Long-term operation of an MBR in the presence of zinc oxide nanoparticles reveals no significant adverse effects on its performance

The role of environmental nanoparticle pollutants (ENPs) in wastewater biological treatment has attracted considerable attention in recent years, while the long-term impacts of ENPs on membrane bioreactor (MBR) performance remain unclear. In this study, zinc oxide (ZnO) nanoparticles were continuously introduced into an MBR system to evaluate their long-term effects. In general, the results showed that ZnO nanoparticles had no obvious adverse effects on the treated water quality and membrane filtration performance. However, subtle changes in membrane fouling behaviors and microbial communities were found between the MBR with nanoparticles (MBR-NPs) and the control MBR. The presence of ZnO nanoparticles could aggravate the membrane pore blocking but alleviate the external fouling (gel layer) in the MBR-NPs. In addition, 454 high-throughput pyrosequencing indicated that Gemmatimonadaceae, Chlorobiales and Nitrospiraceae were the most resistant bacteria genera, while Bacillus, Rhodoplanes and Xanthomonadaceae were the most sensitive groups of which the relative abundances were impaired in MBR-NPs. The results demonstrated that the microbial community could delicately respond to the presence of nanoparticles. Moreover, specific comparison down to the genus level indicated the relative abundance of Nitrospiraceae_Nitrospira affiliated to nitrite oxidizing bacteria was enhanced while the relative abundances of Nitrosomonadaceae (ammonium oxidizing bacteria) and denitrifying bacteria were not evidently changed. It was also found that the relative abundance of Gemmatimonadaceae in MBR-NPs was increased, which could result in the enhanced formation of carbonate minerals on the membrane surface.