Desalination Plants

Desalination plants such as the U.K.’s Thames Gateway facility use reverse osmosis technology.

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.

And it’s much cheaper than it was two decades ago. The first desalination method—and still the most common, especially in oil-rich countries along the Persian Gulf—was brute-force distillation: Heat seawater until it turns to steam, leaving its salt behind, then condense it. The current state of the art, used, for example, at plants that opened recently in Tampa Bay, Florida, and Perth, Australia, is reverse osmosis, in which water is forced through a membrane that catches the salt. Pumping seawater to pressures of more than a thousand pounds per square inch takes less energy than boiling it—but it is still expensive.

Researchers are now working on at least three new technologies that could cut the energy required even further. The closest to commercialization, called forward osmosis, draws water through the porous membrane into a solution that contains even more salt than seawater, but a kind of salt that is easily evaporated. The other two approaches redesign the membrane itself— one by using carbon nanotubes as the pores, the other by using the same proteins that usher water molecules through the membranes of living cells.

None of the three will be a solution for all the world’s water woes. Desalination inevitably leaves behind a concentrated brine, which can harm the environment and even the water supply itself. Brine discharges are especially tricky to dispose of at inland desalination plants, and they’re also raising the salinity in parts of the shallow Persian Gulf. The saltier the water gets, the more expensive it becomes to desalinate.

http://ngm.nationalgeographic.com/big-idea/09/desalination-pg2

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.

desalination membranes-diagrams

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.

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.

Prefab Desalination Plants?

No matter how good the membranes become, however, reverse osmosis plants will need to become cheaper to build and operate if they’re to meet the demands of an increasingly thirsty world, particularly in developing regions.

One way to do this is to standardize plant components and methods and to create smaller, more efficient plants, said Yoram Cohen, a chemical and biomolecular engineering professor at the University of California, Los Angeles (UCLA).

“Why are [personal computers] so cheap?” asked Cohen, who was not involved in the new review. “It’s because the technology is standardized. You can buy parts from anyone and exchange them or combine them into your own design.”

http://news.nationalgeographic.com/news/2011/08/110804-fresh-water-crisis-desalination-environment-science/

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