Why can't we purify ocean water?

scientificamerican.com - Why don't we get our drinking water from the ocean by taking the salt out ...

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Even with all of the water in Earth's oceans, we satisfy less than half a percent of human water needs with desalinated water.* We currently use on the order of 960 cubic miles (4,000 cubic kilometers) of freshwater a year, and overall there's enough water to go around. There is increasing regional scarcity, though.

So why don't we desalinate more to alleviate shortages and growing water conflicts?

The problem is that the desalination of water requires a lot of energy. Salt dissolves very easily in water, forming strong chemical bonds, and those bonds are difficult to break. Energy and the technology to desalinate water are both expensive, and this means that desalinating water can be pretty costly. It's hard to put an exact dollar figure on desalination—this number varies wildly from place to place, based on labor and energy costs, land prices, financial agreements, and even the salt content of the water. It can cost from just under $1 to well over $2 to produce one cubic meter (264 gallons) of desalted water from the ocean. That's about as much as two people in the U.S. typically go through in a day at home. But switch the source to a river or an aquifer, and the cost of a cubic meter of water can plummet to 10 to 20 cents, and farmers often pay far less. That means it's still almost always cheaper to use local freshwater than to desalinate seawater. This price gap, however, is closing. For example, meeting growing demand by finding a new source of water or by building a new dam in a place like California could cost up to 60 cents per cubic meter of water. And sometimes these traditional means of “harvesting” water are no longer available. As such, this cost figure is expected to continue to rise, which is why California is now seriously considering desalination and why the city of Tampa, Fla., decided to build the biggest desalination plant in the U.S. The International Desalination Association says that as of 2007 there were about 13,000 desalination plants operating around the world. They pumped out approximately 14.7 billion gallons (55.6 billion liters) of drinkable freshwater a day. A lot of these plants are in countries like Saudi Arabia, where energy from oil is cheap but water is scarce. So how is energy used to separate salt from water? There are two basic methods for breaking the bonds in saltwater: thermal distillation and membrane separation. Thermal distillation involves heat: Boiling water turns it into vapor—leaving the salt behind—that is collected and condensed back into water by cooling it down. The most common type of membrane separation is called reverse osmosis. Seawater is forced through a semipermeable membrane that separates salt from water. Because the technology typically requires less energy than thermal distillation, most new plants, like Tampa's, now use reverse osmosis.

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There are environmental costs of desalination, as well. Sea life can get sucked into desalination plants, killing small ocean creatures like baby fish and plankton, upsetting the food chain. Also, there's the problem of what to do with the separated salt, which is left over as a very concentrated brine. Pumping this supersalty water back into the ocean can harm local aquatic life. Reducing these impacts is possible, but it adds to the costs. Despite the economic and environmental hurdles, desalination is becoming increasingly attractive as we run out of water from other sources. We are overpumping groundwater, we have already built more dams than we can afford economically and environmentally, and we have tapped nearly all of the accessible rivers. Far more must be done to use our existing water more efficiently, but with the world's population escalating and the water supply dwindling, the economic tide may soon turn in favor of desalination. The Pacific Institute is an Oakland, Calif.–based, nonprofit think tank devoted to solving the world's water needs. The organization reviewed these issues in depth in a 2006 report entitled “Desalination, with a Grain of Salt.” Peter Gleick also authored a book in 2000 called The World's Water, in which he and his colleagues explore desalination and other topics.

*Clarification (8/24/08): This sentence has been modified since the original posting.

scientificamerican.com - Why don't we get our drinking water from the ocean by taking the salt out ...

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