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How is chlorine used to purify public water supplies?
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halogen
Modern version of the periodic table of the elements.
Chlorine is widely used as a water-purification agent to disinfect public water supplies. The process, known as chlorination, involves adding chlorine or chlorine compounds to water to eliminate pathogens.12
Methods of Application
- Gaseous Chlorine: Often the most economical method for large volumes of water, it is applied directly from pressurized steel containers.2
- Chlorine Compounds: Liquid sodium hypochlorite or solid calcium hypochlorite (in tablet or granular form) may also be used.2
- Chloramination: This involves using chloramines—a combination of chlorine and ammonia—which provides a longer-lasting disinfecting effect and reduces the formation of harmful by-products.2
Considerations
Chlorine is sometimes added during the pretreatment stage (prechlorination) to control bacterial growth in pipes and tanks. However, because chlorine can react with natural organic matter to form potentially harmful disinfection by-products, such as trihalomethanes, it is often applied after other treatment steps like coagulation, sedimentation, and filtration to minimize these risks. Maintaining a residual concentration of chlorine throughout the distribution system ensures that water remains safe until it reaches the point of use.32