Extreme Storms and Sewer Overflows

There is growing concern about the impact of wastewater discharges from combined sewer overflows (CSOs) on the quality of surface waters and their vulnerability to a wetter and stormier climate. The UK is already experiencing increased wet weather, and will continue to have more intense and frequent storms with climate change. Stronger and more frequent storms will increase pressure on sewer networks, likely compromising treatment capabilities and increasing discharges via CSOs. Consequently, reduced wastewater treatment means more faecal pollution and further degradation of surface water quality through increased loads of solids, faecal microbes, phosphorus, organics, microplastics, and pharmaceuticals.

Using a 12-month dataset from >1500 overflow monitors across Scottish Water’s Wastewater Intelligence Network (WWIN), the authors will present a novel analysis of real-time monitoring of CSO discharges at the nationwide level. Preliminary analysis involving the WWIN monitoring data and daily regional rainfall intensities show how high intensity storm events were the main driver of significant CSO discharges across Scotland in 2025. This research reveals for the first time that monitoring data has been analysed at a national scale to evaluate the impacts of weather on network performance.  The results demonstrate that the intensity of rainfall, rather than total rainfall volume, is the main driver in causing significant CSO discharge events. This presentation will showcase how real-time CSO monitoring can improve climate-informed decisions in prioritising and evaluating asset performance at a catchment level to ensure maximum return on investment to relation protecting public and environmental health.