A pilot scheme by Thames Water to recycle treated sewage into the River Thames during drought conditions resulted in levels of a toxic chemical exceeding legal limits by up to 13 times, according to internal data seen by media outlets. The scheme, which aims to augment water supplies in south-west London amid climate-driven drought concerns and population growth, was tested in two pilots that showed significant increases in the presence of perfluorooctane sulfonate (Pfos), a type of per- and polyfluoroalkyl substance (Pfas) known as "forever chemicals" due to their persistence in the environment.
The pilot results revealed Pfos concentrations reaching 6.3 and 8.7 nanograms per litre in the respective tests, far above the UK legal limit of 0.65 nanograms per litre for surface waters. The company’s technical notes indicated that the chemical levels rose rapidly, exceeding the threshold within less than a month of the pilot operations. In addition, treated water was found to contain elevated iron levels, which were described as likely to have a detrimental impact on the river ecosystem.
Thames Water’s proposed system would extract up to 75 million litres of water daily from the Thames at Teddington during low-flow periods and replace it with treated sewage from the Mogden treatment plant via a newly constructed tunnel. While the volume planned for recycling is a small fraction of the estimated 571 million litres lost daily to leaks by the company, the pilot data raised concerns about maintaining water quality to current environmental standards. Thames Water has acknowledged that controlling Pfas within the legal limits would be operationally demanding, requiring frequent maintenance of carbon filtration systems and incurring significant additional expenses.
The project, originally budgeted at £350 million, is now expected to exceed £500 million due to unforeseen requirements such as installing extra pumps within a nearby nature reserve. Thames Water is pursuing a development consent order for the scheme, which can be approved as a strategic infrastructure project despite opposition. Local authorities, including the council, and MPs such as Liberal Democrat Munira Wilson, have voiced concerns about potential environmental harm.
Environmental groups like Solar have criticized the scheme, with spokesperson Ian McNuff stating that Thames Water’s characterization of the pilot results as "operationally challenging" reflects an inability to safely implement the technology. He expressed skepticism over the reliability of the system to function effectively during drought conditions, particularly given the strict and evolving standards surrounding chemical contaminants.
Thames Water responded by emphasizing that the pilot’s purpose was to test various technologies and that the data collected does not represent the quality of water to be discharged when the project is fully operational. The Environment Agency, which rejected an earlier version of the plan in 2019 due to environmental concerns, has received the pilot results and reiterated its commitment to enforcing stringent legal protections for rivers and drinking water sources. The agency highlighted the importance of combining water recycling with other measures, such as new reservoirs, to secure future water supplies while maintaining environmental standards.