Reverse osmosis desalination plant

State Water Board update: Addressing the impacts of desalination facilities

Slide 7 DesalWith at least seventeen seawater desalination facilities currently proposed along California’s coast in such locales as Huntington Beach, Monterey Bay, and the Bay Area, desalination is being increasingly seen by some coastal communities as a way to alleviate their water shortages.

Desalination is the process of removing the dissolved salts and minerals from brackish or seawater to produce water that is fit for human consumption.  In order to produce this water, desal plants take in large amounts of seawater; the output of the plant is both potable water and a salty brine that is then discharged to ocean waters.

Currently, California’s water quality control plans for coastal waters and estuaries do not specifically address desalination facilities, so the small number of plants that are operating in California are regulated on a project-specific basis.  Given the number of plants proposed for California, updating the water quality control plans to address these issues has been identified as one of the State Water Resources Control Board’s priorities for 2012-2013.


There are two major areas of concern regarding desalination that fall under the authority of the State Water Board: the impacts of the plant’s intake on marine life, and the impacts to the environment and marine life that are caused by the discharge of brine.  (While the energy usage of desalination facilities is an issue, it is not under the purview of the State Water Resources Control Board.)

ImpactsWater can enter a desalination facility either through surface intakes that take water directly from the water body or through subsurface intakes that draw water in through pipes that are installed underground or under the seafloor.

Surface water intakes can have significant negative impacts on marine life, and screens are necessary to keep larger marine animals and smaller organisms from being taken into the intake pipes.  Even so, many small aquatic organisms on the bottom of the food chain pass through the screens and are killed by the desalination process, making less food available to those that depend on it.  The use of subsurface intakes greatly reduces or eliminates entrainment and impingement of marine life.

The elevated salinity in the brine discharges is of concern as the saltier brine is denser and heavier than ocean water, so there is potential for the brine to accumulate on the sea floor, killing the bottom-living communities of fish, plants, and algae.


In order to address areas of uncertainty, in 2012 the State Water Board commissioned three studies, with a workshop to present the findings in August 2012.


Subsurface intakesThe Moss Landing Marine Laboratories assembled a panel of experts which considered ways to minimize or mitigate damage to marine life caused by method of the intake of seawater, both by power plants and by desalination facilities.  Desalination plant intakes have similar impacts as power plants, but generally use less water.

The panel recommended that desalination plants use ‘the best available site, design, technology and mitigation measures.’  However, even though these technologies can reduce impingement and entrainment of aquatic species, there will still be impacts for which mitigation should be required.  The panel recommended a fee be assessed to mitigate for the loss of marine life that would still occur, with the money being used to create, restore, or protect marine habitats.

Surface intakesProperly designed, constructed and operated subsurface intakes can have minimal impacts, but local geologic conditions can make subsurface intakes infeasible.  Additionally, while large infiltration galleries may have low impingement and entrainment impacts, the construction impacts could be substantial.

When surface water intakes are necessary, intakes should be equipped with 1 millimeter screens and intake velocities of no greater than 0.5 feet per second to minimize impacts to marine life.  Wedgewire screens and other devices are currently being studied; the results of those studies are not yet available.

The use of additional seawater to dilute the brine to meet toxicity levels is not recommended as more organisms are likely killed through impingement and entrainment than would be saved from exposure to high brine concentrations.


Brine impactsThe Granite Canyon Marine Laboratory conducted studies to determine how well specific species tolerate various concentrations of brine. They tested seven species, including bay mussels, purple sea urchins, sand dollars, red abalone, giant kelp, topsmelt, and mysid shrimp.

Testing determined that red abalone, purple sea urchins, and sand dollars were the most sensitive to the elevated salinity at typical brine concentrations, while topsmelt and giant kelp were the most tolerant.

The studies found that concentrations of greater than 5% in the natural waters around the discharge is harmful to marine life.

The panel recommends additional testing be done to answer further questions, such as the effects of various brine and effluent mixtures, effect of long-term chronic exposure to elevated salinity, and the effects of brine discharges in estuaries.


Discharge impactsA science advisory panel assembled by the Southern California Coastal Water Research Project evaluated methods for disposing of the brine as well as monitoring strategies.

The panel determined that brine can be disposed of with minimal environmental effects if sited correctly. Brine disposed of in areas with rapid initial dilution and good flushing action will impact an area only tens of meters from the discharge structure, but if brine is disposed of in an area with low initial dilution and poor flushing action, it can have widespread impacts.

However, the impacts – or lack thereof – vary considerably based on the location, the marine life present, the nature of the brine, and to what degree it is dispersed, so more studies on the effects of long-term exposure to elevated salinity on the animals, algae, and plants that live in estuaries or on the ocean floor are needed.

brine mixingThe panel recommends that the discharged brine have a concentration of no more than 5 percent (approximately 2 parts per thousand) above the salinity of the surrounding water with the mixing zone extending 100 meters from the discharge structure in all directions, including vertically through the water column.  Field monitoring should begin prior to discharges and continue afterwards to evaluate changes, and the effluent monitored to be sure it is within the specified physical and chemical parameters.

The panel recommends prohibiting the use of seawater from a surface water intake for the purposes of diluting the effluent to meet water quality standards; however, blending brine discharges with the discharges from municipal wastewater treatment plants can be an effective way to minimize salinity concentrations.


The State Water Board has been developing amendments the California Ocean Plan and the Enclosed Bay, Estuaries, and Inland Surface Water Quality Control Plan to specifically address the impacts of desalination facilities.  The amendments will include a narrative objective for salinity, limits on desalination intakes, and an implementation plan for discharges, including provision that prohibits the use of seawater for in-plant dilution.

Reverse osmosis desalination plant
Reverse osmosis desalination plant

At the June 4th State Water Board meeting in Monterey, staff updated board members on the status of the amendments.  The water board staff are in the process of drafting a Supplemental Environmental Document  that will analyze the environmental impacts of the amendments. The expert review panel has been reconvened to address remaining questions.  Once the information is gathered, there will be a public hearing in the fall of 2013.

The amendments are tentatively scheduled to be considered for adoption by the State Water Board in 2014.  Once adopted, the regulations will need to be approved by the State Office of Administrative Law as well as the U.S. Environmental Protection Agency to become law, and will be implemented through the National Pollution Discharge Elimination System permits.



  • Click here for the State Water Resources Control Board’s page on amendments pertaining to desalination facilities.
  • The Pacific Institute has a series of publications on desalination which you can find by clicking here.


California Water Plan workshop on desalination:  As part of the update to the California Water Plan, DWR’s statewide planning document for water resources, there will be a workshop on August 28th in Sacramento to discuss the desalination chapter of the Plan, which includes the potential benefits and costs of desalination, implementation issues, and recommendations.  There are online and phone options for those unable to attend in person.  Click here for more information.

Print Friendly, PDF & Email