Smaller sizes of polyethylene terephthalate microplastics mainly stimulate heterotrophic N2O production in aerobic granular sludge systems

Abstract

Widespread polyethylene terephthalate microplastics (PET MPs) have played unintended role in nitrous oxide (N₂O) turnovers (i.e., production and consumption) at wastewater treatment plants (WWTPs). Mainstream aerobic granular sludge (AGS) systems possess potentially strong N₂O-sink capability, which may be reduced by PET MPs stress through altering N₂O-contributing pathways, electron transfer, and microbial community structures. In this study, the effects of PET MPs with two common particle sizes of effluent from WWTPs (0.1 and 0.5 mm) on N₂O turnovers, production pathways and N₂O-sink capability were systematically disclosed in AGS systems by a series of biochemical tests and molecular biological means to achieve the goal of carbon neutrality. The results indicated that 0.1 mm PET MPs could more significantly stimulate N₂O production in AGS systems by inhibiting denitrifying metabolism, compared with control and 0.5 mm PET MPs systems. Specifically, 0.1 mm PET MPs slightly increased the relative abundance of Nitrosomonas, reducing N₂O yields via promoting the hydroxylamine (NH₂OH) oxidation pathway during nitrification. Also, 0.1 mm PET MPs inhibited the electron transport system activities and the relative abundance of N₂O reductase, hindering N₂O reduction during denitrification. Most importantly, 0.1 mm PET MPs more apparently reduced the N₂O-sink capability based on the ratio of N₂O reductase gene and nitrite reductase gene.