Osmosis refers to the phenomenon that water molecules or other solvent molecules pass the semipermeable membrane (a membrane that allows the pass of solvent molecules, while not allowing the pass of solute molecules) from the solution of low concentration to the high concentration. Or it means the phenomenon that water molecules pass though the semipermeable membrane from the higher water to the lower. There are two conditions to cause the phenomenon of osmosis: one is a semipermeable membrane, and the other is the concentration difference existing on both sides of the semipermeable membrane.
2.Reverse osmosis principle
Reverse osmosis is a membrane separation operation that uses the pressure difference as the driving force to separate the solvents from the solution. Place the same volume of diluted solution (eg. fresh water) and concentrated solution (eg. seawater or salt water) on both sides of a container, with a semi-permeable membrane cutting-off in the middle. Thus the solvent in the diluted solution flows to the concentrated solution side through the semipermeable membrane, hence the level of liquid on the concentrated solution side will be higher than that of the diluted solution, forming a certain pressure difference and reaching the osmosis equilibrium state. Such pressure difference is called the osmotic pressure, and the osmotic pressure depends on the varieties of the solution, the concentration and the temperature, but has no relation to the nature of the semipermeable membrane. If a pressure greater than the osmotic pressure is imposed to the concentrated solution side, the solvent in the concentrated solution will flow into the diluted solution, and the solvent flow direction is opposite to the original direction of osmosis. This process is defined as reverse osmosis.
Imposing a certain pressure on the liquid on the membrane side, when the pressure exceeds its osmotic pressure, the solvent permeate against the direction of natural permeation. Thus, a permeated solvent is obtained on the low pressure side of the membrane, that is a permeate; a concentrated solution is obtained on the high pressure side of the membrane, that is a concentrate. When seawater is treated by reverse osmosis, fresh water is obtained on the low pressure side of the membrane, and brine is obtained on the high pressure side.
The reverse osmosis method can remove more than 90% of the dissolved salts and more than 99% of the colloidal microorganisms and organic substances in the water. The reverse osmosis is not only suitable for the seawater desalination, but also for the brackish water desalination. Among the existing desalination methods, desalination by reverse osmosis is considered the most economical, and even superior than the desalination by electrodialysis method. Because the driving force of the reverse osmosis process is the pressure, no phase change occurs in the process, and the membrane only plays the role of "sieving", so the process of separation by reverse osmosis consumes less energy. Reverse osmosis is the most efficient in energy saving among all existing methods of seawater and brackish water desalination.