Eighty-Six Percent, in a Dry Year

The pipe is not new. The numbers are.

London's Thames Tideway Tunnel, the super sewer, was fully connected in February 2025 and officially opened in May 2025. On 12 October 2026 the BBC reported the first annual benefits account. Water Magazine reported it on 14 October. Modelled storm-overflow volumes into the tidal Thames in 2025 were just under 3 million cubic metres. The pre-Tideway average was more than 21 million. That is the 86 percent.

The same report says about 84 tonnes of sewage-derived litter, wet wipes and sanitary products among it, did not reach the river. Across the wider London Tideway system the litter figure is about 130 tonnes. Thames21's foreshore counts, cited in the trade press, put sewage-related items at 30 percent of recorded litter before the tunnel and 18 percent in 2025.

Then the health-risk line splits. The BBC said days of raised health risk linked to sewage overflows fell from 269 in 2023 to 38 in 2025. Water Magazine, Engineering & Technology and the IET printed a rate, not a day count: an average of 75.7 percent of days, down to 10.4 percent in 2025. A reader who adds 269 to 75.7 has mixed a year with a percentage. We will not.

The year was unusually dry. Every serious account says the drop was larger than rainfall alone would explain. That is the claim, and it is also the caveat. One dry year is a bad year to declare a river finished. Operation of the tunnel is passing from Tideway to Thames Water. Tess Fayers, Thames Water's director of waste and bioresources, called the report a milestone and a step toward a cleaner Thames. Henrietta Gibb, Tideway's director of impact and engagement, called the first-year results a profound difference. The BBC priced the build at 5 billion pounds and eight years.

The stretch was once called biologically dead. Thames Water's line now is that it is on the road to recovery. A road is not a destination. Nobody in the report told London to swim.

Key Facts & Figures

• First annual Tideway Benefits Report, covered 12 to 14 October 2026
• Modelled storm-overflow volume, 2025: just under 3 million cubic metres, against a pre-Tideway average above 21 million. About 86 percent lower
• About 84 tonnes of sewage-derived litter kept out of the Thames; about 130 tonnes across the wider London Tideway system
• BBC health-risk days: 269 in 2023, 38 in 2025
• Trade-press health-risk rate: 75.7 percent of days on average, down to 10.4 percent in 2025
• Thames21 foreshore: sewage-related litter from 30 percent of items to 18 percent
• Tunnel fully connected February 2025, officially opened May 2025. Build cost cited by the BBC: 5 billion pounds, eight years
• 2025 was unusually dry. The report says the drop exceeded what rainfall would explain
• Tess Fayers, Thames Water. Henrietta Gibb, Tideway

Why This Matters

A city that spent 5 billion pounds on a tunnel is owed a first-year number, not a slogan. Eighty-six percent is that number. It is modelled. It is one year. It is a dry year. The litter tonnes are the part a person on the foreshore can understand without a model.

What We Don't Know Yet

• How the volume looks in a wet year. The report has not had one.
• Which health-risk figure a reader should carry. The day count and the percentage are both printed. They are not convertible from the copy we have.
• Whether "modelled" volumes will be replaced by a measured series in the second annual report.

Sources

• BBC, 12 October 2026: https://www.bbc.co.uk/news/articles/cw5yn0de1k4ko
• Water Magazine, 14 October 2026: https://www.watermagazine.co.uk/2026/10/14/tideway-tunnel-cuts-storm-overflow-volumes-to-thames-by-86/
• Engineering & Technology, 13 October 2026: https://eandt.theiet.org/2026/10/13/london-super-sewer-delivers-86-reduction-thames-pollution