Chlorine, a strong oxidizing agent, is cheap, reliable, easy to use and monitor, and safe when handled properly. Chlorination with chlorine gas is the oldest method of continuous disinfection for public water supplies. It was initially introduced in 1904. Disinfection by chlorination has been studied extensively and there is a great body of experience upon which to draw. Chlorination is the standard by which other disinfection procedures are judged. Alternative disinfecting forms of chlorine are hypochlorites, chlorine dioxide and products of chlorine-ammonia reactions.

Simple diffuser systems are adequate for distributing chlorine into water, but the supply tank may have to be warmed if high feeding rates result in evaporative cooling of liquid chlorine. Other hazards of working with chlorine are explosions of pressure vessels (especially if corrosion weakens them) and violent reactions when chlorine comes in contact with oxidizable substances.

Chlorination is the final step for most wastewater treatment plants. In addition to disinfection of the effluent, BOD is reduced because reaction with chlorine substitutes for reaction with oxygen. The residual chlorine discourages iron bacteria that form slimes in effluent conduits, and insects may also be killed. Most smelly compounds in treatment plant effluents are easily reacted with chlorine, usually to odorless products. Unfortunately, the products of chlorination may be hazardous, and some are known carcinogens. Concentrations in the environment may be much higher than in the effluents because microorganisms or other life forms ingest and store chlorinated organic compounds.