Sewage treatment works upstream of Port Meadow under investigation

Cassington Sewage Treatment Works in Cherwell, Oxford, is being referred to the Environment Agency for investigations after research by the Oxford Rivers Project found regular sewage spills from the plant into Port Meadow – a popular swimming spot.

Researchers for the river improvement project found that in 2021, Cassington sewage treatment works had spilled on occasions when rainfall was as low as 1mm, and sometimes before any rainfall had occurred. Not only this, but further analysis of the flows at the sewage treatment works indicated that raw sewage was being discharged into the upper River Thames even when the works was operating below full capacity. 

Tim Harris, the principal researcher on the Oxford Rivers Project, said: “We first thought something could be wrong when Cassington showed no spills at all on our live alerts system since March 2021, despite spilling throughout the year in 2019 and 2020.

“Then, on questioning officers at Thames Water, they stated that the capacity of the works had increased 20% in 2021, an unusually large amount, and despite no investment at the works. This prompted us to look more closely at the data and then submit it to the Environment Agency for investigation. I’m looking forward to hearing their conclusions”.

Campaigners in Oxford have submitted an application for designated bathing water status at Port Meadow, a popular swimming and water sports site on the River Thames in Oxford. The status would ensure water quality testing for harmful bacteria in the summer months, and require Thames Water and the City Council to take action to improve the water quality at the site. If granted, it will only be the second river location in the country to receive the status.

Claire Robertson, the Oxford Rivers Project Officer at Thames 21, said: “In our view, it’s totally unacceptable that raw sewage ever goes into our beautiful rivers, but to find out this might be happening illegally, before we’ve even had any heavy rain is even more shocking. So, many people locally rely on the rivers for their physical and mental health, not to mention the wildlife that rely on them for survival. Thames Water need to demonstrate they’re serious about cleaning up their act by telling us what concrete plans they have to invest and upgrade their treatment works so they are fit for purpose.”

Sewage should go through two to three rounds of treatment to remove solid matter and most harmful bacteria before it is released into watercourses or the sea. However, it is currently legal for water companies to discharge raw, untreated sewage into rivers in times of “exceptionally heavy rainfall”. Water companies must monitor their overflows and provide data on spills of untreated sewage each year.

There were 50,000 hours of raw sewage spills in the upper Thames in 2020, and more than 3 million hours across England as a whole.

The Oxford Rivers Project used a team of community volunteers to collect samples of river water from 14 locations in and around Oxford every week of 2021, which were then analysed for the presence of harmful bacteria and other signs of pollution. The final report is expected at the end of this month.

Michelle Walker, Technical Co-Director at The Rivers Trust, said: “Citizen scientists at Port Meadow have helped gather important evidence to highlight the reality of raw sewage discharge into our rivers.  This shows that permit conditions for the sewage works are being breached. It isn’t just illegal, it’s plain wrong. We urge the Environment Agency to complete a thorough investigation into the matter, and ensure routine pollution is stopped.”


If you would like to find out more about the Oxford Rivers Project, click link here.

Further information


Thames21 connects people with rivers by putting healthy rivers back at the heart of everyday life. We improve and restore rivers, educate and empower the community and campaign for positive change for the good of people and the environment. To learn more, visit

The Rivers Trust are conservation experts with a wealth of data and expertise at our fingertips. Through our network of over 63 local Trusts in England, Wales, Scotland, Northern Ireland and the Republic of Ireland, and with over 350 dedicated specialists including over 60 farm advisers giving confidential expert advice, we work in partnership at a catchment scale to make our shared vision of wild, healthy, natural rivers, valued by all a reality.

Media Contact

Liz Gyekye

Communications Manager

+44 (0)7597584349