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The Thames Tideway Tunnel is an under-construction 25 kilometer (16 mile) tunnel running mostly under the tidal section of the River Thames through central London, which will provide capture, storage and conveyance of almost all the combined raw sewage and rainwater discharges that currently overflow into the river. Bazalgette Tunnel Limited (BTL) is the licensed 'Infrastructure Provider' set up to finance, build, maintain and operate the Thames Tideway Tunnel. BTL is a consortium of investors that comprises Allianz, Amber Infrastructure, Dalmore Capital and DIF. From the licence award, BTL trades and is known to the public as 'Tideway'. On 3 November 2015, BTL received its operating licence from Ofwat, ensuring the start of the project.[1]

Starting in 2016, construction of the Thames Tideway Tunnel will take seven to eight years, giving a target completion date of 2023.[2] Once constructed, the main tunnel will have an internal diameter of 7.2 m (24 ft) and will run from −30 m (−98 ft) at Acton in the west of London down to −70 m (−230 ft) at Abbey Mills in the east.

It will connect 34 of the most polluting combined sewer overflows (CSOs), via transfer tunnels, and is expected to reduce the number of overflow events to a maximum of four per CSO per year at time of commissioning, increasing gradually due to effects of climate change and population growth.[3] The tunnel will transfer the captured sewage to the Stratford to East Ham for onward delivery to Beckton Sewage Treatment Works for treatment. The recycled clean water is then released into the River Thames.

The current estimate of its capital cost – excluding unknown financing costs, ongoing operations and maintenance costs – was £4.2 billion in 2012 prices.[4]