## 2 Materials and Methods
### Accounting Method  
Greenhouse gas emissions from wastewater treatment plants (WWTPs) can be divided into two categories: direct and indirect. Direct emissions include dinitrogen monoxide (N\({}_{2}\)O), methane (CH\({}_{4}\)), and carbon dioxide (CO\({}_{2}\)) produced during the treatment process, whereas indirect emissions arise from the carbon footprint linked to electricity usage, heating, and chemical consumption [23, 24]. For the aforementioned emission sources, the main methods for the accounting of greenhouse gas emissions currently include the measurement method, the modeling method, the carbon footprint method, and the emission factor method, each with its applicable scope and limitations [25]. 1. Measurement Method: This method obtains direct emission data through on-site air sampling and continuous monitoring, which offers high precision. However, it is limited by instrument sensitivity and maintenance costs, making it unsuitable to apply on a large scale. 2. Modeling Method: Based on biodynamic models that simulate microbial metabolic processes, this method can quantify the generation of GHG in specific processes. Nevertheless, the model parameters (such as nitrification/denitrification rates) require calibration with localized data, and the universality of this method is relatively poor. 3. Carbon Footprint Method: Using a life cycle assessment framework, this method accounts for both emissions throughout the entire process of WWTP construction, operation, and demolition. Although it can provide a basis for systemic emission reduction, it faces data availability issues during the construction phase, limiting its application scope. 4. Emission Factor Method: This method estimates carbon emissions by combining the activity level (such as electricity consumption) with the corresponding emission factor (such as CH\({}_{4}\)/kgCOD). It has the advantages of being easy to operate and applicable to regional assessments and is widely used for provincial-scale evaluations. However, the regional heterogeneity of emission factors (such as differences in processes and management levels) may lead to calculation deviations, necessitating dynamic corrections with local data. Currently, the greenhouse gas accounting system based on the emission factor method mainly includes the following three guiding documents:  
1. IPCC 2019 Revised Edition: This is applicable for national-level carbon emission accounting. The accounting targets focus on the direct emissions of CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O. 2. Technical Specification for Low-Carbon Operation Evaluation of WWTPs (T/CAEPI 49-2022 [26]): This document is targeted at the operation stage of WWTPs. It can directly calculate the emissions of CH\({}_{4}\) and N\({}_{2}\)O, along with indirect emissions like those from electricity and chemical consumption (calculated in CO\({}_{2}\) equivalents), providing a quantitative basis for low-carbon operation. 3. Standard for Carbon Emission Reduction Assessment of Urban WWTPs (T/CUWA 50055-2023) [27]: This document is based on a life cycle perspective, covering all stages from construction to operation and decommissioning. In addition to accounting for the emissions of fossil-derived CO\({}_{2}\), CH\({}_{4}\), and N\({}_{2}\)O during the operation stage, it also integrates calculation methods for the emission reduction thanks to low-carbon technologies such as heat pumps, photovoltaic applications, and sludge anaerobic fermentation. Provincial carbon emission management is a crucial link in mitigating global climate change. However, existing studies have rarely systematically compared the applicability of different accounting methods, especially empirical analyses based on the actual operational data of multiple plants over the past three years, which are still a blank area.