## 3 Results and Discussion
### MD/MCr Tests with the Wastewater  
The flux for the MCr tests, which were performed for the centrifuged wastewater at different feed temperatures, is shown in Figure 3. Similar to the pure water flux, the flux increased with temperature. The mean flux was measured to be 0.95 \(\pm\) 0.2 L/m\({}^{2}\cdot\)h, 1.50 \(\pm\) 0.2 L/m\({}^{2}\cdot\)h, and 2.43 \(\pm\) 0.2 L/m\({}^{2}\cdot\)h at feed inlet temperatures of 38 \({}^{\circ}\)C, 47 \({}^{\circ}\)C, and 56 \({}^{\circ}\)C, respectively. Even at high recovery factors (>60%), the flux decline was more evident only at a low feed inlet temperature (37 \({}^{\circ}\)C). This observation is consistent with the literature [10], and can be associated with the relatively high viscosity of the used solution at low temperature, which increases the concentration polarization [22], and consequently, possible salt accumulation at the membrane surface. The conductivity of the permeate (Figure 3b) increased slightly at the beginning of the experiments, but remained constant during the rest of the time. For all of the conducted experiments, the final permeate conductivity remained below 250 \(\upmu\)S/cm (corresponding to a minimum rejection toward conductivity of 99.4%), which indicates no or only a partly wetting of the membrane. No clear tendency of permeate conductivity with respect to the driving force (i.e., temperature) was observed during the experiments. In general, the permeate quality is of good standard. Furthermore, this is confirmed by analyzing the composition of the permeate stream (Table 4), which indicates that for all of the conducted experiments, only small amounts of impurities passed through the membrane, which was possibly due to the wetting of some of the membrane pores. MD and MCr tests have also been carried out on the wastewater under various hydrodynamic conditions (Figure 4a). The maximum flux was observed at the highest flow rate. It is well acknowledged that a high feed velocity decreases the temperature polarization and temperature drop along the module in MD [23; 24], which results in a higher flux, as is also observed for the studies on pure water flux. However, there was no significant difference in the flux measured at feed flow rates of 14.8 L/h and 21.6 L/h. The feed inlet temperature for the experiment performed at 14.7 L/h (47.7 \(\pm\) 0.3 \({}^{\circ}\)C) was slightly higher than for that at 21.6 L/h (47.2 \(\pm\) 0.3 \({}^{\circ}\)C). This slight difference in both the temperatures has possibly narrowed down the difference between the observed flux at these \begin{table}
\begin{tabular}{c c} \hline  & **Concentration [mg/L]** \\ \hline Mn\({}^{2+}\) & 0.0043 \\ Mg\({}^{2+}\) & 0.076 \\ P\({}^{3-}\) & 0.0367 \\ Na\({}^{+}\) & 16.6 \\ \hline \end{tabular}
\end{table} 
Table 4: Maximum concentration of ions in the permeate at the end of experiments. The initial concentration was below the detection limit. Figure 3: **(a)** Flux and **(b)** conductivity of permeate as a function of water recovery factor at different feed temperatures, and a flow rate of 21.6 L/h. two flow rates. As expected, the flux decreases slightly with the recovery factor, due to decrease in the water activity and potential scaling. The reason for the disruption in the flux for the higher flow rate is that this experiment was conducted overnight without automatic recording of the mass changes of the permeate. The dependence of conductivity of the permeate does not show any clear trend with respect to the flow rate, and remains below 240 \(\upmu\)S/cm (corresponding to a minimum rejection toward conductivity of 99.4%) in all of the cases. For all of the conducted experiments, a water recovery factor between 84-92% was achieved. This highlights the possibility of minimizing the waste stream with at least a factor of six; simultaneously, this has been converted into a pure water stream. Moreover, precipitation occurred in all of the conducted experiments, potentially producing another valuable resource from the wastewater.