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Combined sewer overflow contribution to microplastic pollution in urban water bodies
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When it rains heavily in cities, sewer systems can overflow directly into rivers and streams, and this study found that these overflow events dump massive amounts of microplastics into urban waterways—especially right at the start of a storm, when pollution levels spiked up to 10 times higher than later in the event. Since these waterways can feed into drinking water sources or places where fish are caught for food, understanding this overlooked pollution pathway matters for tracking how microplastics make their way into the environment and, potentially, our bodies.
Combined Sewer Overflows (CSOs) have recently gained attention as episodic but significant sources of microplastics (MPs) to urban receiving water bodies; however, the occurrence, magnitude and characteristics of MP pollution in this context remains poorly investigated. Through an in-field monitoring campaign in the city of Pisa (Italy), this research aimed to assess the impact of CSOs to microplastic pollution in urban aquatic environment and to predict MP abundance due to hydraulic overflowing event. MP analysis was conducted using Laser Direct InfraRed Chemical Imaging System (LDIR), an innovative technology which enables reliable quantification and characterization of MPs in terms of polymer composition, size and shape, thus supporting source identification. MP abundance ranged between (3.71 ± 0.16) × 10 6 and (1.28 ± 0.12) × 10 8 items/m 3 , with values increasing with the overflow discharge. A positive correlation was observed between MP abundance and the discharged volume, highlighting the influence of precipitation-driver hydrodynamics. In the six monitored events, we observed a first flush effect, with MP concentrations reaching up to an order of magnitude higher than during subsequent stages of the event. Rubber, cellulosic materials and PMMA were identified as the predominant polymers, underlining the mixed sources related to urban runoff and wastewater inputs. Most of the detected MPs had size between 10 and 100 μm (77–93%), with fragment and pellet as predominant shapes (35–81% and 13–48%, respectively). The results highlighted the importance of including microplastics in CSO monitoring programs and provided insights on hydrological and hydraulic factors relevant to understand urban water pollution.
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Microplastic accumulation in sewer systems and their pathways to the environment along with combined sewer overflows
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This study examines how microplastics accumulate in urban sewer systems and the pathways through which they enter the environment, particularly through combined sewer overflows during storm events. Understanding these infrastructure pathways is critical for controlling microplastic release into rivers and coastal waters.
Microplastics in Combined Sewer Overflows: An Experimental Study
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Researchers investigated microplastic concentrations and characteristics in combined sewer overflow events through experimental study, finding that overflow discharges transport significant quantities of microplastic particles directly to surface waters, bypassing wastewater treatment and representing a major urban microplastic pathway.
Change in microplastic concentration during various temporal events downstream of a combined sewage overflow and in an urban stormwater creek
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Researchers examined how microplastic concentrations in urban waterways changed during rain events, snowmelt, and combined sewage overflow episodes, finding that stormwater runoff and sewer overflows substantially increase microplastic loads, with event type and intensity influencing concentration patterns.
Microplastic accumulation in sewer sediments and its potential entering the environment via combined sewer overflows: a study case in Paris
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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.
Microplastic accumulation in sewer sediments and its potential entering the environment via combined sewer overflows: a study case in Paris
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Researchers measured microplastic levels in sewer sediments in Paris and found extremely high concentrations, similar in composition to raw wastewater. During heavy rain events, these contaminated sediments get flushed into rivers through sewer overflows, releasing large amounts of microplastics into the environment. The study reveals that urban sewer systems are a significant hidden reservoir of microplastic pollution that can rapidly contaminate waterways used for recreation and drinking water.
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