Sewage overflows are the result of a lack of investment in infrastructure, studies show

The recent surge in sewage overflows is due to infrastructure not keeping up with demand, according to a study by Imperial College London.

The conclusion suggests that other proposals to deal with the problem – such as preventing blockages and separating rain and dirty water – will not be enough to solve the problem of polluting sewage overflows.

The UK sewerage system combines domestic, commercial and industrial wastewater and stormwater runoff and directs it to wastewater treatment plants (WWTWs). However, in times of extreme flow, such as very heavy rainfall, combined sewer overflows (CSOs) can be used to discharge some of the wastewater into natural watercourses before reaching the WWTW.

There are 14,346 CSOs in England and although they 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. Discharge of raw sewage from CSOs directly into rivers and the sea can pollute water, especially when not used during heavy rainfall, which could dilute the sewage.

These excessive spills can lead to environmental degradation and pose a danger to human health, for example through enteroviruses that cause intestinal infections or the spread of antibiotic resistance. This has an additional effect on tourism and recreational activities such as swimming and boating, as well as the consumption of seafood, which can accumulate toxins and microplastics.

In response to the increased use of CSOs, water utilities were ordered to publish data on the frequency and duration of overflow events for each CSO for the past two years. However, this raw data does not explain the reason for the increase in their use. Without understanding the causes of these spills, the problem cannot be addressed with appropriate solutions.

In the first analysis of this data, Imperial researchers show that most of the increase is due to the lack of capacity of WWTWs to handle increased flows as the population has increased and industrial activities have increased.

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

This refutes assumptions about causes of increased CSO use that blame unpredictable weather, with changes in rainfall causing increased runoff in wet weather. The results are published in the journal Environmental Science: Water Research and Technology.

Lead researcher Professor Nick Voulvoulis, of the Environmental Policy Center at Imperial, said: “Our findings reveal the chronic undercapacity of England’s wastewater systems as a major cause of 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.”

Between 2000 and 2008, just over 1% of sewerage systems in England and Wales were replaced or rehabilitated. Given that much of the infrastructure is built with a lifespan of 60-80 years, at this 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 Center for Environmental Policy at Imperial, said: “Most proposals to tackle the problem of increased sewage overflows propose ways to reduce the impact of CSO discharges rather than targeting the causes of them.”

“The elephant in the room is that most cities have experienced population growth and wastewater system expansion at a rate that has not been matched by the growth of water infrastructure. It shows the need for an open discussion about the sustainability of water infrastructure and investment, issues that perhaps most importantly I believe have already been addressed in high-income countries like the UK.”