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The sewage treatment process

The sewage treatment process

As well as getting safe, clean and reliable water to you, we also have to take it away once you've 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.

In some areas we collect the rainwater that runs off roofs, roads and pavements in a separate system, called a surface water sewer. Surface water goes straight into a river, which is why you must not pour any waste water into surface water drains.

In some areas, including central London, surface water and sewage are mixed together before being treated at our sewage works.

The sewage treatment process diagram

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.

Did you know?

In London we have a team of sewer flushers who regularly inspect the large Victorian sewers to ensure that London’s waste keeps moving!

2. Screening

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.

Did you know?

Over 55,000 sewer blockages each year are caused by people putting the wrong things down the drain. As a result, 7,000 homes and gardens are flooded. The message from our sewer flushers is “Bin it - don’t block it”.

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 centre 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.

Did you know?

We use the sludge to generate renewable energy which on average saves us £15m per year in electricity costs. At our sewage works in Didcot, the sludge is used to generate renewable gas that supplies up to 200 homes in the area - a UK first.

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.

Did you know?

Our 350 sewage treatment works treat 2,800 million litres of sewage every day from our 14 million customers.

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.

Sometimes additional treatment is needed if the river that the treated wastewater will be returned to is particularly sensitive. The treated wastewater is slowly filtered through a bed of sand, which acts as a filter and catches any remaining particles.

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 fertiliser, but we also use it to generate energy. We do this in three ways:

  • Combined heat and power - this process treats the sludge using a process called ‘anaerobic digestion’. This is where the sludge is heated to encourage the bacteria to eat it. This creates biogas that we then burn to create heat, which in turn creates electricity.
  • Gas to grid - we can also clean the biogas to a higher standard (known as biomethane) so that we can put it into the national gas grid to power homes, businesses and schools.
  • Thermal destruction - this process involves drying the sludge into blocks called ‘cake’, which are then burned to generate heat. We capture this heat and turn it into electricity.

Did you know?

We’ve been generating electricity from waste for more than 50 years and have the largest renewable generation capacity inside the M25.*

*excluding the commercial electricity generators.

7. Back to the river

Now the wastewater is clean, it can be returned to local rivers and streams. In some areas, 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.

Did you know?

We have over 43,500 miles of sewer, 2,530 pumping stations and 1.2 million manholes.

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