Combined Sewer Overflows (CSOs)

Combined Sewer Overflows (CSOs)

What are Combined Sewer Overflows (CSOs)?

Combined sewer overflow is the discharge, during rain storms, of untreated wastewater from a sewer system that carries both sewage and storm water (a combined sewerage system).  The increased flow caused by the storm water runoff exceeds the sewerage system’s capacity and the sewage is forced to overflow into streams and rivers in the area through CSO outfalls.

Why, and how often, do CSOs overflow?

Changing rainfall patterns caused by climate change and urban development have meant that London's 140-year-old sewage system is often unable to cope with the combined flow from the city's sewage and storm water system.  Of the 57 CSOs in London, 36 are considered ‘unsatisfactory’ in terms of frequency of discharge and/or environmental impact.  Even during periods of moderate rainfall, the overflows discharge storm water and sewage into the River Thames on average once a week.  An average of 20 million cubic metres of untreated sewage is discharged into the Thames every year.

What are the environmental impacts of intermittent discharges of raw sewage?

These discharges can cause:

• aesthetic pollution due to sewage-derived litter deposited on the foreshore and in the river;
• an increased health risk for recreational river users;
• damage to the ecology of the river;
• reduced levels of dissolved oxygen in the river which has resulted in occasional large fish kills. 

What are the potential solutions to the problem?

A Thames Tideway Strategy Group was set up in 2000 to assess the environmental impact of CSOs and to prepare potential solutions having regard to cost.  Some of the solutions considered include:

• The use of Sustainable Drainage Systems (SUDS) techniques to reduce rainfall inputs into CSOs, such as permeable surfaces, infiltration devices, ponds and reef gardens.  The group concluded that the use of these techniques alone could not deal with the amount of water that enters the system.

• The separation of all existing combined sewerage systems, providing one system for rain and one for sewage.  However, it was determined that such a scheme would be far too expensive.

• A 35km long storage-and-transfer tunnel.  The tunnel would run beneath the Thames from Hammersmith in West London and convey the discharge from 36 CSOs for collection and treatment at the Crossness sewage works in east London where a plant to handle the storm flows could be built.  This was the studies preferred solution, at an estimated cost of £2bn.
  
  

Further Information:

·         For information on the Thames Tideway Strategy - http://www.thamestidewaystrategicstudy.co.uk/

·         The Chartered Institution of water and Environmental Management (CIWEM) -www.ciwen.org.uk/policy/policies/overflows.asp