AbstractIn this study, a sequential fed-batch anaerobic–anoxic–aerobic (A2/O) moving bed biofilm reactor (MBBR) (A2/O MBBR) system will be employed for the simultaneous treatment of highly concentrated phenol, ammonia (NH4), and oxyanions [e.g., nitrate (NO3) and sulfate (SO42-)] from wastewater. The A2/O system efficiently removed phenol (99%) and NH4–nitrogen [NH4–N (95%)] with a 99% reduction in the chemical oxygen demand (COD) of the effluent for 3,000 and 200 mg/L of the maximum initial feed concentration, respectively with a 10-day hydraulic retention time (HRT). However, high removal of SO42- was achieved under anaerobic conditions (96%) for 100 mg/L of the initial feed concentration. The increasing feed phenol concentration (1,500–3,000 mg/L) inhibited NO3 reduction by releasing significant levels of NO3 in the anoxic reactor effluent and increased the SO42- accumulation in the anoxic and oxic reactors. Heterotrophic denitrification was reduced in the anoxic reactor by 15%–17% and increased the phenol concentration from 2,000 to 3,000 mg/L due to the highly toxic effect on the denitrifying microbes. NH4 removal by the anaerobic (R1) and anoxic (R2) reactors reduced from 87% to 65% and 66% to 46%, respectively, and increased the phenol concentration from 1,500 to 3,000 mg/L but the oxic reactor showed stable performance. Scanning electron micrographs (SEM) of the carrier media showed the better adherence of well-grown round and rod-shaped mixed microbes in different environments. Bacillus licheniformis was identified as the dominant species in the anoxic and oxic reactors by the VITEK 2 COMPACT system. Similarly, Kocuria kristinae was the predominant microbe identified in the anaerobic reactor. The outcome of this study revealed that an A2/O MBBR system that had an established indigenous mixed microbial culture could be a feasible technique to treat highly concentrated phenol combined with NH4 and oxyanions that are present in wastewater.