the feasibility of removing nitrogen from mature landfill leachate using the nitrite route was investigated in a two-stage sbr system - (1o first stage, short-cut nitrification; second stage, denitrification via nitrite. with a volumetric exchange rate (n) of 0.3 d-1 and an oxygen concentration of between 1.2 and 1.3 mg/l, the nitrite accumulation ratio - (no2-n)×100/(nox-n)% - was between 32% and 37%. for the same hydraulic retention time, but lower oxygen concentration - between 0.8 and 0.9 mg/l - nitrite accumulation averaged 96%, indicating that ammonia was removed completely via the nitrite pathway. in the second step, an external carbon source was added to promote denitrification. complete removal of nitrite was obtained for a carbon dose of 2.4 mg cod/mg no2-n using acetic acid and 3.8 mg cod/mg no2-n for butyric acid. also, a higher denitrification rate - 14.6 mg no2-n/g vss？h - was observed with butyric acid as compared with acetic acid - 9.12 mg no2-n/g vss？h.