Could Seawater Solve The Freshwater Crisis?

This story is part of a special National Geographic News series on global water issues.

With 1.8 billion people predicted to live in areas of extreme water scarcity by 2025, desalination—the removal of salt from water—is increasingly being proposed as a solution.

But before desalination can make a real difference solving in the looming water crisis, officials and experts need to commit to overcoming obstacles that make the process expensive and inefficient, a new paper argues.

(Read National Geographic magazine’s special water issue online.)

Scientists predict that by 2016, the amount of fresh water produced by desalination plants will exceed 10 billion gallons (38 million cubic meters) a year, or double the rate in 2008.

Modern desalination plants use a technology called reverse osmosis, pressing salty water through ultrathin, semipermeable plastic membranes. Unable to pass through, large molecules or ions, such as salt, are filtered out, so fresh water flows out the other side. (See diagrams of three proposed alternatives to reverse osmosis desalination.)

This method wastes much less energy than earlier desalination techniques, such as heating seawater and harvesting fresh water from the steam. But a typical reverse osmosis plant can still spend up to 40 percent of its operating costs on generating electricity to run the system—a big reason engineers are searching for ways to cut costs and make plants more efficient, starting at the membrane level.

(Interactive: Calculate your water footprint.)

Situation Normal: All Fouled Up?

Reverse osmosis membranes have improved since their invention in the 1960s. Today’s membranes do a better job of allowing water to pass through and keeping salts out, for instance.

The membranes are also more resistant to bacterial contamination, but that doesn’t mean the problem of “membrane fouling” has been completely solved.

“When you operate a membrane, bacteria in the water will accumulate on the thin selective layer, making it more difficult to squeeze water through,” explained Menachem Elimelech, an environmental engineer at Yale University, who co-authored the new paper.

Chlorine can be used to clear away the bacteria, but today’s reverse osmosis membranes are still very sensitive to chlorine and degrade quickly when exposed to the harsh chemical.

“There should be a lot of focus to develop membranes that are chlorine resistant,” Elimelech said.

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