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Thames Tideway Tunnel
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<p>The Thames Tideway Tunnel is a deep-level <a href="/facts/Combined_sewer/UUxl1lwB">sewer</a> along the tidal section of the <a href="/facts/River_Thames/XnQWN52L">River Thames</a> in London, running 25 kilometres (16 miles) from Acton in the west to Abbey Mills in the east, where it joins the <a href="/facts/Lee_Tunnel/bChMksX2">Lee Tunnel</a> which connects to <a href="/facts/Beckton_Sewage_Treatment_Works/2gEGYGno">Beckton Sewage Treatment Works</a>. The tunnel is designed to capture almost all the raw <a href="/facts/Sewage/aC5N1kls">sewage</a> and <a href="/facts/Rain/MLqcQEyx">rainwater</a> from <a href="/facts/Combined_sewer/UUxl1lwB">combined sewers</a> 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. </p><p>Construction of the Tideway Tunnel began in 2016 and the first sewage flowed into the tunnel in September 2024. It became fully operational in February 2025, at a cost of £5bn. </p><p>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 will drain 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. </p><p>Bazalgette Tunnel Limited (BTL) is the licensed infrastructure provider for its finance, building, maintenance and operation. It has as investors: <a href="/facts/Allianz/uGcVXYsy">Allianz</a>, Amber Infrastructure, Dalmore Capital and DIF. Since the licence award, it also trades as Tideway. On 3 November 2015, the award was made by <a href="/facts/Ofwat/vC4a0gxS">Ofwat</a>, ensuring the start of the project. </p><p>Started in 2016, the project was due to be completed by 2024. The <a href="/facts/COVID-19_pandemic_in_the_United_Kingdom/P28V4Ioq">COVID-19 pandemic</a> delayed this to early 2025. </p><p>The estimated capital cost – excluding financing, operations and maintenance – was £3.8bn with an additional £1.1bn for preparatory works. Due to the COVID-19 pandemic, additional costs of £233m were incurred. The 2021-22 annual report gave an updated cost of £4.3bn. The final cost was £5bn. </p>