Civil engineering insights into combined sewer overflows (CSOs)

England has a combined sewage system made up of hundreds of thousands of kilometres of sewers, much of which was built by the Victorians, in many urban centres.

This means rainwater and wastewater from toilets, bathrooms and kitchens are conveyed in the same pipe to sewage treatment works.

During heavy rainfall, the capacity of these pipes can be exceeded.

This means possible inundation of sewage works and the potential to back up and flood homes, roads, and open spaces unless it is allowed to spill elsewhere.

Combined sewer overflows (CSOs, also known as storm overflows) were developed as overflow valves to reduce flooding during heavy rainfall.

Overflows of diluted sewage during heavy rainfall are not a sign that the system is faulty.

CSOs are a necessary part of the existing sewerage infrastructure system, preventing sewage from flooding homes and businesses.

However, the topic of sewage discharges onto UK beaches and rivers has attracted a significant amount of attention.

In 2022, sewer overflows discharged a mix of raw sewage and rainwater into rivers and seas 301,091 times – or 825 times a day on average.

Untreated sewage contains bacteria such as E. coli and viruses like hepatitis, which can be harmful to the public and wildlife.

The issue has resonated very strongly with the public, who are facing the prospect of ongoing sewage discharges and increased bills to fix or mitigate the problem.

The UK government published its storm overflows discharge reduction plan in August 2022.

It commits water companies to spend £56 billion between 2025 and 2050 to reduce spills from CSOs that discharge to inland waterways and designated bathing waters.

Reducing discharges from sewer overflows on the scale planned in an environmentally sensitive way is a huge challenge.

Professionals and policymakers must now ensure that the additional investment delivers the intended improvements to rivers, waterways and the infrastructure system.

However, attempting to rush the process will likely result in ineffective solutions that do more harm than good.

This insight paper, which draws on views and evidence from experts on the ICE’s Water and Sanitation Community Advisory Board, asks:

• What is the true impact and scale of the problem?
• What are the solutions and timescales to implement solutions?
• And what are the challenges to overcome for policymakers and professionals?