What are CSOs?

Sewer flooding

Combined Sewer Overflows (CSOs) are essential features of older combined sewer systems, mainly constructed before 1970, when it was common practice for foul sewage and surface water to be collected in the same pipe.

During times of heavy rainfall, when the sewers and pumping stations become overwhelmed, CSOs enable excess flows to be discharged into the sea, rivers or watercourses in a controlled and regulated way.  This is to protect homes, gardens, highways and open spaces from sewage flooding.

CSOs have to comply with strict European legislation and are regulated by the Environment Agency (EA) through discharge consents which prescribe the conditions under which they can operate. New consents are only permitted under exceptional circumstances.

In this region we have just over 1,700 CSOs and South West Water has invested £75 million in the last 10 years to reduce the volume and improve the quality of discharges in the most sensitive areas including bathing and shellfish waters.

Following new EA consents, all of our intermittent discharges were deemed to be operating within consent.