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- The Thames Tideway Tunnel is a 25 km (16 mi) combined sewer under construction running mostly under the tidal section (estuary) of the River Thames across Inner London to capture, store and convey almost all the raw sewage and rainwater that currently overflows into the estuary. These events occur when rainfall volumes exceed the capacity of Bazalgette's and other engineers' London sewerage system. The tunnelling phase of the project was completed in April 2022. Bazalgette Tunnel Limited (BTL) is the licensed infrastructure provider for its finance, building, maintenance and operation. It has as investors: Allianz, , and DIF. Since the licence award, it also trades as Tideway. On 3 November 2015, the award was made by Ofwat, ensuring the start of the project. Started in 2016, the construction aimed to be completed by 2024. The COVID-19 pandemic has delayed this to early 2025. 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 for over 25 km (16 mi) across the full width of Inner London finally reaching −70 m (−230 ft) at Abbey Mills in the east. This tunnel will connect 34 of the most polluting combined sewer overflows (CSOs) and has been forecast to lead to 3.7% of existing overflows – on a maximal four days per year – at time of commissioning. This may rise, gradually, due to climate change and population growth, but will be cut by the installation for streets, car parks and large roofs, and even for building roofs, of separate surface water drainage, which gives owners a little-mentioned annual saving, and is obligatory for all new development under planning law, which imposes sustainable urban drainage systems. The tunnel will transfer the combined sewer's somewhat diluted foul water to the Stratford to East Ham Lee Tunnel for onward delivery to Beckton Sewage Treatment Works. Following treatment (to the extent currently possible given the greatly increased amount of diluting rain) water will then be released into the saline Thames Estuary. 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. Its outfall, the Lee Tunnel, runs deep under the London Borough of Newham to Beckton Sewage Treatment Works. (en)
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