A membrane separation operation in which a solvent is separated from the solution with a pressure difference as a driving force. Pressure is applied to the material of the membrane side, and when the pressure exceeds its osmotic pressure, the solvent will reverse the reverse direction of the natural permeation. So that a permeated solvent, i.e., a permeated liquid, is obtained on the low pressure side of the membrane; and a concentrated solution, i.e., a concentrated liquid, is obtained on the high pressure side. If the seawater is treated with reverse osmosis, fresh water is obtained on the low pressure side of the membrane and brine is obtained on the high pressure side.
The same volume of dilute solution (such as fresh water) and concentrated solution (such as sea water or salt water) were placed on both sides of a container, the middle with semipermeable membrane barrier, dilute solution of the solvent will naturally through the semipermeable membrane, The liquid side of the solution side, the liquid level on the solution side will be higher than the liquid level of the dilute solution to form a pressure difference to reach the osmotic equilibrium state. This pressure difference is the osmotic pressure. The size of the osmotic pressure is determined by the concentration Species, concentration and temperature, independent of the nature of the semipermeable membrane. If a pressure greater than the osmotic pressure is applied to the concentrated solution side, the solvent in the concentrated solution flows to the dilute solution, and the flow direction of the solvent is opposite to that of the original infiltration. This process is called reverse osmosis.