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ater 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Raw (untreated) water is withdrawn from either a surface water supply (such as a lake or stream) or from an underground aquifer (by means of wells).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 As it is withdrawn from the source, surface water is usually screened through steel bars, typically about 1 in (2.54 cm) thick and about 2 in (5.08 cm) apart, to prevent large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.

Other chemicals may be added at the same time, including powdered activated carbon (to absorb taste- and odor causing chemicals or to remove synthetic chemicals); chemical oxidants such as chlorine, ozone, chlorine dioxide, or potassium permanganate (to initiate disinfection, to oxidize organic contaminants, to control taste and odor, or to oxidize inorganic contaminants such as iron, manganese, and sulfide); and acid or base (to control pH). Coagulated particles are aggregated into large, rapidly settling "floc" particles by flocculation, accomplished by gently stirring the water using paddles, turbines, or impellers. This process typically takes 20 to 30 minutes. The flocculated water is then gently introduced into a sedimentation basin, where the floc particles are given about two to four hours to settle out. After sedimentation, the water is filtered, most commonly through 24-30 in (61-76 cm) of sand or anthracite having an effective diameter of about 0.02 in (0.5 mm).

When the raw water is low in turbidity, coagulated or flocculated water may be taken directly to the filters, bypassing sedimentation; this practice is referred to as direct filtration. Once the water has been filtered, it can be satisfactorily disinfected. Disinfection is the elimination of pathogenic microorganisms from the water. It does not render the water completely sterile but does make it safe to drink from a microbial standpoint. Most water treatment plants in the United States rely primarily on chlorine for disinfection. Some utilities use ozone, chlorine dioxide, chloramines (formed from chlorine and ammonia), or a combination of chemicals added at different points during treatment. There are important advantages and disadvantages associated with each of these chemicals, and the optimum choice for a particular water requires careful study and expert advice.

Chemical disinfectants react not only with microorganisms but also with naturally occurring organic matter present in the water, producing trace amounts of contaminants collectively referred to as disinfection byproducts (DBPs). The most well-known DBPs are the trihalomethanes. Although DBPs are not known to be toxic at the concentrations found in drinking water, some are known to be toxic at much higher concentrations. Therefore, prudence dictates that reasonable efforts be made to minimize their presence in drinking water. The most effective strategy for minimizing DBP formation is to avoid adding chemical disinfectants until the water has been filtered and to add only the amount required to achieve adequate disinfection. Some DBPs can be minimized by changing to another disinfectant, but all chemical disinfectants form DBPs. Regardless of which chemical disinfectant is used, great care must be exercised to ensure adequate disinfection, since the health risks associated with pathogenic microorganisms greatly outweigh those associated with DBPs.

There are a number of other processes that may be employed to treat water, depending on the quality of the source water and the desired quality of the treated water. Processes that may be used to treat either surface water or groundwater include:

  1. lime softening, which involves the addition of lime during rapid mixing to precipitate calcium and magnesium ions;
  2. stabilization, to prevent corrosion and scale formation, usually by adjusting the pH or alkalinity of the water or by adding scale inhibitors;
  3. activated carbon adsorption, to remove taste- and odor-causing chemicals or synthetic organic contaminants; and
  4. fluoridation, to increase the concentration of fluoride to the optimum level for the prevention of dental cavities.

Compared to surface waters, groundwaters are relatively free of turbidity and pathogenic microorganisms, but they are more likely to contain unacceptable levels of dissolved gases (carbon dioxide, methane, and hydrogen sulfide), hardness, iron and manganese, volatile organic compounds (VOCs) originating from chemical spills or improper waste disposal practices, and dissolved solids (salinity). High-quality groundwaters do not require filtration, but they are usually disinfected to protect against contamination of the water as it passes through the distribution system. Small systems are sometimes exempted from disinfection requirements if they are able to meet a set of strict criteria. Groundwaters withdrawn from shallow wells or along riverbanks may be deemed to be "under the influence of surface water," in which case they are normally required by law to be filtered and disinfected.

Hard groundwaters may be treated by lime softening, as are many hard surface waters, or by ion exchange softening, in which calcium and magnesium ions are exchanged for sodium ions as the water passes through a bed of ion-exchange resin. Groundwaters having high levels of dissolved gases or VOCs are commonly treated by air stripping, achieved by passing air over small droplets of water to allow the gases to leave the water and enter the air. Many groundwaters-approximately one quarter of those used for public water supply in the United States-are contaminated with naturally occurring iron and manganese, which tend to dissolve into groundwater in their chemically reduced forms in the absence of oxygen.

Iron and manganese are most commonly removed by oxidation (accomplished by aeration or by adding a chemical oxidant, such as chlorine or potassium permanganate) followed by sedimentation and filtration; by filtration through an adsorptive media; or by lime softening. Groundwaters high in dissolved solids may be treated using reverse osmosis, in which water is forced through a membrane under high pressure, leaving the salt behind. Membrane processes are rapidly evolving, and membranes suitable for removing hardness, dissolved organic matter, and turbidity from both ground and surface waters have recently been developed.