Switching exogenous to endogenous electron donor-driven denitrification via anaerobic/aerobic/anoxic strategy: Key...

Efficient nitrogen removal from low carbon-to-nitrogen (C/N) municipal wastewater remains a major challenge. Although the endogenous denitrification (ED) in the anaerobic/aerobic/anoxic (A/O/A) process attracts increasing attention for saving carbon and energy, achieving reliable start-up and controllable enrichment techniques of key functional microorganisms in practice remains challenging. This study demonstrates the successful transition from exogenous to endogenous electron donor-driven denitrification in an A/O/A sequencing batch reactor (SBR) under low dissolved oxygen (DO) and extended anoxic conditions. After operational optimization, the system achieved a substantial enrichment of Candidatus Competibacter (relative abundance increasing from 0.96% to 35.42%), which played a crucial role in ED. Consequently, the nitrogen removal efficiency increased from 60.2% to 83.2%. Results indicated that the optimal Polyhydroxyalkanoates (PHA) accumulation occurred at 90 min in anaerobic stage. The low DO range (0.5-1.0 mg/L) was key to minimizing aerobic PHA consumption, ensuring its availability for subsequent denitrification. During the extended anoxic duration of 4 h, glycogen was identified as the primary electron donor, with secondary contributions from PHA, extracellular polymeric substances (EPS), and cellular metabolites. Microbial community analysis confirmed Candidatus Competibacter as the dominant organism, facilitating nitrate reduction using internally stored polymers. This enrichment was attributed to sufficient glycogen storage (37.3mmolC/L) and reduced competition with Denitrifying polyphosphate -accumulating organisms (DPAOs). Finally, the combined strategy and selective enrichment of Denitrifying glycogen-accumulating organisms (DGAOs) promoting internal carbon source utilization not only enables efficient nitrogen removal without external carbon addition but also reduces sludge production for treating low C/N wastewater.