Principle Of Reverse Osmosis System

The reverse osmosis system is a membrane separation technology developed in the 1960s. This technology has developed rapidly and has been widely used in fields such as water desalination and desalination, preparation of high-purity water and pure drinking water. Most reverse osmosis membranes have a pore size of 10×10-10m, and their separation objects are dissolved ions and organic matter with a molecular weight of several hundred or more.

A membrane that can only pass through solvent but not solute is often called an ideal semipermeable membrane. When a solvent and a solution (or two solutions of different concentrations) are placed on both sides of this membrane, the pure solvent will naturally pass through the semipermeable membrane and flow spontaneously to one side of the solution (or from a low solution to a high concentrated solution). This phenomenon is called osmosis. When the osmotic process reaches the surface of the solution, pressure is generated to counteract the tendency of the solvent to flow toward the solution, that is, equilibrium is reached. This pressure is called the osmotic pressure of the solution.

Osmotic pressure depends on the type, concentration and temperature of the solution and has nothing to do with the membrane itself. In this case, if an external pressure greater than the osmotic pressure is applied to the solution surface, the solvent will flow from the solution to the solvent side in the opposite direction to the original osmotic pressure. This phenomenon is called reverse osmosis. Any separation method that concentrates or purifies a solution based on this principle is generally called a reverse osmosis process.

Reverse osmosis is the reverse migration movement of osmosis. Mainly driven by pressure, the solvent is forced to separate from the solute through the interception of the semipermeable membrane. The higher the concentration of the solution, the greater the osmotic pressure. In the reverse osmosis process, the applied pressure must be much greater than the osmotic pressure of the solution, generally several times to nearly dozens of times, within the range allowed by the system and membrane strength.

When the aqueous salt solution comes into contact with the surface of the porous semipermeable membrane, a layer of water molecules is selectively adsorbed on the solution interface of the membrane. Under the action of reverse osmosis pressure, pure water flows out of the membrane through capillary action. And the pure water that forms the interface continuously flows out.

For the removal of organic matter, it belongs to the screening mechanism. Therefore, it is related to the molecular weight and shape of the organic matter. In the ultrafiltration range, membranes with larger pore sizes are generally used, which are called ultrafiltration membranes. The pore size of ultrafiltration membrane is 2-10 nanometers, while the pore size of reverse osmosis membrane is 0.3-2 nanometers.

Therefore, reverse osmosis membrane filtration can better remove various bacteria, such as the smallest bacteria "Pseudomonas aeruginosa" (3000×10-10M); it can also filter out influenza viruses (800×10-10M), pyrogens (10-500×10-10m) and other viruses.