in-system storage may be implemented at
all times when the flow is less than the designed capacity of the sewer. For
small storms, the full flow capacity of the sewer may never be reached so that
in-system storage is affected throughout the storm.
However, when in-system storage is used in conjunction with onsite storage,
some trading-off of surcharging and street or parking lot ponding may be
worthwhile and cost effective for combined sewer overflow control or storm-
water attenuation. In this case, in-system storage can be used throughout the
storm.
Controls
Static regulators usually require no controls at all. The regulator is
designed to operate in a single manner with no outside manipulation. These
regulators are designed and sized to provide control and operate over a fixed
range. To change the range or control strategy requires replacement of the
regulator itself.
The control system for effecting in-system storage with dynamic regulators is
usually quite sophisticated. Not only are controls required to operate the
regulator itself but also for monitoring conditions upstream and downstream of
the regulator. This requires that level sensors and overflow detectors be
used as a minimum. It may also require rain gage networks, water quality
sensors, and flowmeters. If more than one in-system storage location is
involved, it may also require a computer to monitor and control the operation
of the storage to maximize its effectiveness.
Resuspension of Sediment
One side effect of in-system storage is the settling of suspended material in
the sewer as the flow velocity is reduced. This is a definite problem in
combined sewer systems, and especially where offline storage is used. To
6-6
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prevent reduced flow capacity problems later on, the sediment must be
resuspended and transported to the treatment plant or to the discharge
location on separate storm drain systems. The operation of inline storage
tends to self-remedy this situation. During periods when there is high flow
(approaching peak design flowrate) in the sewer during a storm, the velocity
is usually sufficiently high to resuspend and transport any sediment.
Sediment can be resuspended also by selectively releasing upstream in-system
storage that is centrally controlled.
For offline storage, resuspension of sediment can be a definite problem unless
special design considerations are included. Special flushing flows may be
required following a storm if flow velocities generated during dewatering of
the facility are not sufficient to resuspend and transport the sediment. A
portion of the pumped discharge can be recycled to the upstream end of the
offline storage unit to resuspend the sediment.
DESIGN PROCEDURE/EXAMPLES
A suggested design methodology for onsite (source control) storage was shown
in Figure 9 and for in-system storage was shown in Figure 10. The two
methodologies are very similar and can be combined together in the discussion
presented in this section. Each of the indicated steps is discussed below and
examples are introduced where applicable.
Step 1 - Identify Functional Requirements
Onsite. As noted previously, the intended operational function of an onsite
storage facility determines its design emphasis. The design emphasis can also
be dictated by the type of development either existing on or planned for the
site.
Information that must be determined at this point includes:
The type of development existing on or planned for the site
t The reason that stormwater detention is needed or desired
The type of development greatly affects the types of stormwater detention
facilities that can be employed for the site. Commercial complexes can
incorporate combinations of rooftop, parking lot, plaza, and underground
structures for stormwater detention. Residential developments usually
incorporate rooftop detention (only if flat roofs are used), underground
structures, and multipurpose reservoirs.
In-System. The frequency with which overflows to the receiving water occur at
various locations in the sewer system determines the design emphasis for in-
system storage facilities. Locations where frequent overflows occur or where
only small amounts of rainfall initiate overflows are prime candidates for in-
system storage implementation.
6-7
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Identification of the need for overflow control is usually based on one or
more of the following:
Overflow frequency reduction
Overflow volume reduction
t Overflow quality improvement
Although in-system storage is used most frequently on combined sewer systems,
it can also be used on separate storm systems. Provision must be made for
removal of any sediment since, unlike a combined sewer system, the stored flow
does not receive treatment before discharge to the receiving water.
Stormwater volume or frequency of discharge to the receiving water can be
reduced if the stored stormwater is later used for groundwater recharge or
some other land application use.
Step 2 - Identify Site Constraints
Sites for onsite and in-system storage facilities should be identified and
cataloged with respect to at least the following criteria:
Accessibility to the channel, sewer, or interceptor to which it
discharges or to the discharge or overflow point.
Total area usable for storage (dimensions, configuration, topography)
including any area needed for construction and operation of any
necessary controls.
Hydraulic and hydrologic data on rainfall intensity, flow levels in the
conduits or channels to which flow is discharged, and allowable
discharge rate for onsite storage. Hydraulic data on receiving water
levels at the overflow point, flow depth ranges and capacities for the
trunk sewers and interceptor, any water level stage and pumping
requirements for the proposed facility, stage and corresponding storage
volume within the sewer system, and the frequency and volume of
overflows for in-system storage.
Environmental setting such as proximity to residences or other
structures, local and surrounding land uses, and visual impacts.
Geotechnical conditions that could affect load bearing capacity, side
slope stability, hazards to adjacent structures and utilities, and
groundwater supplies.
Structural requirements.
Accessibility to utility services and construction and operation
activities.
6-8
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Step 3 - Establish Basis of Design
In the past, several design methodologies have been used for design of storage
facilities. However, the validity of these methods must be measured against
how well they reflect the hydro!ogic cycle and whether or not they include an
inflow hydrograph, a depth-storage relationship, a depth-outflow relationship,
and a routing routine [2].
More than 45 different methods of predicting runoff rates and developing
inflow hydrographs were reported in the APWA survey [1]. The rational
formula, the most commonly accepted method, was approved for use by 80% of the
respondees even though the method yields only a peak flowrate. The Soil
Conservation Service curve number method (the fourth most popular by the
respondees) has been used even though it considers only the 100-year, 24-hour
storm and watersheds smaller than 2,000 acres (810 ha) [9]. Some methods are
applicable only to certain types of situations. Some methods cannot be used
on watersheds containing several detention facilities because they handle only
one detention facility at a time. Only hydrologic methods that include a
channel routing routine can be used for watersheds where channel storage has
an effect on the shape of the hydrograph.
The general approach to the design of any detention facility (for either
onsite control or in-system control) is basically the same as that described
previously for a retention facility in Section 5; it is a storage reservoir
routing problem. This applies to all forms of detention facilities. The use
of a reservoir routing approach can be used for the design or evaluation of
any storage facility. All require an inlet hydrograph, a depth-storage
relationship, and a depth-discharge relationship. Some forms may also require
a channel storage evaluation or other specialized approaches for the analysis.
The purpose of this step is to determine the inflow rate, allowable discharge
rate, pollutant loadings, and storage volume needed to evaluate the
effectiveness in meeting the requirements established in Step 1.
Onsite. The allowable discharge rate may be based on the capacity of the
conduit or channel at the discharge location, or on building regulations. In
the latter case, for example, a municipal ordinance may limit the discharge to
that occurring before development occurs.
Specific criteria to be considered during the design and siting of various
kinds of detention facilities are described here.
Parking Lots and Streets. The parking area should be graded to create
multiple storage areas like saucers. At each low point, a catchbasin or inlet
is used to control the outflow. The outflow control can be accomplished
either by restricting the size of the outlet pipe or by using a special cover
with drilled holes. As a rule, the maximum depth of the detained water at the
low point should not exceed 12 in. (30 cm). Thus, the depth of the stored
water varies between zero at the edges to 12 in. (30 cm) at the low point.
6-9
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Almost half of the public agencies responding to the APWA survey indicated
that they permit designs to provide for some street flooding. Flooding depths
ranging from 6 to 8 in. (15 to 20 cm) were most commonly permitted. The full
range of responses is shown in Table 17.
Table 17. DEPTH OF FLOODING
ALLOWED ON STREETS [1]
Responses
Depth of flooding
permitted. 1n. No. I
0
1-2
3-5
6-8
9-17
18 or tore
No answer
Total
136
14
34
74
16
6
43
325
42
4
10
23
5
2
13
Roof Storage. By placing a parapet all around the edge of a flat roof,
stormwater may be stored on the roof without concern for the structural
integrity of the roof. Most building codes require roofs to withstand 20 to
30 lb/ft^ (958 to 1,436 N/m2) live loads [10]. This is equivalent to 4 to
6 in. (10 to 15 cm) of standing water. The detention is controlled by a drain
ring set around the roof drains. As the roof begins to pond, flow is
controlled by orifices or slits in the ring; extreme flows overflow the ring
to prevent structural damage to the roof.
Multipurpose Detention Basins. Multipurpose basins are usually used to
store stormwater near the site where it is generated. The required basin size
is determined by calculations based on the design storm. Such basins are
generally 3 to 5 ft (0.8 to 1.5 m) deep. To prevent water standing in the
basin between storms, the invert of the outlet structure is at the same
elevation as the bottom of the basin. The outlet structure should incorporate
not only an orifice for controlling the outflow rate but also an overflow
grating to allow discharge if the orifice is blocked with debris. Typically,
a 1 to 2 ft (0.3 to 0.6 m) freeboard should be provided. Sides should be
sloped (generally 1-1.5:1 or flatter) and planted with grass that can
withstand periodic flooding. These basins can be used as baseball or football
fields, tennis courts, or playgrounds. In an Oak Lawn, Illinois, detention
basin, a concrete paved area (antierosion section at the inlet) is flooded
during winter months to serve as an ice-skating rink [11].
Underground Structures. Concrete, fiber glass, or metal storage tanks can
be constructed underground to serve as detention facilities. Such structures
may be located beneath parking lots, buildings, or sidewalks and planter
strips. Oversized storm sewer pipes can be used in place of storage tanks.
Access must be provided to allow removal of any debris that collects in the
tanks.
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Plazas. The basic design approach for plaza storage is the same as for
other forms of detention. The outlet must be construced to allow runoff to
accumulate during peak storm conditions. The depth that can accumulate on
plazas should be limited to about 0.75 in. (1.9 cm) so pedestrians can still
pass, but it is possible to design plazas so that portions can be flooded '
without inconvenience [12].
In-System. Representative inflow characteristics may be developed as for
onsite storage facilities or they may be developed from dry-weather flow data
and analyses of historical overflow samples. Direct field measurements may be
required.
Storage requirements for the drainage areas must be determined. The relative
effectiveness of storage related to overflow volume and/or frequency must be
evaluated for each area. The areas must be ranked by relative effectiveness
to establish the priority for further evaluation.
The alternative methods available for controlling the in-system storage must
be reviewed. The storage can be in multiple, discrete units or integrated
into a centralized location. For example, a long, large diameter pipe located
in the parking strip along a street with several catchbasins discharging to it
can be used to provide storage. Discharge from the storage to the sewer can
be controlled by an orifice or a Hydrobrake, for example. Many discrete units
such as this could be used to provide the total storage volume needed.
A single, centralized facility could be used to provide the needed storage
also. A large, lined basin located adjacent to the sewer (as is the case at
the Spring Creek facility in New York City [6]) can be used. For each site, a
control method must be selected that will satisfy the needs for that
particular site. If more than one site utilizing dynamic control will be
involved in the in-system storage system, the means for monitoring conditions
at each site and for coordinating the controls to optimize the effectiveness
must be selected. This could include simple supervisory control by an
operator or full computerized control.
Provisions should be incorporated to resuspend and transport any sediment
deposited during in-system storage. This can be accomplished by (1) ensuring
that the dry-weather flow has sufficient velocity to resuspend the sediment,
(2) sequential release of stored stormwater, or (3) intoducing flushing water
to the sewer system. Also, auxiliary controls within the sewer to redirect
flow to a single barrel of a multibarrel sewer may be required. The quantity
of sediment in a sewer at the end of a storm is a function of the quantity and
characteristics of suspended solids in the flow, the length of time the in-
system storage was in operation, and the hydraulic characteristics of the
sewer.
Step 4 -_ Select Storage Option(s) and Locations
This step requires that one or more storage methods and sites be selected to
meet the functional requirements from Step 1, the site constraints identified
in Step 2, and the design criteria established in Step 3.
6-11
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Onsite. Depending on the type of development contemplated or existing, one or
more different storage options may be needed to develop the required storage
volume. For example, a combination of rooftop and surface detention might be
required for a single-family residential site to meet the discharge
requirements. In addition, consideration must be given to access to the
storage unit for cleaning. Leaves, twigs, grass, dust, and eroded soil are
only some of the items that find their way into storage facilities in various
quantities. Depending on the storage option selected, provision must be made
to remove these sediments and debris.
In-System. Based on the results of the first three steps, the alternative
approaches and sites should be ranked. Detailed evaluation of the sites
should be performed in accordance with this priority ranking. The operational
concept, storage control method, and projected effectiveness should be
determined for each specific site, based on the functional requirement for
that site. Operational concepts to be considered should include:
Storm anticipation so that runoff from short duration storms or "spot
cell" storms can be completely captured and treated.
Selective detention of flow from portions of the system to allow sewer
inspection, maintenance, or modification.
Selective overflowing at points that will have the least effect on the
receiving waters.
First flush interception of the more heavily contaminated stormwater
from the early part of a storm.
Auxiliary systems for sediment transport and regulator control must be
determined. This should include air handling and odor control, energy (power,
lighting, heating), and instrumentation for controls. Availability of
utilities needed to operate the storage facility must be assessed.
In addition, an operation plan and a maintenance schedule should be prepared
at this time.
Step 5 - Estimate Costs and Cost Sensitivities
Onsite. Detailed cost estimates should be prepared to determine the least
cost option(s) to be incorporated. These costs can then be compared to the
cost difference between providing an outfall sewer able to convey the peak
runoff (usually based on a 5 or 10 year design storm) with that to convey the
reduced flow.
In some cases, no additional outfall sewers may be required. Then, the
objective is to minimize the onsite storage cost by selecting the options that
result in the lowest construction cost.
6-12
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In-System. Once the location or locations have been selected and a
preliminary design of the in-system storage unit(s) completed, a detailed cost
estimate should be prepared with emphasis on component systems and following
value engineering guidelines. Operation and maintenance costs should also be
estimated based on the operation plan and maintenance schedule from Step 4. A
cost-effectiveness analysis can then be prepared evaluating each site and
establishing the priority for implementing construction.
Step 6 - Complete Design
The final step is to confirm that all objectives have been met. Several
iterations back through previous steps may be needed to reach the most cost-
effective solution. This is particularly true once site-specific costs for
the various storage options have been developed.
OPERATION AND MAINTENANCE CONSIDERATIONS
The major operation and maintenance goal for detention storage facilities is
to provide readily available, nuisance-free storage that will operate as
designed whenever needed (even following prolonged periods of non-use). Such
units should utilize as little unproductive space as possible while minimizing
any visual impact.
On site
Onsite storage facilities should not require extensive maintenance after each
use. Debris removal, care of the landscaping (if any), and inspection and
maintenance of the outlet structure are all part of the routine operation of a
storage facility.
Mosquito and algae problems can be eliminated by ensuring tht storage
facilities drain completely and dry out between use. Storage ponds look best
when a grass cover is kept on the basin slopes and floor. However, if the
basin needs to be vegetation-free for any reason, visual screening can be
provided by sight barriers such as trees.
Safety features must also be considered. Hazardous areas must be fenced to
restrict access. Debris must be removed whenever it collects to prevent
interference with the operation of the outlet structure and to eliminate
hazards to users in a multipurpose facility.
In-System
Adequate information on the anticipated operation and effectiveness of the
storage facility should be prepared. Experience has shown that frequent,
periodic maintenance and equipment exercising is required. The nature of the
atmosphere and conditions found where much of the equipment is located
dictates this. Corrosion and clogging by debris are typical of the problems
encountered by in-system facilities. The instrumentation needed to control
the operation of dynamic regulators can be another source of problems.
However, recent advances in the manufacture and design of instrumentation and
controls have greatly reduced the problem.
6-13
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COSTS
Construction costs for in-system storage have been reported for selected
demonstration sites [5]. However, they are highly site specific. Adjusted to
ENR 4000, the range of unit construction coasts for in-system storage is from
$0.04 to $0.94/gal ($10.60 to $221.90/nr) of storage volume. Costs also vary
considerably depending on the complexity of the flow regulators and control
systems. For example, the cost of the control and monitoring system was 47%
of the $0.94/gal for that demonstration project [3].
Construction costs for offline storage for selected demonstration sites was
reported to vary from $0.16 to $0.90/gal ($42.30 to $237.80/mJ) of storage
volume [5],
Detailed operation and maintenance cost data are limited. No rule-of-thumb
guides exist at present. Operation and maintenance costs must be estimated
for specific facilities from the operation plan and maintenance schedule. For
planning level studies, estimates can be developed from costs reported for
operating facilities [5j.
REFERENCES
1. American Public Works Association. Survey of Stormwater Detention
Practices in the United States and Canada. Unpublished report.
2. American Public Works Association. Urban Stormwater Management - Special
Report No. 49. 1981.
3. Leiser, C.P. Computer Management of a Combined Sewer System. USEPA
Report No. EPA-670/2-74-022. NTIS No. PB 235 717. July 1974.
4. Metropolitan Sewer Board - St. Paul, Minnesota. Dispatching System for
Control of Combined Sewer Losses. USEPA Report No. 11020FAQ03/71. NTIS
No. PB 203 678. March 1971.
5. Lager, J.A. et al. Urban Stormwater Management and Technology. Update
and Users' Guide. USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
6. Feuerstein, D.L. and W.O. Maddaus. Wastewater Management Program, Jamaica
Bay, New York; Volume I: Smmary Report. USEPA Report No. EPA-600/2-76-
222a. NTIS No. PB 260 887. September 1976.
7. City of Milwaukee, Wisconsin, and Consoer, Townsend and Associates.
Detention Tank for Combined Sewer Overflow, Milwaukee, Wisconsin,
Demonstration Project. USEPA Report No. EPA-600/2-75-071. NTIS No. PB
250 427. December 1975.
8. Metcal f & Eddy, Inc. City and County of San Francisco Southwest Water
Pollution Control Plant Project. Final Project Report. February 1980.
6-14
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9. Soil Conservation Service. Urban Hydrology for Small Watersheds.
Technical Release No. 55, U.S. Department of Agriculture. January 1975.
10. Uniform Building Code. 1979 Edition.
11. Poertner, H.G. Better Storm Drainage Facilities at Lower Cost. Civil
Engineering. ASCE, 43, No. 10, pp 67-70. October 1973.
6-15
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Section 7
DESIGN OF SEDIMENTATION FACILITIES
The frequency of overflow operations to the total frequency of facility
activations determines the design emphasis for downstream storage/
sedimentation basins. By definition these facilities are intended to be end-
of-the-pipe controls; discharges (overflows) are expected to be directly to
receiving waters with or without further treatment. Storage/sedimentation is
the most common and, perhaps, effectively practiced method of urban stormwater
runoff control in terms of operating installations and length of service.
Conversely, since it parallels historic sanitary engineering practice,
storage/sedimentation is frequently criticized for lack of innovation due to
its simplicity, and high cost due to size and structural requirements.
In this section, design considerations and procedures for downstream
storage/sedimentation basins are presented and illustrated by example and
through references to designed and operated facilities. Planning level costs
and cost considerations are given.
DESIGN CONSIDERATIONS
Functionally, the applications of downstream storage/sedimentation facilities
vary from essentially total containment, experiencing only a few overflows per
year, to flow-through treatment systems where total containment is the
exception rather than the rule. For total containment, the major concerns are
the usable storage volume, the provisions for dewatering, and post-storm
cleanup. For flow-through treatment systems, performance hinges on treatment
effectiveness and design considerations including loading rates, inlet and
outlet controls, short circuiting, and sludge and scum removal systems.
In the case of offline facilities, the option exists to selectively capture a
portion of the stormflow (i.e., first flush contaminant load) and bypass the
balance to avoid the loss of captured solids through turbulance and
resuspension. Examples of representative CSO storage/sedimentation basins and
auxiliary support facilities are shown in Figure 26.
Storage
Required storage volumes to accomplish a specified level of control are best
approximated through use of simplified continuous simulation models. A
summary of representative models and their application is presented in the
REQM Handbook [1]. Statistical methods recently developed by Hydroscience and
by Howard have proved valuable for determining storage requirements for simple
7-1
-------
SCIECNS
STORAGE/SEDIIENTATION
' (IURIEO) I
COARSE FINE
PARALLEL IASINS
STORAGE CAPACITY 1.3 Hfll
BOSTON (COTTAGE FARM), MASSACHUSETTS
STORAGE/SEDIMENTATION (OPEN)
DRAIN
RETURN TO
INTERCEPTOR
L
ONE IASIN
STORAGE CAPACITY
2,1 Mpl
CHIPPEWA FALLS, WISCONSIN
FINE SCREEN
STORAGE/SEDIMENTATION (IURIEO)
U 4
1
4
-! OVERFLOW fc
1
DE1ATER TO
INTERCEPTOR
ONE IASIN
STORAGE CAPACITY
3.1 IM I
MILWAUKEE (HUMBOLT AVE.), WISCONSIN
Figure 26. Representative CSO Storage/Sedimentation
Basins and Auxiliary Support Facilities.
7-2
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TUNNEL
STORA6E
-©-"
STORAGE /SEDIMENTATION (OPEN)
STORAGE CAPACITY 27 MgaI
w
I 3 SEDIMENTATION/
RESUSPENSION
BASINS IN SERIES
n\ h.
L,
_rr
POST -STORM DEWATER
TO INTERCEPTOR
£
OVERFLOW
CONTACT
BASIN
AERATED RETENTION BASIN
LONG-TERM DEWATER TO
TREATMENT AND REUSE
MOUNT CLEMENS
DEGRITTING
CYCLONE
DRAIN AND PUMPED
RETURN TO
INTERCEPTOR
r-
\
STOWAGE
COARSE
SCREEN
STORAGE/SEDIMENTATION
(ENCLOSED)
1 8 PARALLEL BASINS
STORAGE CAPACITY 10 Mga I
NEW YORK C TY (SPRING CREEK)
STORAGE/SEDIMENTATION (COVERED)
DRAIN RETURN TO
INTERCEPTOR
3 BASINS IN SERIES INTERCONNECTED
BY OVERFLOW WEIRS (i.e., BASINS
FILL SEQUENTIALLY) STORAGE
CAPACITY 23
SACRAMENTO (PIONEER RESERVOIR)
Figure 26 (Continued)
7-3
-------
STORAGE/SEDIMENTATION
(ENCLOSED)
COARSE SCREENS
CL2
DRAI
RETU
INTE
{
1 ' '
' 2 PAIRS OF BA
' STORAGE CAPAC
iCEPTOR
^M
- crri-ucni a one
OVERFLOW ^
SINS IN SERIES.
ILL SEQUENTIALLY
TY 3.5 Mga 1
SAGINAW (HANCOCK STREET)
STORAGE/SEDIMENTATION I
(BURIED)
FINE SCREENS
DEWATER TO
DRY-WEATHER
BASINS AS
NECESSARY
__ OVERFLOW
COARSE SCREENS
1 16 PARALLEL BASINS
(TYPICALLY 3 IN SERVICE
N DRY WEATHER)
STORAGE CAPACITY 15 Mga I
CONTINUOUS SLUDGE
PUMPING AVAILABLE
NOTE : JOINT DRY-WEATHER/WET-LEATHER
PRIMARY TREATMENT FACILITY
SAN FRANCISCO (SOUTHWEST WPCP)
LEGEND
©
P } PUMPS
CL, LOCATION OF CHLORINE OR
i HYPOCHLORITE ADDITION
Figure 26 (Concluded)
7-4
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systems [2, 3]. Where the level of control objective is high (i.e., storage-
treatment capacity is large compared to runoff volume) and urban development
is intense, two particularly useful models are EPAMAC [4] and STORM [5]. The
former is an extension of the Simplified Stormwater Model [6] and the areawide
planning model ABMAC'[7]. EPAMAC has been used as the base model for examples
in this text. In cases where short-term flow dynamics are of major concern or
where pervious areas of the watershed play a significant role, other, more
detailed models such as SWMM [8] and NPS [9] may be required.
EPAMAC operates on an hourly timestep and is designed for applications on
combined sewer systems. Inputs include hourly rainfall data, watershed
subareas, runoff coefficients, runoff quantity and quality, routing time
offsets, and network flow routing. At each network control node, the user
specifies the available storage volume and dewatering (treatment) rate with
its associated operating rules (i.e., pumps started and stopped as a function
of filled storage volume for that timestep). When the hourly storage-
treatment capacity is exceeded, an overflow (discharge) occurs.
The user selects trial storage volumes and associated dewatering (treatment)
rates to fit the constraints of his system and through iterative analyses
selects the combination that best satisfies his needs (i.e., overflow
frequency, site limitations, and cost). Note that storage may be distributed
over a series of nodes to represent upstream and inline storage options;
however, only one network control node is allowed per run. For example, if
storage is to be provided within a watershed through a sequential series of
dispersed storage elements such as an upstream surface detention basin, an
intermediate zone of inline storage, and a downstream storage/sedimentation
basin, three sets of computational runs would be required. The first would
consider only the upper watershed and its storage basin. The computed
discharge from this basin would constitute a lateral infow for the second
run. The second run would consider the additional tributary area to the
inline storage facility, plus the lateral inflow generated by the first run.
In turn, the second run would produce a lateral inflow for the third run,
which would reflect the storage benefits of both the upper surface basin and
the inline storage in the intermediate zone. Finally, the third run would add
any new tributary area flows to the lateral inflow generated by the second run
and permit the sizing of the downstream basin.
Treatment Efficiency
Storage units may alter the wastewater characteristics of the applied stream
by gravity separation. The suspended solids removal efficiency approaches a
maximum under steady state, quiescent conditions. Unsteady, storm-induced
flows generally produce velocity and, in some cases, temperature gradients in
the sedimentation basins. These unsteady conditions may reduce the
instantaneous suspended solids removal efficiency but, on an overall storm
basis, may not drastically alter the overall removal efficiency. In its
simplest sense, wastewater is made up of water containing particulates of
varying specific gravities. When the convective forces transporting these
particulates are reduced, those lighter than water start to rise and those
heavier start to fall. This movement may increase collisions between
7-5
-------
particles and by adsorption or floccupation, large particles are formed that
in turn furthers the separation. The movement continues until the particles
settle to the floor of the chamber forming a sludge, rise to the surface
forming a scum, or are carried out in the overflow.
Sedimentation theory and convective forces are lucidly described in wastewater
engineering texts; however, there are two major problems: (1) wastewater tends
to be quite heterogeneous with its makeup of heavier than, equal to, and
lighter than water particulates changing from one moment to the next; and
(2) the theory reflects idealized situations to which a myriad of modifiers
must be applied to reflect real world conditions.
Design variables affecting hydraulic performance in general order of
importance are surface-loading rates or overflow rates, detention time, basin
geometry, inlet and outlet design, and rapid sludge removal. Potentially
controllable parameters adversely affecting sedimentation performance are
density currents due to temperature differentials between the incoming flow
and the basin contents, density currents resulting from marine/estuarine water
intrusion, turbulance generated by flow variations, and wind-induced
currents. Generally, noncontrolTable but important parameters of the raw
wastewater are its suspended solids concentration, the effect of shear forces
or velocities in the sewer on agglomerated organic particles, the proportion
of settleable solids, and its age or septicity.
To estimate the efficiency of any sedimentation basin it is most important to
know not only the suspended solids load but also the settleability charac-
teristics and distribution of other pollutants associated with the solids. In
other words, the particle size distribution, pollutants associated with the
particles, and the density of the particles must be known. Therefore,
detailed stormwater runoff and combined sewer overflow sampling is necessary
to characterize the solids and pollutant distributions.
Samples should be flow weighted to produce a representative sample for
analysis. The time variation in the solids loading is taken into account when
flow weighted samples are used. Efforts should be made to ensure that the
samples are representative with respect to depth within the flow stream.
A long documented but little used test to reflect these unique characteristics
of a particular waste is the settling column test described by Metcalf & Eddy
[10], Camp [11], and others [12, 13, 14, 15]. These tests provide a valuable
aid in projecting storage/sedimentation performance in urban stormwater
systems design.
Application and use of settling column tests are illustrated under the design
procedure discussion to follow. Tests are normally run to record total
suspended solids removal as a function of time and depth; however, it should
be noted that settling column tests can be run on the basis of any quality
parameter for which removals are accomplished by sedimentation. In
association with a standard sieve analysis for particle size distributions,
chemical analyses of the various solids fractions should be performed to
determine the chemical and biochemical pollutants associated with the various
7-6
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particle sizes. This information can be used to estimate the effective
removal efficiency for the various pollutants associated with the sediment.
Biochemical analyses of the liquid portion should also be made so that the
total pollutant load can be determined. The use of settling column test
results along with application of the Storage/Transport Block of SWMM-Version
III is recommended.
Typical removal efficiencies for total suspended solids as related to surface
loading rates and detention times are shown in Figure 27. Each plot
represents a "best fit" curve representation of a broad data scatter from a
number of installations over a large number of events. For example, the
empirical suspended solids removal efficiency, in terms of the surface loading
rate for conventional primary treatment with mechanical sludge removal
according to Smith [16] is:
R = 0.82e-s/2780
where R = TSS removal efficiency, %
S = surface loading rate, gal/ftz-d
(7-1)
The degree of scatter and limitations of theoretical approaches are illus-
trated in Figure 28, which is based on 24-hour influent and effluent samples
from a primary treatment plant in San Francisco receiving storm and sanitary
flows from a combined sewer system [17]. Thus, even a single plant will
exhibit wide day-to-day fluxes in efficiency under the same surface loading
rate. The potential for removal efficiencies to vary during individual storm
events is shown in Table 18 [17], wherein performance over the first 2 storm
hours (first flush) is compared to the average performance over the entire
storm event.
Representative removal efficiencies associated with plain sedimentation of
wastewater in conventional plants are listed in Table 19. Because of the
limited data base, independent performance ranges cannot be presented for
urban stormwater, but the presumption is that they will be similar. In San
Francisco, comparison of heavy metals between filtered and nonfiltered
stormwater samples indicated that the majority of heavy metals were associated
with the solids fraction [17]; thus, effluent quality improvement would be
expected to be associated with sedimentation. In the limited number of storm
composites measured before and after treatment, however, a conclusive trend
was not apparent.
7-7
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80
40
20
FROM REF. [4]
TYPICAL CSO STORAGE/SEDIMENTATION
WITHOUT MECHANICAL SLUDGE REMOVAL
FROM REF
R = 0.82.2780
CONVENTIONAL PRIMARY TREATMENT
WITH MECHANICAL SLUDGE REMOVAL
500
1 000
2000
3000
4000
S= SURFACE LOADINfi RATE, gtl/ft^'d
W 3.6 1-8 0.9 0.6
DETENTION TIME (ASSUMING 10ft AVG DEPTH), hours
0.45
Figure 27, Typical TSS removal efficiencies by sedimentation
100 I
80
60
tn
t^
II 40
20
800
NORTH POJNT WPCP
IET-IEATHER 1977-1878
(DAILY COMPOSITE VALUES
FOR RAINFALL> 0.1 0 in.)
o
LEAST SQUARES FIT
O
1000
2000
3000
4000
S= SURFACE LOADING RATE, gal/ft^-d
Figure 28. Experienced TSS removal efficiency variations [17]
7-8
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Table 18. PILOT PLANT PERFORMANCE ON RICHMOND-SUNSET
STORMWATER (CSO) FLOWS [17]
Total storm
First flush8
Date
Test Surface-loading
duration ratei
h gal/ft*-d
Avg TSS
Avg removal,
Influent Effluent S
Avg TSS
Avg removal,
Influent Effluent t
2/28/79
3/16/79
3/26/79
a.m."
p.m.
a. Average
b. Morning
7
14
22
.5
.5
.5
of all
shower
1
1
2
,500-2,
,600-2,
,000-2,
400
400
400
grab samples over
lasted only 1 hour
128
111
98
first 2
; main
87
78
49
32
30
50
176
173
255
152
92
105
118
42
48
39
61
72
hours of storm unless otherwise noted.
storm started 4-1/2 hours later.
Table 19. COMMON REMOVAL EFFICIENCIES ASSOCIATED
WITH PRIMARY SEDIMENTATION OF SANITARY WASTEWATER [17]
Wastewater
BOD
TSS
Settleable solids
Bacteria
Total nitrogen
Total phosphorus
Grease and oil
Removal efficiency, %
25-40
40-70
85->95
25-75
5-25
5-20
40-60
Studies for Milwaukee have developed process curves for detention tanks.
evaluating pollutant reduction and volumetric efficiency for several tank
volumes. Suspended solids and BOD retention and percent of storm volume
retained for both wet- and dry-year rainfalls are shown in Figure 29 [18]. The
study also showed a decreasing efficiency per unit volume as tank size
increases, as shown in Figure 30. -
Disinfection
Where disinfection is often required in a storage/sedimentation basin, a
minimum contact period is specified. Further, the consumption of disinfectant
(typically chlorine or a chlorine derivative) and its effectiveness are
adversely impacted by solids in the flow. Therefore, where detention periods
dictated by storage requirements are significantly longer than the contact
period required, multistaged basins should be considered with the disinfectant
added to the final stage(s) only. In this manner, the benefits of the
partially clarified wastewater will be realized. Common dosage requirements
are 15 to 30 mg/L of available chlorine.
7-9
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OUT TEH
ill/"' I I ««!!./>
TIKI VOllUE,
Figure 29. Pollution and volumetric retention
versus storage tank volume for wet- and dry-years,
X
20
IOD
I 2
SUE,
Figure 30. Unit removal efficiencies
for combined sewer overflow detention tanks
7-10
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High-rate disinfection of raw and prescreened combined sewer overflows using
sodium hypochlorite with high velocity gradients in the contact chamber or
using chlorine alone or chlorine followed by chlorine dioxide has been tested
[19, 20, 21, 22]. Results equivalent to that of normal practice were achieved
using dosages as low as 8 mg/L and contact times as short as 3 minutes and
less. Flow control must be established and the flowrates known in order to
effectively pace the dosage. Because of the rapid changes in flow typically
received in storage/sedimentation basins, pacing disinfection additions solely
by effluent residual monitoring has not been effective [23].
Site Constraints
Whereas approximately one out of ten conventional wastewater treatment plants
is covered, the reverse is the general rule for downstream storage/
sedimentation basins serving combined sewer areas. This is because available
land along waterfronts within the urban core is typically in very highly
developed or recreation oriented areas; thus, in the public's mind at least,
it is incompatible with open raw sewage basins. Historically, treatment
plants have been built in quasi-isolated areas and development has encroached
on the sites. Conversely, CSO systems and their associated urban development
exist, and it is the treatment facility that must do the encroaching. In
smaller communities (i.e., Chippewa Falls, Wisconsin [23], and Mt. Clemens,
Michigan [24], comparatively isolated centrally located areas have been found
and open basins constructed and operated without reported nuisance. In the
majority of cases, however (i.e., Akron [25], Boston [26], Milwaukee [18], New
York [27], Sacramento, Saginaw [28], and San Francisco), the facilities are
covered and in some cases buried. In the cases of Akron and Saginaw, use is
made of the land above the structure.
In San Francisco, problems with limitations in usable waterfront space were
coupled with the need to intercept a multitude of dispersed overflow points in
arriving at an innovative storage-transport concept. In this case, large,
elongated downstream storage/sedimentation basinssuper sewerswere
constructed that combined storage/sedimentation functions (i.e., pretreatment
for overflows) with interception and transport functions. The North Shore
consolidation project, for example, provides a monolithic box conduit and
tunnel structure snaking along 3 miles of waterfront. The project intercepts
seven overflow points, provides 23 Mgal (87 m3) of storage, conveys flows to a
central location for pumping to treatment, and provides pretreatment of over-
flows (an average of four per year) by sedimentation and skimming. The fact
that "super sewers" can provide significant treatment during periods of
overflow is demonstrated in operating data taken from the 2 mile long, 62 Mgal
(235 m6} capacity, Red Run Drain near Detroit [29] and shown in Table 20.
Even covered facilities vary in that some permit operator access during
operations (i.e., walkways and working space above the free water surface),
where others can be entered (as in sewers) only in a dewatered condition. The
net impact is frequently a doubling and redoubling of the basic functional
cost; however, the alternatives are typically no more acceptable than open
sewers would be.
7-11
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Table 20. PERFORMANCE OF THE RED RUN CSO
SEDIMENTATION/TRANSPORT BASIN [29]
Total suspended solids
Volatile suspended sol Ids
Storm
date
3/14-15/78
3/21-22/78
5/8/78
5/13/78
5/30/78
Influent,
mg/L
116
52
238
114
294
Effluent,
mg/L
102
36
168
38
152
Removal,
X
12
31
29
67
48
Influent,
ng/L
62
32
128
64
170
Effluent,
ng/L
36
20
54
4
26
Removal ,
I
42
38
59
94
85
Limited studies of odor generation from stored urban stormwater (CSO)
conducted at San Francisco [20] for two storms. These studies showed that for
1,500 gallon (5,678 L) samples of first flush stormwater stored in covered but
vented storage, hydrogen sulfide generation peaked 24 hours after collection
but that concentrations had not reached a distinguishable level (0.3 ppm-
volumetric), even after 48 hours. Highest odor potential should be expected
during dewatering operations as settled sludge is exposed. Options for sludge
removal systems, performance assessment under variable flowrates, and other
design details are discussed in the following section.
DESIGN PROCEDURE/EXAMPLE
A suggested design methodology was shown previously in Section 4. Each of the
indicated steps is discussed below and examples are introduced where
applicable.
Step 1 - Identify Functional Requirements
As noted previously, the intended operational function of the downstream
storage/sedimentation basin will determine its design emphasis (i.e., does its
treatment function rank primary or secondary with respect to storage).
Obviously, a facility that will spill or discharge under all but the smallest
of storms (i.e., a typical detention-chlorination facility) should be designed
as a treatment facility. For a discharge frequency of a few times a year, a
design predicated mainly on construction and operation economics would be
warranted. Area hydrology, system hydraulics, and overflow frequency
objectives will determine the volumetric capacity required. As stated
earlier, simplified continuous simulation models are generally best suited to
this task and detailed user manuals [1, 4] have been prepared.
Answers to be provided by the model or developed from the model output
include:
Design volumetric capacity or matrix of storage-dewatering
(treatment) rate combinations that meet overflow frequency criteria.
7-12
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Frequency distribution of unit operations by month, year, and period
of record.
Frequency distribution of operation durations, storage volumes
utilized, treatment rates experienced, overflow events, overflow
rates, overflow durations, and between storm downtime availability.
First cut assessment of solids applied, solids retained or diverted,
and solids overflowed.
First cut assessment of the impact of the reduced solids and
pollutant load on the receiving water quality and the determination
of the cost-effectiveness of the proposed facilities.
Where an NPDES permit has been issued, it must be consulted to identify any
restrictions on discharges in terms of concentrations, mass loadings (i.e.,
basin plan waste load allocations), disinfection requirements, and reporting
criteria. Where NPDES permits have not been issued, target criteria must be
established through meetings with regulatory agencies having jurisdiction, and
must be supported through cost-effectiveness analysis.
Step 2 - Identify Site Constraints
Sites for downstream storage/sedimentation basins should be identified and
cataloged with respect to at least the following criteria:
Accessibility to the collection conduit, the interceptor (for
postevent dewatering), and a suitable overflow point for discharges.
Total usable area and its dimensions and configuration.
t Hydraulic data on influent levels, receiving water levels, (normal
and flood), and interceptor levels to identify stage and pumping
requirements for the proposed facility.
Environmental setting such as proximity to residences, public
facilities, compatible and noncompatible land uses, visual exposure,
and prevail ing winds.
Geotechnical conditions and probable structural requirements (i.e.,
pile supports, hazards to adjacent structures and utilities, etc.)
t Accessibility to utility services and for construction and operation
activities.
Typically, this information will be used in selecting the main treatment
geometry in Step 4.
7-13
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Step 3 - Establish Basis of Design
The purpose of this step is to determine influent characteristics and loading
rates necessary to meet the requirements set down in Step 1. Representative
influent characteristics may be developed, in the case of CSO systems, from
direct field measurements and supplemented by an analysis of dry-weather
wastewater treatment plant influent data during wet-weather operations. These
data should be segregated by (1) storm size (rainfall recorded), (2) seasonal
occurrence, (3) time into the event, etc.
In addition, it is recommended that settling column tests be performed as a
basis for predicting basin performance. These tests have found wide
acceptance in the industrial waste treatment field (where designers freely
admit lack of knowledge of a particular wastes settling behavior). However,
these tests are rarely performed on municipal wastewaters where (1) the
assumption is made that behavior will be typical, or (2) that the
sedimentation unit process will be followed by additional unit processes; thus
the relative importance of primary settling characteristics is small. In
urban stormwater management, the additional testing is certainly warranted.
In addition to knowing the influent suspended solids concentration, it would
be extremely informative to know the range of settling velocities for the
particles and the mass that can be settled within a reasonable time period
when selecting surface loading rates and detention times for design.
In translating the idealized (quiescent) settling column results in design,
texts caution that to account for the less than optimum conditions encountered
in the field, the design settling velocity or surface loading rate obtained
from column studies should be multiplied by a factor of 0.50 to 0.85 and
detention times by a factor of 1.25 to 2.0 [10, 23]. Heinke et al. [30] found
good correlation between settling column results and measured performance of
three municipal plants in Canada monitored over a 4-year period.
Predictions of sedimentation tank performance made from settling column tests
compared closely with actual tank performance under low overflow rates of
about 600 gal/ft *d (24 nr/nr'd). For higher overflow rates, the actual
performance of the settling tanks was much better than the predictions from
the settling tests.
The use of the suspended solids removal efficiencies for various overflow
rates can be used to predict the efficiency of a sedimentation basin for
unsteady flow conditions. A time-step approach utilizing the overflow rate,
and the predicted efficiency at that overflow rate, and the influent suspended
solids concentrations can estimate the overall efficiency for a storm or
series of storms.
Example 1 illustrates the use of settling column test results in establishing
a projected performance curve.
7-14
-------
Example.1. COMPUTE TSS PERFORMANCE CURVE FROM SETTLING COLUMN TEST RESULTS
Specified Conditions
1.
Laboratory test results were obtained from a settling column test of a 2 hour composite
sample of "first flush" CSO. Test column was 6 in. diameter, 10 ft high, with sample
taps at 24 in. centers. The composite sample was premixed and pumped into the column.
Samples were drawn from each tap initially and repeated at specified time intervals.
TSS results for the individual samples were as follows:
TSS results, mg/L
Elapsed time,
Initial
depth, in.
5
29
53
77
101
Mean values
0
202
240
384
384
408
324
30
136
172
148
236
226
184
60
112
122
126
140
154
131
minutes
90
96
126
132
142
138
127
120
_-
110
118
124
118
118
2. Sedimentation tank depth is 10 ft.
Assumptions
1. Assume a surface loading rate scale factor of 0.75 to translate the "idealized" column
results to a field basin.
Solution
1. Calculate TSS removals as a percent of initial mean concentration.
TSS removal results, i
Elapsed time,
Initial
depth, in.
5
29
53
77
101
0 30
58
47
54
27
30
60
65
62
61
57
52
minutes
90
70
61
59
56
57
120
__
66
64
62
64
2. Plot results to scale and sketch in best fit removal curves for 30T, 40*. 50%, 601, and 70%.
SHIFUI
I DOS I.
7-15
-------
3. Compute the percent removal at 30, 60, 90, and 120 minutes by proportionality. For example,
at t = 120 minutes, percent removal
Ahl
Ahi + An2
Time
Calculation
Removal
61.3
55.5
4. Using 10 ft depth, compute surface loadings corresponding to detention times and apply scale
factors (0.75 surface loading and 1.33 to detention time) for projected prototype performance.
Unsealed (ideal) performance
Projected performance
Detention Surface Removal Detention Surface Removal
time, min loading, gal/ftz-d efficiency, t time, min. loading, gal/ftz-d efficiency, %
30 3,591 38 40 2,693 38
60 1,796 56 80 1,347 56
90 1,197 61 120 898 61
120 898 66 160 674 66
5. Plot projected performance results for use in Example 7. .
00
IN
W
>
vt
»-
20
0
^V^^ , nOJECTEl PERFOftWNCE
t I i i 1 1 1 i i i i 1 i 1 I |
9 900 t 000 2000 3000 4000
S^ttl»F»Ct LOAOINO MTE, !! /((*(
II | 1 1
3.0 t.l 0.0 0.0 0.49
DtTKTIOH TIKE (ASJtKINt 10 tl »»8 OEfTN).
Comments
First flush test behavior shows continued good performance at high overflow rates.
Problem in completely mixing the sample in the settling column at t = 0 is evident in
the test results. Ideally, the concentrations at each depth at t = 0 should fall
within 10% of the mean value.
7-16
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Step 4 - Select Main Treatment Geometry
The geometry of a downstream storage/sedimentation basin will be governed by
the constraints identified in Step 2, the loading rates selected from Step 3,
and the operational concept developed from the Step 1 frequency analyses. For
example, if a large number of the total plant operations will use, say 50% of
the storage capacity or less, a compartmented basin with sequential filling
could greatly reduce cleanup operations without any impact on performance.
Also, if the characterization data indicate a pronounced first flush,
segregating this load from the balance of the storm may be beneficial from
both a cleanup and performance aspect.
Basic reasons for dividing storage/sedimentation basins into compartments are:
To reduce short circuiting
To facilitate cleanup and sludge removal from tanks that fill
in series
To permit isolation of slug loads in individual tanks
To provide operational redundancy through parallel units
When compartments are linked in series (Saginaw, Sacramento), short circuiting
is minimized. When compartments are operated in parallel (Boston, New York
City), longitudinal flow (resuspension) velocities are minimized. Camp [11]
notes that it has been common practice to limit design longitudinal velocities
in settling tanks to about 3 ft/min (0.02 m/s). However, he notes that
despite a dearth of experimental data there is increasing evidence that higher
velocities are accompanied by better removals. This phenomenon occurs because
the additional flocculation caused by turbulence in the tank may speed up the
settling to a greater extent than turbulent mixing retards it. Heinke et. al
[30] suggest 8 ft/min (0.04 m/s) as a maximum design value for primary
sedimentation tanks based on field observations. Initial results from Saginaw
[24], as shown in Table 21, suggest potential benefits from the series (two-
stage settling) configuration. However, at present there are no settling
column test results to compare the theoretical and actual removal efficiencies
based on influent characteristics and basin design. One possible explanation
is that CSO contains not only those solids found in sanitary sewage but also
additional grit and sand resuspended from the sewer or flushed into the sewer
from urban areas.
Table 21. PERFORMANCE OF THE HANCOCK STREET
SEDIMENTATION, SAGINAW, MI
Suspended sol Ids
Avg surface Longitudinal
Storm loading rate, velocity Influent, Effluent, Removal,
date ga1/ft2>d ft/m1n ing/L mg/L X
8/19/78
9/13/78
9/20/78
970
1,235
2,270
3.7
4.7
8.7
896
149
420
62
27
232
93
82
45
7-17
-------
When treatment effectiveness is the primary concern, inlet and outlet works
should be designed as in conventional wastewater treatment practice [10, 31],
(i.e., to minimize density currents, short circuiting, resuspension, and
turbulence). The inlet works should spread the influent evenly across the
vertical cross-section of the tank without resuspending the sludge blanket.
Effluent weir loading rates of 10,000 to 40,000 gal/ft'd (125 to 500 m3/nrd)
are representative of conventional design [10], When the basin's function is
basically storage, inlet and outlet works should be as simple as practical,
but effluent and, probably, interstage baffles should be provided to minimize
scum carryover.
Two major, and frequently the most controversial, design decisions will be the
degree and means of covering the basins and the means of solids and floatables
removal. Both are expected to impact cost and aesthetics more than they do
performance; however, past practices may have underestimated the value of
continuous sludge removal. Of the 10 downstream storage/sedimentation basins
reviewed in a recent state-of-the-art assessment [32], all contemplated a
batch (fill-operated-drain) operation; all but two were covered; and none
provided for solids removal until after the event. A potential liability of
allowing solids to accumulate in the basin is the resuspension of those solids
during another operation of the basin before the solids can be removed.
Discharge of any of these solids in the overflow results in an apparent
reduction of the basin efficiency. Data from New York City's Spring Creek
Facility [33], the Cottage Farm facility in Boston [32], and the Whittier
Street facility in Columbus [32] exhibit this behavior, especially under
surface loading rates exceeding 3,000 gal/ft 'd [37.5 nr/nr d].
More important, perhaps, in the design and performance assessment of
facilities such as those in New York at Spring Creek (see Table 22) and Boston
that provide both storage and treatment (ignoring for the present their
primary function for overflow disinfection), are the storm events totally or
substantially contained. As an illustration, Table 23 reflects the total
facility performance at Spring Creek when totally contained events are
credited as 100% removal. Obviously, this illustration could be expanded to
account for the efficiency of the downstream plant and flows retained in the
basin and subsequently returned. However, the greater the percentage of
events totally contained, the less will be the impact on net performance of
the short-term efficiencies or inefficiencies during discharge.
Table 22. PERFORMANCE OF THE SPRING CREEK
AUXILIARY WATER POLLUTION CONTROL PLANT [33]
Events Monthly averages
No. plant operations totally
contained. Influent. Effluent. Removal.
Year Startups Discharges I Parameter mg/L g/L I*
1977
1978
110
52
24
27
78
48
TSS
BOO
TSS
600
166
79
162
56
103
50
71
31
36
37
56
45
a. Removal efficiencies reflect periods of discharge only.
b. Months Nhere average effluent concentrations exceed average Influent concentration.
excluding zero discharge Months.
7-18
-------
Table 23. NET BENEFITS APPROXIMATION OF
SPRING CREEK FACILITIES
Year
1977
1978
Net removal
Parameter Calculation efficiency, X
TSS
BOD
TSS
BOD
86 x
86 x
25 x
25 x
100 + 24
110
100 + 24
110
100 + 27
52
100 + 27
52
x 38 .
x 37 .
x 56.
x 45 .
86.5
86.2
77.2
71.4
Example 2 illustrates the use of continuous simulation model!(EPAMAC) results
and a projected performance curve for assessing the overall treatment
effectiveness of two operational concepts:
Concept 1 Multicompartmented basin with all units committed for
each event.
t Concept 2 Same facilities as Concept 1, but with limiting number
of compartments online to approach but not exceed a maximum
overflow rate objective.
Covering downstream storage/sedimentation basins on CSO systems is frequently
a design requirement for environmental compatibility. Design considerations
include cost, equipment access, and the creation of a potentially hazardous
and corrosive environment. Adamski [34] sums up an operator's perspective of
New York City's experience in covering wastewater treatment plants with the
conclusion that
"...covered treatment plants are difficult and costly to build and
operate. That even by improving the design features, certain difficulties
cannot be overcome. The reason for covering is usually a result of
uneducated planners, architects, and citizens who impose a burden on the
operator because the choices open to them are limited. When evaluating
the need for a roof on a sewage treatment plant, the reason for it must be
clearly defined and remain clear in the course of review and the rhetoric
of protest. If the need is for odor control, then the best solution,
whether operating or structural, should be selected after adequate study
of alternatives. If the need is aesthetic, then the point of viewing must
be kept in mind (whether from the ground or from above). If the need is
land or recreational opportunity, then that should be explored. In all
cases, the cost of satisfying these needs should be spelled out and the
ability to choose other needs to satisfy. Also, the operator should be
considered so that his job can be made easier." [34]
7-19
-------
EXAMPLE 2. COMPARE TREATMENT EFFECTIVENESS OF TWO ALTERNATIVE OPERATIONAL
CONCEPTS: (1) MULTICOMPARTMENTED BASIN WITH ALL AVAILABLE BASIN CAPACITY
ONLINE, AND (2) SAME FACILITIES BUT LIMITING NUMBER OF COMPARTMENTS ONLINE TO
APPROACH BUT NOT EXCEED MAXIMUM SURFACE LOADING RATE OBJECTIVE.
Specified Conditions
1. Maximum surface loading rate objective is 3,000 gal/ft^-d.
2. Average annual operating requirements are as follows (from EPAMAC system analysis):
Flowrate,
Mgal/d
>400
400
320
240
160
80
Average annual
hours of operation
0
260
25
38
89
221
Total 633
Total volume captured and treated by sedimentation - 4,940 Mgal
Total TSS applied - 6,923,000 Ib '
3. The storage/sedimentation facility is to have five parallel, identical basins with average
sidewater depth of 10 feet.
Assumptions
1. For purposes of comparing options, assume TSS influent concentration is constant.
2. Performance curve developed in Example 1 applies.
Solution
1. Compute mean TSS concentration applied.
6.933.000 Ib 1 mq/L ,,R
4,940 M 8.34 Ib/Hgal 168 mg/L
2. Compute required surface area for facility based on maximum surface loading rate.
*00 "qal/d = 133 333 ft2
3,000 gal/ft^-d '">" T*
3. For Concept 1, compute surface loading rates corresponding to design flows (note in Concept 2
surface loading rate 1s always 3,000 gal/ft2-d by definition).
Read removal efficiencies from performance curve' (Example 1) for each of these rates.
Flow, Surface loading rate, Removal
Mgal^d __ ga1/ft2-d efficiency. %
400 3,000 35
320 2,400 41
240 1,800 48
160 1,200 56
80 600 67
2. Compute removal effectiveness of each
Concept 1
(80 Mgal/d)/24 x 8.34 x 168 x 221 h x 0.67 = 0.69 x 106 Ib
(160 Mgal/d)/24 x 8.34 x 168 x 89 h x 0.56 = 0.47 x 106 Ib
(240 Mgal/d)/24 x 8.34 x 168 x 38 h x 0.48 = 0.26 x 106 Ib
(320 Mgal/d)/24 x 8.34 x 168 x 25 h x 0.41 = 0.19 x 106 Ib
(400 Mgal/d)/24 x 8.34 x 168 x 260 h x 0.35 * 2.12 x 106 Ib
Total removed = 3.73 x 106 Ib
Net % removed = 54%
Concept 2
Total TSS applied 6.923 x 10^ Ib x 0.35 = total removed « 2.42 x 106 Ib.
5. Compute net effectless improvement of Concept 1 over Concept 2.
(3.73 - 2.42)/2.42 = 54% improvement in annual TSS removal by adopting
Concept 1 over Concept 2
Comment
For the conditions stated, it is apparent that Concept 1 (maximizing tankage
online) is associated with significant benefits. This might not be the case where the
adopted maximum surface loading is much more conservative, where the performance change
as a function of surface loading rate is less pronounced, or where the required operating
history is significantly different.
7-20
-------
Typically, where flushing is the adopted system (whether through fixed or
movable nozzles), a center dewatering troughinvert slope approximately 1%
is provided running the length of the basin with floor side slopes to the
trough at 5 to 10%. Basins or individual bays range from 27 ft (8 m) to 80 ft
(24 m) in width, and typically 10 to 20 ft (3 to 6 m) in sidewater depth.
Boston provides manual cleanup after dewatering using fire hoses; New York
uses traveling bridge mounted sprays; and Saginaw uses a combination of wall-
mounted fixed sprays and strategically positioned high pressure fire nozzle
stations.
Under the latter case, approximately 5 Mgal (19,000 m3) of washwater (strained
river water) is used per washdown cycle for the 23 Mgal (87 nr) capacity
reservoir [35].
Cited advantages of flushing water systems include low cost, thorough cleaning
performance, and minimum of mechanical equipment exposed to the corrosive
environment. Principal disadvantages would appear to be the inability to
remove sludge in other than a dewatered basin condition, energy requirements
for pressurization, and the increased liquid volume to be treated through the
pump-back system.
In lieu of flushing, Milwaukee uses seven mechanical mixers to resuspend
solids from its 0.7 acre (0.3 ha) floor area during dewatering operations; New
York City uses a series of hydraulic nozzles on a traveling bridge to
resuspend solids; Columbus uses a traveling bridge mounted scraper blade; and
San Francisco proposes to use conventional chain and flight collectors in its
Southwest wet-weather primary treatment plant. In the latter case, virtually
all operations (averaging 633 operating hours per year) will be in a flow-
through treatment mode as the principal storage is provided elsewhere in the
system. To avoid problems experienced in earlier stormwater demonstration
projects where chains and drives have significantly corroded (rust bound)
under the normal fill and draw operations, nonmetallic chains are under
consideration. It is also noted that with the capability for continuous
sludge removal, tanks may not have to be dewatered through most of the wet-
weather season, easing maintenance requirements and maintaining a short
response time (readiness-to-serve) for system activation. In Mt. Clemens,
still another system will use air and water jets from wall mounted headers to
resuspend solids in a slurry as a modification of the more typical flushing
system [24].
One means currently used in Europe for removal of settled solids is a
submersible pump suspended from a traveling bridge. The depth of the pump is
automatically controlled so that sludge of the proper consistency is withdrawn
(without disturbing the pond bottom when unlined earthen ponds are used).
At Chippewa Falls, solids are removed from the dewatered basin by mechanical
equipment (street sweepers, loaders, and dump trucks). The basin is lined
with asphalt and has a ramp down the side for vehicle access. A similar
system is used at several facilities in Europe also.
7-21
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Step 5 - Identify and Select Pretreatment Components
Pretreatment components are selected on the basis of enhancing performance
and/or operations. Typically, the choices include coarse screening and grit
removal. The units, if provided, may be located at the facility or upstream
of pumps serving the facility. In practice, the adopted components include
none, some, or all of the above. The purpose of the coarse bar racks with 2
to 4 in. (5 to 10 cm) clear openings is to remove heavy objects of all
descriptions from the flow to protect downstream equipment and to prevent
travel of objects to more inaccessable locations. Finer screens with 0.75 to
1.5 in. (2 to 4 cm) clear openings typically remove rags and finer solids that
tend to clog process piping, valves, and pumps. They also trap many of the
floatables that otherwise might appear in the effluent. Where basin overflows
are rare, either or both have been omitted. Separate grit removal normally
would be required only where treatment is the primary role of the facility and
where grit is to be handled separately from the sludge. In some facilities,
grit is removed from the sludge after sedimentation by using cyclone grit
separators. Flow measurement and recording is recommended for all facilities
that discharge frequently; whereas stage measurement and recording should
suffice for basins which seldom overflow. Flow measurement is essential for
pacing disinfectant dosages to reduce the chance for ineffective application
and toxic carryovers.
Step 6 - Detail Auxiliary Systems
Auxiliary systems, those which support and complete the primary function,
typically include sludge removal and processing, flushing, disinfection, air
handling and odor control, energy (power, lighting, heating), and instrumen-
tation and control. Sludge processing considerations include (1) the location
and method of ultimate processing, (2) method of transport, (3) the impact on
existing facilities, and (4) constraints (i.e., pumping rates, solids
concentration, pretreatment) that must be observed. The simplest and most
practiced solution is to return the sludge to the interceptor, sometimes with
an intermediate degritting step.
Flushing water system evaluation includes source of supply, quantity and rate
of application, pressure requirements (typically up to 150 lb/in. or
11 kg/cnr), distribution system, and method of control. A conceptual drawing
of the Sacramento system as adapted to San Francisco is shown in Figure 31
[36]. The design flushing water application rate is 30 gal/min ft of basin
length (21 L/m s) with 100 ft (30 m) segments to be flushed sequentially.
When facilities are covered, air handling and gas monitoring (for explosive,
corrosive, and toxic potential) are important considerations. In selecting
air change requirements for enclosed secondary treatment plants, Adamski [34]
notes that two air changes per hour proved inadequate to control misting and
that later New York City designs provided for a minimum of six air changes per
hour. Common practice in covered CSO storage/sedimentation basins seems to be
6 to 12 air changes per hour with the higher figures based on a full liquid
depth condition. Variable rate air handling control through staging or speed
control appears desirable for energy conservation. Standard practices for
odor control should be evaluated [37].
7-22
-------
HEADER (INTERNALLY BR
EXTERNALLY MOUNTED)
FLOOR SPRAT
NOZZLES
45-DEGREE
FJLLET
ALL SPRAY
NOZZLES
(OPTIONAL
CENTRAL DISCHARGE
CHANNEL
NOTE: CONTROL VALVES
IM EACH SPRAY LINE
NOT SHOIN.
Figure 31. Flushing water system concept [36].
As a rule, instrumentation and control systems should be as simple as possible
and should be designed giving full recognition to the corrosive environment
and the level of operation and maintenance to be provided. Areas of study
recommended are:
Status and performance monitoring
System activation
t System deactivation
Auxiliary system control
Step7 - Estimate Costs and Cost Sensitivities
Detailed cost estimates should be prepared with emphasis on component systems
and following the value engineering guidelines. For example, what is the base
cost of the facility to provide the functional requirements identified in
Step 1 on the site selected in Step 2? What was the added cost of covering
including air handling? What was the added cost of pretreatment? sludge
7-23
-------
removal? instrumentation and control? How much do site conditions impact the
base cost? This analysis should lead to a more cost effective total design.
An operations plan and staffing and maintenance schedule should be finalized
at this point and operations and maintenance cost projections made. As noted
in earlier state-of-the art assessments [23, 32], the proximity of the
storage/sedimentation basin to a fully staffed water pollution control plant
may provide for optimum joint staff utilization.
Step 8 - Complete Design
The final step is to confirm that all objectives have been satisfied. This
may require several iterations back through earlier steps including a
reassessment of the basic criteria once the site specific costs for compliance
are known.
OPERATION AND MAINTENANCE CONSIDERATIONS
The major operation and maintenance goal of downstream storage/sedimentation
basins is to provide a facility that is available to its full design capacity
when needed and for as long as needed. Secondary goals include clear, prompt,
and complete records of performance (i.e., NPDES compliance reporting),
reliability to provide for reallocation of personnel and facilities in non-
storm periods, and dual use operations (such as backup treatment and/or flow
equalization for dry-weather plants).
Experience has shown that frequent, periodic maintenance and equipment
exercising is essential to maintain an effective readiness-to-serve. For
obvious reasons, it is recommended that this maintenance be carried out on a
preplanned rather than as-available basis. Staffing requirements will be
unique for each facility and operation and maintenance organization.
Questions to be addressed in the operations plan include:
t Will the facility activate unattended?
What operational staffing is necessary to complete the primary
function?
What staffing is necessary to complete the auxiliary functions?
Will the monitoring-reporting system activate unattended?
What activities require immediate response (multishift availability)
and what activities can be deferred and for how long (to conform to
standard shift)?
What emergency conditions could be encountered and how will they be
addressed?
What operational decisions must be made and who has the
responsibility/control?
7-24
-------
What operational decisions can be implemented remote from the site
and which, if any, require direct observation?
What are the standard operating procedures to ensure safety of the
public, operators, and equipment?
Recognizing that unit activations (storms) can occur at any time and generally
with short warning, adequate provisions must be made for parking, assembly and
briefing, and operating station access. Backup plans for automated actions
should be identified including confirmation feedback and manual override if
necessary.
Again, the operation and maintenance requirements and procedures should be
developed from the operational plan and not from industry wide standards since
there are none.
COSTS
Construction costs of downstream storage/sedimentation basins have been
reported [32, Table 73] for selected demonstration facilities (including
pretreatment and auxiliary systems) and are highly site specific. Adjusted to
ENR 4000, the range of unit costs is from $0.50 to $10.00 /gal ($0.13 to
$2.64/L) of storage capacity with a median value of about $2.50/gal
($0.66/L). As would be expected, facilities whose primary function is storage
fall at a low end of the cost range and those which are in effect primary
treatment plants rank at the high end of the range.
Storage/sedimentation basins estimated by Benjes excluding pretreatment and
auxiliary systems (based on a 20 Mgal or 76,000 nr capacity) ranged from
$0.03/gal ($0.01/L) for open earthen basins, to $0.42/gal ($0.11/L for covered
concrete basins [38]. The discrepancy between Benjes estimates which are
based on unit costs and hypothetical basin designs and actual construction bid
costs demonstrate not only the impact of pretreatment and auxiliary systems,
but the overriding importance of specific site conditions.
Another planning level cost source is a 1978 USEPA publ ication--Construction
Costs for Municipal Wastewater Treatment Plants: 1973-1977 [39], which
presents a regression analysis of construction bid costs by region, unit
process, and construction component. While the emphasis is on secondary
treatment, there is a good deal of potentially applicable information on
preliminary treatment, influent pumping, primary sedimentation, site work, and
special conditions.
Capital Cost Breakdown - Illustrative Examples
Three examples are presented in Table 24: Facilities A and B represent
covered basins where the primary function is storage and Facility C represents
a covered and buried facility where the primary function is sedimentation.
Facility C is also unique in that it provides continuous service as a dry-
weather treatment plant (22 Mgal/d or 1 nr/s average dry-weather capacity) as
7-25
-------
well as 450 Mgal/d (20 nr/s) peak wet-weather flow capacity. The unit cost
summaries at the bottom of the table clearly demonstrate the function/cost
relationship stressed in basin design: the storage units are cost-optimized
on the basis of volumetric capacity and the treatment unit is cost-optimized
on the basis of volume treated and discharged. The premium cost for burial of
Facility C, included in the table costs, to facilitate dual use of the site
above the tanks is estimated as 18% above the cost of a totally enclosed plant
with exposed superstructures.
Table 24. EXAMPLE CAPITAL COST BREAKDOWNS
Facility A [.40J Facility B [41] Facility C [42]
Item
Cost,
$ million
% of
total
Cost,
$ million
% of
total
Cost,
$ million
% of
totil
General, sltework, and
outside piping 3.24
Structural and
architectural
Mechanical equipment,
piping, and plumbing 0.11*
Heating, ventilating,
91
2.1
9.6
3.2
13
59
19
27.1
32.9
14.3
29
35
16
and odor control
Instrumentation
Electrical
Total
Cost per gallon of
storage capacity b
Cost per gallon treated
nd discharged0
0.08
0.12
3.55
0.91
NAd
2 0.5
0.4
4 0.6
100 17.4
0.76
0.27
3
2
4
100
8.4
2.5
7.2
92.4
6.16
0.006
9
3
8
100
l. Equipment carried under General.
b. $/gal.
c. Capital cost divided by average annual volume discharged.
d. NA - not available.
Example 3 illustrates the use of the USEPA regression cost curves as a
crosscheck for Facility C.
Operation and Maintenance Costs
As noted earlier, there are no rule-of-thumb guides for estimating operation
and maintenance costs short of developing an operation and maintenance cost
program for the specific facility. For planning level estimates, first-cut
approximations may be developed from reported costs of operating facilities
[32 (Table 73), 8] or in the case where the primary function is treatment,
from standard sanitary engineering references such as the 1971 USEPA published
regression curves developed by Black and Veatch [43] with adjustment to
reflect intermittent operations.
7-26
-------
EXAMPLE 3. COMPARE COST OF FACILITY IN TABLE 24 WITH EXPECTED COST OF
"EQUIVALENT" PRIMARY TREATMENT PLANT USING REGRESSION CURVES FROM
REFERENCE [38]
Specified Conditions
1. The design flow = 450 Mgal/d
2. The process train includes screening, grit removal, and primary sedimentation.
3. No sludge treatment is included.
4. An operations and maintenance building (with laboratory) is included.
5. Surface loading rate for Facility C is 2,700 gal/ftZ-d at 450 Mgal/d.
Assumptions
1. Design surface loading rate for conventional primary sedimentation is 900 gal/ft^-d.
2. Primary plant component costs without sludge will be 35% of secondary with sludge.
Solution
1. Select appropriate cost curves or regression equations from reference [38].
a. Process - Second order cost curves, page 6-54.
(1) Preliminary treatment C = 5.79 x 10*!)1-17
(2) Primary sedimentation C = 6.94 x lO^Q'-O^
(3) Laboratory/maintenance building C = 1.65 x loSql.02
b. Construction component - second order curves, Tables 6-42 through 6-50 inclusive.
(1) Mobilization C = 4.77 x 10*Ql.15
(2) Sitework Including excavation C = 1.71 x 105Ql-l7
(3) Pilings, special foundation, dewatering C » 3.68 x
(4) Electrical C = 1.36 x
(5) Heating, ventilating, and air conditioning C = 3.10 x 104Q1-24
(6) Controls and Instrumentation C - 5.06 x 10*0.1-12
(7) Yard piping C = 9.96 x 104Ql-03
2. Select "equivalent" design flow for conventional plant.
a. Equate on basis of surface loading rates.
Q = (900/2,700) x 450 = 150 Mgal/d
b. This falls outside range of sample data, use Q = 100 Mgal/d, which is upper limit of
sample data.
3. Compute regional and time adjustment factors.
Base costs are 2nd Quarter 1977 = EPA LCAT Index 134
Current cost 3rd Quarter 1980 * EPA LCAT Index 181
Regional multiplier from Table 7-1, page 7-14, for San Francisco 1s 1.3175
Combined multiplier = (181/134) x 1.3175 = 1.78
4. Compute costs and compare.
Cost, $ million (ENR 4000)
Item Facility C estimate Computed survey cost
General, site work, and outside piping
[Items b.(1),(2),(3), and (7) x 0.35 (for primary)] 27.1 40.2
Structural and architectural
[Items a(l),(2), and 0.4 x (3)] 32.9 50.3
Mechanical equipment, piping, and plumbing
[included under structural and architectural] 14.3
Heating, ventilating, and odor control
[Item b(5)] 8.4 5.9
Instrumentation
[Item b(6)] 2.5 5.4
Electrical
[Item b(4)] 7.2 8.5
Total 92.4 110.3
Coirment
The USEPA guide provides an effective tool for quick cost breakdown comparisons, but application
becomes questionable for plant capacities greater than 50 Mgal/d.
7-27
-------
REFERENCES
1. Basta, D.J. and B.T. Bower, Editors. Analyzing Natural Systems--
Analysis for Residuals for Environmental Quality Management. Johns
Hopkins Press, Baltimore,Md. June 1982.
2. Hydroscience, Inc. Procedures for Assessment of Urban Pollutant Sources
and Loadings, Chapter 3 in Areawide Assessment Procedures Manual. USEPA
Report No. EPA-600/9-76-014. July 1976.
3. Howard, C.D.D., et al. Storm and Combined Sewer Storage Treatment
Theory Compared to Computer Simulation. USEPA Report No. EPA-600/2-81-
125. NTIS No. PB 81 222341. July 1981.
4. Smith, W.G. and M.E. Strickfaden. Macroscopic Planning Model:
Application Guide and Users Manual. USEPA Report (at press).
5. Hydrologic Engineering Center, Corps of Engineers. Urban Stormwater
Runoff: Storm. Generalized Computer Program 723-S8-L2520, Hydrologic
Engineering Center, Army Corps of Engineers. Davis, California. July
1976.
6. Lager, J.A., et al. Development and Application of a Simplified
Stormwater Management Model. USEPA Report No. EPA-600/2-76-218. NTIS
No. PB 258 074. August 1976.
7. Litwin, Y.J., et al. Areawide Stormwater Pollution Analysis with the
Macroscopic Planning (ABMAC) Model. USEPA Report by Association of Bay
Area Governments. Final Report (at press).
8. Metcalf & Eddy, Inc., University of Florida, and Water Resources
Engineers, Inc. Stormwater Management Model, Volume I. USEPA Report
No. 11024DOC07/71. NTIS No. PB 203 289. July 1971.
9. Litwin, Y.J. and A.S. Donigian, Jr. Continuous Simulation of Non Point
Pollution. Journal WPCF, Vol. 50, No. 10, p. 2348. October 1978.
10. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. Second Edition. McGraw-Hill. 1979.
11. Camp, T.R. Sedimentation and the Design of Settling Tanks.
Transactions ASCE, Volume 111, p. 895. 1946.
12. Eckenfelder, W.W. and D.J. O'Connor. Biological Waste Treatment.
Pergamon Press. 1961.
13. White, J.B. and M.R. Allos. Experiments on Wastewater Sedimentation.
Journal WPCF Volume 48, No. 7, p. 1741. July 1976.
14. Dalrymple, R.J., S.L. Hodd, and D.C. Morin. Physical and Settling
Characteristics of Particulates in Storm and Sanitary Wastewaters.
USEPA Report No. EPA-670/2-75-011. NTIS No. PB 242 011/AS. April 1975.
7-28
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15. Environmental Design & Planning, Inc. Data Transmitted to Project
Officer, Office of Research and Development. USEPA, Cincinnati, Ohio.
July 1980.
16. Smith, R. Preliminary Design of Wastewater Treatment Systems. Journal
of the Sanitary Engineering Division, ASCE Vol. 95, No. SA1, p. 117.
February 1969.
17. Metcalf & Eddy, Inc. City and County of San Francisco Southwest Water
Pollution Control Plant Project. Final Project Report. February 1980.
18. City of Milwaukee, Wisconsin, and Consoer, Townsend and Associates.
Detention Tank for Combined Sewer Overflow, Milwaukee, Wisconsin,
Demonstration Project. USEPA Report No. EPA-600/2-75-071. NTIS No. PB
250 427. December 1975.
19. Glover, G.E. and P.M. Yatsuk. Microstrainng and Disinfection of
Combined Sewer Overflows. USEPA Report No. 11023EV006/70. NTIS No. PB
195 674. June 1970.
20. Glover, G.E. and G.R. Herbert. Microstraining and Disinfection of
Combined Sewer Overflows - Phase II. USEPA Report No. EPA-R2-73-124.
NTIS No. PB 219 879. January 1973.
21. Moffa, P.E., et al. Bench-Scale High-Rate Disinfection of Combined
Sewer Overflows With Chlorine and Chlorine Dioxide. USEPA Report No.
EPA-670/2-75-021. NTIS No. PB 242 296. April 1975.
22. Drehwing, F.J., et al. Combined Sewer Overflow Abatement Program,
Rochester, N.Y. - Volume II Pilot Plant Evaluations. USEPA Report No.
EPA-600/2-79-031b. NTIS No. PB 80-159262. 1979.
23. Lager, J.A. and W.G. Smith. Urban Stormwater Management and
Technology: An Assessment. USEPA Report No. EPA-670/2-74-040. NTIS
No. PB 240 687. December 1974.
24. Lynard, W.G. et al. Urban Stormwater Management and Technology: Case
Histories. USEPA Report No. EPA-600/8-80-035. NTIS No. PB 81-107153.
August 1980.
25. Karl R. Rohrer Associates, Inc. Demonstration of Void Space Storage
With Treatment and Flow Regulation. USEPA Report No. EPA-600/2-76-
272. NTIS No. PB 263 032. December 1976.
26. Commonwealth of Massachusetts, Metropolitan District Commission.
Cottage Farm Combined Sewer Detention and Chlorination Station,
Cambridge, Massachusetts. USEPA Report No. EPA-600/2-77-046. NTIS No.
PB 263 292. November 1976.
7-29
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27. Feuerstein, D.L. and W.O. Maddaus. Wastewater Management Program,
Jamaica Bay, New York; Volume I: Summary Report. USEPA Report No. EPA
600/2-76-222a. NTIS No. PB 260 887. September 1976.
28. Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan on
Preliminary Design of the Hancock Street Combined Sewage Overflow
Storage and Treatment Facility. March 16, 1973.
29. Southeastern Oakland County Sewerage Disposal System, MI. Communication
from Mr. C. McKinnon, Chief Chemist to Dr. J. Finnemore, Metcalf & Eddy,
on Red Run Drain Retention Basin. June 1978.
30. Heinke, G.W. et al. Effects of Chemical Addition on the Performance of
Settling Tanks. Journal WPCF, Volume 52, No. 12, p. 2946. December
1980.
31. ASCE-Manual of Engineering Practice No. 36 Wastewater Treatment Plant
Design. Lancaster Press, Inc. 1977.
32. Lager, J.A., et al. Urban Stormwater Management and Technology: Update
and Users' Guide. USEPA Report No., EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
33. City of New York, Department of Environmental Protection. Communication
from Mr. H. Tschudi, Deputy Director Bureau of Water Pollution Control
to Mr. M.P. Chow, Section Engineer San Francisco Clean Water Program on
Performance Data from Spring Creek Auxiliary Water Pollution Control
Plant. September 1979.
34. Adamnski, R.E. New York City's Experience in Covering Sewage Treatment
Plants. Presented at 50th Annual Winter Meeting of the New York State
WPCA. January 1979.
35. Sacramento Area Consultants, Sacramento, California Contract Documents
for Pioneer Reservoir Sump 1 Modifications. Contract No. 1108,
Sacramento Regional County Sanitation District. September 1977.
36. Caldwell-Gonzalez-Kennedy-Tudor Consulting Engineers. Bayside
Facilities Planning Project Draft Project Report for San Francisco Clean
Water Program. December 1980.
37. WPCF Manual of Practice No. 22. Odor Control for Wastewater
Facilities. 1979.
38. Benjes, H.H., Jr. Cost Estimating Manual - Combined Sewer Overflow
Storage Treatment. USEPA Report No. EPA-600/2-76-286. NTIS No. 266
359. December 1976.
39. Dames & Moore. Construction Costs for Municipal Wastewater Treatment
Plants: 1973-1977. USEPA Report No. EPA-430/9-77-013, MCD-37. January
1978.
7-30
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40. Consoer, Townsend & Associates. City of Milwaukee, Wisconsin, Humbolt
Avenue Pollution Abatement Demonstration Project. Final Report Draft.
USEPA Project No. 11020-FAU. May 1973.
41. Brown and Caldwell - Sacramento Area Consultants. Pioneer Reservoir Bid
Tabulation and Engineer's Breakdown. February 1978.
42. Metcalf & Eddy, Inc. San Francisco Southwest WPCP. Engineer's Estimate
at 75% Design Level. February 1981.
43. Black and Veatch. Estimating Costs and Manpower Requirements for
Conventional Wastewater Treatment Facilities. USEPA Report No. 17090
DAN 10/71. October 1971.
7-31
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Section 8
INTERNATIONAL PERSPECTIVE
INTRODUCTION
The application of storage/sedimentation controls to urban stormwater problems
is not unique to the United States. In fact, in this era of excellent
communications and increasing technology-sharing on an international scale,
basically similar approaches are found in many areas of the world. This is
particularly evident in the highly productive and densely developed nations
for which receiving water quality is of great concern.
Stahre, in his comprehensive manual on storage/sedimetation practices in
Europe, ranks principal urban stormwater problems as: (1) flooding,
(2) discharge of untreated wastewater (CSO), and (3) shock loadings of the
wastewater treatment plants [1]. Similarly. Kuribayashi and Nakamura identify
combined sewer problems in Japan as threefold: (1) flooding, (2) pollution as
a result of excess overflow, and (3) pollution by primary effluent [2]. The
latter is a result of limitations in secondary process treatment capacities to
1.3 to 1.5 times average dry-weather flow and the common practice of returning
supernatants from sludge processing facilities to the primary units. Because
of this supernatant return, excess wet-weather flows discharged from secondary
plants after only primary treatment may be heavily contaminated.
Chambers and Tottle have documented the benefits of onsite detention
facilities as an alternative to conventional storm sewer systems in Winnipeg,
Canada [3]. The impoundments were found to be well suited to the low relief
topography, and the impermeable nature of the soil made attractive wet ponds
feasible and recreationally as well as technically effective. Source controls
emphasizing infiltration and percolation, although introduced only in the
early 1970s, now number several hundred installations in Sweden, according to
Stahre.
In the United Kingdom, traditional design of combined sewers has been to
provide sewer capacity equivalent to six times the average dry-weather flow.
Excess flows are allowed to overflow directly to the receiving water.
Wastewater treatment plants are designed to provide biological treatment to
3.0 times dry-weather flow. Flows in the range of three to six times dry-
weather flow are treated in storage/sedimentation tanks, sized to provide a
minimum of 2 hours detention before overflowing to the receiving water. This
system is particularly effective in the United Kingdom because of the
uniformity and low intensity of its rainfall.
8-1
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In West Germany, where many of the cities are also served by combined sewers,
the climate, topography, and sewer catchment configuration often combine to
produce a pronounced first flush effect. A series of dispersed upstream
storage basins with simple diversions and flowrate controls was found to offer
a promising solution for water quality protection. Detailed guidelines have
been prepared by the state agencies to cover the planning, design, and
operation of these and alternative facilities [4]. Where accommodated by
existing hydraulics, many basins have been designed to facilitate self
cleaning.
In Japan, Kuribayashi and Nakamura conclude that onsite and offsite storage of
stormwater (70% if the sewered areas are served by combined sewers) and the
bleeding of it back to the treatment works during low dry-weather flow periods
seems to be one of the most feasible and effective solutions. They note that
because the storage of stormwater can solve pollution problems caused by CSO
as well as flooding, this measure is gradually becoming accepted by many city
engineers.
In each of the above examples, hydrology, topography, and existing facilities
and practices have been important factors in determining the direction of
cost-effective approaches.
Typical of the international urban stormwater runoff and combined sewer
overflow control and treatment techniques and practices are the following
European examples of a storage/sedimentation practices manual, several flow
control devices, and two innovative technology applications presented in this
section.
STORAGE/SEDIMENTATION PRACTICES MANUAL
Despite the fact that detention basins have been in use for a long time in
many countries, the authorities in Sweden began to accept such faciltiies as
an adequate alternative to sewer separation only in recent years. A review of
the storage/sedimentation practices in Sweden along with a detailed analysis
of the technical configuration, design, and layout of various storage/
sedimentation arrangements was prepared by Dr. Peter Stahre [1]. The book,
directed toward municipal water and sewer engineers and consulting engineers,
includes four parts: (1) systemization of facilities with respect to
technical configuration and placement within the sewerage system,
(2) regulation of flow from storage/sedimentation facilities, (3) design of
facilities with respect to the primary function of the facility, and
(4) planning requirements and cost-effectiveness analysis for storage/
sedimentation facilities. The book can also be of benefit to researchers and
government workers who deal with stormwater and combined sewer overflow
problems. The content of each part of the book is stuctured so that it is
possible to go directly to the section of interest. Also included in the book
are the results and evaluations of several hydraulic modeling tests on flow
control devices.
8-2
-------
FLOW CONTROL DEVICES
For certain cases, the flow from storage/sedimentation facilities can be
controlled by means of specially designed flow regulators. These provide more
effective regulation of the flow than can be accomplished with a fixed
throttle section. Four different regulation arrangements are described
briefly:
Flow regulator
Hydrobrake
Wirbeldrossel
Flow valve
a common feature is
special exterior
Although the arrangements operate somewhat differently,
that they are completely self-regulating and require no
control equipment
Flow Regulator
The Steinscruv flow regulator for temporarily impounding flow in the pipelines
upstream of the regulator was developed by Stein in Sweden in the mid-1970s.
The flow regulator consists of a stationary, anchored screw-shaped plate that
is turned through 270° installed in a pipe, as shown in Figure 32. In that
part of the plate which fits against the bottom of the pipeline, there is an
opening to release a certain specified base flow. The opening is sized so
that the flow that passes through the regulator is sufficient to maintain the
self-cleaning velocity for the pipeline. The length of the flow regulator is
approximately three times the diameter of the pipeline.
. Figure 32. Flow regulator [5].
Damming takes place when the inflow to the regulator exceeds the capacity of
the base opening. The extent of the damning and the volume detained are
dependent on the slope of the pipe. When the flow depth reaches the crown of
the pipe, the flow capacity becomes practically equal to the unregulated
capacity as shown in Figure 33. It is possible to further regulate the flow
by using several flow regulators in series.
8-3
-------
h
-D
100
60
40
7
20
40
60
n
Discharge curve with flow reg.
Discharge curve without flow regulator
Figure 33. Comparison of discharge curves for unrestricted
pipe and pipe with flow regulator [5].
The flow regulator can be used in either separate storm sewers or combined
sewers to control storage. However, to prevent clogging of the regulator by
debris, a diameter of 30 in. (800 mm) has been reported as the suitable
minimum dimension [1],
Hydrobrake
The Hydrobrake, developed in Denmark in the mid-1960s
outflow from a storage structure.
is used to control
The Hydrobrake consists of an eccentric vertical cylindrical housing with an
inlet opening located on the surface of the cylinder and an axial outlet pipe
located on one base of the cylinder (see Figure 34). The Hydrobrake is
installed within the storage structure so that the axial outlet pipe
discharges into a downstream pipe or other conveyance structure. Several
different configuations are available depending on the specific application
required.
When the water level rises in the storage structure, hydrostatic pressure sets
the water in motion In the Hydrobrake, as shown in Figure 35. -Since the
outlet pipe opening is perpendicular to the direction of rotation of the
water, the flow tends to assume a helical motion and the discharge is
significantly less than if the flow had taken place through a fixed throttled
section. A comparison of the discharge from a Hydrobrake with a circular pipe
of the same size is shown in Figure 36.
Hydrobrakes are in use presently in the United States, Canada, and Sweden.
8-4
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OUTFLOW
INFLOW
Figure 34. Schematic of a Hydrobrake [1]
Figure 35. Schematic of flow patterns during
Hydrobrake operation [6].
8-5
-------
Filling
height (ni)
3,0-
2,5-
Hole 150 nun diameter
ii 1ii 1r
0 W 20 30 40 SO 60 70 80 90 KM 110 UO
Flow
*0/t)
Figure 36. Discharge curve comparison for Hydrobrake
and short pipe of same diameter [7].
Wirbeldrossel
The Wirbeldrossel, or turbulent throttle, developed in Germany in the mid-
1970s, is another means for regulating outflow from a storage facility. In
many respects, it is similar to a horizontal Hydrobrake but located
immediately downstream of the storage facility (see Figure 37). The
Wirbeldrossel is made up of a symmetrical cylinder having a tangential inlet
and a circular outlet on the base of the cylinder. An aeration pipe is
provided on the top of the unit.
A similar unit, the Wirbenventil or turbulence valve, was later developed for
applications where there is a continuous base flow through the storage unit.
The construction is essentially the same as for the Wirbeldrossel but consists
of an obliquely positioned rotational chamber that does not require as great a
head!oss.
The Wirbeldrossel functions basically the same as a Hydrobrake in that rotary
motion imparted to the water limits the discharge rate. The discharge is
further limited by the core of air formed around the axis of rotation in the
cylinder housing blocking a great part of the outlet opening.
8-6
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Inlet
AERATION
PIPE
OUTLET
Figure 37. Schematic of flow pattern
in a Wirbeldrossel [1].
The discharge is dependent on the pressure head, size of the inlet pipe, and
the outlet opening. A comparison of the discharge curves for a Wirbeldrossel
and a circular pipe both having the same outlet opening is shown in Figure 38,
h(m)
2 -
1.5 -
1.0
0.5 -
Inlet pipe 200 mm diair.eter
Diaphragm opening 150 mr. diameter
Horizontally arranged outlet
hole 150 mm diameter
100 Q(l/s)
Figure 38. Discharge curves for Wirbeldrossel and
circular outlet of the same size [8].
8-7
-------
Flow Valve
The flow valve was developed in the late 1970s in Sweden as a device for
holding the outflow from a detention facility constant. The flow valve is
essentially a central outlet pipe surrounded by a pressure chamber filled with
air as shown in Figure 39. The top part of the pressure chamber and its
connection to the central outlet pipe are made of flexible rubber fabric; the
rubber fabric is braced at the inlet and outlet of the center pipe.
FLEXIBLE
RUBBER
FABRIC
y
Figure 39. Diagram of a flow valve [9],
Water pressure on the upper portion of the rubber fabric is propagated through
the pressure chamber displacing the fabric at the outlet section. Thus, the
hydraulic capacity of the outlet is throttled by the change in outlet cross-
sectional area. The resultant effect is that the discharge through the flow
valve remains constant and independent of the pressure head as shown in
Figure 40.
Filling height ;m>
3,0-
2,5
2,0
1,0-
0,3
Flow valve
.Horizontally arranged
outlet hole
_Flow
10
20
Figure 40. Typical discharge curve for flow valve [1]
8-8
-------
INNOVATIVE TECHNOLOGY APPLICATIONS
Flow Balance System
An innovative approach to urban stormwater treatment for the protection of
lakes has been developed and applied at several locations in Sweden by Karl
Dunkers. The patented system uses a portion of the lake volume to store and
settle urban runoff before discharge. A schematic of the system in shown in
Figure 41.
A grid of wooden platforms floating on the lake's surface support flexible
PVC fiber glass cloth that extends to the lake bottom and forms a series of
baffled rectangular cells isolating the main body of the lake from the urban
stormwater runoff. To ensure plug flow through the facility, the openings in
the baffles are placed alternately at the top and bottom in adjacent cells.
Stormwater runoff entering the cells displaces lake water from sequential
cells. Stored runoff is withdrawn from the inlet cell and treated prior to
its discharge to the receiving water. Lake water replaces the withdrawn
stored runoff at the grid's outlet, so the stormwater in the grid flows back
toward the inlet cell. This system is an excellent example of low cost, but
effective, stormwater storage for receiving water quality protection.
An operating facility is located at Lake Trehormingen at Huddinge/Stockholm,
Sweden. To prevent eutrophication of the lake resulting from phosphorus
loadings, a treatment plant removes phosphorus from the stored stormwater.
Self-Cleaning Storage/Sedimentation Basin
Typically, removal of settled solids from an inline storage facility has been
a problem that required an auxiliary flushing system of some sort. An example
of an innovative approach to eliminating this problem is included in an inline
storage/sedimentation tank in Zurich, Switzerland. A continuous dry-weather
channel, which is an extension of the tank's combined sewer inlet, is formed
by a number of parallel grooves connected at their end points similar to that
shown in Figure 42.
The bottom groove must be given a certain slope for the water to flow by
gravity through the basin. Thus, the outlet can be significantly lower than
the inlet depending on the size of the basin. This approach should only be
considered for small basins, less than about 132,000 gal (500 nr) [1]. Any
solids that have settled in the basin during its storage operation are
resuspended by the channelized flow during the drawdown following a storm
event.
For small storms, the runoff is completely captured. If the basin fills, the
overflow is discharged over a weir at the end of the basin.
8-9
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fe«d punipf)11 "\. . . i' ;
^ ^V_^/^ A : plasticjcloth:-f-
: floating tank
Figure 41. Schematic of pontoon tank system
at Lake Tehorningen, Sweden.
8-10
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Submerged screen
Spillway
Figure 42. Self-Cleaning Storage/Sedimentation Basin
used 1n Zurich, Switzerland [10],
8-11
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REFERENCES
1. Stahre, Peter. Flodesutjamningi Avloppsnat (Flow Balancing in Waste
Water Nets). Byggforskningsradet (Construction Research Council). 1981.
2. Kuribayaski, M. and E. Nakamura. Challenging Combined Sewer Problems in
Japan. Journal of the Water Pollution Control Federation, Vol. 52,
No. 5. May 1980.
3. Chambers, G.M. and C.H. Tottle. Evaluation of Stormwater Impoundments in
Winnipeg. Report SCAT-1 Environmental Protection Service, Environment
Canada. April 1980.
4. Technical Wastewater Union E.V. Guidelines for the Sizing and Design of
Stormwater Discharges in Combined Sewers. Working Instruction A-128.
1978.
5. Janson, I.E. and S. Bendixen. New Method for Detaining Flow Variations
in Gravity Flow Lines. Stadsbyggnad No. 6. 1975.
6. Hydro-Storm Sewage Corporation. Hydrobrake, Liquid Flow Controls.
Brochure. 1978.
7. Hydro-Storm Sewage Corporation. Hydrobrake Installations. Brochure.
1979.
8. Quadt, K.S. and H. Brombach. Operational Experiences with the Turbulence
Throttle in Storm Water Overflow Basins. 1978.
9. Kungl Paten och Registreringsverket. Inlet and Outlet Regulator for
Flowing Media. Patent No. 770601-0. 1980.
10. Koral, J. and C. Saatci. Rain Overflow and Rain Detention Basins.
Second Edition. Zurich. 1976.
1-12
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Appendix A
POLLUTANT CHARACTERIZATION AND ESTIMATION OF REMOVAL
INTRODUCTION
Characterization of the pollutants in the flow (either stormwater runoff or
CSOs) is important to the estimation of the pollutant removal by storage or
sedimentation or both. In this appendix, the characterization of pollutants
is discussed from the standpoint of sample collection and sample analysis.
The approach and methodology to be used to collect representative samples is
presented. Also presented are discussions of the selection of pollutants to
be analyzed, particle size determination, and the pollutant distribution
versus particle size and specific gravity.
A suggested analytical method for flow routing and for pollutant routing is
described. Pollutant removal can be simulated by (1) characterization by
magnitude, or (2) characterization by particle size and specific gravity
distribution. Both of these methods are presented along with the appropriate
equations and figures.
POLLUTANT CHARACTERIZATION
Characterization studies for stormwater runoff and CSO are conducted to
determine (1) the physical, biological, and chemical characteristics, and the
concentration of constitutents in the wastewater; and (2 ) the best means of
reducing the pollutant concentrations. Procedures for wastewater sampling and
methods for sample analysis are described in this section.
Sample Col lection
The sampling techniques used in wastewater characterization studies must
assure that representative samples are obtained because the data from the
analysis of the samples will ultimately serve as a basis for designing
treatment facilities. Sampling programs must be individually tailored to fit
each situation. Suitable sampling locations must be selected, and the
frequency and type of sample to be collected must be determined since the
composition of most stormwater runoff and combined sewer overflows varies
considerably with time.
Sampling Locations. Sampling locations should be selected where flow
conditions encourage a homogeneous mixture. In sewers and deep, narrow
channels, samples should be taken from a point one-third the water depth from
the bottom. The collection point in wide channels should be rotated across
A-l
-------
the channel. At all times, the flow velocity at the sample point should be
sufficient to prevent deposition of solids. When collecting samples', care
should be taken to avoid creating excessive turbulence that may liberate
dissolved gases and yield an unrepresentative sample.
Sample Intervals. The amount of flowrate variation dictates the time interval
for sampling; the interval must be short enough to provide a true
representation of the flow. Even when flowrates vary only slightly, the
concentration of pollutants may vary widely. Frequent sampling {10- or 15-
minute uniform intervals) allows estimation of the average concentration
during the sampling period.
Sampling Procedure. To adequately characterize the pollutant concentration
variation with time, discrete samples must be collected. These discrete
samples must be of sufficient volume so that the desired analyses can be
performed. The samples can be obtained by automatic samplers or by individual
grab samples. The number and size of the samples required are determined by
the analyses to be performed. The physical, chemical, and biological
integrity of the samples must be maintained during the interim period between
sample collection and sample analysis; provision must be made for preserving
the samples. Preservation techniques and maximum holding periods for some
selected parameters are shown in Table A-l II].
Sample Analysis
Effective data analysis should include, as a minimum, the definition of
(1) flow extremes .(e.g., the ratio of dry-weather flow to maximum conduit
capacity); (2) frequencies of occurrence of flowrates and parameter loadings;
( 3 ) types and frequencies of samples; (4) mean values and ranges of
characteristics; J5) rates of change patterns and prestorm impacts; |6) site
and time dependency [e.g., size of area, land use, time of day, and seasonal
effects); and (7) special conditions or qualifications (e.g., construction
impacts, plant bypasses, atypical flows or source area management, snowmelt
versus rainfall-runoff) [3].
Careful thought should be given to the selection of the pollutants to be
included in the analysis of samples. The analysis should include only those
pollutants that may be affected by storage or sedimentation or both.
Standard, easy to perform analyses of the physical, biological, and chemical
characteristics should be included. Additional, more difficult to perform
analyses should be done only if they are of specific interest.
Typically, analyses for BOD5, SS, dissolved oxygen, total nitrogen, total
phosphorus, and total coliforms are included in stormwater runoff and CSO
characterizations. Less frequently incorporated analyses include various
heavy metals, VSS, COD, and grease.
Where storage, sedimentation, or both are to be used as a means of treatment
or control, the suspended solids (both total and VSS,) become important
parameters. The effectiveness of the sedimentation process depends greatly on
the size and specific gravity of the particles. In addition, the
A-2
-------
effectiveness of the pollutant removal by sedimentation is determined by the
pollutant distribution associated with the particle size and specific gravity
distribution. Thus, for storage or sedimentation facilities, the suspended
solids and particle size distribution analyses become very important to the
quality characterization.
Table A-l. PRESERVATION OF WASTEWATER SAMPLES
Parameter
Preservative
Maximum
holding period
Acidity-alkalinity
BOD
Calcium
COD
Chloride
Color
Cyanide
Dissolved oxygen
Fluoride
Hardness
Metals, total
Metals, dissolved
Nitrogen, ammonia
Nitrogen, kjeldahl
Nitrogen, nitrate-nitrite
Oil and grease
Organic carbon
pH
Phenolics
Phosphorus
Solids
Specific conductance
Sulfate
Sulfide
Threshold odor
Turbidity
Refrigeration at 4°C 24 h
Refrigeration at 4°Ca 6 h
None required
2 ml/I H2S04 7 d
None required
Refrigeration at 4°C 24 h
NaOH to pH 10 24 h
Determine onsiteb No holding
None required
None required
5 mL/L HN03 6 mo
Filtrate: 3 mL/L 1:1 HN03 6 mo
40 mg/L HgCl2, 4°C 7 d
40 mg/L HgCl2, 4°C Unstable
40 mg/L HgCl2, 4°C 7 d
2 mL/L H2S04, 4°C 24 h
2 mL/L H2S04 (pH 2) 7 d
None available
1.0 g CuS04 + H3P04 to pH 4.0, 4°C 24 h
40 mg/L HgCl2, 4°C 7 d
None available
None required
Refrigeration at 4°C 7 d
2 mL/L Zn acetate 7 d
Refrigeration at 4°C 24 h
None available
a. Slow-freezing techniques (to -25°C) can be used for preserving samples
to be analyzed for organic content.
b. For some methods of determination, 4 to 8 h preservation can be
accomplished with 0.7 mL cone. H2S04 and 20 mg NaNOo. Refer to
Standard Methods [2] for prescribed applications. (Footnote not
in original reference.)
Note: 1.8(°C) + 32 = °F
mg/L = g/mj
A-3
-------
FLOW AND POLLUTANT ROUTING
A description of the flow and pollutant routing in the Storage/Treatment Block
of SWMM Version III is presented in this section. The approaches described
for both flow and pollutant routing can be used for either a desktop analysis
or a computer simulation analysis. Much of the remaining material in this
appendix is adapted from Huber et al . [4].
Description
Flow and pollutants are routed through one or more storage/treatment units by
several techniques. The flows into, through, and out of a unit are shown in
Figure A-l . The units may be arranged in any fashion, restricted only by the
requirements that inflow to the plant enters at only one unit and that the
products (treated outflow, residuals, and bypass flow) from each unit not be
directed to more than three units. Both wet- and dry -weather facilities may
be simulated by the proper selection of unit arrangement and
characteristics. Units may be modeled as having a detention capability or
instantaneous throughflow. Pollutants or sludges may be represented as a
simple mass or further characterized by a particle size distribution. A unit
may remove pollutants (or concentrate sludges) as a function of particle size
and specific gravity, detention time, incoming concentration, the removal rate
of another pollutant, or a constant percentage. All flows and pollutants are
assumed to be averages over a time step. This includes the input data and
internal calculations [4J.
Flow Routing
A storage or sedimentation unit may be modeled to handle flow in one of two
ways: as a detention unit .'(reservoir) or a unit instantaneously passing all
flow. In this report, discussion is limited only to the detention or
reservoir application.
Flow routing for a simple reservoir requires that three relationships for the
reservoir be known: (1) an inflow hydrograph, (2) a depth-storage
relationship, and (3) a depth-discharge relationship. Routing is the solution
of the storage equation which is an expression of continuity
T - 0 = AV/At (A-l)
where T = average rate during At, ft^/s
TJ = average outflow rate during At, ft3/s
AV = reservoir volume, ftj
At = time step, s
Using subscripts 1 and 2 to represent the beginning and end of the period,
respectively,
(Ij + I2)/2 - (D! + 02)/2 = (V2 - V^/At (A-2)
A-4
-------
'tot
BYPASS EXCESS
(YES)
fby
s/
'res
LEGEND
Qtot = TOTAL INFLOW, ft3/s
Qmax s MAXIMUM ALLOWABLE INFLOW,
ft3/s::
= BYPASSED FLOW, ft3/s
= DIRECT INFLOW TO UNIT, ft3/*
= TREATED OUTFLOW, ft3/s
Ores r RESIDUAL STREAM, ft3/s
Qby
Qin
Qout
Figure A-l. Flows into, through, and out of a storage/treatment unit [4],
A-5
-------
Equation A-2 may be transformed to
V2 . Vj_ = C(Ij + I2)/2lAt - 'ilQ1 + 02l/2]At C.A-3I
For a given time step, I, I2> Op and Vi are known and 02 and V2 must be
determined. Grouping the unknowns on trie left side of trie equation and
rearranging yields one of the two required equations:
0.5(02)At + V2 = 0.5(1] + Itf t - (0.5(Oi)At - V] ) (A_4 )
The second equation is found by relating 02 and V2, each of which is a
function of the reservoir depth. The procedure is illustrated in the
following example.
The data in Table A-2 give 02 and V2 as functions of depth over the range of
depths for which outflow exists. In this hypothetical case, outflow occurs
only if the reservoir depth exceeds 8.0 feet. The 11 data triplets cover the
range of interest. From this information, the corresponding values of 0,502At
^outflow volume', and 0.502 At + V2 (outflow and reservoir volume) can be
calculated. The depth-discharge relationship can be a composite made up of
the relationships, for multiple outlets.
Table A-2. ROUTING DATA FOR HYPOTHETICAL RESERVOIR
(1) (2) (3) (4) (5)
Elevation Depth Volume Discharge
n h y V2 02
ft ft 1000 ft3 ft3/s
1
2
3
4
5
6
7
8
9
10
11
Column
351.0
351.2
351.4
351.6
351.8
352.0
352.2
352.4
352.6
352.8
353.0
: (1)
(2)
(3)
(4)
8.0
8.2
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
10.0
Counter
1,720
1,850
2,000
2,220
2,400
2,650
2,900
3,100
3,400
3,700
3,900
Elevation from
Depth = h - 343
Volume measured
0
10.
20.
35.
50.
65.
80.
105.
130.
165.
200.
topographic map
.0
or calculated
(6)
02DT2
0.502At
1000 ft3
0
108.
116.
378.
540.
702.
804.
1,134.
1,404.
1,782.
2,160.
vol ume
(7)
SATERM
1000 ft3
1,720.
1,958.
2,216.
2,598.
2,940.
3,352.
3,764.
4,234.
4,804.
5,482.
6,060.
(5) Measured data or calculated from discharge formulas
(6) Calculated using 02 (column 5), At = 21,600 s
(7) Calculated using column 4 and column 6
A-6
-------
The computations procedure is summarized as follows:
1. Know values of
now values of 1^, I2, Oi, At, and V^ are substituted into the right
ide of Equation B-4. The result is the first value of 0.502At + V2.
2. Knowing (0.502At + V2), the value of 0.502At is obtained by
interpolation between adjacent values of the outflow volume (column
6) and the outflow and reservoir volume (column 7).
3. The values of V2 and 02 are determined and become the values of V1
and 0}, respectively, in the next time step.
4. Add 0.5(Ii + I2)At to the new value of O.SOiAt - Vi to get the new
value of 0.502At + V2.
5. Continue this process until all inflows have been routed [4].
This flow routing procedure has been adapted for computer simulation in SWMM-
Version III but it can be applied also for hand computation or graphical
methods.
Pollutant Routing
Pollutants are routed through a detention unit by one of two modes: complete
mixing or plug flow.
Complete Mixing. For complete mixing, the concentration of the pollutant in
the unit is assumed to be equal to the effluent concentration. The mass
balance equation for an assumed well mixed, variable volume reservoir is:
d(VC)/dt = I(t) CZ(t) - 0(t) C(t) - K C(t) V(t) (A-5)
where V = reservoir volume, ft
C = influent pollutant concentration, mg/L
C .= effluent and reservoir pollutant concentration, mg/L
I = inflow rate, ft /s
0 = outflow rate, fWs
t = time, s
K = decay coefficient, s
Equation A-5 may be approximated by writing the mass balance equation for the
pollutant over the interval, At:
Change in Mass entering Mass leaving Decay during
mass in basin = during At - during At - At
during At
Ci1 Ii + C2 12 Cl°l + C2°2 C1V1 + C2V2^ (A_6)
- CiVi = g At " 2^ " 2
A-7
-------
where subscripts 1 and 2 refer to the beginning and end of the time step,
respectively.
From the flow routing procedure discussed earlier, Ij, I2, 0^, 02, Vi, and V2
are known. The concentration in the reservoir at the beginning of the time
step, Cj_, and the influent concentrations, Cj and Co are also known as are the
decay rate, K, and the time step, At. Thus, the only unknown, the end of time
step concentration, C2, can be found directly by rearranging Equation B-6 to
yield
c,v,
,t(c'
T + p* y } r o
J. -I ' \jn IQ 1 V>-1 U-.
At /
p At 0 <
, KAt, + U2
V2U + 2 j + 2 At
K C]V1
J 2
<>-"v;n^,^t*
Equation A-7 is the basis for the complete mixing model of pollutant routing
through a detention unit. This is applicable to small detention units with
turbulent flow.
Plug Flow. The inflow during each time step, called a plug, is labeled and
queued through the detention unit. There is assumed to be no transfer of
pollutants between plugs. The outflow for any time step is comprised of the
oldest plugs or fractions thereof or both present in the unit. This is
accomplished by satisfying continuity for the present outflow volume (see
previous section on Flow Routing):
LP
_ZJpVJ'fJ = Vo (A-8)
where V0 = volume leaving unit during the present time step, ft _
V.: = volume entering unit during jtn time step (plug j), ftj
f^ = fraction of plug j that must be removed to satisfy continuity
J with Vp, 0 i fj | 1
JP = time step number of the oldest plug in the unit
LP = time step number of the youngest plug required to satisfy
continuity with VQ
The detention time (s) for each plug j is calculated as
(td).j = (KKDT - j)At (K-9)
where KKDT = present time step number
For a plug j leaving the unit, the amount of pollutant leaving is
(P0)j = (pi)j fj (1.0 - Rj) (A-10)
where (P0)j = amount of pollutant leaving unit in plug j, Ib
(P.j)j = amount of pollutant entering unit with plug j, Ib
R.; = removal fraction for plug j
A-8
-------
The manner in which Rj is calculated is decided by the user; however, as with
the complete mixing option, Rj should be a function of (t^h. The technique
for developing removal equations is discussed later. The remaining pollutants
in each plug leaving the unit are totaled to give the total amount discharged
during the present time step.
In either the complete mixing or plug flow mode, the removed pollutants are
accumulated in the unit and combined with the water drained or drawn from the
unit to form the residuals stream. The water draw-off rate can be either a
fraction of the remaining storage or a constant rate.
Sludge can be assumed to consist of the removed suspended solids and water
drawn from the storage or sedimentation unit. Sludge should be treated simply
as one of the flows leaving the unit. The residuals stream from the unit can
only be termed "sludge" if suspended solids are routed.
POLLUTANT REMOVAL SIMULATION
Pollutants may be characterized by their magnitude (i.e., mass flow and
concentration) or by particle size and specific gravity distributions.
Describing pollutants by their particle size distribution is especially
appropriate where small or large particles dominate or where several storage
or sedimentation units are operated in series. For example, if the influent
is primarily sand and grit, then a sedimentation unit would be very effective;
if clay and silt predominate, sedimentation may be of little use. Also, if
several units are operated in series, the first units will remove a certain
range of particle sizes thus affecting the performance of downstream units.
The pollutant removal mechanism peculiar to each characterization is discussed
below.
Characterization by Magnitude
If pollutants are characterized only by their magnitude, then removal of any
pollutant may be simulated as a function of detention time (in minutes,
detention units only), incoming concentration, inflow rate, the removal
fraction of another pollutant, the incoming concentration of another
pollutant, or any combination of the above "removal factors." Two functional
forms can be used to construct the desired removal equation:
R =
or
/ (a]x1 + a2x2 + a3x3 + a^)
R =(age X5
A-9
-------
where x^ = removal equation variables
a^ = coefficients
R = removal fraction 0 _<^ R _<^ 1 .0
The removal equation variables, x^ , may be assigned to be parameters such as
detention time, flowrate, inflow concentration of the parameter being removed,
inflow concentration of another parameter, etc. These are parameters that are
computed at each time step. (If they are not assigned to be specific
parameters, the remaining x^ are set equal to 1.0 for the duration of the
simulation.) The cofficients, a j , are directly specified by the user. There
is considerable flexibility contained in these two forms, and with a judicious
selection of coefficients and factors, the user can probably create the
desired equation. Example applications of Equations A-ll and A-12 are given
below to illustrate the procedure.
One version of the Storage/Treatment Block of SWMM employed the following
removal equation for suspended solids in a sedimentation tank [5].
Rss V(1 - e"Ud) (frl3)
where R$s = suspended solids removal fraction, 0 | RSS ^ Rmax
Rmax = maximum removal fraction
= detention time, min
K
= first order decay coefficient, L/min
This same equation could be built from Equation A-ll by setting a9 = Rmax,
= -Rmax, 33 = -K, a^3 = 1.0, and letting x^ = detention time, t^.
All other coefficients, 3j, would equal zero.
Another example is taken from a study by Lager et al . [6]. Several curves for
suspended solids removal from microstrainers with a variety of aperture sizes
were derived. Fitting a power function to the curve representing a 35 micron
microstrainer yields
Rss = 0.0963 SS°-286 (A-14)
where R<^ = suspended solids removal fraction, and 0 | Ro$ * 1.0
Ss = influent suspended solids concentration, mg/L
Equation A-12 can be used to duplicate this removal equation by setting ag
= 0.0963, a5 = 0.286, a10 = l-°> anc* *s = influent suspended solids
concentration, SS. All other a,- are zero.
Characterization by Particle Size and Specific Gravity Distribution
Distribution. If pollutants are characterized by their particle size and
specific gravity distribution, then they are removed from the waste stream by
particle settling or obstruction. Many storage or treatment processes use
these physical methods to treat wastewater; sedimentation and screening are
among the most obvious examples.
A-10
-------
In this mode, pollutants are apportioned over several particle size/specific
gravity ranges (e.g., 10% of the BOD is found in the range from 10 to 50
microns). Each of the selected ranges is assigned an upper and lower bound on
the particle diameter and a value for specific gravity. The apportionment of
pollutants over the various ranges as they enter the storage or sedimentation
unit must be specified. This distribution is modified as it passes through
the unit. The analysis is simplified if the particle size distribution
entering the unit is assumed to remain constant over time. While the previous
assumption is not usually true for actual flows, the use of flow weighted
composite samples in the determination of the particle size and specific
gravity distributions provides a reasonable approximation of a constant
distribution.
The storage or sedimentation unit removes all or some portion of each range;
the associated removal of pollutants is easily determined. For example, if a
sedimentation unit removes 50% of the particles in the 50 to 100 micron range
and 10% of the pollutant in question is found in this range, then 5% of the
total pollutant load is removed. The total removal is determined by summing
the effects of several ranges passing through the unit. Once certain
particles are removed, the distribution of particle sizes for the outflow can
be determined. The removed particles constitute the size distribution for the
residuals stream. The next several paragraphs describe the two mechanisms
available to the user for pollutant removal when pollutants are characterized
by particle size.
Particle Settling. There are several forms of settling: unhindered settling
by discrete particles, settling by flocculating particles, hindered settling
by closely spaced particles, and compression settling within the sludge mass
[7]. For simplicity, the unhindered settling of discrete particles will be
the removal mechanism presented here.
The settling of discrete, nonflocculating particles can be analyzed by means
of the classic laws of sedimentation formed by Newton and Stokes. Netwon's
law yields the terminal particles velocity by equating the gravitational force
of the particle to the frictional resistance or drag. Equating the
gravitational force to the frictional drag force for spherical particles
yields [8]:
_
vs = ^(4/3)[(gd/CD)(Sp-l)]
where vs = terminal velocity of particles, ft/s
g = gravitational constant, 32 ft/s2
CQ = drag coefficient
SD = specific gravity of particle
a = diameter of particle, ft
Additionally,
CD = 24/NR, if NR < 0.5 (A-16)
A-11
-------
CD = 24/NR + 3/^RR + 0.34, if 0.5 ^ NR < 104 (A-17)
CD = 0.4, if NR * TO4 (A-18)
where NR = Reynolds number, dimension!ess,
NR = vs d/v (A-19)
and v = kinematic viscosity, ft/s
A procedure for finding v_ under any of the above conditions has been
demonstrated by Sonnen [9J. The average of the high and low ends of each
particle size range should be used as the representative particle size in the
above calculations.
A range of conditions may exist in an actual detention unit, from very
quiescent, to highly turbulent and nonquiescent. Camp's [10] ideal removal
efficiency, EQ, can be used for quiescent conditions, and an adaptation of his
sedimentation trap efficiency curves [10, 11, 12] as described by Chen [13]
can be used to make the extension to nonquiescent conditions, as described
below.
For quiescent conditions,
EQsm1nL/v.. (A-20)
where EQ = particle removal efficiency as a fraction 0 _<_ EQ _<^ 1
vs = terminal velocity of particle, ft/s ~ ~
vu = overflow velocity, ft/s
Additionally,
vu = Q/A = (Ay/td)/A = y/td (A-21)
where Q = flowrate, ft/s
A = surface area of detention unit, ft
y = depth of water in unit, ft
t^ = detention time, s
Equation A-21 assumes a rectangular detention unit with vertical sides.
However, a circular unit (with vertical sides) may also be modeled when
characterizing pollutants by particle size. In other words, Equation A-21 is
restricted to units that allow the surface area to remain constant at any
depth. Applying this equation (and, thus, the entire particle size
methodology) to other unit types should only be done when the surface area is
independent of depth.
A-12
-------
Equation A-20 represents an ideal quiescent basin in which all particles with
settling velocities greater than Vy will be removed. Deviations from
quiescent conditions can be handled explicitly based on Camp's [10]
sedimentation trap efficiency curves, which were developed as a complex
function of particle settling velocity, and several basin parameters, i.e.,
E = f(vsy/2e, vsA/Q = vs£/vty = ys/vu (A'"22)
where E = particle removal efficiency, 0 ^ E ^ 1
e = vertical turbulent diffusivity or mixing coefficient, ft /s
y = flowthrough velocity of detention unit, ft/s
£* = travel length of detention unit, ft
Other terms are defined previously.
Camp [10] solves for the functional form of Equation A-22 assuming a uniform
horizontal velocity distribution and constant diffusivity, e. A form of the
advective-diffusion equation then results in which local changes in
concentration at any vertical elevation are equal to the net effect of
settling from above and diffusion from below. The diffusivity will be
constant if the horizontal velocity is assumed to have a parabolic
distribution, (although this assumption is clearly at variance with the
uniform velocity distribution assumption above). For the parabolic
distribution, sis then found from
e = 0.075 y/ro7p (A-23)
^
where TQ = boundary shear stress, lb/ftd
p = density of water = 1.94 slug/ft3 (1,00 g/cnr)
The term A0/p is known as the shear velocity, u*, and can be evaluated using
Manning's equation for open channel flow [12].
u* = ^h = (vtn/g)/l -49 y1/6
where n = Manning's roughness coefficient.
The flowthrough (horizontal) velocity, vt, is also given by
vt = Vtd (A-25)
where £= travel length of detention unit, ft
tjj = detention time, s
Equations A-23 and A-24 are then used to convert v$y/2e to a more usable form,
a = 0.1(vsy/2e) = vs/1.5u* = (vsy1/6)/(vt n/g) (A"26)
A-1.3
-------
where a= turbulence factor, dimensionless when all parameters are in
units of feet and seconds
Camp's sedimentation trap efficiency curves are the solution to the advective-
diffusion equation mentioned previously and are shown in Figure A-2 as a
function of a. Based on early work of Hazen [14] and the Bureau of
Reclamation as described by Chen [13], it is assumed that an upper limit on
turbulent conditions is given by a= 0.01. Removal efficiency under these
conditions is accurately represented by the function fitted to the ordinate of
Figure A-2.
Et =
or
E = 1 -
(A-27)
(A_28)
where Et = particle removal efficiency under turbulent conditions, 0 = Et = 1
1.00
u
2
LJ
U
u!
u.
IU
0.
cc
Ul
s
a
u
to
.80
.60
.40
.20
.00
0.01
0.10
TURBULENCE
oc = V8y>/6
f
ny
10 2 C
10.0
Figure A-2. Camp's sediment trap efficiency curves [12, 13]
A-14
-------
Quiescent conditions are assumed to exist for a= 1.0 for which removal is
given by Equation A-20. Equations A-20 andA-27 are shown in Figure A-3. The
parameter a may now be used as a weighting factor to obtain the overall
removal efficiency, E,
E = Et + [(In a - In 0.01)/(ln 1 - In 0.01)](Eg - ET)
= EQ + [(In a)/4.605](EQ - Et)
(A-29)
Thus a linear approximation (with respect to In a) can be made of the curves
shown in Figure A-2. Within reasonable accuracy, the values of the turbulence
factor can be limited to 0.01 - a ^ i.o.
100
_ r~ i r i i i rri
DO - IDEAL QUIESCENT
- CONDITIONS
TURBULENT FLOW
CONDITION (CC«O.OI)
Et » l-oxp(-vf/v
i I i I 1 i >
.3 .4 .6 .8 1.0
SlO
tO
Figure A-3. Limiting cases in sediment trap efficiency.
To summarize, the particle settling computations should proceed as follows:
1. For each size and specific gravity range, a settling velocity is
computed using Equations A-15 to A-19. Then for each range, all
steps below are performed.
2. The turbulence factor, a , is computed from Equation A-26.
3. EQ is computed using Equation A-20.
4. Et is computed using Equation A-27 or A-28.
5. Finally, the removal efficiency for the particular particle size and
specific gravity range is computed from Equation A-29.
A-15
-------
In a normal simulation, several plugs leave the detention unit in any given
time step. The effluent is all or part of a number of plugs depending on the
required outflow as determined by the storage routing techniques discussed
earlier. Thus, the effluent particle size distribution is a composite of
several plugs. This composite distribution .is determined by taking a weighted
average (by pollutant weight in each plug) over the effluent plugs. This
distribution is then routed downstream for release or further treatment. The
particles that were removed from each plug are also composited and are used to
characterize the residuals stream.
Comment on Characterization by Particle Size Distribution. Pollutants
characterized by a particle size distribution are most easily simulated by the
two removal mechanisms discussed above. The types of units that could be
considered in this case would include sedimentation tanks and regular storage
basins. However, these units probably represent the processes most frequently
applied to the problem of combined sewer overflow and stormwater runoff.
Thus, limits of the applicability of this mode are probably not too severe.
REFERENCES
1. FWPCA Methods for Chemical Analysis of Water and Wastes. U.S. Department
of the Interior. Federal Water Pollution Control Administration. 1969.
2. Standard Methods for the Examination of Water and Waste Water. 14th Ed.
American Public Health Association. 1975.
3. Lager, J.A. and W.G. Smith. Urban Stormwater Management and Technology:
An Assessment. USEPA Report No. EPA-670/2-74-040. NTIS No. PB 240-687.
December 1974.
4. Huber, W.C., et al. SWMM User's Manual - Version III. Draft USEPA
Report. April 1980.
5. Huber, W.C., et al. Storm Water Management Model User's Manual,
Version II. USEPA Report No. EPA-670/2-75-017. December 1975.
6. Lager, J.A., et al. Urban Stormwater Management and Technology: Update
and Users' Guide. USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
264. September 1977.
7. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. Second Edition. McGraw Hill Book Company. 1979.
8. Fair, G.M., et al. Water and Wastewater Engineering, Volume 2. John
Wiley and Sons, Inc. New York. 1968.
9. Sonnen, M. B. Subroutine for Settling Velocities of Spheres. Journal of
the Hydraulic Division, ASCE, Vol. 103, No. HY9. September 1977.
A-16
-------
10. Camp. T.L. Sedimentation and the Design of Settling Tanks. Transactions
ASCE, Vol. 111. 1945.
11. Dobbins, W.E. Effect of Turbulence on Sedimentation. Transactions ASCE,
Vol. 109. 1944.
12. Brown, C.B. Sedimentation Engineering. Chapter XII in Engineering
Hydraulics. H. Rouse, Ed. John Wiley and Sons. New York. 1950.
13. Chen, C.N. Design of Sediment Retention Basins. Proceedings of the
National Symposium on Urban Hydrology and Sediment Control, University of
Kentucky, Lexington, Kentucky. July 1975.
14. Hazen, A. On Sedimentation. Transactions ASCE, Vol. 53. 1904.
A-17
-------
APPENDIX B
ASSESSMENT METHODS
The relationship between rainfall and runoff is a complex and variable
phenomenon, sensitive to many factors. For most stormwater analyses, the
planner must model the physical system, usually with some type of
mathematical model, to predict response to varying watershed conditions. A
number of mathematical stormwater simulation models are available. They
range in complexity from the very simple, involving desktop calculation
techniques, to the highly sophisticated, involving computer simulators.
MODEL CATEGORIES
Assessment models may generally be divided into three application categories:
1. Preliminary assessment models provide indications of the existence,
source, and nature of a stormwater pollutant problem. Desktop
computational procedures that make use of simple equations and
nomographs are usually adequate. The data requirements for these
models are minimal, consisting of mean annual precipitation,
watershed area, land use, population density, and sewer types. The
results of preliminary assessment models are used to direct
subsequent study efforts, to screen alternatives, to assess flow
and quality data needs, and to aid in the selection of more
sophisticated models, if necessary.
2. Continuous simulation models may help describe the variation of
pollutant loadings from storm event to storm event. Variations
within a single event are not described. Computer or combination
desktop-computer methods generally are needed. Typically, the data
requirements are long-term hourly rainfall records, overflow
structure characteristics, treatment rates and storage volumes,
actual overflow quantity and quality data, and varying details of
drainage area characteristics, streamflow data, etc. Continuous
simulation models are used to assess the magnitude of water quality
problems and to evaluate alternative solutions.
3. Single event simulation models are sophisticated computer models
that can be used to describe temporal and spatial variations in
runoff quantity and quality within a single storm event. They
require extensive and detailed data specific to the watershed,
including rainfall records and runoff hydrographs, catchment and
B-l
-------
transport system details, system maintenance information, and dry-
weather and combined flow characteristics. Results obtained with
these models are used in detailed planning and in the design of
facilities.
Operational models, a fourth major stormwater management model category, are
not assessment models, but provide real-time control of sewerage systems
based on telemetered rainfall and sewer system data.
The categories of assessment models are illustrated in Figure B-l. The
categories tend to blend into one another on an ascending scale of complexity
and detail. More than 30 models are available. A number of the more
commonly used assessment models, listed by category in order of increasing
complexity are presented in Table B-l. The basic capabilities of each model
are also shown. More detailed and up-to-date descriptions of capabilities
and data requirements may be obtained from reports comparing and summarizing
the application of such models [la, Ib] or from the documentation or current
user's guides for each model.
MODEL SELECTION
A mathematical model may be a useful tool in analyzing stormwater problems and
possible solutions. The tool is most useful when carefully matched to the
specific study needs. The important considerations are the level of the
assessment required to meet the study objectives, the availability of the
input data required, the usefulness of the model output, and the overall cost
of the model. The important criteria to be considered when selecting a model
are presented in Table B-2. The criteria are based on a method developed by
Systems Control, Inc., for selection of water quality models [2].
In most cases, a sophisticated model should be employed only after its use has
been justified by results from a simpler one. The first step in model
selection is to perform a preliminary assessment of the problem. The results
can be used to assess whether the water quality problems are storm-generated,
or if other point and nonpoint discharges are the source. A preliminary
assessment will also help to identify the pollutants and the time period of
concern, the need for and appropriate level of subsequent models, and the data
requirements of the study.
If justified, a list of models that might be used is then assembled. The
models listed in Table B-l are a starting point. Additional models may be
identified through current publications. Adequate details of model
capabilities can be obtained from the literature so that a preliminary
screening can be performed. Models appropriate to the level of the study and
capable of simulating the pollutants and time period of interest are given
further consideration. A detailed description of the capabilities of each of
these models should be obtained from the model originator. Each candidate
model should then be evaluated according to the criteria listed in Table B-2.
B-2
-------
POLLUTANT
MASS/n
TOTAL YEARLY »t
POLLUTANT LOAD
(COMPARABLE TO A
CONTINUOUS LOAD)
1 TEAR
«) PRELIMINARY ASSESSMENT MODEL
ACTUAL EVENT DISTRIBUTION OF LOAD
POLLUTANT
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t i
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, AVERAGE LOAD PER EVENT »R
SEVERAL
HOURS
c) SINGLE EVENT SIMULATION MODEL
Figure B-l. Assessment model categories,
B-3
-------
Table B-l. CHARACTERISTICS OF ASSESSMENT MODELS,
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B-4
-------
Table B-2. MODEL SELECTION CRITERIA
Applicability
What features are necessary to the analysis?
What features are desirable?
What are the capabilities of the candidate models?
Accuracy
What level of accuracy is required?
What accuracy is justified by the available data?
How appropriate to the Individual situation are the representations and
assumptions of each model?
Usability
Is the available model documentation sufficient?
How usable are the output form and content?
How easily can data be changed to simulate alternative conditions?
How easily can the model be modified?
What are the capabilities of the planning staff to apply the model and
analyze results?
Cost
Model acquisition cost
Equipment requirements and costs
Data acquisition costa and time requirements
Manpower costs
MODEL APPLICATION
For models beyond the preliminary assessment category, application may be
thought of as consisting of three steps: calibration, verification, and
analysis. In the calibration step, the known watershed characteristics and
hydrologic data for a selected set of storm events are input and the unknown
or uncertain model parameters adjusted so that the model output corresponds to
observed runoff and receiving water responses. The model is verified when a
significantly different set of conditions is input, and the model again
satisfactorily predicts observed system behavior. If the model prediction for
the subsequent simulation is not satisfactory, then further calibration is
necessary.
Data acquisition for calibration and verification usually represents a major
share of the cost of single-event and many continuous simulation models. To
ensure that the usefulness of model results justifies the expenditure,
consideration should be given to the parameters to be monitored and to data
collection techniques. An excellent guide to monitoring and sampling for
runoff data is Methodology for the Study of Urban Storm Generated Pollution
and Control, [3T
B-5
-------
REFERENCES
1. U.S. Environmental Protection Agency. Urban Stormwater Management and
Technology: Update and Users Guide. EPA/600/8-77-014. 1977.
la Resources for the Future. REQM Manual. Edited by Daniel J. Basta.
Unpublished.
Ib Brandstetter, Albin. Assessment of Mathematical Models for Storm and
Combined Sewer Management. EPA Report No. EPA/600/2-76-175a.
August 1976.
2. Grimsrud, G.T., E.J. Finnemore, and H.J. Owen. Evaluation of Water
Quality Models: A Management Guide for Planners. EPA/600/5-76-004.
1976.
3. Envirex, Inc. Methodology for the Study of Urban Storm Generated
Pollution and Control. EPA/600/2-76-145. 1976.
B-6
-------
APPENDIX C
INFILTRATION MEASUREMENT TECHNIQUES
The infiltration rate value that is required in design of stormwater
percolation facilities is the long-term acceptance rate of the entire soil
surface on the proposed site for the actual stormwater to be applied. The
value that can be measured is only a short-term equilibrium acceptance rate
for a number of particular areas within the overall site. It is strongly
recommended that hydraulic tests of any type be conducted with the actual
runoff whenever possible. Such practice will provide valuable information
relative to possible soil-stormwater interactions that might create future
operating problems. If suitable runoff is not available at the site, the
ionic composition of the water used should be adjusted to correspond to that
of the runoff. Even this simple step may provide useful data on the swelling
of expansive clay minerals due to sodium exchange.
There are many potential techniques for measuring infiltration including
basin flooding, sprinkler infiltrometers, cylinder infiltrometers, and
lysimeters. The technique selected should reflect the actual method of
application being considered. For design of stormwater retention facilities,
the preferred techniques are basic flooding and cylinder infiltrometers. The
area of land and the volume of stormwater used should be as large as
practical.
Before discussing the two techniques, it should be pointed out that the
standard U.S. Public Health Service (USPHS) percolation test used for
establishing the size of septic tank drain fields [1] is not recommended
except for very small subsurface disposal fields or beds. Comparative field
studies have shown that the percolation rate from the test hole is always
significantly higher than the infiltration rate as determined from the double-
cylinder (also called double ring) infiltrometer test. The difference
between the two techniques is of course related to the much higher percentage
of lateral flow experienced with the standard percolation test. The final
rates measured at four locations on a 30 acre (12 ha) site using the two
techniques are compared in Table C-l. The lower coefficient of variation
(defined as the standard deviation divided by the mean value, C = s/M for
the double-cylinder technique is especially significant. A plausible
interpretation is that the measurement technique involved in inherently more
precise than the standard percolation test.
C-
-------
Table C-l. COMPARISON OF INFILTRATION MEASUREMENT USING
STANDARD USPHS PERCOLATION TEST AND DOUBLE-CYLINDER INFILTROMETER'
Location
Equilibrium infiltration
rate, in./h
Standard USPHS Double-cylinder
percolation test inflltrometer
1
2
3
4
Mean
Standard
deviation
Coefficient
of variation
48.0
84.0
60.0
138.0
82.5
40.0
0.48
9.0
10.8
14.4
12.0
11.6
2.3
0.20
a. Using sandy soil free of clay.
FLOODING BASIN TECHNIQUES
Where pilot basins have been used for determination of infiltration, the
plots have generally ranged from 10 ft (0.9 m) to 0.25 acre (0.1 ha).
Larger plots are provided with a border arrangement for application of the
water. If the plots are filled by hose, a canvas or burlap sack over the end
of the hose will minimize disturbance of the soil [2]. Although basin tests
are desirable, and should be used whenever possible, there probably will not
arise many opportunities to do so because of the large volumes of water
needed for measurements. In at least one known instance, pilot basins of
large scale (5 to 8 acres or 2 to 3.2 ha) were used to demonstrate feasibility
of wastewater percolation and then were incorporated into the larger full-
scale system [3].
CYLINDER INFILTROMETERS
A useful reference on cylinder infiltrometers is Haise, ^t a]_. [4]. The basic
technique, as currently practiced, is to drive or jack a metal cylinder into
the soil to a depth of about 6 in. (15 cm) to prevent lateral or divergent
flow of water from the ring. The cylinder should be 6 to 14 in. (15 to 35 cm)
in diameter and approximately 10 in. (25 cm) in length. Divergent flow is
further minimized by means of a "buffer zone" surrounding the central ring.
The buffer zone is commonly provided by another cylinder 16 to 30 in. (40 to
75 cm) in diameter driven to a depth of 2 to 4 in. (5 to 10 cm) and kept
partially full of water during the time of infiltration measurements from the
inner ring. Alternatively, a buffer zone may be provided by diking the area
around the intake cylinder with low (3 to 4 in. or 7.5 to 10 cm) earthen
dikes.
C-2
-------
The quantity of water that might have to be supplied to the double-cylinder
system during a test can be substantial and might be considered a limitation
of the technique. For highly permeable soil, a 1,500 gal (5,680 L) tank truck
might be needed to hold a day's water supply for a series of tests. The basic
configuration of the equipment during a test is shown in Figure C-l.
This technique is thought to produce data that are at least representative of
the vertical component of flow. In most soils, the infiltration rate will
decrease throughout the test and approach a steady state value asymtotically.
This may require as little as 20 to 30 minutes in some soils and several hours
in others. The test cannot be terminated until the steady state is attained
or else the results are meaningless.
The following precautions concerning the cylinder infiltrometer test are
noted.
1. If a more restrictive layer is present below the intended plane of
infiltration and this layer is close enough to the intended plane to
interfere, the infiltration cylinders should be embedded into this
layer to ensure a conservative estimate.
2. The method of placement into the soil may be a serious limitation.
Disturbance of natural structural conditions (shattering or
compaction) may cause a large variation in infiltration rates
between replicated runs. Also the interface between the soil and
the metal cylinder may become a seepage plane, resulting in
abnormally high rates. In cohesionless soils (sands and gravels),
the poor bond between the soil and the cylinder may allow seepage
around the cylinder and cause "piping." This can be observed easily
and corrected, usually by moving a short distance to a new location
and trying again. Variability of data caused by cylinder placement
can largely be overcome by leaving the cylinders in place over an
extended period during a series of measurements [2],
Knowledge of the ratio of the total quantity of water infiltrated to the
quantity of water remaining directly beneath the cylinder is essential if one
is interested only in vertical water movements. If no correction is made for
lateral seepage, the measured infiltration rate in the cylinder will be well
in excess of the "real" rate [5]. Several investigators have studied this
problem of lateral seepage and have offered suggestions for handling it [5,
6, 7,].
As pointed out by Van Schilfgaarde [8], measurements of hydraulic
conductivity on soil samples often show wide variations within a relatively
small area. Hundred-fold differences are common on some sites. Assessing
hydraulic capacity for a project site is especially difficult because test
plots may have adequate capacity when tested as isolated portions, but may
prove to have inadequate capacity after water is applied to the total area
for prolonged periods. Parizek has observed that problem areas can be
anticipated more readily by field study following spring thaws or prolonged
periods of heavy rainfall and recharge [9]. Runoff, ponding, and near
saturation conditions may be observed for brief periods at sites where
drainage problems are likely to occur after extensive application begins.
C-3
-------
SUFFER POND
LEVEL
GAGE INOEJt
ENGINEER'S SCALE
WELDING ROD
NOOK
ATER SURFACE
INTAKE CYLINDER
GROUND LEVEL
Figure C-l . Cylinder infiltrometer in use
C-4
-------
Although far too few extensive tests have been made to gather meaningful
statistical data on the cylinder infiltrometer technique, one very
comprehensive study is available from which tentative conclusions can be
drawn. Burgy and Luthin reported on studies of three 40 by 90 ft (12.2 x
27.4 m) plots of Yolo silt loam characterized by the absence of horizon
development in the upper profile [10]. The plots were diked with levees 2 ft
(0.6 m) high. Each plot was flooded to a depth of 1.5 ft (0.5 m), and the
time for the water to subside to a depth of 0.5 ft (0.15 m) was noted. The
plots were then allowed to drain to the approximate field capacity and a
series of cylinder infiltrometer tests--357 totalwere made.
Test results from the three basins located on the same homogeneous field were
compared. In addition, test results from single-cylinder infiltrometers with
no buffer zone were compared with those from double-cylinder infiltrometers.
The inside cylinders had a 6 in. (15 cm) diameter; the outside cylinders,
where used, had a 12 in. (30 cm) diameter.
For this particular soil, the presence of a buffer zone did not have a
significant effect on the measured rates. Consequently, all of the data are
summarized on one histogram in Figure C-2. The calculated mean of the
distribution shown is 6.2 in./h (15.7 cm/h). The standard deviation is
5.1 in./h (12.9 cm/h).
Burgy and Luthin suggest that the extreme high values, while not erroneous,
should be rejected in calculating the hydraulic capacity of the site.
Physical inspection revealed that these values were obtained when the
cylinders intersected gopher burrows or root tubes. Although these phenomena
had an effect on the infiltration rate, they should not be included in the
averaging process since they carried too much weight.
As a criterion for rejection, Burgy and Luthin suggest omitting all values
greater than three standard deviations from the mean value. They further
suggest an arbitrary selection of the mean and standard deviation for this
procedure based on one's best estimate of the corrected values rather than
the original calculations. From inspection of the histogram, these values
might be selected as about 5 in./h (12.7 cm/h) and 3.5 in./h (8.9 cm/h),
respectively. Thus, all values greater than 5.0 + 3(3.5), or 15.5 in./h
(39.4 cm/h) are arbitrarily rejected: a total of 12 of the 357 tests made
(3.4%).
Because it is important to provide conservative design parameters for this
work, however, it is recommended that all values greater than two standard
deviations from the mean be rejected. For the example, this results in the
rejection of all values greater than 5.0 + 2(3.5), or 12 in./h (30.5 cm/h)
from the average. A recomputation using this criterion provides a mean of
5.1 in./h (12.5 cm/h) and a standard deviation of 2.8 in./h (7.1 cm/h). This
average value is within 16% of the "true" mean value of 4.4 in./h (11.2 cm/h)
as measured during flooding tests of the entire plot.
C-5
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The main question to be answered now is, how many individual tests must be
made to obtain an average that is within some given percent of the true mean,
say at the 90% confidence interval? The answer has been provided by
statisticians using the Student "T" distribution. Details of the derivation
are omitted here but can be found in most standard texts on statistics.
The results of two typical sets of computations are summarized in Figures C-3
and C-4. The two sets of curves are for 90% and 95% confidence intervals.
The confidence interval and the desired precision are, of course, basic
choices that the engineer must make. A 90% confidence in the measured mean,
which is within 30% of the true mean', may be sufficient for small sites where
neighboring property is available for expansion if necessary. On the other
hand, 95% confidence that the measured value is within 10% of the true mean
may be more appropriate for larger sites or for sites where expansion will not
be easily accomplished once the project is constructed.
The coefficient of variation will have to be estimated from a few preliminary
tests because it is the main plotting parameter in these figures. As an
example, for the adjusted distribution of Burgy and Luthin's data with a
coefficient of variation estimated at 0.55, at least 23 separate tests would
be required to have 90% confidence that the computed mean would be within 20%
of the true mean value of infiltration. Obviously, time and budget
constraints must be considered i'n making the confidence and accuracy
determinations; 3 to 4 man-days of work might be required to make 23 cylinder
infiltrometer tests.
C-7
-------
«o 20
0.\ 0.2 0. 3 0.4 0-5 0.6
COEFFICIENT OF VARIATION
0.7
O.B
Figure C-3. Number of tests required for 90%
confidence that the calculated mean is
within stated percent of the true mean.
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0.2
0.3 0.4 0.5 0.6
COEFFICIENT OF VARIATION
0.7
0.8
Figure C-4 Number of tests required for 95%
confidence that the calculated mean is
within stated percent of the true mean.
-------
REFERENCES
1. Manual of Septic-Tank Practice. U.S. Public Health Service. Publication
No. 526. U.S. Gov't Printing Office, 1969.
2. Parr, J.F. and A.R. Bertrand. Water Infiltration Into Soils. In:
Advances in Agronomy. Norman, A.G. (ed.). New York, Academic Press.
1960. pp 311-363.
3. Wallace, A.T., et al. Rapid Infiltration Disposal of Kraft Mill
Eflluent. In: Proceedings of the 30th Industrial Waste Conference,
Purdue University, Ind. 1975.
4. Haise, H.R., et al. The Use of Cylinder Infiltrometers to Determine the
Intake Characteristics of Irrigated Soils. U.S. Dept. of Agriculture,
Agricultural Research Service. Publication No. 41-7. 1956.
5. Hills, R.C. Lateral Flow Under Cylinder Infiltrometers: A Graphical
Correction Procedure. Journal of Hydrology. 13:153-162, 1971.
6. Swartzendruber, D. and T.C. Olson. Model Study of the Double-Ring
Infiltrometer as Affected by Depth of Wetting and Particle Size. Soil
Science. 92:219-225, April 1961.
7. Bouwer, H. Unsaturated Flow in Ground-water Hydraulics. In:
Proceedings of the American Society of Civil Engineers. 90:HY5:121-141,
1964.
8. Van Schilfgaarde, J. Theory of Flow to Drains. In: Advances in
Hydroscience. Chow, V.T. (ed.). New York, Academic Press. 1970. pp 43-
103.
9. Parizek, R.R. Site Selection Criteria for Wastewater Disposal - Soils
and Hydrogeologic Considerations. In: Recycling Treated Municipal
Wastewater and Sludge Through Forest and Cropland. Sopper, W.E. and L.T.
Kardos (eds). Pennsylvania State University Press. 1973. pp 95-147.
10. Burgy, R.H. and J.N. Luthin. A Test of the Single- and Double-Ring Types
of Infiltrometers. Trans. Amer. Geophysical Union. 37:189-191, 1956.
C-9
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