Combined Sewer Overflows

What is a combined sewerage system?

Across much of the UK, our sewerage network is set up as a ‘combined system’. This means that our sewage water and surface water flow into the same pipe system before travelling to a treatment plant. However, when the sewerage system becomes overloaded – because of heavy rainfall for example – water companies are allowed to use sewage overflows (including Combined Sewer Overflows (CSOs) and pumping stations) to discharge untreated human sewage and waste water into the environment.

How often are storm overflows being used?

Sewage overflows are meant to act as an emergency relief valve for our sewerage system to stop sewage backing up into our homes. Legislation states they should only be used in ‘exceptional circumstances’.

Despite this, we’re seeing hundreds of thousands of discharges happening every year. In 2023 alone, water companies discharged sewage into rivers and seas over 600,000 times.

And that’s just the discharges that water companies tell us about. At SAS we’ve become increasingly concerned that discharges are being used to regularly dispose of untreated sewage, even during times of low rainfall – or none at all.

All this sewage entering the environment is wreaking havoc on our precious environment, and putting our health at risk. Check out how storm overflows are making us sick.

How do I find out where sewage overflows are?

There are around 22,000 sewage overflows in the UK. Sometimes they take the form of very obvious pipes going straight out to sea, or large grates opening onto beaches, but most are well-hidden, out of sight of unsuspecting eyes.

To see the locations of CSOs in your area please click below (if data is available):


Northern Ireland 


South West Water Region

Thames Water Region

Southern Water Region

Yorkshire Water Region

Anglian Water

Wessex Water Region

United Utilities Region

Northumbrian Water Region

Can we get rid of storm overflows?

We won’t be able to get rid of storm overflows overnight. But there are lots of ways we can reduce our reliance on them. We can:

  • Install separated sewage and surface water systems for new developments.
  • Reduce pressure on the sewage network by planting trees and installing sustainable urban drainage systems (SUDS) that show water down.
  • Introduce nature-based solutions, such as reinstating wetlands.
  • Increase storage capacity.
  • Only flushing the ‘three Ps’ – poo, pee, paper – down the loo.