## 3 Results and Discussion
### Indirect Carbon Emissions Assessed Using Three Different Methods  
2.1 Comparison of Chemical Carbon Emission Calculation Results Using T/CAEPI 49-2022 and T/CUWA 50055-2023  
The addition of chemical agents increases indirect carbon emissions [30; 31; 32]. In the process of wastewater treatment, phosphorus-removing agents need to be added for deep phosphorus removal, flocculants are required for sludge dewatering, disinfectants must be added before the treated water is discharged, and when the influent carbon source does not meet the denitrification requirements, carbon sources also need to be added. The carbon emissions generated during the production and use of chemicals are usually estimated by emission factors. However, due to the different emission factors used in different accounting systems, the results show notable discrepancies. The chemical carbon emissions calculated using T/CAEPI 49-2022 were 42,651.58 \(\mathrm{tCO_{2}}\)-eq, with a carbon emission intensity of 0.0688 \(\mathrm{kgCO_{2}}\)-eq/m\({}^{3}\), accounting for 15.32% of the total greenhouse gas emissions. In contrast, the chemical carbon emissions calculated using T/CUWA 50055-2023 were 37,862.65 \(\mathrm{tCO_{2}}\)-eq, with a carbon emission intensity of 0.0611 \(\mathrm{kgCO_{2}}\)-eq/m\({}^{3}\), accounting for 13.51% of the total greenhouse gas emissions. There is a certain difference between the results obtained from these two sets of standards. The main reason is that T/CUWA 50055-2023 remodeled the chemicals based on the ecoinvent database according to the literature tracing of the wrong chemicals, reconstructed the chemical emission factors, and the recommended values are more in line with the actual situation, resulting in lower calculation results. Upon further analysis, it was discovered that the carbon-emission contributions of diverse chemical types vary significantly. The dosages and carbon-emission contributions of chemicals in 49 wastewater treatment plants were categorized and tallied (For details, see Figure 3). The findings indicated that the chemical with the most substantial carbon-emission contribution was the carbon source, which accounted for 27.6%. Next was the phosphorus-removing agent, representing 15.2% of the TC emission contribution. The three chemicals with the highest carbon emissions were glucose, PAC, and sodium hypochlorite, among which the carbon emissions of glucose were nearly twice that of PAC. Due to the widespread problems of aging, damage, and misconnection in China's sewage collection network, serious rainwater and sewage overflow pollution, and low sewage collection rate, the influent carbon/nitrogen ratio is generally insufficient, which significantly increases the dosage of carbon sources. [PERSON] [33] and others found in their study that the annual carbon emissions generated by the addition of sodium acetate in Gaobeidian WWTPs in Beijing accounted for 2.63% of the TC emissions of the plant. Among various carbon sources, the emission factor of sodium acetate is the highest (2.90), while that of methanol is the lowest (0.65), indicating significant differences in the impact of different carbon sources on carbon emissions. Therefore, it is recommended to prioritize the usage of carbon sources with lower emission factors (such as methanol or glucose), reduce the use of sodium acetate, or develop alternative carbon sources (such as high BOD wastewater, kitchen wastewater, etc.). A plausible alternative is the use of wastewater. [PERSON] [34] used tofu wastewater as a carbon source for a sewage plant with a scale of 20,000 m\({}^{3}\)/d and found that the carbon-substitution effect was significant. It reduced 337.9 tCO\({}_{2}\)-eq, accounting for about 15.8% of the total emissions of the sewage plant, and saved 7-10% of the total operating costs. The carbon emissions of chemical phosphorus-removing agents should not be ignored. Optimizing the biological phosphorus-removing process is an available way to reduce the dosage of PAC.