Textile dye biodecolourization and ammonium removal over nitrite in aerobic granular sludge sequencing batch reactors

[Display omitted] •Aerobic granules had compact and stable microbial structure at low DO≤2mgL−1.•Simultaneous removal of COD, ammonium and azo dye by aerobic granular sludge.•Ammonium was removed mainly through ammonium removal via nitrite pathway.•Azo dye biodecolourization by aerobic granules under microaerophilic conditions.•Azo dye decolourizing and nitrifying (Nitrospirae) bacteria were detected. Biodecolourization of azo dye and removal of ammonium by aerobic granular sludge (AGS) was investigated under different growth conditions. AGS not previously exposed to azo dye was able to effectively decolourize azo dye under anaerobic and microaerophilic conditions. Azo dye, total organic carbon and ammoniacal nitrogen removal efficiencies of 89–100%, 79–95% and 92–100%, respectively, were achieved in the AGS reactor operated for 80days under microaerophilic conditions. Removal of carbon, nitrogen and phosphorus was not impacted by azo dye loading. Azo dye, organic carbon and ammonium were majorly removed in the anoxic period wherein bulk dissolved oxygen was ranged from 0.5 and