Thames Water has invested more than £4 million in new ultraviolet (UV) treatment technology at Cassington sewage treatment works as efforts continue to improve water quality at the Wolvercote Mill Stream bathing spot.
The investment, together with several years of detailed investigations by Thames Water, the Environment Agency (EA) and the local catchment partnership, is part of ongoing collaborative efforts to better understand and address the factors affecting water quality at the Oxfordshire site.
Wolvercote Mill Stream was designated as a bathing water in 2022 following an application from Oxford City Council, Thames21, the Rivers Trust, and Thames Water. Since then, the bathing water has been classified as poor each year, although monitoring has shown that water quality can vary significantly throughout the season.
To support potential improvements, Thames Water commissioned a new UV disinfection system at Cassington sewage treatment works, located approximately 3.5km upstream of the bathing water. The technology has been operational since May 2026 and neutralizes bacteria and other pathogens before treated wastewater is returned to the environment.
Investigations carried out since 2023 have included extensive catchment sampling, scientific studies, and computer modeling. The findings indicate that Cassington Sewage Treatment Works has the potential to affect the bathing water but is not the only contributing factor.
Instead, the results suggest that other pressures closer to Wolvercote Mill Stream are having a greater impact through intermittent inputs, including agriculture and urban run-off, which can be difficult to identify and capture through routine monitoring.
Paul Hampton, Healthy River Community Manager at Thames Water, said the company has spent several years investigating the factors affecting water quality at Wolvercote Mill Stream through extensive sampling, modeling, and monitoring across the catchment.
He said the investigations have not identified a single cause for the bathing water’s classification but have improved understanding of the range of pressures affecting the site. He added that Thames Water invested more than £4 million in UV treatment at Cassington sewage treatment works to help move the bathing water towards an improved classification.
Thames Water will continue working with the Environment Agency, Oxford City Council, and local stakeholders to better understand the remaining sources of bacteria and identify where further improvements can be made.
Professor Jeremy Biggs, Chief Executive Officer at Freshwater Habitats Trust, welcomed Thames Water’s investment in upgrading its works to improve water quality at the Wolvercote Mill Stream bathing water site.
He said the organization looks forward to continued collaborative work with Thames Water and other partners through a catchment-based approach to address the complex and interconnected causes of water pollution in the area and across the wider Ock catchment.
Thames Water, the Environment Agency, and the local catchment partnership will continue to review monitoring results throughout the 2026 bathing season and work together to better understand the factors influencing water quality at Wolvercote Mill Stream.
Source: Thames Water
