The water treatment process turns a variable raw water source into water that is suitable for a defined use. For a public supply, that means producing wholesome drinking water and protecting its quality through storage and distribution. For commercial and industrial systems, treatment may control hardness, corrosion, suspended solids, microorganisms or dissolved salts. Effective design begins with representative source-water analysis and a measurable finished-water specification. The Drinking Water Inspectorate describes a multiple-barrier approach in which each process prepares water for the next. A typical sequence may include abstraction, intake screening, coagulation, flocculation, clarification, filtration, activated carbon or contaminant-specific treatment, disinfection, final conditioning, storage and distribution. Membrane processes such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis may be used where their separation performance is justified. Treatment performance must be verified through representative operational monitoring, calibrated instruments and laboratory analysis. Sludge, filter backwash, membrane concentrate and regenerant waste also need lawful management. Commercial treatment systems follow the same principles: define the objective, analyse the feed water, select compatible barriers, monitor critical limits and investigate failures rather than repeatedly treating symptoms.