Ever wonder what happens when you Flush? We all take for granted having access to toilet facilities, but are you aware what happens after you flush? We investigate the water treatment processes to explain what our water companies do for us.

This month, we take a look into the processes involved after we Flush and how the water and waste is managed and recycled.

Water Collection Process

Rainwater is collected from roofs, roads and pavements in a system, called a surface water sewer.

Surface water goes straight into a river, which is why you must not pour any wastewater into surface water drains.

However, in some areas, including central London, surface water and sewage are mixed together, before being treated at the sewage works.

1 . Taking the water 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 an underground network of other sewers which take the wastewater towards the sewage treatment works. Sometimes it needs to be pumped there.

At the sewage works, sewage goes through several cleaning processes so that it can be put back safely into rivers.


In London there are people called sewer flushers who regularly inspect the large Victorian sewers to ensure that London’s waste keeps moving! Some might say it’s a Big Job 😉

Recently, News reporters have brought attention to a Gigantic ‘FatBerg’ in London which has become a major problem. Due to what people are flushing down toilets and sinks, the build up of grease and fats has created an underground monster of lard, reported to be a ‘Fat Burg’ of epic proportions.

Monster Sewer FatBerg weighs 130 tonne

A fatberg weighing the same as 11 double decker buses and stretching the length of two football pitches is blocking a section of London’s ageing sewage network.

The solid mass of congealed fat, wet wipes, nappies, oil and condoms was found in the Victorian-era tunnel in Whitechapel in September 2017.

2. Screening Process

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! Nappies, face wipes, sanitary items and cotton buds are often found and surprisingly, bottles and bricks.

The wastewater often contains a lot of grit that gets washed into the sewer, so special equipment is used 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.

This is why it’s important to use Flush Hygiene for Sanitary Waste Collection services. Bin it, don’t flush it….

3. Primary Treatment

The wastewater still contains poo (or if you haven’t had breakfast yet, human waste 🙂

The next stage is to separate this from the water, so wastewater is put into large settlement tanks, which cause solids to sink to the bottom.

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?

Sludge from the bottom of the tanks is used to generate renewable energy, which on average saves the water company £15m per year in electricity costs. At Thames Water sewage works in Didcot, the sludge is used to generate renewable gas that supplies up to 200 homes in the area.

4. Secondary Treatment

Although the visible bits of sludge have been removed, it is imperative the invisible nasty bugs are also taken out.

In 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

5. Final treatment

The treated wastewater is passed through a final settlement tank, where the good bacteria sinks 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. This is the final stage of the water treatment process.

6. Sludge treatment

The sludge collected at the start of the process is then treated and put to good use:

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.

7. Back to the river

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

Water Treatment Process

This information was extracted from relevant water companies to educate our readers and customers as to the process which takes place. It’s in all our interests, to avoid flushing anything other than toilet roll down toilet systems, to avoid blockages and problems.