Water treatment-or the purification and sanitation of water-varies as to the source and kinds of water. Municipal waters, for example, consist of surface water and ground water, and their treatment is to be distinguished from that of industrial water supplies. Municipal water supplies are treated by public or private water utilities to make the water potable (safe to drink) and palatable (aesthetically pleasing) and to insure an adequate supply of water to meet the needs of the community at a reasonable cost. Except in exceedingly rare instances, the entire supply is treated to drinking water quality for three reasons: it is generally not feasible to supply water of more than one quality; it is difficult to control public access to water not treated to drinking water quality; and a substantial amount of treatment may be required even if the water is not intended for human consumption.
Raw (untreated) water is withdrawn from either a surface water supply (such as a lake or stream) or from an underground aquifer (by means of wells). The water flows or is pumped to a central treatment facility. Large municipalities may utilize more than one source and may have more than one treatment facility. The treated water is then pumped under pressure into a distribution system, which typically consists of a network of pipes (water mains) interconnected with ground level or elevated storage facilities (reservoirs). As it is withdrawn from the source, surface water is usually screened through steel bars, typically about 1 in (2.54 cm) thick and about 2 in (5.08 cm) apart, to prevent large objects such as logs or fish from entering the treatment facility. Finer screens are sometimes employed to remove leaves. If the water is highly turbid (cloudy or muddy), it may be pretreated in a large basin known as a pre-sedimentation basin to allow time for sand and larger silt particles to settle out.
All surface waters have the potential to carry pathogenic (disease-causing) microorganisms and must be disinfected prior to human consumption. Since the adequacy of disinfection cannot be assured in the presence of turbidity, it is first necessary to remove the suspended solids causing the water to be turbid. This is accomplished by a sequence of treatment processes that typically includes coagulation, flocculation, sedimentation, and filtration. Coagulation is accomplished by adding chemical coagulants, usually aluminum or iron salts, to neutralize the negative charge on the surfaces of the particles (suspended solids) present in the water, thereby eliminating the repulsive forces between the particles and enabling them to aggregate. Coagulants are usually dispersed in the water by rapid mixing.