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ENVIRONMENTAL IMPACTS OF DESALINATION PLANT
INTAKES AND DISCHARGES AND HYDRAULIC PLANNING
Mehmet Berkün
İstanbul Kültür University Civil Engineering Department, Istanbul, Turkey
Ümmükülsüm Özel Akdemir
Giresun University Civil Engineering Department, Giresun, Turkey
Summary
Seawater intakes and discharges are integral parts of every seawater desalination plant. The
purpose of this paper is to provide an overview of potential impingement, entrainment and
discharge impacts associated with the operation of seawater intakes and discharges for seawater
desalination plants.
Introduction
Most of the desalination technologies rely on either distillation or membranes to separate salts
from the product water. Ultimately, the selection of a desalination process depends on site-
specific conditions, including the salt content of the water, economics, the quality of water
needed by the end user, and local engineering experience and skills.
Desalination, like any other major industrial process, has environmental impacts that must be
understood and mitigated. These include effects associated with the construction of the plant
and, especially, its long-term operation, including the effects of withdrawing large volumes of
brackish water from an aquifer or seawater and discharging large volumes of highly
concentrated brine. Indirect impacts associated with the substantial use of energy must also be
considered. Each desalination facility must be individually evaluated in the context of location,
plant design, and local environmental conditions.
The possible sources of impacts of desalination plants on coastal ecosystems can be sub-
divided into three general categories: (1) physical changes; (2) enrichment with organic
material, inorganic nutrients, heat or salt; and (3) introduction of toxic materials.
Physical changes in structure or dynamics of the coastal zone are mostly related to the
changes in hydrodynamics and erosion/sedimentation patterns resulting from the construction
of the plant and the required dredging of intake and outfall channels. Another important source
of impacts is the flow of water towards the intake structure. Millions of fish and invertebrates
may become trapped (impinged) on the plant intake screens each year or may pass through the
plant (entrainment), often with lethal consequences.
The discharge of desalination plants consists of slightly heated brine. In the area close to the
outlet the primary and secondary production, the species composition, biomass and nutrient
dynamics of aquatic ecosystems may change considerably as a result of changes in
physiological processes and behavior of individual species. Toxic materials discharged by the
plant can either be acutely toxic or cause chronic or sub-lethal effects. The toxins in
desalination plant discharges are heavy metals, dissolved from the plant's piping, and biocides
used to control biofouling on heat exchangers. The most severe impacts on the marine and
coastal environment are usually brought about by the destruction of support ecosystems
(Pacific Institute, 2013; Lattemann, 2008).
Impacts of Brine Intakes and Design Parameters
The principal types of seawater intake structures are given in Fig.1. Intake water design and
operation have environmental and ecological implications. Large marine organisms, such as
adult fish, invertebrates, birds, and even mammals, are killed on the intake screen
(impingement); organisms small enough to pass through the intake screens, such as plankton,
eggs, larvae, and some fish, are killed during processing of the salt water (entrainment). The
impinged and entrained organisms are then disposed of in the marine environment.
Decomposition of these organisms can reduce the oxygen content of the water near the
discharge point, creating additional stress on the marine environment. More specifically,
impingement and entrainment may adversely affect recruitment of juvenile fish and
invertebrates to parent or resident populations or may reduce breeding stocks of economically
valuable fishes below their compensation point resulting in reduced production and yield
(Brining et al., 1981 ; Edinger, 2000).
The magnitude of environmental impacts on marine organisms caused by impingement and
entrainment of seawater intakes is site specific and varies significantly from one project to
another. Open ocean intakes are typically equipped with coarse bar screens (Fig.1), which
typically have openings between the bars of 20 mm to 150 mm followed by smaller-size (fine)
screens with openings of 1 mm to 10 mm (Fig. 2), which preclude the majority of the adult and
juvenile marine organisms (fish, crabs, etc.) from entering the desalination plants. After
screening, the water is typically processed by finer filters for pretreatment of seawater, which
typically have sizes of the filtration media openings (pores) between 0.01 microns to 0.2
microns for membrane ultra- and micro-filters and 0.25 to 0.9 mm for granular media filters.
Since subsurface intakes collect source seawater through the ocean bottom and coastal
aquifer sediments (Fig.2), they are not expected to exert an impingement type of impact on the
marine species contained in the source seawater. However, the magnitude of potential
entrainment of marine species into the bottom sediments caused by continuous subsurface
intake operations is not well known and has not been systematically and scientifically studied
to date.
Fig. 1. Marine intake systems for seawater desalination plants.
(a) (b) (c)
Fig.2. a. Open intake, b.Wedgewire screen, c. Subsurface intake.
Open Intakes
Intakes in the littoral zone (i.e., the near-shore zone encompassed by low and high tide levels)
have the greatest potential to cause elevated impingement and entrainment impacts. The US
EPA considers extending intakes 125 meters outside of the littoral zone a good engineering
practice aimed at reduced impingement and entrainment. In addition, installing the intake to
depths where there is a lower concentration of living organisms (i.e., at least 20 meters) is also
expected to decrease environmental impacts associated with intake operations.
Impingement occurs when the intake through-screen velocity is so high that species such as
crab or fish cannot swim away and are retained against the screens. The US EPA has
determined that if the intake velocity is lower or equal to 0.15m/sn, the intake facility is
deemed to have met impingement mortality performance standards. Therefore, designing intake
screening facilities to always operate at or below this velocity would adequately address
impingement impacts.
Use of bar screens with a distance between the exclusion bars of no greater than 23cm is
recommended for preventing large organisms from entering the seawater intake
After entering the bar screen, the seawater has to pass through fine screens to prevent debris
from interfering with the downstream desalination plant treatment processes. The fine screen
mesh size is a very important design parameter and should be selected such that it is fitted to
the size of a majority of the larval organisms it is targeting to protect. Typically, the openings
of most fine screens are 9.5 mm or smaller because most adult and juvenile fish are larger than
10 mm in head size.
Wedgewire screens are cylindrical metal screens with trapezoidal-shaped “wedgewire” slots
with openings of 0.5 to 10 mm. They combine very low flow-through velocities, small slot size,
and naturally occurring high screen surface sweeping velocities to minimize impingement and
entrainment.
The conversion from the vertical flow to the horizontal flow reduces by between 80 - 90% the
suction of organisms. The quality of the water is greater and we avoid that the deposition of any
organic or inorganic waste material may enter into the seawater intake circuit.
The greatest possible distance is sought between the intake and the water surface for various
reasons: to achieve a water intake without floating particles and to avoid the entrance of sestonic
species (for example, jellyfish) that are located close to the water surface. Furthermore, with
distancing the intake from the surface, the amount of light is reduced, and accordingly certain
organisms will be unable to live on the surfaces of the structure. In the case of refrigeration circuits,
at a greater depth, lower the water temperature. The actions of the waves against the tower are,
furthermore, reduced when the depth of the intake tower is greater. The only disadvantage in
placing the intake tower at great depths is the cost thereof, given that it implies the installation of a
much longer intake pipeline, with the corresponding additional costs ofconstruction and the
operation (pumping) thereof.
A velocity cap is a configuration of the open intake structure that is designed to change the
main direction of water withdrawal from vertical to horizontal (Fig.3). This configuration is
beneficial for two main reasons: (1) it eliminates vertical vortices and avoids withdrawal from
the more productive aquatic habitat which usually is located closer to the surface of the water
body; and (2) it creates a horizontal velocity pattern which gives juvenile and adult fish an
indication for danger-most fish have receptors along the length of their bodies that sense
horizontal movement because in nature such movement is associated with unusual conditions.
Based on a US EPA technology efficacy assessment, velocity caps could provide over 50%
impingement reduction and can minimize entrainment and entrapment of marine species
between the inlet structure and the fine plant screens.
Fig.3. Velocity cap for entrainment reduction.
Subsurface Intakes
Subsurface intakes (vertical and horizontal directionally drilled wells, slant wells and
infiltration galleries) are considered a low-impact technology in terms of impingement and
entrainment. However, to date there are no studies that document the actual level of
entrainment reduction that can be achieved by these types of intakes. In addition, the potential
application of a subsurface intake is very site specific and highly dependent on the project size;
the coastal aquifer geology (aquifer soils, depth, transmissivity, water quality, capacity, etc.);
the intensity of the natural beach erosion in the vicinity of the intake site; and on many other
environmental and socioeconomic factors.
The percent of source seawater converted to fresh water during the desalination process is
known as plant recovery. Typically, seawater desalination plants are designed to recover 45 to
55% of the seawater collected by the intake.
Brine Discharge
Sea water desalination plants discharge a concentrated brine effluent into coastal waters. It is
estimated that for every 1 m3 of desalinated water, 2 m3 is generated as reject brine. Modern,
large capacity plants require submerged discharges, in form of a negatively buoyant jet, that
ensure a high dilution in order to minimize harmful impacts on the marine environment. The
impacts of a desalination plant discharge on the marine environment depend on the physical
and chemical properties of the desalination plant reject streams, and the susceptibility of coastal
ecosystems to these discharges depending on their hydrographical and biological features.
To avoid impacts from high salinity, the desalination plant brine can be pre-diluted with
seawater or power plant cooling water. To avoid impacts from high temperature, the outfall
should achieve maximum heat dissipation from the waste stream to the atmosphere before
entering the water body (e.g. by using cooling towers) and maximum dilution following
discharge. Negative impacts from chemicals can be minimized by treatment before discharge,
by substitution of hazardous substances, and by implementing alternative treatment options.
Especially biocides such as chlorine, which may acutely affect non-target organisms in the
discharge site, should be replaced or treated prior to discharge. Chlorine can be effectively
removed by different chemicals, such as sodium bisulfite as practiced in RO plants, while
sulfur dioxide and hydrogen peroxide have been suggested to treat thermal plant reject streams
(Khordagui, 1992). Filter backwash waters should be treated by sedimentation, dewatering and
land deposition, while cleaning solutions should be treated on-site in special treatment facilities
or discharged to a sanitary sewer system. The current options for reject brine management are
rather limited and have not achieved a practical solution to this environmental challenge. The
chemical reaction of reject brine with carbon dioxide is a new approach that promises to be
effective, economical and environmental friendly (El-Naas et al, 2010). After reaching their
terminal levels, the flows becomes primarily horizontal, may undergo an internal hydraulic
jump, and entrain further seawater, but eventually the turbulence collapses under its own
induced density stratification. All these processes are commonly referred to as near field
processes, i.e. determined by the discharge itself under parameters under the control of the
outfall designer. Beyond the near field, the plume drifts with the ocean currents and is diffused
by oceanic turbulence; this region is referred to as the far field.
Near field processes typically operate on time scales of minutes and over length scales of tens
of meters. Far field processes operate under time scales of hours to days and length scales of
tens of meters to kilometers.
Characteristics of Negatively Buoyant Diffuser Discharges
In order to effect high dilution of negatively buoyant effluent it will be necessary to discharge
it as high velocity jets through a diffuser (Fig.4). Because the jet is dense, it falls back to the
seabed where it then spreads as a density current. The highest seabed salinity occurs where the
centerline of the jet impacts the seabed. Additional dilution occurs beyond this point before the
flow collapses under the influence of the induced density stratification. The point where this
collapse occurs is the end of the near field, and the dilution at this point is the near field
dilution. The processes in the near field operate over small scales: distances of order tens of
meters and times of order minutes. Different flow regions are the ascending jet phase, terminal
rise current, and finally into the far field (Fig.5). The degree of dilution depends on the exit
velocity and jet diameter, the effluent and receiving water densities, and ambient currents. It
can be estimated in stagnant environments by semiempirical equations. The far field is located
farther away from the discharge point, where the brine becomes a gravity current that flows
down the seabed slope or horizontally in the case of a flat seabed. Mixing depends primarily on
ambient (oceanic) turbulence and is affected by currents, breaking waves, etc. The difference in
density between the spreading layer and receiving waters results in a density stratification that
reduces vertical mixing. Because of these effects, the rate of mixing is much slower than in the
near field. Flow and mixing characteristics are dominated by larger scales: distances of order
hundreds of meters to kilometers, and times of order hours (Bleninger, 2008).
The separate near and far field models must be coupled to predict overall plume behavior
(Fig.5). The main question is how and where to introduce the effluent flow and its pollutant
mass into the far field model.
Fig. 4. Negatively buoyant jet discharging into stagnant ambient with sloping bottom.
(a) (b)
Fig.5. Separate and coupled near and far field models.
Discussion and Conclusions
Intakes
One of the key environmental impacts of seawater reverse-osmosis desalination plants is
associated with their intakes, which generally withdraw two cubic meters of water for every
cubic meter of freshwater produced. Appropriately sited, designed, and operated seawater
desalination plant intakes can have minimal environmental impacts on the marine environment
and resources.
The majority of desalination plants extract water directly from the ocean through open water
intakes which have a direct impact on marine life. Fish and other larger marine organisms are
killed on the intake screens (impingement); organisms small enough to pass through the intake
screens, such as plankton, fish eggs, and larvae, are killed during processing of the salt water
(entrainment). The impacts of impingement and entrainment on the marine environment are not
fully understood but are likely to be species- and site-specific. Additionally, impingement and
entrainment rates, even for a single desalination plant, may be subject to daily, seasonal,
annual, and even decadal variation. Several operational, design, and technological measures are
available to reduce impingement and entrainment from open water intakes. These measures
generally fall into two broad categories: physical barriers and behavioral deterrents. Physical
barriers, e.g., mesh or wedgewire screens, block fish passage into the desalination plant and
may be coupled with some sort of fish collection and return system. Behavioral deterrents, e.g.
strobe lights or air bubble curtains, provide a signal to keep fish and other organisms away
from the intake area or prevent them from crossing a threshold where they may be impinged.
Additionally, subsurface intakes offer an alternative to open water intakes and can virtually
eliminate impingement and entrainment. The choice of intake design will ultimately be site-
specific. While some project developers contend that subsurface intakes are infeasible due to
their higher construction costs, desalination plants in many other countries have made use of
these systems, including beach wells and onshore and offshore infiltration galleries. Subsurface
intakes, however, may not be appropriate in all locations because their installation depends on
having the proper geology and sediment characteristics, such as sand and gravel, with a
sufficiently high porosity and transmissivity. However, with new drilling technologies, e.g.,
directional drilling, it may be possible to find a pocket with the right conditions surrounded by
generally unfavorable ones. When the appropriate site conditions are present, the advantages
are clear. These systems can virtually eliminate impingement and entrainment; they also
provide a level of pre-filtration that can reduce plant chemical and energy use and operating
costs over the long term.
Brine Discharge
Safe disposal of the concentrated brine produced by desalination plants presents a major
environmental challenge. All large coastal seawater desalination plants discharge brine into
oceans and estuaries. Brine, by definition, has a high salt concentration, and as a result, it is
denser than the waters into which it is discharged. Once discharged, brine tends to sink and
slowly spread along the ocean floor. Mixing along the ocean floor is usually much slower than
at the surface, thus inhibiting dilution and resulting in elevated salt concentrations near the
outfall. Diffusers can be placed on the discharge pipe to promote mixing. Brine can also be
diluted with effluent from a wastewater treatment plant or with cooling water from a power
plant or other industrial user, although these approaches have their own drawbacks. The
impacts of brine on the marine environment are largely unknown. The majority of studies
available focus on a limited number of species over short time periods and lack baseline data
which would allow a comparison to pre-operation conditions. The laboratory and field studies
that have been conducted to date, however, indicate the potential for acute and chronic toxicity
and changes to the community structures in marine environments. The ecological impacts of
brine discharge, however, vary widely and are a function of several factors, including the
characteristics of the brine, the discharge method, the rate of dilution and dispersal, and the
sensitivity of organisms. Despite the long history of seawater desalination plants operating in
some regions, data on their ecological impacts are limited.
References
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