Last updated: 20 June 2018


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.


Sir Bazalgette’s design used London’s natural drainage system of ‘lost rivers’ – the Fleet, the Tyburn, and the Effra, etc. – 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.

Western Pumping Station in Battersea discharging untreated sewage in to the Thames - December 2013

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.

Stephen Halliday discusses The Great Stink and Sir Joseph Bazalgette

The need for a solution

In 2001, the ‘Thames Tideway Strategic Study Group’ was set up to investigate the impact of sewage into the River Thames – a number of options to tackle the problem were proposed.

Of the options (which included sustainable drainage solutions, ‘bubbler’ vessels to add oxygen to the river, and work to separate the sewer system into sewage and rainwater run-off), the Government chose a full-length tunnel as the most timely and cost-effective solution.

The situation now

The Victorian sewerage network simply cannot keep up with the demands of 21st Century London and need future-proofing.

The sewers built by Sir Joseph Bazalgette in the 1860s form the backbone of London's sewerage system today. They are in excellent working condition, but have simply run out of capacity. Built when London's population was two million and designed for four million, they are now struggling to serve a capital city with more than eight million people; a figure that continues to rise.

Population of London

By 2031, there will be 10 million people living in London. To cope with this increase, it is estimated that at least 600,000 new homes are needed. In order that these homes can be built the sewerage network, which is already under severe pressure, needs to be upgraded.

Project timeline