Osmosis refers to the phenomenon where solvent (water molecules) on the dilute solution side permeates through a semi-permeable membrane (reverse osmosis membrane or nanofiltration membrane) and enters the concentrated solution side under the drive of osmotic pressure.
Osmotic pressure is defined as the pressure difference between the concentrated solution and the dilute solution during the natural osmosis process. The liquid level rises continuously while the liquid level falls accordingly until the pressure potential energy of the water column on both sides equals the osmotic pressure potential energy of the solvent. At this point, the liquid levels on both sides of the solution no longer change, that is, the osmosis process reaches an equilibrium point.
The principle of reverse osmosis is that an operating pressure is applied to the concentrated solution side to overcome the osmotic pressure. When an operating pressure higher than the osmotic pressure is applied to the concentrated solution side, the natural flow direction of water molecules will be reversed, and water molecules will pass through the semipermeable membrane into the dilute solution side, which is reverse osmosis.
Figure 3.1 Schematic diagram of reverse osmosis
Nanofiltration membranes do not completely block dissolved salts, but rather have a unique selectivity. The ability of these solutes to pass through the nanofiltration membrane depends on the solute and the type of nanofiltration membrane.
