Distribution and Removal Efficiency of Microplastics in Urban Wastewater Treatment Plants

Urban wastewater treatment plants (WWTPs) act as the final barrier for municipal sewage, effectively eliminating conventional pollutants. However, they currently lack dedicated units designed for the removal of emerging contaminants such as microplastics (MPs), and the removal efficiency of MPs in WWTPs remains insufficiently studied. In this study, wastewater samples were collected from 12 treatment stages of a large municipal WWTP and analyzed using high-throughput laser direct infrared (LDIR) spectroscopy to identify the composition, size, and shape of MPs. MPs were detected in 12 tanks ranging from 3540 to 54,600 items/L, and an 83.0% reduction in MP abundance from raw influent to final effluent, indicating considerable effectiveness of the existing treatment processes in removing MPs. The two clarifier tanks were identified as the stages contributing most significantly to MP reduction, suggesting that gravity sedimentation serves as the primary mechanism for MP removal. This conclusion is further supported by the notably higher MP abundance observed in the pre-acidification tank, which results from the sludge recovery process. Additionally, the anaerobic/oxic tank and aeration tank, which represent the core treatment processes in this WWTP, also demonstrated effective MP removal. The treatment processes demonstrated higher removal efficiency for large-sized, high-density, and fibrous MPs, compared to small-sized, low-density, and granular MPs. These findings highlight the nuanced removal efficiencies across different MP characteristics, providing crucial insights for optimizing existing infrastructure, informing the design of future targeted removal strategies, and reducing the environmental risks of MPs discharge.