Abundance, characteristics and seasonal variation of microplastics in a domestic sewage treatment plant in Nanjing, China

Sewage treatment plants are a potential route of microplastics (MPs) into the environment, hence it's urgent to investigate MPs in wastewater and sludge through lengthy sampling campaigns. In this study, a sewage treatment plant in Nanjing, China was chosen, seasonal variation of MPs in influent was analyzed by collecting samples during a 12-month sampling campaign (at monthly intervals over a one-year period). Abundance and characteristic of MPs from different treatment stages were also investigated. The results showed that there was a seasonal variation in the amount of MPs in influent and the abundance of MPs was significantly greater in summer than other seasons. The average MPs abundance was 41.0 items/L and 3.0 items/L in influent and effluent, with a total 92.6 % removal rate. Overall, 125-355 mu m were the main size in each treatment section, with fibers dominant shape and transparent main color. Additionally, the primary polymer compositions of MPs were polyethylene (PE) and polypropylene (PP) using micro-Roman spectroscopy. The daily MPs emission from effluent and sludge was estimated to be 300 million and 1274 million items/d, demonstrating that the sewage treatment plant is an important source of MPs into the environment despite the high removal rate of MPs.