Combined overflows

Combined sewer overflows

Combined sewers transport sewage from homes and industry as well as carrying surface water run off from gutters, drains and some highways.

Heavy or prolonged rainfall can rapidly increase the flow in a combined sewer until the amount of water exceeds sewer capacity.

Combined sewer overflows (CSOs) act as relief valves allowing excess stormwater to be released to rivers or the sea. This helps protect properties from flooding and prevents sewage backing up into streets and homes during heavy storm events.

As CSOs should only operate during periods of unusually intense rainfall, any foul water released from them will be very dilute because of the large volumes of rainwater within the system.

Flows are further diluted by the receiving watercourses that will also be swollen by the same heavy rain. CSOs are also fitted with screens that help remove debris from the flow and minimise any environmental impact.

Our most critical CSOs are continuously monitored and licensed by the Environment Agency to ensure they only operate when the system is heavily surcharged by rainfall.