Climate change, population growth, and the paving over of green spaces that provide natural drainage are putting increasing pressure on our sewerage network.
This is often made worse by people putting unsuitable products down the sink or toilet, increasing risk of sewer flooding.
Many areas have separate sewers to take foul waste and rainwater, but in much of London and some other urban areas, the sewer system is combined, meaning that foul sewage from kitchens and toilets mixes with rainfall.
This means that, during a heavy storm, the flow in the sewer is much greater and can reach maximum capacity.
Our sewers are designed to cope with the vast majority of storms but occasionally rainfall can be so heavy that it overwhelms the system.
When this happens, sewage can overflow from manholes and gullies and flood land, rivers and gardens. In the worst cases, sewage can even flood homes.