Recurrent droughts and uncertainties about future water supplies have led several California communities to look to saltwater for supplemental supplies through a process known as desalination.
Desalination removes salt and other dissolved minerals from water and is one method to reclaim water for other uses. This can occur along the coast and in the interior at spots that draw from ancient salt water deep under the surface.
There are two desalination processes that use semi-permeable membranes to separate salt and other dissolved minerals from water. One method is reverse osmosis, which forces pressurized saline water through a semi-permeable membrane that inhibits the passage of dissolved solids, including salt. The other, electro-dialysis, uses a combination of electrical charges and membranes to extract the salt and then separate the treated water into a fresh pool and a brine pool.
Desalination plants have treated brackish groundwater, irrigation runoff, seawater and domestic wastewater for decades in California. During the drought of 1987-1992, for instance, desalination was seen as a way of supplying additional water. The West Coast’s first seawater desalination facility was on California’s Catalina Island in 1991.
Along the coast in Carlsbad, the largest seawater desalination facility in the Western Hemisphere formally launched in December 2015 after more than 10 years of development. The plant desalts and then pumps out 50 million gallons of fresh water each day into the San Diego’s local supply, prompting questions about the role seawater desalination will play in California’s future.
In Santa Barbara, the city’s old desalination plant is being rehabilitated for a scheduled re-launch in October 2016. Spending $55 million to renovate the plant was heavily influenced by the drought but its utility extends further. Reactivating the facility means the city will revisit the role of desalination in meeting its water needs.
Desalination Going Forward
Desalination is energy intensive but has become more cost competitive with other water supply options in recent years, thanks to improved filtration technology. (Water from the Carlsbad Desalination Project is estimated to cost approximately $2,000 an acre-foot.)
More controversially, the California Coastal Commission has found desalination has a direct adverse environmental impact on marine life through entrainment and highly concentrated, salty discharges.
Potential drawbacks, however, include the impact on marine life and related eco-systems. The California Coastal Commission has found that desalination has a direct adverse environmental impact on marine life through entrainment and highly concentrated, salty discharges. The State Water Resources Control Board has also found that desalination plant pipes kill millions of fish and other marine life annually.
The Ocean Plan amendment requires subsurface intakes unless a regional water board determines they are not feasible. In that case, open ocean intakes are acceptable but must utilize the best available technology. This must be equipped with a 1-millimeter wedge wire screen that limits the size of any marine life that can be pulled into the system.
The intake water velocity has to be equal or less than one-half foot per second, which is less than natural ocean current velocities. The brine discharge is to be commingled with wastewater or, if wastewater is not available, released into the water with via multiport diffusers, which rapidly dilute and disperse brine within a small area to facilitate its dispersal into the ocean
California’s water supply has always teetered between the extremes of drought and flood and heavily influenced by the demands of a growing population. Massive public works projects helped make the state’s system what it is today but the harsh drought and a changing climate are game changers – calling for a revised view of water sue that that includes greater water conservation, reuse and, possibly, desalination. There are many things to consider – cost, energy use, environmental impacts and public acceptance. It’s safe to say desalination will remain an option that is pursued but to what degree remains to be seen.