Endogenous-denitrification-driven AOA-like processes for carbon-limited wastewater treatment: mechanisms, intensification strategies and application boundaries.
Abstract
Carbon-limited wastewaters remain challenging for biological nitrogen removal because organic electron donors are scarce, poorly biodegradable, or temporally mismatched with denitrification demand. Conventional responses, including external carbon addition, intensified aeration, and extended hydraulic retention, can improve effluent quality but increase chemical use, energy demand, sludge production, and greenhouse-gas burdens. Endogenous-denitrification-driven anaerobic-oxic-anoxic-like (AOA-like) processes offer a different strategy by separating anaerobic carbon capture or transformation from its later post-anoxic use, thereby redistributing the reducing power already present in wastewater. Yet rapid diversification has outpaced mechanistic resolution because anaerobic COD removal is often conflated with functionally available donor-pool formation, and electron-donor origin with nitrogen-transformation pathways. This review develops a carbon-electron management framework in which "ED-driven" denotes verified post-anoxic use of internally generated or retained donors, without requiring endogenous denitrification (ED) to dominate nitrogen removal. Across studies, performance depends on forming and preserving accessible donor pools, matching them with post-anoxic NOX⁻, and reducing organic-electron demand through nitrite-based or autotrophic routes. Intensification should therefore target the dominant carbon-electron constraint rather than universal operating setpoints, while application boundaries reflect wastewater biodegradability, temperature and loading disturbances, inhibition, and whole-process nitrous oxide (N2O) trade-offs. Available evidence supports delayed endogenous-carbon use, although direct pathway-flux measurements remain scarce. AOA-like systems can reduce external-carbon and aeration demand, but transferable design requires wastewater-specific verification of donor fate, pathway contributions, and environmental performance.