The long-term effects of different influent Cr(VI) concentrations (0-0.5 mg L −1 ) on the nitrification activities and microbial community structures of nitrifying activated sludge system were investigated in this study. Results showed that the performance of ammonia oxidation was significantly inhibited, and the effluent concentration of ammonia nitrogen (NH 4 + -N) increased markedly when the influent Cr(VI) loading was equal or greater than 0.2 mg L −1 . The specific oxygen utilization rate

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... SOUR), specific ammonium oxidation rate (SAOR), and specific nitrite oxidation rate (SNOR) of the system decreased from 53.24, 6.31, and 7.33 mg N g −1 VSS h −1 to 18.17, 1.68, and 2.88 mg N g −1 VSS h −1 , respectively, with an increase of Cr(VI) concentration from 0 to 0.5 mg L −1 . The protein/polysaccharide (PN/PS) ratio increased with the increasing Cr(VI) concentration, indicating that excessive PN secreted by microorganisms was conducive to resisting the toxicity of Cr(VI). High-throughput sequencing revealed that the relative abundance of ammonia-oxidizing bacteria (Nitrospira) and nitrite-oxidizing bacteria (Nitrosomonas and Nitrosospira) all decreased with the increasing Cr(VI) concentration, and ammonia-oxidizing bacteria were more sensitive to heavy metal toxicity than nitrite-oxidizing bacteria. The activities of nitrifying activated sludge system could not be completely recovered after a 30-d recovery process.