## 3 Results and Discussion
### Analysis of Influencing Factors on Carbon Emission Intensity
#### 3.2.3 Treatment Process  
Significant differences in carbon emission intensity exist among various sewage treatment processes (Figure 7). While carbon emission intensity is influenced by factors such as load rate, influent water quality, aeration intensity, and operational modes, the study found that among 98 STPs, the anaerobic-anoxic-oxic (AAO) process exhibited the lowest carbon emission intensity, with an average value of 0.370 kgCO\({}_{2}\)-eq/m\({}^{3}\). This was lower than the average carbon emission intensity of all 98 STPs (0.399 kgCO\({}_{2}\)-eq/m\({}^{3}\)). This outcome is closely associated with the high nitrogen and phosphorus removal efficiency of the AAO process. In contrast, the Sequencing Batch Reactor (SBR) process, due to its intermittent operation mode, consumes more electricity, leading to a higher carbon emission intensity [30; 31]. Direct Carbon Emission Disparities Between AAO and SBR Processes (1) Nitros Oxide (N\({}_{2}\)O) Emission Variability During Denitrification AAO Process: The continuous-flow configuration with dedicated anaerobic, anoxic, and aerobic zones spatially separates nitrification and denitrification processes. Enhanced carbon utilization efficiency in the anoxic zone promotes complete denitrification, minimizing N\({}_{2}\)O accumulation and emissions. SBR Process: Sequential batch operation induces frequent environmental fluctuations (e.g., dissolved oxygen, carbon availability), destabilizing nitrifier and denitrifier communities. Incomplete denitrification under dynamic conditions exacerbates N\({}_{2}\)O generation. (2) Methane (CH\({}_{4}\)) Production Potential AAO Process: Rapid organic matter adsorption by polyphosphate-accumulating organisms (PAOs) in the anaerobic zone and subsequent aerobic oxidation reduce residual organic content in excess sludge, thereby diminishing CH\({}_{4}\) emission potential during downstream sludge treatment. SBR Process: Intermittent aeration may leave residual organic substrates in sludge, increasing CH\({}_{4}\) emissions during subsequent stabilization processes. Indirect Carbon Emission Disparities Between AAO and SBR Processes (1) Aeration Energy Consumption and Efficiency AAO Process: Continuous aeration in aerobic zones allows for optimized dissolved oxygen (DO) control (e.g., step-feed aeration, fine-bubble diffusers), reducing energy waste. Figure 7: Carbon emission intensity of different treatment processes. SBR Process: Frequent blower start/stop cycles during intermittent aeration incur energy losses (e.g., motor inrush currents), while prolonged aeration per treatment cycle elevates overall electricity consumption. (2) Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT) Management  
AAO Process: Continuous-flow operation enables extended SRT, facilitating nitrifier enrichment and stable nitrogen removal without chemical supplementation. Prolonged SRT also reduces excess sludge yield, mitigating sludge treatment emissions. SBR Process: Limited SRT due to batch operation risks nitrifier washout during de-canting phases, necessitating prolonged aeration or external carbon addition to maintain treatment efficacy, thereby increasing energy and chemical footprints. Process Stability and Carbon Utilization Efficiency  
AAO Process: Spatial segregation of functional microbial guilds (PAOs, nitrifiers, denitrifiers) ensures optimal carbon allocation, minimizing competition for resources. Stable carbon pathways reduce supplementary carbon requirements for denitrification. SBR Process: Carbon substrates must simultaneously support anaerobic phosphorus release, denitrification, and aerobic phosphorus uptake within a single reactor, often leading to carbon limitations and necessitating external carbon dosing. In summary, the AAO process demonstrates superior carbon emission performance through process compartmentalization, enhanced carbon utilization efficiency, and stable operational conditions. These attributes collectively minimize direct N2O/CH4 emissions and indirect energy-related emissions.