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Environment & Climate

Thames Water pilot tests show elevated forever chemical levels in proposed recycling scheme

Internal files reveal drought relief testing yielded chemical concentrations up to 13 times legal limits, though Thames Water says the data does not reflect final design performance.

Updated Aug 27, 2026 · 05:17 UTC1 reports

The short version

  • Pilot testing for Thames Water's proposed drought relief scheme in south-west London revealed levels of the forever chemical PFOS reached up to 13 times the UK surface water legal limit.
  • The project seeks to abstract up to 75 million liters of water daily from the River Thames during droughts and replace it with treated sewage effluent.
  • Thames Water stated that pilot data does not reflect final operational output and that the facility will require Environment Agency approval before opening.

Key facts

  • Internal technical documents from Thames Water show pilot test results for its water recycling scheme recorded PFOS levels of 6.3 and 8.7 nanograms per liter, exceeding the UK surface water limit of 0.65 nanograms per liter.[The Guardian]
  • The proposed Teddington Direct River Abstraction project involves diverting up to 75 million liters of Thames water daily during droughts and replacing it with treated sewage from the Mogden treatment plant.[The Guardian]
  • Company technical notes indicated the pilot tests produced elevated iron levels and showed rapid chemical breakthroughs within a month, noting that maintaining legal limits would require significant additional operational expenses.[The Guardian]
  • Sources close to the project report that estimated costs have risen from £350 million to over £500 million due to requirements for additional mechanical pumps.[The Guardian]
  • The Environment Agency stated the site must undergo environmental assessments and receive a permit before development or operation can begin.[The Guardian]

What remains uncertain

  • Thames Water stated the pilot study was designed to evaluate technologies and that test data does not reflect the water quality that would be released when the project becomes fully operational.[The Guardian]

Sources