Severe droughts are forcing researchers to rethink how technology can increase the supply of fresh water.
Even in drought-stricken California, San Diego stands out. It gets less rain than parched Los Angeles or Fresno. The region has less groundwater than many other parts of the state. And more than 80 percent of water for homes and businesses is imported from sources that are increasingly stressed.
The Colorado River is so overtaxed that it rarely reaches the sea; water originating in the Sacramento River delta, more than 400 miles north, was rationed by state officials this year, cutting off some farmers in California’s Central Valley from their main source of irrigation. San Diego County, hot, dry, and increasingly populous, offers a preview of where much of the world is headed. So too does a recent decision by the county government: it is building the largest seawater desalination plant in the Western Hemisphere, at a cost of $1 billion.
The massive project, in Carlsbad, teems with nearly 500 workers in yellow hard hats. When it’s done next year, it will take in more than 100 million gallons of Pacific Ocean water daily and produce 54 million gallons of fresh, drinkable water. While this adds up to just 10 percent of the county’s water delivery needs, it will, crucially, be reliable and drought-proof—a hedge against potentially worse times ahead.
The county is betting on a combination of modern engineering and decades-old desalination technology. A pipe trench under construction leads to a nearby lagoon inlet; 18 house-size concrete tanks await loads of sand and charcoal to treat the salt water before it is ready for desalination; pressurizers lead to a stainless-steel pipe one meter in diameter. This final piece of gleaming hardware will convey water under high pressure into 2,000 fiberglass tubes, where it will be squeezed through semipermeable polymer membranes. What gets through will be fresh water, leaving brine behind.
The process is called reverse osmosis (RO), and it’s the mainstay of large-scale desalination facilities around the world. As water is forced through the membrane, the polymer allows the water molecules to pass while blocking the salts and other inorganic impurities. Global desalination output has tripled since 2000: 16,000 plants are up and running around the world, and the pace of construction is expected to increase while the technology continues to improve. Carlsbad, for example, has been outfitted with state-of-the art commercial membranes and advanced pressure-recovery systems. But the plants remain costly to build and operate.
Seawater desalination, in fact, is one of the most expensive sources of fresh water. The water sells—depending on site conditions—for between $1,000 and $2,500 per acre-foot (the amount used by two five-person U.S. households per year). Carlsbad’s product will sell for around $2,000, which is 80 percent more than the county pays for treated water from outside the area. One reason is the huge amount of energy required to push water through the membranes. And Carlsbad, like most desalination plants, is being built with extra pumps, treatment capacity, and membrane tubes, the better to guarantee uptime. “Because it is a critical asset for the region, there is a tremendous amount of redundancy to give high reliability,” says Jonathan Loveland, vice president at Poseidon Water, the owner of the plant. “If any piece fails, something else will pick up the slack.”
Already, some 700 million people worldwide suffer from water scarcity, but that number is expected to swell to 1.8 billion in just 10 years. Some countries, like Israel, already rely heavily on desalination; more will follow suit. In many places, “we are already at the limit of renewable water resources, and yet we continue to grow,” says John Lienhard, a mechanical engineer and director of the Center for Clean Water and Clean Energy at MIT. “On top of that we have global warming, with hotter and drier conditions in many areas, which will potentially further reduce the amount of renewable water available.” While conservation and recycling will help, you can’t recycle what you don’t have. “As coastal cities grow,” he says, “the value of seawater desalination is going to increase rapidly, and it’s likely we will see widespread adoption.”
Against this grim backdrop, there is some good news. In short, desalination is ripe for technological improvement. A combination of sensor-driven optimization and automation, plus new types of membranes, could eventually allow for desalination plants that are half the size and use commensurately less energy. Among other benefits, small, mobile desalination units could be used in agricultural regions hundreds of miles away from the ocean, where demand for water is great and growing.
Smart Water
Every two weeks, Yoram Cohen, a chemical engineer who heads the Water Technology Research Center at the University of California, Los Angeles, hits the road for the drought-blasted San Joaquin Valley. Part of the state’s vast agricultural midsection that grows much of the country’s produce, the region has suffered badly. Last year, 2014, was the third straight drought year—at a time when demand for water has reached an all-time high. I joined Cohen for a recent outing: a car ride from his labs at UCLA to the small valley town of Firebaugh, in one of the hardest-hit agricultural regions in the state. Along I-5, the highway that connects the cities of California’s southern coast with its central valley, we saw vast water-engineering edifices built in the 1950s, including four vast pipes traversing the Tehachapi
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