Are combined sewer overflows from wastewater treatment plants a critical source of microplastics discharge into water bodies?

Microplastic (MPs) pollution has emerged as a significant environmental concern due to its widespread presence in aquatic ecosystems (Yang et al., 2022). Wastewater treatment plants (WWTPs) could play a crucial role in mitigating microplastic contamination, with removal efficiencies ranging from 60% and 99.9% (Krishnan et al., 2023). However, substantial amounts of MPs can still enter the environment, especially during combined sewer overflow (CSO) events. CSOs occur when intense rainfall exceeds the capacity of WWTPs, leading to the direct discharge of untreated wastewater into the environment. Precipitation events can significantly influence the concentration and composition of microplastics in wastewater, as stormwater runoff mobilizes plastic debris from urban surfaces while resuspension processes within the sewer network further contribute to their abundance. Nevertheless, few studies are available about this topic. Understanding the impact of CSO events on microplastic contamination is essential for assessing the environmental risks associated with untreated stormwater discharges. For this purpose, this study investigated the variations in MPs concentrations under different weather conditions, highlighting the role of CSOs in amplifying their presence in wastewater discharges.