BACKGROUND: High SO42-, COD, and nitrogen-containing wastewater treatment requires different bacterial groups and environmental conditions, which can be achieved by sequential separate processes. This study examined the simultaneous removal of SO42-, COD and nitrogen in a four-compartment ABR, which allowed enrichment of different group of bacteria in each compartment. Feed COD and SO42- concentrations were kept constant at 1200 and 1500 mg L- (1), corresponding to COD/SO4-2 ratio of 0.8. Initially, ABR was tested at varying influent ammonium concentrations (25-1500mg NH4+-N/L). Further, the third compartment of ABR was supplemented with nitrate under varying loading rates (60-300 mgNO(3)-N L-1 d(-1)) for autotrophic denitrification with the sulfide produced in the previous compartments. This kind of application mimicked the internal recirculation of nitrate, which can be generated via nitrification of ABR effluent.