Forever Chemicals at 13 Times the Legal Limit: Thames Water's Recycling Trial Raises Alarm
Pilot tests for Thames Water's planned water-recycling scheme recorded levels of the forever chemical PFOS at up to 13 times the UK legal limit, according to documents from the trials. The findings, reported on 27 August 2026, emerged from two pilot runs of the Teddington Direct River Abstraction (TDRA) scheme. The project would take up to 75 million litres of water per day from the River Thames at Teddington during low-flow conditions and replace that volume with treated sewage effluent from the Mogden treatment works via a new tunnel into the river. Thames Water is currently seeking a development consent order — the planning permission needed for major infrastructure projects — for a project estimated to cost at least £350 million.
The UK legal limit for PFOS (perfluorooctane sulfonate) in surface waters is 0.65 nanograms per litre. The two pilot tests returned PFOS concentrations of 6.3 ng/l and 8.7 ng/l respectively. Pilot documents recorded a "rapid breakthrough of PFOS" above the legal threshold in less than one month of operation. In plain terms, the carbon filtration system designed to capture the chemical saturated far faster than expected under real-world conditions — think of a sponge that fills up with contaminants much more quickly than the manufacturer promised. Thames Water itself acknowledged in the documents that staying within legal PFOS limits would be "operationally challenging" and would require significant additional expense, including frequent regeneration of carbon filters.
PFOS belongs to a larger family called PFAS (perfluoroalkyl substances) — synthetic compounds widely used in industrial and consumer products, from non-stick pans to firefighting foam. They are dubbed "forever chemicals" because they do not break down in the environment. PFOS is a suspected carcinogen, meaning it may cause cancer. The UK has been gradually building its regulatory framework around PFAS in water: testing requirements for drinking water were introduced in 2021, and the Drinking Water Inspectorate (DWI) has since set a guideline value of 0.1 micrograms per litre for the combined total of 48 named PFAS in drinking water. In November 2025, the DWI ordered water companies to act over forever chemicals linked to cancer and other conditions. The DWI has also completed a study into how effectively conventional treatment methods remove PFAS from drinking water (Water Magazine).
The TDRA scheme is not designed as a direct potable reuse project — that is, it does not turn sewage directly into tap water. Instead, treated effluent from Mogden would be discharged into the river to maintain river levels, and the water abstracted at Teddington would then feed downstream supply systems. A related Thames Water proposal, the Thames to Affinity Transfer, takes a different approach: it assumes that effluent from the Beckton sewage treatment works would be treated to a high degree by a proposed Advanced Water Recycling Plant before transfer to Affinity Water. The TDRA scheme, by contrast, discharges recycled water directly into the Thames.
The pilot tests also flagged elevated iron levels in the treated water. Pilot documents warned this would have a "significant detrimental impact" on the river, adding a second water-quality concern beyond PFOS.
Thames Water has publicly insisted the recycling system would not harm the river, its habitats, or pose a threat to the public. The company is pressing ahead with the development consent order process despite the pilot results. The backdrop is one of mounting supply pressure: Thames Water currently leaks 571 million litres of water per day from its network. In July 2026, the UK water industry warned that the country will not have enough water for future datacentres, offering pointed criticism of the government's AI growth plans (The Guardian).
The broader regulatory and environmental context extends beyond this single scheme. Millions of tonnes of treated sewage sludge are spread on UK farmland every year despite containing forever chemicals and microplastics, a practice that predates the current focus on PFAS in aquatic systems (The Guardian). The challenge of removing persistent contaminants from wastewater is not unique to the UK. A series of pilots in Switzerland conducted before 2016 found that removing 80% of pollutants from waterways was the maximum achievable for a reasonable cost, suggesting there are hard technical and economic ceilings to what even advanced treatment can deliver (The Guardian).
What the pilot results lay bare is a gap between the treatment performance Thames Water needs and what its current technology can deliver under sustained operation. The "rapid breakthrough" finding is particularly significant because it suggests the carbon-based filtration model may not be viable at scale without either a step-change in filter regeneration frequency or a fundamentally different treatment architecture. Thames Water's own characterisation of compliance as "operationally challenging" is a notable concession from a company that has maintained the scheme poses no environmental risk.
The tension here is structural. London faces a projected supply deficit driven by population growth and climate-driven drought risk, and the water industry has flagged datacentre demand as an additional constraint. Schemes like the TDRA are designed to bridge that gap. But the pilot data suggest the bridge may carry its own environmental cost: a suspected carcinogen discharged into a river system at concentrations an order of magnitude above the legal threshold, and iron levels the company's own documents describe as harmful. The development consent order process will need to weigh whether operational mitigations can close that gap or whether the scheme's design assumptions need revision before construction proceeds.
