£5bn Thames tunnel leads to big fall in sewage spills

By the Editorial Team · Tuesday, October 13, 2026

London’s Thames Tideway Tunnel has cut raw sewage discharges into the River Thames by 86 percent since opening last year, significantly lowering pollution levels. This reduction has improved river health and public safety, though the project has yet to reach its goal of a 95 percent cut in sewage spills.

London’s £5 billion Thames Tideway Tunnel, often referred to as a “supersewer,” has significantly reduced raw sewage discharges into the River Thames since its opening last year, according to Thames Water’s first annual performance report. Now under Thames Water’s ownership, the tunnel is designed to hold excess sewage during heavy rainfall, preventing it from spilling into the river.

The report indicates that sewage spills dropped to around 3 million cubic metres last year, representing an 86 percent reduction compared to levels before the tunnel’s construction. However, this falls short of the project’s original goal to cut discharges by more than 95 percent. Experts note that 2025 was an unusually dry year across the UK, with fewer significant storms and more dry days, contributing to the lower volumes of sewage overflow.

Despite the shortfall in meeting its target, Thames Water described the tunnel as marking a “new chapter” for the Thames, a river once declared biologically dead during the 1950s. Environmental charity Thames21 reported that sewage-related litter accounted for just 18 percent of all foreshore waste last year, down from 30 percent prior to the tunnel’s operation. The tunnel also intercepted approximately 84 tonnes of sewage-derived debris, including wet wipes, sanitary products, and plastic waste. This interception has helped prevent the formation of large accumulations of waste, such as the “wet wipe islands” historically observed on the Hammersmith foreshore.

While the Thames Tideway is not intended to make the tidal Thames safe for swimming—due to strong currents and heavy shipping traffic—it has substantially reduced health risks tied to sewage spills. The number of days with elevated levels of harmful bacteria fell from 75.5 percent before the tunnel became operational to 10.4 percent last year.

Alison Debney, conservation lead for UK nature recovery at the Zoological Society of London, which monitors the river’s wildlife, said the reduction in untreated sewage inflows is an encouraging development. She emphasized the importance of this progress for the health of the river ecosystem and the people and wildlife that rely on it.

Though more progress is needed to fully meet the project’s environmental targets, the Thames Tideway Tunnel is widely seen as a major step toward improving water quality in one of London’s most important natural landmarks.

This article was reported by synthesizing coverage from multiple independent news outlets to present a balanced, unbiased account. Read about our editorial standards. Spotted an error? Let us know.