v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-99-040
September 1999
Combined Sewer Overflow
Technology Fact Sheet
Screens
DESCRIPTION
In 1994, the U. S. Environmental Protection Agency
(EPA) recognized the importance of controlling
solid and floatable materials under the "nine
minimum controls" described in the Combined
Sewer Overflow (CSO) Control Policy. CSOs can
contain high levels of floatable materials, suspended
solids, biochemical oxygen demand (BOD), oils and
grease, toxic pollutants, and pathogenic
microorganisms. Floatables are often the most
noticeable and problematic CSO pollutant. They
create aesthetic problems and boating hazards,
threaten wildlife, foul recreational areas, and cause
beach closures. There are numerous methods
available for floatables control, including baffles,
catch basin modifications, netting systems,
containment booms, skimming processes, and
screening and trash rack devices. These
technologies are summarized in EPA's CSO
Technology Fact Sheet entitled "Floatables Control"
(EPA 832-F-99-008). This fact sheet focuses on
screens and trash racks for CSO floatables control.
Screens are considered an effective and
economically efficient method of removing solids
and floatables from CSOs. CSO screens are
typically constructed of steel parallel bars or wires,
wire mesh (wedgewire), grating, or perforated plate;
some screens, however, are constructed of milled
bronze or copper plates. In general, the openings
are circular or rectangular slots, varying in size from
0.25 to 15.24 centimeter (0.1 to 6 inch) spacings.
The amount and size of the solids and floatables
removed is dependent on the type of screen and the
size of the screen openings. Solids are removed
from the flow by two basic treatment mechanisms:
• Direct straining of all particles larger than
the screen openings.
• Filtering of smaller particles by straining
flow through the mat of solids already
deposited on the screen.
Generally there are two types of bar screens- coarse
and fine. Both are used at CSO control facilities,
with each different type providing a different level
of removal efficiency. While there is no industry
standard for classifying screens based on aperture
size coarse bar screens generally have 0.04 to 0.08
meter (1.5 to 3.0 inch) clear spacing between bars
and fine screens generally have rounded or slotted
openings of 0.3 to 1.3 centimeters (0.1 to 0.5 inch)
clear space.
Coarse Screens
Course screens are constructed of parallel vertical
bars and are often referred to as bar racks or bar
screens. In CSO control and treatment facilities,
coarse screens are usually the first unit of equipment
in the system. These screens are usually set at 0 to
30 degrees from vertical and are cleaned by an
electrically or hydraulically driven rake mechanism
that removes the material entrained on the screen on
a continuous or periodic basis. There are three
types of bar screens used at CSO control facilities:
trash racks; manually cleaned screens; and
mechanically cleaned screens.
Trash racks
Trash racks (also known as trash grates) are
intended to remove only very large objects from the
flow stream. Trash racks are generally provided at
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the intersection of the combined sewer and the
sanitary interceptor to prevent major blockages in
the interceptor or to protect pumping equipment.
Since both dry and wet weather flows pass through
this type of screening device, daily cleaning is
usually required. Trash racks typically have 0.04 to
0.08 meter (1.5 to 3.0 inch) clear spacing between
bars. Figure lisa diagram of a typical trash rack.
Manually cleaned bar screens
Manually cleaned bar screens have a 2.54 to 5.08
centimeter (1.0 to 2.0 inches) clear spacing between
bars. The bars are set 30 to 45 degrees from the
vertical and the screenings are manually raked onto
a perforated plate for drainage prior to disposal.
Mechanically cleaned bar screens
Mechanically cleaned bar screens have a 0.64 to
2.54 centimeter (0.25 to 1.0 inch) clear spacing
between bars. The bars are set 0 to 30 degrees from
the vertical. Electrically driven rake mechanisms
will either continuously or periodically remove
material entrained on the bar screen itself. The three
common types of mechanically cleaned screens are:
(1) chain driven, (2) climber type rake, and (3)
catenary.
Chain driven mechanical raking systems consist of
a series of bar rakes connected to chains on each
side of the bar rack. During the cleaning cycle, the
rakes travel in a continuous circuit from the bottom
to the top of the bar rack, removing materials
retained on the bars and discharging them at the top
of the rack. A disadvantage of chain-driven systems
is that the lower bearings and sprockets are
submerged in the flow and are susceptible to
blockage and damage from grit and other materials.
Accelerated chain wear and corrosion can also be a
problem.
Climber-type systems employ a single rake
mechanism mounted on a gear driven rack and
pinion system. The gear drive turns cogwheels that
move along a pin rack mounted on each side of the
bar rack. During the cleaning cycle, the rake
mechanism travels up and down the bar rack to
remove materials retained on the bars. Screenings
are typically discharged from the bars at the top of
the rack. This type of bar screen has no submerged
bearings or sprockets and is, therefore, less
susceptible to blockages, damage and corrosion than
chain driven units.
Catenary systems also employ chain-driven rake
mechanisms, but all sprockets, bearings, and shafts
are located above the flow level in the screenings
Removable section
\ Plate, weld to rack, Cover plate
( 1 cut to fit channel
Clearance line
for bar rake
Bars I in x 2 in
o
Notch rack bars
and weld cross bars
in notches
Flow
Concrete fill
Drainage plate
A
1
Clip angle,
- bolt to channel
with stainless
steel bolts
^ Masonry expansion
* anchor, stainless
steel bolt
Buttonhead bolts
"^ o o o o o o
OO O O O O OO
oooooooo
oooooooo
oooooooo
oooooooo
oooooooo
oooooooo
oooooooo
* o o o o o o fl
Curve
this
section
Drainage plate
Source: Metcalf and Eddy, 1991.
FIGURE 1 DIAGRAM OF TRASH RACK USED FOR TREATMENT OF CSOs
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channel. This in turn reduces the potential for
damage and corrosion and facilitates routine
maintenance. During the cleaning cycle, the rakes
travel in a continuous circuit from the bottom to the
top of the bar rack to remove materials retained on
the bars. Screenings are typically discharged from
the bars at the top of the rack. The cleaning rake is
held against the bars by the weight of its chains,
allowing the rake to be pulled over large obj ects that
are lodged in the bars and that might otherwise jam
the rake mechanism.
Fine Screens
Fine screens at CSO facilities typically follow coarse
bar screening equipment and provide the next level
of physical treatment in removing the smaller solid
particles from the waste stream. Both fixed (static)
and rotary screens have been used in CSO treatment
facilities.
Fixed fine screens are typically provided with
horizontal or rounded slotted openings of 0.02 to
1.27 centimeters (0.010 to 0.5 inches). The screens
are usually constructed of stainless steel in a
concave configuration, at a slope of approximately
30 degrees. Flow is discharged across the top of the
screen. The flow then passes through the slotted
openings and solids are retained on the screen
surface. Solids are discharged from the screen
surface by gravity and by washing onto a conveyer
belt or other collecting system.
Rotary fine screens include externally and internally
fed screens. Externally fed screens allow
wastewater to flow over the top of the drum
mechanism and through the screens while collecting
solids on the screen surface. As the screen rotates,
a system of cleaning brushes or sprayed water
removes debris from the drum. Internally fed
systems discharge wastewater in the center of the
drum, allowing water to pass through the screen
into a discharge channel, while solids are removed
from the screen surface by cleaning brushes or a
water spray. Screened material is usually washed
from the screen with a high pressure spray into a
discharge trough. Screen diameters can range from
0.5 to 2 meters (1.6 to 6.6 feet), while the lengths
can vary from 2 to 6 meters (6.6 to 19.7 feet).
There are three modes of operation which include:
• Low Flow- no drum movement.
• Intermediate Flow- drum moves a short
distance and stops with brush coming on as
head loss rises.
• High Flows- continuous operation where the
drum rotates at 1 rpm and brush at 10 rpm.
In response to the need for solids and floatables
control during storm events, proprietary screen
products, such as the ROMAG™ screen (Figure 2),
have been designed for wet weather applications.
The ROMAG™ screen partitions the flow, sending
screened flow to the CSO discharge point, while
keeping solids and floatables in the flow directed
towards the sanitary sewer.
The ROMAG™ screen works as follows: excess
flow enters the screening chamber, flows over a spill
weir and proceeds through the screen into a channel
which discharges flow to a receiving water body.
Floatables trapped by the screen move laterally
along the face of the screen via combs/separators to
the transverse end section of the pipe where they
can be directed to the sanitary sewer line for
ultimate removal at the wastewater treatment plant.
Screen blinding is prevented by a hydraulically-
driven rake assembly.
The ROMAG™ screen surface is accessible from
ROMAG SCREEN
Source: Pisano, 1995.
FIGURE 2 ROMAG™ "COMBING"
MECHANICAL SCREEN (VERTICAL) FOR CSO
FLOATABLES CONTROL
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both sides to facilitate inspections and maintenance.
The screen consists of horizontal bars with 4 mm
(0.16 inches) openings that are mounted on a weir
in the collection system. Screens range from 2 to 9
meters (6.6-29.5 feet) in length and 330-1200 mm
(13- 47.2 inches) in height. Units can be stacked to
create a customized mesh opening for a specified
design flow at a particular location. The nominal
velocity through the bar openings is approximately
1.5 meters per second (4.9 feet per second).
The hydraulically driven mechanical combs used to
clean the screen move laterally along the front face
of the screen when activated by a level control,
which detects rising water. As the screen surface is
cleaned, captured material is transported forward to
the end section for storage and subsequent removal.
The hydraulic combing unit is located outside the
screen and consists of an oil tank, pump and control
valves.
The ROMAG™ screen may be designed for a
variety of flow scenarios. Water may pass through
the screen horizontally (RSW type), as shown in
Figure 2; over the top of the screen (RSO type) or
up from under the screen (RSU) type. This unit has
proven useful in remote settings and is capable of
handling flows from 300-6100 L/sec (6-140 MOD).
APPLICABILITY
While screening is widely used to control solids and
floatables at the headworks of wastewater treatment
plants, screening for solids at remote locations,
such as at CSO or storm water overflow points, is
less common. However, some types of screens are
effective for remote solids and floatables control due
to their large aperture size and self-cleaning ability.
As a result, mechanically-cleaned bar screens have
proven to be a relatively simple and inexpensive
means of removing floatables and visible solids.
They are typically the screen of choice in many CSO
treatment facilities, and are widely used or
implemented at a large number of CSO facilities
across the country and abroad.
There has been less success in removing fine solids
from storm water and CSO overflows. However,
proprietary methods, such as the Romag™ screen,
have addressed this issue. More than 250 Romag™
screens have been installed in Europe since 1990.
Recently, several Romag™ screens have been
installed in the U.S. The first was installed in
Rahway, NJ, in 1997.
In addition, Deerfield, Illinois has had success
utilizing rotating fine screens at their overflow
facilities. Their fine screens have 1.02 millimeter
(0.04 inch) openings that remove all large solids and
floatables. The screened wastewater is discharged
inside the screen and conveyed to a chlorine contact
tank for disinfection prior to discharge to the
receiving stream. The screenings are conveyed by
internal conveyors to a discharge chute for storage
and eventual return to the POTW at the end of the
overflow event. The entire operation is automatic
(Westetal., 1990).
ADVANTAGES AND DISADVANTAGES
Since screening is a physical treatment process, it
will remove only those objects that are larger than
the screen openings. Screening systems are very
effective in removing floatable and visible solids, but
do not remove a significant amount of suspended
solids. In cases where water quality evaluations
indicate the need for removal of suspended solids or
oxygen demanding materials, additional treatment
processes downstream from the screening units
would be required.
Because screens at CSO control facilities remove
debris, rags, and other floatables that would
otherwise be discharged into a receiving stream,
they are vital in preserving water quality and
aesthetics. Unscreened material in CSOs can
become a nuisance if the floatables, and other solids
end up in receiving waters. They can create
navigational hazards, attract nuisance vectors, and
retain bacteria and other pollutants.
Properly screened and removed materials in CSSs
prevent materials from settling out in the system,
thus preventing potential back ups and possible
overflows elsewhere. The screenings and debris
that are removed from the screens are typically not
hazardous and can be disposed of in a licensed
landfill or incinerated. Negative environmental
impacts can occur from improper disposal of
screened materials, such as by stockpiling in areas
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adjacent to receiving waters or in areas where they
may be seen by the public.
DESIGN CRITERIA
Grit classifiers are effective in separating,
washing, and dewatering grit, sand, finds,
and silt from an effluent flow normally
downstream form the screens.
Hydraulic losses through bar screens are a function
of approach velocity and the velocity through the
bars. The headloss through a clean bar screen can
be estimated using the following equation:
hL = (1/0.7) *((V2-v2)/2g)
where:
hL = headloss, ft (m)
0.7 = an empirical discharge coefficient to
account to turbulence and eddy losses
V = velocity of flow through the openings
of the bar racks, ft/s (m/s)
v = approach velocity in upstream channel,
ft/s (m/s)
g = acceleration due to gravity, ft/s2 (m/s2)
Headloss increases as the bar screen becomes
clogged, or blinded. For coarse screens, the
approach velocity should be at least 0.38 meters per
second (1.25 feet per second) to minimize
deposition, while the velocity through the bars
should be less than 0.91 meters per second (3 feet
per second) to prevent entrained solids from being
forced through the bars. Instrumentation provided
with mechanically-cleaned screens is configured to
send a signal to the cleaning mechanism so the
headloss across the screen is limited to 6 inches.
The following general factors should be considered
in the design and operation of coarse and fine
screens:
Coarse screens with moving parts out of the
flow stream are preferable to coarse screens
with submerged parts.
• Fine screens using steel wire mesh or
perforated panels are very prone to clogging
from fibrous materials and are not easily
cleaned. Plastic mesh panels have proven to
be effective, are resistant to clogging and
are easily cleaned with water sprays.
Pumping or conveying large amounts of large and
small solids typically removed by screening systems
has proven to be very difficult and a major
maintenance problem. Screw conveyors and
compactor type screws have been shown to be
effective in handling solids, especially those
removed by fine screens. Design parameters for
different types of screens are given on Tables 1, 2,
and 3.
Additional design issues to consider include:
• Backwater from a storage/sedimentation
tank effluent weir can create quiescent
settling conditions in the bar screen channel.
Therefore, a means of flushing or
backwashing the screenings channel should
be provided.
A redundant or back-up bar screen should
be provided so that peak flow to the facility
can be maintained with one unit out of
service. Providing stop grooves or slide
Grit will tend to accumulate upstream and
downstream of screens. Provisions must be
made for easy access to such areas and
alternative methods of grit removal,
including vacuum systems, high pressure
water cannons or spray systems.
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TABLE 1 DESIGN PARAMETERS FOR
STATIC SCREENS
Hydraulic loading, gal/min/ft of width 100-180
Incline of screens, degrees from vertical* 35
Slot space, |jm 250-1600
Automatic controls None
*Bauer Hydrasieves ™have 3-stage slopes on each
screen: 25°, 35°, 45°.
Note: gal/min/ft X 0.207 = l/m/s
gates in the channel allows the user to
isolate the screen from the flow for
maintenance.
• Guards, railings, and gratings should be
provided in the area around the screening
equipment to ensure operator safety.
Electrical fittings and equipment associated
with the screening equipment must
conform to the exposure rating for the
space in which the equipment is located.
PERFORMANCE
Removal efficiency is a function of bar screen
spacing and floatable solids characteristics.
Removal efficiency increases as the size and
concentration of the solids increases and the spacing
dimension decreases. Screenings typically
containing 10-20 percent dry solids will typically
have a bulk density ranging from 640 to 1100
kilograms per cubic meter (40 to 70 pounds per
cubic foot). Typical floatable removal rates for
coarse screens range from 3.5 to 84 liters per 1000
cubic meters (0.469 to 11.2 cubic feet per MG).
The quantity of screenings can vary greatly and, in
general, depends on the following factors:
Configuration of the drainage system.
• Time of year.
• Interval between storms.
• Intensity of the storm.
TABLE 2 DESIGN PARAMETERS FOR
DRUM SCREENS AND ROTARY SCREEN
Parameter
Drum/Band
Screen
Rotary Screen
Screen spacing,
Screen material
Drum speed,
r/min
Speed range
Recommended
speed
Peripheral
speed, ft/s
Submergence of
drum, %
Flux density,
gal/ft2/min of
submergence
screen
Hydraulic
efficiency, % of
inflow
Headless, in.
Backwash
Volume, % of
inflow
Pressure, Ib/in2
100-420
stainless
steel or
plastic
2-7
5
74-167
105 recommended
stainless steel or
plastic
30-65
55
14-16
60-70
20-50
70-150
75-90
6-24
0.5-3
30-50
0.02-2.5
50
Note: gal/ ft2/ min x 2.44 = m3/h/m2
in. X2.54 = cm
ft X 0.305 = cm; Ib/in.2 X 0.0703 = kg/cm2
• Velocity of the flow through the screens.
• Screen aperture.
Studies have found average CSO screenings loads
varying from approximately 3.7xlO"9- 8.23xlO"8
cubic meters per liter (0.5 to 11 cubic feet per
million gallons), with peaking factors based on
hourly flows ranging from 2:1 to greater than 20:1.
Field studies performed in Canada and Europe have
revealed the following floatable removal efficiencies:
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• Samplings taken at different CSO outfalls
in Montreal, Canada showed that up to 80
percent of floatable material can be retained
by properly designed bar screens with 6.35
millimeters (0.25 inch) bar spacing.
• A year-long study was conducted in
Germany to determine the efficiency of an
externally fed rotary screen in controlling
downstream floatable pollution. The
screen, which was activated by high flows,
received 42 percent of the CSO discharge,
with no visible solids reported after
frequent inspections of river banks.
A pilot study in Great Britain tested a 4
mm ROMAG™ bar spaced "weir mount"
storm overflow screen. The average solids
loading before the screen was 2369 grams
per minute, while the solids concentration
after the screen was 3.5 grams per minute,
exhibiting a 98.5 percent deflection rate. In
a similar study, on 11 different occasions
during a 12 week period, average mass
reduction of floatables and solids material
greater than 6 millimeters (0.24 inches) was
98.5 percent.
OPERATION AND MAINTENANCE
Instrumentation and control of screens typically
includes some combination of the following:
Manual start/stop.
Automatic start/stop on timer.
Automatic start/stop on differential head.
Activation of mechanically cleaned screens is
triggered by remote sensing of flow into the
screenings channel, or the water level in the
screening channel.
As screens are subject to blinding from grease and
the "first flush" in a CSO event, the screen should
be kept clean to minimize headloss. Due to the
intermittent nature of CSOs it is important for the
screening units spray system to be working properly
to prevent solids from drying and sticking to the
screens, thus increasing headlosses. Fine screens
can be cleaned with high pressure water, steam, or
cleaning agents to maintain performance. Screening
systems should be regularly inspected to ensure that
chains and roller mechanisms are lubricated and
functioning. The trunnions associated with fine
screens are the least reliable component due to the
abusive forces they receive. The manufacturer's
operation and maintenance manual should be
consulted for the maintenance requirements and
schedules.
COSTS
The cost for CSO screens varies and depends on
such factors as:
• The size of the screen.
• The means of cleaning (manual or
automatic).
• The materials of construction (e.g.,
aluminum or stainless steel).
• The flow rate that the screen will be
required to physically treat.
• Whether the construction is new or
retrofit construction.
The costs included in Table 4 are presented as a
guide only and may not be applicable for all
conditions. Other costs may include costs for
handling and disposal of residual solids. EPA has
summarized this data in the Storm Water O&M Fact
Sheet "Handling and Disposal of Collected
Solids/Residuals from Storm Water and Sediment
Control Practices" (EPA 832-F-99-032).
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TABLE 4
COST SUMMARY OF SELECTED SCREENING ALTERNATIVES
Type of Screen
Climber Bar Screen (5 mm
plastic media rotary drum)
Rotary screen
Drum screen
Static screen
Microstrainer
with chemical addition
without chemical addition
ROMAG™ RSW 2X2
5X5
8X8
Project Location
Atlanta, GA
Belleville, Ont. (1)
Seattle, WA (2)
Syracuse, NY (3)a
Fort Wayne, IN (4)
Cleveland, OH (5)
Racine, Wl (4)
Syracuse, NY (3)a
Fort Wayne, IN (4)
Fort Wayne, IN (4)
Belleville, Ont (1)
Mount Clemens, Ml (6)
Philadelphia, PA (4)
Vendor Specified
Screening
Capacity
(MG/d)
375
300
200
1.8
5.4
7.2
25
5
18
25
50
100
200
3.9
10
18
18
0.75
5.3
7.5
1.0
7.4
7.4
5.9
40
100
Capital
Cost ($)
2,230,300
1,926,200
1,774,150
91,800
267,800
352,000
1,645,200
355,000
1,603,300
1,668,600
2,434,200
4,785,300
9,159,200
62,000
704,700
697,900
746,900
40,800
262,100
358,400
71,800
249,000
405,800
55,000b
105,000b
185,000b
Cost
($/MG/d
)
5,948
6,421
8,900
51,000
49,600
48,900
65,800
71,000
89,100
66,700
48,700
47,900
45,800
15,900
70,500
38,700
41,500
54,400
49,500
47,800
71,80
33,600
54,800
Annual O&M
($1,000 gal)
0.08
0.08
0.08
0.23
0.23
0.23
0.27
0.13
0.11
0.06
0.12
0.12
0.12
0.13
0.13
ENR = 5484
(a) Estimates not including supplemental pumping stations and appurtenances.
(b) Unit cost and does not include installation, freight or start-up assistance.
(1) Operational data for the Belleville Screening Project, Ontario Ministry of the Environment, August 6, 1976.
(2)EPA11023fdd03/70
(3) EPA 600/2-76-826
(4) EPA 60018-77-014. As provided in EPA 960018-77-014.
(5)EPA11023EY104/72
(6) EPA 670/2-75-010
Note: Conversion factors: MG/d x 0.0438 = m3/s; $/1,000 gal x 0.264 = $/m3
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REFERENCES
9.
Gavle, Barrel R., and David G. Mitchell,
1995. Innovative and Economical SSO
Treatment Utilizing Fine Screens and
Chlorination. Presented at the EPA
National Conference on Combined Sewer
Overflows, Washington, D.C.
Couture, M., J. Lamontagne, and B. Gagne,
John Meunier, Inc.; O. Dalkir, Cegeo
Technologies; and C. Marche, University of
Montreal; 1997. Abstract of a presentation
at the New York Water Environment
Association, New York, NY.
Metcalf and Eddy, 1991. Wastewater
Engineering - Treatment, Disposal, and
Reuse. McGraw-Hill, Inc., New York.
Northumbrian Water, LTD., 1994.
"Effectiveness of Romag™ Screen Test
Report." Engineering Department,
Stockton-on-Taes, Cleveland, U.K. TS17
OEQ.
Pisano, William C., 1995. "Comparative
Assessment: Vortex Separators, Rotary
Sieves, and "Combing" Screens for CSO
Floatable Control." Presented at the Water
Environment Federation Annual Conference,
Miami, FL.
U.S. EPA, 1977. Urban Storm Water
Management and Technology: Update and
User's Guide. EPA-960018-77-014.
U.S. EPA, 1993. Combined Sewer Overflow
Control Manual. EPA-625R-93-007.
Water Environment Federation and the
American Society of Civil Engineers, 1991.
Design ofMunicipal Wastewater Treatment
Plants, Volumes 1 & 2. WEF Manual of
Practice No. 8. ASCE Manual and Report
on Engineering Practice No. 76.
West et. al., 1990. Control and Treatment of
Combined Sewer Overflows, "Design of
Combined Sewer Overflows (CSO) Facilities
for the City of Atlanta, Georgia,"
Presented at the Water Pollution Control
Federation 63rd Annual Conference.
ADDITIONAL INFORMATION
Deerfield Wastewater Reclamation Facility
Jon Kaeding
Chief Operator and Foreman
850 Waukegan Rd.
Deerfield, IL 60015
City of Kingston, New York
Paul Van Wagen
Brinnier & Larios
Hasbrouck and Wilbur Avenues
Kingston, NY 12401
North Vernon Wastewater Department
Russell Vaught
Wastewater Treatment Plant Superintendent
725 N. Greensburg St.
North Vernon, IN 47265
Rahway Valley Sewerage Authority
Artie Wright
Plant Superintendent
1050 East Hazel wood Ave.
Rahway, NJ 07065
City of Savannah, Georgia
Don Atwell
City of Savannah Stormwater Management
P.O. Box 1027
Savannah, GA 31402
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
Excelence fri compliance through optimal technical soLtfbns
MUNICIPAL TECHNOLOGY BRANCH
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