Comparative Analysis of CH4 and N2O Generation and Emission Characteristics in A2/O and A2/O-MBR Wastewater Treatment Plants
The A2/O-MBR process, owing to its superior effluent quality and smaller footprint, is increasingly adopted in newly built and upgraded wastewater treatment plants. However, systematic studies on its greenhouse gas (GHG) emissions remain scarce, and direct comparisons with the conventional A2/O process are lacking. In this study, two full-scale wastewater treatment plants employing the A2/O and A2/O-MBR processes under identical influent conditions, climate, and discharge standards were investigated. A high-frequency monitoring system covering the entire treatment train was established, and combined with measurements of dissolved CH4 and N2O, water quality parameters, and operational parameters, to elucidate the differences in GHG emission characteristics. Results showed that the daily average CH4 emission intensities were not significantly different between the two plants [(0.67 ± 0.22) and (0.65 ± 0.18) g/m3, respectively]. CH4 emissions mainly originated from sewer-derived anaerobic production and subsequent release in the pretreatment units (accounting for over 70% of the total emissions), with partial in-plant oxidation by methanotrophs. Temperature and aeration-induced stripping were identified as key driving factors, as CH4 emissions were positively correlated with ambient temperature and dissolved oxygen (DO). In contrast, more than 90% of N2O emissions occurred in the biological treatment units. The A2/O-MBR plant exhibited significantly higher daily N2O emission intensity [(0.132 ± 0.055) g/m3] than the A2/O plant [(0.060 ± 0.046) g/m3], largely due to intensive aeration and oxygen-enriched internal/external recirculation in the membrane tank, which enhanced N2O production and stripping. Correlation analysis further revealed that N2O emissions in the A2/O plant were positively related to influent COD and BOD5, indicating dominance of heterotrophic denitrification, whereas in the A2/O-MBR process they were mainly driven by NH3-N loading and DO, reflecting a nitrification-based pathway. Importantly, both processes exhibited CH4 and N2O emission factors that were significantly lower than the reference values recommended by the IPCC and industry guidelines, underscoring the necessity of localizing emission factors for accurate carbon accounting.