&EPA
United State3
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S-98/004 May 1998
ENVIRONMENTAL
RESEARCH BRIEF
Storage/Sedimentation Facilities for Control of Storm and
Combined Sewer Overflows: Design Manual
Joyce M. Perdek*, Richard Field*, and Shih-Long Liao**
Abstract
This report describes applications of storage facilities in wet-
weather flow (WWF) control and presents step-by-step proce-
dures for the analysis and design of storage-treatment facili-
ties. Retention, detention, and sedimentation storage are clas-
sified and described. International as well as national state-of-
the-art technologies are discussed.
Retention storage facilities capture and dispose of stormwater
runoff through infiltration, percolation, and evaporation. Deten-
tion storage is temporary storage for stormwater runoff or
combined sewer overflow (CSO). Stored flows are subse-
quently 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 treat-
ment. 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 pre-
sented in the report includes the following steps: identifying
functional requirements; identifying site constraints; establish-
ing basis of design; selecting a storage and/or treatment op-
tion; 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unpublished report of the same title.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
'National Risk Management Research Laboratory, Edison, NJ
"Oak Ridge Institute for Science and Education, Edison, NJ
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 drain-
age 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 sys-
tems, 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 signifi-
cance 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 pollu-
tion from WWFs are substantial. The cost of meeting water
quality standards for stormwater discharges has been pro-
jected 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.
The variable nature of WWFs makes controlling them difficult.
Transport and treatment facilities for controlling excess WWF,
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which generally are designed to handle medium-intensity, me-
dium-duration storm-flow volumes, frequently are idle during
dry periods and overflow during large storms. Temporary stor-
age 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 appli-
cations (I.e., physical treatment for removal of settleable and
suspended solids and floatable material). Storage and/or sedi-
mentation facilities are and should be the backbone of such an
integrated WWF management plan. The following aspects of
Cpmbined Sewer Overflow
Mixture of Municipal
Wastewater and Stormwater
Discharging into the
Receiving Waters
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 stor-
age and/or sedimentation.
General planning conditions include determining whether stor-
age/sedimentation is the best solution for dealing with the
problems involved in terms of the type of WWF and the treat-
ment 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 over-
flow volume, and desired detention time.
The basic planning methodology includes the following steps:
(1) identify functional requirements, (2) identify site constraints,
(3) establish basis of design, (4) select storage and/or treat-
ment option, (5) estimate costs and cost sensitivities, (6) check
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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 param-
eters (e.g., settling time and facility size) on flow control must
be determined. Methodologies for approaching these calcula-
tions include the following: desktop hand computations; statisti-
cal analysis of rainfall and flow data; simple, continuous simu-
lation 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, com-
puterized continuous simulated models can be used.
To evaluate the different storage/sedimentation alternatives,
the degree to which each alternative achieves the goals devel-
oped 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 estab-
lished at the lowest cost.
The process of integrating WWF control into a pollution control
system involves initial planning where existing facilities are
Identify Functional
Requirements
A. Control of Runoff
Rate
B. Reduction of
Runoff Volume
Identify Site
Constraints
A. Area
B. Hydraulic
C. Environmental
D. Structural
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) estab-
lish system needs, (3) identify applicable control alternatives,
and (4) determine control method compatibility.
Facility Design
Storage/sedimentation facility design procedures for both com-
bined 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 strat-
egy for design, and (8) instrumentation and control strategy for
operation.
Design of Retention Storage Facilities
Stormwater retention is the storage of excess runoff for com-
plete removal from the surface drainage and discharge system.
The water collected percolates through the bottom of the reten-
tion facility and may reach the groundwater. Stormwater reten-
tion 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.
Size and location are important design considerations for both
types of ponds. Size requirements include not only volumetric
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
Shift to Alternate
Site or Method
.'\ N_°
•''' "x Modify and
Repeat?
Estimate Costs and
Cost Sensitivities
A. Capital
B. Operation and
Maintenance
Figure 2. Source Control Design Methodology.
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capacity but also both surface area and soil interface area
requirements. The pond configuration depends on (1) the run-
off 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 ad-
equate 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 con-
sists of the following steps: (1) quantify functional require-
ments, (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 con-
figuration. 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 prac-
tice, 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 pollut-
ant loadings depends heavily on the underlying soil as a treat-
ment 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 alterna-
tive 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, de-
tention storage holds excess water until the inflow decreases
and releases it during low-flow periods. Because of sedimenta-
tion 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 de-
tention storage holds storm flow either in series or in parallel
within the collection system. In-system detention storage in-
cludes 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 uHlined funnels. Func-
tionally, 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 de-
crease the frequency and volume of overflows from combined
sewer systems.
Factors to be considered in the design of onsite storage facili-
ties 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 discus-
sion 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) estab-
lish 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 re-
ported 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 sim-
plicity and high cost due to size and structural requirements.
The report presents detailed design considerations and proce-
dures 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 auxil-
iary support facilities are shown in Figure 3.
Functionally, the applications of downstream storage/sedimen-
tation facilities vary from essentially total containment, experi-
encing 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 post-
storm cleanup. For flow-through treatment systems, perfor-
mance hinges on treatment effectiveness and design consider-
ations 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/sedimenta-
tion facilities include the following: (1) storage volume, (2)
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Tunnel
Storage
Storage/Sedimentation (Open)
Storage Capacity 27 Mgal
t
| _
3 Sedimentation
Resuspension
Basins in Series
Post-Storm Dewater
to Interceptor
JH
Overflow
Contact
Basin
Aerated Retention Basin
Long-Term Dewater to
Treatment and Reuse
Mount Clemens
Degritting
Cyclone
. In-Sewer
Storage
I Storage/Sedimentation '
(Enclosed) |
Drain and Pumped
Return to
Interceptor
Coarse
Screen
8 Parallel Basins
Storage Capacity 10 Mgal
New York City (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 Mgal
Sacramento (Pioneer Reservoir)
Figure 3. Representative CSO Storage/Sedimentation Basins and Auxiliary Support Facilities.
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 con-
figuration, (5) identify and select pretreatment, (6) detail auxil-
iary systems, (7) estimate costs and cost-effectiveness analy-
sis, 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
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 re-
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quirements 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 inter-
nationally are introduced below. The review includes flow-
control devices developed in Sweden, Denmark, and Ger-
many; an in-receiving water flow balancing system developed
and applied in Sweden; and an innovative self-cleaning stor-
age/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 associ-
ated 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 cham-
ber 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 devel-
oped 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 sche-
matic of the system is shown in Figure 4.
Typically, removal of settled solids from an inline storage facil-
ity 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 con-
tinuous 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 configura-
tion 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. Stor-
age/Sedimentation Facilities for Control of Storm and Com-
bined Sewer Overflows: Design Manual, EPA/600/R-98/006
PB98-132228. Cincinnati, OH: U.S. Environmental Protection
Agency, 1998.
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Figure 4. Schematic of Pontoon Tank System at Lake Tehorningen, Sweden.
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Submerged Screen
Spillway
Figure 5. Self-Cleaning Storage/Sedimentation Basin used in Zurich, Switzerland.
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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