BACKGROUNDN(2)H(4) and NO are the important intermediates of Anammox. The appropriate addition of N2H4 weakens the inhibitory effect of NO2- on anaerobic ammonium oxidation bacteria (AnAOB) activity. The effect of long-term addition of micro-amounts of N2H4 was investigated in this study. RESULTSA sequencing batch reactor (SBR) performing Anammox could run stably during long-term addition of micro-amounts of N2H4. The nitrogen removal rate and efficiency of the SBR with micro-amounts of N2H4 addition were higher than the SBR without N2H4 addition. The molar ratio of NO3- production to NH4+ removal (MRNN) decreased to 0.12, significantly lower than the expected stoichiometry of Anammox. The NO emission decreased by more than 15%. The addition of micro-amounts of N2H4 could weaken the inhibition of NO2- to the activity of AnAOB. CONCLUSIONThe electrons released from the oxidation of added N2H4 by AnAOB acclimated under long-term micro-amounts of N2H4 addition replenish the loss of electrons due to the diffusive loss of intermediates in AnAOB metabolism, enhance the activity of AnAOB and accelerate the NO consumption with resulting decreased NO emission. The oxidation of added N2H4 partially substitutes the oxidation of NO2- to NO3- for the anabolism of AnAOB with resulting significantly decreased production of NO3-.(c) 2014 Society of Chemical Industry