Phase-resolved monitoring reveals a dominant dissolved greenhouse gas footprint in the AOA process treating low C/N municipal wastewater.

The anaerobic/aerobic/anoxic (AOA) process offers a low-carbon pathway for biological nitrogen removal from low carbon-to-nitrogen (C/N) ratio municipal wastewater, yet its complete greenhouse gas (GHG) footprint across gas and liquid phases remains unresolved. This study systematically quantified gaseous and dissolved CH4, CO2, and N2O at each operational stage of a laboratory-scale AOA system using phase-resolved monitoring. The system achieved stable nitrogen removal of 86.2%, with a CO2-equivalent direct gaseous emission factor was 25.2%-36.6% lower than that of the conventional wastewater treatment processes. However, phase-resolved accounting revealed that dissolved GHGs constituted the dominant fraction of total GHG footprint, with an intensity of 0.780 kg CO2e m⁻³-3.3-fold higher than direct gaseous emissions (0.237 kg CO2e m⁻³)-exposing a critically underreported hidden carbon reservoir. N2O was the principal climate driver, contributing 88.7% of total GHG intensity (1.010 kg CO2e m⁻3). The aerobic stage emerged as the primary emission hotspot (0.424 kg CO2e m⁻3), with N₂O accounting for 93.1% (0.395 kg CO2e m⁻3) of its intensity, potentially associated with hydroxylamine-related pathways under partial nitritation conditions and the enrichment of Nitrosomonas. In the anoxic stage, Bacteroidota and GAOs may participate in endogenous carbon cycling and denitrification, facilitating dissolved N2O reduction. These findings demonstrate that accurate GHG accounting for AOA systems must integrate concurrent gas-liquid phase monitoring, and that targeted aeration control to suppress aerobic N2O generation is essential for achieving genuinely carbon-neutral nitrogen removal.