The Thames Tideway Tunnel has reduced sewage discharges into the River Thames by 86%, according to the first annual assessment of the environmental benefits delivered by London’s £4.5bn “super sewer”.

Published as responsibility for operating the 25km tunnel transfers from Tideway to Thames Water, the report shows significant improvements in river health during the tunnel’s first full year of operation in 2025.

The Tideway Benefits Report shows storm overflows to the tidal Thames have been reduced by 86%, compared to pre-Tideway levels. The tunnel has prevented approximately 84 tonnes of sewage-derived litter items, such as wet wipes and sanitary products, from entering the river in 2025, helping improve conditions for wildlife and river users.

Across the wider London Tideway Tunnel system, around 130 tonnes of sewage-derived litter were intercepted before reaching the river.

The ‘super sewer’ that runs beneath London, was built to increase capacity, allowing more rainwater and waste to be stored and reducing overflows into the tidal stretch of the Thames. It took eight years to construct with around 25,000 individuals having worked on the project – including tunnellers, engineers and divers – contributing 40 million work hours.

The report re-emphasises that while weather can influence discharge volumes, with last year being unusually dry, the reduction that was seen in 2025 substantially exceeds what would be expected from rainfall patterns alone and demonstrates the impact of the tunnel.

Days associated with elevated health risks linked to storm overflows have reduced drastically from an average of 75.7% of days to just 10.4% in 2025, compared to a time when the Thames Tideway Tunnel didn’t exist.

Monitoring by environmental charity Thames21 recorded a significant reduction in sewage-derived litter on the foreshore, with sewage-related items falling from 30% of litter recorded before the tunnel became operational to 18% in 2025.

The report marks a significant milestone for a river that was declared biologically dead in the late 1950s. Thames Water said  the stretch of river was now “on the road to recovery”.

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.

“The River Thames is one of the world’s great urban rivers and a vital habitat for wildlife. This is an important milestone for the river and a significant step towards a cleaner, healthier and more resilient Thames for future generations.”

The full Tideway Benefits can be accessed here