United States
                       Environmental Protection
                       Off ice of Water
EPA 832-F-99-037
September 1999
Combined Sewer  Overflow
Technology  Fact  Sheet
Netting  Systems for Floatables Control

This fact sheet  describes  netting  systems for
controlling discharges of floatable materials from
combined sewer  overflows  (CSOs).  Control of
floatable material is an important component of
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 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. The Netting TrashTrap™ system is a
modular floatables collection system located at the
CSO outfall.   It  uses the passive energy  of the
effluent stream to drive the floatable materials into
disposable mesh bags. These bags are suspended
horizontally  in the CSO flow stream  within  a
support structure. Construction methodology and
method of installation at the outfall is determined
on a site by site basis.  The demonstration projects
in Newark, NJ, and New York City used floating
Netting TrashTrap™ systems attached to the ends
of the outfalls.  Since then, end-of-pipe and in-line
configurations  have been  developed   and
implemented. Figure 1 represents drawings of the
floating, end-of-pipe, and in-line configurations of
the Netting TrashTrap™ systems.

The standard nets used in the system are designed
to hold up to 0.7 cubic meters (25 cubic feet) of
floatables and  a  weight of 227 kilograms (500
pounds) each.

For the floating  units, the  effluent  stream  and
entrained floatables are directed into the bags by
                     two floating boom and curtains which run from the
                     front corners of the pontoon to either side of the
                     outfall where they attach to a vertical piling with a
                     roller mechanism or a  shoreline support.  This
                     design allows the boom to float and accommodate
                     changes in the water level.  The extended curtains
                     are weighted to conform  to  the water bottom.
                     Curtain depth is determined by the maximum high
                     water level expected at the site.

                     Modifications to the outfall may include: adding
                     structural support, attaching structural struts and
                     strut anchor support, and adding foundations.


                     The Netting TrashTrap™ technology has proven to
                     be applicable to  a wide  spectrum  of weather
                     conditions, including freezing  conditions. In one
                     case during the demonstration projects,  the water
                     surface froze, preventing  the bags from being
                     changed. During this period, CSO events occurred,
                     and floatables were transported beneath the ice and
                     into the bags.  The entire system, including the
                     bags, remained intact and held the floatables for 16
                     weeks until the ice thawed and the bags could be
                     changed.   High velocity nets, which  are  more
                     expensive than standard nets, are necessary at
                     outfalls where CSO discharge velocities exceed
                     2 meters per second (7 feet per second) because of
                     the risk of tearing. The netting design is typically
                     based on the peak flow and pounds of floatables per
                     million gallons projected to be discharged from the

                     While one of the configurations of the Netting
                     TrashTrap™ technology is applicable to most types
                     of outfalls, certain parameters should be considered

Source:  Fresh Creek Technologies, Inc., 1999.

when implementing such a system. The following
criteria may help in  determining the most cost
effective locations for a Netting TrashTrap™.

•  The location  should be accessible by a road
   capable  of  accommodating medium  sized
trucks, and easily accessible to maintenance

Site topography should allow for placement of
prefabricated structures by a crane.

•  For the floating units, the water should be a
   minimum of 0.9 meters  (3 feet) at low tide,
   although the system  may sit on the bottom

   The area around the net support structures and
   pontoon arrangement should be  cleared of
   protruding rocks or debris.

   The system  should not  be  situated where it
   would  interfere  with  heavily   navigated

   The system  should be  located where it  is
   protected from  extremely  strong  currents,
   severe wave action, and high winds.


The Netting TrashTrap™ technology can have a
significant  positive  impact  on  the  aquatic
environment if several cautions are observed.  The
impact of construction activity should be minimized
wherever  possible.   Nets  should be  changed
regularly  to prevent  odors  and  other  aesthetic
impacts. Netting structures should be off-limits to
the public in order to prevent health hazards from
contact with floatables.

While  the  Netting  TrashTrap™  is   designed
specifically to remove floatables and is not intended
to remove  other  pollutants,  preliminary  data
suggests that it does remove other pollutants (such
as suspended solids),  although this has not been
quantified. This may limit its applicability in cases
where  implementation  of  the  nine   minimum
controls entails removal of non-floatable objects.
In addition, if the unit is suspended in the receiving
water, it may be difficult to provide disinfection at
the  same  location.     However,  the  in-line
configuration  is  capable of  working with  a
disinfection system.   The need for removal of
submerged solids for disinfection to meet water
quality  standards  should be  evaluated during
implementation of the nine minimum controls and
during long-term CSO control planning studies.
The  Netting  TrashTrap™  is  supplied  as  a
prefabricated unit which is delivered to the site and
typically can be assembled and installed in less than
two days.  The system is fabricated from type 316
stainless steel. The floatation is provided by U.S.
Coast Guard-approved closed-cell foam injected
into the side chambers of the system.  The hanging
curtain is made of polyvinyl chloride (PVC) and is
reinforced with  polyester filament  fabric.   The
curtain weighs 930 grams (30 ounces) per 84 square
meters (1 square yard), has a minimum thickness of
30 mm  and a tear strength of 992 kilograms (450
pounds). The seams of the curtain are heat-welded,
and steel grommets are used to reinforce the points
of attachment. The standard net mesh material is a
knotless synthetic weave  that  produces  a bar
strength of  165  kilograms (75  pounds) with a
square mesh aperture.  The netting system design is
flexible and can be modified according to site-
specific conditions. The most common systems
utilize  two  nets but  can   be  expanded  to
accommodate  larger  outfall  areas.   The  entire
system  is  designed and manufactured  to have a
minimum life expectancy of at least 20 years.

Nets with captured floatables can be removed from
the systems by several methods. Nets can be lifted
by a boom  truck crane and  placed in a carting
container for proper disposal.  Alternatively nets
could be  floated out  through the  back of the
pontoon structure and picked  up by a skimmer or
work boat.  The City of New York has installed a
rail mounted hoist and cart to facilitate placement
of full nets in an adjacent dumpster  at the Fresh
Creek site.  The  crane used for changing the nets
should  be  capable of lifting 2204  kilograms
(1000 pounds) and have the reach to access nets
from outfalls on  a site specific basis. Depending
upon the potential for vandalism, fencing may be
needed to secure the area.


Performance  data  collected for  the Netting
TrashTrap™ are based on demonstration proj ects at
one location in New York and two locations in New
Jersey.  The goals of the demonstration projects
were: to evaluate the  technology for eliminating
floatables during CSO events; to define conditions
under which the technology should perform; and to

obtain capital  and operation and  maintenance
(O&M) cost data.

New York City Demonstration Project

The first Netting TrashTrap™ system was installed
at the Fresh Creek outfall, a tributary to Jamaica
Bay in New York City.  The outfall drains 880
hectares (2170 acres) and is one of the city's largest.
During the two years of this study, flow volumes at
the site ranged from 3,785 cubic meters (1 MG) to
264,950 cubic meters (70 MG). Flow rate averaged
4.25 cubic meters per second (150 cubic feet per
second) and ranged from 0.6 to 40 cubic meters per
second (21 cubic feet per second to 1412 cubic feet
per second).   While monitoring the  collection
system, 20 CSO events resulted in the discharge of
1.6 million cubic meters (423 MG) of wastewater.
The system was installed on two of the four outfall
barrel,  and  consisted  of  two  8  foot  long,
1.3  centimeter  (0.5 inch) mesh bags attached  to
each outfall barrel. Therefore, approximately half
of the CSO volume passed through the netting
system. From April to November, 1993, roughly
3,855 kilograms (8,500 pounds) of floatables were
removed using the netting system.  An average
295 kilograms  (650 pounds) of floatables  were
removed per 37,850 cubic meters (10 MG) of CSO
discharge.    System  efficiency  for  capturing
floatables was  determined by using a secondary
boom  with  an  attached curtain  to capture all
fugitive floatables.  Total floatable discharge per
event was determined by adding fugitive floatable
weight to the weight of the floatables captured by
the system. The efficiency of the nets ranged from
90 to 95 percent for floatable capture. All captured
floatables and used nets were disposed as municipal
solid waste by the City of New York. The system
was subsequently expanded to eight bags spanning
the entire outfall.

Newark, NJ, Demonstration Project

In the  City of Newark,  Netting  TrashTrap™
structures were installed at two sites. The Peddie
outfall by Newark Airport  drains  635 hectares
(1570  acres).   It  has four tide  gate  structures
measuring 1.8 meters (6 feet) by 2.4 meters (8 feet)
to accommodate  the  166  MGD  CSO design
capacity.  The outfall's four-net system includes a
"curtain" under   the  front of the unit  and two
additional curtains at the sides from the headwall of
the outfall to each corner of the unit.  These curtains
help funnel floatables into the unit, but during high
flows (flows exceeding 1.06 meters per second (3.5
feet per second)), they "lift" from  the bottom to
prevent damage  to the unit.  By lifting from the
bottom of the unit, this feature makes it unlikely
that floatables will escape.  Both 0.65  centimeter
(0.25  inch) aperture nets and 1.3 centimeter (0.5
inch)  aperture nets were tested at the site over a
variety of flows.

The  other  demonstration location was at  the
Saybrook outfall  on the Passaic River, which drains
116 hectares (287 acres.)  As with the Peddie
system, the two  net system used at this site was
designed to lift during intense storm events  when
discharge flow velocity exceeded 2.13  meters per
second (7 feet per second). Again, 0.65 centimeter
(0.25 inch) aperture and 1.3 centimeter (0.25 inch)
aperture  nets were  evaluated at the  site over a
variety of flow conditions.  System efficiency for
capturing floatables was  determined in a similar
way to the New York demonstration project.

Data collected on both 0.65 centimeter (0.25 inch)
aperture and 1.3 centimeter (0.5 inch) aperture nets
from  both the Saybrook and Peddie  outfalls in
Newark, New Jersey, provided similar floatable
removal efficiencies.

The 0.65 centimeter (0.25 inch) aperture nets at the
Saybrook outfall were monitored during 10 CSO
events.   During  this  time, the  0.65  centimeter
(0.25  inch)  nets screened  93,508 cubic meters
(3,302,456 cubic feet) of CSO discharge and were
93 percent efficient at removing 781 kilograms
(1,723 pounds) of floatables. The 0.65 centimeter
(0.25  inch)  aperture  nets removed  an average
316 kilograms per 37,850 cubic meters (697 pounds
per 10 MG) of CSO discharge. The 1.3 centimeter
(0.5 inch) aperture nets experienced 18 CSO events
at  Saybrook  treating  352,668   cubic  meters
(12,452,714 cubic feet) of CSO discharge. The nets
were   94   percent   effective  and   removed
2,074 kilogram (4,562 pounds) of floatables from
the  outfall  during  CSO  discharges.     The
1.3 centimeter (0.5 inch) aperture nets removed
floatables at  an  average  289 kilograms  per

37,850 cubic meters (637 pounds per 10 MG) of
CSO discharge.  The maximum  flow  and peak
velocity experienced by both nets can be found in
Table 1.

The four 0.65 centimeter (0.25 inch) aperture nets
at the  Peddie  outfall  captured 4,619 kilograms
(10,184 pounds) during  10  CSO events.  The
0.65  centimeter (0.25 inch) nets had a 97  percent
floatables   removal  efficiency   and  removed
4,619 kilograms (10,184 pounds) of floatables from
206,967 cubic meters (7,309,504 cubic feet) of
CSO discharge. The nets removed floatables at an
average of 844 kilograms per 37,850 cubic meters
(1,862 pounds per 10 MG) of CSO discharge. The
1.3 centimeter (0.5  inch)  aperture nets  were
97 percent effective and removed 8,742 kilograms
(19,273 pounds) of floatables from 817,910 cubic
meters (28,886,223 cubic feet) of CSO discharge.
The  nets   removed  floatables  at  an  average
404 kilograms per 37,850 cubic meter (891 pounds
per 10 MG) of CSO discharge. Peak flow and peak
velocity for these nets, as well as a summary of the
data  collected  during the study can be found in
Table 1.


Maintenance  for  a  Netting  TrashTrap™  is
dependent upon the number and frequency  of CSO
events and the capacity of the system.  Maintenance
includes net removal, installation of new nets, trash
cleanup, and boom/curtain inspections. Nets should
be changed and disposed at approved facilities, on
average, after three CSO events but never less than
once a month.  Inspection of booms and nets is
important during the initial weeks of installation of
the system to ensure that all equipment functions
according to design. Any adjustments to the netting
structure should be made during this time.  Net
inspections may be required after intense storms to
check for damage.


Costs for planning  and  construction of a Netting
TrashTrap™  system  are likely  to  range from
$75,000 to $300,000, depending on site conditions.
A typical two- net  system with 1.4 cubic meters
(50  cubic  feet)  capacity,  handling   about
227 kilograms (500 pounds) of damp weight per net
and spanning 4.5 meters (15 feet) of CSO outfall,
has an estimated capital cost of $125,000.  This
includes the cost of fabrication  and  installation,
which can take three to six months. The land-based
materials  handling   system   (trash
collection/disposal) associated with the system has
an additional estimated capital cost of $25,000 to

The  cost for a  sewage  treatment plant staff to
operate  and maintain  a typical two-net  system
during the Newark demonstration project  was
estimated at $1,500 per month.  The cost for  nets
                          SAYBROOK AND PEDDIE OUTFALLS

0.65 cm nets
1.3 cm nets
0.65 cm nets
1.3 cm nets
Peak Flow




Volume of


Total Weight
of Captured






 Source: Fresh Creek Technologies, Inc., 1999.

and labor for a two-net system, excluding floatables
disposal costs, was approximately $570  per CSO
event.  Replacement nets designed to capture the
high velocity  discharge at Saybrook totaled $200
for two nets ($100 per net). Note that O&M at the
Peddie  and   Saybrook  site  occurred  under
demonstration conditions; therefore, compared to
normal operating conditions, more hours were spent
on flow monitoring, data collection, miscellaneous
adjustments, and repositioning equipment, as well
as net changes after every CSO event, in order to
obtain site-specific data.

The  total  cost per CSO event at Peddie under
demonstration conditions (nets  and labor)  was
$850. The replacement cost for four nets required
at the Peddie site totaled $380 ($95 per net).

Disposal  costs  for captured materials  and  nets
should also be considered when calculating O&M
costs. The quantity of captured floatables will vary
from site  to site; during the  13-month Newark
demonstration project, for example, approximately
2,800 kilograms (6,172 pounds) were captured at
Saybrook  and   over   13,361   kilograms
(29,455 pounds) at Peddie. Used nets, which are
disposed   with  the  captured  floatables,  add
approximately 28 Ibs wet weight per net. The city
of Newark paid $109.85 per ton for disposal of nets
and captured floatables, or approximately $2,270
over the 13-month demonstration project.


1. Forndran, A., etal., 1994. "TrappingFloatables
   in a Combined Sewer Outfall Netting System."
   In A Global Perspective for Reducing CSOs:
   Balancing  Technologies,  Costs,  and Water
   Quality.    Proceedings  Water  Environment
   Federation Specialty Conference.

2. HydroQual, Inc., 1993. City-Wide Floatables
   Study. Draft  Report  to  New  York  City
   Department of Environmental Protection, New
   York, NY.
4.  Parsons  Engineering  Science,  1994.  CSO
   Floatable  Control Demonstration  Project.
   Revised Interim Report. City of Newark.

5.  Parsons  Engineering  Science, 1995.   Final
   Report, CSO Floatable Control Demonstration
   Project.  City of Newark, NJ.

6.  Sudol, F.,  1995.  "Newark Nets Floatables".
   Water Environment and Technology.

1.  Turner, R., 1995.  "Floatables Control and the
   New  USEPA CSO Policy."  In A Global
   Perspective for reducing CSOs:  Balancing
   Technologies,  Costs,  and  Water  Quality.
   Proceedings Water Environment Federation
   Specialty Conference.


City of Madison, Indiana
Wayne Turner
1213 West 1st Street
Madison, IN 47250

City of Nashville, Tennessee
Mrs. Lyn Fontana, P.E.
Metropolitan Government
1600 2nd Avenue North 4th Floor
Nashville, TN  37208

City of New York, New York
Eric Delva
Bureau of Clean Water
New  York  City Department of Environmental
96-05 Horace Harding Express Way
Corona, NY 11368

City of Philadelphia, Pennsylvania
Gene Foster
Fox and Roberts Streets
Philadelphia, PA 19129
3.  Newark Floatable Flow  Monitoring Report,
Fresh Creek Technologies, Inc.
Richard Turner
P.O.Box 1184

West Caldwell, NJ 07007-1184

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
                                                         Excellence fn compliance through optimal technical solutions
                                                         MUNICIPAL TECHNOLOGY BRANCH