The increasing use of engineered nanoparticles (NPs) poses an emerging challenge to biological wastewater treatment. The long-term impact of CuNPs on anaerobic ammonium oxidation (anammox) process was firstly investigated in this study. The nitrogen removal capacity of anammox reactor was nearly deprived within 30 days under the stress of 5.0 mg L-1 CuNPs and the relative abundance of anammox bacteria (Ca. Kuenenia) was decreased from 29.59% to 17.53%. Meanwhile, copper resistance genes associated with the Cus, Cop and Pco systems were enriched to eliminate excess intracellular copper. After the withdrawal of CuNPs from the influent, the nitrogen removal capacity of anammox biomass recovered completely within 70 days. Overall, anammox biomass showed susceptibility, resistance and resilience to the stress of CuNPs. Therefore, the potential impacts of ENPs on anammox-based processes should be of great concern. (C) 2017 Elsevier Ltd. All rights reserved.