Process — How drinking water is produced

The diagram shows the eight stages by which raw water is turned into safe drinking water and delivered to homes.

Overall, the water passes through a sequence of increasingly fine cleaning steps — from screens that trap rubbish, to settling tanks, sand filters and finally chemical disinfection — before being stored and pumped through underground pipes to consumers.

The process begins with water collected in a reservoir, which is fed by rivers and rainfall. From there it passes through screens, which trap large debris such as leaves, sticks and rubbish. The water then flows into a sedimentation tank, where it is held still long enough for mud and grit to sink slowly to the bottom. Next, it is filtered downwards through layers of sand and gravel, which capture the fine particles that remain.

At the fifth stage a small dose of chlorine is added to kill any remaining bacteria, making the water safe to drink. The clean water is then kept in a covered storage tank until it is needed. A pumping station maintains pressure in the network, and finally the water travels through underground pipes to homes, schools and other buildings.