A record 50% more raw sewage was discharged into rivers in England by Thames Water last year compared with the previous 12 months, data seen by the Guardian reveals.
Thames, the largest of the privatised water companies, which is teetering on the verge of collapse with debts of £19bn, was responsible for almost 300,000 hours of raw sewage pouring into waterways in 2024 from its ageing sewage works, according to the data. This compares with 196,414 hours of raw effluent dumped in 2023.
The data, obtained by the analyst Peter Hammond in answer to an environmental information request to the company, comes after Thames Water won approval from the court of appeal for a £3bn emergency debt bailout to avoid collapse.
Environmental campaigners and a small group of Thames’s creditors unsuccessfully challenged the bailout loan on the grounds it was not in the public interest and would continue a “Thames Water debt doom loop”. They argued temporary nationalisation under a special administration regime would be a better option.
The raw sewage discharge data from event duration monitors on Thames Water combined sewer overflows, pumping stations and treatment works reveals the scale of the investment challenge ahead to fix its crumbling infrastructure.
To pay for the investment Thames Water is seeking to raise household bills by 59% over the next five years in an appeal to the Competiton and Markets Authority, far higher than the 35% that the regulator, Ofwat, has approved.
Thames Water’s sewage treatment works were responsible for almost 90% of the 298,081 hours of raw sewage it released into rivers in 2024, the data shows. The works are already subject to a criminal Environment Agency (EA) investigation into illegal sewage dumping and a parallel inquiry by Ofwat, which has ordered the water company to move at pace to put things right.
Thames has admitted it has left its sewage treatment works to crumble for decades as a result of underinvestment. Its own business documents say it has “sweated these assets” by failing to invest in their upkeep, and as a result its infrastructure poses a risk to public safety, water supply and to the environment.
The data also shows that raw sewage was discharged from its network of combined sewer overflows, which are supposed to be used only in exceptional circumstances, and from sewage pumping stations. According to the spill data:
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The Amersham balancing tanks in Buckinghamshire, which are supposed to safely store excess sewage after heavy rain, discharged 4,842 hours of raw sewage in 2024.
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Amersham was the scene of the longest unbroken individual discharge, when the equivalent of 154 days of raw sewage spilled into the River Misbourne, a chalk stream, last year.
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Marlborough sewage treatment works dumped raw sewage for 2,786 hours.
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At the Chesham sewage treatment works there were 2,681 hours of sewage discharges.
Steve Reed, the environment secretary, has promised to clean up rivers and has set a target for water companies to achieve an average of 10 spills per sewage overflow by 2050. There was an average of 45.2 spills per overflow in 2024, according to the latest Thames data.
Ash Smith, of Windrush Against Sewage Pollution, who monitors raw sewage pollution into the River Windrush, said: “Thames Water has become reliant on dumping sewage and paying dividends and bonuses while the billpayer funds everything.
“The regulator’s inactivity and inertia plays a vital role in allowing this and to call it a scandal is an understatement.”
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The analysis suggests discharges are taking place in breach of the permit rules for the treatment works and could be illegal. For example, according to Hammond’s analysis, Marlborough sewage treatment discharged raw sewage for 26 days into the River Kennet, a chalk stream that is a site of special scientific interest. The spills were illegal, Hammond believes, because they took place on dry days in breach of the legal permit to operate.
Thames Water said the data had to be verified by the EA and could be subject to change. “In 2024 we experienced one of the wettest years on record. Storm discharges are closely correlated with rainfall and groundwater conditions, and we therefore experienced an increase in the frequency and duration of storm discharge events during 2024,” the water company said.
“We know how much people enjoy and appreciate rivers, which is why over the next five years we will deliver a record amount of investment to address our ageing infrastructure. This is an enormous undertaking; we are responsible for the oldest and most complex infrastructure of any company in the sector.”
It added: “While all storm discharges are unacceptable, the sewage system was historically designed to prevent sewage backing up into people’s homes. Transparency is at the heart of what we do, and we were the first water company to publish a real time data map on our website, before it became legal requirement to do so.”
Hammond, a professor of computational biology at University College London, now retired, is an expert on water company raw sewage discharges. His evidence to MPs on the environmental audit committee in 2021 showed the scale of illegal raw sewage discharges by water companies was 10 times greater than the EA estimated, prompting the biggest criminal investigation by the agency into illegal spills and the parallel Ofwat investigation. Both investigations are ongoing.
