Optimization of an HRT-Fixed Plug-Flow Anaerobic-Oxic-Anoxic (AOA) Reactor: Changing the Specific Anaerobic/Oxic/Anoxic HRT Ratios

Anaerobic-Oxic-Anoxic (AOA) is a promising process that addresses the increasingly stringent requirements for advanced nitrogen (N) and phosphorus (P) removal in wastewater treatment plants (WWTPs). Plug-flow AOA systems have received much attention due to the similarity of their application scenarios to those of WWTPs; however, the understanding of the AOA process remains incomplete. In this study, a plug-flow AOA reactor was operated for 142 days under different A/O/A hydraulic retention time (HRT) ratios at a short HRT (13.3 h). Efficient nutrient removal performance was achieved at an A/O/A HRT ratio of 1:2:2, with total inorganic nitrogen (TIN), P, and chemical oxygen demand (COD) removal efficiencies of 78.3 ± 5.5%, 96.0 ± 3.7%, and 79.8 ± 4.9%, respectively. Predominant functional bacteria, including Candidatus_Competibacter (2.1%) and Defluviicoccus (8.0%), as typical glycogen accumulating organisms (GAOs) contributed to good endogenous denitrification (approximately 37% TIN removal). Additionally, the reasonable A/O/A HRT ratio ensured synergistic interactions among multiple functional bacteria, enabling the stable operation of the efficient and cost-effective AOA system.