How To Purify Salt Water In The Wild

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

Peter Gleick, president of the Pacific Constitute, distills an answer:

Even with all of the water in Earth'due south oceans, we satisfy less than half a percent of human water needs with desalinated water.* Nosotros currently utilize on the society of 960 cubic miles (4,000 cubic kilometers) of freshwater a year, and overall there'south plenty water to go around. In that location is increasing regional scarcity, though.

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

The problem is that the desalination of water requires a lot of energy. Salt dissolves very easily in h2o, forming strong chemical bonds, and those bonds are difficult to pause. Energy and the engineering science to desalinate water are both expensive, and this ways that desalinating water can be pretty plush.

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, country prices, financial agreements, and fifty-fifty the salt content of the water. It tin can cost from just under $1 to well over $2 to produce i cubic meter (264 gallons) of desalted water from the body of water. That'due south about as much as two people in the U.S. typically go through in a twenty-four hour period at home.

But switch the source to a river or an aquifer, and the cost of a cubic meter of h2o can plummet to ten to 20 cents, and farmers often pay far less.

That ways it's notwithstanding nigh always cheaper to use local freshwater than to desalinate seawater. This price gap, nonetheless, is closing. For example, meeting growing demand by finding a new source of water or past building a new dam in a place like California could cost upwards to sixty cents per cubic meter of h2o.

And sometimes these traditional means of "harvesting" h2o are no longer available. As such, this cost effigy is expected to continue to rise, which is why California is at present seriously considering desalination and why the city of Tampa, Fla., decided to build the biggest desalination institute in the U.S.

The International Desalination Clan says that equally of 2007 there were virtually 13,000 desalination plants operating around the world. They pumped out approximately 14.7 billion gallons (55.6 billion liters) of drinkable freshwater a twenty-four hours. A lot of these plants are in countries similar Saudi Arabia, where energy from oil is cheap but h2o is scarce.

So how is energy used to separate table salt from water?

There are 2 bones methods for breaking the bonds in saltwater: thermal distillation and membrane separation. Thermal distillation involves oestrus: Humid water turns information technology into vapor—leaving the salt backside—that is collected and condensed back into water past cooling it downward.

The nearly common type of membrane separation is called opposite osmosis. Seawater is forced through a semipermeable membrane that separates salt from water. Because the technology typically requires less energy than thermal distillation, nearly new plants, like Tampa's, at present use contrary osmosis.

At that place 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 exercise with the separated salt, which is left over every bit a very concentrated alkali. Pumping this supersalty h2o back into the ocean can damage local aquatic life. Reducing these impacts is possible, simply it adds to the costs.

Despite the economic and ecology hurdles, desalination is condign increasingly attractive as we run out of water from other sources. We are overpumping groundwater, nosotros have already built more dams than nosotros tin beget economically and environmentally, and we have tapped nearly all of the accessible rivers.

Far more must be done to use our existing water more than efficiently, but with the world'due south population escalating and the water supply dwindling, the economic tide may soon turn in favor of desalination.

The Pacific Plant is an Oakland, Calif.–based, nonprofit think tank devoted to solving the globe's water needs. The arrangement 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'due south Water, in which he and his colleagues explore desalination and other topics.

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