Only 3% of the water on earth is fresh. Three-fourths of that fresh water is in the form of ice. As we waste more of our freshwater supply, water desalination will become a big part of our health. Although water desalination is expensive, it is sometimes cheaper than shipping freshwater long distances. Desalted water provides the primary source of municipal supply in many areas of the world, and its use is increasing in the United States. (Nathanson 144) Some techniques for desalination are already used widely, while others are now being developed and refined. There are four main methods of desalination: electrodialysis, reverse osmosis, flash distillation, and solar humidification. Some methods are more effective in different parts of the world. Also, some methods are only used to treat brackish water as they do not remove very much salinity. If a liquid rich in ions is subjected to an electrical field be means of two electrodes with a continuous potential difference applied between them, the cations will be attracted to the cathode and the anions will be attracted to the anode. (Degremont 303) In the electrodialysis process, the water to be treated is placed in a large tray. A series of membranes (some negat
This instantaneous boiling and the generation of vapor without a heat supply is known as flashing. Distillation takes place and the hot sea water is cooled in the process. Further flash steam can be released at a lower pressure and temperature in a series of stages. (How It Works 729) Usually, a flash distillation plant is consisted of a series of chambers that are running at lower temperatures than the previous one. There are also condensing coils that are banks of tinned tubes. The heat that starts the process is steam that has already completed a job, such as a steam turbine in another part of the facility. The sea water flows from one chamber to the next producing water vapor. The vapor is condensed and the freshwater falls into collecting trays. Then the water is put into bulk storage. Some very large plants have been built for this type of desalination. One, in Tijuana, Mexico, was built in 1970. It uses local sea water and produces 27 million liters of distilled water per day. Abu Dhabi has six facilities distilling nine million liters of water per day. This method is cheaper and more effective than reverse osmosis or electrodialysis. There have been problems with membrane breakage in those methods, hampering production. The last method of desalination is solar humidification. This is a very cheap and simple method, but production is limited. Sea water is placed in shallow trays with black lining. Black lining is used because it collects heat better than any other color. Over the trays is a ro
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