Drink Up: Taking the Salt Out of Seawater

Almost three quarters of Earth's surface is covered with water, but most of it is too salty to drink. And the 2.5 percent that is freshwater is locked up either in soil, remote snowpacks and glaciers or in deep aquifers. That leaves less than 1 percent of all freshwater for humans and animals to drink and for farmers to use to raise crops—and that remnant is shrinking as rising global temperatures trigger more droughts. The upshot: it's becoming increasingly difficult to slake the world's thirst as the population grows and water supplies dwindle. Analysts at the investment bank Goldman Sachs estimate that worldwide water use doubles every 20 years.

So the search for new water sources is on. One proved candidate is desalinization—technologies that extract the salt from brine drawn from the oceans or saline aquifers to create potable water. But the historically high price of desalinization has largely kept it at bay, a situation that's changing as technology improves and growing demand squeezes freshwater supplies .

"The two main desalinization techniques are distillation and reverse osmosis, or RO," says Menachem Elimelech, an environmental engineer at Yale University. "Distillation, in which the raw water is evaporated and then condensed as freshwater, is energy-intensive, so it's mainly used in the Middle East where oil is abundant." Thermal salt-removing processes require high temperatures so they tend to be expensive (more than $1 per cubic meter of freshwater), but the use of rejected "waste" heat from other industrial or power plant operations for co-generation can cut energy expenditure.

More commonly, however, desalinization plants rely on RO, which is based on high-tech polymer membranes that are permeable to water, but reject the passage of dissolved salts, Elimelich says. When a saline solution sits on one side of a semipermeable membrane and a less salty solution is on the other, he explains, water diffuses through the membrane from the less concentrated to the more concentrated side. Scientists call this phenomenon osmosis, which tends to equalize the salinity of the two solutions.

In the 1950s and '60s researchers realized that they could reverse the process by applying pressure to the more concentrated solution, causing water molecules there to traverse the membrane, leaving behind a condensed brine. To counter the osmotic pressure that arises between the solutions and force water back through the membrane, desalinization plants must utilize high pressures of 7,000 to 8,300 kilopascals (71 to 86.5 kilogram-force per square centimeter or 1,000 to 1,200 pounds per square inch), he notes.

Common RO membranes are thin-film composites that combine a mechanically robust support layer made of microporous polysulfone with a micron-thick polyamide "filter" layer through which water molecules can pass but nothing else. The latter substance is "a second cousin to DuPont's Kevlar—the super-strong aramid polymer fiber used in lightweight body armor," says Bill Mickols, senior research scientist at Dow Water Solutions (DWS) of Edina, Minn., the biggest supplier of such products. RO membranes have matured during the past two decades, he says, with marked improvements in water permeability, salt-rejection capability, operating life (now as long as three to five years) and cost.

These advances, in combination with energy-recovery devices that take pressure from the concentrated brine stream and transfer most of it to the incoming water flow, have made desalinization more affordable. Current RO facilities desalinize seawater for 68 to 90 cents per cubic meter. The average delivery price of municipal water in the U.S. is around 60 cents a cubic meter, according to the American Water Works Association.

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