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Extreme Storms and Sewer Overflows
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When heavy rainstorms hit, sewer systems can overflow and dump untreated wastewater—including harmful bacteria, chemicals, and microplastics—directly into rivers and lakes. New research tracking over 1,500 sewer monitors across Scotland found that it's the intensity of rainfall (how hard it rains) rather than the total amount that causes these dangerous overflows. As climate change brings more intense storms, these toxic spills will likely happen more often, threatening both drinking water sources and recreational waters where people swim and fish.
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.
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This review examines how combined sewer overflows — when heavy rain causes sewers to release untreated waste directly into rivers — deliver microplastics, micropollutants, and pathogens into waterways, and evaluates management strategies for protecting water quality as climate change makes intense storms more frequent.
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This review of existing research found that when heavy rainstorms hit cities, they wash dangerous pollutants like germs, metals, and particles into waterways within just the first few minutes. These pollution spikes can make people sick through contaminated water, and green infrastructure like rain gardens helps reduce the problem but works less well during extreme storms or when not properly maintained. The findings suggest cities need better monitoring and maintenance systems to protect public health as climate change brings more intense rainfall.
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During heavy rainfall events in Paris, sewer sediments containing densely accumulated microplastics — up to 178,000 particles per kilogram of dry sediment — are resuspended and discharged directly into rivers via combined sewer overflows. This previously overlooked pathway means that stormwater overflow events can rapidly transfer large quantities of microplastics stored in urban sewer networks into freshwater ecosystems.
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