At least half of Thames Water’s sewage plants lack the pipes and tanks to process enough waste water, leading untreated effluent to spill into rivers and waterways across its network, according to the company’s own data.
Some 181 of the utility’s 351 waste water plants did not have enough capacity to treat incoming material last year, with some able to process only a third of the quantity required by local residents.
The company, which serves a quarter of the UK’s population, failed to provide data for about a third of its plants, meaning the overall number of sites that do not have enough capacity and therefore spilling waste material may be far higher.
Mark Hull, a water economist at the Oxford Rivers Improvement Campaign (Oric), which used environmental information requests to collect the data from the company, said the findings were “shocking”.
“A century after sewage treatment became practical and normal in larger towns, Thames Water does not have enough hydraulic sewage treatment capacity to handle the flows arriving at our sewage works . . . leading to a rising tide of effluent in our rivers,” he said.
The data also shows that the number of sites pouring effluent into rivers for more than 38 hours a week — equivalent to 2,000 hours a year — has almost doubled from 22 to 40 between 2020 and last year.
While sewage outflows increase with heavy rain, Thames’ own findings showed that there were lengthy discharges even in dry years such as 2022. Oric said the “treatment may be perfunctory”, or “key parts of the works can be periodically flooded”.
The utility has been granted permission by regulator Ofwat to raise average household bills by a third to £639 from this April in order to invest in its network.
Although the business pledged to use previous price increases to fund improvements at 108 sewage treatment plants — including its largest non-London plant at Oxford — the work that had been due to start by this year is running behind schedule and the company now aims to begin by the end of the decade.
Data provided to Oric also showed that Thames does not know when the secondary sewage capacity at the Oxford plant was upgraded — something that has constrained planning permission for thousands of new homes near the town.
The state of Thames’ network — including its lack of information about a large part of its system — is hampering the company’s efforts to raise financing to stave off bankruptcy.
The business needs to find new owners and invest in its infrastructure while also servicing its £19bn debt mountain.
But potential bidders have expressed concerned about the lack of information over the state of the infrastructure, as well as the risk of fines and other unknown costs, according to several people with knowledge of the process.
The company is already facing a £104mn fine from Ofwat for failing to manage its waste water treatment network, while it is also being investigated for its improvement delays.
More than 30 per cent of Thames’ sewage pipe network is not mapped, while the company has already warned that it has £18bn of ageing assets that pose “a risk to public safety, water supply and the environment”.
Although construction of the sewer networks began in the Victorian era, most treatment plants were built in the 20th century — with a significant expansion in capacity during the post war period and in the 1960s and 1970s when the water utilities were still nationalised and under local authority control.
Thames Water is seeking court approval for a £3bn loan from its existing lenders to tide it over until it can find new owners for the business.
The company has also been closing down treatment works. Environmental information requests by the Financial Times show that Thames Water sold or closed at least 45 works in the decade after privatisation, reducing its number from 398 works in 1990 to 353 by 2000 and just 351 now.
Thames said: “We are responsible for the oldest and most complex infrastructure of any company in the sector and we recognise that we need to make investment to meet the demands that come with population growth and climate change.”
It said that this year’s price increase would allow it to “deliver a record amount of investment to address our ageing infrastructure” over the next five years. It added that “we continue to execute our plans to upgrade over 250 of our sites across the region”.