Thames Tideway Tunnel

The Thames Tideway Tunnel is a deep-level sewer along the tidal section of the River Thames in London, running 25 kilometres (16 miles) from Acton in the west to Abbey Mills in the east, where it joins the Lee Tunnel which connects to Beckton Sewage Treatment Works. The tunnel is designed to capture almost all the raw sewage and rainwater from combined sewers which would otherwise overflow into the river during heavy rain. The sewage can be stored in the tunnel until it can be treated at Beckton.

Bazalgette Tunnel Limited (BTL), backed by investors Allianz, Amber Infrastructure, Dalmore Capital and DIF, is the licensed infrastructure provider for the tunnel's finance, building, maintenance and operation. On 3 November 2015, the license award was made by Ofwat, ensuring the start of the project.^[4] Since then, BTL also trades as Tideway.

Construction of the Tideway Tunnel began in 2016 and the project was due to be completed by 2024,^[2][5][6] but the COVID-19 pandemic delayed this to early 2025.^[3] The first sewage flowed into the tunnel in September 2024.^[7] It became fully operational in February 2025,^[1][8] and was officially opened in May 2025.

The estimated capital cost – excluding financing, operations and maintenance – was £3.8bn with an additional £1.1bn for preparatory works.^[9] Due to the COVID-19 pandemic, additional costs of £233m were incurred.^[3] The 2021-22 annual report gave an updated cost of £4.3bn.^[10] The final cost was £5bn.^[8]

The main tunnel has an internal diameter of 7.2 m (24 ft) and runs at a depth of between 30 m (98 ft) at the western end, and 70 m (230 ft) in the east. The tunnel drains 34 of the most polluting combined sewer overflows and is expected to lead to the overflows operating for 3.7% of the time on a maximum of four days per year at the time of commissioning.