## 3 Results and Discussion  
### Effect of Permeate Recirculation Velocity and Trans-Membrane Temperature  
Figure 2A illustrates the permeate flux with respect to the permeate recirculation velocity at feed and permeate temperature of 55 and 20 \({}^{\circ}\)C, respectively. The permeate recirculation velocity was calculated by dividing recirculating permeate flow rate by cross-sectional area of fiber lumen. The results show that permeate flux increases with increasing permeate recirculation velocity but approaches an asymptotic value when the velocity is higher than 0.5 m/s. An increase in the permeate recirculation velocity could prevent temperature drop in MD permeate while it was flowed through fiber lumen. Thus, vapor pressure difference can be maintained so that it can result in stable permeate flux [31; 32]. As the permeate recirculation velocity increases, the Reynolds number (Re) also increases along the hollow-fiber membrane. When the permeate recirculation velocity increased from 0.2 to 0.8 m/s, the value of Re was increased from 332 to 1162 based upon water density and viscosity at 20 \({}^{\circ}\)C. Since the conductivity in MD permeate was very low (less than 30 \(\upmu\)S/cm) as observed in this study, the experimental error in estimating Re at 20 \({}^{\circ}\)C was within 0.2% only. The permeate flux increased proportionally with Re number, but the flux approached to the steady value as it was higher than 664. The tendency of experimental result shows that the thickness of thermal boundary layer in permeate stream is reduced by increasing the permeate recirculation velocity even under laminar region (Re \(<\) 2100). As a result, an increase of permeate recirculation velocity enhances transverse vapor flux through membrane [33; 34; 35]. Figure 2A also shows the effect of mixing intensity driven by the rotational speed using a magnetic bar in the feed tank on MD performances. Enhancing the mixing intensity of feed wastewater tends to reduce concentration and temperature polarization layer on membrane surface [36; 37]. Nevertheless, the beneficial effect of mixing intensity on the permeate flux was not observed as the rate was higher than 600 rpm. In the SDCMD process, inducing mixing intensity in feed wastewater should be the only way to control thermal polarization formed on membrane due to temperature gradient. Higher mixing in feed wastewater is associated with more turbulence on the membrane surface, thereby improving the permeate flux across membrane. However, there was no beneficial effect on enhancing permeate flux as the mixing intensity was higher than the critical value probably due to the limitation of the size of the wake region in fluid mixing which creates secondary flows and/or local mixing on membrane. The permeate fluxes through the hydrophobic membrane in SDCMD process vary dependent upon the feed temperature due to water vapor pressure change [38; 39]. Figure 2B shows that the permeate flux increases as trans-membrane temperature increases from 15 to 40 \({}^{\circ}\)C almost exponentially under constant permeate recirculation velocity and mixing intensity of 0.5 m/s and 600 rpm, respectively. The extent to which the enhancement of permeate flux was more pronounced by higher feed temperature rather than that at lower permeate temperature. Permeate flux increased linearly with the permeate temperature as the feed temperature was fixed at 55 \({}^{\circ}\)C, indicating that controlling feed temperature should affect permeate flux more dominantly. The theoretical value of permeate flux was calculated by using the MD coefficient from the preparatory experiment using DI water under the corresponding operational condition with Equations (3) and (4). The mass transfer coefficient was estimated as an indicator to observe the impact of temperature on permeate flux using Equation (3) as below.