United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F99-008
September 1999
v>EPA Combined Sewer Overflow
Technology Fact Sheet
Floatables Control
DESCRIPTION
This fact sheet describes various technologies for
controlling discharges of floatable materials from
combined sewer overflows (CSOs). Control of
floatable material is an important component of the
U.S. Environmental Protection Agency's (EPA's)
CSO Control Policy.
Combined sewer systems (CSSs) are wastewater
collection systems designed to carry both sanitary
sewage and storm water runoff in a single pipe to a
wastewater treatment plant. CSOs generally occur
during wet weather periods when the hydraulic
capacity of the CSS becomes overloaded.
Floatables control technologies are designed to
reduce or eliminate the visible solid waste that is
often present in CSO discharges.
Example floatables control technologies include:
Baffles
Screens and trash racks
Catch basin modifications
• Netting
• Containment booms
• Skimmer vessels
Baffles
Baffles are simple floatables control devices that are
typically installed at flow regulators within the CSS.
They consist of vertical steel plates or concrete
beams that extend from the top of the sewer to just
below the top of the regulating weir. During an
overflow event, floatables are retained by the baffles
while water passes under the baffles, over the
regulator, and into the receiving water body. When
the flow recedes below the bottom of the baffle,
floatable material is carried downstream to the
wastewater treatment plant. Figure 1 presents an
example of a typical baffle in a CSO regulator.
Discharge L _^;—,
FIGURE 1 BAFFLE IN CSO REGULATOR
Screens and Trash Racks
Screens and trash racks consist of a series of vertical
and horizontal bars or wires that trap floatables
while allowing water to pass through the openings
between the bars or wires. Screens can be installed
at select points within a CSS to capture floatables
and prevent their discharge in CSOs. Screens used
for CSO control include mechanically cleaned
permanent screens, static screens, traveling screens,
or drum screens. Screens can also be divided into
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three categories according to the size of floatable
material they are designed to capture. These are:
Bar screens ( > 2.5 centimeter [1 inch]
openings)
Coarse screens (0.5 - 2.5 centimeter [0.19 -
1 inch] openings)
Fine screens (0.01 - 0.5 centimeter [0.004 -
0.19 inch] openings)
The screens most commonly used to control CSOs
are trash racks (a type of bar screen primarily used
as an end-of-pipe control) and coarse screens. See
EPA's fact sheet "Screens" (EPA 832-F-99-027) for
additional information on screens for CSO control.
FIGURE 2 MODIFIED CATCH BASIN WITH
HOOD
Catch Basin Modifications
Catch basins are surface-level inlets to the sewer
system that are often used to allow runoff from
streets and lawns to enter the CSS. These basins are
often modified to prevent floatables from entering
the system. Inlet grates installed at the top of the
catch basins reduce the amount of street litter and
debris that enters the catch basin. If floatables enter
the basin through these grates, they can be collected
in colander-like structures called trash buckets
installed in the basin beneath the grate. These
structures retain floatables while letting water flow
through to the CSS.
Other catch basin modifications, such as hoods,
submerged outlets, and vortex valves, alter outlet
pipe conditions and keep floatables from entering
the CSS. Hoods are vertical cast iron baffles
installed in catch basins. Submerged outlets are
located below the elevation of the CSS and are
connected by a riser pipe. The original intent of
both hoods and submerged outlets was to serve as
gas traps, but they have also proven to be effective
barriers for retaining litter and other floatables
within catch basins. A vortex valve is a discharge
throttling device that is able to reduce the frequency
and the volume of CSO events. Vortex valves have
also proven capable of controlling floatables. A
typical modified catch basin with hood is presented
in Figure 2.
Netting
Two types of netting systems can be used to collect
floatables in a CSS: in-line netting, and floating
units.
In-line netting can be installed at strategic locations
throughout the CSS. The nets would be installed in
underground concrete vaults containing one or
more nylon mesh bags and a metal frame and guide
system to support the nets. The mesh netting is
sized according to the volume and types of
floatables targeted for capture. The CSO flow
carries the floatables into the nets for capture. Bags
are replaced after every storm event.
Floating units consist of an in-water containment
area that funnels CSO flow through a series of large
nylon mesh nets. Mesh size depends on the volume
and type of floatables expected at the site. This
system is passive and relies on the energy of the
overflow to carry the floatables to the nets.
However, nets must be located some distance from
the outfall (often 15 meters [50 feet] or more) to
allow floatables entrained in the turbulent CSO flow
to rise to the flow surface and be captured. The nets
are single use, and after an overflow, the nets are
typically removed and taken to a disposal area.
Additional information on one type of floating unit,
the TrashTrap™ system, is provided in a separate
fact sheet (EPA 832-F-99-024).
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Containment Booms
Booms are containment systems that use specially
fabricated floatation structures with suspended
curtains designed to capture buoyant materials.
Booms can also be designed to absorb oils and
grease. They are typically anchored to a shoreline
structure and the bottom, and they can be located
downstream of one or more outfalls. Booms are
sized based upon the expected volume of floatables
released during a design-storm event. After a storm
event, material captured in the boom can be
removed manually, or with a vacuum truck or a
skimmer vessel.
Skimmer Vessels
Skimmer vessels are specially-designed boats used
to collect floating debris, including material
contained behind booms. Skimmer vessels almost
always require companion equipment, including a
shore conveyor for offloading, a truck for disposal,
and a trailer for land transport. Floatables are
brought on board the skimmer vessel with moving
screens on a conveyor belt system, or by lowering
large nets into the water. Skimmer vessels are used
primarily in lakes, harbors, and bays. Figure 3
shows a schematic diagram of a skimmer vessel.
APPLICABILITY
Baffles
The effectiveness of baffles depends on the design
of the flow regulator. Baffles should be considered
if new regulators are being designed and
constructed. Baffles can also be retrofitted to
existing regulators in many collection systems. In
some situations, baffles may restrict access to
regulating structures, making maintenance more
difficult. However, baffles are considered a low
maintenance alternative, and require only occasional
cleaning to remove debris and reduce odors.
The city of Columbus, GA, uses baffles for CSO
control in parts of its sewer system. The city has
installed concrete baffles at the CSO outfalls for
each of its 12 new diversion structures. These
baffles retain floatables during high flows, and then
release the floatables to the treatment plant through
Control Cab
ELEVATION
^- Propulsion
Unit (Typ.)
PLAN.
FIGURE 9. TYPICAL SKIMMER VESSEL
PLAN AND ELEVATION
FIGURE 3 TYPICAL SKIMMER VESSEL
a vortex box during low flows. The city has found
that the baffles effectively remove most floatable
material from the overflow, with minimal costs.
Screens and Trash Racks
Trash racks and coarse screens can be used
effectively for CSO control because they capture a
significant amount of aesthetically undesirable
floating debris and trash contained in the CSS.
Removal efficiencies are tied closely to the design
size, and can range from 25-90 percent of the total
solids. Finer screens have higher removal
efficiencies, but are more susceptible to clogging
and tearing and may require maintenance after each
overflow event. The effectiveness of screening
units is reduced significantly by the presence of oil
and grease in the flow.
Catch Basin Modifications
The ability of catch basins to control floatables
ultimately depends upon their regular maintenance
and cleaning. While most catch basins need to be
cleaned only once per year, catch basins that have
been modified to trap floatables may require
cleaning and maintenance after each storm event.
The physical placement of the inlet grates is
important to their efficiency. For example,
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installing basin gratings with openings parallel to
flow direction will optimize the flow hydraulics.
However, grates of this type may be hazardous to
bicyclists.
Hoods or submerged outlets can be included as key
components in standard catch basin design. Hoods
can also be retrofitted in catch basins in many
situations.
Containment Booms
Containment boom efficiency can range from 60 to
90 percent. Site conditions such as receiving water
velocity and CSO flow velocity should be
considered when evaluating containment boom
design, placement, and anchoring. Although booms
will float and therefore accommodate water level
fluctuations, high river velocities and winds may
dislodge them. Booms cannot be employed during
the winter in waters that are subject to freezing.
Maintenance requirements for booms are moderate
relative to other floatables control technologies.
Containment booms must be cleaned after storm
events, and this can be costly. Special consideration
should be given to booms located in highly visible
public areas. Booms potentially create unsightly
conditions near the outfalls, and may therefore be
inappropriate in areas with waterfront development.
In addition, resource and regulatory agencies may
have concerns about the presence of floating booms
within the natural boundaries of waterways.
Netting
The netting system most appropriate for a given
situation depends on three sizing requirements: the
absolute peak flow expected, the maximum flow
velocity, and the volume of floatable material per
million gallons of CSO. In-line units are widely
applicable, and can be adapted to most combined
systems. Floating units are more suitable for use in
lakes, estuaries, and tidal waters at outfalls or close
to the water level. Since floating units collect
floatables after they enter the receiving water, they
can potentially create unsightly conditions near the
outfall, and may be inappropriate in areas with
waterfront development. In addition, resource and
regulatory agencies may have concerns about
floating units that lie within the natural boundaries
of waterways.
Netting systems require a high amount of
maintenance. The frequency with which bags must
be changed depends on site-specific conditions,
including the frequency and volume of overflows,
the volume of floatables, and the overall water
quality. In some places, bags may need to be
changed as frequently as 30 to 60 times per year. In
many instances bags are changed to remove
captured waste before it gets old and moldy, rather
than because the bags are full. It can take 30
minutes to two hours to service each unit,
depending on the number and size of the netting
bags. Field test results indicate netting can provide
removal efficiencies of up to 90 percent for
floatables.
Skimmer Vessels
Skimmer vessels are a very visible floatables control
method that are easy for the general public to
understand and support. Skimmer vessels are
typically used to clean broad areas of open water.
As a result, the floatable debris and litter collected
comes from a variety of sources including CSOs,
separate storm water systems, and upstream sources.
Financial assistance from sources other than the
owner and operator of the CSS may be warranted.
Although the U.S. Coast Guard does not require a
specific license in order to operate a skimmer
vessel, operation requires considerable skill. Most
skimmers require a crew of two, and usually do not
fare well in high winds, in the wakes of other
vessels, or in strong currents. Ice impedes
navigation and the collection of floatables. It is also
important to be aware of minimum depth and
clearance height requirements specific to each
vessel.
PERFORMANCE
Baffles
Columbus, GA uses baffles as part of their CSO
control system. However, their specific
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performance is not tracked. See below for a
discussion of the overall performance of Columbus'
CSO controls.
Bar Screens and Trash Racks
Columbus, GA uses bar screens and trash racks in
both of its treatment plants to mitigate the effects of
CSOs. Columbus' Uptown Park CSO facility,
which receives flow from the Cooks Branch line of
its combined sewer system, has enhanced its CSO
system as part of a CSO demonstration project for
EPA. The Uptown Park CSO facility uses bar
screens and trash racks to filter CSOs, as well as to
retain floatables released to the plant after storms
from baffles in other parts of the system. During
storms, all flow (including excess flow) in the
Cooks Branch line is filtered through the bar screens
and trash racks located at the Uptown Park CSO
facility, and overflows go directly to the receiving
water body. Floatables from the Cooks Branch line
are retained on the bar screens and trash racks and
are taken by conveyor to a dumpster for
characterization and disposal. In addition,
floatables originally retained during storms by
baffles in other parts of the system are eventually
released to the Uptown Park CSO facility, where
they are removed by the trash racks.
Both the bar screens and the trash racks at the
Columbus facility have 3.8 centimeter (1.5 inch)
openings. The system effectively captures large
solids and floatables, and the facility meets its
permit requirements for floatables and fecal
coliform. The screens and trash racks are self-
cleaning, and so maintenance is minimal. Annual
maintenance costs are below $15,000 for the entire
sewer system.
Catch Basin Modifications
As part of a city-wide floatables study in New York
City, the amount of litter released to the CSS was
compared for catch basins with and without hoods,
under identical flow regimes. The hooded catch
basins retained approximately 85 percent of the
litter delivered to the CSS, while unhooded catch
basins captured only 30 percent of the litter.
Netting
End-of-pipe netting was installed by New York City
at the Fresh Creek outfall, a tributary to Jamaica
Bay. Fresh Creek is one of the city's largest CSO
outfalls. The floating netting system, using a total
of eight bags, removed an average of 295 kilograms
(650 pounds) of floatables for every 37.85 million
liters (10 million gallons) of CSO filtered. The net
is designed to trap material 1.3 centimeters (0.5
inches) or greater in size, and has a capture
efficiency of 90 to 95 percent.
In Kentucky, the Louisville and Jefferson County
Metropolitan Sewer District (MSD) installed three
in-line netting units in their CSS at a total capital
cost of $75,500. City personnel changed the bags
after two to three overflow events. They found that
even though the bags were not always full after
three events, they become clogged with dried
materials and needed to be replaced. The MSD
estimates that the operation and maintenance costs
associated with changing the bag(s) in a unit
approaches $900, including personnel, machinery,
and disposal fees. Additionally, the local landfill
does not accept the bags until they have been
dewatered, which requires a suitable location for the
bags to drain overnight.
Containment Booms
A four-boom containment system was tested by
New York City during a two-year pilot study in
Jamaica Bay, NY. Floatables were contained by the
booms and collected using a skimmer vessel. An
assessment of the effectiveness of the booms was
made by measuring the quantities of floatable
material in the waters and on the shorelines of the
Bay before and after installation of the booms.
Results showed substantial improvements from the
pre-boom conditions, and indicated that
containment booms provide a floatables retention
efficiency of approximately 75 percent. During the
two-year test period, more than 40,640 kilograms
(40 tons) of trash were removed from the
containment area.
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Skimmer Vessels
Catch Basin Modifications
The Anacostia Floatable Debris Removal Program
in Washington, DC ,employs five full-time workers
and uses two skimmer boats to maintain more than
ten miles of shoreline. In 1998, the skimmer vessels
removed 406,400 kilograms (400 tons) of debris
from Washington waterways. Overall, the program
has removed 3.02 million kilograms (2,975 tons) of
debris from waterways since 1992. The program
has enj oyed marked success and is hoping to expand
in the future. Two additional personnel were hired
into the program in late 1998, and the program plans
to purchase another skimmer vessel in 1999.
COSTS
Baffles
Capital costs for baffles depend on the size of the
structure and the design storm. Sewers that are
retrofitted with baffles typically employ stainless
steel or aluminum curtains at an average cost of less
than $10,000 each. Concrete baffles can be
considerably less expensive, but are usually
reserved for use in new construction. In complex
situations where proper installation requires
substantial structural alterations, costs may exceed
$25,000 per outfall.
Screens and Trash Racks
Screen installation, operation, and maintenance
costs vary widely, and depend upon the flow rate
and the type of screen selected. Construction costs
for screen systems include costs for installing a
specialized housing unit for the screen within the
pipe. This may require costly structural alterations
to regulators and outfalls. In general, screens and
trash racks have moderate maintenance
requirements. All units need regular inspection and
cleaning. However, these costs are generally low,
especially for self-cleaning units. For example,
Columbus, GA, spent approximately $15,000 on
maintaining their CSO abatement system in 1998.
These costs included costs for cleaning screens at
the Cooks Branch treatment facility, as well as
maintaining other CSO treatment works in other
parts of the system.
The costs associated with different modifications
vary greatly. Trash buckets can cost as little as
$100, while installing vortex valves in catch basins
can cost as much as $700 per basin. Additionally,
the modified basins require regular maintenance at
$50 to $150 per unit, including equipment and labor
costs.
Netting
Typical purchase, construction, and installation
costs for commercially available netting systems
range from $25,000 to $150,000 per site. Operating
and maintenance costs are estimated at $1,000 per
changeout, including $ 100 for the disposable netting
bags.
Containment Booms
The installed cost of a containment boom can run as
high as $100,000 to $150,000 per site. Capital costs
for the 4-boom system pilot-tested in New York
City (excluding engineering costs) were $240,000,
while O&M costs were $5,000 over eighteen
months. This figure does not include expenses
incurred when removing floatables from behind the
boom. This was done using a skimmer vessel, and
costs for skimmer vessels are discussed elsewhere
in this fact sheet. Disposal costs for removing
floatables are heavily dependent on the type of
system used for removal, the boom's accessibility,
the travel time between locations, and fuel use.
Skimmer Vessels
Skimmer vessels can range in cost from $300,000 to
almost $700,000, including shore conveyors for off-
loading, and a trailer for transporting the vessel
from site to site. Annual operating costs average
$75,000 to $125,000 per boat and include vessel
maintenance and repair, crew wages, fuel,
insurance, and land disposal fees for the collected
matter. Operating costs for the eighteen month
New York City study were $280,000. Of these
operating costs, $26,000 in costs related to
offloading collected floatables were recorded in 9
months.
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Skimmer vessels can be expensive to maintain. For
example, while the skimmer vessel program in
Washington, D.C. has been very successful, the
vessel itself has required a great deal of mechanical
maintenance. Maintaining the vessel requires
mechanics who are knowledgeable in repairing
boats.
REFERENCES
1. Columbus Water Works, 1999. Cliff
Arnett, Senior Vice President of
Operations, Columbus Water Works,
personal communication with Parsons
Engineering Science, Inc.
2. Heath, G. R. CSO Floatables Control for
the Greater Boston, Massachusetts, Area.
Metcalf & Eddy, Inc.
3. Municipal Environmental Research
Laboratory, September, 1981. A Planning
and Design Guidebook for Combined Sewer
Overflow Control and Treatment.
4. New York City Department of
Environmental Protection, 1995. City-Wide
Floatables Study. Bureau of Environmental
Engineering, Division of Water Quality
Improvement, prepared by HydroQual, Inc.
5. Washington, D.C. Water and Sewer
Authority, 1999. Dunbar Regis, Chief,
Inspection and Maintenance Section, Bureau
of Sewer Services, Washington D.C. Water
and Sewer Authority, personal
communication with Parsons Engineering
Science, Inc.
ADDITIONAL INFORMATION
City of Columbus, GA
Cliff Arnett
Columbus Water Works
1501 13th Avenue
Columbus, GA 31902
Fresh Creek Technologies, Inc.
Richard Turner
P.O.Box 1184
West Caldwell, NJ 07007-1184
Jefferson County Metro Sewer District
Dan Knowles
700 West Liberty Street
Louisville, KY 40203
Massachusetts Water Resources Authority
David Kubiak
Senior Program Manager, CSOs
Charlestown Navy Yard
100 First Avenue
Boston, MA 02129
City of New York, New York
Eric Delva
Bureau of Clean Water
New York City Department of Environmental
Protection
96-05 Horace Harding Express Way
Corona, NY 11368
City of Washington, DC
Regis Dunbar
Bureau of Sewer Services
125 O Street, SE
Washington, DC 20003
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
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