London sewerage system

The London sewerage system is part of the water infrastructure serving London, England. The modern system was developed during the late 19th century, and as London has grown the system has been expanded. It is currently owned and operated by Thames Water and serves almost all of Greater London.

History

During the early 19th century the River Thames was an open sewer, with disastrous consequences for public health in London, including cholera epidemics. These were caused by enterotoxin-producing strains of the bacterium Vibrio cholerae. Proposals to modernise the sewerage system had been made during 1856, but were neglected due to lack of funds. However, after the Great Stink of 1858, Parliament realised the urgency of the problem and resolved to create a modern sewerage system.

Joseph Bazalgette, a civil engineer and Chief Engineer of the Metropolitan Board of Works, was given responsibility for the work. He designed an extensive underground sewerage system that diverted waste to the Thames Estuary, downstream of the main centre of population. Six main interceptor sewers, totalling almost 100 miles (160 km) in length, were constructed, some incorporating stretches of London's 'lost' rivers. Three of these sewers were north of the river, the southernmost, low-level one being incorporated in the Thames Embankment. The Embankment also allowed new roads, new public gardens, and the Circle Line of the London Underground.

The intercepting sewers, constructed between 1859 and 1865, were fed by 450 miles (720 km) of main sewers that, in turn, conveyed the contents of some 13,000 miles (21,000 km) of smaller local sewers. Construction of the interceptor system required 318 million bricks, 2.7 million cubic metres of excavated earth and 670,000 cubic metres of concrete.

Gravity allows the sewage to flow eastwards, but in places such as Chelsea, Deptford and Abbey Mills, pumping stations were built to raise the water and provide sufficient flow. Sewers north of the Thames feed into the Northern Outfall Sewer, which feeds into a major treatment works at Beckton. South of the river, the Southern Outfall Sewer extends to a similar facility at Crossness.

During the 20th century, major improvements were made to the sewerage system and to the sewage treatment provision to substantially reduce pollution of the Thames Estuary and the North Sea.

Modern development needs

The original system was designed to cope with 6.5 mm (1/4”) per hour of rainfall within the catchment area, and supported a smaller population than today’s. London's growth has put pressure on the capacity of the sewerage system. During storms, for example, high levels of rainfall (in excess of 6 mm per hour) in a short period of time can overwhelm the system. Sewers and treatment works are unable to cope with the large volumes of rainwater entering the system. Rainwater mixes with sewage in combined sewers and excess mixed water is discharged into the Thames. If this does not happen quickly enough, localised flooding occurs (surcharge). Such sanitary sewer overflow can mean streets becoming flooded with a mixture of water and sewage, causing a health risk.

In redeveloping the Isle of Dogs and Royal Docks areas of east London during the late 1980s and early 1990s, the London Docklands Development Corporation invested in major new drainage infrastructure to manage future sewage and surface water run-off from proposed developments. Consulting engineer Sir William Halcrow & Partners designed a system of large diameter tunnels served by new pumping stations. In the Royal Docks, approximately 16 miles (25 km) of foul and surface water drains were built, plus pumping stations at Tidal Basin (designed by Richard Rogers Partnership) and North Woolwich (architect: Nicholas Grimshaw). The Isle of Dogs drainage network is served by a stormwater pumping station situated in Stewart Street, designed by John Outram Associates.

Thames Tideway Scheme

Increasing the carrying capacity of London’s sewerage system has been debated for some years. Proposals for the ' Thames Tideway' include a wide diameter storage-and-transfer tunnel (internal diameters of 7.2 m and 9 m have been suggested), 22 miles (35 km) long, underneath the riverbed of the Thames between Hammersmith in the west and Beckton/ Crossness in the east, but as the cost of such a megaproject is likely to be substantial (estimated at £1.7 billion in 2004), investment decisions have been slow in forthcoming. In March 2007 the Mayor of London announced that the project will proceed with completion expected by 2020.

Because design and construction of such a tunnel will take an estimated 15 years, a shorter-term (and slightly lower cost) interim solution has also been developed. This £1.6 billion scheme (2006 prices) involves two shorter tunnels (one taking storm water from Hammersmith to Battersea for treatment or storage, the other carrying water from Abbey Mills south to the river at Beckton) and improvements to associated treatment facilities.