WATER PURIFICATION

treatment of water to make it safe and acceptable for human use. Such treatment has grown vastly in importance in the 20th century because of the growth of cities and development of industry and, consequently, of pollution. The first treatment plant was built in 1829 to purify Thames River water by filtration. After the London cholera epidemic of 1854 was traced to a public well, the problem of water treatment received widespread attention. A dramatic proof of the value of treatment came in 1892, when a cholera epidemic that ravaged Hamburg was escaped by the neighbouring city of Altona, Ger., thanks to its filtration system. Cholera outbreaks soon led to development of filtration plants in most of the world's cities, beginning with western Europe and the United States. Filtration is still the most widely used method of purification. In slow filtration, the water is allowed to pass through a deep layer of fine sand; most of the impurities are removed by the top inch or two of sand, which is removed and cleaned from time to time or, in modern plants, is washed in place by special wash water. In rapid-filtration plants, the water is treated with a coagulant, such as aluminum sulfate, ferric chloride, or ferric sulfate, which flocculates particles, carrying most suspended matter to the bottom in sedimentation tanks. After this preparation, the water is passed at a relatively rapid rate through small beds of coarse sand that are washed from time to time. Heavily polluted waters may be chlorinated both before and after filtration. Aeration (mixing air with the water) is carried out if undesirable amounts of iron and manganese are present; they are held in solution in water only in the absence of oxygen. See also water softener; desalination.