Storm overflows

Storm overflows (also referred to in the media as combined sewer overflows or CSOs) have been in place for many years to protect people’s homes from flooding, but the spotlight is increasingly falling on them, and the perception of the damage they do to the environment.

Many storm overflows are decades, if not hundreds of years old, and this legacy infrastructure is an antiquated system for the modern world. Removing them completely would be complicated, disruptive and expensive, with an estimated price tag of £600 billion to effectively re-plumb major towns and cities across the UK.


While replacing storm overflows can’t be achieved overnight, we are focussed on finding ways to optimise the system to increase the capacity of our sewer network, tackle surface water flooding, and reduce storm spills. Storm overflows are not classed as a pollution. To find out more about pollutions and how we are tackling them, visit our Pollution Incident Report Plan here.


As part of our Get River Positive commitment, we've pledged to ensure storm overflows and sewage treatment works do not harm rivers. We’ve also committed to be as transparent as possible with the data we collect about our water recycling network and the improvements that we're making, especially around storm overflows.


The map here shows our latest storm overflow data.

What are storm overflows?

Data-led improvements

Storm overflow performance is measured by how often and for how long storm overflows discharge into the environment. Anglian Water has been working closely with the Environment Agency to install Event Duration Monitors (EDMs) across our network to provide a robust and consistent way of monitoring this data.


Our overall EDM coverage is now at 86%, up from 54% in 2021, and we will have 100% coverage by the end of this year. This data helps inform our overall strategy of reducing environmental storm overflows and their impact.  


Our 2022 EDM data shows that we’re making good progress in reducing spills to an average of 20 per year by 2025 across all storm overflows with EDMs in place.  This year we have reduced spills to 15 (from 25 in 2021) against an industry average of 29, effectively meeting the 2025 target.


We have also reduced the total duration of those spills by 54% from the year before. That means that the monitored storm overflows in our region spilled for around 1% of the entirety of 2022. That’s still 1% too much, and we recognise one year of performance ahead of target does not mean the job is complete, but the progress and positive downward trajectory is clear.


Year-on-year data changes

The rapid decline in spills we’ve seen this year can be attributed to a number of reasons. 2022 was predominantly an exceptionally dry year (though the end of 2022 saw above average rainfall in parts of the region). While this gives us confidence that our overflows are only operating when they are permitted to – in extreme rainfall – the decline is also a result of the improvements we’re making with targeted investment and action on-the-ground.  
Since 2021, we have also thoroughly interrogated and cleaned the EDM data to improve accuracy, speed and provide quality assurance. By cross-referencing EDM data with other telemetry systems we have in situ, and by investing in more sensors on our network, we have been able to verify genuine spills.

When is a spill not a spill?

As a result of our data accuracy improvements, we’ve found some EDM recorded ‘spills’ are not genuine spills.  The sensors can be triggered by a number of normal things, such as losing communications because of intermittent or poor signal strength, equipment faults, or simply a spider crawling over the sensor – all of which can cause false readings. This is why it’s essential we cross reference this data with other telemetry data and local investigations.


The data accuracy improvements made this year have contributed to the dramatic year-on-year change in spill frequency. With a more accurate dataset, we have been able to make the right investment decisions to address priority storm overflows and therefore create real operational improvements.


Targeted investments for environmental gain

We recognise that any harm caused by our assets is unacceptable, and despite the improvements our EDM data shows, we know there is still more to do. We continue to work hard and collaborate with others to reach our shared ambition of eliminating harm from storm overflows. We have committed to invest close to £220million between 2020 and 2025 to further improve our performance.


Latest developments include: 


  • Substantial improvements to our sewer network, installing over 100 storm tanks to increase capacity and thereby reduce storm spills, and by using more intelligent technology to plan our maintenance and jetting programme.
  • Telemetry investments with 22,000 sewer monitors which alert us proactively to issues on our network using artificial intelligence (AI). This is helping us to prevent spills before they occur 
    Significant improvements to the EDM monitors themselves and the ability to cross reference data with other sensors on our network, means we can more easily verify genuine spills over false activations of monitors.
  • Targeting our highest spillers from rainfall in 2021, through our Storm Overflow Assessment Framework (SOAF), we’ve conducted 40 investigations so far to understand what, if any, environmental impact they have. As a result, 10 priority schemes have been put forward which will reduce spills by 2025 (mainly by installing storm tanks), and one has already been delivered.
  • A host of additional investment is being ploughed in to improve river health in other ways through Get River Positive schemes such as wetlands and river restoration.


Dr Robin Price, Director of Quality and Environment, Anglian Water

Whilst our data shows that overall number of hours of spills from our storm overflows reduced by over 50 per cent last year compared to 2021, we are not complacent about the work that still needs to be done to reduce the impacts of spills on our rivers and waterways. This year, we are investing £39m to reduce the impacts from storm overflows, and as part of our Get River Positive initiative, we have committed to eliminate all serious pollutions by 2025, a goal we are currently on track to meet.