## 3 Results and Discussion
### Low-Carbon Operation Evaluation  
The Technical Specification for Low-Carbon Operation of Sewage Treatment Plants (T/CAEPI 49-2022), China's inaugural low-carbon group standard in the sewage treatment sector, marks a pioneering effort in advancing low-carbon practices for STPs. Applying this standard, 98 STPs were evaluated for their low-carbon operations. Based on the evaluation criteria, STPs were initially screened, requiring an average load rate exceeding 50% and a daily average COD contribution from industrial wastewater to total influent COD below 10% during the evaluation period. After screening, 72 STPs met the evaluation conditions. The low-carbon operation evaluation index system comprises quantitative evaluation and qualitative evaluation. Quantitative evaluation focuses on assessing carbon  
Figure 10: Relationship between influent water quality, TN and TP removal, and carbon emission intensity. (**a**) The relationship between influent BOD/TN and carbon emission intensity. (**b**) The relationship between influent COD/TN and carbon emission intensity. (**c**) The relationship between TN removal rate and carbon emission intensity. (**d**) The relationship between influent BOD/TP and carbon emission intensity. (**e**) The relationship between TP removal rate and carbon emission intensity. emission intensity, while qualitative evaluation examines low-carbon practices such as equipment/facility upgrades, operational optimization, monitoring, and accounting. Due to challenges in obtaining qualitative evaluation data, this study prioritizes quantitative evaluation, which provides a data-driven, objective reflection of STPs' low-carbon performance. The carbon emission intensity is calculated using Formulas (15)-(17) below. \[E_{P}=E_{dc}+E_{ic} \tag{15}\]  
\[E_{dc}=E_{N2O}\times k_{1}+E_{CH4}+E_{CO2} \tag{16}\]  
\[E_{ic}=[E_{e}\times(k_{2}\times k_{5})+E_{h}+E_{c}]\times k_{3}\times k_{4} \tag{17}\]  
where:  
\(E_{p}\): Evaluation of carbon emission intensity (kgCO2-eq/m\({}^{3}\)). \(E_{dc}\): Direct carbon emission correction intensity (kgCO2-eq/m\({}^{3}\)). \(E_{ic}\): Indirect carbon emission correction intensity (kgCO2-eq/m\({}^{3}\)). \(E_{N2O}\): Direct N2O carbon emission (kgCO2-eq/m\({}^{3}\)). \(E_{CH4}\): Direct CH4 carbon emission intensity (kgCO2-eq/m\({}^{3}\)). \(E_{CO2}\): Direct CO2 carbon emission intensity (kgCO2-eq/m\({}^{3}\)). \(k_{1}\): Total nitrogen removal rate correction factor (dimensionless). \(k_{2}\): Treatment capacity correction factor (dimensionless). \(k_{3}\): Oxygen-demanding pollutant reduction correction factor (dimensionless). \(k_{4}\): Effluent discharge standard correction factor (dimensionless). \(k_{5}\): Odor control level correction factor (dimensionless). As shown in Figure 11, the carbon emission correction intensity of the 72 STPs exhibited a significant negative correlation with their low-carbon operation evaluation scores. Specifically, the higher the carbon emission intensity of an STP, the lower its low-carbon operation evaluation score. The average corrected carbon emission intensity was 0.338 kgCO2-eq/m\({}^{3}\), with a maximum value of 0.672 kgCO2-eq/m\({}^{3}\) and a minimum value of 0.187 kgCO2-eq/m\({}^{3}\). The direct carbon emission correction intensity ranged from 0.046 to 0.218 kgCO2-eq/m\({}^{3}\), with an average of 0.106 kgCO2-eq/m\({}^{3}\). The indirect carbon emission correction intensity ranged from 0.116 to 0.560 kgCO2-eq/m\({}^{3}\), with an average of 0.232 kgCO2-eq/m\({}^{3}\). Figure 11: Carbon emission modification intensity and low carbon operation evaluation scores of 72 sewage treatment plants. After adjusting the correction factors--including the total nitrogen removal rate correction factor, treatment scale correction factor, oxygen-consuming pollutant reduction correction factor, effluent discharge standard correction factor, and odor control level correction factor--the carbon emission correction intensity of the 72 STPs decreased by 12% compared to pre-correction levels.