Proposed solution

Last updated: 30 April 2015

Proposed solution

The Thames Tideway Tunnel project will upgrade London’s sewerage system to cope with the demands of the city well into the 22nd century.

A decade of study has concluded that the most timely and cost-effective solution to the combined sewer overflows (CSOs) discharge problem is a 25 kilometre interception, storage and transfer tunnel running up to 65 metres below the river - the Thames Tideway Tunnel.

It will be 7.2 metres in diameter and have a capacity of 1.6 million cubic metres.

Starting in west London, the proposed route for the main tunnel generally follows the route of the River Thames to Limehouse, where it then continues north-east to Abbey Mills Pumping Station near Stratford. There it will be connected to the Lee Tunnel, which will transfer the sewage to Beckton Sewage Treatment Works.

Overflows of sewage into the tidal River Thames add up to tens of millions of tonnes every year. This is clearly unacceptable and also contravenes the European Urban Wastewater Treatment Directive.

Route and tunnel


What happens now?

London’s sewerage system relies on an integrated network of combined sewer overflows (CSOs). At the time it was built, the CSOs would release flows through discharge points along the river during heavy storms. Originally, this would happen once or twice a year, it now happens on a weekly basis and no longer requires a storm to trigger overflows. It can currently take as little as two millimeters of rain to trigger a discharge.

Our plan


By intercepting the sewage before it enters the river, the Thames Tideway Tunnel would prevent the tidal River Thames from being polluted with untreated sewage which can stay in the river for up to three months before the ebb and flow of the tide finally takes it out to sea.

The solution in brief


Built from three main construction drive sites in Fulham, Battersea and Southwark, the Thames Tideway Tunnel is expected to take seven years to build, at a cost of £4.2 billion (2011 prices). It will require the use of 24 construction sites, 11 of which would be located along the river bank. The main tunnelling work is anticipated to start in 2016.

The Thames Tideway Tunnel is part of Thames Water’s London Tideway Improvements. Alongside the Lee Tunnel, a shorter tunnel in east London which is due to become operational in 2015, and the upgrades at London’s five main sewage treatment works. 


Volumes of discharges from London's combined sewer overflows (CSOs)

Existing system

CSO Existingsystem

N.B. This diagram is for illustration purposes only.
For a more accutrate visualisation, please download the chart set

Lee Tunnel and STW improvements

CSO Stwimprovements

N.B. This diagram is for illustration purposes only.
For a more accutrate visualisation, please download the chart set

Tideway Tideway Tunnel and STW improvements

CSO Tidewayimprovements

N.B. This diagram is for illustration purposes only.
For a more accurate visualisation, please download the chart set.