Like many old cities around the world, most of London is served by a ‘combined’ sewerage system, which collects not just the sewage from loos, sinks, showers and washing machines, but also the rainwater run-off from roads, gutters and pavements – hence, ‘combined’.
The magnificent system we rely on today was designed by Sir Joseph Bazalgette after The Great Stink of 1858. The 318 million bricks used to construct the 1,100 miles of underground sewers are still in good condition, but are simply overwhelmed by today’s population.
The Thames Tideway Tunnel will offer the increased capacity needed to tackle the current pollution problem, and provide a system fit for the future. However, alongside the tunnel, sustainable drainage systems will also play their part. Whilst it is simply not viable to separate the sewer system now, reducing the amount of rainwater entering the sewers will extend the life of the tunnel.
Sir Joseph Bazalgette’s design used London’s natural drainage system of ‘lost rivers’ – including the Fleet and the Tyburn, . – which had been built over before Victorian times, to flow into his new sewers and on to balancing tanks in east London. However, during severe storms and times of heavy rainfall, the system was designed to overflow into the River Thames, rather than flooding streets and homes. In Bazalgette’s day, this happened once or twice a year. Now, this happens every week, on average.
While they are still in excellent condition, London's Victorian sewers now lack the capacity to meet the demands of modern-day living.
In the mid-nineteenth century, more than two million people lived in London. Bazalgette had the foresight to design his system to serve four million, but today the city's population is nearing nine million – and continues to grow.
Back in the 1850s, not only were there fewer people living in London, but they also used less water per person and there was considerably more green space available to soak up rainfall. This meant that overflows occurred only very occasionally.