The WateReuse Desalination Committee's White Papers are living documents. The intent of the Committee is to enhance the

content of the papers periodically as new and pertinent information on the topics becomes available. Members of the

desalination stakeholder community are encouraged to submit their constructive comments to white‐papers@watereuse.org

and share their experience and/or case studies for consideration for inclusion in the next issuance of the white papers.

Desalination Plant Intakes  

Impingement and Entrainment

Impacts and Solutions

White Paper

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

 WATEREUSE ASSOCIATION 

DESALINATION COMMITTEE 

Desalination Plant Intakes – 

Impingement and Entrainment Impacts and Solutions 

White Paper 

INTRODUCTION 

Seawater intakes are an integral part of every seawater desalination plant. The purpose of this 

white paper is to provide an overview of potential impingement and entrainment (I&E) impacts 

associated with the operation of open ocean intakes for seawater desalination plants and to 

discuss alternative solutions for efficient and cost effective I&E reduction. For information on 

alternative intakes for seawater desalination plants, refer to the WateReuse Association’s white 

paper titled “Overview of Desalination Plant Intake Alternatives.” 

WHAT IS IMPINGEMENT 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 avoid going through the screens 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 organisms (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 and 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 impacts associated with offshore intake 

structures connected to an on-shore intake screen and pump station via long conveyance pipeline 

or tunnel. Organisms that enter the offshore intake and cannot swim back out of it are often 

referred to as entrapped1

. Such marine organisms could either be impinged on the intake screens 

or entrained if they pass through the screens and enter the downstream facilities of the 

desalination plant. 

                                                           

1 http://www.waterlink-international.com/download/whitepaper_uploadfile_21.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 2 

Attention to seawater intake impingement and entrainment issues is partially prompted by the 

Section 316(b) of the 1972 Clean Water Act that regulates cooling water intake of the steam 

electric industry by the environmental scrutiny associated with the public 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 specific and varies significantly from one project to 

another. Open ocean intakes are typically equipped with coarse bar screens (Figure 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 (Figure 2), which preclude the majority of the adult 

and juvenile marine organisms (fish, crabs, etc.) from entering the desalination plants. While 

coarse screens are always stationary, fine screens could be two types – stationary (passive) and 

periodically moving (i.e., rotating) screens. Figure 2 depicts a 3-mm rotating fine screen. Most 

marine organisms collected with the source seawater used for production of desalinated water are 

removed by screening and downstream filtration before this seawater enters the reverse osmosis 

desalination membranes for salt separation. 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. 

 Source: GHD 

Figure 1 – Intake Bar Screen 
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 3 

 Source: Water Globe Consulting 

Figure 2 – Fine Intake Screen 

By comparison, intake wells and infiltration galleries pre-filter aquatic life through the ocean 

bottom sediments. In this case, the ocean bottom provides a natural separation barrier for adult 

and juvenile marine organisms. Since subsurface intakes collect source seawater through the 

ocean bottom and coastal aquifer sediments (see Figure 3), 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. An ongoing side-by-side study of the I&E effects of a subsurface 

intake and an open ocean intake equipped with a passive wedgewire screen at the West Basin 

Municipal Water District’s desalination demonstration plant is expected to provide more detailed 

information on this topic2

. 

 

                                                           

2 http://www.watereuse.org/node/978
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 seawater for once-through cooling completed by the 

California State Water Resources Control Board in 2010 provides important insight into the 

magnitude of these intake-related environmental impacts3. Based on this study, the estimated 

total average annual impingement of fish caused by the seawater intakes 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 Station); and for all 19 plants it averaged 6.63 

lbs./MGD. Taking into consideration 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 Control Board impingement and entrainment study 

results as a baseline, for a large 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./day4

. The comparison illustrates the fact that the impingement 

impact of seawater desalination plants with open ocean intakes is not significant and would not 

have measurable impact on natural aquatic resources (Figure 4). 

                                                           

3 http://www.watereuse.org/sites/default/files/u8/Quote 3.pdf 4 http://www.sandiegozoo.org/animalbytes/t-pelican.html
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 fish 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 impact is very site-specific. 

As per the same report, the average annual entrainment is estimated at 2.14 million of fish larvae 

per MGD of intake flow. Prorated for a 110 MGD intake of a 50 MGD seawater desalination 

plant, this annual entrainment impact is 235.4 MM of larval fish/yr. While this number seems 

large, based on expert evaluation and research, large entrainment numbers do not necessarily 

equate to a measurable impact to adult fish populations because of the enormous amount of eggs, 

fish larvae and other zooplankton in seawater5

. 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 predation, exposure to destructive forces of nature 

such as wind and wave action, and the inability to find appropriately-sized pray during the 

                                                           

5

http://www.swrcb.ca.gov/water_issues/programs/ocean/cwa316/docs/workshop_oakland2005/pres_tenera.pdf 6 http://www.scwd2desal.org/documents/Presentations/Nov_10_2010/02_Tenera_nov10_web.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 6 

critical period of their development (i.e., after their yolk sack 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 Survival 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 lifetime7

. In simple terms, the annual entrainment impact of 

one 50 MGD desalination plant would be comparable 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 water 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 fresh surface water (rivers, lakes, etc.). However, 

damaging long-term environmental impacts may also result from continued over-depletion of 

those conventional water supplies, including inter-basin water transfers. 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; wetlands 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 southern regions of these countries), has resulted in 

substantial environmental impacts to the traditional fresh water resources in these regions. One 

                                                           

7

http://www.watereuse.org/sites/default/files/u8/Quote%207%20-%20Presentation.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 7 

such specific example of dramatic environmental impact is the reduction of the habitat of delta 

smelt as a result of over-pumping caused by California State Water Project’s intake facilities.8

Such long-term fresh water transfers have affected the ecological stability in the fresh water 

habitats to the extent that the long-term continuation of current water supply practices may result 

in significant and irreversible damage of the ecosystems 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 desalination projects should be weighed against the 

environmentally damaging consequences from the continued expansion of the existing freshwater supply practices. 

A responsible approach to water supply management must ensure that sustainable and droughtproof local supplies are available, and long-term reliance on conventional water supply sources 

(i.e., surface water, groundwater) is reconsidered in favor of a well-balanced and diversified 

water supply portfolio which combines surface water, groundwater, recycled 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 Board, Tampa Bay Water, and other 

agencies in the United States. 

IMPINGEMENT AND ENTRAINMENT SOLUTIONS 

While impingement and entrainment associated with seawater intake operations 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 productivity and vitality of the aquatic environment in the vicinity of the 

intake. 

Prudent Open Intake Design 

Installation of Intake Inlet Structure Outside of the Littoral Zone

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 (410 feet) outside of the littoral zone a good engineering 

practice aimed at reduced impingement and entrainment9

. According to the Office of Naval 

Research, the littoral zone extends 600 feet from the shore10. Thus, intakes with an inlet structure 

located at least 1100 feet from the shore could result in reduced environmental impacts. In 

addition, installing the intake to depths where there is a lower concentration 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

10http://www.onr.navy.mil/focus/ocean/regions/littoralzone1.htm
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 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 0.5 feet per second (fps), the intake facility is deemed to 

have met impingement mortality performance standards11. 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 greater than 9 inches is 

recommended for preventing large organisms from entering the seawater intake12. 

 

Suitable Fine Screen Mesh Size

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 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 typically collect seawater for 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 pretreatment 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 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. Designing the desalination plant to operate closer to the 

upper limits of recovery (i.e., 50 to 55%) would require collecting less water and therefore, 

would reduce impingement and entrainment associated with seawater intake operations. Longterm testing completed by the Affordable Desalination Collaboration, aimed to identify the most 

suitable operational conditions for low-energy SWRO desalination, indicates that optimum 

                                                           

11 http://edocket.access.gpo.gov/cfr_2008/julqtr/pdf/40cfr125.94.pdf 12 http://www.watereuse.org/sites/default/files/u8/Quote%2012%20-%20Policy.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 9 

energy consumption is achieved at a membrane flux 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 additional 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 approach has cost implications but is a prudent 

design practice aimed at reducing overall plant seawater 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 adequate dilution of the saline discharge. This 

additional flow intake could be eliminated by designing 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 concentrate dissipation into the ambient marine 

environment without additional dilution. 

If the desalination plant production capacity has to vary diurnally, the design and installation of 

variable frequency drives on the intake pumps could also allow decreasing impingement and 

entrainment of the plant intake by closely matching collected source seawater volume to the 

plant production needs. 

Use of Low-Impact Intake Technologies 

Impingement and entrainment of marine organisms could be minimized by using various 

subsurface and open intake technologies. Currently, there are no federal and state regulations 

which specifically define requirements for reduction of impingement and entrainment caused by 

desalination plant intakes. However, the US EPA Section 316(b) of the Clean Water Act federal 

regulations have stipulated national performance standards for intake impacts from power 

generation plants which require 80 to 95% reduction of impingement and 60 to 90% reduction of 

entrainment as compared to those caused by uncontrolled intake conditions14. Technologies that 

can meet these impingement and entrainment performance standards are defined by US EPA as 

Best Technology Available (BTA). 

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 

                                                           

13 http://www.affordabledesal.com/home/news/ADC%20Completes%20Profile%20of%20SWRO%203-28-08.pdf 14 http://edocket.access.gpo.gov/cfr_2008/julqtr/pdf/40cfr125.94.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 10 

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. 

Because optimal conditions for subsurface intakes are often impossible to find in the vicinity of 

the desalination plant site, the application of this type of intake technology to date worldwide has 

been limited to plants of relatively small capacity. 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 the first 17 MGD phase of 

the 34 MGD San Pedro Del Pinatar (Cartagena) desalination plant in Spain. For this project, sitespecific hydrogeological constraints made it impossible 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 subsurface intakes in Long Beach and Dana Point, 

California are expected to provide comprehensive data that would allow completing a 

scientifically-based analysis of the viability and performance benefits of subsurface intakes for 

larger-size applications. The tested subsurface intake technologies are currently under evaluation 

and do not yet have established performance, reliability, and environmental track records. 

Wedgewire Screen Intakes

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. This is the only open intake technology 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 juveniles of any fish and 

shellfish to be protected at the plant intake site16. 

Wedgewire screens are designed to be placed in a water body where significant prevailing 

ambient cross flow current velocities (≥ 1 fps) exist. This high cross-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/julqtr/pdf/40cfr125.99.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 11 

An integral part of a typical wedgewire screen system is an air burst back-flush system, which 

directs a charge of compressed air to each screen unit to blow-off 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 Beckton desalination plant is equipped with seven 

(7) 3-mm wedgewire screens installed on the suction pipe of each of the plant intake pumps. 

Total screen length is 11.55 ft. (3500 mm) and the screen diameter is 3.6 ft. (1100 mm). The 

plant intake is under significant influence of tidal exchange of river water and seawater. To 

capture the ebb tide and minimize entrainment, the intake 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 development by the City of Santa Cruz Water Department and Soquel 

Creek Water District in California17. 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. 

                                                           

17http://www.scwd2desal.org/documents/Reports/Open_Ocean_Intake_Effects/Open%20Ocean%20Intake%20Effec

ts%20Study%20Final%20Dec%202010.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 12 

The tested wedgewire screen had 2.0 mm of slot openings and was constructed of copper-nickel 

alloy. The diameter of the screen was 8-5/8 inches; the overall screen length 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 comprehensive I&E study indicate that: 

• No endangered, threatened, or listed 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 proportional mortality that could be attributed to the screen 

operation for the top 80% of the fish larvae in the source water area at 7.0 MGD intake 

flow was 0.06%. 

• The greatest projected proportional 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 fish, shrimp and crab populations indicate that 

the full-scale wedgewire intake screen operation at 7.0 MGD will not cause significant 
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 plant operation are a very small fraction of 

the reproductive output of the source populations of marine organisms inhabiting the 

intake area. For example, for the white croaker – 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 (reproductive yield) of a single female fish. 

To study the behavioral responses of different species swimming near 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 recorder (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 allow the conclusion that 

operating the wedgewire screen intake at a through-screen velocity of 0.33 fps eliminates 

impingement. 

 Source: Tenera Environmental

Figure 8 – Rockfish Sitting on Screen 
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 14 

 Source: Tenera Environmental

Figure 9 – Shrimps Swimming Near Screen 

 Source: Tenera Environmental

Figure 10 – School of Juvenile Rockfish Swimming Near Screen 
Seawater Desalination Plant Intakes – Impingement and Entrainment  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, CA18. 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 currently being tested at the West Basin Municipal Water District 

seawater desalination demonstration 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 Figure 11). 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. 

This natural indication combined with maintaining low through-screen velocity (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 track record and is widely used worldwide. This 

is the original configuration of many power plant intakes in Southern 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 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 screens19. 

                                                           

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

19 http://www.epa.gov/waterscience/316b/phase1/technical/ch5.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 offshore conduit cannot return back to the open ocean 

because they are stranded between the intake inlet structure and the downstream fine screens. 

The use of velocity caps and low velocity through both the coarse screen of the intake structure 

and the downstream fine screens could reduce this entrapment effect. 

Other Impingement and Entrainment Reduction Technologies

In addition to the intake technologies described above, there are a number of other technologies 

which have been demonstrated to reduce the impingement and entrainment 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 technologies 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, these technologies could be used in synergistic 

combination to achieve project-specific environmental 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, the WateReuse Research Foundation initiated a 

research study to document and evaluate the impingement and entrainment reduction efficiency 

of these and other technologies (WateReuse-10-04). 
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 17 

Table 1 – Potential Open Intake Impingement 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 reduction technologies is the previously referenced 40 

MGD Beckton desalination plant in London. Besides wedgewire screens, the intake structure of 

this plant is equipped with an acoustic fish deflection 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 (dB). 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 only 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 Entrainment  Page 18 

 Source: Acciona Agua 

Figure 12 – Beckton Desalination Plant Intake Acoustic Fish Deflection System 

Fine mesh screens are one of the technologies equally popular for both seawater desalination and 

power plant intakes. One type of fine mesh screen associated with the operations of the 25 MGD 

Tampa Bay seawater desalination plant is shown on Figure 13. This desalination 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 reduce impingement and entrainment of fish eggs and 

larvae through the downstream conventional 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 the power plant discharge outfall from where the 

desalination plant collects source seawater. As a result, the screenings can find their way to the 

desalination plant intake and impact desalination plant pretreatment system performance. This 

challenge necessitated the need for the remediated desalination plant 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.pdf 
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 Filter 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 modules (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 area21. It should be 

pointed out that this type of screen has a limited track record because the plant began operation 

in November 2008 and has not been operating at its full 5 MGD production capacity as of yet. 

                                                           

21 http://www.watereuse.org/sites/default/files/u8/Quote%2021.pdf
Seawater Desalination Plant Intakes – Impingement and Entrainment  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 impingement and entrainment reduction to sustain the 

biological balance of the marine habitat in the area of the intake. Examples of types of activities 

that may be implemented by desalination facilities to provide environmental impact mitigation 

include: 

• Wetland Restoration; 

• Coastal Lagoon Restoration; 

• Restoration of Historic Sediment Elevations to Promote Reestablishment 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 selected to create a new habitat capable of sustaining types of species 
Seawater Desalination Plant Intakes – Impingement and Entrainment  Page 21 

and levels of biological productivity comparable to those lost as a result 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 coastal wetlands is the preferred impingement and 

entrainment mitigation alternative 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 significant, and such habitat restorative measures are 

typically used when the impingement and entrainment reduction measures described in the 

previous sections are not readily available or viable for a given project. 

Some environmental groups do not consider mitigation 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 Carlsbad and Huntington Beach SWRO projects, 

however, indicate that mitigation by environmental restoration is a viable method for 

supplementing the use of best technologies available and operational measures to address the 

potential environmental impacts associated with collecting seawater for desalination. 

CONCLUDING REMARKS 

In summary, appropriately sited, designed, and operated seawater desalination plant intakes can 

have minimal environmental impacts on the marine environment and resources. In fact, based on 

recent studies, impingement and entrainment resulting from well-planned and designed open 

ocean intakes would be minor: the equivalent of the daily food intake of one pelican and the loss 

of the annual bio-productivity of five adult female halibut, respectively. Ongoing developments 

in impingement and entrainment reduction technology, combined with the existing wealth of 

knowledge and experience in this field, both domestically and internationally, pave the way for 

maintaining sustainable and environmentally safe production of fresh water from the ocean. With 

over 20 years of successful operational experience at more than 8000 desalination plants 

worldwide, seawater desalination is currently a well-established drinking water production 

technology of proven performance which will play an increasingly prominent role in well 

balanced and sustainable water supply portfolios of coastal communities in the US and abroad.