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Storm overflows

We are ready to embark on the biggest programme for reducing overflows in the world


will be invested in England and Wales to 2030 in an intensive programme to reduce sewage overflows


of storm overflows in England and Wales are now monitored

100,000 kilometres of England’s sewer pipes were built before the mid-1960s and use an old-fashioned design that carries two kinds of waste: rainwater from street drains, and sewage from homes.

These combined sewers were built many decades before modern rates of population growth and climate change were understood, which means that there is often now more sewage and rainwater going into these systems than first envisaged.

Storm overflow diagram

What are Storm Overflows?

Storm Overflows are an outlet from the public sewer that is designed to spill excess rainwater and sewage into the environment if sewers become inundated with rainwater. They do this to prevent the combined mixture flooding back up into homes.

Plans for Urgent Change

In early 2024, we will release a National Overflows Plan that updates the public on the actions industry intends to take. Companies have confirmed that they are ready to back the plan with investment worth more than £10bn - a more intensive investment than any country has ever attempted on overflows, and triple the current rate. The Plan will include:

  • Increasing the capacity of sewage treatment works, allowing them to treat higher volumes of rainfall and sewage;
  • Installing storm overflow tanks to hold surges in rainwater that would otherwise overload the system, with new storage equivalent to several thousand new Olympic swimming pools;
  • Replacing concrete with grass and ponds to reduce rainfall run-off entering sewers, protecting them against the overloading that causes spills; and
  • Improvements to the sewer network, including expanding and improving pipes, allowing them to safely carry more during peak times.

Transparency and accountability

Every water and sewage company in England will have their own map showing the operation of all of their overflow spills. Building on that, as an industry we are also working to:

  • Bring individual companies’ data together into a single national picture, available to any member of the public that wants to see what’s happening in their area – in near real-time 
  • Ensure that the map has independent oversight to ensure people can be confident in what they’re seeing
  • Add further data as it becomes available, for example from monitors checking the quality of river water so everyone can see the impact sewage is having

This means that the nation’s swimmers, canoeists, tourists, and anglers will all be able to see exactly what’s happening, virtually as it’s happening, across every river and beach. This will be the first time anywhere in the world that live storm overflow information will be made available right across the country.

Our world-leading plan to remove 4 million spills from rivers and seas

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Our plan, published in early 2024, sets out when each of the 14,187 overflows in England will benefit from our unprecedented levels of proposed investment – we are ready to more than triple what we’re spending (up from £3.1bn – which includes Thames Tideway – to £10.2bn).  

We also recognise that transparency is an important part of how we will reassure people about the progress being made. That’s why we are also committing to the world’s first ever national data hub to provide data on the operation of all 15,000 sewage overflow sites across England. This will empower people will information and improve accountability.

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The rules that determine the legality of a spill are set out in each overflow’s permit, with which compliance is very high (about 97% in 2022). We get though that this also misses the point because regardless of legality, people want these things gone.

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The engineering solutions for the removal of CSOs are complicated as many of the older sewers serve towns and cities running under buildings and roads. The ‘replacement of all CSOs’ would take many years to achieve and result in major disruption. This is not a reason not to do it, but there is no quick fix.

The cost to re-plumb these areas is estimated to be up to £600 billion nationwide. This level of investment needs to be agreed by regulators and balanced with keeping customer bills affordable. Water companies have been addressing CSOs for years, taking a prioritised approach to those causing environmental harm first.  

If CSOs are removed, then solutions are required to address excess surface water which will cause flooding. Adequate flood defense systems must be installed which include solutions for the storage of rainwater from highways and buildings. This will require land acquisition which can be difficult in urban areas. 

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The biggest impact to the environment when Storm Overflows operate is when non-flushable items cause blockages in sewers. These discharges are more harmful as they are not predominantly rainwater. Wet wipes, cotton buds, sanitary products and Fats Oils and Greases (FOGs) all create blockages in the pipes that make Storm Overflows discharges more likely.  

Reducing the likelihood of blockages caused by non-flushable items such as wipes, and FOGs is a key way that individuals can help. 

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Companies have published their Drainage and Waste Water Management Plans (DWMPs) which have informed their investment for PR24.  

DWMPs are the plans for the future of drainage, wastewater and environmental quality and provide the basis for more collaborative and integrated long-term planning by organisations that have interests and/or responsibilities relating to drainage, flooding and protection of the environment. The production of DWMPs is led by water companies, all of these organisations have a part to play in their creation. By planning together, organisations are more likely to work together to deliver improvements that benefit the communities we live and work in. 

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No, they are in many other European countries too. The UK actually has a much lower number of CSOs than other countries and in fact, some countries do not even know how many they have. England and Wales achieved 100% monitoring at the end of 2023.

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Companies first published their plans for investing in sewage works and overflows in October 2023. Since then, they have been discussing the detail with regulators and the government. We have now published an updated and consolidated version summary of these plans with detailed of planned activity across England.

Storm overflows Q&A


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Every water and sewage company in England has developed a plan to expand drains and sewers and reduce sewage going into rivers

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