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Membrane distillation (MD) uses a microporous hydrophobic membrane to separate dissolved molecules from a liquid stream. Notwithstanding the great potential, membrane distillation is not applied on an industrial level yet, because of the lack of specifically developed membranes, modules, and techno-economic data at full scale. This review gives a comprehensive overview of the optimal membrane properties and can serve as a guideline for the development of new membranes, specifically for membrane distillation. Optimization of the membrane is needed to sufficiently resist wetting. Generally, a pore diameter of 0.3 μm is recommended to balance between a high liquid entry pressure and flux. Since vacuum membrane distillation is more sensitive to wetting, a smaller pore diameter could be appropriate for this configuration to avoid membrane wetting. An optimal membrane thickness is found between 10 and 700 μm, depending on process conditions, balancing between mass transport and energy loss. To improve the mass transfer and energy efficiency, membrane porosity should preferably be as high as possible (>75%), while low tortuosity (1.1–1.2) and thermal conductivity (>0.06 w·m–1·K–1) are recommended as well.

Membrane distillation is an emerging thermal membrane technology for the separation of salts and other non-volatile inclusions from water streams. The process offers a solution for the treatment of concentrated solutions, which are out of the scope of reverse osmosis. However, only few studies focused on the optimal membrane properties and operational conditions in the high concentration regime. In this paper, membranes with variations in thickness, porosity and structure are experimentally investigated in direct contact membrane distillation (DCMD) as well as simulated, using the Dusty Gas Model. Operational conditions, including the temperature difference over the membrane, the flow velocity and the feed stream salinity up to saturation are varied. It is confirmed that for pure water, thinner membranes show higher fluxes, while energy efficiency is unaffected by membrane thickness. At higher salinities, an optimal membrane thickness depending on membrane parameters and process conditions exists. The optimal membrane thickness is calculated in this article for concentrations of NaCl ranging from 0 up to 320 g/l and variations in bulk temperature difference and flow velocities for four different membranes. In this paper, membranes with thickness ranging from 20 -188 μm, variations in porosity and different structure are experimentally investigated and simulated using the Dusty Gas Model.Multiple process conditions are used for exploring the entire solubility range of NaCl. In addition, the optimal membrane thickness is computed, depending on process conditions and membrane structure. Furthermore, guidelines are proposed for the choice of membrane and operational conditions to optimize the DCMD process.All the authors mutually agree on submitting our manuscript to Journal of Membrane Science and the manuscript is an original work of the authors. Moreover, none of the work described in this manuscript has been submitted earlier to Journal of Membrane Science. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 INFLUENCE OF MEMBRANE THICKNESS AND PROCESS CONDITIONS ON DIRECT CONTACT MEMBRANE DISTILLATION AT DIFFERENT SALINITIES AbstractMembrane distillation is an emerging thermal membrane technology for the separation of salts and other non-volatile inclusions from water streams. The process offers a solution for the treatment of concentrated solutions, which are out of the scope of reverse osmosis.However, only few studies focused on the optimal membrane properties and operational conditions in the high concentration regime. In this paper, membranes with variations in thickness, porosity and structure are experimentally investigated in direct contact membrane distillation (DCMD) as well as simulated, using the Dusty Gas Model. Operational conditions, including the temperature difference over the membrane, the flo...