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How does chlorination disinfect drinking water?

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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.

Chlorination is a critical water treatment process used to disinfect drinking water by destroying pathogenic bacteria and other disease-causing microorganisms. This practice, which became a standard public health measure in the early 20th century, was instrumental in the virtual elimination of waterborne diseases such as cholera and typhoid in developed nations.12

The process involves adding chlorine or chlorine compounds—such as liquid sodium hypochlorite or calcium hypochlorite—to water. Gaseous chlorine is often used for large-scale operations due to its economic efficiency. A key advantage of chlorination is that a residual concentration of chlorine can be maintained throughout the water distribution system, providing ongoing protection against contamination until the water reaches the point of use.3

While effective, the process requires careful management. Chlorine can react with naturally occurring organic matter in raw water to form potentially harmful disinfection by-products, such as trihalomethanes. To mitigate these risks, chlorine is typically applied after stages like coagulation, sedimentation, and filtration.34