Flow and spill reporting

What do we mean by 'flow' and 'spill'?

The amount of wastewater leaving our treatment works is measured and often referred to as ‘flow’. If there’s heavy rain, the sewer network may struggle to cope with the amount of wastewater and rainwater in it. The screened wastewater, which can be around 95% rainwater, is released through storm overflows.

The storm overflows act as a pressure valve to release excess water through outfalls into rivers and the sea. This protects homes and communities from flooding. Releases from storm overflows are called spills.

We publish our annual flow and spill figures here (last updated March 2023).

We want to reduce storm overflow releases, so we have set up a Clean Rivers and Seas Task Force to look at ways to do this. This includes slowing the flow of water entering the sewers, making better use of existing infrastructure or building bigger infrastructure to cope with the amount of water.

Our flow data

Flow data required by permit, is reported every year to the Environment Agency. The graph below shows the total amount of treated water that left these sites each year in cubic metres, divided by county.

As the population of our region continues to grow, the volume of wastewater received by our sites increases too. As a result, our flow figures fluctuate year-on-year based on the number of people who choose to live in the South East.

Although this would suggest our flow should increase year-on-year, other factors impact our flow too. For instance, some of our sewers carry rainwater away from gutters and street gullies to our treatment works to prevent flooding. As a result, our flow is also higher during years with a wet winter.

Our spill data

In some areas, sewers are ‘combined’ meaning rainwater from gutters and street gullies is also channelled to our wastewater treatment works during a storm, to protect properties and streets from flooding.

If our wastewater treatment works receives more rainwater and wastewater than we're able to process or store, our network could back up which could cause flooding via manholes, drains and toilets. To prevent this, where we can, we store and filter the surplus through a screen before releasing it through outfalls into rivers and the sea – This is called a spill.

Water companies are permitted by the Environment Agency to spill only under strict conditions, to protect homes, protect the environment and manage storms.

Sometimes, we also have to spill if wastewater stops moving through our network – for example, due to a pump failure. Again, wastewater with nowhere else to go can flood homes and communities, so we release it to prevent this happening.

Total number of spills per county, per year

We report our spill data every year to the Environment Agency. The graph below shows the total number of spills from our wastewater treatment sites each year, divided by county.

Last year the levels of rainfall were truly exceptional, especially in March, April and July and a prolonged wet winter that all contributed to our network being under severe pressure.

Groundwater also plays a part. We had 150% more rainfall than normal between November 2022 and January 2023, starting the year with exceptionally high groundwater levels (the highest ever recorded at one of the sites in our region). This has added further complications to an already complex issue.

Key

The acronyms used in the data files have the following definitions:

• CEO – Combined Emergency Overflow: A storm overflow and an emergency overflow from a wastewater pumping station that discharge via the same outlet.
• SSO – Settled Storm Overflow: An overflow designed to discharge heavily diluted and settled wastewater via an outfall pipe directly to controlled waters, when flows to a wastewater treatment works exceed the pass forward rate capacity for the site and the storm tanks are full, due to rainfall and/or snowmelt.
• EMO – Emergency Overflow: An overflow at a wastewater pumping station which allows spillage of foul sewage to a watercourse or other waters in an emergency, in the event of mechanical or electrical failure of the pumping station, or due to failure of the downstream rising main.