Thames Tideway Project, London, United Kingdom

The Thames Tideway scheme – coupling improvements to London’s three main wastewater treatment plants (WWTPs) with the construction of a storage-and-transfer tunnel nearly 35km long – forms one of the city’s most ambitious wastewater projects since its first sanitation system in the mid-19th century.

With an eight-year estimated construction schedule, the tunnel is intended to improve the capacity of London’s sewerage system and help prevent sewage discharge into the River Thames. In addition to reducing the potential for wet weather overflows, the work will allow more wastewater to be treated to a higher standard, in line with water utility Thames Water’s policy aim of making the river cleaner and improving the quality of both water and the environment.

The improvement work at the plants in Beckton in north London, Crossness in south London and Mogden in west London will require an investment of some £400m, spread over nine years. The tunnel is expected to cost around £2.5bn, with full completion provisionally scheduled for 2020.

SEWER CONSTRUCTION

London’s lack of sewer provision made cholera epidemics commonplace throughout most of the early 19th century and although proposals to remedy the problem were put forward in 1856, it was not until "The Great Stink" of 1858 that the idea of a modern sewerage system took root.

Joseph Bazalgette – chief engineer of the Metropolitan Board of Works – designed an extensive underground system to divert wastewater, which included constructing interceptor sewers between 1859 and 1865, to serve both the north and south of the Thames. The scale of this Victorian scheme was enormous, requiring the excavation of more than 3.5 million tonnes of earth, nearly 700,000mm³ of cement and around a third of a million bricks to complete.

Despite major improvements in the 20th century, London’s physical expansion and population growth has placed increasing demands on system capacity – particularly in wet weather, when high rainfall can overwhelm the city’s sewers and treatment works.

An overflow system diverts the excess flow to the Thames, but this can also lead to untreated sewage being discharged directly into the water. With an estimated 20 million cubic meters of untreated sewage – albeit diluted with surface water run-off – entering the river annually, the situation became increasingly unacceptable, particularly in the light of the demands of the Urban Wastewater Treatment Directive.

For some years there has been discussion over how to increase the carrying capacity of the system. Thames Water first proposed a combined sewer overflow back in the early 1990s – an idea which subsequently gave rise to the Tideway Tunnel scheme.

THAMES TIDEWAY TUNNEL

The Thames Tideway Strategic Study (TTSS) – a multi-agency group comprising Thames Water, the Department for Environment, Food and Rural Affairs (Defra), the Environment Agency and the Greater London Authority (GLA), with water service regulator Ofwat as an observer – was formed in 2000 to examine and evaluate possible solutions to the problem. In due course, they recommended a 34.5km storage tunnel to intercept the overflows at the river and convey the wastewater for treatment.

After a subsequent request by the UK Government to examine possible ways to address the problem more quickly – particularly in view of the 2012 London Olympics – TTSS presented their findings in February 2005, followed up by a supplementary report in the November. After extensive consideration of all the suggested options and sub-options, approval was given to a scheme to build a tunnel some 32.2km long from Hammersmith to Beckton, with an additional spur tunnel running the 5.5km from Abbey Mills to Beckton.

The tunnel will lie more than 60m below ground for most of its route, starting out at a depth of 40m at Hammersmith in the west and dropping to around 80m as it travels east to Beckton. Tunnel Boring Machines will create the tunnel and a series of strategically placed access shafts, with excavated material being transported by barge along the river to minimise road haulage. Once completed, it will be lined to isolate it from the surrounding earth and eliminate the chance of seepage.

There will be a single pump out shaft – at Beckton, the UK’s largest WWTP – and a total storage volume in excess of 1.6 million cubic meters. The Abbey Mills to Beckton tunnel is expected to be completed first.

IMPROVING WATER TREATMENT

Increasing the treatment capacity of the WWTPs is intended to provide sufficient slack to enable greater volumes of rain-swollen influent to be fully treated when necessary, while ensuring that the dissolved oxygen levels in the river are not adversely affected. This will also allow the frequency of storm tank usage to be reduced, which itself further reduces the likelihood of overflows.

Designing the programme – which includes improving screening and grit removal, primary and secondary settlement, aeration and odour control – took projections of future growth into account. The figures for Dry Weather Flow (DWF) and Flow to Full Treatment (FTFT), for instance, reflect expected population increases up to 2021– five years more than had previously featured in earlier schemes.

Other measures that Thames Water are implementing to support their wider programme of improvements include introducing new ‘skimmer’ boats to remove floating litter from the river and ‘bubbler’ vessels to pump extra oxygen into the water after heavy storms.

KEY PLAYERS

Thames Water own and operate the plants, being members of the TTSS, along with Defra, the EA, GLA and Ofwat. Jacobs Babtie were responsible for the review of the TTSS recommendations on behalf of Ofwat. Rotork provided automation systems for part of the plant upgrades with Hydro International supplying their Zickert longitudinal scrapers, scum scrapers and other associated equipment to improve sludge settlement.

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