Transfer and fate of microplastics during the conventional activated sludge process in one wastewater treatment plant of China

Municipal wastewater treatment plants (WWTP) are considered as a significant point source of microplastics (MPs) in the aquatic environment. The objective of this study was to investigate the transport and fate of MPs particles in one WWTP of China based on the conventional activated sludge process. The results exhibited that the abundance of MPs in wastewater declined sharply, from 79.9 n L−1 in the influent to 28.4 n L−1 in the effluent, with a removal rate of 64.4%. MPs removed were mostly transferred and stored into the sludge, and the abundance of MPs in dewatered sludge was 240.3 ± 31.4 n g−1 (dry sludge) with an average size of 222.6 μm. Larger size fraction of MPs in the effluent was reduced compared to that in the influent due to mechanical erosion and sedimentation into sludge. Fiber and fragment were main MPs particles in four wastewater sampling sites, with the average percentage ranged from 33.5 to 56.7% and 30.4 to 45.6%, respectively. An interesting finding is that the ellipses with the size ranged from 100 to 800 µm (average size of 348.1 µm), seldom reported before, were abundantly seen in the influent with a percentage of 4.4%, but not observed in the effluent. A higher fraction of microbead and foam in sludge (17.1% and 12.9%) indicates MPs with the smaller size (average size of 90.3 and 240.1 µm, respectively) in wastewater are prone to be adsorbed and transferred into sludge. Polyamide (nylon) was found to be the main plastic component in wastewater with 54.8% based on Raman spectra, indicating that the MPs particles are primarily originated from the wastewater discharged by washing clothes and polymer manufacturing and processing industries, followed by personal care products.