How do we clean sewage?
Sewage is the dirty waste product generated from water after it's been used – and we treat a massive 1.4 billion litres of the stuff every day! Besides human waste, sewage also contains food, detergents, oils, metal residues, sand and sometimes even harmful chemicals. It’s our job to remove all these so we can return the cleaned sewage back into rivers or streams.
Stage 1 – Primary treatment
Sewage passes through screens to remove any large pieces of debris such as plastic, paper, rags or wipes. The liquid then flows through channels to remove grit that's been washed in from places like roads. The sewage then rests in large settlement tanks where fine particles sink to the bottom to form sludge. We then take the sludge away for further treatment, using it to generate electricity and fertiliser.
Stage 2 – Secondary treatment
Next up, we treat the settled sewage using special bacteria. The bacteria feed on organic matter and turn it into carbon dioxide, water or nitrogen. There’s two ways we do this.
The first method uses biological filters that encourage bacteria to grow in deep beds of irregularly shaped stones. The gaps between the stones are full of air, which allow the bacteria to grow and breathe as it breaks down the settled sewage. This allows the liquid to then pass through.
The second, more modern method we use is called activated sludge treatment. Air is bubbled through the liquid to encourage bacteria to grow. Once the bacteria have cleaned the sewage, the liquid is passed into settlement tanks before flowing back into the river.
Stage 3 – Final polish
In order to meet even higher standards set by the Environment Agency, treated water can be put through a final filtration process using either shallow beds of gravel or reed beds. This gives the water a final clean before it's returned to watercourses.
Time to clear things up even more
Sewage treatment poster
From catchment to sludge recycling, here’s everything you need to know about treating sewage.
Package plant fact sheet
Learn the key differences between the two types of packing plants we use at Severn Trent.
Sewage treatment gallery
Get a snapshot of the many processes Severn Trent use to clean sewage.