As well as getting safe, clean and reliable water to you, we also have to take it away once you have used it. So, when you empty the bath, flush the toilet or use your washing machine, it’s our job to make sure that all that used water – now called ‘sewage’ or ‘wastewater’ – is put safely back into rivers.
The sewage treatment process
1. Taking the wastewater away
When you flush the toilet or empty the sink, the wastewater goes down the drain and into a pipe, which takes it to a larger sewer pipe under the road.
The sewer then joins our network of other sewers and takes the wastewater to a sewage treatment works – sometimes it needs to be pumped there.
At the sewage works we put the wastewater through several cleaning processes so that it can be put back safely into rivers.
• The first stage of cleaning the wastewater is to remove large objects that may block or damage equipment, or be unsightly if allowed back into the river. This includes items that should never have been put down the drain in the first place – such as nappies, face wipes, sanitary items and cotton buds – but often can be things like bricks, bottles and rags!
• The wastewater often contains a lot of grit that gets washed into the sewer, so we have special equipment to remove this as well.
3. Primary treatment
The wastewater still contains organic solid matter – or human waste. The next stage is to separate this from the water, and to do this, we put the wastewater into large settlement tanks, which causes the solids to sink to the bottom of the tank. We call these settled solids ‘sludge’.
In a circular tank, large arms, or scrapers, slowly move around the tank and push the sludge towards the center where it is then pumped away for further treatment.
The water passes over a wall near the top of the tank and is taken to the next stage of the treatment process.
4. Secondary treatment
Although the visible bits of sludge have been removed, we have to ensure that the smaller and sometimes invisible nasty bugs are also taken out.
At our larger sewage treatment works, the wastewater is put into rectangular tanks called ‘aeration lanes’, where air is pumped into the wastewater. This encourages the good bacteria to break down the nasty bugs by eating them. The more they eat, the more they grow and multiply until all the nasty bugs have gone.
5. Final treatment
The treated wastewater is then passed through a final settlement tank, where the good bacteria sink to the bottom. This forms more sludge – some of it is recycled back to the ‘secondary treatment’ stage, and the rest goes to ‘sludge treatment’. The now clean water passes over a wall near the top of the tank.
6. Sludge treatment
The sludge we collect at the start of the process is then treated and put to good use. Most of it is recycled to agricultural land for farmers to use as fertilizer
7. Back to the river
Next the water passes through a series of UV lights, now the wastewater is clean, it can be returned to local rivers and streams. The water we put back into the river is very important as it helps to keep them healthy.
The quality of the cleaned wastewater is strictly regulated by the Environment Agency, and we test it to make sure that it meets high-quality standards.
Watch how the process works