## 3 Results and Discussion  
### Overall Evaluation of Carbon Emissions  
In 2024, the total treated water volume of 98 sewage treatment plants (STPs) in East China reached 1.29 \(\times\) 10\({}^{9}\) m\({}^{3}\), with a total carbon emission of 5.17 \(\times\) 10\({}^{8}\) tCO\({}_{2}\)-eq. Among these emissions, direct emissions amounted to 1.96 \(\times\) 10\({}^{8}\) kgCO\({}_{2}\)-eq, while indirect emissions totaled 3.21 \(\times\) 10\({}^{8}\) tCO\({}_{2}\)-eq. The carbon emissions of individual STPs ranged from 0.29 \(\times\) 10\({}^{3}\) to 25.80 \(\times\) 10\({}^{3}\) tCO\({}_{2}\)-eq, with an average of 5.27 \(\times\) 10\({}^{3}\) tCO\({}_{2}\)-eq and a median of 3.43 \(\times\) 10\({}^{3}\) tCO\({}_{2}\)-eq. The average carbon emission intensity of the 98 STPs was 0.399 kgCO\({}_{2}\)-eq/m\({}^{3}\), varying from 0.160 to 1.749 kgCO\({}_{2}\)-eq/m\({}^{3}\). Compared to the carbon emission intensity of STPs in China reported in other literature (0.360-1.439 kgCO\({}_{2}\)-eq/m\({}^{3}\)) [25; 26; 27; 28; 29; 30], the carbon emission intensity of STPs in East China was lower. It was also lower than the average values reported for Poland, Finland [31], and several developing countries [32]. The average carbon emission intensity of wastewater treatment plants (WWTPs) in Poland and Finland is reported as 31 kg CO\({}_{2}\)eq/PE and 63 kg CO\({}_{2}\)eq/PE, respectively, when normalized to population equivalent (PE). When recalculated using treated water volume as the functional unit, the average emission intensity for both countries converges to approximately 0.7 kg CO\({}_{2}\)eq/m\({}^{3}\). In comparison, [PERSON] et al. [33] estimated an average greenhouse gas emission intensity of 0.45 kg CO\({}_{2}\)-eq/m\({}^{3}\) across 5282 Chinese WWTPs. Notably, this value \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Chemical Reagent**} & **Emission Factor EF\({}_{c}\)** & **Emission Factor EF\({}_{c}\)** \\  & **(kgCO\({}_{2}\)\(\cdot\)kg\({}^{-1}\))** & **Chemical Reagent** & **(kgCO\({}_{2}\)\(\cdot\)kg\({}^{-1}\))** \\ \hline Glucose & 1.40 & Polyacrylamide (PAM) & 2.85 \\ Sodium Acetate & 2.90 & Sodium Bicarbonate & 1.17 \\ Methanol & 0.65 & Sodium Hydroxide & 0.46 \\ Acetic Acid & 1.92 & Calcium Hydroxide & 1.11 \\ Polyaluminum Chloride & 6.19 & Sodium Carbonate & 1.46 \\ (based on Al\({}^{3+}\) mass) & & & \\ Ferric Chloride & 2.86 & Quicklime (Calcium Oxide) & 1.18 \\ (based on Fe\({}^{3+}\) mass) & & & \\ Ferrous Chloride & 0.22 & Hydrochloric Acid & 1.20 \\ Aluminum Sulfate & 0.30 & Citric Acid & 8.17 \\ Iron Sulfate & 0.23 & Sodium Hypochlorite & 2.99 \\ (based on Fe\({}^{3+}\) mass) & & & \\ Magnet & 0.0081 & Sodium Chlorate & 5.11 \\ Activated Carbon Powder & 7.96 & Other Chemical Agents & 1.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Carbon emission factors of different chemicals. exceeds the emission intensity observed in eastern China's facilities, which is attributed to the inclusion of sludge disposal-related emissions in [PERSON] et al. 's national-scale assessment. By contrast, the present study focuses on direct process emissions and chemical-associated carbon outputs, thereby providing a refined baseline for targeted regional mitigation strategies. As shown in Figure 2, the distribution of carbon emission intensity in East China's STPs generally followed a normal distribution. Specifically, five STPs (5.1%) had a carbon emission intensity of less than 0.2 kgCO\({}_{2}\)-eq/m\({}^{3}\), 82 STPs (83.1%) had an intensity between 0.2 and 0.6 kgCO\({}_{2}\)-eq/m\({}^{3}\), and only three STPs (3.1%) had an intensity higher than 1.0 kgCO\({}_{2}\)-eq/m\({}^{3}\). The proportion of each carbon emission category in East China's STPs is illustrated in Figure 3. Indirect carbon emissions were the primary source of carbon emissions in STPs, with electricity consumption contributing the largest proportion, reaching 47.9%. This finding aligns with the research trends identified by [PERSON] et al. [34,35] and other scholars.