## 4 Results  
### Air Stripping without Liquid Circulation  
Ammonia stripping was performed using the stripping column in Figure 1. In the column reactor, the contact surface between liquid and gas is the interface of the air bubble [14]. The mass transfer interfacial area is related to the size and number of air bubbles. Therefore, it is possible to compare the mass transfer coefficient in the constant conditions: total liquid volume; air flow rate; temperature [14]. As an experimental condition, the airflow rate was constant of 900 L/h on 1 L of artificial wastewater. The stripping efficiency and mass transfer rate of ammonia were analyzed according to initial concentration and pH. Ammonia removal efficiency and mass transfer coefficient were calculated by Equations (10), (15), and (16). Figure 3 shows the results of stripping at different ammonium hydroxide concentrations at 20 \({}^{\circ}\)C and pH 9.4. Figure 3a shows the ammonia removal rate according to the concentration. The removal rate increased with increasing time. After 3 h, similar results of 38.4%, 38.5%, and 38.3% were observed for concentrations of 1500, 2200, and 2900 mg/L, respectively. Figure 3b shows the natural logarithm of the relative change in total ammonia concentration over time. A linear regression analysis was conducted against time; the mean slope was 0.16 \(\pm\) 20.011. The R\({}^{2}\) values were 0.980, 0.973, and 0.996 for each respective concentration, indicating that the results were reliable. The overall volumetric mass transfer coefficients on the liquid phase (\(K_{L,\ NH_{3}}a\)) were calculated from the slope of Figure 3a and Equation (16). The mean \(K_{L,\ NH_{3}}a\) was 0.319 h\({}^{-1}\), which remained constant regardless of the initial ammonia concentration in the solution. Therefore, the ammonium hydroxide concentration had little effect on the removal efficiency or rate. The change of pH in the ammonia solution causes a change of ammonia fraction, which means that the higher pH, the more ammonia can be removed. pH is an important parameter affecting the stripping efficiency and mass transfer coefficient. \begin{table}
\begin{tabular}{c c c} \hline
**Design Parameter** & **Value** & **Unit** \\ \hline Tower diameter & 0.2 & m \\ Tower height & 1.5 & m \\ Packed layer height & 0.48 & m \\ Packed materials & PP pall ring & - \\ Liquid tank volume & 147 & L \\ \hline \end{tabular}
\end{table} 
Table 2: Design parameters of the stripping tower. \begin{table}
\begin{tabular}{c c c c} \hline
**Initial NH\({}_{3}\) Conc.** & **Liquid Flow Rate** & **Gas Flow Rate** & **Liquid-Gas Ratio, L/G** \\
**(mg/L)** & **(L/h)** & **(L/h)** & **(L-Liquid/m\({}^{3}\)-Gas)** \\ \hline
1100–3700 & 117.0 & 26,200 & 4.5 \\
1100 & 117.0 & 4367 & 26.8 \\
1100 & 117.0 & 2183 & 53.6 \\
1100 & 117.0 & 1092 & 107.2 \\ \hline \end{tabular}
\end{table} 
Table 3: Experimental conditions of NH\({}_{3}\) stripping using a packed tower. Figure 4 represents the results of ammonia stripping according to pH at 2900 mg/L ammonia concentration and 20 \({}^{\circ}\)C. The ammonia removal rate was shown as a linear increase over time; after 2.5 h of stripping, the removal rate increased from 13.9% to 72.6% when pH was increased from 8.9 to 10.8. To analyze the influence of pH on ammonia removal, the ratio of removal rate to ammonia fraction (\(\eta/f_{NH_{3}}\)) was compared. \(\eta/f_{NH_{3}}\) was approximately constant for pH 9.4-10.8; The ratios were 72.0%, 71.2%, and 75.2% at pH 9.4, 10.2 and 10.8, respectively. From these results, it was confirmed that the ammonia removal rate was linearly affected by the ammonia fraction according to the pH change. Figure 4b shows the natural logarithm of the relative change in total ammonia concentration against time; the slopes for each pH value were obtained by linear regression (R\({}^{2}\) = 0.997, 0.980, 0.993, 0.989). As a result, the slope and mass transfer coefficient increased proportionally with increasing pH. The amount of NaOH which was used to adjust the pH was determined.