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 negative and only permeable by cations, others positive and permeable by anions) are arranged alternately in the trough to trap the separate ions. So, the water rich in salt is forced in one section of the tray, while the freshwater is in another section. However, salinity can hardly be reduced by more than 50% be movement through one electrodialysis cell without a loss in efficiency. Consequently, most units comprise several stages in series, and the required reduction in salinity is achieved by successive stages. (Degremont 305) The United States has electrodialysis plants converting 2.25 million liters of brackish water per day. Electrodialysis is more effective on water with a lower salt content.
