Year-round evaluation of pilot-scale Anaerobic/Oxic/Anoxic system treating municipal wastewater under low temperature: performance and microbial response.

Although the Anaerobic/Oxic/Anoxic (AOA) process has been widely investigated, comprehensive research on its performance under low temperature remains limited. In this study, a pilot-scale AOA system with treating capacity of 84.2 m3/d was developed to treat municipal wastewater over 395 days. Despite fluctuations in water temperature (10.4-31.2℃) and influent BOD5/TN (1.1-7.7), the system achieved average effluent total nitrogen (TN) and total phosphorus (TP) concentrations of 4.9 mg/L and 0.3 mg/L without chemical addition. Notably, the anoxic zone contributed up to 74.9 % of TN removal. Extending anoxic hydraulic retention time to 11 h mitigated the adverse impacts of low temperature on endogenous denitrification rate. With intracellular carbon storage efficiency exceeding 78.7 %, carbon consumption was minimized through the integrated control of dissolved oxygen and residual NH4+-N. Consequently, the AOA system delivered effluent TN and TP of 6.5 mg/L and 0.2 mg/L under low-temperature (14.1 ± 1.6℃) and low BOD5/TN (3.6 ± 1.0) conditions. Microbial and metabolic analyses revealed adaptive response to seasonal cooling and carbon limitation, with stable glycogen-accumulating organisms (GAOs) and denitrifiers (0.9 % and 9.0 %), accompanied by increased abundances of genes involved in intracellular carbon and denitrification pathways. Compared with an Anaerobic/Anoxic/Oxic (AAO) system operated at wastewater treatment plant receiving identical influent, the AOA system yielded decreased effluent TN by 2.3 mg/L, external carbon expenditure by $46.4 per 10,000 m3 wastewater and aeration volume fraction by 21.9 %. This study provided insights into the practical application of the AOA process in regions characterized by low temperature and carbon limitation.