## 2 Background and Methods
### Carbon Emission Accounting Boundaries and Methodologies
#### 2.2.2 Carbon Emission Accounting Methodologies  
Currently, China lacks a unified national standard for carbon emission accounting in the wastewater treatment sector [21; 22]. Existing practices are primarily guided by industry group standards. The Assessment Standard for Carbon Mitigation in Municipal Wastewater Treatment Plants (T/CUWA 50055-2023) [23], published by the China Urban Water Association on 21 December 2023, was adopted as the primary methodological basis for this study. Compared to the IPCC 2019 guidelines, the Chinese standard offers a more comprehensive accounting scope by explicitly incorporating carbon emissions from chemical consumption and fossil-derived CO\({}_{2}\), thereby enhancing its applicability to individual WWTPs. In contrast, the IPCC 2019 framework is better suited for national- or city-scale emissions inventory development. Furthermore, the default CH\({}_{4}\) and N\({}_{2}\)O emission factors in IPCC guidelines--derived predominantly from European and North American WWTP operational data--exhibit limitations in reflecting China's treatment process characteristics. A comparative analysis of the three GHG accounting methodologies is summarized in Table 3.  
The T/CUWA 2023 standard expands the accounting scope compared to IPCC 2019 and T/CAEPI 2022 by explicitly incorporating fossil-derived CO\({}_{2}\) emissions and chemical consumption-related carbon emissions, thereby enhancing its applicability to individual WWTPs in China. This study developed a high-precision carbon emission accounting framework for WWTPs by integrating three key methodological components:  
1. The \"Carbon Emission Reduction Assessment Standard for Urban Wastewater Treatment Plants\" (T/CUWA 50055-2023) issued by the China Urban Water Supply and Drainage Association (21 December 2023);
2. The 2021 power sector CO\({}_{2}\) emission factors published jointly by the Ministry of Ecology and Environment and the National Bureau of Statistics;
3. Process-specific CH\({}_{4}\) and N\({}_{2}\)O emission factors for different treatment configurations (Table 4), derived as weighted averages from literature values in the \"Technical Guide for Carbon Accounting and Emission Reduction Pathways in Urban Water Systems\". \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Accounting Category** & **IPCC 2019** & **T/CAEPI 2022** & **T/CUWA 2023** \\ \hline Pretreatment & \(\surd\) & \(\surd\) & \(\surd\) \\ Biological Treatment & \(\surd\) & \(\surd\) & \(\surd\) \\ On-site Sludge Dewatering & \(\surd\) & \(\surd\) & \(\surd\) \\ Off-site Pipeline Network & \(\times\) & \(\times\) & \(\times\) \\ Chemical Consumption & \(\times\) & \(\surd\) & \(\surd\) \\ Biogenic CO\({}_{2}\) Emissions & \(\times\) & \(\times\) & \(\times\) \\ Fossil-derived CO\({}_{2}\) Emissions & \(\times\) & \(\times\) & \(\surd\) \\ Off-site Sludge Disposal & \(\times\) & \(\times\) & \(\times\) \\ \hline \hline \end{tabular} Notes. “\(\surd\)” indicates inclusion in the accounting boundary; “\(\times\)” indicates exclusion. T/CAEPI 2022: Technical Specification for Low-Carbon Operation Evaluation of Wastewater Treatment Plants (T/CAEPI 49-2022) [24]. T/CUWA 2023 [24]: Assessment Standard for Carbon Mitigation in Municipal Wastewater Treatment Plants (T/CUWA 50055-2023).  
\end{table} 
Table 3: Comparison of accounting boundaries across different methodologies. Direct Carbon Emission Quantification Protocols  
(1) Biochemical Process CH4 Emissions  
The annual CH4 emission from wastewater treatment biochemical processes was calculated using Equation (1):  
\[\mathrm{m_{CH_{4}}=\left(1+F_{CH_{4}}\right)\times Q\times(COD_{in}-COD_{eff}) \times EF_{CH_{4}}\times 10^{-3}} \tag{1}\]  
where:  
\(m_{CH_{4}}\): Annual CH4 emission (kg CH4/a). \(F_{CH_{4}}\): Fetitive emission adjustment factor (%) for pumping stations and grit chambers (range: 10.25-26.19%; 20% adopted here). Q: Influent flow rate (m3/a). \(\mathrm{COD_{in}/COD_{eff}}\): Influent/effluent chemical oxygen demand (mg/L).