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Digging bigger sewers under London is expensive, but vital

Trouble is brewing underneath the city. Kirsty Wigglesworth/PA

London’s sewerage system is one of the wonders of the industrial world, and a prize example of great Victorian feats of engineering. The system was designed by the visionary Chief Engineer of the Metropolitan Board of Works - Sir Joseph Bazalgette - in 1856, but languished unfunded, until the Great Stink of 1858. The miasma theory popular at the time held that “bad air” spread disease, and in this case the smell was so bad that Members of Parliament, fearing for their lives, finally focused on tackling the project.

Bazalgette’s original sewer system was designed for a population of four million people - to accommodate for the growth of what was already the world’s largest city at two and a half million citizens. The design also accommodated for rainfall of up 6.5mm per day and was – at a staggering cost of £4.2 million - the world’s most sophisticated system. The same system, using today’s high technology construction systems would cost about £60m.

Until London’s sewers were built, tens of thousands of people were killed by frequent outbreaks of cholera. During the middle of the 19th century, 40,000 people died due to an outbreak of this disease. Sanitation such as Bazalgette’s London sewers is not just about dealing with dirty water - it has made an immense contribution to the health and well-being of Londoners, and saved many millions of lives where implemented worldwide.

To this day, London relies on this original system, but the city’s population of more than eight million today is twice its designed capacity. Adding to complications is that much of the permeable surface that could have helped absorb and slow run-off water has disappeared under the ever expanding concrete and tarmac. Today, even light rainfall of 2mm can trigger a discharge of sewage into the Thames, polluting the water.

Following a decade of study and evaluation of various options, the Thames Tideway Tunnel project has been proposed. This is the first significant change to the capital’s 150-year-old sanitation system, and is designed to prevent the current rates of around 39m cubic metres of untreated sewage discharged into the river every year.

The Thames Tideway scheme is built around the main tunnel, 7.2m in diameter and 16 miles long, that will run mostly under the River Thames between about 30 and 70 metres below ground level. It also includes the new Lee Tunnel, and upgraded sewage treatment facilities, which are already in progress. Once completed in 2023, the main tunnel will reduce the need for sewage discharge and will reduce rainwater to under three million cubic metres – considerably reducing the risk to public health and the environment in the river itself.

The cost of this project is, at £4.2 billion, a thousand times more expensive than its predecessor, and almost half the cost of the 2012 London Olympics. This is a significant sum of money in these hard-pressed economic times. But it’s likely the alternatives will cost significantly more and some may not be feasible in a city as densely populated as London.

Other large cities with very long histories, such as Paris, face the same challenge. A solution similar to the proposed tunnel in London is being considered.

An increasingly common approach is to construct separate drainage systems for sewage and rain water. This system reduces the pressure on drainage infrastructure and allows for savings on sewage treatment costs and reuse of rainwater. However, retrofitting dual drainage systems in cities the size and complexity as London is prohibitively expensive and impractical.

The other alternatives include Sustainable Urban Drainage Systems (SUDS), which use measures to minimise the amount of rainwater that enters the sewage system. For example, grassed “living” roofs absorb water, or soakaways in the ground. But these require large areas of land and without a major disruption to the city and its economy, these simply cannot be implemented. They also take much longer, as they rely in part on natural processes.

Another approach is to build distributed shallow rain water drains connecting to an underground storage tank. The collected rain water can be treated and reused or - if this is uneconomical - released into the main sewage system at a controlled rate. This would avoid the need to release raw sewage into the Thames. But again, this solution is likely to prove expensive and difficult to implement in the densely-built city centre, but offers an alternative to less densely populated areas of Greater London.

Thames Tideway Tunnel is a traditional engineering solution to accumulated problems over the last 150 years. It does not propose any innovations to dealing with the problem. But if steps are not taken to improve London’s sewers, it’s very likely the combined overflow into the Thames will to continue to rise, perhaps as high as 70 million cubic meters. This would seriously damage the river water quality, and potentially pose a public health risk.

While the Thames Tideway Tunnel is not necessarily the ideal answer to London’s rising tide of sewage, it is currently the best, practicable solution on the table.

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