Levels of a toxic 'forever chemical' rose to 13 times the legal limit during pilot tests for Thames Water's controversial water recycling scheme, according to data from the trials. The utility company plans to draw tens of millions of litres of water a day from the River Thames and replace it with treated effluent during drought conditions, a multimillion-pound project it says will not harm the riverine environment.
The pilot data, obtained exclusively by this publication, stands in stark contrast to Thames Water's public statements about the project in south-west London. During the trials, concentrations of perfluorooctane sulfonate (PFOS), a member of the PFAS family of chemicals known as 'forever chemicals' because they do not break down in the environment, spiked dramatically above the legal threshold. PFOS has been linked to a range of health problems, including liver damage, thyroid disease, and certain cancers.
The water recycling project is designed to secure water supplies for London during periods of drought, a growing concern as climate change brings hotter, drier summers to the region. Thames Water has argued that the scheme is essential to maintain supply for its 15 million customers and has emphasised the environmental benefits of reducing reliance on over-extracted groundwater sources. However, environmental groups have repeatedly warned that pumping treated sewage into a freshwater river could introduce a cocktail of pollutants, including pharmaceuticals, microplastics, and PFAS chemicals, into the ecosystem.
The new data appears to confirm those fears. During the pilot phase, which tested the treatment processes that would be used in the full-scale plant, PFOS levels in the water destined for the river exceeded the legal limit by a factor of 13. The finding raises serious questions about the adequacy of the proposed treatment technology and the company's environmental impact assessments.
Thames Water has not yet responded publicly to the data, but the revelation is likely to intensify opposition to the scheme. Local campaigners and environmental organisations have already called for a full public inquiry into the project, arguing that the risks to the river's wildlife and to downstream water users have been underestimated. The River Thames supports a rich ecosystem, including salmon, otters, and hundreds of bird species, and is also a source of drinking water for communities further downstream.
The company has previously stated that its recycling plant would use advanced treatment processes, including ozonation and activated carbon filtration, to remove contaminants before the water is returned to the river. Yet the pilot results suggest that these processes may not be sufficient to prevent PFAS chemicals from passing through, particularly during periods when the incoming sewage contains higher concentrations of industrial pollutants.
The issue is not confined to London. PFAS contamination has become a major concern across the UK and globally, with regulators under pressure to tighten limits on these chemicals in drinking water and the environment. The UK government has set legally binding targets for PFOS in surface waters, and the new data suggests that the Thames Water scheme could breach those targets on a regular basis during drought operations.
For Thames Water, already under scrutiny for leaks, sewage discharges, and financial difficulties, the pilot data represents another significant reputational challenge. The company has insisted that the recycling scheme is a necessary part of its long-term water resource plan, but the evidence from its own trials may now undermine that position. The final decision on whether to proceed with the full-scale plant will rest with the Environment Agency and the Secretary of State for Environment, Food and Rural Affairs, who will have to weigh the urgent need for water security against the demonstrated risks to the riverine environment.
