## 4 Conclusions  
This study constructed the first provincial-level multi-standard carbon emission assessment framework for WWTPs in China. By systematically comparing the IPCC 2019, T/CAEPI 49-2022, and T/CUWA 50055-2023 accounting methods, and based on the actual operation data of 49 WWTPs in Shandong Province for the whole year of 2022, the study revealed the differential impacts of different accounting systems on greenhouse gas assessment results and proposed carbon accounting optimization strategies suitable for the Chinese sewage treatment industry. (1) Methodological innovation and standard adaptability  
The study constructed the first provincial-level multi-dimensional accounting framework that integrates international guidelines with domestic standards, breaking through the spatial applicability limitations of traditional single-method approaches. It was found that T/CUWA 50055-2023, by including fossil-derived CO\({}_{2}\) (contributing 5.0%) and optimizing emission factors (CH\({}_{4}\): 0.00575 kg/kgCOD, N\({}_{2}\)O: 0.01 kg/kgN, with CH\({}_{4}\) and N\({}_{2}\)O emission factors reduced by 23.3% and 37.5%, respectively), resulted in a 19.6% decrease in calculated emissions compared to IPCC 2019 (\(\Delta\) = 68,576 tCO\({}_{2}\)-eq, \(p\) = 0.002 \(<\) 0.01), making it more in line with the actual operating characteristics of Chinese WWTPs. It is recommended to prioritize the use of the T/CUWA 50055-2023 method. This study achieved dynamic traceability analysis of chemical carbon emission factors, quantifying the emission contribution differences in key chemicals such as glucose (3.1% of total emissions), PAC (1.6%), and sodium hypochlorite (1.3%), revealing the shortcomings of the chemical emission factor database in existing standards. (2) Key emission characteristics and driving mechanisms  
Indirect emissions dominated the total greenhouse gas emissions (56.8%), with electricity consumption (43.3%) and chemical consumption (13.5%) being the core sources of contribution. Carbon sources (27.6% of chemical emissions) and phosphorus-removing agents (15.2%) constituted the main targets for emission reduction. Using low-emission-factor carbon sources (such as replacing sodium acetate with methanol) could reduce carbon emissions by 77.6%. The scale effect showed a non-linear characteristic: small-scale plants with a capacity of \(\leq\)20,000 m\({}^{3}\)/d, due to low equipment efficiency, had an indirect emission intensity (0.258 kgCO\({}_{2}\)-eq/m\({}^{3}\)) that was 23.4% higher than the average carbon emission intensity of the 49 WWTPs. Large-scale plants with a capacity of \(\geq\)100,000 m\({}^{3}\)/d, on the other hand, did not show a scale effect due to fluctuations in influent load. Based on marginal abatement cost analysis, replacing sodium acetate with methanol could reduce carbon-source-related emissions by 77.6% (\(\Delta\) = 2.25 kgCO\({}_{2}\)-eq/kg carbon source). It is recommended to formulate  
Figure 8: The impact of influent water quality and emission standards on emission intensity. the \"Technical Guidelines for the Selection of Low-Carbon Chemicals\" and accompany it with carbon-credit incentive policies. This study provided a theoretical basis and practical paradigm for establishing a precise carbon accounting system for the Chinese WWTPs and has important reference value for promoting the green transformation of the industry under the \"dual-carbon\" goals. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15116175/sl](https://www.mdpi.com/article/10.3390/app15116175/sl)]([https://www.mdpi.com/article/10.3390/app15116175/sl](https://www.mdpi.com/article/10.3390/app15116175/sl)): S1. Accounting method; S1.1. Accounting method for direct greenhouse gas emissions from wastewater treatment plants; S1.1.1. Accounting method for CH\({}_{4}\) emissions; S1.1.2. Accounting method for N\({}_{2}\)O emissions; S1.1.3. Accounting method for fossil-derived CO\({}_{2}\) emissions; S1.2.