According to new research from Imperial College London, the recent uptick in sewage overspill events is due to infrastructure not keeping up with demand.

The conclusion suggests other proposals for dealing with the problem – such as preventing blockages and separating rain and foul water – will not be enough to solve the issue of polluting sewage overspills.

The UK sewer system combines wastewater from domestic, commercial, and industrial activities and stormwater runoff and funnels it to (WWTWs).

Combined Sewer Overflows (CSOs) may be used by the water companies to discharge some of the sewage to natural watercourses before it reaches a Wastewater Treatment Works (WWTW). Raw sewage overspill from CSOs directly into rivers and the sea can pollute the water, especially when not used at times of high rainfall that could dilute the sewage.

Although the 14,346 CSOs in England have been assessed for their environmental risk, their increasing use, even in dry spells, has led many to question their role in the country’s wastewater systems.In times of extreme flow however, such as very heavy rainfall,

These overspills can lead to environmental deterioration and present a human health hazard, for example through enteroviruses that cause gut infections, or the proliferation of antibiotic resistance. This has a knock-on effect on tourism and leisure activities like swimming and boating, as well as the consumption of seafood that can accumulate toxins and microplastics.

Data-based solutions

In response to increased use of CSOs, water companies were ordered to release data on the frequency and duration of overspill events for each CSO for the last two years. However, this raw data doesn’t explain what the cause of the increase in their use has been. Without understanding the causes of these spills, the problem cannot be tackled with appropriate solutions.

In the first analysis of this data, Imperial researchers show that most of the increase is due to WWTWs’ lack of capacity to deal with increased flows as populations have risen and industrial activities have scaled up.

Aerial view of a wastewater treatment works

The team matched the CSO use data with the relevant WWTWs and combined this with data on their treatment capacity. The analysis shows the capacity of many WWTWs is not sufficient, even in the absence of extreme rainfall.

According to the Imperial College rersearchers, this refutes suggestions of reasons for increased CSO use that blame unpredictable weather, with changes in rainfall causing increases in wet weather runoff. The results are published in the journal Environmental Science: Water Research & Technology.

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.”

Rates of population growth and wastewater system expansion in cities not matched by infrastructure growth

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.

First author Dr Theodoros Giakoumis, also from the Centre for Environmental Policy at Imperial, commented:

“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.”

Click here to read ‘Combined sewer overflows: relating event duration monitoring data to wastewater systems' capacity in England’ by T. Giakoumis and N. Voulvoulis in full - published in Environmental Science: Water Research & Technology.