Chlorination is by far the most frequently used disinfection method in United States municipal drinking water treatment plants. The acting disinfectant may be chlorine or a chlorine derivative, such as hypochlorous acid (most commonly), chloramines, or chlorine dioxide. Several treatment methods have been developed. Simple chlorina-tion involves adding chlorine after filtration or as the only treatment. Chlorine-ammonia treatment utilizes the addition of both ammonia and chlorine and the germicidal action of chloramines. Residual chlorination is applied to provide residual chlorine in the water. Breakpoint chlori-nation adds sufficient chlorine to react with ammonia and all other chemicals present as well as to assure a free chlorine residue.

Liquid chlorine is the least expensive form of chlorine. It was used in most large municipal water works until several large cities restricted or prohibited transportation and storage of large volumes of liquid chlorine to prevent accidental release into the atmosphere. Chlorine can be used and stored more safely in its solid form as Ca(OCl)2. However, the cost is substantially higher.

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