## 2 Materials and Methods
### Accounting Method
As the third-largest economy in China (with a GDP of 921 billion yuan in 2023), Shandong Province has a sewage treatment scale of 14.931 million m\({}^{3}\)/d, and its carbon emission characteristics hold significant reference value for the entire country. Nevertheless, most existing literature focuses on theoretical models or single-case studies, lacking a provincial-scale systematic assessment based on actual production data. This research, based on the complete operational data of 49 WWTPs in Shandong Province for the year 2022, constructs a multi-method comparison framework using the IPCC 2019, GS 2022, and GS 2023 methods. It analyzes the differences in accounting and identifies key influencing factors (such as process type and scale effects), ultimately proposing an optimized carbon accounting path that matchesregional characteristics to fill the dual gap in methodology and empirical research. The details of the accounting system for this study can be found in the Supplementary Materials. The boundaries of this study's accounting are confined to the internal operations of the wastewater treatment plant, covering processes such as preliminary treatment, biological treatment, advanced treatment, disinfection, and in-plant sludge dewatering. Off-site pipeline networks and the final disposal of sludge are excluded from the accounting scope. For specific details on the accounting boundary division, refer to Table 1. During the wastewater treatment process, CO2 emissions are mainly generated from the oxidation and decomposition of organic matter. Based on their sources, CO2 emissions can be classified into biogenic and fossil-derived categories. Since biogenic CO2 has a negligible impact on global warming [28], this study only includes fossil-derived CO2 in the accounting, and biogenic CO2 is not counted. In China, there is currently no unified national standard for carbon emission accounting in the sewage treatment industry. At present, relevant work is mainly guided by industry group standards. This study adopts the following standards and guidelines for the accounting of greenhouse gas emissions from WWTPs: the \"Technical Specification for Low-Carbon Operation Evaluation of Sewage Treatment Plant\" (GS 2022) released by the China Environmental Protection Industry Association on 6 June 2022 and the \"Carbon Emission Reduction Assessment Standard for Municipal Wastewater Treatment Plants\" (T/CUWA 50055-2023) (GS 2023) released by the China Urban Water Association on 21 December 2023. Leveraging the three aforementioned methods, this research computed and contrasted the greenhouse gas emissions of 49 WWTPs located in Shandong Province. These emissions consist of the direct releases of CH4, N2O, and CO2, along with the indirect emissions stemming from electricity utilization and chemical consumption. The specific accounting results are shown in Tables S4, S5, and S6, respectively. To ensure the consistency of the accounting scope and results, this study has standardized the parameters for the calculation of indirect emissions as follows:  
1. Electricity Consumption: All three accounting methods use the latest 2021 China electricity carbon dioxide emission factor published to calculate the indirect emissions related to electricity consumption, ensuring consistency in the accounting of indirect emissions from electricity use. 2. Chemical Consumption: T/CAEPI 49-2022 and T/CUWA 50055-2023 use the chemical emission factors provided in their respective appendices for calculation. Specifically, T/CAEPI 49-2022 uses Table S9, and T/CUWA 50055-2023 uses Table S4 in Supplementary Materials. Since IPCC 2019 does not provide accounting parameters related to