Combined Sewer Overflows

What are combined sewer overflows?

Sewer overflows are an important part of the sewerage network. Combined sewer overflows, or CSOs, act as a pressure relief valve when there is too much rainfall, allowing rain water, mixed with sewage, to rise inside the sewer and eventually enter a separate pipe which flows into a river or the sea. Sewers operate this way to help prevent the flooding of streets, homes and businesses. When we do need to use them, they can sometimes affect river and bathing water quality, albeit temporarily.

Sewer networks have been designed this way for decades but, in recent times, more people have told us they do not like the idea of untreated sewage going into our rivers and seas, no matter how diluted. We agree. However, it will take many years to change how excess rainfall is managed, especially in our region, because:

  • Analysis of Met Office data shows that average annual water runoff in the North West is 28% higher than the average for England and Wales which means more water runs into our sewers;
  • We have a significantly higher proportion of combined sewers than any other water company. Over 54% of our public sewers combine foul and surface water compared to an average of 33%. Combined sewers respond quicker to a storm with the capacity filling up more quickly when compared to more separate systems; and
  • We have 40% more sewer overflows than the industry average.

What is United Utilities doing to reduce spill frequency?

Since 2000, we have invested £1.2bn to improve overflow discharges to reduce spill frequency, volume and impact upon the natural environment.

Our investment has improved the operation of over 1,200 intermittent overflows.

Between 2020 and 2025, we will conduct 195 investigations to understand high spilling overflows to inform spill reduction schemes for future investment.

With this greater number of overflows and higher levels of rainfall in the North West, it means spills happen more often in our region than in other areas of the UK. These key factors, combined with our comprehensive data collection, has resulted in a higher level of reported spills compared to the industry average.

CSO Performance Data

You can see how many times CSOs have operated over the last year on our CSO Performance Data Page. We record every time each CSO operates, regardless of how small the spill or how short the duration. 

We’re learning lessons from the rapid roll-out of monitors and the data capture. We have a greater understanding of the vast 78,000 km wastewater system across the North West than at any point in history. Now that we have had time to interrogate it, the data is showing some anomalies.

Currently, we are over-stating the number of spills as our new monitors are incorrectly recording water that overflows into storm tanks as a spill – in fact this water is stored safely until there is capacity at the wastewater treatment works to treat it to the usual high standards so it does not flow into rivers and seas untreated.

The Environment Agency estimates that sewer overflows lead to around 30 per cent of river and sea pollution in the North West, with water quality in the natural environment affected by rain running off highways and farm land and private drainage being incorrectly connected. We are committed to improving the water environment and working with partners to tackle these issues.

We want to work with river and beach users, regulators and politicians to plan how we can reduce the need for these overflows in the future. We will continue to work with stakeholders across our water catchment areas to set clear environmental and social objectives for improving water quality. You can find out more on our catchment systems thinking page.

The public sewer system has too long been the catch-all last defence for managing surface water in our communities. Better managing the flow of water and reducing its interaction with the piped sewer network is key to helping improve river quality.

You can read more about what we're doing to keep our waters clear of pollution on the following page: What we are doing to keep Bathing Waters clean.