London’s ambitious 25km “super sewer,” the Thames Tideway Tunnel, has reached a major milestone: The first four of its 21 sites have been switched on.
This marks the beginning of a new era for the River Thames, which will soon benefit from significantly reduced sewage pollution. The £5 billion project, under construction since 2016, is designed to intercept, store and ultimately transfer the raw sewage and rainwater that currently overflows into the River Thames during heavy rainfall. London’s Victorian-era sewer system, built in the 1860s by visionary engineer Sir Joseph Bazalgette, cannot handle the city’s growing population and urbanization demands. This results in millions of tonnes of untreated sewage spilling into the iconic river yearly.
Bazalgette’s original design included a network of interceptor sewers that collected both sewage and rainwater, with a series of combined sewer overflows (CSOs) acting as relief valves during heavy rainfall. While innovative, the system became overwhelmed as London’s population grew from 2 million in the 1860s to over 8 million today. Even as little as 2mm of rain can trigger sewage discharges into the Thames, with an average of 39 million tonnes of untreated sewage entering the river annually.
The Thames Tideway Tunnel, spanning from Acton in the west to Abbey Mills in the east, will capture this overflow through a network of shafts and transfer it to Beckton Sewage Treatment Works for processing. With a 7.2m internal diameter and depths of up to 70m, this massive engineering project is set to drastically improve the water quality and ecological health of the Thames, bringing it closer to its state in the late 1800s.
The tunnel is the final piece of a three-part solution known as the London Tideway Improvements. The first part, the 6.9km Lee Tunnel, has been operational since 2016, capturing 16 million tonnes of sewage annually from the city’s largest CSO at Abbey Mills Pumping Station. The second part involved upgrading London’s five primary sewage treatment works to increase capacity and treatment standards.
The Thames Tideway Tunnel is an engineering marvel. The main tunnel spans 25km, with an internal diameter of 7.2m, running at depths between 30m in the west and 70m in the east. It will intercept sewage from 34 of the most polluting Combined Sewer Overflows (CSOs).
Construction required the use of six tunnel boring machines (TBMs):
- Four machines for the primary 25km tunnel
- Two machines for connecting tunnels. The TBMs were named after influential women:
- Rachel Parsons (Fulham) – engineer and advocate for women’s employment rights
- Audrey ‘Ursula’ Smith (Central area) – cryobiologist
- Millicent Fawcett (Central area) – suffragist
- Charlotte Despard (Frogmore Connection Tunnel) – suffragist
- Selina Fox (East section from Bermondsey) – a doctor who set up a medical mission for the poor
- Annie Scott Dill Russell (Greenwich Connection Tunnel) – first female scientist at Greenwich Observatory
Over 20 shafts, some as wide as St. Paul’s Cathedral, were constructed along the route to divert sewage flows and lower the TBMs into the ground. As the TBMs progressed, they installed precast concrete segments to form the tunnel walls. The deepest shafts reach down to 70m.
The project also includes upgrades to London’s sewage treatment infrastructure. Key improvements were made at Mogden, Crossness, Beckton, Riverside, and Long Reach sewage treatment works to increase capacity and treatment quality.
With the first connections activated, data shows the tunnel captured 589,000m3 of sewage during heavy rainfall on September 23rd, preventing 219,000m3 from entering the Thames. The existing Lee Tunnel captured an additional 370,000m3. The system’s impact will grow as more than 21 sites come online in the coming months.
Stakeholders have praised the progress. Tideway CEO Andy Mitchell called it “an important moment for the Thames,” noting that while there are “more connections to make and further testing to come,” the tunnel’s “positive influence on the health of the Thames will increase over the coming months.”
The Thames Tideway Tunnel is expected to be fully operational in 2025, reducing the number of overflow events to a maximum of four per year. This will bring the UK into compliance with the European Union’s Urban Waste Water Treatment Directive, which sets standards for collecting, treating, and discharging urban wastewater.
The tunnel’s impact extends beyond improving water quality. It has also created jobs, with over 4,000 workers involved in construction at peak periods. Furthermore, the project includes building new public spaces along the river, with seven landscaped embankment areas for community use.
Of course, the project has not been without controversy. Concerns have been raised about the cost, which will be funded through increases in London households’ water bills, and the disruption caused by construction in densely populated areas of the city.
Despite these challenges, the Thames Tideway Tunnel should be a significant step forward in protecting one of London’s most iconic natural assets. As the city’s grows, this infrastructure project will be critical in maintaining a cleaner, healthier River Thames. It showcases British engineering capability and underscores a commitment to environmental stewardship in the heart of the nation’s capital
TLDR:
- London’s 25km Thames Tideway Tunnel has begun operations to reduce sewage overflows into the River Thames.
- The city’s growth overwhelmed Victorian-era sewer systems, leading to millions of tonnes of sewage pollution annually.
- £5bn tunnel will intercept sewage from 34 combined sewer overflows (CSOs) and transfer it for treatment
- The first four of 21 sites were activated, capturing 589,000m3 of sewage during heavy rain on September 23rd
- Tunnel showcases British engineering and commitment to cleaning up the iconic river, expected fully operational in 2025
