How does a sewage treatment works clean the water you flush away?

A sewage treatment works cleans flushed water in stages: first it strains out the big objects, then it lets solids settle, then it hands what is dissolved to microbes, and sometimes it polishes the result before it goes back into a river or the sea. The aim is simple: to remove contaminants from sewage so that the water can be returned to the environment safely, instead of raw sewage pouring into a river.

What arrives at the works is a mix of everything that goes down the drain from homes and businesses. A bar screen at the entrance catches cans, rags, sticks and plastic packets. Sand and gravel settle out in a grit channel. This first step is there to protect the pumps and tanks that follow, not to clean the water.

Then comes the clever part. In primary treatment the sewage drifts slowly through a big basin: heavy solids sink, while grease and light solids float to the top and get skimmed off. In secondary treatment, microorganisms feed on the organic matter that is left. They grow, multiply and clump together into a sludge that can be settled out, leaving a liquid almost free of solids with far fewer pollutants. Some plants add a tertiary stage, a final polish such as extra nutrient removal or disinfection.

None of this is universal. Globally, an estimated 52% of sewage is treated, and the gap between rich and poor countries is wide.