Combined sewer overflows (CSOs) are contributing far more pollution than previously thought to England’s waterways, according to a new study published in Environmental Science: Water Research & Technology.

CSOs are a legacy of Victorian-era infrastructure. They were designed to carry a combination of rainwater and wastewater through the same pipes. When rainfall is heavy they overflow, releasing untreated or partially treated sewage into rivers and coastal waters.

This research – undertaken by scientists from Imperial College London and Brunel University London – provides the first national assessment of pollution loads released when CSOs exceed their capacity and release sewage. 

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. 

These findings come as England’s regulatory bodies are coming under great public scrutiny, in part due to Channel 4’s docudrama ‘Dirty Business’.

The monitoring of these overflow systems has largely focused on recording the frequency in which they are used, rather than quantifying the amount of pollution actually released.

Policies have historically prioritised pollution released by wastewater treatment plants, rather than these overflow systems, in part because they were assumed to only operate occasionally.

But this analysis shows that the total quantity of sewage being discharged from these overflow systems is comparable to and, even more worryingly, sometimes larger than the pollution released from wastewater treatments.

In fact, English waterways received on average four times more organic waste matter and twice as much solid material from CSO overflow events compared to the wastewater treatments.

Waste water treatments have faced increased scrutiny, due to discharging untreated sewage into storm overflows, even during relatively dry periods.

Researchers said their findings point to the fact that current investment and policies – which tend to target treatment plants – overlook a significant source of pollution.

The study also found that some overflows contribute a disproportionately large share of the total pollution in English rivers and waters, suggesting location-dependent, targeted interventions would be a beneficial way of delivering substantial environmental improvements. 

The researchers involved in the project said their findings reveal that systems with insufficient treatment capacity tend to experience longer and more frequent spills – therefore highlighting aging infrastructure as a key driver of the problem.

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