Pull to refresh
Logo
Daily Brief
Following Why Sign Up
London super sewer cuts tidal Thames sewage overflows in its first year

London super sewer cuts tidal Thames sewage overflows in its first year

Built World

First annual benefits report measures an 86% drop in storm overflows, though dry 2025 weather muddies the long-term picture

3 days ago: Findings reported publicly

Overview

Updated 2 hours ago

London's £4.6 billion 'super sewer' cut sewage overflows into the tidal Thames by 86% in its first year of operation, according to the first annual Tideway Benefits Report. The 25-kilometre tunnel, fully operational since February 2025, is the final piece of a three-phase effort to stop London's combined sewers dumping raw sewage into the river.

The measured gains go beyond raw volume. About 84 tonnes of sewage-derived litter stayed out of the river in 2025, and days with elevated health risks for river users fell from roughly 76% to 10%. One caveat runs through the report: 2025 was unusually dry, so whether the reductions hold in a wetter year remains the open question.

Why it matters

If the reductions persist, the Thames becomes a safer river for recreation and a case study for cities with aging sewer systems.

Questions about this story

Free account needed to ask — your question is kept and asked for you right after sign-up. Answers are public.

No questions yet — be the first to ask.

Key Indicators

86%
Reduction in storm overflow volumes entering the tidal Thames
Modelled annual overflow volumes fell from about 21 million to under 3 million cubic metres versus the pre-Tideway average.
84 tonnes
Sewage-derived litter kept out of the river in 2025
Wet wipes, sanitary products and plastics intercepted by the tunnel before reaching the Thames.
10.4%
Days of elevated health risk for river users
Down from 75.7% of days before the tunnel existed, based on storm-overflow-linked risk.
£4.6B
Cost of the super sewer
Funded through customer bills and the largest infrastructure project in Thames Water's network.

Voices

Curated perspectives — historical figures and your fellow readers.

Ever wondered what historical figures would say about today's headlines?

Sign up to generate historical perspectives on this story.

Log in Sign Up

Organizations Involved

Tideway
Tideway
Infrastructure delivery company
Handing operational control of the tunnel to Thames Water

The company created to build and commission the Thames Tideway Tunnel, now transferring operations to Thames Water.
Thames Water
Thames Water
Water utility
Taking over operation of the super sewer

London's wastewater utility, owner of the sewer network the tunnel plugs into and its new operator.
TH
Thames21
Environmental charity
Independent monitor of river litter and water quality

Charity whose foreshore surveys corroborate the tunnel's litter-reduction results.

Timeline

2015 October 2026

8 events Latest: 3 days ago
Tap a bar to jump to that date

  1. Findings reported publicly

    Latest Report

    Human Progress reports the benefits report, noting the reduction is based on modelled volumes.

  2. First annual benefits report published

    Report

    Tideway reports an 86% reduction in storm overflow volumes in the tunnel's first year.

  3. Super sewer officially opened

    Milestone

    The Thames Tideway Tunnel is formally opened.

  4. Tunnel fully operational

    Milestone

    The super sewer becomes fully operational, completing the third phase of improvements.

  5. First sewage enters the tunnel

    Milestone

    First flows captured as step-by-step commissioning begins across the network.

  6. Lee Tunnel completed

    Infrastructure

    Second phase completed, diverting overflows away from the River Lea.

  7. Super sewer construction begins

    Infrastructure

    Construction starts on the 25-kilometre Thames Tideway Tunnel under the river.

  8. Treatment upgrades completed

    Infrastructure

    London's wastewater treatment works upgraded as the first phase of Thames Tideway improvements.

Scenarios

1

Super sewer's benefits hold up in a wetter year

Possible Resolves by Oct 31, 2027

Discussed by: Tideway and Thames Water, whose reports flag that 2025's dry weather inflates the reduction

The report's own caveat is that 2025 saw fewer storms than a typical year. If 2026 brings closer-to-normal rainfall and the tunnel still holds storm overflow volumes well below the pre-Tideway average of about 21 million cubic metres a year, the long-term case is confirmed. The next annual benefits report would show the reduction persisting.

2

Wetter 2026 shrinks the headline reduction

Possible Resolves by Oct 31, 2027

Discussed by: Analysts noting the 86% figure is partly weather-driven

A wetter 2026 means more storm events and more combined sewage reaching the system, pushing overflow volumes back up even with the tunnel working correctly. The reported reduction could fall well below 86%, complicating the success narrative. This would not mean the tunnel failed, only that its measured benefit depends on rainfall.

3

Thames Water completes formal handover of the tunnel

Likely Resolves by Q1 2027

Discussed by: Tideway Annual Report 2025/26, which tracks System Acceptance planning

Thames Water is taking over operation of the tunnel while Tideway retains maintenance and optimisation during the System Acceptance phase. The Handover milestone marks the formal transfer, once storm testing and commissioning finish. Some commissioning tests tied to high fill levels remain pending because inflows were lower than expected.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

Late 1950s

The Thames declared biologically dead (late 1950s)

The tidal Thames through London was declared biologically dead, with oxygen levels too low to support fish. Decades of untreated sewage discharges had smothered the river through the industrial era.

Then

The declaration made the Thames a global symbol of urban pollution.

Now

A slow cleanup followed, with fish returning by the 1960s and the river now among the most improved urban waterways in the world.

Why this matters now

The super sewer's measured benefits continue that decades-long recovery, returning life to a river once written off.

1975 to present

Chicago's Tunnel and Reservoir Plan (1975 to present)

Chicago began building one of the largest civil-engineering projects in the world: deep tunnels and reservoirs to capture combined sewer overflows before they reached Lake Michigan and the Chicago River. Phases have come online over decades.

Then

Each phase delivered incremental reductions in river and lake pollution.

Now

The plan became the model for deep-tunnel sewage capture, and parts are still under construction today.

Why this matters now

It is the closest US counterpart to London's super sewer, showing the same mechanism and the same lesson: deep tunnels work but take decades to build and prove.

2015–2016

Lee Tunnel and London treatment upgrades (2015–2016)

Thames Water completed major treatment upgrades in 2015 and the Lee Tunnel in 2016, cutting overflows into the River Lea and the eastern Thames. They formed the first two phases of the wider Tideway program.

Then

Overflows fell across parts of the network even before the super sewer opened.

Now

Together with the Tideway Tunnel they secure a 95% reduction in sewage entering the tidal Thames.

Why this matters now

They are the direct predecessors of the super sewer and the reason the 2026 report measures the final phase of a single, staged cleanup.

Sources

(7)
+1
Human Progress Tideway Tideway Thames Water Who Gains from Saving Nature Wikipedia Tideway