Reverse Osmosis (abbreviation: RO, from Reverse Osmosis), also called hyperfiltration, is the process in which the passage of solvent molecules from the more concentrated solution to the less concentrated solution is forced by applying a pressure greater than the osmotic pressure to the more concentrated solution.
In practice, Reverse Osmosis is accomplished with a membrane that retains the solute on one side preventing its passage and allowing pure solvent to be obtained from the other side. This phenomenon is not spontaneous and requires the performance of as much mechanical work as is needed to cancel the effect of osmotic pressure.

This process represents the finest water filtration technique in that it does not simply consist of a physical obstacle, determined by pore size, to the passage of molecules, but exploits the different chemical affinity of the species with the membrane, in fact allowing the passage of hydrophilic (or water-like) molecules, that is, chemically similar to water, for example, short-chain alcohols. Plant-wise, the method uses the principle of tangential filtration, as well as other membrane separation techniques such as microfiltration, ultrafiltration and nanofiltration.

Reverse Osmosis is used in water treatment both for desalination and for the removal of trace amounts of phosphates, calcium and heavy metals, phytochemicals, radioactive materials and almost all polluting molecules.

In recent years, “zero liquid discharge" plants are being built in which the Reverse Osmosis section increases the concentration of chemical species in the wastewater to values close to or above their solubility (over-saturated solutions).

Thin film composite membranes (TFC or TFM) are used in the Reverse Osmosis process. These membranes are semipermeable and manufactured primarily for use in water purification or desalination systems. They also have uses in chemical applications such as batteries and fuel cells.

In essence, a TFC material is a molecular sieve constructed in the form of a film of two or more layered materials.

Membranes used in RO are generally made of polyamide, a substance chosen primarily for its water permeability and relative impermeability to various dissolved impurities, including salt ions and other small molecules that cannot be filtered out. Another example of a semipermeable membrane is the one used in dialysis.