Thames Water has begun a new water quality trial to safeguard drinking water supplies to millions of households and businesses across south west London yesterday (14 October.)

The UK's largest water company is testing the tertiary treatment process that will be used as part of its nationally significant Teddington Direct River Abstraction (TDRA) project.  

Tertiary treatment provides an extra cleaning layer to wastewater during the sewage treatment process, ensuring that water can be safely recycled and transferred back into watercourses to help protect the natural environment.  

Thames Water is proposing to use this technology to support its river abstraction and water recycling project in Hounslow, Richmond and Kingston. 

Thames Water forecasts a shortfall of 1 billion litres of water per day by 2050 and expects to supply an additional two million customers in London by that time, up from eight million to ten million. 

 Its TDRA project is designed to protect Greater London's drinking water supplies during periods of drought.

The project proposes to take water from the River Thames and transfer it along an existing pipeline to the Lee Valley Reservoirs, so it can be used for drinking water. 

 To replenish what is taken, and balance water levels and flows, highly treated recycled water would then be transferred via a new tunnel and pipeline from Mogden sewage treatment works, back into the River Thames, upstream from Teddington Weir. 

This graphic shows how Thames Water's TDRA project will work (Credit: Thames Water)

If approved, the project will protect water supply for millions of Londoners. 

It is designed to deliver up to 75 million litres of water each day and would spare London's economy from the catastrophic effects of a severe drought, which could cost as much as £500 million a day.  

 Once operational, the project will also provide immediate protection against the risk of drought ahead of a new reservoir being built for the region. 

 Simon Adams, Project Director for Thames Water's Strategic Resource Options said: "We're committed to maintaining a healthy flow of water in the River Thames during drought, enabling local wildlife and habitats to thrive.

"This project will use recycled water to maintain river levels during extremely hot and dry summers, mitigating against the effects of climate change and population growth, which are already evident across the South East, while protecting water supply for millions of people."

 He continued: "It's vital that we invest in new infrastructure to protect people, the economy, and the environment from the potentially serious effects of drought.

"Our Water Resources Management Plan, which was recently approved by the Government, paves the way for this vital project.

"We are committed to ensuring our proposals protect the health of the River Thames. That matters to local communities, and it matters to us too." 

Replenishing local rivers and streams with clean wastewater is already a very important part of the wastewater treatment process, as it helps to keep water flowing and to protect the environment.  

Primary treatment involves removing organic solid matter - otherwise known as human waste – from wastewater. 

The waste is separated from the water by putting it into large settlement tanks, where solids sink to the bottom. The settled solids are also referred to as 'sludge'.

 Large arms or scrapers help to push the sludge towards the centre, where it's then pumped away for further treatment.

The cleaner water passes over a wall near the top of the tank ready for the next stage of the treatment process. 

During secondary treatment, the smaller and sometimes invisible organisms are removed.

Wastewater is put into rectangular tanks called 'aeration lanes', which pump air into the water.

This encourages the useful bacteria to break down and eat the harmful bacteria. The more the useful bacteria eat, the more they grow and multiply, until all the bugs are gone.  

Tertiary treatment removes tiny, suspended particles from wastewater that has already undergone primary and secondary treatment, such as dissolved organic and inorganic substances, and additional contaminants. 

 Following the successful installation of temporary plant equipment at Mogden sewage treatment works, the tertiary treatment trial is now underway and is expected to run continuously for at least 12 months.  

 The trial will be supported by off-site laboratory testing and the results will be reviewed by the Environment Agency to help determine whether a discharge licence should be granted in the future.

Importantly, wastewater treated and discharged through the tertiary treatment process would not cause a deterioration in the quality of the water in the River Thames.

Recycled water discharged into the Thames, through the abstraction process, would help to enhance the river's water quality. 

 Thames Water intends to submit an application for development consent to the Planning Inspectorate in 2026. 

If the TDRA project is approved for construction, the Environment Agency will closely monitor its ongoing operation under the terms of the license they grant.  

 Results from the tertiary treatment trial will be published and shared with local communities as part of the statutory consultation process next year.  

•  Thames Water has robust plans in place to reduce water leakage, and the Government has pledged to support water companies to encourage customers to use less water.

But the fact remains, these two initiatives are not enough. Thames Water must look at new ways to use water more wisely. Abstraction, supported by water recycling, is a viable solution to the challenge we face. 

• Water recycling is widely used in other water-stressed countries, and we must learn from this as we strive to protect and enhance our existing supplies. 
• Thames Water is striving to improve its resilience to extreme drought events to one in 200 years on average in the 2030s, and one in 500 years on average by 2040. 
• A severe drought would have extensive economic and social impacts, potentially devastating the environment and affecting the health and quality of life of all those who live and work in the city. 
• Over 2,300 people responded to Thames Water's non-statutory public consultation in autumn 2023.

To address the public's concerns about surface level impacts in Hounslow, Richmond and Kingston, Thames Water announced that it would change the proposed construction method for its new recycled water pipeline. 

• The move from pipejacking to tunnel boring significantly reduces visual impacts, as four ventilation shafts and construction compounds are removed from the project design.

This also reduces the number of traffic movements that would have been associated with the original shaft and associated compound sites. 

• Thames Water continues to act on feedback and has also set out a solution that would remove the need to continually discharge treated water upstream of Teddington Weir.

It would instead discharge the maintenance flow, which keeps the water recycling process in good working order, at Isleworth, using existing infrastructure.

As a result of this change, treated water would only be discharged upstream of Teddington Weir when the TDRA project is in operation – on average, once every two years. 

• 2026 – DCO submission 
• 2028/9 – Construction commences 
• 2033 - Construction and commissioning works complete 
• 2033 – Project begins operation