Thursday, April 19, 2012
Solutions Document
Subsurface
Intakes
Subsurface
intakes’ water is naturally pretreated by passing through the sandy ocean
floor. This water traveling through the bottom sediment contains low levels of
solids, silt, and contaminants that are filtered by the sediment. However,
because of the location, subsurface,
these intakes are not only invasive in the marine environment because of the
requirement that the bottom sediment be dredged, but also pose a large threat
of entrainment. Therefore, subsurface intakes should be used in areas with a
low risk of entrainment and are a solution to areas with a high level of
impingement. Some methods of intake offer a simple solution to this problem,
depending on the size of the treatment plant.
Vertical
and Horizontal Wells
Vertical
wells can be implemented to supply water for small desalination plants of 1
million gallons per day or less. For larger plants, horizontal wells that have
a higher capacity can be used. Radial Ranney wells of this type have intake
pipes radiating out of a center point to intake maximum amounts of water. The
wells also implement directionally drilled collectors that fan out under the
seabed. The method of using wells can effectively alleviate the risk of
impingement and lower the effects subsurface intakes have on marine organisms.
Offshore
Intakes
More
economically feasible and less environmentally invasive than subsurface intakes
are offshore intakes, which contain an inlet structure, placed underwater
offshore, connected to a submerged pipe, which extends to the onshore concrete
structure. While a more economically savvy than subsurface intakes, offshore
intakes have a much greater risk for entrapment and impingement. However,
several solutions to these problems are plausible.
Passive
Wedge-Wire Screens
Passive
wedge-wire screens minimize impingement and entrainment because of their unique
shape and eliminate the need for coarse and fine screens on the onshore
concrete structure. The wedge-wire screens use cylindrical screens with
trapezoidal shaped slots with openings from 0.5 mm to 10 mm, following data
that states that the openings should be 3/8 inch or less to prevent entrapment
of adult and juvenile fish. The screens also form low flow-through velocities
to minimize impingement and entrainment. In addition, passive wedge-wire
screens can be placed in locations where high natural cross flow in the current
exists; the natural flow of water will prevent aquatic life from being impinged
on the screen.
Active
Traveling Screens
To
reduce the intake force on the surrounding organisms, active traveling screens
can be implemented. They are placed after course bar screens, and the screens’
segments move in a rotational pattern. This rotating motion aids in preventing
the impingement of fish by creating a cross flow that lessens the pull on the
organisms around the area of intake.
Collocated
Intakes
In
situations where desalination plants are located near power plants, collocated
intakes can be used, reducing the number of intake valves needed for both
plant. Because existing power plants require water intake for cooling purposes,
new desalination plants can connect to the discharge water from these plants to
use in the treatment process. This allows two plants to use one intake valve,
therefore minimizing the amount of entrapment and impingement.
Causes Document
Impingement
Many
shortcomings are evident in the large-scale process of converting seawater into
fresh, clean water. Specifically, a detrimental outcome of the process is that
marine organisms can be killed during the process. Through one cause,
impingement, fish get stuck to the intake screen due to a high intake velocity.
Impingement affects large aquatic and benthic organisms that are large enough
to be caught by the intake screens, such as adult fish, crabs, and marine
birds. Surface intake valves often have 150 mm outer layer bar screens followed
by an inner fine screen with openings ranging from 1 mm to 20 mm. Although the
majority of fish are prevented from entering and are removed by the screening
and downstream filtration, many can get trapped against these screens due to
the speed of the water being pulled inward.
Entrainment
Another
cause of harm to marine organisms is entrainment, through which organisms that
are smaller than the screen are drawn into the intake, and end up going through
the treatment process. This inevitably results in the organisms’ deaths.
Entrainment affects smaller aquatic and benthic organisms that are not removed
by the finer screens. When treatment processes utilize subsurface intake valves
and intake wells, the water is filtered by ocean sediments above the intake,
and organisms living in bottom sediment can be affected.
Entrapment
Organisms
can also be harmed during entrapment. Certain intake valves have an offshore
pumping station that is connected to an onshore intake valve. The intake water
is pumped from offshore stations through conveyance pipelines. If an organism
travels through the pipeline to the onshore intake valve, it can be entrained
into or impinged on the onshore intake valve’s retention screen.
Influences
Ultimately,
three factors largely influence whether or not impingement and entrainment will
occur: intake velocity, screen opening sizes, and intake flow. If the speed of
the intake is too fast, organisms will become caught against the screens meant
to filter them out. Depending on the size of the screens, especially the finer
ones used in the process, organisms may be able to pass through and killed
during the treatment process. Finally, the larger the amount of water volume
collected continuously, the greater the chance is that organisms will be
negatively affected by the water treatment process.
Research Document
·
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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