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Abstract: 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 (25–1500 mg NH4+‐N/L). Further, the third compartment of ABR was supplemented with nitrate under varying loading rates (60–300 mgNO3‐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. RESULTS: High sulfate reduction efficiencies (over 86%) were observed up to 1000 mg L‐1 NH4+‐N, however, 1500 mg L‐1 NH4+‐N caused decrease in reduction efficiency. Optimum nitrate loading rate was determined as 146 mgNO3‐N L‐1 d‐1 at molar N/S ratio of 0.42, corresponding with 100% nitrate, 83% sulfate, and 79% COD removals. Nitrate removal decreased at higher nitrate loadings and high sulfate generation arising from sulfide oxidation was observed. CONCLUSION: Sulfate and COD removals together with efficient sulfide‐based autotrophic denitrification succeeded in a single reactor configuration at S/N ratio of 0.42. The developed process has the potential to be used in lab‐scale applications. © 2017 Society of Chemical Industry [ABSTRACT FROM AUTHOR]