each of these processes. Electrodialysis is a voltage driven process and uses an electrical potential to move salts selectively through a membrane, leaving fresh water behind as product water. Reverse osmosis is a pressure-driven process, with the pressure used for separation by allowing fresh water to move through a membrane, leaving the salts behind. Scientists have explored both of these concepts since the turn of the century, but their commercialization for desalting water for municipal purposes has occurred in only the last 30 to 40 years. Electrodialysis was commercially introduced in the early 1960s, about 10 years before reverse osmosis. The development of electrodialysis provided a cost-effective way to desalt brackish water and spurred considerable interest in the whole field of using desalting technologies for producing potable water for municipal use. Electrodialysis depends on the following general principles: Most salts dissolved in water are ionic, being positively (cationic) or negatively (anionic) charged. These ions migrate toward the electrodes with an opposite electric charge. Membranes can be constructed to permit selective passage of either anions or cations. Another means of purification is the freezing of the water. Extensive work was done in the 1950s and 1960s to develop freezing desalination. During the process of freezing, dissolved salts are naturally excluded during the initial formation of ice crystals. Cooling saline water to form ice crystals under controlled conditions can desalinate seawater. Before the entire mass of water has been frozen, the mixture is usually washed and rinsed to remove the salts in the remaining water from adhering to the ice crystals. The ice is then melted to produce fresh water. Theoretically, freezing has some advantages over distillation, which was the predominant desalting process at the time the freezing process was developed. These advantages include a lower theoretical e...
