As California plans for a new desalination plant, take a look at how these facilities work

The Doheny Ocean Desalination Project, estimated to be completed in 2027, will provide 5 million gallons of drinking water a day to residents in Orange County.
laguna beach
Derek Liang / Unsplash

California, a state that has been facing devastating droughts and wildfires, approved a $140 million desalination plant on October 13 that would enable it to convert seawater into fresh water. It will join a cadre of 12 other facilities currently operating off the coast of California. 

While getting plans for the Doheny Ocean Desalination Project past the California Coastal Commission was a major regulatory hurdle that has since been cleared, the project will still need other state permits before construction can begin, according to Reuters. The proposed facility (estimated to be completed in 2027) will provide 5 million gallons of drinking water for 40,000 people in Orange County daily. This will reduce the district’s reliance on water imported from  the State Water Project and the Colorado River by up to 70 percent and bolster the emergency supply, LA Times reported. 

Desalination plants are useful for times of drought, but can often face pushback from environmental concerns that are directly tied to how the plants work. 

[Related: In photos: Dubai’s massive desalination plant]

Let’s take a look at how a desalination facility turns ocean water potable, and explore the workings of an already existing facility, the Lewis Carlsbad Desalination Plant, the biggest of its kind in the state (it produces 50 million gallons of drinking water a day). 

After seawater is sucked in, it goes through sand pretreatment filters that remove large objects and other solid materials. Then, the water goes through a reverse osmosis process, where membranes (which are semi-permeable barriers that only let molecules of certain sizes, shapes, and charges pass across) and filters separate out dissolved minerals, salts, and other impurities. Then chemicals are added in, and it’s tested before it’s sent out to consumers. 

Plans for a larger, privately owned plant called Poseidon were rejected just a few months ago by Coastal Commission because that plant would’ve sucked in water from above the ocean floor, which would be harmful to animals and other organisms that lived there. Experts told PopSci in April that a slew of small desalination plants would be considerably better than one massive plant. These plants could also function like “washing machines” that help recycle groundwater and other dirty water. 

Doheny, the new plant, would not only be smaller in size compared to the proposed Poseidon, but it will use advanced slant wells to pull water from beneath the ocean floor, Cal Matters reported. However, more research is needed to assess its impact on deep ocean floor creatures. 

The other big ecological concern with desalination plants in general is the hypersaline brine discharged at the end of the reverse osmosis process. This is made up of salt, minerals, and other byproducts from desalination. Since brine is more dense than the water that’s sucked into the plant in the beginning, it could settle onto the seafloor and negatively impact the organisms there. This presents an issue, especially in areas with sensitive ecosystems. Doheny said that it will mix its brine with the district’s existing wastewater lines and dilute it before expelling it about two miles out into the ocean, LA Times reported. 

Finally, there are questions around how much energy these plants use. According to the Department of Energy, “large-scale desalination systems require tens of megawatts to run and provide tens of million gallons of desalinated water per day. Small-scale systems vary in size from tens to hundreds of kilowatts and provide hundreds to thousands of gallons of water per day.” 

Some researchers have been looking into ways to hook up the facility to wind turbines or wave energy systems. On that end, Doheny hopes to power around 15 percent of the energy-intensive process through solar panels, and integrate an energy recovery process (likely by capturing hydraulic energy from the high pressure pumps used during reverse osmosis). It says on its project website that this would result in “45 to 55 percent less energy usage than systems without that feature.”