The long-term addition of N2H4 to completely autotrophic nitrogen removal over nitrite (CANON) sequencing batch reactors (SBRs) recovered and enhanced their autotrophic nitrogen removal capacity while simultaneously reducing their production of NO3-. The total nitrogen (TN) removal rate and TN removal efficiency of the process increased from 0.202 +/- 0.011 to 0.370 +/- 0.016 kg N/m(3)/d and from 65.1 +/- 3.75% to 77.4 +/- 3.8%, respectively, and the molar ratio of NO3- production to NH4+ removal (MRNN) decreased to 0.058. The most effective concentration of N2H4 addition was approximately 3.99 mg/L. N2H4 could increase the specific growth rate of anaerobic ammonium-oxidizing bacteria (AnAOB) and inhibit aerobic ammonia oxidation. The electrons released from the oxidation of additional N2H4 using hydrazine dehydrogenase (HDH), which substituted the electrons from NO2- oxidation to NO3- replenished the consumption of AnAOB anabolism and significantly reduced the consequent NO3- production. (c) 2013 Elsevier Ltd. All rights reserved.