The Thames Tideway Tunnel is a major new sewer, urgently needed to protect the tidal River Thames from pollution.
London's sewerage system is no longer fit for purpose and spills millions of tonnes of sewage into the tidal section of the river every year.
Tideway is building the tunnel to tackle the problem of overflows from the capital's Victorian sewers for at least the next 100 years, and enable the UK to meet European environmental standards.
The tunnel overview
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