Chlorine (Cl) and water
Chlorine is widely used to disinfect drinking water. It can inactivate many disease-causing microorganisms and, when appropriately managed, leave a residual that helps protect water as it moves through a distribution system.
Chlorine species in water
Water-treatment “chlorine” usually refers to free chlorine produced when chlorine gas or hypochlorite is added to water. It forms hypochlorous acid (HOCl) and hypochlorite ions (OCl−); the relative amounts depend mainly on pH. These disinfecting species are different from chloride ions (Cl−), which are common dissolved salts and do not provide a disinfectant residual.
Disinfection and residual protection
Chlorine is used as a primary disinfectant and is also commonly applied to maintain a residual in distribution. Effective disinfection depends on the chlorine concentration, contact time, pH, temperature, turbidity and the water’s chlorine demand. Operational targets and regulatory requirements vary by jurisdiction and supply system; operators monitor the process to balance pathogen control, residual protection, taste and by-product formation.
Disinfection by-products
Chlorine can react with natural organic matter in source water and form disinfection by-products (DBPs), including trihalomethanes (THMs) and haloacetic acids. DBP concentrations depend on source-water composition and treatment conditions. Utilities can manage precursors and optimize disinfection to reduce DBP formation while maintaining effective microbial control.
Monitoring and treatment considerations
Free and total chlorine residuals are monitored at treatment works and, where required, throughout distribution. When a process needs dechlorinated water, a suitable removal step can be selected based on the chlorine species, water composition and downstream requirements. Chlorination decisions should be part of a water-safety and treatment strategy rather than based on chlorine concentration alone.
Last reviewed: 31 October 2026 by Lenntech.