Distribution characteristics of microplastics in wastewater treatment plants in mega cities–the case study of Chengdu City

With the increasingly serious problem of microplastic pollution, wastewater treatment plants (WWTPs) have attracted much attention as a key node for intercepting microplastics into the natural environment. In this study, the distribution characteristics of microplastics and their removal efficacy in wastewater treatment processes were systematically investigated in four typical wastewater treatment plants in Chengdu. The abundance, particle size, morphology and composition of microplastics in the influent and sludge were quantitatively analyzed by improving the sampling device and combining body-view microscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that the microplastic concentration in the influent water showed significant spatial heterogeneity, with microplastic concentrations ranging from 177 to 517 particles/L. The Chengdu Cooperative Plant, which is located in the central business district, had the highest percentage of fibers at 38.5%, while the PET fibers of the Hedong Plant, which is located in the textile industrial area of Jianyang City, had a percentage of 15%. In this study, the removal rate is defined as the difference in microplastic concentrations between influent and effluent divided by the influent concentration.The removal rate of > 1 mm microplastics in the primary treatment reached 85%~92%, but the removal rate of < 0.2 mm ultrafine particles was only 37%~45%, and the percentage of < 0.1 mm particles in the secondary treatment increased inversely, revealing the fragmentation effect caused by hydraulic shear. Microplastics in sludge were dominated by PP and PE, accounting for 76%, and the percentage of small particles (< 0.2 mm) in secondary sludge was significantly higher, with a maximum of 58%. The study showed that the overall removal rate of Chengdu wastewater treatment plant reached 89.3%~94.6%, but the residual small particle size and high density microplastics (e.g., PET fibers) in the effluent still constituted a potential ecological risk. It is recommended to optimize the process for industrial source characteristics and to strengthen the control of sludge microplastics. This study provides a scientific basis for microplastics control and process upgrading in mega-city wastewater treatment plants.