The process of changing sea water into drinking water involves removing unwanted salt, and is referred to as desalination. Desalination is used to produce water on some oceangoing surface ships and submarines, and is also used industrially on the shore to produce drinking water for permanent populations there. In general, however, desalination always costs much more than the sanitation of ground water supplies with chlorine or other methods, and therefore it is only used in those unusual circumstances where there simply is no adequate groundwater supply available, especially in parts of the Middle East.
- Desalination Methods -
The most basic method of changing sea water into drinking water is simply to boil the water and collect the steam. Water has a much lower boiling point than salt; as a result, as it is boiled, the salt is left behind. If the steam is then collected and allowed to condense back into water, the product is purer than the original salt water. This method of distillation is also what must be done, for example, if you find yourself in an emergency situation with only salt water on hand and a need to quickly convert it into something drinkable.
In an industrial or laboratory setting, obviously, the process can be much less crude or improvised than in the above scenario. In particular, boiling the water in a low-pressure chamber is more cost-effective because the water will boil at a lower temperature, and therefore save the desalination facility in heating costs.
In addition, more modern desalination plants may make use of other, newer techniques, such as reverse osmosis. In these plants, salt water is forced through a special membrane so that the salt is separated out.
- Problems with Large-Scale Desalination -
In theory, desalination offers an easy solution for the problem of scarce fresh water supplies in the world today: the (in many cases nearby) oceans dwarf the lakes and rivers in terms of the water available, and desalination plants make the oceans available to us for drinking water. The drawback is that, at present, no methods for changing sea water into drinking water are comparable in cost to the more common process of changing unclean groundwater into purified drinking water (through, for example, chlorination or ozone). For several years, Poseidon Resources has been promoting plans for a plant in Carlsbad, California, which could produce tens of millions of gallons of water per day, making it the largest such plant on the continent - but at the intimidating cost of $300 million.
In addition, large-scale desalination is not without environmental consequences. The output of the plant is not simply clean drinking water, but also extremely salty brine - the byproduct into which all of the salt has been concentrated. Unless this can then be turned into producing table salt (which may be an option in some cases), this waste must eventually be pumped back into the ocean. Even marine life, adapted for a saltwater environment, cannot survive in such extremely salty water, so if waste salt water is simply pumped back into the ocean, this would at the very least lead to a dead zone around the plant. Moreover, taking in water, unless special precautions are taken, can also endanger nearby fish life.
Today, major desalination plants nevertheless exist in a number of locations, particularly in the U.S. and the Middle East. The United Arab Emirates has a trio of desalination plants, each an impressive facility capable of producing hundreds of millions of litres of water per day. Australia, Britain, and Israel also operate major desalination plants. In America, such plants can be found in Texas (El Paso), Florida (Tampa), and Arizona (Yuma).