## 4 Discussion
### Performance Prediction  
The concentration of odor gases varied with time. Therefore, the removal performance was predicted at different concentrations of H\({}_{2}\)S and NH\({}_{3}\). A numerical analysis was conducted by assuming three cases: (1) H\({}_{2}\)S and NH\({}_{3}\) were introduced at the same concentration of 1000 ppm, (2) there was a concentration of 1000 ppm for H\({}_{2}\)S and 500 ppm for NH\({}_{3}\), (3) and a concentration of 500 ppm for H\({}_{2}\)S and 1000 ppm for NH\({}_{3}\). A pH of 8 and a current density of 40 mA cm\({}^{-2}\) were applied. Figure 4 shows the results of the numerical analysis. The removal efficiency of gaseous H\({}_{2}\)S was determined regardless of the NH\({}_{3}\) concentration because H\({}_{2}\)S is oxidized prior to NH\({}_{3}\). In contrast, the NH\({}_{3}\) removal efficiency was determined as a function of the inlet concentrations of both NH\({}_{3}\) and H\({}_{2}\)S. When the inlet concentrations of H\({}_{2}\)S and NH\({}_{3}\) were reduced from 1000 to 500 ppm, there were no significant changes in the NH\({}_{3}\) removal performance. This is because NH\({}_{3}\) is mainly removed by absorption, whereas H\({}_{2}\)S is removed by absorption and oxidation. Consequently, the H\({}_{2}\)S removal performance can be improved by increasing the current density, regardless of the NH\({}_{3}\) concentration. On the contrary, NH\({}_{3}\) can be easily removed by absorption. In other words, the H\({}_{2}\)S removal rate would be improved by applying a higher current density, while the NH\({}_{3}\) removal rate would be enhanced by increasing the liquid-gas ratio. Despite applying a high current density, the aqueous concentration of ammonia increased linearly due to difficult oxidation. In the simulation results in which EWS operated under the loading rate of 83.46 g m\({}^{-3}\) h for H\({}_{2}\)S and 41.7 g m\({}^{-3}\) h for NH\({}_{3}\), the aqueous concentration of ammonia approached saturation after 10 days [11]. If the current density increased to higher than 80 mA cm\({}^{-2}\), the period needed to approach saturation was prolonged to approximately one month. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{2}{c}{**Sensitivity Index**} & \multirow{2}{*}{**Rank**} \\ \cline{2-3} \cline{5-4}  & \(-\)**20\%** & & \\ \hline K\({}_{\text{L}}\)aS & 0.2 & 5.2 & 2 \\ K\({}_{\text{S}}\) & 9.4 & 6.1 & 1 \\ H\({}_{\text{S}}\) & 0.7 & 2.9 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensitivity index for H\({}_{2}\)S removals. Figure 5 summarizes the results of the numerical analysis conducted when the current density was increased to 50-70 mA cm\({}^{-2}\) for shock loading so as to confirm the benefits of the EWS, which can flexibly correspond to an increase in the loading rate by increasing the current density. As the loading rate was increased by 1.5 times, the outlet concentration of H\({}_{2}\)S sharply increased to approximately 780 ppm of H\({}_{2}\)S, despite the current density of 40 mA cm\({}^{-2}\). The concentration of aqueous H\({}_{2}\)S was retained at 27 mg-S dm\({}^{-3}\). However, as soon as the current density increased to 50 and 60 mA cm\({}^{-2}\), the removal efficiency of H\({}_{2}\)S improved. When 70 mA cm\({}^{-2}\) was applied, all the H\({}_{2}\)S was absorbed and oxidized within 30 min of the shock loading injection. Although the inlet concentration increased by a factor of 1.5, there were no significant changes in the slopes of the gas-phase outlet concentration and liquid-phase concentration. The removal efficiency remained approximately similar, even when the current density increased to 60 mA cm\({}^{-2}\). This is because the aqueous H\({}_{2}\)S remained at a concentration of approximately 8 mg-S dm\({}^{-3}\), even at a current density of 60 mA cm\({}^{-2}\). As a result of the initiation of the oxidation reaction, the removal performance changed with the application of a current density of 70 mA cm\({}^{-2}\).