Thames Tideway tunnel … the super sewer for London.


London’s current sewer network, which was built by Joseph Bazalgette in the second half of the 19th century, was hailed as a major engineering achievement in its day and collects sewage and rainwater runoff together.

Sewer network overload 

Today, with the climate changing and the city expanding, the system is under pressure and 52 million cubic metres of untreated sewage and rainwater pollute the Rivers Thames and Lee each year – enough to fill the Albert Hall about 525 times. Of this, 32 million cubic metres comes from sewer network overflows, which provide the only safety valve to prevent the overloaded system from backing up and flooding homes and streets.

The discharges kill fish, create a higher health hazard for users of the river and damage the aesthetic appeal of the Thames, reducing its attractiveness to tourists.

The solution

Thames Water has appointed CH2M HILL as programme manager for the Thames Tideway Tunnel scheme, to help deliver the ‘super-sewer’ that will prevent overflows from London’s sewers entering the River Thames.

The project will involve the construction of London’s deepest ever tunnel, the Thames Tunnel up to 80 metres beneath ground level, and spanning 32 kilometres beneath the River Thames from Hammersmith in West London, to Beckton, in East London. An additional tunnel, the Lee Tunnel (7 kilometres) will run from Abbey Mills in Stratford to Beckton. The tunnels will capture and transfer sewer discharges for treatment, which would otherwise enter the River Thames and its tributaries during heavy rainfall.

London has a combined sewerage system, built during the Victorian era to carry both rainfall and sewage. Overflow points were constructed along the river banks as part of the system to prevent flooding during heavy rainfallby directing sewage into the River Thames and its tributaries.

The detail
Steve Walker, Major Projects Director for Thames Water, said: “The Tideway Tunnel scheme is Thames Water’s biggest single investment project by far. These exceptional tunnels will have enough capacity to store millions of litres of diluted sewage and transfer it to our Beckton sewage treatment works. This scheme is essential if we are to improve the quality of the river and reduce the environmental impact of sewage overflows.

Both tunnels will be more than seven metres wide, running beneath a vast network of existing tunnels, including six Tube lines and utilities. The overall programme includes constructing numerous collection and diversion facilities, a large high-head underground pumping station, and a major upgrade at Beckton sewage treatment works.

Construction of the Lee Tunnel — which will capture half of all Thames’ Tideway discharges from a single overflow point at Abbey Mills pumping station, is expected to start in 2009. The tunnel is due for completion by 2014.  Construction of the Thames Tunnel, which will prevent discharges from 35 overflow points along the Thames Tideway, is not expected to start until 2012 with completion in 2020.

The Thames Tideway Strategic Study was set up in 2000 to assess the environmental impact of sewage overflows, to identify objectives for improvement, and to propose potential solutions. The steering group included Thames Water (the sewerage service provider), the Environment Agency (the environmental regulator), Defra, the Greater London Authority, and with observer status Ofwat, the economic regulator of the water industry.

Mobile oxygenation vessels operated by Thames Water, and deployed at the Environment Agency’s request, will continue to be used along with three land based hydrogen peroxide dosing plants at strategic locations, to protect fish species by re-oxygenating stretches of the river following significant sewage overflow discharges.

CH2M HILL (HQ in Englewood, Colorado) has managed similar projects in Singapore, where the firm provided programme management for one of the world’s largest wastewater reclamation projects, and in Milwaukee, Wisconsin, for the first successful city-wide deep-tunnel combined sewer overflow system implemented in the U.S. The Milwaukee tunnel system has reduced combined sewer overflow events from more than 60 per year to an average of two per year, greatly enhancing the water quality in Lake Michigan.

Olympic games 2012

Consideration has also been given to whether the Abbey Mills to Beckton link tunnel could be completed by the time of the Olympic and Paralympic Games. In case a permanent solution to Abbey Mills discharges cannot be in place by the time of the Olympic and Paralympic Games, interim contingency measures to mitigate the effects of the discharge are being investigated in parallel.

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