## 2 Background and Methods
### Carbon Emission Accounting Boundaries and Methodologies
#### 2.2.2 Carbon Emission Accounting Methodologies
\(EF_{CH_{4}}\): Process-specific methane emission factor (kg CH4/kg COD). Measured values are preferred. When unavailable, recommended values range from 0.004 to 0.0075 kg CH4/kg COD, with higher values applicable for sludge-accumulating units. The median value of 0.00575 kg CH4/kg COD (from AAO process guidelines) was used here. (2) N2O Emissions from Nitrogen Transformation  
Direct N2O emissions during wastewater treatment were calculated using Equation (2):  
\[\mathrm{m_{N_{2}O}=Q\times(TN_{in}-TN_{eff})\times EF_{N_{2}O}\times 10^{-3} \times\frac{44}{28}} \tag{2}\]  
where:  
\(m_{N_{2}O}\): Annual direct N2O emission (kg N2O/a). \(\mathrm{TN_{in}/TN_{eff}}\): Influent/effluent total nitrogen concentration (mg/L). \(EF_{N_{2}O}\): N2O emission factor (kg N2O/kg N). Priority should be given to values derived from mechanistic models or continuous monitoring. When neither is feasible, the default value of 0.01 kg N2O/kg N may be applied. Carbon emissions from fossil carbon mineralization during wastewater treatment were quantified using Equation (3), while those from external carbon source mineralization were calculated using Equation (4):  
\[\mathrm{m_{FOO_{2}}=Q\times(COD_{in}-COD_{eff})\times EF_{CO_{2}}\times FCF \times 10^{-3}} \tag{3}\]  
where:  
\(m_{FOO_{2}}\): Annual fossil carbon-derived CO2-equivalent emission (kg CO2-eq/a). \(\mathrm{EF_{CO_{2}}}\): Emission factor for biochemical CO2-equivalent. emissions (kg CO2-eq/kg COD). Recommended range: 0.366-0.560 kg CO2-eq/kg COD; 0.463 kg CO2-eq/kg COD used here. FCF: Fossil carbon fraction in influent organic matter (%), ranging from 5% to 20% (10% adopted here). \[\mathrm{m_{eCO_{2}}=EF_{CO_{2}}\times M_{e}\times r} \tag{4}\]  
where: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Process Type** & \
\begin{tabular}{c} **CH4 Emission Factor** \\ **//(kgCH4\_kgCOD^-1)** \\ \end{tabular} &
\begin{tabular}{c} **N2O Emission Factor** \\ **//(kgN2O\_kgN^-1)** \\ \end{tabular} \\ \hline AAO & 0.0097 & 0.0045 \\ SBR & 0.0042 & 0.0276 \\ OD & 0.0092 & 0.0017 \\ AO & 0.0027 & 0.0115 \\ UCT & 0.0180 & 0.0005 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Emission factors of CH4 and N2O for WWTPs with different treatment processes. \(\rm m_{eCO_{2}}\): Annual external fossil carbon-derived CO\({}_{2}\) emission (kg CO\({}_{2}\)-eq/a). \(\rm M_{e}\): Annual external carbon source dosage (kg/a). \(\rm r\): Effective carbon content (%). Assuming two-thirds of external carbon is catabolized and one-third is anabolized, direct CO\({}_{2}\) emission factors for common carbon sources (glucose, sodium acetate, acetic acid, methanol) were calculated using Equations (5)-(8):  
\[\rm EF_{cH_{2}O_{6}}=\frac{2}{3}\times 6\times\frac{44}{180}=0.98(kgCO_{2}/kgC _{6}H_{12}O_{6}) \tag{5}\]  
\[\rm EF_{CH_{3}COON_{4}}=\frac{2}{3}\times 2\times\frac{44}{82}=0.72(kgCO_{2}/kgCH_{3} COON_{a}) \tag{6}\]  
\[\rm EF_{CH_{3}COOH}=\frac{2}{3}\times 2\times\frac{44}{60}=0.98(kgCO_{2}/kgCH_{3}COOH) \tag{7}\]  
\[\rm EF_{CH_{3}OH}=\frac{2}{3}\times 1\times\frac{44}{32}=0.92(kgCO_{2}/kgCH_{3} OH) \tag{8}\]  
Total CO\({}_{2}\)-equivalent emissions from biochemical reactions (\(\rm M_{CO_{2}}\)) were calculated using Equation (9):  
\[\rm M_{CO_{2}}=m_{fCO_{2}}+m_{eCO_{2}}+m_{CH_{4}}\times 28 \tag{9}\]  
where:  
\(M_{CH_{4}}/M_{N_{2}O}\): \(\rm CH_{4}/N_{2}O\)-equivalent CO\({}_{2}\) emissions (kg CO\({}_{2}\)-eq/a).