The occurrence, fate and removal of microplastics (MPs) in a wastewater treatment plant (WWTP) in Central Italy were investigated together with their potential adverse effects on anaerobic processes. In the influent of the WWTP, 3.6 MPs.L-1 were detected that mostly comprised polyester fibers and particles in the shape of films, ranging 0.1-0.5 mm and made of polyethylene and polypropylene (PP). The full-scale conventional activated sludge scheme removed 86% of MPs, with the main reduction in the primary and secondary settling. MPs particles bigger than 1 mm were not detected in the final effluent and some loss of polymers types were observed. In comparison, the pilot-scale upflow granular anaerobic sludge blanket (UASB) + anaerobic membrane bioreactor (AnMBR) configuration achieved 94% MPs removal with the abatement of 87% of fibers and 100% of particles. The results highlighted an accumulation phenomenon of MPs in the sludge and suggested the need to further investigate the effects of MPs on anaerobic processes. Accordingly, PP-MPs at concentrations from 5 PP-MPs.gTS-1 to 50 PP-MPs.gTS-1 were spiked in the pilot-scale UASB reactor that was fed with real municipal wastewater, where up to 58% decrease in methanogenic activity was observed at the exposure of 50 PP-MPs.gTS-1. To the best of our knowledge, the presented results will be the first to report of PP-MPs inhibition on anaerobic processes.

The authors would like to acknowledge Matthew Chistolini for his valuable contribution to the investigation of PP-MPs in the pilot-scale UASB operation. The authors also would like to thank the water utility Viva Servizi for their support throughout the study. This study was supported by the Polytechnic University of Marche - MICROWASTE Strategic Project 2017 and by the European Commission with the LIFE program - "LIFE BLUE LAKES" under the Grant Agreement No. LIFE18 GIE/IT/000813. Alessia Foglia kindly acknowledges the Fondazione Cariverona for funding her PhD scholarship.