Combined sewer overflows contribution to microplastics pollution: Long-term monitoring evidences in a context of climatic extremes

Microplastics (MPs) pollution has emerged as a major environmental concern due to its widespread presence in aquatic ecosystems. During intense rainfall, combined sewer overflow (CSO), which partially bypasses the wastewater treatment plant (WWTP), could be an important pathway for MPs to enter aquatic ecosystems. Therefore, this study aimed to evaluate the potential impact of CSO on the release of MPs into the environment during extreme rainfall. To address this, monitoring was conducted for more than 1 year at a municipal WWTP in southern Italy. The concentration of MPs in CSO samples was almost twice as high as that of raw wastewater samples collected during dry weather, and 65% higher than the concentration measured during rainy events without CSO activation. Additionally, CSO discharges contained, on average, a concentration of MPs 12 times higher than the treated effluent of the WWTP. This study highlighted that CSO discharges play a major role in increasing the release of MPs into the environment, especially as extreme weather events driven by climate change are expected to increase the frequency of CSO events.