Integration of partial nitrification, endogenous denitrification and anaerobic ammonia oxidation in low dissolved oxygen anaerobic/oxic/anoxic-aerobic granular sludge reactor treating low carbon to nitrogen ratios wastewater.

The anaerobic/aerobic/anoxic-aerobic granular sludge (AOA-AGS) process effectively removes nitrogen while tolerating limited oxygen and carbon. However, integrating anaerobic ammonia oxidation (Anammox), which thrives under low organic carbon and oxygen conditions, with AOA-AGS remains challenging. This study investigated nitrogen removal performance and community changes in an AOA-AGS sequencing batch reactor with low carbon to nitrogen ratios (C/N) wastewater and reduced dissolved oxygen (DO) from 5-7 mg/L to 0.5 ± 0.2 mg/L. The total inorganic nitrogen removal rate stabilized at 82 ± 9 % under low DO, driven by partial nitrification and endogenous denitrification through dominant denitrifying glycogen-accumulating organisms (DGAOs), such as Candidatus_Competibacter (43.09 %). Anammox bacteria (mainly Candidatus_Brocadia) were enriched under long solids retention time (128 days) and low DO, synergizing with DGAOs for enhanced nitrogen removal. This study demonstrated that AOA-AGS under low DO enables efficient nitrogen removal through the synergistic endogenous denitrification by DGAOs and Anammox in low C/N wastewater, offering a sustainable strategy.