Thames Water Ring Main
London water supply infrastructure
About
The Thames Water Ring Main (TWRM, formerly the London Water Ring Main) is a system of approximately 80 km (50 mi) of concrete tunnels which transfer drinking water from water treatment works in the Thames and River Lea catchments for distribution within central London.
A major part of London's water supply infrastructure, the initial ring was constructed by Thames Water between 1988 and 1993 at a cost of £248 million (equivalent to £534 million in 2025), and when completed, it was the longest tunnel in the UK. Two extensions were constructed between 2007 and 2010.
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