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Membrane distillation is a membrane separation/concentration process which has vapor pressure (temperature) gradient/ difference between the feed and the permeate side of a hydrophobic membrane as the driving force for the process, in other words, Membrane distillation is a thermally driven separation process in which separation is enabled due to phase change. A hydrophobic membrane acts as a barrier for the liquid phase, letting the vapor phase (e.g. water vapor) pass through the membrane's pores. In simple terms, heated salt water (feed) flows across one side and chilled salt free water (permeate) flows across the other side of a hydrophobic membrane at pressures less than the breakthrough pressure of the membrane. Liquid Water does not penetrate the pores of the membrane since it is hydrophobic. The vapor pressure of salt water is lower than the vapor pressure of salt free water. This vapor pressure differential encourages salt water to vaporize. Pure water vapor diffuses across the hydrophobic membrane from the feed to the permeate side and condenses into the chilled permeate stream. The PMI Membrane distillation machine is capable of measuring rate of increase in volume of the permeate stream under a wide variety of experimental conditions including pressure, circulation flow rate and temperature. Quality of feed and permeate can be compared using conductivity measurements of both streams. Clean permeate may be collected in flowing water, in flowing gas or under vacuum.