Reverse osmosis, also known as reverse osmosis, is a membrane separation process that uses pressure difference as the driving force to separate the solvent from the solution.
Pressure is applied to the feed solution on one side of the membrane. When the pressure exceeds its osmotic pressure, the solvent will permeate in the opposite direction of natural osmosis. Thus, the permeated solvent, i.e., permeate, is obtained on the low-pressure side of the membrane, while the concentrated solution, i.e., concentrate, is obtained on the high-pressure side. If reverse osmosis is used to treat seawater, fresh water is obtained on the low-pressure side of the membrane, and brine is obtained on the high-pressure side.
During reverse osmosis, the solvent permeation rate, i.e., the liquid flow energy N, is: N = Kh(Δp - Δπ), where Kh is the hydraulic permeability coefficient, which increases slightly with increasing temperature; Δp is the hydrostatic pressure difference across the membrane; and Δπ is the osmotic pressure difference of the solution across the membrane. The osmotic pressure π of a dilute solution is: π = iCRT, where i is the number of ions generated by the ionization of the solute molecule; C is the molar concentration of the solute; R is the molar gas constant; and T is the absolute temperature. Reverse osmosis typically uses asymmetric membranes and composite membranes. The equipment used for reverse osmosis is mainly hollow fiber or spiral wound membrane separation equipment.
Reverse osmosis membranes can retain various inorganic ions, colloidal substances, and large molecular solutes in water, thereby obtaining purified water. It can also be used for the pre-concentration of solutions containing large molecular organic matter. Due to its simplicity and low energy consumption, reverse osmosis has developed rapidly in the past 20 years. It is now widely used in seawater and brackish water (see brine) desalination, boiler water softening, and wastewater treatment. Combined with ion exchange, it is used to produce high-purity water, and its application scope is expanding, now including the concentration of dairy products and fruit juices, as well as the separation and concentration of biochemical and biological agents.