Reverse osmosis (abbreviation: RO, from the English Reverse Osmosis), also called hyperfiltration (abbreviation: IF), is the process in which the passage of solvent molecules from the more concentrated solution to the less concentrated solution obtained by applying to the solution is forced more concentrated a higher pressure than osmotic pressure.
In practice, reverse osmosis is carried out with a membrane that retains the solute on one side, preventing its passage and allowing the pure solvent to be obtained on the other. This phenomenon is not spontaneous and requires the completion of a mechanical work equal to that necessary to cancel the effect of osmotic pressure.
This process represents the finest water filtration technique, as it does not simply consist of a physical obstacle (determined by the size of the pores) to the passage of the molecules, but exploits the different chemical affinity of the species with the membrane, allowing the passage hydrophilic (or water-like) molecules, i.e. chemically similar to water (e.g. short-chain alcohols). From the plant engineering point of view, the method exploits the principle of tangential filtration, as well as other separation techniques using membranes such as microfiltration, ultrafiltration and nanofiltration.
Reverse osmosis is used in water treatment, both for desalination and for the removal of traces of phosphates, calcium and heavy metals, as well as pesticides, radioactive materials and almost all polluting molecules.
In recent years, “zero liquid discharge” plants have been built in which the reverse osmosis section increases the concentration of chemical species present in the waste water to values close to or higher (supersaturated solutions) than their solubility.
In the reverse osmosis process thin film composite membranes (TFC or TFM, Thin Film Composite Membrane) are used.
These membranes are semi-permeable 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 osmosis are generally made of polyamide, chosen mainly for its water permeability and relative impermeability to various dissolved impurities, including saline ions and other small molecules that cannot be filtered.
Another example of a semipermeable membrane is that used in dialysis.
