## 3 Results and Discussion
### Overall Evaluation of Carbon Emissions
However, the proportion of carbon emissions from electricity consumption in East China (47.9%) was significantly lower than the value reported in their study (58.3%). This discrepancy may be attributed to the higher degree of grid cleanliness in East China. The proportion of carbon emissions from chemical consumption was 14.2%, indicating that the impact of chemical-related carbon emissions on the overall carbon footprint of STPs cannot be overlooked. Notably, fossil-derived CO\({}_{2}\), which was not considered in the IPCC accounting framework, accounted for 4.6% of carbon emissions in this study. Therefore, we recommend incorporating fossil-derived CO\({}_{2}\) into the carbon emission accounting system of STPs to provide a more comprehensive assessment of their environmental impact. From the perspective of processing units, the biochemical system (64.2%) and the water intake and lifting system (22.4%) were identified as the main contributors to carbon emissions in STPs. As shown in Figure 4, a positive correlation was observed between power consumption per ton of water treated and carbon emission intensity in STPs. Figure 2: Carbon emission intensity distribution of wastewater treatment plants in East China. Direct carbon emissions accounted for 37.9% of the total carbon emissions, with CH\({}_{4}\) and N\({}_{2}\)O emissions contributing 16% and 17.3%, respectively. It is noteworthy that the proportion of N\({}_{2}\)O emissions exceeded that of CH\({}_{4}\), primarily due to two factors: first, the global warming potential (GWP) of N\({}_{2}\)O is significantly higher than that of CH\({}_{4}\); second, the low influent C/N ratio of the 98 STPs included in this study, with an average value of 2.36, resulted in suboptimal biological nitrogen removal efficiency and increased production of N\({}_{2}\)O as an intermediate byproduct [36; 37; 38]. Consequently, despite the direct N\({}_{2}\)O emissions (337.3 t) being far lower than CH\({}_{4}\) emissions (2953.7 t), the contribution of N\({}_{2}\)O to the overall carbon footprint remained significant. A sensitivity analysis was conducted within this study's GHG accounting framework to evaluate the influence of critical parameters, including CH\({}_{4}\) and N\({}_{2}\)O emission factors, and the influent COD/TN ratio. The analysis revealed the following:  
1. A 1% variation in CH\({}_{4}\) or N\({}_{2}\)O emission factors resulted in a proportional 1% change in CH\({}_{4}\) or N\({}_{2}\)O emissions, respectively. Figure 4: The relationship between power consumption per ton of water and carbon emission intensity. Figure 3: The proportion of carbon emissions from different sources (**a**) and different treatment stages (**b**) of sewage treatment plants. 2. A 10% fluctuation in the influent COD/TN ratio led to a 10.1% alteration in total carbon emissions. These findings indicate comparable sensitivity coefficients across the evaluated parameters, suggesting that uncertainties in CH\({}_{4}\)/N\({}_{2}\)O emission factors and influent wastewater characteristics exert equivalent levels of influence on the overall carbon accounting accuracy. This underscores the necessity of region-specific emission factor databases and real-time influent quality monitoring for robust GHG inventories.