Desalination

Desalination, also called desalting, removal of dissolved salts from seawater and in some cases from the brackish waters of inland seas, highly mineralized groundwaters, and municipal waste waters. This process renders such otherwise unusable waters fit for human consumption, irrigation, industrial applications, and various other purposes. Existing desalination technology requires a substantial amount of energy, usually in the form of fossil fuels, and so the process is expensive. For this reason it is generally used only where sources of fresh water are not economically available. In addition, the amount of greenhouse gas emissions and brine wastewater generated by desalination plants pose significant environmental challenges. The desalting of seawater is an ancient notion. Desalination methods can utilize either thermal processes or membrane processes. Multistage flash distillation is a thermal process for desalting relatively large quantities of seawater. Based on the fact that the boiling temperature of water is lowered as air pressure drops, this process is carried out in a series of closed tanks set at progressively lower pressures. When preheated seawater enters the first stage, some of it rapidly boils, forming vapour that is condensed into fresh water on heat-exchange tubes. Fresh water is collected in trays as the remaining seawater flows into the next stage, where it also flashes, and the process is continued. In small communities where salt water and intense sunlight are both abundant, a simple thermal process called solar humidification can be used. The heat of the Sun partially vaporizes salt water under a transparent cover. On the underside of the cover, the vapour condenses and flows into a collecting trough. The principal difficulty in this process is that large land areas are required, and energy is needed for pumping the water. Another thermal process makes use of the fact that, when salt water is frozen, the ice crystals contain no salt. In practice, however, objectionable amounts of salt water remain trapped between the crystals, and the amount of fresh water needed to wash the salt water away is comparable to the amount of fresh water produced by melting the crystals.