Four common stages of conventional drinking-water treatment are coagulation and flocculation, sedimentation, filtration, and disinfection. This educational grouping combines coagulation and flocculation; it is not a universal classification or a requirement that every system use four stages. The CDC describes five common steps when coagulation and flocculation are listed separately. Coagulation destabilises fine particles and flocculation encourages them to form larger groups. Sedimentation separates settleable flocs and produces sludge requiring management. Filtration reduces remaining material through selected media or membranes; different filter types have different capabilities. Disinfection uses a suitable chemical or physical process to inactivate susceptible microorganisms. Its effectiveness depends on appropriate conditions and verification, while UV does not provide a continuing distribution residual. These processes form complementary barriers rather than interchangeable solutions. The Drinking Water Inspectorate emphasises selecting treatment to suit source-water properties and contaminants. Some supplies require additional adsorption, ion exchange, membranes or chemistry adjustment. Wastewater treatment uses a different preliminary, primary, secondary and tertiary framework and does not automatically produce potable water. Building systems require ongoing hygiene controls after the incoming supply has been treated. Effective treatment needs representative sampling, monitoring, maintenance, defined failure responses and responsible management of residuals. Reviewed 7 October 2026 using CDC and DWI guidance.