## 2 Materials and Methods
### Membrane Distillation and Crystallization Setup  
MD/MCr experiments were carried out by using polypropylene (PP) hollow fiber membranes (Membrane Accurel(tm) PP S6/2) assembled into a transparent plastic module. A detailed description of the membrane fibers and modules can be found in Table 3. MD and MCr setup was operated in direct contact configuration mode with feed in the lumen side of fibers (in counter current with permeate on the shell side). The feed side consisted of a feed tank with a stirrer. The feed solution was pumped with a peristaltic pump (Masterflex L/S) from the feed tank through a heater followed by the membrane module and back into the feed tank. Permeate side pumps distillate water from the permeate tank through a cooler and into the membrane module and back into the permeate tank. The temperature is measured at the membrane inlet and outlet at the feed and permeate sides. A schematic diagram of the MD/MCr setup has been presented in Figure 1.  
The experiments were conducted both using distillate water and the supernatant from the centrifuged wastewater described in Table 2 as the feed solutions. Feed temperatures at \(\sim\)38 \({}^{\circ}\)C, \(\sim\)47 \({}^{\circ}\)C, and \(\sim\)56 \({}^{\circ}\)C with a flow rate of 21.6 L/h (corresponding to feed and permeate velocities of 1.2 m/s and 0.8 m/s, respectively) and permeate temperature of \(\sim\)15 \({}^{\circ}\)C were used in the experiments. Feed and permeate flow rates of 14.8 L/h, 21.6 L/h, and 30.7 L/h (corresponding to feed velocities of 0.8 m/s, 1.2 m/s, and 1.7 m/s, respectively) were tested while fixing the feed and permeate temperature at \(\sim\)47 \({}^{\circ}\)C and \(\sim\)15 \({}^{\circ}\)C, respectively. The experimental time varied from 40 to 100 hours for each test, depending upon the operating conditions applied; therefore, all of the experiments have been conducted only once. The feed solution was concentrated until precipitation, where the crystallization occurred in the feed tank. The crystallization process was continuously monitored by using a ParticleView V19 microscope (PVM) inserted into the feed tank. When crystals start appearing in the feed tank, a sample was taken from the feed tank to analyze the crystals through an optical microscope. The crystals were recovered by filtering off the solution after the completion of the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**pH** & 7.36 \\
**Redox potential [mV]** & 102 \\
**Conductivity [mS/cm]** & 37.61 \\
**Dry matter [g/L]** & 64.4 \(\pm\) 0.4 \\
**Inorganic matter [\%]** & 88.7 \\
**Organic matter [\%]** & 11.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of wastewater solution before centrifugation.  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Supernatant [mg/L]** \\ \hline
**Dry matter [g/L]** & 43.3 \\
**Inorganic matter [\%]** & 97.9 \\
**Organic matter [\%]** & 2.1 \\
**Conductivity [ms/cm]** & 39 \\
**Mn2+** & 3.648 \\
**Fe2+** & 6 \\
**Mg2+** & 7.813 \\
**P3-** & 90 \\
**Na*** & 2302 \\
**SO42–** & 18990 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Composition of supernatant after centrifugation of the wastewater.  
experiments for X-ray diffraction (XRD), ICP-OES, and scanning electron microscopy (SEM) analysis. Further details about these procedures have been provided in Section 2.5.