Abstract: Based on the shortcut nitrification-denitrification technology， the AO4 micro-oxygen aeration bioreactor was adopted to treat the landfill leachate. The denitrification performance of the bioreactor was investigated， and the denitrification principle of the bioreactor was further explored by qPCR and Illumina MiSeq sequencing. The results showed that when the DO concentration of the micro-oxygen aeration unit was controlled in the range of 0~0.60 mg/L， the nitrification rate could reach more than 90%. The average removal rates of CODCr， [ＮＨ+4]-N and TN were 85%， 99% and 87%， respectively. The research on the mechanism of microbiology suggested that the abundance of functional genes of each unit in AO4 micro-oxygen aeration system was significant while the microbial community structure was little. The abundance of nirS functional genes in each unit was absolutely dominant and counted more than any other functional genes. In the nitrification process， the AOB functional genes of ammonia oxidizing bacteria were dominant， while the nitrite oxidation process dominated by NOB functional genes was inhibited to a certain extent. The relative abundance of Bacteroidetes was lower than that of the conventional aerobic segment reported in similar reports， which indicated that although the micro-oxygen aeration system had high nitrogen removal efficiency， but it had poor metabolic ability of organic matter. Ten denitrifying bacteria genera were detected in the study， but Thauera bacteria were mainly used to achieve denitrifying nitrogen removal in the system due to the high salinity in landfill leachate.

Key words: landfill leachate, micro-oxygen aeration, shortcut nitrification-denitrification, simultaneous nitrification and denitrification