How To Filter Ocean Water

Why don't we become our drinking water from the ocean by taking the salt out of seawater?

Peter Gleick, president of the Pacific Institute, distills an reply:

Why don't we get our drinking water from the ocean by taking the salt out of seawater? — Credit: Getty Images

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

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

The trouble is that the desalination of h2o requires a lot of energy. Salt dissolves very easily in water, forming stiff 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 exist pretty plush.

It's difficult to put an exact dollar effigy on desalination—this number varies wildly from place to place, based on labor and energy costs, land prices, fiscal agreements, and even the salt content of the water. Information technology tin can toll from just under $1 to well over $2 to produce 1 cubic meter (264 gallons) of desalted water from the body of water. That's about as much equally 2 people in the U.S. typically get through in a twenty-four hours at dwelling.

But switch the source to a river or an aquifer, and the cost of a cubic meter of h2o tin can plummet to 10 to xx cents, and farmers oft pay far less.

That means it'south still almost always cheaper to use local freshwater than to desalinate seawater. This price gap, however, is closing. For case, meeting growing demand by finding a new source of h2o or by building a new dam in a place like California could price upward to 60 cents per cubic meter of water.

And sometimes these traditional means of "harvesting" water are no longer available. Equally such, this cost effigy is expected to keep to ascent, which is why California is now seriously considering desalination and why the city of Tampa, Fla., decided to build the biggest desalination found in the U.S.

The International Desalination Association says that every bit of 2007 there were about 13,000 desalination plants operating around the world. They pumped out approximately 14.7 billion gallons (55.half-dozen billion liters) of drinkable freshwater a day. A lot of these plants are in countries like Saudi arabia, where energy from oil is inexpensive merely water is deficient.

Then 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 mutual type of membrane separation is chosen opposite osmosis. Seawater is forced through a semipermeable membrane that separates table salt from water. Because the technology typically requires less energy than thermal distillation, well-nigh new plants, like Tampa's, now use reverse osmosis.

There are environmental costs of desalination, equally well. Sea life can get sucked into desalination plants, killing small body of water 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 equally a very full-bodied brine. Pumping this supersalty h2o back into the ocean can impairment 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 nosotros run out of water from other sources. We are overpumping groundwater, we have already built more than dams than nosotros tin can beget economically and environmentally, and we take tapped nearly all of the accessible rivers.

Far more must be washed to use our existing water more efficiently, but with the globe's population escalating and the h2o supply dwindling, the economic tide may presently turn in favor of desalination.

The Pacific Institute is an Oakland, Calif.–based, nonprofit think tank devoted to solving the world'due south water needs. The organization reviewed these problems in depth in a 2006 report entitled "Desalination, with a Grain of Common salt." Peter Gleick also authored a book in 2000 chosen 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.