A membrane that selectively allows substances to pass through is called a semipermeable membrane. A membrane that allows only the solvent to pass through but not the solute is generally called an ideal semipermeable membrane. When equal volumes of a dilute solution (e.g., fresh water) and a concentrated solution (e.g., salt water) are placed on opposite sides of a semipermeable membrane, the solvent in the dilute solution will naturally flow through the membrane to the concentrated solution side. This phenomenon is called osmosis.
When osmosis reaches equilibrium, the liquid level on the concentrated solution side will be higher than that on the dilute solution side, creating a pressure difference called osmotic pressure. The magnitude of osmotic pressure depends on the inherent properties of the solution, namely the type, concentration, and temperature of the concentrated solution, but not on the properties of the semipermeable membrane. If a pressure greater than the osmotic pressure is applied to the concentrated solution side, the solvent will flow in the opposite direction to the original osmotic direction, starting from the concentrated solution to the dilute solution side. This process is called reverse osmosis.
Reverse osmosis is a reverse migration process of osmosis. It is a separation method that uses the selective retention of a semi-permeable membrane to separate the solute and solvent in a solution under pressure. It has been widely used in the purification and concentration of various liquids. The most common application is in water treatment processes, where reverse osmosis technology is used to remove impurities such as inorganic ions, bacteria, viruses, organic matter and colloids from raw water to obtain high-quality pure water.