Has London’s £5bn ‘super sewer’ improved water quality in the Thames?
Ministers pledged the 25km-long sewer would mitigate health risks, encourage recreational river use and restore wildlife
When London’s new £5bn Thames Tideway Tunnel “super sewer” got the go-ahead more than a decade ago, the project was sold as an environmental renaissance for the capital’s main river artery.
Ministers pledged that the tunnel would encourage greater recreational use of the River Thames, mitigate public health risks and restore wildlife to its turbid brown waters.
Yet although the 25km sewer has diverted 22 million tonnes of sewage away from the central section of the river since it first started operating two years ago, there is no firm evidence that the water quality has significantly improved.
Campaigners have argued the limited official data raises questions over whether the Thames Tideway represents value for money, especially at a time when regulator Ofwat is using it as a model for billions of pounds of new water infrastructure projects.
A report this week by Thames Water, which took operational control of the tunnel in August, stated that it had reduced spills by 86 per cent, prevented 84 tonnes of sewage-derived litter from going into the river and reduced the days of “increased health risk” from combined sewage overflows from 269 in 2023 to 38 in 2025.
But Thames Water also admitted it had not carried out any bacterial or pathogen testing since the sewer became fully operational in February 2025, and said it would be offering “no guidance” as to whether that part of the river was safe for recreational use.
Meanwhile, a long-awaited environmental impact report from Tideway, the independent consortium that built and owns the super sewer, has been postponed.
In the absence of official monitoring, the Fulham Reach Boat Club, based within the tunnel’s catchment area, is doing its own water testing.
Relying on the same equipment used to monitor the Seine ahead of the 2024 Paris Olympics, the club’s testing has found “no clear and convincing improvement in E. coli levels”, a bacteria that can cause severe illness, since the super sewer began operating.
The club’s weekly data found that 62.5 per cent of samples taken between February 2025 and October 2026 contained E. coli levels exceeding 1,000 coliform unit equivalents per 100ml, a threshold classifying the water as a human health risk.
Adam Freeman-Pask, chief executive of Fulham Reach Boat Club, called the results a “depressing read for river users, especially given the expense of the project”.
Tideway argued that assessing water quality trends required multiyear datasets, and that a single year of observation “would not provide a sufficiently robust picture”.
“The Thames is a complex tidal river, and water quality is influenced by a range of factors beyond the combined sewer overflows that Tideway was designed to address,” it added.
“Individual E. coli readings can fluctuate depending on location, rainfall, tides and other inputs. These readings cannot, in isolation, provide a complete picture of the tunnel’s impact.”
Independent experts said the persistent high bacterial levels probably stemmed from untreated sewage discharging further upstream or entering via contaminated tributaries including the Lee and Wandle.
While the super sewer is designed to capture 90 per cent of effluent from 36 combined sewage overflows in the central section, there are a further 21 that continue to discharge into that part of the river.
Furthermore, because this stretch of the Thames is tidal, pollutants wash back and forth twice daily.
Amy Fairman, head of campaigns at environmental charity River Action, called the independent pollution figures “deeply shocking” and demanded that Tideway publish its internal data in full.
“The public deserves to know, given they are paying for this tunnel through their water bills, that it is actually delivering improvements,” Fairman said.
The findings will also concern London Mayor Lord Sadiq Khan, who launched a 10-year plan in July to restore the Thames’s wildlife habitats and improve river health.
There is a chance that results will get better over time. Leon Barron, professor of environmental science at Imperial College London, said water quality metrics could lag and needed to be taken over multiple seasons and years to get an accurate reading.
But the slow pace of improvement will resurface an old debate that questioned the suitability of Thames Tideway, initially conceived by the government and Thames Water after the UK was found in 2012 to have breached the European wastewater directive and was facing multimillion-pound fines.
Environmental experts at the time argued that improvements to the existing sewage infrastructure, including upgrades to overloaded treatment plants, would be cheaper and more effective, while Sir Ian Byatt, a former chief executive of regulator Ofwat, also opposed the project, including the decision to keep it off Thames Water’s balance sheet.
Instead, the super sewer is owned and maintained by Tideway, an independent utility set up specifically for the project, whose shareholders include Allianz, Dalmore Capital and Amber Infrastructure.
Although Thames Water has operational control, Tideway retains its own corporate board, staff and management structure alongside maintenance responsibilities, which include using drones to check for any cracks in the tunnel.
Thames Water is struggling under £20bn of debt and fighting off renationalisation, while Tideway has amassed £5bn in debt — including shareholder loans — since construction started in 2015, paid for via a surcharge on the bills of Thames Water customers — currently £33 a year. Tideway has a licence to receive payments from customers throughout the 120-year lifespan of the tunnel.
According to Tideway’s annual report, Thames Water’s 16mn customers paid £129mn towards the project in the 2025-26 financial year, bringing total customer contributions since 2015 to £746mn — outstripping total equity injections from shareholders.
Investor returns paid in lieu of dividends rose to £65mn for the year ending 31 March 2026, up from £26mn the prior year.
Ian McNuff, a water campaigner and a previous adviser to private equity firm Star Capital, said it was unacceptable that Londoners were forced to rely on community groups for information on water quality.
“Given that Thames Water’s customers have already paid such a huge amount for this sewer, it seems wholly unprofessional that we should have to rely on a local community boat club to measure the impact of the tunnel,” he added.
The Environment Agency has been approached for comment.