Seawater has been converted into drinkable freshwater for decades in the Middle East and in the Caribbean. Only a few major seawater desalination plants exist in the U.S.; the largest operates in Tampa, Fla., and a project twice the size is being developed in Carlsbad, Calif. But that number could grow quickly as millions of people move to coastal communities, which often have insufficient groundwater. “Almost 20 desalination plants are proposed for California alone,” says Tom Pankratz, a desalination consultant in Houston. Installations are being considered in Texas and Georgia, as are more in Florida.
On average, seawater contains about 35,100 milligrams per liter of dissolved solids, 99 percent of which are salts. The World Health Organization considers water potable when that level drops below 500 mg/L. Various processes can achieve the conversion, but today two contenders together account for about 88 percent of worldwide capacity: multistage flash distillation and reverse osmosis.