Adding exogenous protein relieves the toxicity of nanoparticles to anammox granular sludge by adsorption and the formation of eco-coronasElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d1en01138c

MgO nanoparticles (NPs) are widely used in bactericides, fuel cells and electronic materials and are frequently detected in aquatic environments. Anaerobic ammonium oxidation (anammox) is a highly efficient and economical autotrophic nitrogen removal process. To date, the effects of MgO NPs on anammox granular sludge have remained unknown. This work systematically evaluated the short- and long-term effects of MgO NPs on anammox process performance and explored an effective strategy to relieve stress. The half maximal inhibitory concentration (IC50) of MgO NPs to anammox granular sludge was 69.5 mg L−1. During the long-term experiment, the anammox process experienced a reduction in nitrogen removal ability under the stress of 50.0 mg L−1MgO NPs. Moreover, the enzymatic activity and synthesis of heme c significantly decreased. Adding 40.0 mg L−1bull serum albumin (BSA) effectively relieved the stress caused by 50.0 mg L−1MgO NPs, and the anammox activity recovered to 97.0% of the original level. Multiple spectral analyses verified the adsorption of MgO NPs by BSA. Furthermore, the formation of an eco-corona was observed by transmission electron microscopy, which also contributed to decreasing the biotoxicity of the MgO NPs. The results of this study provide guidance for treating NP-containing wastewater during the anammox process.