xvEPA
United States
Environmental Protection
Agency
Off ice of Water
Washington,D.C,
EPA 832-F-99-037
September 1999
Combined Sewer Overflow
Technology Fact Sheet
Netting Systems for Floatables Control
DESCRIPTION
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.
APPLICABILITY
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
outfall.
While one of the configurations of the Netting
TrashTrap™ technology is applicable to most types
of outfalls, certain parameters should be considered
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Source: Fresh Creek Technologies, Inc., 1999.
FIGURE 1 CONFIGURATIONS OF THE NETTING TRASHTRAP™ SYSTEM.
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
crews.
Site topography should allow for placement of
prefabricated structures by a crane.
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• 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
surface.
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
waterways.
The system should be located where it is
protected from extremely strong currents,
severe wave action, and high winds.
ADVANTAGES AND DISADVANTAGES
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.
DESIGN CRITERIA
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
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
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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
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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.
OPERATION AND MAINTENANCE
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
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
$75,000.
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
TABLE 1 SUMMARY OF PERFORMANCE DATA FOR TRASHTRAP™ SYSTEM AT
SAYBROOK AND PEDDIE OUTFALLS
Saybrook
0.65 cm nets
1.3 cm nets
Peddie
0.65 cm nets
1.3 cm nets
Peak Flow
(m3/s)
6.29
7.01
28.07
24.08
Peak
Velocity
(mis)
3.40
3.30
0.94
1.25
Volume of
CSO
Discharge
(m3)
93,508
257,640
206,967
817,910
Total Weight
of Captured
Floatables
(kg)
781
2,074
4,629
8,760
Floatables
Caught
(kg)/1.720m3
Discharge
(Ibs/MG)
3,898
2,740
10,424
4,990
Removal
Efficiency
(%)
93
94
97
97
Source: Fresh Creek Technologies, Inc., 1999.
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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.
REFERENCES
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.
ADDITIONAL INFORMATION
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
Protection
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,
1995.
Fresh Creek Technologies, Inc.
Richard Turner
P.O.Box 1184
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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
1MTB
Excellence fn compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY BRANCH
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