A membrane that is selective for the substance to be permeated is called a semi-permeable membrane. Generally, a membrane that can only pass through a solvent but not a solute is called an ideal semi-permeable membrane. When the same volume of dilute solution (such as fresh water) and concentrated solution (such as salt water) are placed on both sides of the semi-permeable membrane, the solvent in the dilute solution will naturally pass through the semi-permeable membrane and flow spontaneously to the side of the concentrated solution. This phenomenon is called penetration. When the osmosis reaches equilibrium, the liquid level on the side of the concentrated solution will be higher than the liquid level of the dilute solution by a certain height, that is, a pressure difference is formed, and this pressure difference is the osmotic pressure. The size of the osmotic pressure depends on the inherent properties of the solution, that is, it is related to the type, concentration and temperature of the concentrated solution and has nothing to do with the properties of the semipermeable membrane. If a pressure greater than the osmotic pressure is applied on the side of the concentrated solution, the flow direction of the solvent will be opposite to the original permeation direction, and start to flow from the concentrated solution to the dilute solution. This process is called reverse osmosis. Reverse osmosis is a reverse migration movement of osmosis. It is a separation method that separates the solute and the solvent in the solution by means of the selective retention effect of a semipermeable membrane under pressure. It has been widely used in various liquids. Purification and concentration, the most common application example is in the water treatment process, the use of reverse osmosis technology to remove impurities such as inorganic ions, bacteria, viruses, organic matter and colloids in raw water to obtain high-quality purified water.
