Combined sewer overflows: relating event duration monitoring data to wastewater systems' capacity in England

Abstract

Water pollution caused by the frequent use of combined sewer overflows (CSOs) has been attracting increased media and political coverage in England as in other places in the world. Considering that each of the country's 14 346 CSOs has been assessed for their environmental risk potential, as defined by the Environment Agency, and they have each been permitted to act as a storm overflow is indicative of a more systemic problem than currently perceived. While looking at the duration and frequency of discharges from individual CSOs not much can be said about their causes nor about what needs to be done to reduce them, here through an extensive investigation of event duration monitoring (EDM) data for 2021 and 2020, CSO spills are shown to be an issue across all sewerage companies related to how they operate their systems. By analysing EDM data considering the type and location of CSOs, and the sewerage networks they are connected to, 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. Other than pumping stations, 82% of the CSOs with the maximum spill duration per system were located at storm tanks and inlets of treatment works and had on average significantly higher spill durations in systems with insufficient hydraulic capacity both in 2020 and 2021, suggesting that CSOs are used to protect the works under peak dry weather flow conditions. Such frequent, and in some cases independent of rainfall, use of CSOs, could have detrimental effects for the receiving environment, as well as put thousands of water users at risk.