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£20million Thames Water Chesham Sewage Treatment Works upgrade boosts water quality in the River Chess

  • Multi-million-pound scheme has reduced phosphorous entering the Chess by 85 per cent. In concentrated amounts, the substance can damage river health

  • The improvements are part of Thames Water's broader initiative to modernise its infrastructure and enhance river health

  • Upgrade is described as ‘the single most important step to date’ to improve the Chess’ water quality

  • Additional river restoration work is under way to improve habitats and biodiversity.

Thames Water’s £20million upgrade to the Chesham Sewage Treatment Works that has cut phosphorous levels by 85 per cent has already had “a measurable impact on water quality” in the River Chess.

Lower phosphorus levels support healthier, more balanced river ecosystems and the new reduction scheme at Chesham reduced the level of the substance from 2 milligrams per litre to 0.25 milligrams per litre.

Steph Horn, Chess Smarter Water Catchment Project Coordinator at Chilterns National Landscape, described it as being the “single most important step taken to date in improving water quality of the Chess”, while Professor Mike Bowes, Nutrient Hydrochemist for the Centre for Hydrology and Ecology, added “the phosphorus reduction scheme at Chesham Sewage Treatment Works has already made a measurable impact on water quality”.

The site has also been upgraded to increase flow-to-treatment capacity from 241 litres per second to 353l, meaning the site can now hold and treat more wastewater, reducing the risk of storm overflows during periods of heavy rainfall or prolonged groundwater infiltration, when sewer systems can become overwhelmed.

Thames Water is also progressing a series of river restoration projects along the River Chess that aim to create a more resilient and sustainable chalk stream environment, with increased vegetation, enhanced habitats and improved overall biodiversity.

This work complements the River Chess Smarter Water Catchment programme, which has received £3 million of investment from Thames Water over the past five years. The initiative brings together local partners and stakeholders to deliver greater improvements to river health than any organisation could achieve alone.

A further £2 million has now been committed to the catchment, enabling the partnership to continue working with local communities to deliver its flagship Chalk Stream Restoration Plan and enhance river habitats for the long term.

Professor Bowes, Nutrient Hydrochemist for the UK Centre for Ecology & Hydrology, said: 

“The phosphorus reduction scheme at Chesham Sewage Treatment Works has already made a measurable impact on water quality, significantly lowering phosphorus concentrations in the River Chess since its introduction in late 2024. We will continue to monitor its progress as part of our Floods and Droughts Research Infrastructure programme and hope to work closely with Thames Water as part of ongoing efforts to improve water quality and river health.” 

Chris McKenna, Colne and Maidenhead Operations Manager at Thames Water, said: 
“Our purpose is to deliver life's essential service, so our customers, communities and the environment can thrive.  The upgrades at Chesham Sewage Treatment Works are part of our ongoing commitment to delivering reliable services for our customers while also protecting the health of rivers and waterways, in the face of climate change and growing demand.

“We are committed to seeing healthy and thriving waterways, however we can’t do it alone. While we play an important role in this, we aren’t the sole custodians of river health. We need everyone working together to help our waterways thrive.”

Steph Horn, Chess Smarter Water Catchment Project Coordinator at Chilterns National Landscape, said:

“The funding provided to the Chess Smarter Water Catchment Initiative has enabled partnership working at a catchment scale, and driven improvements to the Chess at a level not seen for over a century. It has supported farmers in land management changes that increase biodiversity and reduce run-off, delivered river restoration schemes, and created long-lasting connections with the community through educational and citizen science programmes.

“By reducing the phosphate levels entering the river, the upgrade of Chesham Sewage Treatment Works represents the single most important step taken to date in improving the water quality of the Chess.  This investment is a crucial contribution towards improving the Water Framework Directive (WFD) status of the Chess, safeguarding the long-term future of this globally important chalk stream in the Chilterns National Landscape. However, while this marks a major milestone, there remains much more work to be done by both Thames Water and partner organisations to secure lasting recovery.”

Paul Jennings, Chair of the River Chess Association, said:

“The River Chess faces many challenges around water quality and flow. Since 2019 we have seen improvements on the back of the Thames Water investment in the Chesham Sewage Treatment Works and through Affinity Water reducing abstraction. This, in conjunction with the work supported by the Thames Water Smarter Water Catchment Project, has created tangible improvements to wildlife numbers. Water Voles have expanded their territory, along with Kingfishers and Little Egrets are being spotted in the urban environments of Chesham and Rickmansworth. Otters are now being seen along the river, while wild Brown Trout populations have increased in recent years. Education programmes have also created a strong awareness of this fragile Chalk Stream habitat for future generations. 

“Further work is required as we face up to emerging issues, we still need to resolve the challenge of groundwater ingress into the sewer network and need further reductions to abstraction, particularly at Hawridge in the top of the catchment but through partnership working we are sure we can rise to these challenges.”

Thames Water is committed to improving the health of waterways across its area. Last month, the Zoological Society of London (ZSL) published their latest State of the Thames report which found dissolved phosphorous concentrations continue to fall with significant decreases seen in the River Thames and the River Lee. The report indicates that the most substantial reductions are likely linked to the improvements made at sewage treatment works.