A high-rate A2O bioreactor with airlift-driven circulation and anoxic hybrid growth for enhanced carbon and nutrient removal from a nutrient rich wastewater

Within this research, a one-stage hybrid dual internal circulation airlift A2O (DCAL-A2O) bioreactor was designed and operated to simultaneously remove carbon, nitrogen and phosphorous (CNP) from milk processing wastewater (MPW) in different operational circumstances. The substantial operating variables monitored in this work were including hydraulic retention time (HRT), airflow rate (AFR) and aeration volume ratio (AVR) ranged from 7 to 15 h, 1–3 L/min and 0.324–0.464, respectively. From the view point of economics and process function, the optimum conditions were obtained at the HRT, AFR and AVR of 10 h, 2 L/min and 0.464, respectively. At the optimum conditions TCOD, TN, TP removal efficiencies and effluent turbidity were reported to be 97 %, 90 %, 92 % and 9 NTU, respectively. The impact of wastewater biodegradability (BOD5/COD) was evaluated on the bioreactor performance using two other wastewaters i.e. soft drink (SDW) and soybean oil plant wastewaters (SOW) in comparison with the MPW. Removal efficiencies for TCOD and TN exceeding 80 % were observed. The feeding location revealed a prominent impact on the TN and phosphorous removal efficiencies (both ≥80 %) related to the availability degree of the readily biodegradable organic substrate to denitrifiers and PAOs. The rise in HRT, AFR and AVR resulted in reducing microbial secretions as SMP present in sludge and bioreactor effluent as well as loosely bounded EPS (LB-EPS), reported to be 26, 28, 32.5 and 194.4 mg/L TOC, respectively. Different bacteria species were present at optimum conditions confirming concurrent CNP removal in a single body. Finally, the operating cost evaluation verified the effectiveness of the hybrid airlift A2O treating the MPW.