The method most often used is rapid sand filtration that operates at rates of 125 to 250 million gallons per acre. Filters consist of a layer of sand about two feet deep that rests on graded layers of gravel that overlays an underground drainage system. The sand used is coarse, and the actual grain size is determined by the raw water quality. The sand bed is made with the same uniform grain size in order to assure proper backwashing. The underground drain system serves to collect the filtered water, and to distribute the wash-water back through the filter media when the system is backwashed.
Chlorination is used almost universally in U.S. public water supplies.
Chlorine gas is most often used in large systems. In small water works, calcium or sodium hypochlorite is frequently used. The amount used varies with the level of contaminating microorganisms. Relatively uncontaminated water can be disinfected with small doses, usually less than one part per million. More contaminated water requires chlorination of several parts per million.