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A combined process of a two-stage up-flow anaerobic sludge blanket (UASB), an anoxic/aerobic (A/O) reactor and a sequencing batch reactor (SBR) was used to treat mature landfill leachate, and the influencing factors on short-cut nitrification in A/O reactor were also researched. The operational mode of the process was as follows: Denitrification took place in the first stage UASB (UASB1). The residual nitrite and nitrate of UASB1 by using the residual chemical oxygen demand (COD) was further removed in the second stage UASB (UASB2). The ammonia nitrogen was removed by short-cut nitrification in the aerobic zone of A/O reactor. The nitrite and nitrate produced from the nitrification were removed in SBR. The experiment results show that the COD and ammonia nitrogen mass concentrations of raw leachate are about 3.0 g/L and 2.0 g/L, respectively. The influent leachate is mixed by raw leachate as well as domestic wastewater in a ratio of 1. The external carbon source (sodium acetate anhydrous) is added in order to enhance the ρ(C)/ρ(N) ratio from 1.7 to 3.0. Through the cooperative inhibition of the free ammonia (FA) and free nitrite acid (FNA), the short-cut nitrification is achieved stably in the A/O reactor with more than 70% of the nitrite accumulation ratio. The nitrite and nitrate produced from the nitrification could be removed completely in SBR. The ammonia mass concentration of final effluent is less than 2 mg/L, and its removal efficiency is 99%. The total nitrogen mass concentration of final effluent is about 26 mg/L, and its removal efficiency is about 98%.
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