‘Forever chemicals’ surge during pilot of Thames Water plan to combat drought

Exclusive: utility company wants to draw off tens of millions of litres of water a day from Thames and replace it with treated effluent

Levels of a toxic “forever chemical” rose to 13 times the legal limit during Thames Water pilots for a controversial multimillion pound water recycling scheme that will pump millions of litres of treated sewage into the River Thames during drought.

The data from the pilots is in stark contrast to public comments from Thames Water that their water recycling project in south-west London will not harm the riverine environment.

In technical notes in the company’s files for the project, seen by the Guardian, the results from two pilots reveal the recycling system rapidly and significantly increased levels of a particularly toxic type of Pfas chemical perfluorooctane sulfonate (Pfos), a suspected carcinogen – breaching the legal limits by as much as 13 times.

As standard wastewater treatment plants are not designed to filter or destroy them, treated and untreated sewage contains Pfas chemicals (short for per- and polyfluoroalkyl substances). They are known as “forever chemicals” because they do not break down in the environment, and the detrimental impact on rivers has been found to be widespread. New figures show every English water body is polluted with toxic chemicals. They are also considered a threat to human health.

Thames Water’s recycling system is intended to provide more water to cope with shortages resulting from both a rising population, and predicted droughts caused by climate breakdown, over the coming decades.

It involves abstracting up to 75m litres of water a day from the Thames at Teddington, south-west London, when the river is in low flow at times of drought, and replacing it with the same amount of treated sewage from the Mogden treatment plant via a new tunnel into the river. The 75m litres a day is a fraction of the 571m litres of water the company leaks each day.

The results of the company’s tests, however, reveal the scale of the treated sewage input would make it very difficult for the water to meet current standards for forever chemicals and pesticides.

Thames Water, which is seeking a development consent order for the scheme, has insisted publicly that the water recycling system – which will cost at least £350m – would not harm the river, its habitats or be a threat to the public.

But the technical note on the pilots shows levels of Pfos rose to 6.3 nanograms per litre in test one and 8.7 nanograms per litre in test two of the recycling technology. The UK legal limit for surface waters is 0.65 nanograms per litre.

The documents show there was a “rapid breakthrough of Pfos” above the legal threshold in less than one month of operation. The company said keeping within legal limits would be “operationally challenging”, requiring carbon filters to be frequently regenerated, reactivated and replaced, and would require “significant additional operational expense”.

In addition to the forever chemical, the pilots also showed the treated water contained very high levels of iron, which the document states would have a “significant detrimental impact” on the river.

Ian McNuff, of the campaign group Solar , which opposes the project because of what they say will be irreversible environmental damage to the river, said: “Thames Water described the results for the pilot as ‘operationally’ challenging.

“In essence, they can’t make it work properly. Test failures were rapid, and the idea that they can switch this on and off as a completely reliable asset during drought is unrealistic against current standards, let alone the higher ones expected around Pfas and pesticides.”

Costs for the water recycling scheme are already rising for Thames, which is struggling under debts of £20bn and teetering on the brink of a government-handled administration.

The original cost of £350m is now expected to rise to more than £500m, as Thames Water has discovered it needs to install more mechanical pumps within a nature reserve near Teddington to pump the water, according to sources close to the project.

The Environment Agency has been given the results of the two pilots carried out by Thames Water. The agency rejected an earlier scheme in 2019 because of the unacceptable impact on the environment of releasing millions of litres of treated effluent into the river.

The project is opposed by the local council and local MPs, including the Liberal Democrat MP Munira Wilson. But as a strategic infrastructure project, it can be pushed through without local consent.

An Environment Agency spokesperson said: “We have strict legal requirements in place to protect our rivers, groundwater and drinking water catchments from toxic chemicals and the Teddington site will continue to undergo rigorous environmental assessments to ensure these conditions are met before any development work can begin.

“Environmental protection and sustainable development must go hand-in-hand and water recycling projects, combined with new reservoirs, will be critical to securing future water supplies effectivity.”

A Thames Water spokesperson said: “The Teddington Direct River Abstraction (TDRA) project will not be allowed to operate without securing an environmental permit from the Environment Agency.

“The pilot plant study was set up to test a range of technologies to help inform and refine the final design of the tertiary treatment plant. The data from the pilot plant study is not indicative of what would be released back into the river when the project is operational.”

The spokesperson added: “We want to see Pfas banned and the development of a national plan to remove it from the environment, which should be paid for by manufacturers as opposed to water bill payers.”