London’s sewers: A busted flush
LONDON’S sewerage system was one of the great engineering projects of the Victorian age. The 21,000-kilometre network, designed by Joseph Bazalgette, helped to banish cholera and make the capital habitable and fragrant. It was made to last: the sewers were built for a population nearly twice that of the time. But 150 years on, three times as many people live in London and there is less greenery to soak up rain. Sewage and run-off go into the same system, which is full to bursting.
And burst it does. Bazalgette’s design included a safety valve: when too much waste enters the system, raw sewage runs into the River Thames. The supposedly-exceptional circumstance now happens once a week, on average. Two millimetres of rain in an hour can trigger a discharge; 39m tonnes of untreated sludge flushes into the river every year, says Thames Water, the utility firm that supplies London. Because the Thames is tidal, it can take four weeks for the muck to reach the sea.
In 2005 an independent commission proposed a solution. A giant 7.2 metre-wide sewer running beneath the river, known as the Thames Tunnel, would intercept all but two of the most polluting overflows and ferry waste to east London for treatment. Similar tunnels exist in Milwaukee and Portland, Oregon. Thames Water has adopted the scheme. On November 4th it launches the second phase of its consultation on a preferred 25km route and construction sites. The company hopes to begin digging in 2013 and finish in 2020.
The government is eager for the project to start: it needs to clean up the river to comply with European rules on treating waste water. It judges London’s tunnel to be “nationally important”, which means that planning should be streamlined. Ministers are also keen on a project that will employ people: this week they reiterated their thirst for infrastructure schemes.
But not everyone supports the super-sewer. Chris Binnie, who chaired the commission that proposed the tunnel, has retreated from his conclusion. He suggests considering a shorter tunnel along a 9km route from Hammersmith to Heathwall (see map). This would be cheaper and would allay the worst of the problem in the most-used stretch of the Thames. Others argue for green solutions like more porous surfaces and grassy trenches.
Yet such modest alternatives might still leave the Thames down in the dumps. A shorter tunnel would tackle only 19 of the 34 polluting overflows. Sewage would be stored until space was available in existing sewers, but with little spare capacity in the current network, waste could still spill into the river. Flows would be screened to remove big chunks, but filthy water would remain untreated. The tidal river would push detritus into the most-used spots.
That does not make it an easy decision to go ahead with the long tunnel. Thames Water’s 14m customers will pay for the scheme. Because the firm has invested so little in infrastructure historically, local bills have been lower than the national average. Thames Water says bills will rise “for the foreseeable future”. In 2006 the project was estimated to cost £3.6 billion. That figure is sure to go up.
And 24 construction sites must be picked. That will set off a fierce battle: suggested locations include some of the most populous and well-heeled parts of London. Local groups are already pitched against each other, as well as against Thames Water. Such a major engineering project will certainly cause a lot of disruption and fuss. But, judging by Bazalgette’s record, a good sewerage system can last 150 years. That’s not to be sniffed at.