Reverse osmosis (RO) membranes feature the smallest pores and involve, appropriately enough, the reversal of osmotic pressure in order to drive water away from dissolved molecules. Strictly speaking, RO is not a size exclusion process based on pore size; it depends on ionic diffusion to affect separation. One of its common applications is seawater desalination, in which pure water is produced from a highly saline feed stream, similar to evaporation with far better economy.
RO is also used in cheese whey concentration, fruit juice concentration, ice making, car wash reclamation, wastewater volume reduction, and other industrial processes, with the goal of producing a pure filtrate (typically water) or retaining the components. Because the osmotic pressure of many process streams is quite high, RO membranes must operate at pressures of 400-1,200 psi (29-83 bars), which restricts available membrane geometries.