A super-sewer for London is not a choice but a necessity, and must go ahead despite opposition in order to preserve the Thames and cope with London’s rising population, according to Lord Chris Smith, chairman of the Environment Agency.

He said a super-sewer was “absolutely necessary” and would prevent the current “unacceptable” situation in which raw sewage frequently spills into the Thames. The new 32km super-tunnel would run under most of the length of the river through London, taking the pressure from the often overloaded pipes that have been in place since the mid-19th century. Thames Water estimates it could add around £50 a year to the average water bill, but other estimates put the figure much higher.

Smith said: “London needs a 21st-century sewerage system to bring it up to the standards of other UK and EU cities. The Thames Tideway offers the best value for money and the environment, and will help prevent sewage overflowing into the river after heavy rainfall.”

The proposal for a super-sewer to run under the Thames has been in development for more than five years, but only last year were detailed plans drawn up, and they involve extensive construction work across London for at least seven years, at a cost of about £3.6bn. There were immediate objections from councils, groups affected by the proposed building works, and residents near some of the parks that will have to be the site of building works if the construction goes ahead.

But Smith said the new proposals, which are under consultation, were the only way to solve the problems of sewage entering the Thames, and to avoid falling foul of European regulations. As the population of the south-east of England increases, the pressure on the current sewage system – the capacity of which has not been increased since early in the last century – grows greater. Smith said alternatives – such as building piecemeal additions to the existing sewers – had been evaluated, but the “Thames tideway tunnel” represented the best long-term solution and the best value for money.

London’s sewers were built by Sir Joseph Bazalgette in the 1860s, one of the first programmes of major public works to improve the capital’s infrastructure, instituted after years of complaints about the filth and stink, and the diseases caused. The sewers were deliberately designed to empty into the Thames if heavy rain caused them to overflow, to prevent sewage backing up or flowing into the streets. But under increasing pressure of population, the system is close to capacity and now overflows of sewage happen on a weekly basis, killing fish and other river life.

Speaking at a ceremony to mark the 30th anniversary of the London Wildlife Trust, Smith said: “The current situation is unacceptable, with 39m cubic metres of raw sewage mixed with surface water run-off discharged in an average year. In wetter years the figure can increase threefold. These discharges occur, on average, once a week – up to 60 times a year – and after as little as 2mm of rain.”

Smith said that, a century and a half later, a major new programme of works was needed. He said it could not be achieved without disruption, but that the works would be worth it as they would provide a long-term solution to London’s sewage problems, improve the environment, cope with a rising population, and prevent fines being levied by the European Union for the pollution of the Thames.

He said: “This is the worst sewage pollution problem in the UK. There are chronic, long-term effects that occur to fish species that reside, or migrate through, reaches that are affected by overflows. Fish populations do not live out their full lifecycle in the Thames and improvements need to be made to the water quality.”

While the price tag will be hefty, and the works could last until 2021, the work will provide long-term capacity for London to expand, and will last for at least a century. The system is also likely to be needed to cope with the effects of climate change – a government-sponsored study found this week that floods would be the worst risk for the UK from global warming.

Smith added: “The tunnel will help the UK implement commitments under the water framework directive and ensure continued compliance with the urban waste water treatment directive, thereby avoiding unlimited EU fines.”

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