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How is chlorine used to disinfect drinking water?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

Chlorine is used to disinfect drinking water by killing harmful microorganisms, such as bacteria, viruses, and protozoa, that may be present in the supply. This process, known as chlorination, is the final step in water purification.12

Chlorine can be applied in several forms:
* Gaseous chlorine: Often the most economical method for disinfecting large volumes of water, it is applied directly from pressurized steel containers.2
* Chlorine compounds: Liquid sodium hypochlorite or calcium hypochlorite in tablet or granular form are also used.2
* Chloramines: Chlorine combined with ammonia is increasingly used for public water supplies because its disinfecting effect lasts longer throughout the distribution system.2

To ensure safety, chlorine is typically applied after coagulation, sedimentation, and filtration to minimize the formation of potentially harmful by-products like chloroform. Maintaining a residual concentration of chlorine throughout the distribution system helps ensure the water remains safe until it reaches the point of use.2