Occurrence, characteristics and removal of microplastics in wastewater treatment plants with different treatment processes

The abundance of microplastics (MPs) in wastewater from three wastewater treatment plants (WWTPs) were determined in Hangzhou, Zhejiang Province, China. The MPs abundance was 140–350 particles per litre in the influent and 10–30 particles per litre in the effluent. Four shapes of MPs in the influent were observed, while mainly only debris was left in the effluent. The percentage of small (≤100 μm), medium (100–500 μm), and large-sized (≥500 μm) plastics in the raw leachate of the three WWTPs were 54.3%, 8.6%, and 37.1%, 28.6%, 64.3%, and 7.1%, and 41.4%, 24.1%, and 34.5%, respectively. Mainly only the size of ≤100 μm was left in the effluent of all. The removal efficiencies of MPs in a range of 78.6% to 96.6% were achieved. Polypropylene, polystyrene, polyethylene, polyethylene terephthalate and polyvinyl chloride were the main types and detected in all wastewater samples, accounting for over 75% of all types. The plastic components contained in different industrial wastewater were more complex. The distribution of MPs was significantly positively correlated with most conventional indicators such as chemical oxygen demead, ammonia nitrogen, and total phosphorus, but not with heavy metals. Similar wastewater, different treatment processes, or similar processes but different wastewater (industrial wastewater proportion varied) could all lead to differences in MPs removal. The MPs abundance measured in this experiment was similar to some previous studies, but relatively high. The three WWTPs can discharge up to 6.0 × 10⁸–1.8 × 10⁹ plastics of MPs per day, which poses potential ecological risks. This study indicates that the source control of MPs and optimizing the process design of existing WWTPs are crucial for preventing and controlling MPs pollution.