Source: BBC Research, Natural England

UK - Water firms illegally spilled sewage on dry days - data suggests

Three major water companies illegally discharged sewage hundreds of times last year on days when it was not raining, a BBC investigation suggests. The practice, known as "dry spilling", is banned because it can lead to higher concentrations of sewage in waterways.

Thames, Wessex and Southern Water appear to have collectively released sewage in dry spills for 3,500 hours in 2022 - in breach of their permits.

Water UK, the industry body, said the spills "should be investigated".

The BBC requested the same data from the other water companies in England, which said they could not respond due to being under an Environment Agency (EA) criminal investigation.

Releasing sewage into rivers and seas is allowed in the UK to prevent pipe systems becoming overwhelmed - but it has to have been raining.

Without rainwater the sewage is likely to be less diluted - leading to build-ups of algae which produce toxins "that can be fatal to pets and pose a health risk to swimmers", says Dr Linda May, a water ecologist at the UK Centre for Ecology and Hydrology.

Discharging in dry conditions is therefore illegal under environmental law.

Graphic showing how illegal dry spills occur - in normal conditions, wastewater flows smoothly through the system to the sewage treatment plant. However, sometimes sewage spills straight out into rivers and seas even when there is no rain.

Collectively throughout 2022, Thames, Southern and Wessex illegally started releasing sewage on dry days 388 times - research by the BBC's climate and data teams suggests - including during last summer when these regions were in drought.

There even appears to have been spills by all three companies on 19 July 2022, the hottest day on record, when temperatures topped 40C in some places and many people tried to cool off in rivers.

Environment Secretary Therese Coffey told BBC News: "It does seem extraordinary on the hottest day of the year that there may be releases. The EA is the regulator; they are the people who do the detailed investigation of why that has happened."

All nine English water companies were sent environmental information requests for data on when their spills started and stopped. Only Thames, Southern and Wessex provided details - which the BBC then cross-referenced with Met Office rainfall data to identify dry spills.

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