Inside a treatment plant

A city drinking-water plant takes river or reservoir water and makes it fit to drink. It does not do one big magic step. It runs a chain of steps, and each step targets a different kind of pollutant. If one step is skipped or poorly run, the next steps cannot fully make up for it.

1. Intake and screening. Bars and screens stop leaves, twigs and fish. Some plants add a gentle pre-treatment.

2. Coagulation. Most clay and silt particles carry a small negative electric charge, so they repel each other and stay floating. The plant adds a coagulant, usually alum (aluminium sulfate) or an iron salt. Its positive ions cancel the charges, and the tiny particles can finally touch and stick together. Water is mixed hard for a few seconds so the chemical reaches every particle.

3. Flocculation. The water is then stirred slowly. The sticking particles grow into fluffy clumps called flocs, many times bigger than the starting particles. Stirring too fast would break them up again.

4. Sedimentation. In a wide, calm basin the heavy flocs sink to the bottom as sludge, which is removed. The water on top is already much clearer.

5. Filtration. The water trickles down through a bed of sand, often topped with crushed anthracite (a hard coal), and the remaining particles stick to the grains. Over days the filter clogs, so it is cleaned by backwashing: water is pushed up through the bed in reverse. Many plants add granular activated carbon, a very porous material whose huge surface adsorbs (holds on its surface) dissolved organic compounds, which improves taste and smell and removes many chemicals.

Finally comes disinfection (next lesson), a last adjustment of pH, and storage in a clean covered tank, from which the water is pumped to homes.

How clear is clear enough? Clarity is measured as turbidity in NTU (nephelometric turbidity units), using how much light the particles scatter. Rules vary by country; in the United States, for example, the Environmental Protection Agency requires filtered water from conventional plants to be no more than 0.3 NTU in at least 95% of samples, and never above 1 NTU. This matters because germs can hide inside or behind particles, and because some parasites, such as Cryptosporidium, are very resistant to chlorine, so a good filter is the main defence against them.

In the Walkthrough you will tour five stations (intake, coagulation and flocculation, sedimentation, filtration, disinfection), each with its own dial. Watch how each station changes one measurement.