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Applicability and sizing of a Netting TrashTrap®
unit to an outfall site
Q. What types of outfalls are applicable to Netting
TrashTrap® solutions?
A. With three models and scalable unit sizing, Netting
TrashTrap® technology can be applied
effectively to virtually all CSOs and stormwater outfalls.. Units
are designed to withstand flow velocities up to 5 feet per second
without a velocity brake. Site specific parameters must be considered.
For further guidance, refer to "Technology Fact Sheet - Netting
Systems for Floatables Control", USEPA, Office of Water, Washington, DC September, 1999
Q. Are there any CSO outfalls which are not suitable
for Netting TrashTrap® technology?
A. Only in unusual site specific conditions is one of
the three Netting TrashTrap® models
not applicable. For example, (1) unusually high velocities that
cannot be reduced (by velocity brake) to less than five feet per
second, and (2) unusually high volumes that would necessitate a
system too large to fit at the site. For further guidance, refer
to "Technology Fact Sheet - Netting
Systems for Floatables Control", USEPA, Office of Water, Washington, DC September, 1999
Q. How do I determine the size of a Netting TrashTrap®
unit which will be required for a particular outfall?
A. The sizing of any of these versions requires knowledge
of the peak flow volume (Q) anticipated, peak velocity (V) impacting
the Netting TrashTrap® structure, and the concentration
of trash and floatables in the CSO discharge. In most cases, for the peak
Q and V, basic conveyance capacity of the pipes can be used for
sizing without the need for modeling and monitoring. Trash and floatables
content of the CSO can be estimated based on the monitoring data
from systems we have already installed and from inspection of the
drainage basin of the outfall. In all of the cases we have examined
to date, the trash and floatables content of the CSO has not been the dominant
sizing parameter.
Q. Are there any competitive technologies addressing
trash and floatables control?
A. Yes, there are several other technologies or approaches.
For more information, refer to "Combined Sewer Overflow BMP
Fact Sheet – Floatables Control", USEPA, Office of Water, Washington,
DC 20460, September, 1999.
Q. Are there odors or visual aesthetic conditions to
be considered?
A. Floating units collect trash and floatables after they have
entered the receiving water and therefore can be subject to odors
or unsightly conditions. The floating model may not be applicable if the
outfall is close to waterfront development. In Line systems are
enclosed and therefore have little or no odor or displeasing aesthetics.
End of Pipe systems may be open on top (but covered with grating)
and therefore may experience odors if the flows do not scour the
system clean.
Q. How efficient is trash and floatables capture using Netting
TrashTrap® technology?
A. Field tests sponsored by U.S. EPA indicate that Netting
TrashTrap® technology can provide
removal efficiencies of greater than 90% for trash and floatables when properly
operated and maintained. No other system or approach has similar
data.
Q. Is Netting TrashTrap®
technology effective for outfalls other than CSOs?
A. Yes. CSOs have been emphasized because of the cleanup
of pollution from Combined Sewer Systems is a national priority.
However, Netting TrashTrap® systems are also
fully effective in removing trash and floatables from any point-source pipe
discharge outfall, particularly stormwater overflows.
Q. Does Netting TrashTrap®
technology provide any benefits in addition to removal of trash and floatables?
A. Yes. In addition to capturing trash and floatables the system
also drains a significant amount of pollutants which otherwise would
flow through to the receiving waterbody. Also, the floatable collection
process naturally provides a much more consistent and treatable
liquid effluent flow.
Q. How does the cost of Netting TrashTrap®
technology compare with other floatable solutions?
A. Netting TrashTrap® systems generally provide lower installed
cost, shorter lead times, quicker installation, and lower servicing
costs than other trash and floatables solutions.
Q. What is the amount of head loss in a typical unit
installation?
A: For End of Pipe and In-Line units, a maximum of four
inches, typically much less. For Floating units one inch or less.
Q. Are your head loss calculations supported by modeling
or simulation?
A. Yes.
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