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Why is a chlorine residual maintained in water-distribution systems?

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Diagram of a water treatment plant showing the process from water inlet through mixing, coagulation, flocculation, settling, filtering, chemical treatment with chlorine, storage, and high-lift pumping to the distribution system. — water purification Water from inlets located in the water supply, such as a lake, is sent to be mixed, coagulated, and flocculated and is then sent to the waterworks for purification by filtering and chemical treatment. After being treated, the water is pumped into water mains for storage or distribution.

A chlorine residual is maintained in water-distribution systems to ensure that the water remains safe for consumption at the points of use. By keeping a residual concentration throughout the system, water suppliers provide ongoing protection against pathogenic bacteria, which is essential for preventing the spread of waterborne diseases.12

While disinfection is typically the final stage of treatment, maintaining this residual is a critical public health measure to protect water quality as it travels through pipes and tanks to reach consumers. Some systems utilize chloramines—a combination of chlorine and ammonia—because their disinfecting effect lasts longer than that of chlorine alone, offering extended protection throughout the distribution network.12