Desalination Of Ocean Water

ocean water-There is about 35g of salt per liter of ocean water, but the salinity of the water varies from place to place and depending on the depth of the water.

lake water- What do we mean by "saline water?" Water that is saline contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm). If water has a concentration of 10,000 ppm of dissolved salts, than one percent of the weight of the water comes from dissolved salts.

Here are our parameters for saline water:

Fresh water - Less than 1,000 ppm

Slightly saline water - From 1,000 ppm to 3,000 ppm

Moderately saline water - From 3,000 ppm to 10,000 ppm

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

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.

SEE