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# Desalination Plant Intakes
Impingement and Entrainment Impacts and Solutions

White Paper

March 2011; Revised June 2011Seawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 1 

### WATEREUSE ASSOCIATION
DESALINATION COMMITTEE

Desalination Plant Intakes – Impingement and Entrainme nt Impacts and Solutions

White Paper

INTRODUCTION Seawater intakes are an integral part of every seawater desalinati on plant. The purpose of this white paper is to provide an ov erview of potential impingement and entrainment (I&E) impacts associated with the operation of open ocean in takes for seawater desalination plants and to
discuss alternative solutions for efficient and cost effective I&E reduc tion. For information on alternative intakes for seawater desalination plan ts, refer to the WateReuse Association’s white paper titled “Overview of Desalinat ion Plant Intake Alternatives.”

WHAT IS IMPINGEMEN T AND ENTRAINMENT? As with any other natural surface water source currently used for fresh water supply around the globe, seawater contains aquatic organisms (algae, plankton, fish, bacteria, etc.). Impingement occurs when organisms sufficiently large to a void going through the screen s are trapped against them by the force of the flowing source water – i. e., algae, plankton and bacteria are not exposed to impingement. On the other hand entrainment occurs when marine organisms enter the desalination plant intake, are drawn into the intake system, and pass through to the treatment
facilities.

Impingement typically involves adult aquatic orga nisms (fish, crabs, etc.) that are large enough to actually be retained by the intake screens, while entrainment mainly affects aquatic species small enough to pass through the particular size a nd shape of intake screen mesh. Impingement and entrainment of aquatic organisms are not unique to open intakes of seawater desalination plants only. Conventional open freshwater intakes from surface water sources (i.e., rivers, lakes, estuaries) may also cause measurable impingement and entrainment.

A third term, “ entrapment, ” is then used when describing imp acts associated with offshore intake
structures connected to an on-shore intake sc reen and pump station vi a long conveyance pipeline or tunnel. Organisms that enter the offshore in take and cannot swim ba ck out of it are often referred to as entrapped 1 . Such marine organisms could either be impinged on the intake screens or entrained if they pass through the screen s and enter the downstream facilities of the desalination plant.

1 http://www.waterlink-international.com/ download/whitepaper_uploadfile_21.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 2 Attention to seawater intake im pingement and entrainment issues is partially prompted by the Section 316(b) of the 1972 Clean Water Act that regulates cooli ng water intake of the steam electric industry by the environmental scrutiny a ssociated with the pub lic review process of desalination projects in California.

MAGNITUDE OF ENVIRONMENTAL IMPACTS The magnitude of environmental impacts on marine organisms caused by impingement and entrainment of seawater intakes is site specif ic and varies significantly from one project to another. Open ocean intakes are typically equipp ed with coarse bar screens (Figure 1), which typically have openings between the bars of 20 mm to 150 mm fo llowed by smaller-size (“fine”) screens with openings of 1 mm to 10 mm (Figur e 2), which preclude the majority of the adult and juvenile marine organisms (fish, crabs, et c.) from entering the desalination plants. While coarse screens are always stati onary, fine screens could be two types – stationary (passive) and periodically moving (i.e., rotati ng) screens. Figure 2 depicts a 3-mm rotating fine screen. Most marine organisms collected with the source seawater used for pr oduction of desalinated water are removed by screening and downstream filtration befo re this seawater enters the reverse osmosis desalination membranes for salt separation. After screening, the water is typically processed by finer filters for pretreat ment 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.

Source: GHD

Figure 1 – Intake Bar ScreenSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 3 Source: Water Globe Consulting

Figure 2 – Fine Intake Screen

By comparison, intake wells and infiltration galle ries pre-filter aquatic life through the ocean bottom sediments. In this case, the ocean botto m provides a natural separation barrier for adult and juvenile marine organisms. Since subsurf ace intakes collect source seawater through the ocean bottom and coastal aquifer sediments (see Fi gure 3), they are not e xpected to exert an impingement type of impact on th e marine species contained in the source seawater. However, the magnitude of potential entr ainment 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. An ongoing side-by- side study of the I&E e ffects of a subsurface intake and an open ocean intake equipped with a passive wedgewire screen at the West Basin Municipal Water District’s desalin ation demonstration plant is exp ected to provide more detailed information on this topic 2

.

2 http://www.watereuse.org/node/978Seawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 4 Source: Kennedy/Jenks Consultants

Figure 3 – Subsurface Intake Schematic

A comprehensive multi-year impingement and entrainment assessment study of the open ocean intakes of 19 power generation plants using se awater for once-through c ooling completed by the
California State Water Resources Control Boar d in 2010 provides important insight into the magnitude of these intake-related environmental impacts 3 . Based on this study, the estimated total average annual impingement of fish cause d by the seawater intake s varied between 0.31 pounds (lbs.) per million gallons a day (MGD) of collected seawater (Diablo Canyon Power Plant) and 52.29 lbs./MGD (Harbor Generating Sta tion); and for all 19 plants it averaged 6.63 lbs./MGD. Taking into considerat ion that this amount is the total annual impact, the average daily impingement rate is estimated to be 0.018 lbs./MGD of intake flow (6.63 lbs./365 days = 0.018 lbs./MGD).

Using the California State Water Resources Cont rol Board impingement and entrainment study results as a baseline, for a la rge desalination plant of 50 MGD production capacity collecting 110 MGD of intake flow, the daily impingement impact is projected to be 2 lbs. per day (0.018 lbs./MGD x 110 MGD = 2 lbs./day). This impingement impact is less than the daily food intake
of one pelican – up to 4.0 lbs./day 4

. The comparison illustrates the fact that the impingement impact of seawater desalination plants with open ocean intakes is not si gnificant and would not have measurable impact on natura l aquatic resources (Figure 4).

3 http://www.watereuse.org/sit es/default/files/u8/Quote 3.pdf

4 http://www.sandiegozoo.org/animalbytes/t-pelican.htmlSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 5 Source: US Fish and Wildlife Service

Figure 4 – Average Daily Desalination Intake Impingement Impact Is Less than the Daily
Fish Intake of One Pelican

The California State Water Resources Control Board report mentioned earlier also gives a baseline for assessment of the entrainment impact of seawater intakes. The study indicates that the magnitude of such annual impact on larval fi sh can vary in a wide range – from 0.08 million (MM)/MGD (Contra Costa Power Plant) to 5.8 MM/MGD (Encina Power Plant) and illustrates the fact that the entrainment imp act is very site-specific.

As per the same report, the aver age annual entrainment is estimated at 2.14 million of fish larvae per MGD of intake flow. Prorated for a 110 M GD intake of a 50 MGD seawater desalination plant, this annual entr ainment impact is 235.4 MM of larval fish/yr. While this number seems large, based on expert evaluati on and research, large entrainm ent numbers do not necessarily equate to a measurable impact to adult fish populations because of the enormous amount of eggs,
fish larvae and other zo oplankton in seawater 5 . Due to the large natural attrition of larval fish, very few larval fish actually develop to juvenile and adult stages in the natural environment (see Figure 5) 6 . The majority of larvae are lost to predati on, exposure to destructive forces of nature such as wind and wave action, and the inabilit y to find appropriately-s ized pray during the

5 http://www.swrcb.ca.gov/wa ter_issues/programs/ocean /cwa316/docs/workshop_oa kland2005/pres_tenera.pdf

6 http://www.scwd2desal.org/documents/Prese ntations/Nov_10_2010/02_Tenera_nov10_web.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 6 critical period of their developmen t (i.e., after their yolk s ack is empty). All of these forces have several orders of magnitude higher impact on fish populations than seawater intakes.

Source: Tenera Environmental

Figure 5 – Typical Reproduction and Sur vival of Larval Producing Organisms

For example, a single female halibut produces as many as 50 million eggs per year for as long as 20 years, or one billion eggs over a lifetime 7 . In simple terms, the annual entrainment impact of

one 50 MGD desalination plant would be comparab le to the annual bio-productivity of five
adult female halibut fish (i.e., the “environmental impact” which five fishermen can cause with their daily halibut catch quota of one fish each).

The environmental impact of desalination plant operations should be assessed in the context of the environmental impacts of wate r supply alternatives that may be used instead of desalination. Desalination projects are typically driven by the limited availability of alternative lower-cost water supply resources such as groundwater or fres h surface water (rivers, lakes, etc.). However, damaging long-term environmental impacts may al so result from continued over-depletion of those conventional water supplies, including inte r-basin water transfer s. For example, overpumping of fresh water aquifers over the years in a number of areas worldwide (i.e., the San Francisco Bay Delta in Northern California; wetl ands in the Tampa Bay region of Florida; and fresh water aquifers, and rivers and lakes in northern Israel and Spain, which supply water to sustain agricultural and urban centers in the southe rn regions of these count ries), has resulted in substantial environmental impacts to the traditional fresh water resources in these regions. One

7 http://www.watereuse.org/site s/default/files/u8/Quote%2 07%20-%20Presentation.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 7 such specific example of dramatic environmental im pact is the reduction of the habitat of delta smelt as a result of over-pumping caused by Califo rnia State Water Project’s intake facilities. 8

Such long-term fresh water transfers have affect ed the ecological stability in the fresh water habitats to the extent that the long-term conti nuation of current water su pply practices may result in significant and irreversible damage of the eco systems of traditional fresh water supply sources and even the intrusion of saline water into the freshwater aquifers, such as the case in Salinas Valley, Monterey County, California. In such instances, the environmental impacts of construction and operation of new seawater desa lination projects should be weighed against the environmentally damaging consequences from th e continued expansion of the existing freshwater supply practices.

A responsible approach to water supply manage ment must ensure that sustainable and droughtproof local supplies are availabl e, and long-term reliance on c onventional water supply sources (i.e., surface water, groundwater) is reconsidered in favor of a well-balanced and diversified water supply portfolio which combines surf ace water, groundwater, r ecycled water, water conservation, and desalination. For example, this type of reliability-driven, balanced water supply program is currently implemented by West Basin Municipal Water District ( www.westbasin.org ), the Texas Water Development Bo ard, Tampa Bay Water, and other agencies in the United States.

IMPINGEMENT AND ENT RAINMENT SOLUTIONS While impingement and entrainment associated with seawater intake opera tions are not expected to create biologically significant impacts under most circumstances, best available site

, design

, technology, and when needed, mitigation measures, are prudent for minimizing loss of marine life and maintaining the pr oductivity and vitality of the aquatic environment in the vicinity of the
intake.

Prudent Open Intake Design
Installation of Intake Inlet Struct ure Outside of the Littoral Zone Intakes in the littoral zone (i.e., the near-sho re 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 me ters (410 feet) outside of the littoral zone a good engineering practice aimed at reduced impingement and entrainment 9 . According to the Office of Naval Research, the littoral zone extends 600 feet from the shore 10 . Thus, intakes with an inlet structure
located at least 1100 feet from the shore could result in reduced envir onmental impacts. In addition, installing the intake to depths where th ere is a lower concentra tion of living organisms

8 http://www.science.calwater.ca.gov/pdf/eco_restor_delta_smelt.pdf

9 http://water.epa.gov/lawsregs/lawsguidance/cwa/316b/phase1/upload/2009_04_02_316b_phase1_support_contents. pdf

10 http://www.onr.n avy.mil/focus/ocean/regions/littoralzone1.htmSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 8 (i.e., at least 20 meters) is also expected to decrease environmental impacts associated with intake operations.

Low Through-Screen Velocity Impingement occurs when the intake through-screen ve locity is so high that species such as crab or fish cannot swim away and are retained agains t the screens. The US EPA has determined that if the intake velocity is lower or equal to 0.5 feet per second (fps), the intake facility is deemed to have met impingement mortality performance standards 11 . Therefore, designing intake screening facilities to always operate at or below this velocity would adequately address impingement impacts.

Small-Size Bar Screen Openings
Use of bar screens with a distance between the exclusion bars of no great er than 9 inches is recommended for preventing large organi sms from entering the seawater intake 12

.

Suitable Fine Screen Mesh Size After entering the bar screen, the seawater has to pass through fine scre ens to prevent debris from interfering with the downstream desalinati on plant treatment processes. The fine screen mesh size is a very important design parameter and s hould be selected such that it is fitted to the size of a majority of the larval organisms it is targeting to protect. T ypically, the openings of most fine screens are 3/8 inch (9.5 mm) or smaller because most adult and juvenile fish are larger
than 10 mm in head size.

Design Enhancements for Collection of Minimum Intake Flow Membrane reverse osmosis desalination plants typi cally collect seawater fo r one or more of the following three purposes: (1) to use it as a source water for fresh water production; (2) to apply it as a backwash water for the source water pretreat ment system; and (3) to pre-dilute concentrate generated during the salt separation process down to environmentally safe salinity levels before it is discharged to the ocean.

The percent of source se awater converted to fresh water dur ing the desalination process is known as plant recovery. Typically, seaw ater desalination plants are desi gned to recover 45 to 55% of the seawater collected by the intake. Designing the desalination plant to operate closer to the upper limits of recovery (i.e., 50 to 55%) woul d require collecting less water and therefore, would reduce impingement and entrainment associ ated with seawater intake operations. Longterm testing completed by the Affordable Desali nation Collaboration, aime d to identify the most suitable operational conditions for low-energy SWRO desalination, indi cates that optimum

11 http://edocket.access.gpo .gov/cfr_2008/ju lqtr/pdf/40cfr125.94.pdf

12 http://www.watereuse.org/sites/defau lt/files/u8/Quote%2012%20-%20Policy.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 9 energy consumption is achieved at a membrane fl ux of 9.0 gallons per square feet per day (gfd) and RO system recovery of 48% 13

.

Most desalination plants collect 4 to 10% of add itional water to wash their pretreatment filtration systems and discharge the spent filter backwash water back to the ocean. A design approach which may allow reducing this water use significantly is treatment and reuse of the backwash water. Such a backwash treatment and reuse appr oach has cost implications but is a prudent design practice aimed at reducing overall plant seaw ater intake flow and associated impingement
and entrainment.

Collecting additional seawater for concentrate pre-dilution may be needed when existing wastewater intake or power plant outfalls are used for concentrate discharge and the existing outfall volume is not sufficient to produce adequa te dilution of the saline discharge. This additional flow intake could be eliminated by de signing facilities for storing concentrate during periods of low outfall flows when adequate dilution is not available, or by installing a discharge diffuser system which allows enhancing con centrate dissipation into the ambient marine
environment without additional dilution.

If the desalination plant production capacity has to vary diurnall y, the design and installation of variable frequency driv es on the intake pumps could also allow decreasing impingement and entrainment of the plant intake by closely matc hing collected source seawater volume to the plant production needs.

Use of Low-Impact Intake Technologies Impingement and entrainment of marine orga nisms could be minimized by using various subsurface and open intake technologies. Currentl y, there are no federal and state regulations which specifically define requirements for reduc tion of impingement and entrainment caused by desalination plant intakes. However, the US EP A Section 316(b) of the Clean Water Act federal regulations have stipulated na tional performance standards fo r intake impacts from power generation plants which require 80 to 95% reduction of impingeme nt and 60 to 90% reduction of entrainment as compared to those caused by uncontrolled intake conditions 14 . Technologies that can meet these impingement and entrainment pe rformance standards are defined by US EPA as Best Technology Available (BTA).

Subsurface Intakes Subsurface intakes (vertical and horizontal directi onally 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 documen t the actual level of entrainment reduction that

13 http://www.affordabledesal.com/h ome/news/ADC%20Completes%20Profile%20of%20SWRO%203-28-08.pdf

14 http://edocket.access.gpo .gov/cfr_2008/ju lqtr/pdf/40cfr125.94.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 10 can be achieved by these types of intakes. In a ddition, the potential app lication of a subsurface intake is very site specific and highly dependent on the project size; the coastal aquifer geology (aquifer soils, depth, transmissivity, water qualit y, capacity, etc.); the intensity of the natural beach erosion in the vicinity of the intake site; and on many other environmental and socioeconomic factors.

Because optimal conditions for subsurface intakes ar e often impossible to find in the vicinity of the desalination plant site, the appl ication of this type of intake technology to date worldwide has been limited to plants of relatively small cap acity. As indicated in WateReuse Association’s White Paper titled “Overview of Desalination Plant Intake Alternatives,” 15 the largest seawater desalination facility with a subsurface intake in operation at present is th e first 17 MGD phase of the 34 MGD San Pedro Del Pinatar (C artagena) desalination plant in Spain. For this project, sitespecific hydrogeological constraints made it impo ssible to use intake wells for plant expansion, and the second 17 MGD phase of this project was constructed with an open intake.

Ongoing long-term studies of innovative subsur face intakes in Long Beach and Dana Point, California are expected to provide comprehe nsive data that would allow completing a scientifically-based analysis of the viability and performance be nefits of subsurface intakes for larger-size applications. The tested subsurface intake technol ogies are currently under evaluation and do not yet have established performance, reliability, and environm ental track records.

Wedgewire Screen Intakes Wedgewire screens are cylindrical metal screens w ith trapezoidal-shaped “wedgewire” slots with openings of 0.5 to 10 mm. They combine very lo w flow-through velocities, small slot size, and naturally occurring high screen surface sweepi ng velocities to minimize impingement and entrainment. This is the only open intake tech nology approved by US EPA as Best Technology Available. Such approval, however, is granted provided that sufficient ambient conditions exist to promote cleaning of the screen face; the through screen design intake velocity is 0.5 feet/sec or less; and the slot size is appropriate for the size of eggs, larvae, and juve niles of any fish and shellfish to be protected at the plant intake site 16

.

Wedgewire screens are designed to be placed in a water body where significant prevailing ambient cross flow current velocities ( ≥ 1 fps) exist. This high cr oss-flow velocity allows organisms that would otherwise be impinged on the wedgewire screen intake to be carried away
with the flow.

15 http://www.watereuse.org/node/1340

16 http://edocket.access.gpo .gov/cfr_2006/ju lqtr/pdf/40cfr125.99.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 11 An integral part of a typical we dgewire screen system is an ai r burst back-flush system, which directs a charge of compressed ai r to each screen unit to blow-o ff debris back into the water body, where they are carried away from the screen unit by the ambient cross-flow currents.

Figure 6 presents a schematic of the wedgewire screen intake used at the 40 MGD Beckton desalination plant in London, England. The Beckt on desalination plant is equipped with seven (7) 3-mm wedgewire screens installed on the suc tion pipe of each of th e plant intake pumps. Total screen length is 11.55 ft. (3500 mm) and th e screen diameter is 3.6 ft. (1100 mm). The plant intake is under significant influence of ti dal exchange of river water and seawater. To capture the ebb tide and minimize entrainment, the in take adjusts as it also targets lower salinity

waters.

Source: Acciona Agua

Figure 6 – Wedgewire Screen Intake of Beckton Desalination Plant

An I&E study of a cylindrical wedgewire screen (Figure 7) was conducted over a 13-month period from April 2009 through May 2010 by Tenera Environmental for a seawater desalination project currently under de velopment by the City of Santa Cr uz Water Department and Soquel
Creek Water District in California 17 . The intake for the full-scale desalination project would be designed to collect of up to 7.0 MGD of source seawater in order to produce an average of 2.5 MGD of fresh drinking water.

17 http://www.scwd2des al.org/documents/Reports/Open_Ocean_In take_Effects/Open%20O cean%20Intake%20Effec ts%20Study%20Final%20Dec%202010.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 12 The tested wedgewire screen ha d 2.0 mm of slot openings and was constructed of copper-nickel alloy. The diameter of the screen was 8-5/8 inch es; the overall screen le ngth was 35 inches; and the outer flange was 6-5/8 inches. Seawater was pumped from a depth of 15 to 20 feet beneath
the sea surface.

Source: Tenera Environmental

Figure 7 – Wedgewire Screen Used in Santa Cruz I&E Study

The results of this comprehe nsive I&E study indicate that:

• No endangered, threatened, or li sted species were entrained.

• At an average intake velocity of 0.33 fps, the screen was successful in completely eliminating impingement.

• The wedgewire prevented entrainment of adult and juvenile fish species.

• The greatest projected proportiona l mortality that could be attributed to the screen operation for the top 80% of the fish larvae in the source water ar ea at 7.0 MGD intake
flow was 0.06%.

• The greatest projected proportiona l mortality for the caridean shrimp and cancrid crab
larvae in the source water area for 7.0 MGD intake flow was 0.02%.

• The extremely low proportional losses of fis h, shrimp and crab populations indicate that the full-scale wedgewire intake screen ope ration at 7.0 MGD will not cause significantSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 13 environmental impact considering that the natural mortality rates of these species are over
99.9%.

• The absolute numbers of larvae projected to be entrained annually due to the collection of 7.0 MGD of source seawater for desalination pl ant operation are a very small fraction of the reproductive output of the source popula tions of marine organisms inhabiting the intake area. For example, for the white croa ker – a fish frequently encountered in the intake area – the potential larval losses (fecundity losses) are 3.6 million larvae, which are comparable to the total lifetime fecundity (re productive yield) of a single female fish.

To study the behavioral responses of differe nt species swimming n ear or contacting the wedgewire screens, two underwater video cameras were installed to view the surface of the screens during operation. One camera was oriented to provide a lengthwise view of the screen’s surface while a second camera videotaped a top view of the screen’s surface. Videos were displayed and recorded to a digital video record er (DVR) when the intake pump was operated. Figures 8, 9 and 10 present still photographs from the impingement video. The video footage shows that all fish, amphipods, and shrimps that encountered the screen were able to free themselves after contacting the screen. The video observations allo w the conclusion that operating the wedgewire screen intake at a th rough-screen velocity of 0.33 fps eliminates impingement.

Source: Tenera Environmental

Figure 8 – Rockfish Sitting on ScreenSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 14 Source: Tenera Environmental

Figure 9 – Shrimps Swimming Near Screen

Source: Tenera Environmental

Figure 10 – School of Juvenile Rockfish Swimming Near ScreenSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 15 A wedgewire screen intake I&E study has also been completed at the Marin Municipal Water District SWRO pilot plant near San Francisco, CA 18 . The results of this study indicated that no impingement was observed and the larval entrainment losses were found to be less than 0.2% of the total larval population in the intake area of the desalination plant. The use of cylindrical wedgewire screens is also curren tly being tested at the West Basin Municipal Water District
seawater desalination demonstr ation plant in California.

Offshore Intake Velocity Cap 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 (see Fi gure 11). This configuration is beneficial for two main reasons: (1) it eliminates vertical vortices and a voids withdrawal from the more productive aquatic habitat which usually is located closer to th e surface of the water body; and (2) it creates a horizontal velocity patte rn 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 move ment is associated with unusual conditions.
This natural indication combined with maintainin g low through-screen velo city (0.5 fps or less) provides fish in the area of the intake ample warning and opportunity to swim away from the
intake.

The velocity cap intake configuration has a long tr ack record and is widely used worldwide. This is the original configuration of many power plant intakes in Sout hern California and of all new large seawater desalination plants in Australia, Spain, and Israel constructed over the last five years. Based on a US EPA technology efficacy asse ssment, velocity caps could provide over 50% impingement reduction and can minimize entr ainment and entrapment of marine species
between the inlet structure and the fine plant screens 19

.

18 http://www.marinwater.org/controller?action=menuclick&id=446

19 http://www.epa.gov/waterscience/316b/phase1/technical/ch5.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 16 Source: US EPA

Figure 11 – Velocity Cap for Entrainment Reduction

As indicated previously, open intakes may also exhibit an entrapment effect – fish and other marine organisms that are drawn into the offs hore conduit cannot return back to the open ocean because they are stranded between the intake inle t structure and the downstream fine screens. The use of velocity caps and low velocity throu gh both the coarse screen of the intake structure
and the downstream fine screens coul d reduce this entrapment effect.

Other Impingement and Entrainment Reduction Technologies
In addition to the intake tec hnologies described above, there ar e a number of other technologies
which have been demonstrated to reduce the impingement and entrainm ent of open intake operations, mainly based on testing at existing power plant intakes. Table 1 below provides a summary of such technologies. Not all of the t echnologies listed in the table can meet the US EPA performance targets under all conditions and circumstances or deliver both impingement and entrainment benefits. However, if needed, th ese technologies could be used in synergistic combination to achieve project-specific enviro nmental impact reduction targets. Some of the technologies listed in Table 1 (such as velocity caps, acoustic barriers, wedgewire screens and fine mesh travelling screens) have found full- scale applications for recently implemented seawater desalination projects. In mid-2011, th e WateReuse Research Foundation initiated a research study to document and evaluate the imp ingement and entrainment reduction efficiency
of these and other technol ogies (WateReuse-10-04).Seawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 17 Table 1 – Potential Open Intake Impingemen t and Entrainment Reduction Technologies

Type of
I&E
Reduction

Measures How Do They Work? Technologies Impact Reduction Potential

Impingement Entrainment

Physical
Barriers By Blocking Fish Passage and Reducing Intake Velocity • Wedgewire Screens

• Fine Mesh Screens

• Microscreening Systems

• Barrier Nets

• Aquatic Filter Barriers Yes Yes

Collection
&

Return Systems Equipment is Installed on
Fine Screens for Fish
Collection and Return to
the Ocean • Ristroph Travelling
Screens

• Fine Mesh Travelling
Screens Yes No

Diversion Systems Devices Which Divert Fish
from the Screens and Direct
Back to the Ocean • Angled Screens with

Louvers

• Inclined Screens Yes Yes

Behavioral

Deterrent
Devices Repulsing Organisms from the Intake by Introducing Changes that Alert Them • Velocity Caps

• Acoustic Barriers

• Strobe Lights

• Air Bubble Curtains Yes No

An example of the synergistic use of I&E reducti on technologies is the pr eviously referenced 40 MGD Beckton desalination plant in London. Besides wedgewire screens, the intake structure of this plant is equipped with an acoustic fish de flection system. This system includes eight low frequency sound generation units that deflect fish movement away from the wedgewire intake structure (Figure 12). The scale at the bottom of this figure indicates the sound level of the acoustic fish deflection system in decibels (d B). The low frequency (25 – 400 Hz) sound level is maintained at a level of 150 dB or more, which gives a clear cue for danger to fish entering the area of the intake. This acoustic system is onl y operated for short periods, twice daily, during pump startup. At this time, no published data are available regarding the I&E reduction efficiency of this technology.Seawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 18 Source: Acciona Agua

Figure 12 – Beckton Desalination Plant Inta ke Acoustic Fish Deflection System

Fine mesh screens are one of the technologies eq ually popular for both se awater desalination and power plant intakes. One type of fine mesh scre en associated with the operations of the 25 MGD Tampa Bay seawater desalination plant is sh own on Figure 13. This de salination plant is collocated with the 1200 MW Big Bend Power Plant and uses cooling water from this plant as source seawater for desalination. The Tampa Bay desalination plant does not have a separate seawater intake. However, the intake of the power plant is equipped with 0.5-mm Ristroph finemesh screens, which have been proven to reduc e impingement and entrainment of fish eggs and larvae through the downstream c onventional bar and fine screens of the power plant intake by
over 80% 20

.
Unfortunately for the desalination plant, these screens are periodically bypassed (as allowed by permit) and/or screenings are conveyed to th e power plant discharge outfall from where the desalination plant collects sour ce seawater. As a result, the scr eenings can find their way to the desalination plant intake and im pact desalination plant pretreatment system performance. This challenge necessitated the need fo r the remediated desalination pl ant to be equipped in 2005 with another set of fine screens located just upstream of the pretreatment facilities.

20 http://www.epa.gov/waterscience/316b/phase1/technical/ch5.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 19 Source: Water Globe Consulting

Figure 13 – 0.5 mm Fine Mesh Screens of the Tampa Bay Power Plant Intake

Another example of a full-scale implementation of an intake with advanced impingement and
entrainment reduction features is the Filtrex F ilter Intake System of the 10 MGD Taunton River Desalination Plant in Dighton, Massachusetts (Figure 14). This plant is planned to be constructed in two 5 MGD phases. The 30 MGD intake system for this plant is comprised of 30 racks with 96, 4.6-inch long individual plastic filtration modul es (candles) per rack, through which saline
water is withdrawn.

The candles have a pore size of 0.04 mm (40 microns) and very low (0.2 feet/sec) through-pore velocity. These intake features allow complete avoidance of the impingement of adult fish; a reduction of impingement of fish eggs down to less than 15%; and a minimization of entrainment of larval organisms and fish eggs to less than 3% of the species in the intake area 21 . It should be pointed out that this type of screen has a lim ited track record because the plant began operation in November 2008 and has not been operating at its full 5 MGD production capa city as of yet.

21 http://www.watereuse.org/sites /default/files/u8/Quote%2021.pdfSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 20 Source: TRI-MONT Engineering Company

Figure 14 – Taunton River Desalination Plant Intake

Environmental Impact Mitigation Measures Environmental impact mitigation is typically applied if the site, design, and technology measures described above do not provide adequate impinge ment and entrainment reduction to sustain the biological balance of the marine habitat in the ar ea of the intake. Examples of types of activities that may be implemented by desalination faciliti es to provide environmental impact mitigation
include:

• Wetland Restoration;

• Coastal Lagoon Restoration;

• Restoration of Historic Sedime nt Elevations to Promote Rees tablishment of Eelgrass Beds;

• Marine Fish Hatchery Enhancement;

• Contribution to a Marine Fish Hatchery Stocking Program;

• Artificial Reef Development; and

• Kelp Bed Enhancement.

The type and size of the mitigation alternative or combination of alternatives most suitable for a given project are typically select ed to create a new habitat capabl e of sustaining types of speciesSeawater Desalination Plant Intakes – Impingement and Entrainme nt  Page 21 and levels of biological productivity comparable to those lost as a re sult of the intake operations.

Coastal wetlands are the nursery areas for many of the species impacted by desalination intakes. Wetland restoration is, therefore, a common mitigation measure for large seawater intake systems. For example, development of new coas tal wetlands is the preferred impingement and entrainment mitigation alternativ e for the 50 MGD Carlsbad seawater desalination project in
California.

The time and cost expenditures involved in the permitting, implementation, maintenance, and monitoring of such mitigation measures are signifi cant, and such habitat restorative measures are typically used when the impingement and entr ainment reduction measures described in the previous sections are not readily availa ble or viable for a given project.

Some environmental groups do not consider m itigation as an acceptable I&E management alternative and have challenged the legality of the use of I&E mitigation measures for both power plant and desalination plant intakes. Court resolutions to recent legal challenges associated with the permitting of the 50 MGD Ca rlsbad and Huntington Beach SWRO projects, however, indicate that mitigat ion by environmental restoration is a viable method for supplementing the use of best te chnologies available and operati onal measures to address the potential environmental impact s associated with collecting seawater for desalination.

CONCLUDING REMARKS In summary, appropriately sited, designed, and ope rated seawater desalination plant intakes can have minimal environmental impacts on the marine environment and resources. In fact, based on recent studies, impingement and entrainment re sulting from well-planned and designed open ocean intakes would be minor: the equivalent of th e daily food intake of one pelican and the loss of the annual bio-productivity of five adult fema le halibut, respectively . Ongoing developments in impingement and entrainment reduction tec hnology, combined with the existing wealth of knowledge and experience in this field, both domes tically and internationa lly, pave the way for maintaining sustainable and environmentally safe production of fresh water from the ocean. With over 20 years of successful operational experien ce at more than 8000 desalination plants worldwide, seawater desalina tion is currently a well-established drinking water production technology of proven performance which will pl ay an increasingly prominent role in well balanced and sustainable water supply portfolios of coastal communities in the US and abroad.