Chlorination is the controlled addition of chlorine to water to reduce harmful microorganisms and, where appropriate, maintain a protective disinfectant residual. DWI guidance explains chlorine sources, free and combined chlorine, demand, mixing and contact-system design. Effective treatment depends on water quality, pH, temperature, chlorine concentration and sufficient effective contact time. Continuous chlorination of a flowing supply differs from one-off disinfection of isolated tanks or pipework during commissioning or remedial work. Chlorination forms part of a wider treatment and risk-management strategy; it does not remove every chemical contaminant or replace clean systems and competent maintenance. Operators should monitor the specified chlorine parameter at representative locations, maintain reliable equipment and investigate failed limits. Taste, odour, disinfection by-products, chemical hazards and environmental discharge must also be managed. Building managers should receive records of the scope, method, treatment conditions, verification, limitations and return-to-service decisions. The full guide includes DWI government guidance, two related technical articles and a homepage link. Reviewed 3 October 2026.