In an average year, 39 million cubic metres of untreated sewage overflows into the River Thames through London’s combined sewer overflows (CSOs).

London’s sewerage system dates from the 19th Century and was designed as a combined system. This means that a single pipe carries both foul water (from homes and businesses) and rainwater run-off (from streets, roofs and parks) to sewage works for processing before being discharged into the River Thames.

Increasingly, when it rains in London there is not enough capacity in the sewerage network to convey all the rainwater as well as foul flows. The system was designed to overflow into the River Thames so that peoples’ homes and streets are not flooded with untreated sewage. The system does this through the CSOs on the banks of the River Thames.

Combined Sewerage System

When Sir Joseph Bazalgette designed London’s sewerage system in the 19th Century, the CSOs operated once in a while – only when there were major storms. This meant that when overflows occurred, the sewage was diluted by large volumes of rainwater.

Some CSOs discharge untreated sewage into the River Thames on average more than once a week, and after only 2mm of rainfall. However, the growth of London’s population, as well as an increase in building developments and paved surfaces, has meant that overflows from the CSOs happen more and more frequently.

P2 Overflow PIP by thamestunnel

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