What do we mean by 'flow' and 'spill'?
The amount of wastewater which leaves our treatment works after being fully treated is measured and often referred to as ‘flow’.
If there’s heavy rain or sudden influxes of groundwater, the volumes entering sites can increase quickly and overload our capacity. When this happens, we screen these highly diluted volumes, made up of around 95% rainwater, and release them into the environment without full treatment, via storm overflows. Releases from storm overflows are called spills.
In this way, the historic network of storm overflows, which operates throughout the UK and around the world, acts as a pressure valve to release excess flows into rivers and the sea. This prevents sewers from backing up and flooding customers’ homes and communities.
We agree with our customers that this system should not be relied upon as much as it has been. That is why we are working hard to reduce storm overflow releases, with this work being led by our Clean Rivers and Seas Task Force. They are exploring a range of engineering and nature-based solutions to stop or slow the flow of rainwater and groundwater entering sewers in the first place – keeping them free to deal with wastewater only wherever possible.
Our flow data
Flow data 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 catchment.
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 wastewater treatment works to prevent flooding. As a result, our flow is also higher during years with a wet winter.
Our spill data
The graph below shows the total number of spills from our wastewater treatment sites each year, divided by county.
Water companies are permitted by the Environment Agency to spill only under strict conditions, to protect properties, protect the environment and manage storms.
Sometimes, we have to spill if our wastewater treatment works receives more rainwater and wastewater than we're able to process or store, usually during a storm and when groundwater levels are exceptionally high. We may also have to spill when wastewater stops moving through our network – for example, due to a pump failure. Wastewater with nowhere else to go can flood homes and communities, so we release it to prevent this happening.
