Pollution levels and characteristics of microplastics in influent of two wastewater treatment plants in Hangzhou

Background Urban sewage may operate as a route for microplastics entering the water environment. Studying the characteristics of microplastic pollutants can help to understand the possibility of human activities affecting the production of microplastics.

Objective This study investigates the levels and characteristics of microplastic pollutants in the influent water of two wastewater treatment plants (WWTPs) in Hangzhou.

Methods In four quarters of 2020, water samples were collected from the influent of selected two WWTPs once each quarter. Microplastics were extracted using hydrogen peroxide to digest impurities in the samples at a constant temperature of 60℃, and then stainless-steel sieves to remove large particles. The extracted microplastics were identified under a stereo microscope and a Fourier transform infrared spectroscopy microscope, and the samples were counted and classified under a stereo microscope.

Results The average number concentrations of microplastics in the influent of the two WWTPs were (8.20±0.28) and (11.01±0.59) particles·L^-1 respectively. No difference was found among the four quarters in the two WWTPs (P > 0.05). The results of size fractionation of microplastics showed that 0.5-1 mm particles accounted for the largest proportion, and the proportions were 57.5% and 54.0% respectively in WWTP A and B; the second place was > 1 mm, and the proportions were 25.0% and 28.0% respectively; and microplastics with a size of 0.098-0.2 mm accounted for the least proportion, which were 4.5% and 6.0% respectively. Fibers were the main form in the collected samples, followed by fragments. The average proportions of fibers in the two WWTPs were 47.5% and 49.0%, and the average proportions of fragments in the two WWTPs were 30.5% and 29.0%, respectively. Black was the dominant color. Black microplastics accounted for 57.0% and 54.0% of total microplastics, respectively. The digestion and fractional extraction methods adopted in the study showed a good dose-effect relationship, and the scatter fitting curve was ŷ=211x+3.4083, R²=0.995 2 (P < 0.001).

Conclusion Fibers produced in daily life activities are the main form of microplastics in the sewage from WWTPs. Seasons are not found to have an association with the characteristics of microplastics in sewage. The digestion and fractional extraction methods adopted in the study are feasible.