A combined system consisting of an upflow anaerobic sludge blanket(UASB)and anoxic/oxic bioreactor was adopted for treating mature municipal landfill leachate rich in ammonia nitrogen(NH+4N).On the premise of the COD and nitrogen being removed simultaneously，the sludge distribution and DO concentration in UASB and A/O

denitrification and methanogenic activity in UASB

nitrite accumulation ratio variation in A/O

organic matter and nitrogen transition behavior in the combined system were analyzed at different recycling ratios（100%—600%).The results with the recycling ratio maintained at an appropriate level indicated that the sludge distribution characteristics and DO concentration in respective reactors were in a relatively better state.Gas production analysis showed that denitrifying bacteria and methanogen activity in UASB were maintained at homeostasis balance.Under the premise of ensuring the efficiency of nitrification

the selective inhibition of FA on ammonia oxidizing bacteria and nitrite oxidizing bacteria realizing control on nitrification rate to achieve a high rate of nitrite accumulation in A/O. According to the characteristics of liquid water and considering energy consumption

an optimal recycling ratio of 300% was suggested to achieve optimum system performance with the removal efficiencies of NH+4N

TN

COD reaching 99.3%，85.9% and 90.6% respectively and NO-2N accumulation ratio reaching 94.4%.