Sewage overflows may pose greater threat to England's rivers than previously thought

Sewage Overflows May Pose Greater Threat to England's Rivers Than Previously Thought

A groundbreaking study conducted by researchers at Imperial College London and Brunel University London reveals that combined sewer overflows (CSOs) may be contaminating England's rivers far more severely than previously acknowledged. This research, published in Environmental Science: Water Research & Technology and spearheaded by Professor Nick Voulvoulis of Imperial's Centre for Environmental Policy, represents the first comprehensive national evaluation of the pollution loads released during sewage overflow events and their impact on England's water ecosystems.

A Clearer Picture of Pollution

Combined sewer overflows comprise a component of the antiquated infrastructure established during the Victorian era, designed to transport both rainwater and wastewater via the same pipelines. During intense rainfall, these systems are prone to becoming overloaded, resulting in the discharge of untreated or partially treated sewage into rivers and coastal waters. While there has been ongoing monitoring of the frequency of these overflows, this new study seeks to quantify the actual pollution released during these events.

By analyzing data gathered from over 13,900 overflow points across England, the researchers estimate that CSOs discharged approximately 420,000 tonnes of organic pollution (measured as biochemical oxygen demand) and 360,000 tonnes of suspended solids in 2023 alone. Notably, these quantities are frequently comparable to, or even exceed, the pollution output from wastewater treatment works (WWTWs).

Challenging Long-Held Assumptions

Historically, CSOs have been regarded as a secondary concern in water quality management, stemming from the assumption that their occurrences are infrequent. This study refutes such assumptions, demonstrating that the cumulative pollution emitted from CSOs often surpasses that from WWTWs. The resulting pollution loads affecting water bodies are, on average, four times greater for biochemical oxygen demand and twice as high for suspended solids.

Not All Overflows Are Equal

The research further indicates that not all overflows contribute uniformly to pollution levels, with a limited number of overflows being responsible for a disproportionately large share of total contamination. This finding suggests that prioritizing interventions at the most problematic systems could lead to significant environmental enhancements. Additionally, systems that suffer from inadequate treatment capacity tend to experience longer and more frequent spills, highlighting the urgency of addressing deteriorating infrastructure.

A New Way to Assess Risk

To assess the impact of these discharges, the research team devised a national risk framework linking pollution sources, pathways, and the sensitivity of receiving water bodies. Their comprehensive analysis revealed that 44.7% of wastewater systems in England fall into high or very high environmental risk categories. This innovative method not only accounts for the frequency of spills but also addresses the magnitude of pollution and the susceptibility of affected rivers.

Implications for Policy and Investment

These findings emerge amidst rising public concern regarding river pollution in England. By integrating monitoring data with environmental and regulatory insights, the study establishes a robust evidence base for informed decision-making, enabling regulators and water companies to concentrate investments where they will exert the most significant impact. It underscores the need for a transition from reactive monitoring practices to proactive, system-wide strategies in sewage management. As the pressures on aging infrastructure increase and climate change exacerbates rainfall events, a precise understanding and timely response to CSO pollution are essential for safeguarding the health of England's rivers.

For further details, refer to: Theodoros Giakoumis et al, "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," Environmental Science: Water Research & Technology (2026). DOI: 10.1039/d5ew00860c