·
Background
o
Impingement
§ Definition:
“Fish get stuck to intake screen due to high intake velocity”
§ Organisms
get caught through screens and are trapped
§ Usually
involves large aquatic and benthic organisms that are large enough to be caught
by intake screens i.e. adult fish, crabs etc.
o
Entrainment
§ Definition:
“Organisms that are smaller than the screen are drawn into the intake”
§ Organisms
go through treatment process
§ Involves
aquatic and benthic organisms that are too small to be caught by intake
screens.
o
Entrapment
§ Certain
intake valves have an offshore pumping station that is connected to an onshore
intake valve. The onshore intake valve has the retention screen, and intake
water is pumped from the offshore stations through conveyance pipelines
§ Describes
incidents where organisms are trapped in an offshore intake valve and cannot
fight the current and swim out.
§ If
organism travels through pipeline to the onshore intake valve it can be then
entrained or impinged
·
Causes of Impingement and Entrainment
o
Varies site to site, different methods of
intake.
o
Surface intake (Impingement)
§ Openings
on intake valve can range from outer layer bar screens containing 150mm
followed by inner fine screens with openings from 1mm-20mm.
§ Prevent
majority of fish population from entering
§ Coarse
outer screens are stationary, inner screens can be rotating.
§ Most
organisms removed by screening and downstream filtration.
o
Subsurface intake valves and intake wells
§ Water
filtered by ocean sediments above intake.
§ Sediment
barrier between organisms and intake valve
§ Organisms
living in bottom sediment can be effected (Entrainment)
o
Factors that influence Impingement and
Entrainment
§ High
intake velocity – Speed of water entering intake valve
§ Large
screen openings - How fine pre-intake screens are
§ High
intake flow – Amount of water volume collected
Solutions
·
Subsurface Intakes
o
Overview
§ Intake
water is naturally pretreated through passing through sandy ocean floor
§ Water
passing through bottom sentiments contains low levels of solids, silt, and
contaminants
§ Bottom
sentiments filter water before reaching intake
§ Unknown
whether organisms in bottom sentiment can be entrapped
§ Subsurface
intakes offer a low yield, low intake flow, low debris and entrapment
possibility
§ Invasive
installment, requires bottom sentiment to be dredged where the intake valve
will be placed
o
Solutions
§ Vertical
wells
·
Can supply water for small desalination plants
(1 MGD or less)
§ Horizontal
wells
·
Higher capacity than vertical wells
·
Radial Ranney-type wells have intakes pipes
radiating out of a center point
·
Directionally drilled collectors (HDD wells) fan
out under the seabed
·
Off-shore intakes
o
Overview
§ Contain
inlet structure, submerged pipe, onshore concrete structure
§ Inlet
structure placed underwater offshore and is connected to the onshore concrete
structure through pipe.
§ Passive
screens are stationary while active screens are rotating
§ Greater
risk for entrapment and impingement, however are more economically feasible and
less invasive to the environment
o
Intake system solutions
§ Passive
wedge-wire screens can be used
·
They eliminate the need for coarse and fine screens on the
onshore concrete structure
·
Use cylindrical screens with trapezoidal shaped
slots
·
Openings from .5 to 10mm
·
Form low through screen flow velocities that
minimizes impingement and entrainment
·
Also can be placed in locations with high
natural cross flow in the current exists.
·
High cross flow prevents organism to be impinged
on intake
·
Natural flow of the water prevents aquatic life
from being impinged
§ Active
traveling screens can be used
·
Placed after course bar screens
·
Screen segments move in rotational pattern
·
Rotating motion prevents impingement of fish
·
Creates cross flow which reduces the intake
force on surrounding fish
§ Collocated
Intakes
·
For desalination plants located near power
plants
·
Existing power plants require water intake for
cooling purposes
·
New Desalination plants can connect to the
discharge water from these plants for use as drinking water
·
Minimizes entrapment and impingement by using
one intake valve rather than two intake valves
·
Additional information
o
Most impingement and entrainment occurs in the
Littoral Zone
o
US EPA advises extending intakes 410 feet out of
the littoral zone, 1100 feet from shore
o
US EPA determines that low through-screen
velocities should be minimized and lower or equal to .5 feet per second to meet
impingement mortality performance standards
o
Coarse bar screen openings should be no larger
than 9inches
o
Small fine screen openings should be 3/8 inch or
less to prevent entrapment of adult and juvenile fish
Citations:
Brush B., Yager E., Rich F. (May
2011), Con Edison East River Generating
Station Aquatic Life Preservation [PowerPoint Slides]. Retrieved April 10,
2012.
WaterReuse Association (March 2011), Desalination Plant Intakes: Impingement and
Entrainment, Impacts and Solutions. Retrieved on April 1, 2012 from http://www.watereuse.org/sites/default/files/u8/IE_White_Paper.pdf.
WaterReuse
Association (June 2011), Overview of
Desalination Plant Intake Alternatives. Retrieved on April 5, 2012 from
http://www.watereuse.org/sites/default/files/u8/Intake_White_Paper.pdf
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