Evaluation of anaerobic/anoxic/oxic (A2/O) and reverse A2/O processes in biological nutrient removal

A performance comparison between the conventional anaerobic/anoxic/oxic (A2/O) process and the anoxic/anaerobic/oxic (reverse A2/O) process was conducted in lab-scale at the hydraulic retention time of 0.75 d and solids retention time of 10 d for more than 200 days. Both processes demonstrated excellent removal efficiency for organic matter (> 96%) and total nitrogen (> 85%), with no significant difference in microbial activities, sludge phosphorus (P) content, sludge settling property, and nitrifying community (dominated by Nitrosomonas and Nitrospira). However, there was significant difference in P removal, with the total P removal efficiencies in the reverse A2/O and A2/O systems of 76 ± 6% and 70 ± 6%, respectively. Placing an anoxic stage before the anaerobic stage in the reverse A2/O process resulted in a lower oxidation-reduction potential (ORP = -268 ± 45 mV) of the anaerobic zone, which contributed to higher P uptake by bacteria under subsequent aerobic respiration.