Many cities use three basic processes: (1) coagulation and settling, (2) filtration, and (3) disinfection.
Coagulation and settling
The raw (untreated) water flows into the treatment plant and is mixed with chemicals. Some of these chemicals are coagulants. The most widely used coagulant is a fine powder called aluminum sulfate or alum. In the water, the alum forms tiny, sticky globs called flocs. Bacteria, mud, and other impurities stick to the flocs. The water then passes into a settling basin, where the flocs settle to the bottom. Coagulation and settling remove most impurities.
Filtration
The water is then passed through a filter. The filter consists of a bed of sand, or of sand and coal, usually about 21/2 feet (76 centimeters) deep on top of a bed of gravel about 1 foot (30 centimeters) deep. As the water trickles down through the filter, any remaining particles are screened out. The water then flows to huge reservoirs for a final treatment that kills bacteria.
Disinfection
kills disease-carrying bacteria. Most plants disinfect water by adding a substance called chlorine. Chlorine sometimes is added before coagulation and settling, and often after filtration. Most cities chlorinate their water, even if they do not treat it in any other way.
