ENVIRONMENTAL

RESEARCH BRIEF

Storage/Sedimentation Facilities for Control of Storm and

Combined Sewer Overflows: Design Manual

Research and Development EPA/600/S-98/004 May 1998

Joyce M. Perdek*, Richard Field*, and Shih-Long Liao**

United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati OH 45268

Abstract

This report describes applications of storage facilities in wetweather flow (WWF) control 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-ofthe-art technologies 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 (CSO). 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 CSO must be

characterized to estimate the efficiency of any sedimentation

basin.

The detailed design methodology for each type of facility presented in the report includes the following steps: identifying

functional requirements; identifying site constraints; establishing basis of design; selecting a storage and/or treatment option; and conducting a cost analysis.

This research brief summarizes a 1997 revision of an earlier

unpublished report of the same title. The original report was

prepared between September 1979 and October 1981 by

Metcalf & Eddy, Inc., under the sponsorship of the U.S.

Environmental Protection Agency’s Office of Research and

Development in Cincinnati, OH, in partial fulfillment of

Contract No. 68-03-2877. The 1997 revision is being released

currently to provide information to communities in support of

their stormwater and CSO management efforts. Despite the

revision, some of the content may no longer be entirely

current. The authors of the full report are W. Michael Stallard,

William G. Smith, Ronald W. Crites, and John A. Lager.

Copies of the report are available from the National Technical

Information Service, 5285 Port Royal Road, Springfield, VA

22161

Introduction

Among the earliest examples of public works are urban drainage systems designed to convey urban storm flow or WWF

away from populated areas to receiving waters. WWF may

consist of stormwater alone, or it may consist of both stormwater

and sanitary or domestic wastewater in combined sewer systems, which is known as CSO. Common elements of a typical

combined sewer system, generally found in older cites, are

illustrated in Figure 1.

Discharges from WWFs conveyance systems have significant

impacts on receiving-water quality. Recognition of their significance has increased as the quality of effluents from municipal

wastewater treatment plants has improved as a result of the

Clean Water Act. National cost estimates for controlling pollution from WWFs are substantial. The cost of meeting water

quality standards for stormwater discharges has been projected to be as high as $400 billion in capital costs and $540

billion/year in operation and maintenance (O&M) costs. Capital

costs for CSO abatement are estimated to be more than $50

billion for eleven hundred communities served by combined

sewer systems.

*National Risk Management Research Laboratory, Edison, NJ

** Oak Ridge Institute for Science and Education, Edison, NJ
Figure 1. Common Elements of a Combined Sewer System.

The variable nature of WWFs makes controlling them difficult.

Transport and treatment facilities for controlling excess WWF,

which generally are designed to handle medium-intensity, medium-duration storm-flow volumes, frequently are idle during

dry periods and overflow during large storms. Temporary storage of excess WWF can be an effective and economical

method of controlling flooding and pollution. Excess WWF

stored during large storms or during more intense rainfall

periods can be released slowly when capacity in the drainage

and treatment system is available. As a result, overflows occur

less often.

Planning Methodology

The solution to WWF problems is most often a combination of

various best management practices (i.e., nonstructural and

low-structurally intensive alternatives) and unit process applications (i.e., physical treatment for removal of settleable and

suspended solids and floatable material). Storage and/or sedimentation facilities are and should be the backbone of such an

integrated WWF management plan. The following aspects of

planning a storage/sedimentation facility are described: (1)

general planning conditions, (2) establishment of goals, (3)

planning methodology, (4) cost optimization methodologies, (5)

storage-volume determination methods, and (6) effect of storage and/or sedimentation.

General planning conditions include determining whether storage/sedimentation is the best solution for dealing with the

problems involved in terms of the type of WWF and the treatment goals. The feasibility of locating such a facility must be

examined. Treatment goals include, but are not limited to, the

maximum number of yearly overflow events, maximum overflow volume, and desired detention time.

The basic planning methodology includes the following steps:

(1) identify functional requirements, (2) identify site constraints,

Receiving Waters

Outfall

Combined Sewer Overflow

Mixture of Municipal

Wastewater and Stormwater

Discharging into the

Receiving Waters

Sanitary Interceptor to

Water Pollution Control

Plants for Treatment

Weir

Manhole

Regularor

Structure

Building

Sewer

Combined

Sewer

Basement

Drain

Roof

Drain

House Waste

(Including

Roof Drainage)

Rainfall

Catchbasin

Surface

Runoff
(3) establish basis of design, (4) select storage and/or treatment option, (5) estimate costs and cost sensitivities, (6) check

that facilities satisfy objectives, and (7) refine and complete or

modify and repeat. A typical design methodology for source

control options is illustrated in Figure 2. 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.

The Mass-Diagram Method and Production Theory Method are

described in the manual.

In determining storage volumes, the effect of different possible

combinations of storage and/or sedimentation design parameters (e.g., settling time and facility size) on flow control must

be determined. Methodologies for approaching these calculations include the following: desktop hand computations; statistical analysis of rainfall and flow data; simple, continuous simulation of WWF systems; and detailed, continuous or single

event simulation of WWF systems. Deciding on the approach

to be used depends on the size and complexity of the drainage

area and/or sewerage system. For small and simple systems,

hand computations can be used. For detailed systems, computerized continuous simulated models can be used.

To evaluate the different storage/sedimentation alternatives,

the degree to which each alternative achieves the goals developed must be compared. Cost and performance of each should

be considered. Thus, the best apparent alternative should be

the most cost-effective one meeting the technical goals established at the lowest cost.

The process of integrating WWF 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. These

steps are to (1) identify existing system and needs, (2) establish system needs, (3) identify applicable control alternatives,

and (4) determine control method compatibility.

Facility Design

Storage/sedimentation facility design procedures for both combined sewer and separate storm sewer systems are discussed

in detail in the manual. The main steps to be followed in

designing these 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, (7) operating strategy for design, and (8) instrumentation and control strategy for

operation.

Design of Retention Storage Facilities

Stormwater retention is the storage of excess runoff for complete removal from the surface drainage and discharge system.

The water collected percolates through the bottom of the retention facility and may reach the groundwater. Stormwater retention facilities may take a variety of forms such as ponds and

perforated culverts. This section describes design procedures

and operation considerations for the most common retention

storage facility types — dry and wet ponds.

Figure 2. Source Control Design Methodology.

Identify Functional

Requirements

A. Control of Runoff

Rate

B. Reduction of

Runoff Volume

Identify Site

Constraints

A. Area

B. Hydraulic

C. Environmental

D. Structural

Establish Basis of Design

A. Design or Continuous

Storm(s)

B. Inflow Rate

C. Outflow Rate

D. Storage Volume

E. Pollutant Removal

(Overflow Rate)

Select Detention Option ($)

A. Operational Concept

B. Inlet/Outlet Works

C. Area/Depth

D. Cleaning Access

Estimate Costs and

Cost Sensitivities

A. Capital

B. Operation and

Maintenance

Yes

Yes

No No

Shift to Alternate

Site or Method

Modify and

Repeat?

Facilities

Satisfy

Objectives?

Refine and

Complete
Size and location are important design considerations for both

types of ponds. Size requirements include not only volumetric

capacity but also both surface area and soil interface area

requirements. The pond configuration depends on (1) the runoff storage volume needed; (2) the surface area, configuration,

and weir length required to assure adequate settling during

sedimentation operation; (3) the surface area needed for adequate transfer of oxygen into the pond water to allow aerobic

decomposition of organic pollutants; (4) the soil-water interface

area needed for adequate percolation of stored runoff between

storm events; and (5) the area needed to serve whatever dual

uses the basin may have. The suitability of a site within a

drainage area for locating a retention pond facility depends on

(1) site availability, (2) compatibility of surrounding land uses with

a stormwater retention facility and its other functions, (3) the area

required, (4) soil permeability, (5) tributary catchment size, and

(6) its relationship to other sewer or drainage facilities.

The procedure presented for design of retention facilities consists of the following steps: (1) quantify functional requirements, (2) identify waste load and flow reduction, (3) determine

preliminary basin size, (4) identify feasible pond sites, (5)

investigate most promising sites, (6) establish basin sizes, (7)

design solids removal facilities, and (8) determine pond configuration. The approach, which should make use of existing

experience, known concepts, or developed theories, must be

integrated to insure that the desired functions of the ponds

(sediment removal, infiltration and percolation, flood control, or

flow reduction) are compatible with the type of flow reaching

the pond (stormwater runoff or CSO) and any other multi-use

aspects (recreation, irrigation, aesthetics, etc.). In actual practice, retention ponds are very seldom used for CSOs because

the organic solids tend to seal the pond bottom and reduce the

soil infiltration capacity.

The efficiency of retention ponds in reducing stormwater pollutant loadings depends heavily on the underlying soil as a treatment medium. The mechanisms of removal include settling,

filtering, biological activity, coagulation, adsorption, and chemical

reaction. The major operational problems with ponds center

around handling captured solids. Other operational concerns are

the inlet and outlet structures, maintenance of vegetative cover

through alternating wetting and drying periods, insect control,

odor control, and maximizing availability of the pond for alternative uses. Cost curves for pond construction are presented in the

manual. Operational costs are site specific.

Design of Detention Facilities

Detention storage delays excess runoff and attenuates peak

flows in the surface drainage system. During peak flows, detention storage holds excess water until the inflow decreases

and releases it during low-flow periods. Because of sedimentation that occurs during detention, detention storage in tanks or

basins can also be considered a treatment process for high

storm flow volumes that create tank or basin overflow. Site

constraints to be considered for detention storage facilities

include tributary area, topography, local land use, and area

available for the structure or basin.

Types of detention storage include onsite and in-system. Onsite

detention is the detention of stormwater or CSO at the source

before it reaches a sewer network or receiving water. Onsite

detention occurs in natural ditches, open ponds or basins,

rooftops, parking lots, or recreational facilities. In-system detention storage holds storm flow either in series or in parallel

within the collection system. In-system detention storage includes inline storage and offline storage. Inline storage can be

accomplished by using the available volume in trunk sewers,

interceptors, wet wells and tunnels to store excess WWF.

Excess flows are stored off line in open or uncovered basins,

caverns, mined labyrinths, and lined or unlined tunnels. Functionally, the application of onsite detention differs little from insystem 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.

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 detention storage

facilities are (1) size and slope of sewers, (2) peak flow rates,

(3) controls required for system operation, and (4) resuspension

of sediment.

The design methodologies for onsite storage and in-system

storage are very similar and combined together in the discussion presented in this section of the manual. The design

procedure described consists of the following steps: (1) identify

functional requirements, (2) identify site constraints, (3) establish basis of design, (4) select storage options and locations,

(5) estimate costs, and (6) complete design.

The construction costs for in-system storage have been reported for selected demonstration sites. However, they are

highly site specific. Costs also vary considerably depending on

the complexity of the flow regulators and control systems.

Detailed O&M cost data are limited. O&M costs must be

estimated for specific facilities from the operation plan and

maintenance schedule.

Design of Sedimentation Facilities

Storage/sedimentation is the most commonly and perhaps

most effectively practiced method of urban CSO and stormwater

runoff control in terms of the number of operating installations

and length of service. Conversely, storage/sedimentation is

frequently criticized for lack of innovation because of its simplicity and high cost due to size and structural requirements.

The report presents detailed design considerations and procedures for downstream storage/sedimentation basins, which are

illustrated by example and through references of designed and

operated facilities. Cost information is also provided. Examples

of representative CSO storage/sedimentation basins and auxiliary support facilities are shown in Figure 3.

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

large storage volume, the provisions of dewatering, and poststorm 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 the

portion of storm flow with the highest pollutant load, referred to

as the first flush, and bypass the balance of the flow to avoid

the discharge of much of the pollution.
Factors to be considered in the design of storage/sedimentation facilities include the following: (1) storage volume, (2)

treatment efficiency, (3) disinfection, and (4) site constraints.

The following sedimentation facility design procedures are (1)

identify functional requirements, (2) identify site constraints, (3)

establish basis of design, (4) select sedimentation facility configuration, (5) identify and select pretreatment, (6) detail auxiliary systems, (7) estimate costs and cost-effectiveness analysis, and (8) complete design.

The major O&M goal of downstream storage/sedimentation

basins is to provide a facility that is available to its full design

capacity as long as needed. Secondary goals include clear,

prompt, and complete records of performance, reliability to

Figure 3. Representative CSO Storage/Sedimentation Basins and Auxiliary Support Facilities.
provide for real location of personnel and facilities in non-storm

periods, and dual-use operations such as backup treatment

and/or flow equalization for dry-weather plants. The O&M requirements and procedures should be developed from the

operational plan; there are no industry-wide standards.

International Perspective

The application of storage/sedimentation controls for urban

WWF problems is not unique to the United States. In this era of

excellent communications and increasing technology-sharing

on an international scale, similar approaches are found in

many areas of the world. Several technologies developed internationally are introduced below. The review includes flowcontrol devices developed in Sweden, Denmark, and Germany; an in-receiving water flow balancing system developed

and applied in Sweden; and an innovative self-cleaning storage/sedimentation basin used in Zurich, Switzerland.

For certain cases, the flow from storage/sedimentation facilities

can be controlled by means of specially designed flow-control

devices, which provide more effective flow control than can be

accomplished with conventional static devices. Four devices

are described. An advanced static device, the Steinscruv flow

regulator, developed in Sweden in the 1970s by Stein

Bendixsen, consists of a stationary, anchored, screw-shaped

plate that is installed in a pipe. In that part of the plate which

fits against the bottom of the pipe, there is a bottom opening to

release a specified base dry-weather flow. The Hydrobrake,

developed in Denmark in the mid-1960s, is used to control

outflows from storage structures. Hydrostatic pressure associated with the water level controls the rate of flow through this

device. A device with a similar operating principle, the

Wirbeldrossel or turbulent throttle, developed in Germany in

the mid-1970s, also regulates flow from a storage facility.

Another flow regulator valve, developed in Sweden in the late

1970s, is a central outlet pipe surrounded by a pressure chamber filled with air. Water pressure on the upper portion of the

device displaces the fabric at the outlet, which controls the

discharge volume.

The Flow Balance Method, an innovative approach to urban

WWF treatment for the protection of lakes, has been developed and applied at several locations in Sweden by Karl

Dunkers. The Flow Balance Method, which is also being used

in other locations, uses a portion of receiving-water volume

within a hanging curtain to store runoff, while allowing for

suspended solids sedimentation, before discharge. A schematic of the system is shown in Figure 4.

Typically, removal of settled solids from an inline storage facility has been a problem that requires an auxiliary flushing

system of some sort. An innovative approach to eliminating this

problem has been implemented 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 the configuration shown in Figure 5. Any solids that have settled in the basin

during its storage operation are resuspended by the channelized

high-velocity flow during the drawdown following a storm event.

Reference

Stallard, W.M., W.G. Smith, R.W. Crites, and J.A. Lager. Storage/Sedimentation Facilities for Control of Storm and Combined Sewer Overflows: Design Manual, EPA/600/R-98/006

PB98-132228. Cincinnati, OH: U.S. Environmental Protection

Agency, 1998.
Figure 4. Schematic of Pontoon Tank System at Lake Tehorningen, Sweden.

Opening

Pontoons

Plastic Cloth

Pressure Line

Feed Pump

Floating Tank

Urban Runoff

Discharge

Treatment Plant

Outlet Pipe

Drainage

Tank Section

Tank Side

Weights

Plastic Cloth

Pontoons

Tank Side

Weights

Plastic Cloth

Pontoons

Lake Side

Sludge Drying Beds
United States BULK RATE

Environmental Protection Agency POSTAGE & FEES PAID

Center for Environmental Research Information EPA

Cincinnati, OH 45268 PERMIT No. G-35

Official Business

Penalty for Private Use

$300

Inlet Outlet

Submerged Screen

Spillway

Figure 5. Self-Cleaning Storage/Sedimentation Basin used in Zurich, Switzerland.

EPA/600/S-98/004