Effect of anaerobic duration on biological phosphorus removal in reversed AAO process (anoxic-anaerobic-oxic)

In this study, the effect of anaerobic duration on phosphorus (P) removal in reversed AAO (anoxic-anaerobic-oxic) process was investigated using synthetic wastewater (with different volatile fatty acid (VFA) ratios) and real wastewater. The P, poly-hydroxyalkanoates (PHAs), dehydrogenase activity (DHA), polyphosphate kinases (PPK), electron transfer system (ETS), and adenosine 5'-triphosphate (ATP) were determined as indicators. The highest P removal efficiencies were achieved at an anaerobic duration of 3, 4, and 6 h for 15, 30, and 60% VFA ratio in synthetic wastewater. The amount of the released P and stored PHAs can be manipulated by different anaerobic durations, but the P removal efficiency cannot be guaranteed with higher stored PHAs. Additionally, the energy metabolism confirmed the significance of anaerobic duration extension on microbial activity. The highest values of four indicators were all achieved at anaerobic duration of 4 h with 30% VFAs ratio which achieved the highest P removal efficiency. Real wastewater experiments also proved the reproductivity of these results. We defined this phenomenon as the "hunger response" where microorganisms responded to suppression (anaerobic duration extension) with higher activity after the end of the anaerobic condition. These results can provide references for better design and operation of biological phosphorus removal in RAAO process.