Why did London's river stink in 1858, and why did it need a giant new sewer in 2025?

London's river stank in 1858 because the city used the Thames as its sewer, and it needed a giant new one in 2025 because the first sewer was built for a much smaller city. The Great Stink was a summer when hot weather made the smell of untreated human waste and industrial effluent on the Thames banks much worse. It was not a surprise. In 1855 Michael Faraday, shocked at the Thames, had dropped white paper into the water to test how opaque it was, and in 1857 the government poured chalk lime, chloride of lime and carbolic acid into the river to ease the stench.

The fix was an underground system. The authorities accepted Joseph Bazalgette's proposal to carry the waste eastwards through interconnecting sewers to outfalls beyond the metropolitan area. His work ended the dumping of sewage on the Thames shores and brought an end to the cholera outbreaks.

But Bazalgette's system was built between 1859 and 1875 for the sewage of four million people. It had a safety valve: in heavy rain it could overflow into the river through 57 overflows. Then London grew. Its population density in 2001 was 18,457 people per square kilometre, against 6,825 in Bazalgette's day, and more rain ran into the sewers as hard surfaces replaced soil.

By 2011, overflows entered the river more than once a week, and as little as 2 mm of rain could trigger one. The mean total of storm sewage entering the river was put at about 39 million tonnes. The answer was a second system: the Thames Tideway Tunnel, a deep-level sewer 25 km long, designed to capture almost all the raw sewage and rainwater that would otherwise overflow. Its first sewage flowed in September 2024, and it was officially opened in May 2025.

The river itself is busier with life than you might expect: up to 700 grey and harbour seals live in the Thames Estuary.