Newly published pilot-test information has brought PFAS treatment into sharper focus for Thames Water’s proposed Teddington Direct River Abstraction (TDRA) scheme, a major drought-resilience project intended to support London’s water supplies.
The project would take up to 75 million litres a day from the River Thames upstream of Teddington Weir during drought conditions, with the river flow replaced by recycled water from Mogden Sewage Treatment Works. Thames Water says the scheme would operate principally in drought periods, with a lower-volume maintenance flow used outside those periods to keep the treatment system ready.
Reporting published on 27 August 2026 drew attention to results from pilot trials associated with the proposed tertiary treatment plant. The underlying technical note, now available through Thames Water’s project documents, gives a more detailed picture: it records a rapid loss of PFOS removal performance in the tested biological activated carbon and granular activated carbon process, rather than confirming that the future full-scale discharge will necessarily have the same quality.
What the pilot results show
PFOS is one of the better-known per- and polyfluoroalkyl substances, or PFAS. It is persistent in the environment and is currently the only PFAS with a statutory Environmental Quality Standard for surface waters in England.
In the Thames Water pilot, the 95th-percentile PFOS concentration in the feed water to the extended carbon-treatment process was reported as 6.3 nanograms per litre in the first test period and 8.7 nanograms per litre in the second. The technical note identifies the annual-average surface-water Environmental Quality Standard for PFOS as 0.65 nanograms per litre.
The carbon process achieved PFOS removal of up to 81% during the test periods. However, the report says PFOS broke through above the annual-average standard in less than one month of operation. It concludes that a conventional biological activated carbon/granular activated carbon arrangement would need frequent media regeneration, reactivation or replacement to pursue tighter PFOS targets at full scale, with associated operational and cost implications.
This is an important distinction for project teams. The 6.3 and 8.7 nanograms per litre figures are feed-water results, not stated final discharge concentrations. Thames Water has said the pilot was designed to test alternative technologies and refine the final tertiary-treatment design, and that pilot data should not be treated as representative of the eventual operational discharge.
Why this matters for strategic water infrastructure
The findings do not mean the TDRA scheme has been approved, nor do they amount to a finding that it would breach a permit. They do, however, show that PFAS control is likely to be a material design, operational and consenting issue rather than a peripheral water-quality consideration.
For water-reuse and effluent-replacement schemes, treatment selection cannot be separated from the expected lifecycle of treatment media, source-water variability, analytical detection limits, waste handling and resilience during peak operating conditions. Where activated carbon is part of the control strategy, the rate at which adsorption capacity is exhausted can determine not only operating expenditure but also the credibility of compliance margins.
The wider regulatory context is also evolving. The Environment Agency states that PFOS remains the only PFAS with an Environmental Quality Standard, while separate work has developed science-based thresholds for PFOA, PFHxS, PFNA and PFOS to support assessment of combined risks. Those thresholds are not replacements for the existing statutory PFOS standard, but they point towards a more sophisticated assessment of PFAS mixtures and bioaccumulation risks over time.
Consent and permitting timetable
TDRA remains a proposal. Thames Water held its statutory consultation from 17 June to 26 August 2025, followed by a supplementary consultation from 21 May to 18 June 2026. The company now expects to submit its Development Consent Order application in early 2027, later than previously anticipated. The application is expected to include an Environmental Impact Assessment and would be examined by the Planning Inspectorate on behalf of the Secretary of State.
Separately, Thames Water has confirmed that the project cannot operate without an environmental permit from the Environment Agency. That is a distinct regulatory gateway from development consent. The Environment Agency’s published guidance requires applicants for new or changed bespoke permits involving hazardous chemical discharges to surface water to undertake a surface-water pollution risk assessment, including screening and, where necessary, more detailed modelling.
Accordingly, the key question is not simply whether PFOS is detectable. It is whether the final scheme design, its proposed operating regime and the receiving-water assessment can demonstrate that the relevant environmental protections will be met.
Actions for consultants, operators and affected organisations
- Interrogate the treatment basis: distinguish pilot influent, intermediate process and final-effluent data; identify whether proposed design changes alter the risk profile.
- Build a robust PFAS evidence plan: use representative sampling over seasonal, drought and maintenance-flow conditions, with clear quality assurance and treatment-media performance monitoring.
- Assess mixtures, not just PFOS: PFOS has a statutory standard, but wider PFAS screening and cumulative-risk evaluation will be increasingly relevant to due diligence and stakeholder confidence.
- Link design to permit evidence: develop mass balances, receiving-water models, sensitivity testing and contingency measures early enough to inform the consent application.
- Consider residuals management: frequent regeneration or replacement of carbon media may transfer PFAS into a concentrated waste stream, creating separate transport, treatment and disposal questions.
- Engage early with stakeholders: local authorities, river users, landowners and environmental groups will expect transparent explanation of pilot limitations, mitigation measures and operational controls.
The TDRA pilot results are therefore best understood as a live design-risk signal. They reinforce the need for strategic water-resource schemes to demonstrate not only additional drought yield, but also durable control of persistent contaminants across the full treatment and environmental-permitting case.
Further information is available from Thames Water’s pilot plant technical note, the TDRA project page and Environment Agency PFAS threshold work.