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Chlorination
Chlorination is one of the greatest advances in water purification. It is a supplement, not a substitute to sand filtration. Chlorine kills pathogenic bacteria, but is has no effect on spores of certain viruses (e.g. polio, hepatitis) except when used in high doses. It oxidises iron, manganese and hydrogen sulphide; it destroys some taste and odour-producing constituents; it controls algae and slime organisms and aids coagulation.

The disinfecting action of chlorine is mainly due to the hypochlorous acid, and to a small extent due to the hypochlorite ions. The hypochlorous acid is the most effective form of chlorine for water disinfection. It is more effective (70-80 times) than the hypochlorite ion. Chlorine acts best as a disinfectant when the pH of water is around 7 because of the predominance of hypochlorous acid. When the pH value exceeds 8.5 it is unreliable as a disinfectant because about 90% of the hypochlorous acid gets ionised to hypochlorite ions.