Reverse Osmosis (RO)

In the ordinary osmosis apparatus of section 9.9.4, pure water spontaneously moves through the semipermeable membrane into the NaCl solution side. This natural direction of flow can be reversed by applying an external pressure -- greater than the osmotic pressure -- to the solution side. Once that happens, pure water instead moves from the solution side back to the solvent side. This reversed process is called reverse osmosis (RO): the passage of solvent through a semipermeable membrane in the direction opposite to ordinary osmosis, driven by a hydrostatic pressure that exceeds the osmotic pressure.

Application: water purification (Figure 9.15). Reverse osmosis is widely used both to desalinate sea water and to purify drinking water. When a pressure higher than the osmotic pressure is applied on the solution side (e.g. sea water), water molecules are forced through the semipermeable membrane to the solvent side -- opposite to the natural osmotic-flow direction -- and pure water can be collected on that side. …