vvEPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA 832-F-99-042
September 1999
Combined Sewer Overflow
Technology Fact Sheet
Retention Basins
DESCRIPTION
In older communities, where combined sewer
systems are still common, storm water flows often
exceed the sewer system's hydraulic capacity.
Redevelopment of urban areas can also increase the
impervious area in the sewershed, which, in turn,
increases storm water flows to combined sewers.
Storage is often the best measure for attenuating
peak combined sewer flows. Storage facilities have
been used extensively for Combined Sewer
Overflow (CSO) mitigation (Urbonas and Stahre,
1993; Field, 1997). Specific CSO retention
methods include underground storage (e.g.,
tunnels), in-receiving water storage, and retention
basins (RBs). RBs may be built in-line or off-line.
This fact sheet describes near-surface, off-line RBs.
CSO RBs capture and store some of the excess
combined sewer flow that would otherwise be
bypassed to receiving waters. Stored flows are
subsequently returned to the sewer system during
dry weather periods, when in-line flows are reduced
and capacity is available at the treatment facility.
RBs can be designed to control both flow rate and
water quality. Figure 1 shows an example of a
multi-stage CSO RB with treatment capabilities.
This facility handles peak flows by routing them
through a mechanical bar screen and then pumping
them into the first compartment. The main
function of the first compartment is to allow for
EFFLUENT SEWER
/ PUMP STATION
* J FX. MARKET AVF. J ,— —
^=^= pi IMPIN^ ^TATPN
^*~\— SANITARY SEWERS ___^^
\, A taj
/ ''
Source: City of Grand Rapids, Michigan, 1992.
FIGURE 1 CITY OF GRAND RAPIDS-MARKET AVENUE CSO RETENTION BASIN
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primary settling and grit removal. If the flows
continue to rise, the first compartment fills and then
spills over into the second compartment. This
compartment is designed specifically to store most
of the overflow from the first compartment.
Compartment two is also equipped with a floor
wash system that flushes all settled sediments into
a collection trough. If the flows continue to rise,
the water spills over into a series of troughs where
sodium hypochlorite is applied for disinfection.
The flow is then routed to a contact tank in
compartment three which eventually returns the
water to the Grand River.
In addition to minimizing water quality impacts and
attenuating peak flows, CSO storage eliminates or
reduces sewer backups, improves the efficiency of
existing treatment capacity, and improves effluent
quality at the treatment facility (WEF/ASCE, 1992).
Near-surface storage in open or covered basins is
the most common method of CSO retention. RBs
may be placed online or offline from the combined
sewer. Online RBs are connected in series to the
combined sewer and retain excess flows when the
inlet flow surpasses the outlet capacity. Off-line
RBs are connected in parallel to the combined
sewer and receive flows only during wet weather
periods.
Off-line RBs are typically earthen basins or covered
or uncovered concrete tanks. Covered basins are
widely used because they provide better odor
control and better safety conditions.
Off-line storage is more costly than online storage
because parallel lines must be constructed and
facilities for pumping the stored wastewater back to
the sewer are usually required. However, off-line
storage is required where head loss in the
downstream sewer is a concern and sedimentation
or other treatment methods are desired.
Offline RBs may be located at upstream or
downstream locations in the combined sewer
system. Advantages of upstream control include
greater flexibility in selecting sites for facilities and
more efficient control of flows to the downstream
treatment facility. The primary advantage of
downstream storage is that fewer facilities are
required, resulting in lower construction and
operation and maintenance costs. It may be
possible to minimize costs further if storage
capacity is available at the wastewater treatment
plant.
Primary concerns in CSO retention are:
• Managing flows to and from the retention
basin.
• Preventing the combined sewage from
becoming septic (or handling the
wastewater after it has become septic).
Removing accumulated solids and
floatables.
• Disinfecting basin overflows to receiving
waters.
CSO RBs have been designed to limit these
problems. In offline facilities, flows are regulated
by limiting the amount of flow that is diverted to
the RB (Figure 1). The sidestream flow is regulated
by a device located downstream of the diversion or
at the basin's outlet. Many types of fixed and
movable flow regulators may be used for CSO
retention basins.
Average BOD concentrations of combined sewage
are usually high enough to promote septic
conditions if the wastewater is retained for an
extended period of time. Therefore, the stored
wastewater must be either aerated at the RB or
returned to the sewer system in a timely manner.
Sedimentation treatment is often incorporated into
the RB design. RBs may be compartmentalized so
that the first flush flow can be retained in one or
more tanks long enough to remove suspended
solids, BOD, and nutrients, while the remainder of
the flow is handled in subsequent compartments. In
some cases, the discharged flow can bypass the
primary sedimentation process at the wastewater
treatment facility.
An RB can remove accumulated solids by inducing
scouring or agitating the stored wastewater during
discharge to the sewer, by flushing with a potable or
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effluent water supply, or by using mechanical
scrapers (WEF/ASCE, 1992). Flushing systems
release a wave of water to sweep accumulated
solids and debris to a disposal channel at the end of
the basin.
In cases where CSO flows exceed the capacity of
RBs, an overflow structure diverts the excess flows
to nearby surface waters. Disinfection of KB
overflows may be required to minimize the risk of
pathogens being discharged to receiving waters.
High-rate disinfection is often required to ensure
adequate removal of pathogens from short-term,
high volume overflows. The disinfection process
must be adaptable to intermittent high flows with
variable temperatures, suspended solids
concentrations, and microorganism levels (Field,
1997).
APPLICABILITY
Offline storage is a common structural CSO control
because it is less costly and more easily adapted to
site conditions than other structural approaches
(e.g., storm sewer separation). Several
municipalities, including Boston, MA, Atlanta, GA,
Los Angeles, CA, Wayne County, MI, and
Saginaw, MI, have constructed networks of offline
retention basins to minimize CSOs. Other
municipalities, including Richmond, VA, and
Grand Rapids, MI, are using one or a few large RBs
for CSO control.
The City of Richmond's RB has a storage capacity
of 36 million gallons (MG). It is designed to
capture the first flush of combined sewage for a
one-month design storm. The basin includes jet
aeration and momentum headers, which prevent the
retained wastewater from becoming septic and keep
solids in suspension. The stored combined sewage
is pumped to the wastewater treatment plant within
48 hours after the end of each event.
Environmental assessments are required to evaluate
the environmental impact of constructing and
operating RBs. The results of these studies can
influence the location and the design of facilities,
and even the decision of whether or not to construct
retention basins (Munger and Toll, 1996).
Experience has shown that public perception is an
important factor in the design of CSO retention
basins. Recent designs of CSO retention basins
have incorporated aesthetic elements, such as parks
or recreational areas, on top of covered basins, as
well as more elaborate odor control systems
(Wayne County, 1997; AMSA, 1994).
Offline retention basins are often used to manage
CSOs after existing storage in the sewer system has
been optimized. Offline RBs are usually
implemented where online facilities are not
practical (e.g., where head loss in downstream
sewers is a concern) or some level of treatment is
required.
Engineering design studies should evaluate
subsurface soil conditions, depth of construction,
constraints on use and access to the site, and
environmental impact. Detailed evaluation should
consider the means of disposal of screenings and
residual solids, handling and storage of chemicals,
capacity and routing of sewer/force main, and the
need for a pump station.
ADVANTAGES AND DISADVANTAGES
The primary limitations of CSO retention basins are
their costs and the environmental impact of
construction. Because retention basin construction
can be expensive, other source control and sewer
optimization measures should be implemented
before considering the addition of storage controls.
Construction of large retention basins may require
the destruction of sensitive habitats for terrestrial
and aquatic life. Environmental assessments are
generally required to select sites that will have the
least impact on the environment and the public. In
some cases, mitigation of unavoidable
environmental impacts may be required.
Construction of RBs can have a significant impact
on the environment. Environmental assessments
are usually performed to identify sites and
construction methods that will minimize damage to
the environment and disturbance of the public.
These assessments will also specify steps that can
be taken to mitigate unavoidable environmental
impacts.
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The elimination or reduction of CSOs can improve
receiving water quality. As shown in Figure 2,
fecal coliform concentrations in the Grand River
below the CSO outfalls and wastewater treatment
plant have exhibited a long-term decline. The
improved water quality appears to be related to the
30 MG retention basin that came online in 1992.
Additional information on the effects of CSO
storage on water quality will become available
when CSO long-term monitoring programs are fully
implemented as part of EPA's CSO Control
Strategy (U.S. EPA, 1994). Recommended
performance measures for the National CSO
Control Program are described by AMSA (1996).
DESIGN CRITERIA
Source: Grand Rapids, 1996.
Source: City of Grand Rapids, Michigan, 1996.
FIGURE 2 TREND OF FECAL COLIFORMS IN
GRAND RAPIDS, Ml
The primary purpose of storage is to optimize
treatment of the combined flow at the downstream
wastewater treatment plant; therefore, RB design
should be developed with consideration of the
treatment capabilities of the wastewater treatment
plant. The RB design should also ensure that stored
wastewater does not become septic and
accumulated solids and floatables are effectively
removed. In some cases, RB overflows to receiving
waters must be disinfected.
As with all CSO control approaches, the evaluation
of CSO retention basins should start with a detailed
characterization of the sewer system, including a
review of rainfall and sewer flow records,
monitoring of selected CSO locations, receiving
water quality monitoring, and mathematic modeling
(Moffa, 1997). This information is used to
establish a design standard such as the frequency of
RB overflows to surface waters (e.g., one, three, or
five times a year) or the percent reduction in CSO
volume. The size of the basin may be based on data
on rainfall intensity, duration, and frequency;
standard design storms with a specified recurrence
interval (e.g., one, five, or ten years); the
chronologic record of rainfall; or a chronologic
series of measured flows at the site (Urbonas and
Stahre, 1993).
Storage Volume
Several methods for calculating storage volumes
have been summarized by Urbonas and Stahre
(1993). One of the most common is flow route
modeling. These models have become popular for
calculating storage volumes because of their ability
to simulate runoff under a variety of conditions.
Commonly used models include EPA's Storm
Water Management Model (SWMM); the U.S.
Army Corps of Engineers' Storage, Treatment,
Overflow, Runoff Model (STORM); and the
Hydrological Simulation Program-FORTRAN
(HSPF). Useful summaries of these models are
given by Huber and Heaney (1980, 1982). SWMM
is most commonly used to simulate urban runoff
processes and combined sewer systems (EPA,
1985; James, 1993). In addition to estimating
surface runoff, SWMM and other models can
estimate pollutant loads in response to precipitation
and surface pollutant accumulations.
Basin Shape
Rectangular basins are the least expensive to
construct and maintain. Circular and octagonal
basins are more expensive to build, but are
advantageous because they can be configured to
self-clean settled solids.
Inlet Type
The choice of inlet type will depend on whether the
RB is designed to remove solids. If solids removal
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is desired, the inflow velocity mustbe reduced (e.g.,
by using baffles) to prevent resuspension of settled
solids. Alternatively, the inlet can be configured to
create turbulence and circular flow to keep solids in
suspension if sedimentation is not desired.
Bottom Configuration
The bottom configuration of an KB can enhance
cleaning of sand, silt, and other settled solids.
Three types of bottom configuration are used for
rectangular RBs: flat bottoms; parallel longitudinal
grooves; and a single continuous groove. Flat
bottoms are the easiest to construct, but require a
mechanism for solids removal. Urbonas and Stahre
(1993) suggest that flat-bottomed basins should
have a minimum bottom slope of 3 percent and a
width equal to one-half to two-thirds of the length.
A series of parallel, longitudinal grooves may be
constructed to convey solids out of the basin;
however, experience has shown that supplemental
cleaning is occasionally required (Urbonas and
Stahre, 1993). Basins with bottoms that have a
single continuous groove can also be self-flushing.
Outlet Type
Outlet structures must be closely matched to the
specifications of downstream facilities. It is usually
desirable to maintain a constant outlet flow rate to
minimize the effects on downstream structures. KB
outflow can be controlled using a fixed outlet
opening, a choked outlet pipe, adjustable gates,
pumps, and special regulators. Fixed outlet
orifices, flow-restricting pipes, and overflow weirs
are often chosen to regulate the KB outlet flow
because they have predictable hydraulic
characteristics and are simple to design (Field,
1997).
Remotely controlled gates offer more direct control
of the KB outlet flow. These outlets generally
consist of a flow-monitoring system, movable gates
that are adjusted by electrical, hydraulic, or
pneumatic controls, and a data processing and
control unit (Urbonas and Stahre, 1993). Coupled
with the use of a model to estimate runoff from
rainfall, this system can be adjust the gate opening
well in advance of high-flow events. However, gate
systems consist of many moving parts and require
instrumentation, which can be expensive to operate
and maintain.
Pumps are used to regulate KB outflow where the
topography does not allow gravity flow or more
operational control is needed. The primary
disadvantages of this system are the costs of
operating and maintaining the pumps and the
potential for pump failure.
In Europe, several self-regulating outlet systems
have been developed for RBs, including
float-activated gates, floating outlets, and bending
weirs. A float-activated gate developed in
Germany, called the Hydroslide™, restricts outlet
openings during high flow events so that a constant
discharge rate is maintained (Figure 3). A floating
outlet is another self-regulating outlet. This outlet
floats on the water surface and maintains a constant
water depth at the KB outlet. The removal of
surface water helps to trap settled solids in the RB.
Bending weirs also operate automatically without
the use of auxiliary power (Field, 1997). One type
of bending weir used for RBs, called the
Hydrobend™, is a bending flap controlled by a
counterweight and an eccentric control disc. As
shown in Figure 4, the disc is designed to balance
between the forces of water pressure on the
overflow face of the bending weir and the
counterweight (GNA, 1996).
Source: GNA, 1996.
FIGURE 3 HYDROSLIDE™ FLOW
REGULATOR
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Eccentric Control Disc
Monitoring System
Counterweight
Source: GNA, 1996.
FIGURE 4 HYDROBEND™ FLOW
REGULATOR
Overflow Structures
RBs are usually equipped with emergency spillways
or outlets. Spillways are sized to pass the
maximum inflow under the worst-case condition of
a clogged or non-operational outlet. Spillways
should be located away from the KB outlet to
minimize the loss of floatables that often collect
near outlets (Urbonas and Stahre, 1993).
Disinfection
In some cases, KB overflows may be disinfected to
prevent the discharge of pathogens to surface
waters. Chlorine gas (C12) or sodium chloride
(NaCl) is commonly used for wastewater
disinfection. However, these disinfectants can react
with ammonia (NH3) in the combined sewage to
form chloramine compounds, which are toxic to
aquatic life. Chlorine dioxide (C1O2), a more
rapidly-acting disinfectant, does not react with NH3.
It may be necessary to dechlorinate the KB
overflow to eliminate the potential for instream
toxicity due to chlorine.
Operational Control
CSO retention basins are operated to minimize
overflow and maximize the capacity of the sewer
system and treatment facility. Three levels of
operational control may be practiced:
• Local (closed-loop) control of regulators,
gates, or pumps.
Regional coordination of several local
controllers.
• Global control of multiple components,
including rainfall and flow data recorders,
telemetered controllers for regulators, gates,
and pumps, and a computer system for
overall system control.
The City of Grand Rapids (MI) uses a global CSO
monitoring system that consists of sewer level
sensors, a bridged telephone network and telemetry
system, and a PC-based data acquisition and
processing system (Spykerman, 1996).
PERFORMANCE
CSO retention basins can significantly reduce or
eliminate CSO volumes. Construction of a 30 MG
retention and treatment basin by the City of Grand
Rapids (MI) helped to reduce CSO discharge
volumes by about 90 percent (Grand Rapids, 1995).
Similar results have been achieved at other
municipalities.
Concentrations of coliform bacteria, suspended
solids, nutrients, and BOD can be reduced in RBs
designed to induce sedimentation. Estimated
removal rates of BOD5 and suspended solids are
determined by the detention time and overflow rates
(based on average daily flows). Figures shows the
average percent removal of BOD5 and suspended
solids for municipal wastewaters. However, the
solids characteristics and settling velocities of
CSOs vary by site and it is recommended to
develop a settling velocity distribution for each site
(Field, 1997). Chemicals can be added to improve
solids removal by coagulation (Adams, et. al,
1981).
OPERATION AND MAINTENANCE
Basins, especially those with covered
compartments, are subjected to extreme conditions,
including high humidity, corrosive gases, and
sludge deposits. Operation and maintenance
requirements can be reduced through appropriate
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I
I
£
20 -
2345
Detention Time, hr
Source: Steel, 1960.
FIGURES PERCENT BOD5 AND
SUSPENDED SOLIDS REMOVAL FOR
VARIOUS OVERFLOW RATES VERSUS
DETENTION TIME IN TREATING MUNICIPAL
WASTEWATERS
planning and design (e.g., using corrosion-resistant
materials and providing adequate ventilation).
However, routine inspection and maintenance
remain necessary to ensure proper operation of the
basin.
Sediment removal systems include traveling bridge
nozzles, fixed nozzles, mechanical mixers, and
water flushing systems. Studies have shown that
flushing systems are the most efficient and cost-
effective (Novae and Grande, 1992; Parente, etal.,
1995). Two types of flushers have been widely
used: a flip gate flusher such as Hydroself™ or
Hydrass™, and tipping flushers (Field, 1997). The
Hydroself™ system operates by discharging water
through a hydraulically operated flap gate. The gate
creates a flushing wave that sweeps settled solids
and debris from the basin floor (GNA, 1996). More
than 300 Hydroself™ units are used in Europe for
cleaning CSO storage tanks (Field, 1997).
As shown in Figure 6, the tipping flusher is a
cylindrical vessel that is placed above the maximum
water level on the back wall of the retention basin.
The vessel fills with water up to a pre-determined
depth, and then the vessel rotates on a center axis,
spilling the water into the basin and creating a
flushing wave. Tipping flusher units have been
used in North America since the early 1990s.
Source: GNA, 1996.
FIGURES TIPPING FLUSHER UNIT
Access must be provided for basin cleaning and for
removing blockages from the outlet and spillway.
Walkways are also recommended for inspection of
spillways, outlets, and the interior of covered basins
(Urbonas and Stahre, 1993). Monitoring
information is essential for operational control and
future design considerations; therefore, the water
level in the basin should be recorded.
COSTS
Storage and treatment of CSOs is structurally
intensive and costly, and should be used only after
CSO sources have been controlled and sewer use
has been optimized. Costs of CSO controls for
selected communities throughout the U.S are
summarized in Table 1. The cost of basin
construction can also be estimated from standard
cost curves like those provided by Lager, et al.
(1977).
As shown in Table 1, capital costs for CSO
retention basins are significant. Before
implementing CSO controls, a cost-benefit analysis
should be done to compare the costs of the controls
to the anticipated benefits of improved water
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TABLE 1 COSTS OF CSO RETENTION BASINS FOR SELECTED MUNICIPALITIES
Municipality
Grand
Rapids, Ml
Richmond,
VA
Oakland
County, Ml
San
Francisco,
CA
Retention
Basin
Market
Avenue RB
Shockhoe
RBand
Diversion
System
Acacia
Park
Birmingha
m
Bloomfield
Village
North
Shore
Mariposa
Sunnydale
Yosemite
Year
Constructed
June 1992
First
Compartment
Second
Compartment
Third
Compartment
-1988
1997
1997
1997
1984
1992
1991
1989
Basin
Capacity
(MG)
30.5
10.68
16.68
3.14
41
4.5
9.6
(includes
tunnel)
10.2
24
0.7
6.2
11.5
Covered/
Uncovered
covered
uncovered
uncovered
covered and
uncovered
covered
covered
covered
covered
(underground)
covered
(underground)
covered
(underground)
covered
(underground)
Type of
Facility
offline
hydraulic
retention,
sediment
return to
sewerage
system, no
disinfection
offline with
sedimentation
and
disinfection
capabilities
offline with
sedimentation
and
disinfection
capabilities
offline with
sedimentation
and
disinfection
capabilities
transport/
storage box3
transport/
storage box3
transport/
storage box3
transport/
storage box3
Construction
Cost
$30 million
covered:
$1 ,077,9002
uncovered:
$343,9002
$13.9 million
$35.6 million
$28.9 million
$69.08 million
$10.17 million
$19.29 million
$19.16 million
O&M
Cost
$40,000
$500,096
$207,000
$370,000
$500,000
not
available
not
available
not
available
not
available
Design
Criteria
10yr-1
hr storm
1 month
design
storm,
7500 cfs
first flush
30
minute
detention
for
1 year/
1 hour
storm
30
minute
detention
for
1 year/
1 hour
storm
30
minute
detention
for
1 year/
1 hour
storm
4 CSOs/
year4
10
CSOs/
year4
1 CSO/
year4
1 CSO/
year4
1 includes operators, time, supplies, chemicals, parts, and utility charges
2 costs adjusted to 1994 costs
3 CSO discharges from transport storage facilities receive flow through treatment with weirs to settle solids and baffles to skim floatables
4 Transport storage facilities were designed in conjunction with pumping and treatment facilities to meet annual average CSO criteria
specified in NPDES permits; local sewers tributary to the transport/storage boxes are designed for the 5 year storm
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quality. For example, King County, WA is
currently performing a CSO assessment to identify
the water quality benefits of CSO controls and to
determine the most effective method for improving
water quality (Munger and Toll, 1997).
Novae and Grande (1992) surveyed the costs of
various methods for cleaning CSO storage basins.
Tipping flushers had less capital and operation and
maintenance costs than traveling bridge nozzles,
fixed nozzles, and mechanical mixers. The average
capital cost for tipping flushers was $19.59 per
cubic meter of storage ($14.96 per cubic yard of
storage), compared with $51.66 and $52.44 per
cubic meter of storage ($39.45 and $40.04 per cubic
yard) for traveling bridge nozzles and fixed nozzles
(adjusted to 1998 costs). Parente, et al., (1995)
determined that Hydroself™, a flushing gate
system, would be cost-effective compared to
tipping flushers and spray methods. Capital and
operation and maintenance costs were $109.38 and
$0.08 per square meter of area ($91.44 and $0.07
per square yard) for Hydroself™ versus $164.08
and $0.12 per square meter of area ($137.17 and
$0.10 per square yard) for tipping flushers (adjusted
to 1998 costs).
Capital and operating costs for high-intensity
chlorination systems are described by Field (1997).
A system with short chlorine contact times and
intense mixing is more economical and can be as
effective as a conventional process with longer
chlorine contact times (Field, 1997).
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Eckenfelder, 1981. Development of Design
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Washington, D.C.
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communication with Parsons Engineering
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Spykerman, M., 1996. Real-Time
Combined Sewer Overflow Monitoring.
Michigan Water Environment Association
"Need to Know" Workshop.
Urbonas, B. and P. Stahre, 1993.
Stormwater: Best Management Practices
and Detention for Water Quality, Drainage,
and CSO Management. PTR Prentice Hall,
Englewood Cliffs, NJ.
U. S. EPA, 1985. Storm Water
Management Model (SWMM) Bibliography.
EPA/600/3-85/077, Athens, GA.
U. S. EPA, 1994.
Overflows Policy.
Washington, D.C.
Combined Sewer
EPA 830-B-94-001,
Wayne County Department of Environment,
1998. "Rouge River National Wet Weather
Demonstration Project: CSO Demonstration
Project." Internet site at
http://208.220.235.35/wayne/build/rpo/tec
htop/cso/csocont.html, accessed June, 1998.
WEF/ASCE, 1992. Design and
Construction of Urban Stormwater
Management Practices. Water
Environment Federation, Alexandria, VA,
American Society of Civil Engineers,
Washington, D.C.
-------
ADDITIONAL INFORMATION
City of Grand Rapids, Michigan
Chuck Shroeder
Grand Rapids Wastewater Treatment Facility
1300 Market St., SW
Grand Rapids, MI 49503
Milwaukee Metropolitan Sewerage District
Wayne St. John
Director of Operations
260 W. Seeboth St.
Milwaukee, WI 53201
Oakland County, Michigan
Gary R. Aho
Oakland County Drain Commissioner
No. 1 Public Works Drive
Waterford, MI 48328
City of Richmond, Virginia
Mac McConico
600 E. Broad St., Rm 831
Richmond, VA 23219
City of San Francisco, California
Beth Goldstein
Public Utilities Commission
1212 Market St., 2nd floor
San Francisco, CA 94102
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for the use by the U.S.
Environmental Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
!MTB
Excellence in compliance through optvnal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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