## 3 Results and Discussion
### Variations of Solution pH with Operational Conditions  
Figure 3 shows the changes of solution pH measured in both feed wastewater and permeate produced by SDCMD process with changes of operating parameter. Experiments were performed under the permeate recirculation velocity of 0.5 m/s, mixing intensity of 600 rpm, feed temperature of 55 \({}^{\circ}\)C and permeate temperature of 20 \({}^{\circ}\)C for each experimental condition. As shown in Figure 3A, increasing a permeate recirculation velocity as mixing intensity, feed and permeate temperature are constant reduces a permeate pH from 7.1 to 6.7. This can be explained by the transport of CO\({}_{2}\) from the feed wastewater toward permeate side through hydrophobic MD membrane [41]. Since higher recirculation velocity through fiber lumen can mitigate the increase in permeate temperature during MD operation, it allows more CO\({}_{2}\) dissolved in MD permeate. Furthermore, mass transfer through the membrane walls can be improved by increasing permeate recirculation velocity. It has been reported that enhancing mass transfer allows the passage of volatile fatty acid (VFA) such as sodium acetate and sodium propionate in feed wastewater through membrane easily [42]. However, there was no significant impacts of mixing intensity on solution pH when other operational parameters  
Figure 2: Permeate flux changes with operational parameters (**A**) permeate recirculation velocity and mixing intensity, (**B**) trans-membrane temperature. were constant as shown in Figure 3B. This may be caused by the fact that mixing in feed wastewater through SDCMD process can reduce the concentration or thermal polarization on the outer surface of membrane more rather than mass transfer of CO2 towards the fiber lumen. The results shown in Figure 3C indicates that the increase of feed temperature leads to an increase of the solution pH in both feed wastewater and permeate as recirculation velocity and mixing intensity are 0.5 m/s and 600 rpm, respectively. However, as shown in Figure 3D, increasing the permeate temperature did not change the pHs in both feed wastewater and MD permeate. Ammonia transfer is driven by the difference in partial pressure of ammonia gas between feed wastewater and permeate. An explanation is that higher feed temperature can allow more ionization of the bicarbonate ions (HCO3-) into carbonate ions (CO3-), leading to increase a feed wastewater pH [43]. In addition, an increase in temperature of feed wastewater should facilitate the volatilization of ammonium ion (NH4+) into ammonia gas which can be passed through the hydrophobic membrane easily. With synthetic wastewater tested in this study, ammonia nitrogen can exist mainly as two forms of nitrogen consisting of volatile ammonia and ammonium ions. The solubility of ammonia in feed wastewater can be decreased by increasing its temperature. The vaporization of dissolved ammonia across the membrane should reduce the rejection of ammonium nitrogen. As a result, permeate pH should be increased due to the formation of ammonia nitrogen as shown below [44]. \[\text{NH}_{3}+\text{H}_{2}\text{O}\leftrightarrow\text{NH}_{4}{}^{+}+\text{OH} ^{-} \tag{5}\]  
Figure 3: Variations of pH in feed wastewater and permeate after 1 h of operation time with respect to the permeate recirculation velocity (**A**), mixing intensity (**B**), feed temperature (**C**) and permeate temperature (**D**) (background operational condition; permeate recirculation velocity of 0.5 m/s, mixing intensity of 600 rpm, feed temperature of 55 °C and permeate temperature of 20 °C). At the 25 \({}^{\circ}\)C feed temperature, the rate constant in forward reaction (k\({}_{\rm A}\)) in Equation (5) is \(1.8\times 10^{-5}\) while the constant in backward reaction (k\({}_{\rm B}\)) is \(5.6\times 10^{-10}\). This means that the rate of forward reaction with a formation of ammonium ion is almost \(3.2\times 10^{4}\) times faster than that occurring toward the formation of ammonia gas [45].