Conventional drinking water treatment

The classic surface water treatment train. Each stage is a barrier, and the design principle is that no single stage is relied upon alone.

Multiple barriers: Each stage removes different things. Filtration handles Cryptosporidium; chlorination handles bacteria and viruses; carbon handles organics. No single barrier covers everything.
Order matters: Organic matter is removed before chlorination, because chlorine reacting with organics forms disinfection by-products.
Residual: A small chlorine concentration is deliberately maintained in the network to guard against contamination between the works and the tap.

Groundwater sources are often much simpler — sometimes disinfection alone — because the aquifer has already provided filtration.

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Raw water passes through screening, then coagulation and flocculation where a chemical dose makes fine particles clump, then sedimentation where flocs settle out, then rapid gravity filtration through sand, then activated carbon for taste, odour and micropollutants, then disinfection by chlorine or ultraviolet light, then pH adjustment and the addition of a chlorine residual before entering the distribution network.