Water treatment means changing water quality so that it is suitable for a defined purpose. It may remove unwanted substances, inactivate harmful microorganisms, adjust chemistry or control conditions that damage equipment. Applications include drinking-water supplies, wastewater discharge or reuse, commercial and industrial processes, and building-water hygiene. Treatment is broader than purification, filtration or disinfection, and suitability for one use does not automatically imply suitability for drinking. The Drinking Water Inspectorate describes a multiple-barrier approach in which individual stages prepare water for subsequent treatment. Physical, chemical and biological processes address different problems and must be selected against representative source-water analysis and a finished-water specification. Wastewater treatment reduces pollutants rather than automatically producing potable water. Building systems need continued management even when the incoming supply has been treated. Performance must be verified with suitable operational measurements and laboratory analysis. Competent maintenance, records, failure responses and responsible residual management are essential. Reviewed 7 October 2026 using DWI and HSE guidance, with historical Defra material checked only for established wastewater process descriptions.