History of London's sewer system
By the mid-19th century, London was overcrowded and polluted. It faced deadly cholera outbreaks. The River Thames acted as the city’s open sewer. Waste from cesspits, drains, and overflowing privies flowed directly into the river. This contributed to repeated cholera outbreaks and caused the Great Stink in 1858. That summer was so awful that Parliament hung lime-soaked sheets at Westminster to mask the smell. This crisis finally prompted action. London needed a complete rethink of sewage management for a modern city.
Between 1859 and 1875, Sir Joseph Bazalgette, Chief Engineer of the Metropolitan Board of Works, designed a huge network of intercepting sewers. This system is so well built that much of it still supports London’s wastewater infrastructure today. It included about 82 miles of main brick-lined tunnels, fed by over 1,100 miles of street sewers. These were built to catch waste before it reached the Thames and to push it east, away from the city. Bazalgette famously oversized the system for a population of around 4 million. He believed London would only build it once, and that foresight is why much of it still works today.
How the sewer system works
Sir Joseph Bazalgette used gravity and pumping stations to move sewage. Key stations included Western, Abbey Mills, Deptford, and Crossness. Large beam engines helped where gravity was not enough. Crossness and Abbey Mills are famous for their architecture and ironwork. Today, Crossness is a museum and Abbey Mills is still active, using electric motors.
Modern-day issues
London grew and changed. Bazalgette's sewers still work, but they can't handle today's rain and wastewater. Heavy storms overwhelm the system, causing untreated wastewater to flow into the Thames. This happens because London is now too big and its surfaces are too hard, causing more water to run off into the sewers.
The next phase
The network had to expand. First, in 1891, the mid-level 2 and low-level 2 sewers were built. These secondary intercepting sewers were made at different depths. They collected extra flows from growing urban areas and eased the load on the original system. Then, in 1905, 2 new barrels were added to the Northern Outfall Sewer. This sewer is one of London’s main trunk sewers. Bazalgette designed it to carry sewage east from central and north London to treatment works downstream. These upgrades increased the trunk sewer size by about 40% of the original network. This change allowed for greater flow capacity before storm discharge into the River Thames.
Next came the Thames Tideway Improvement Programme, which began in the early 2000s. The Thames Tideway Strategic Study found an urgent need for a new solution. Even small amounts of rain caused the combined sewer system to overflow. This overflow sent millions of tonnes of untreated sewage into the River Thames each year from many discharge points.
To tackle this, we created a 3-part improvement program. In the first phase, we upgraded 5 major sewage treatment works, finishing in 2014. These were Beckton, Crossness, Riverside, Long Reach, and Mogden. The second phase involved Thames Water building the Lee Tunnel in 2016. This tunnel intercepts the largest combined sewer overflow (CSO) at Abbey Mills on the tidal Thames. The Lee Tunnel is 7.2m in diameter and 7km long. It runs from Abbey Mills pumping station to Beckton sewage treatment works.
Thames Tideway Tunnel
In 2015, a new company, Tideway (formerly Bazalgette Tunnel Limited), was created to finance and build the Thames Tideway Tunnel. This tunnel is the third phase of the program. It’s a 25km ‘super sewer’ that will greatly reduce sewage pollution in the River Thames. The infrastructure follows Bazalgette’s original ideas for a growing city. In many ways, it continues his work. It’s the 21st-century sequel to the Victorian system that made London a modern city.
Above ground, the Thames Tideway Tunnel project leaves a lasting mark on London. It has created new public spaces along the river, managed by Thames Water. These areas allow the public to enjoy and connect with the iconic river. From widened sections of the Thames Path to new river spots, these places offer a chance to pause and appreciate the river.
Tideway’s public art program, created with artists and local communities, enhances this legacy. Sculptures, poetry, crafted details, and site-specific artworks share the river's story. They highlight the engineering below and the lives of those along its banks.
Discover our riverside spaces
Find out how you can explore and enjoy new riverside spaces across London.