Long-term effects of Fe3O4 NPs on the granule-based anaerobic ammonium oxidation process: Performance, sludge characteristics and microbial community

[Display omitted] •Long-term effects of Fe3O4 NPs on anammox UASB reactor were firstly investigated.•The anammox performance was not obviously influenced under exposure of 2−200 mg L−1 Fe3O4 NPs.•The sludge characteristics were increased with Fe3O4 NPs concentration elevated.•The Fe3O4 NPs could enhance the relative abundance of Candidatus Kuenenia. The performance of anaerobic ammonium oxidation (anammox) granules were studied under long-term exposure to Fe3O4 NPs. The Fe3O4 NPs had no negative impacts on nitrogen removal performance with the addition of 2−200 mg L−1. The specific anammox activity (SAA) slightly decreased from 287.0 ± 13.2 to –253.0 ± 9.2 mg TN g-1VSS d-1 with the increase in Fe3O4 NPs level from 2 to 60 mg L-1, and then significantly enhanced to 381.8 ± 15.7 mg TN g-1VSS d-1 at 200 mg L-1 Fe3O4 NPs. And the change trends of the heme c content, extracellular polymeric substance amount and settling velocity were consistent with that of SAA. The Candidatus_Kuenenia was the dominant species during the entire experiment and its relative abundance was up to 33.4 % at the end the experiment. The results provide some useful information for comprehending the impact of Fe3O4 NPs on the performance of wastewater biological treatment systems.