Long-term effects of Fe₃O₄ NPs on the granule-based anaerobic ammonium oxidation process: Performance, sludge...

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.