Photo description: Looking towards London’s The Shard across the Thames River in London at dusk. Photo by Dan Stephens on Unsplash
86% reduction in raw sewage entering the river, reports Thames Water
The health of the River Thames in London improved significantly in the first year of the new “super sewer”, the Thames Tideway Tunnel, according to Thames Water.
First annual environmental benefits report
The Tideway Benefits Report, the project’s first annual environmental benefits report, found that annual storm-overflow volumes fell to just under three million cubic metres in 2025, an 86% drop compared with a pre-Tideway average of more than 21 million cubic metres. The sewer was built to increase capacity, allowing more rainwater and waste to be stored and reducing overflows into the tidal stretch of the Thames.
The sewer also reportedly prevented about 84 tonnes of litter, such as wet wipes and sanitary products, from reaching the waterway, with approximately 130 tonnes of sewage-derived litter intercepted across the wider London Tideway Tunnel system. Additionally, the report found that the proportion of days associated with elevated health risks linked to storm overflows fell from an average of 75.7% before the tunnel was operational to 10.4% in 2025. The report recorded a single dissolved oxygen depletion event linked to sewage-overflow activity during 2025, with the tunnel helping to improve conditions for fish and other aquatic wildlife in the tidal Thames.
Monitoring by environmental charity Thames21, which campaigns for healthier waterways, also recorded a fall in sewage-related items as a proportion of litter found on the river foreshore, from 30% before the tunnel became operational to 18% in 2025.
Climate change and extreme weather remain “big challenges”
The reduction comes despite the report acknowledging that 2025 was unusually dry, with weather conditions influencing discharge volumes. It states that the reduction substantially exceeded what would be expected from rainfall patterns alone. The report warned that pollution and climate change remained “big challenges” and said continued action was needed.
Thames21 said the tunnel was “an impressive piece of engineering”, however warned other methods of water management would be needed if extreme weather events caused by climate change became more frequent.
What’s next?
The report assesses the environmental benefits delivered during the tunnel’s commissioning phase, as operational responsibility for the infrastructure transfers from Tideway, the company established to deliver the project, to Thames Water.
Tess Fayers, Director of Waste and Bioresources at Thames Water, said: “This report shows that the Thames Tideway Tunnel is already delivering real benefits for the River Thames.
“As responsibility for operating the Thames Tideway Tunnel passes to Thames Water, we are seeing dramatically fewer sewage discharges entering the river, less sewage-derived litter reaching our foreshores and significantly improved river water quality.”
Thames Water and Tideway will continue to publish annual reports tracking the tunnel’s environmental benefits as the system moves into long-term operation.
Read more:
Thames Water: Tideway Benefits Report,
The BBC: coverage here
Water Magazine: coverage here