Last updated: 07 November 2017


Did you know? Sir Joseph Bazalgette's brick sewers are made up of 318 million bricks laid by hand

Enough sewage discharges into the River Thames each year to fill the Albert Hall 450 times over

Like many older cities around the world, the vast majority of London is served by a combined sewerage system, collecting sewage from toilets, sinks, showers and washing machines, together with rainwater run-off from roads, roofs and pavements.

The magnificent interceptor sewers, constructed by Sir Joseph Bazalgette following the ‘Great Stink’ of 1858, are still the backbone of London’s sewer network today. Rebuilding and separating this system, using modern methods, would cost £50-60 billion at today’s prices.

Sir Bazalgette's design

Bazalgette's design centred around using the city's natural drainage system of 'lost rivers', such as the Fleet and the Tyburn, which had already been built over before Victorian times, to flow into his new interceptor sewers and transfer to balancing tanks in east London. However, in times of severe storms, the system was designed to overflow through discharge points on the banks of the river into the River Thames, rather than flooding streets and homes. When designed, this would have happened once or twice a year. It now 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 Bazalgette's day, just over two million people lived in London. Sir Joseph had the foresight to design his system to serve four million, but today the city's population is near to eight 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 head 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


In 2001, the Thames Tideway Strategic Study Group was commissioned to investigate the impact of sewage discharges into the tidal River Thames. It was as a result of their work that options into how best to tackle the problem were proposed.

A full-length tunnel solution was selected by Government as the most timely and cost-effective solution to tackle the tens of millions of tonnes of untreated sewage which discharge into the tidal River Thames every year.


The need for a solution

The need for a solution


Why a tunnel?

Action before water enters the sewer:

Source Control and Sustainable Drainage Solutions (SuDS)

Within the sewer network:

Localised storage and separation


More bubbler and skimmer vessels

Interception of overflow:

Central storage and transfer

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