## 4 Results
### Packed Tower Air Stripping with Liquid Circulation  
To analyze the effect of liquid-to-gas contact efficiency on the stripping tower operating factors, the stripping performance was assessed using the packed tower in Figure 2. In the packed tower, the mass transfer interfacial area is a water film on the surface of packed materials. In this study, tower design, packed layer height, and liquid flow rate were constant as shown in Tables 2 and 3. In consequence, a relative comparison of mass transfer coefficients is possible using overall volumetric mass transfer coefficients. Stripping was performed on 100 L of artificial wastewater at ammonia concentrations of 1100 and 3700 mg/L with the packed tower. The liquid flow rate was 117.0 L/h and the gas flow rate was 26,200 L/h. The stripping results for ammonia concentration in the packed tower were shown in Figure 5.  
Figure 5a shows the ammonia removal rate over time. After 1 h, the removal rate reached 30.1% and 18.4%, in 1100 and 3700 mg/L solutions, respectively. The reason for the difference in ammonia removal rate is that pH of the 1100 mg/L solution decrease to 8. Thereafter, NaOH was used to maintain the pH at approximately 8.5, and the ammonia removal rate showed similar trends over total stripping time. Figure 5b shows the natural logarithm of the change in the ammonia concentration ratio. As a result of linear regression analysis against time, the R\({}^{2}\) values were relatively low (0.923-0.957), but reliable results were obtained. The slopes were similar between the two solution concentrations and the mass transfer coefficients of liquid ammonia were also similar. At 70 \({}^{\circ}\)C, pH 8.5, and 4.5 L/m\({}^{3}\) L/G ratio, the mean mass transfer coefficient was 0.192 \(\pm\) 0.003 h\({}^{-1}\). Stripping using the packed tower exhibited similar patterns regardless of the solution concentration as shown in Figure 3. Finally, the ammonia concentration of solution had little effect on the removal rate or mass transfer rate in both the stripping column and packed tower. To identify the optimal liquid-to-gas contact range for the tower, the stripping performance was compared at different gas flow rates in the range of 4.5-107.2 L/m\({}^{3}\). Figure 6a shows the removal rate for 1100 mg/L ammonium hydroxide solution at different L/G ratios. The ammonia removal rate tended to increase as the L/G ratio decreased. A decrease in the L/G ratio corresponds to an increase in the volume of air supplied per unit effective liquid capacity, which improves the ammonia removal efficiency [14; 15]. However, when the L/G ratio was excessively reduced (4.5 L/m\({}^{3}\) level here), airflow rate was too much, resulting in a reduction in ammonia removal efficiency. In Figure 6a, the removal rates  
Figure 5: Ammonia stripping performance according to ammonia concentration for the packed tower at 70 \({}^{\circ}\)C and pH 8.5: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. after 6 h were 57.1%, 63.0%, and 57.1% for L/G of 4.5, 26.8, 53.6 L/m\({}^{3}\), respectively. The most effective removal performance was shown at L/G 26.8 L/m\({}^{3}\). Figure 6b shows the natural logarithms of the relative concentration according to L/G ratios. Figure 6b indicates that the slope and mass transfer coefficients increased with decreasing L/G ratio, except for 4.5 L/m\({}^{3}\). The reason the mass transfer coefficient increases with reducing L/G is that contact efficiency improves at the liquid-to-gas interface due to increased airflow rate. Base on the L/G ratio of 107.2 L/m\({}^{3}\), when it decreases by 1/2 or 1/4, the gas flow rate increases by two or four times, and the mass transfer coefficient increased 1.4 or 1.6 times, respectively. However, when L/G reduced to 4.5 L/m\({}^{3}\), the gas flow rate increases significantly by 24 times, whereas the mass transfer coefficient increases by 1.5 times. As a result, it was derived that the mass transfer coefficient increased according to L/G decrease but decreases below a certain level.