“Chronic under-capacity” of wastewater systems to blame for storm spills, researchers find

Storm overflows are discharging frequently because wastewater treatment capacity has not kept up with population growth and increased industrial activity according to findings from researchers at Imperial College London.

The researchers matched water company Event Duration Monitoring data with the relevant wastewater treatment works, and combined this with data on their treatment capacity. This analysis showed the capacity of many wastewater treatment works is not sufficient, even in the absence of extreme rainfall. This suggests solutions such as preventing blockages and separating rainwater from combined systems will not eliminate sewage spills.

Lead researcher, Professor Nick Voulvoulis, from the Centre for Environmental Policy at Imperial, said: “Our findings reveal the chronic under-capacity of the English wastewater systems as a fundamental cause behind the increased frequency and duration of CSO spills. We hope this work can help the water industry demonstrate the need for capital investment in infrastructure. It is often taken for granted but investing in infrastructure is critical to our future prosperity.”

First author Dr Theodoros Giakoumis, also from the Centre for Environmental Policy at Imperial, said: “Most proposals to deal with the problem of increased sewage overspills suggest ways to reduce the impact of CSO discharges instead of targeting their causes. The elephant in the room is that most cities have experienced population growth and wastewater system expansion, at rates that have not been matched by water infrastructure growth.

“This shows the need for an open discussion on water infrastructure resilience and investment, issues that perhaps most believe had already been addressed in high-income countries like the UK.”

Imperial said between 2000 and 2008, just over 1% of the sewers in England and Wales were replaced or rehabilitated. “Considering that much of the infrastructure was built with a lifespan of 60-80 years, at that rate of replacement, it would take 800 years for this to happen for all the sewers in England and Wales.”

Giakoumis and Voulvoulis' report: Combined sewer overflows: relating event duration monitoring data to wastewater systems' capacity in England was published in Environmental Science: Water Research & Technology.