Where does tap water come from, and how is it treated?

From rivers, reservoirs or underground aquifers, through a treatment sequence that has changed remarkably little in principle for a century — and which is among the largest public health interventions ever made.

The sources:

Surface water — rivers and reservoirs — which is abundant, variable in quality, and needs the most treatment.

Groundwater, pumped from aquifers, naturally filtered by rock and usually cleaner, sometimes needing little beyond disinfection but vulnerable to contamination that persists for decades.

Desalination, energy-intensive and used where nothing else is available.

The treatment sequence:

Screening, removing debris, fish and leaves.

Coagulation and flocculation. A coagulant is added, neutralising the charges that keep fine particles suspended so they clump into larger flocs. This is the step that makes the rest possible.

Sedimentation, where flocs settle out in large tanks.

Filtration, through sand and gravel or membranes, removing what remains — including most pathogens.

Disinfection, usually chlorine, sometimes ozone or ultraviolet. Chlorine is retained deliberately as a residual through the pipe network, so contamination entering a cracked pipe downstream is still dealt with. That residual is why tap water sometimes smells faintly of chlorine, and it is a feature.

Adjustments — pH correction to stop pipes corroding, and in some places fluoridation.

Why the network matters as much as the plant. Water is kept at pressure precisely so that any leak pushes water out rather than drawing contamination in. This is why a boil notice follows a pressure loss.

Lead remains an issue in older properties with lead service pipes or fittings; treatment adds phosphate to line the pipes, but replacement is the real answer.

Bottled water is not meaningfully safer in countries with treated supplies, is typically regulated as a food rather than to drinking water standards, and costs hundreds of times more.