The drought currently shriveling the West Coast comes with an irony that the poet Samuel Taylor Coleridge would recognize. There's water, water, everywhere out there—literally an ocean's worth—but you can't drink it or irrigate with it for the salt. That used to be an insoluble technical and economic obstacle. But now, with snowpacks at zero percent and reservoirs looking more like puddles, engineers in San Diego are preparing to hook up a new $1 billion desalinization plant that will provide enough water for 300,000 thirsty people each day.

First commercialized in the 1950s, desalinization has two main approaches: distillation (a variant on the process for making booze), which heats seawater and collects the salt-free vapor for recondensation into a liquid, and reverse osmosis, which pushes water at high pressure through membranes that leave clean water on one side and a concentrated brine on the other.

Today, 300 million people get some of their water from desalinization, according to the International Desalinization Association. Desal is big in arid nations like Saudi Arabia, Israel, and Australia, and on tropical islands that don’t get much rainfall.

For decades, US environmentalists have fought desalinization projects. They feared that big seawater intake pipes would kill fish larvae and other microscopic marine life. And all that salt has to go somewhere; those environmentalists worry that the salty brine discharge kills bigger fish as well. But some changes in the way plants discharge water—not to mention that prolonged drought—are forcing many California communities to reconsider their opposition.

Santa Barbara’s mothballed plant will start running again after sitting idle for 23 years. Cambria, Calif., opened an emergency $9.5 million desal plant in November 2014, while Monterey County approved “atmospheric water generators” (just like Luke’s uncle ran on Tatooine) to supply water for some businesses and industrial parks.

Poseidon Water

But San Diego’s big new desal plant, the largest in the Western Hemisphere, isn’t a reaction to the latest water crisis, says Scott Maloni, spokesman for Poseidon Water, the privately-held project developer. It was first proposed back in 1998, and the firm has been dealing with permits and financing since then.

“Ninety-eight percent of the water for San Diego has to be imported from Northern California or the Colorado River,” Maloni says. “Southern California is an arid region, and climate scientists say this drought cycle will be more severe and more frequent in the future. Even if it starts raining tomorrow, we will still need it.”

Maloni says desalted water is slowly getting cheaper. The polyamide nylon membranes that remove marine salts and minerals last longer now—seven years instead of wearing out after three or four—and it now takes less pressure to push the water through because of technical improvements, including new ceramic pressure exchangers that boost the efficiency of the process.

The developers are building the plant next to a gas-fired power plant in Carlsbad, about 35 miles north of San Diego. That way it can take use some of the power station’s coolant water to dilute the salty brine discharge. After passing through several treatments to remove marine salts and bacteria, the water will go 10 miles inland to an aqueduct, where it'll join the rest of the San Diego County Water Authority’s system. By 2020, desalted water is supposed to make up 7 percent of the total supply.

Despite the technical improvements to the process, desal water costs twice as much as existing imported water. San Diego ratepayers will fork over an extra $5 to $7 per month for the next 30 years to cover the difference.

Of course, it'd be nice if those costs kept going down. At MIT, Rohit Karnik’s team has been working on making single-layer membranes from graphene, the nearly two-dimensional, carbon-based material that all kinds of scientific fields hope will change the world. Graphene is tougher than polyamide mesh, doesn’t break down in the presence of the chlorine used to treat water, and has potential for higher throughput and lower energy cost. It's also quite cheap. “If we want quantum leaps in performance, we have to look for other materials,” said Karnik, a mechanical engineer. “We are just starting to make these membranes. But it will be a few years before we truly know their potential.”

In the meantime, maybe San Diego can squeeze out a few drops to drink from the ancient sea.