Here's a link to how a current membrane might be reconfigured. The vacuum (29 hg's ?) could be generated and maintained by pumping (drawn, possibly using a mechanical piston type pump, like one used for wells) water out of the top leaving very little air in it. Discharge pumps (both brine and desalinated) would draw sea water into membrane. Since membranes pores are so small, the vacuum above the water may separate the water so that individual molecules would be drawn through the membrane. If this works, then the vacuum acting on the water would cause the water's own molecules to lose their cohesion. http://i979.photobucket.com/.../reconfig_zps96c7b1d9.jpg What might be missed with vacuum is that having an extreme vacuum above the water would not effect it's flow, just it's physical characteristics. And while a true vacuum pump might lower the air pressure only, if a column of water were 2 meters tall and held 2 liters of water with an are of 20 cm^3 that it is taking suction on, drawing it down with a piston type pump would mean that if 20 cm^3 of water were pumped out, the air pressure would be 1/2.Evacuating water would create vacuum equal to the vol. of air it is expanding.