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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