London’s ambitious ‘Super Sewer’, officially known as the Thames Tideway Tunnel, reached a historic milestone on May 7, 2025, with a royal visit from King Charles III.

The monarch arrived via the hybrid Mars Clipper vessel, highlighting the project’s commitment to sustainability. At Bazalgette Embankment, he unveiled a commemorative plaque, officially marking the completion of the decade-long, £4.5 billion infrastructure project.

During his visit, King Charles toured the tunnel’s high-tech facilities, including real-time tracking systems monitoring waste diversion. He met with some of the 25,000 workers who contributed over 40 million hours to the project and spoke with poet Dorothea Smartt, whose verses adorn the tunnel’s ventilation columns.

Designed to address the chronic sewage pollution of the River Thames, the 25-kilometer tunnel stretches from Acton in West London to Stratford in the east. It aims to intercept and divert over 90% of storm sewage overflows, which have historically discharged tens of millions of tonnes of untreated waste into the river annually.

Since partial operations began in August 2024, the system has already redirected nearly two billion gallons of wastewater, significantly improving water quality and biodiversity in the Thames.

London’s population has surged from about 6.6 million in the 1980s to over 10 million by 2025, putting immense strain on the city’s infrastructure, including its Victorian-era sewage system. This increase, combined with urban expansion, means that the sewage system can no longer handle even small amounts of rainfall — as little as one millimetre — without overflowing into the Thames.

Tideway is the private company delivering this public infrastructure, and shared its story with ITN Business as part of our recent programme The Future of Investing.

The tunnel, now fully operational, is designed to be resilient and last for at least 120 years, helping protect the Thames from sewage spills long into the future. 

The King’s visit and official opening of the Thames Tideway Tunnel shines a light on his long-running passion for the environment and support for projects that make cities cleaner, greener, and better prepared for the future.

The Super Sewer’s unique funding model

One unique aspect of this project has been its innovative funding model, which benefits not only the environment but also shareholders and bill payers. 

The project was initiated with a £1.3 billion equity investment, and additional funds were raised through debt and green bonds, with Allianz being the largest shareholder.

In our interview with Andrew Cox, Head of Infrastructure at Allianz Capital Partners, he explained that risk-sharing measures between the government and investors have played a crucial role in the project’s financial success.

This model enabled investors to provide low-cost capital, significantly reducing the cost for consumers. Early estimates suggested the project would add £70–80 to the annual Thames Water customer’s bill, but due to the efficiency of the funding model, the actual increase is around £20–25.

The tunnel is not just an engineering project but also a step towards improving public spaces along the river. Sites like Blackfriars are being transformed into areas where Londoners can reconnect with a cleaner, healthier Thames. And this is just the beginning; with more connections and testing ahead, the tunnel’s impact will only grow as more sites come online in the coming months.

The Thames Tideway Tunnel is also aligned with the UK’s broader environmental goals, including the government’s push towards achieving net-zero emissions by 2050. This project stands as an example of how private investment can contribute to public infrastructure while supporting sustainability initiatives.

Image: Tideway’s Blackfriars site

Measuring its impact

The total cost of the tunnel is expected to be around £4.5 billion, a significant investment in London’s environmental future, and one that is poised to improve both the urban landscape and the health of its waterways for generations to come.

Recent data indicated that during heavy rainfall on 23 September, 2024, the tunnel successfully captured 589,000 cubic metres of wastewater, demonstrating its immediate impact​. Once fully operational, the Thames Tideway Tunnel will possess a capacity equivalent to 640 Olympic-sized swimming pools.

The Environment Agency has estimated that the project will lead to a reduction in storm sewage spills into the tidal Thames by approximately 95 percent.

Chairman Alan Lovell expressed confidence that the project will significantly benefit both wildlife and the public, stating, “We are regulating the system with environmental permits and will continue to monitor water quality and aquatic life in our capital’s river.”

Water Minister Emma Hardy last year praised the tunnel as a prime example of the “transformation and investment” that the government aims to foster across the country, highlighting its importance in improving urban infrastructure and environmental health.

Sewage overflows into the Thames pose serious environmental and health risks; untreated sewage containing harmful bacteria, pathogens and chemicals, can have devastating effects on both aquatic ecosystems and human health.

Swimmers and rowers, or anyone in close proximity to the river, could be exposed to bacteria such as E. coli and viruses that can cause gastrointestinal illnesses, respiratory problems, and skin infections. There’s also the potential for the spread of antibiotic-resistant bacteria, which poses a significant public health concern.