In this study COD, ammonia and nitrate were treated through methanogenesis, nitrification denitrification and anammox processes in anaerobic-aerobic and anaerobic/anoxic sequential in leachate samples produced from municipal solid waste in an anaerobic simulated landfilling bioreactor. The experiments were performed in an upflow anaerobic sludge blanket reactor (UASB), aerobic completely stirred tank reactor (CSTR) and upflow anaerobic/anoxic sludge blanket reactor (UA/A(N)SB). Hydraulic retention times in anaerobic, aerobic and anaerobic/anoxic stages were 1, 3.6 and 1 days, respectively, through 244 days of total operation period with 168 days of adaptation period of microorganisms to the reactors. The organic loading rates increased from 5.9 to 50 kg COD m(-3) day(-1). The total COD and TN removal efficiencies of the anaerobic-aerobic-anoxic system were 96% and 99%, respectively, at an influent OLR as high as 50 kg COD m(-3) day(-1). The maximum methane percentage in the UASB reactor was 82% while the methane percentage was zero in UA/A(N)SB reactor for the aforementioned OLR at the end of steady-state conditions. NH4-N removal efficiency of the aerobic reactor was 90% while anaerobic ammonia oxidation was measured as 99% in the anoxic reactor. The denitrification efficiency was 99% in the same reactor. Total TN removal of the whole system was 99%.