## 3 Results and Discussion
### Direct Carbon Emissions Assessed by Three Different Methods
#### 3.1.3 Calculation of Fossil-Derived CO\({}_{2}\) Emissions Using T/CUWA 50055-2023  
In 2022, the fossil-derived CO\({}_{2}\) emissions of 49 WWTPs in Shandong Province were computed in accordance with the guidelines of T/CUWA 50055-2023. These emissions were composed of two primary components: carbon emissions from fossil-derived mineralization and fossil-derived CO\({}_{2}\) stemming from the mineralization of external carbon sources. The carbon emissions from fossil-derived mineralization attained 7046.44 tCO\({}_{2}\), with an emission intensity of 0.0114 kgCO\({}_{2}\)/m\({}^{3}\). The mineralization of external carbon sources resulted in carbon emissions of 6967.18 tCO\({}_{2}\), with an emission intensity of 0.0112 kgCO\({}_{2}\)-eq per cubic meter. Overall, fossil-derived CO\({}_{2}\) emissions total 14,013.62 tCO\({}_{2}\), with an emission intensity of 0.0226 kgCO\({}_{2}\)-eq per cubic meter. Although fossil-derived CO\({}_{2}\) accounted for only 5.0% of the total carbon emissions from WWTPs, its inclusion in carbon emission assessments is crucial. Therefore, it should be incorporated  
Figure 2: CH\({}_{4}\) emissions and emission intensity. (**a**) Comparison of CH\({}_{4}\) emissions among the three standards (IPCC 2019 vs. T/CAEPI 49-2022/2023), (**b**) CH\({}_{4}\) emissions per unit mass of COD removed calculated using IPCC 2019 for different treatment processes, (**c**) CH\({}_{4}\) emissions per unit mass of COD removed calculated using T/CAEPI 49-2022 for different treatment processes.  
into the accounting framework. Research has indicated that the share of fossil-derived CO\({}_{2}\) emissions in Italian WWTPs varies between 16% and 26% [29].  
Currently, the IPCC guidelines and T/CAEPI 49-2022 do not include fossil-derived CO\({}_{2}\) in their accounting. On the one hand, this is because the boundary between biogenic carbon and fossil carbon is blurred, and there is uncertainty in distinguishing between them during accounting. On the other hand, the existing accounting standards do not definitely stipulate whether to include fossil-derived CO\({}_{2}\), which may result in an underestimation of the carbon footprint of WWTPs and weaken the reference value of the accounting results for policy-making and the optimization of emission reduction measures.  
In addition, the lower emission factors in T/CUWA 50055-2023 are more in line with the actual emission levels of N\({}_{2}\)O and CH\({}_{4}\), and it also takes into account the emissions of fossil-derived CO\({}_{2}\). Although fixed emission factors cannot reflect the differences in emissions between different processes, the precision and applicability of the accounting can be improved by updating and refining the process emission factors. In addition, incorporating fossil-derived CO\({}_{2}\) enables a more all-encompassing evaluation of the TC emissions in WWTPs. It furnishes a scientific foundation for formulating carbon emission reduction policies and optimizing management strategies. As a result, this approach becomes the favored choice for current direct carbon emission accounting.