Combined storm overflows

CSOs, what are they and why do we need them?

Most of the UK has a combined sewerage system, meaning that both rainwater and wastewater (from toilets, bathrooms and kitchens) are carried in the same pipes to a sewage treatment works.

When our sewer system is operating normally, combined sewers collect rain water that runs off gutters, drains and roads, as well as sewage. We call this wastewater, which then gets taken to our wastewater treatment works, where it is cleaned, treated and returned safely to the environment to rivers or the sea.

Most of the wastewater network was built over 100 years ago during the Victorian times, if we were designing a system now, we would do it very differently and have separate pipes for sewerage and rainwater. This is how new housing developments are designed.

During heavy rain storms, more water enters the pipes of these older ‘combined’ systems than they are designed to cope with, so they have been designed to safely relieve the pressure through release points - known as Combined Storm Overflows or CSOs. CSO release the flows – which is around 95% surface water - into river of the sea. Without these release points, the sewerage system would back up, and cause sewage flooding to streets, highways and cause toilets to overflow inside properties.

It is important to point out that CSOs are designed to operate during heavy rain, so that if they release wastewater then any sewage present is heavily diluted with rain and surface water into waterbodies which should also be in flood.

The operation of our CSOs is highly regulated and is permitted and monitored by our environmental regulators Natural Resources Wales and Environment Agency. CSOs have always existed on the sewer network and are not a new creation. It is possible that they are releasing more due to increased storm frequency linked to climate change, as well as increased amounts of impermeable surfaces like paving or concrete. These surfaces means more water runs off into the drains than on to an absorbent surface such as grass.

What have we done so far?

Welsh Water has installed more than 2,300 monitors on CSOs since 2015. There has been no material change in performance since then, but their operation varies year on year with the weather. The monitors tell us more about how and when our CSOs operate than we have ever known before. Some people ask why can’t CSOs be removed from the system completely. While we would not build a combined sewerage system today, removing CSOs is not an option. Firstly there is the cost implication as it would cost anywhere between £9 billion and £14 billion to separate our systems or remove CSOs completely. This would have significantly detrimental effect on customers bills and make them unaffordable. Secondly there is the disruption that would be caused as it would involve digging up just about every street and even green areas in our communities which would also take decades to complete. CSOs are essential infrastructure but we must reduce our reliance on them.

We now have spill monitors on 99% of all of our CSOs. The monitors record the number and duration of spills and this data – known as EDM data - is published on our website and reported to our environmental regulators. You can see our EDM map below.

We also provide real time spill information for key bathing waters to interested bodies, including Surfers Against Sewage and Rivers Trust.

If a CSO isn’t operating the way we would expect or a pollution incident is reported, we’ll investigate to understand what is happening and what action we need to take. To make sure this is acceptable to Natural Resources Wales and the Environment Agency, we share and discuss our EDM data with them on an annual basis. They can then help us decide and prioritise what, if anything, we need to do about the overflows and by when.

In the next few years we will:

  • Build on and expand our bathing water CSO alert systems to community and representative groups- We will provide real time spill updates within an hour of a CSO operating by 2025.
  • We will continue to lobby for Welsh Government to ban wet wipes as part of their new regulations on single use plastics. Wipes containing plastic do not dissolve if flushed away and gather in the sewer pipe to cause blockages which can cause pollution incidents, including from CSOs. Blockages currently account for around 40% of the pollution incidents we deal with each year.

2022 EDM Data

Event Duration Monitors (EDM) record the number of times and duration of time our storm overflows have operated. With monitors now on over 99.5% of our overflows, we are able to provide one of the most comprehensive reports on how our storm overflows perform. We report this information to our environmental regulators and also provide real time spill information for key bathing waters to interested bodies, including Surfers Against Sewage and Rivers Trusts

Our data for 2022 shows that we saw:

  • 12% reduction in the average number of storm releases per storm overflow compared 2021, from 43.5 to 38.3
  • 25% reduction in total spill duration in 2022, from 807512 hrs to 602987.5 hrs

While we are pleased to see the reduction, which in part is due to the investment work we are undertaking to reduce spills, the reduction has also been influenced by the weather we saw last year. The annual rainfall for the year was 10% below the long term average, we saw the driest January-August period since 1976 and drought conditions were declared across parts of Wales and England.

The total removal of combined storm overflows from our system is unaffordable and would take decades and therefore is not an option but what is in our control is the ability to target investment to CSOs which have the biggest environmental impact. That is why we are investing significantly to improve CSOs with £140m being invested between 2020-2025 and a further £420m planned from 2025 to 2030. You can read more about our investment plans here.

The interactive map below shows the information we have reported from our monitors in 2022.

We are currently developing a new map which will provide information of a storm overflows operating within an hour - for bathing waters and high amenity sites initially. We aim to have this live within the next year.


Annual Emergency Overflow Annual EDM return - DCWW 2022

192.8kB, XLSM

Annual Storm Overflow EDM return - DCWW 2022

196.3kB, XLSM

NRW Annual Emergency Overflow Unpermitted Storm Overflow EDM return - DCWW 2022

87kB, XLSX

NRW Annual incl Shellfish Waters Storm Overflow EDM return DCWW 2022

369.3kB, XLSX


EDM Return Dwr Cymru Welsh Water Emergency Overflow Annual 2021

51kB, XLSX

EDM Return Dwr Cymru Welsh Water Storm Overflow Annual 2021

50.3kB, XLSX

EDM Return DCWW_Wales Water Annual 2021

270.8kB, XLSX