London’s older sewers carry rainwater and wastewater in the same pipes. In heavy rain, they can fill faster than treatment works can take the flow, so overflow outlets discharge the excess into the Thames to stop sewage backing up into streets and homes. By 2020, after treatment works upgrades and the Lee Tunnel had come into operation, typical annual overflow had fallen from 39 million m³ to about 18 million m³. Tideway now catches most of what remained, reducing the comparable figure to up to 2.4 million m³.

That is a 94% fall from the 2010 baseline. But 2.4 million m³ is still a lot of sewage: enough to fill about 960 Olympic-sized swimming pools in a typical year.

Methodology. These are comparable typical-year figures, not three measured annual totals. The 2.4 million m³ figure is Tideway’s modelled residual overflow for a typical year with the full tunnel system operating. The chart places it at 2025 because that is when Tideway was fully connected, not because 2.4 million m³ was measured during calendar year 2025.