• • Daily average CH4 emission intensities were statistically indistinguishable between A2/O and A2/O-MBR plants: (0.67 ± 0.22) g/m3 vs. (0.65 ± 0.18) g/m3, with over 70% of emissions originating from sewer-derived anaerobic production and subsequent stripping in pretreatment units. This indicates that upstream sewer management, not the biological process, dominates CH4 emissions, and mitigation should focus on pretreatment capture.
• • A2/O-MBR exhibited significantly higher daily N2O emission intensity: (0.132 ± 0.055) g/m3 vs. (0.060 ± 0.046) g/m3 for A2/O, a 2.2-fold increase. The membrane tank's intensive aeration and oxygen-enriched recirculation enhanced N2O production and stripping, highlighting the need to optimize membrane aeration strategies to reduce N2O emissions.
• • N2O emission pathways diverged: in A2/O, emissions correlated positively with influent COD and BOD5 (heterotrophic denitrification), while in A2/O-MBR they correlated with NH3-N loading and DO (nitrification-based). This mechanistic difference implies that tailored operational controls—carbon source addition for A2/O vs. aeration optimization for A2/O-MBR—are required for effective mitigation.
• • Measured emission factors were substantially lower than IPCC and industry defaults: CH4 factors were 0.0082 and 0.0084 kg/kg BOD5 for A2/O and A2/O-MBR, respectively, versus IPCC default of 0.018 kg/kg BOD5; N2O factors were 0.0014 and 0.0026 kg/kg TN, versus IPCC default of 0.016 kg/kg TN. This underscores that using default factors overestimates emissions by up to 11-fold for N2O, emphasizing the need for local, measurement-based factors to accurately reflect advanced process performance and avoid overestimation in carbon accounting.