Storm overflows FAQs

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Message last updated - Friday 09^th May 2025

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Message last updated - Friday 09^th May 2025

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What is a storm overflow and how do they work?

A storm overflow (previously known as a Combined Sewer Overflow or CSO) acts as a safety valve, allowing excess water into rivers and the sea to prevent sewers overflowing during periods of heavy or prolonged rainfall or snow melt.

Storm overflows protect homes and businesses from flooding. Without them this excess storm water would back up through toilets, drains and manholes. Because of the job they do, we know the majority of what comes out of them is rainwater.

Sewers have not been built like this for years, since well before the water industry was privatised, but each of our storm overflows is permitted to operate by the Environment Agency (EA) under certain conditions as they are recognised as an important part of the sewer system.

What is an EDM monitor?

Event Duration Monitoring is the collection of data about storm overflow events. This is measured by the number of spills at each storm overflow point and the number of hours an overflow discharged for.

How much sewage typically overflows?

99.7% of the waste water we treat, goes through our process, exactly as it should, roughly 0.3% escapes via storm overflows.Under extreme weather conditions storm overflows prevent sewers from becoming overloaded, acting as the pressure release that allows floodwater to escape into a river or the sea, and not back up into the toilets and shower trays of ground floor bathrooms. Because of the job they do, most of what they release is extremely diluted and largely rainwater – approximately 95% - not raw sewage.

Each of our overflows has been assessed for the environmental risk potential, as defined by the Environment Agency, and they have each been permitted to act as a ‘storm overflow’ under certain conditions. We report all spills to the Environment Agency.

The Environment Agency has calculated that storm overflows are responsible for just 1-4% of poor river health.

How much are you investing in addressing storm overflows?

We’ve been investing in dealing with storm overflows for many years, prioritising that investment to where it will have the most benefit to the environment. From 2025, we proposing a further £1bn investment will go directly into reducing storm overflow spills, by creating new storage and ways to prevent surface water from entering the sewer network, installing additional monitoring and increasing the capacity of our treatment sites to deal with more rainfall as a result of unpredictable weather. We aim to reduce discharges from storm overflows by 17% in the next five years, in line with the Storm Overflows Discharge Reduction Plan.

Will customer bills need to rise further to cover cost of investment to reduce spills and clean up the environment?

Yes. While we think we have achieved the right balance between investing in our environment in the most targeted and effective ways, to bills at relatively low levels, the scale of investment needed to eradicate, or find alternatives to CSOs, means customer bills would need to rise.

Surely stopping all storm overflows should be the priority?

We recognise customers want us to take swift action to end storm overflow discharges. We intend to meet these expectations and have geared our Business Plan from 2025 to 2030 to address the highest priority overflows soonest.

Have you achieved 100% EDM coverage?

As of the 14 December, yes all of our permitted Storm Overflows now have monitors on them.

Does this data tell us when rivers are safe to swim in?

There are many potential hidden dangers to swimming in open water, as well as considerations about water quality impacted by agricultural and urban run off or storm overflow activity, especially after heavy rainfall. And the impact of bacteria and viruses from animals in the river environment too.

In addition to water company assets, there are many private sewage assets that are owned and operated by individual homes and businesses across the region. These also discharge to the environment, but their activity is currently not tracked or reported on.

We would always encourage people to contact their local wild swimming group for more specialist advice. The Rivers Trust also has useful contacts on its website, as does the EA Swimfo website.

There is lots of useful information around wild swimming from The Rivers Trust. Read more here. And here.

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