Combined sewer overflows (CSOs) could be contributing far more pollution to England's rivers than previously recognised, according to new research involving scientists from Imperial College London and Brunel University London.
The study, published in Environmental Science: Water Research & Technology, was led by Professor Nick Voulvoulis from Imperial's Centre for Environmental Policy. Bringing together expertise in environmental policy and water systems, the research provides the first national assessment of the pollution loads released during sewage overflow events, and the risks they pose to England's waterbodies.
A clearer picture of pollution
CSOs are a legacy of Victorian-era infrastructure, designed to carry both rainwater and wastewater through the same pipes. During heavy rainfall, these systems can exceed capacity, releasing untreated or partially treated sewage into rivers and coastal waters.
While monitoring in recent years has tracked how often these overflows occur, the new research goes further, quantifying the actual pollution they release.
Analysing data from more than 13,900 overflow points across England in 2023, the researchers estimate that CSOs discharged around 420,000 tonnes of organic pollution (biochemical oxygen demand) and 360,000 tonnes of suspended solids in a single year.
Crucially, these figures are not just large, they are often comparable to, or greater than, the pollution coming from wastewater treatment works (WWTWs) themselves.
Challenging long-held assumptions
For decades, CSOs have been treated as a secondary concern in water quality management, partly because they were assumed to operate only occasionally.
This study challenges that view.
The researchers found that, for 2023, estimated aggregated CSO loads frequently surpassed those from the effluents of their WWTWs, with affected waterbodies receiving loads from CSOs four times higher for BOD and double for suspended solids on average.
These findings suggest that focussing policy and investment primarily on treatment plants may overlook a significant source of pollution.
Not all overflows are equal
The study also reveals that pollution is unevenly distributed. While many overflows contribute relatively small amounts, a small proportion are responsible for a disproportionately large share of total pollution.
This opens the door to more targeted interventions, prioritising the worst-performing systems could deliver substantial environmental improvements.
The research further shows that systems with insufficient treatment capacity tend to experience longer and more frequent spills, highlighting ageing infrastructure as a key driver of the problem.
A new way to assess risk
To better understand the impact of these discharges, the team developed a national risk framework linking pollution sources, pathways and the sensitivity of receiving waterbodies.
Their analysis found that 44.7% of wastewater systems in England fall into high or very high environmental risk categories.
This approach moves beyond simple measures of spill frequency, instead capturing the scale of pollution and the vulnerability of affected rivers.
Implications for policy and investment
The findings arrive at a time of growing public concern over river pollution in England and increasing scrutiny of water infrastructure.
By combining monitoring data with environmental and regulatory information, the study provides a more robust evidence base for decision-making. It could help regulators and water companies prioritise investments where they will have the greatest impact.
More broadly, the research highlights the need to rethink how sewage systems are managed, shifting from reactive monitoring to proactive, system-wide solutions.
As pressure grows on ageing infrastructure and climate change intensifies rainfall patterns, understanding, and addressing, the true scale of CSO pollution will be critical to improving the health of England's rivers.
The full study, "The pollution load of combined sewer overflows and risks to England's waterbodies: relating event duration monitoring data to discharge consents from wastewater treatment works," is available in Environmental Science: Water Research & Technology.
