Reverse osmosis (abbreviation: RO), also known as hyper filtration (abbreviation: HF), is the process by which solvent molecules pass from the most concentrated solution to the least concentrated solution. This is obtained by applying a pressure higher than the osmotic pressure to the most concentrated solution. In fact, the reverse osmosis is achieved through a membrane that retains the solute on one part, preventing its passage and to extract the pure solvent from the other part. This phenomenon is not spontaneous and it requires carrying out a mechanical work similar to that necessary to cancel the effect of the osmotic pressure.

This process represents the finest water filtration technique in the sense it does not consist simply of a physical obstacle, determined by the size of the pores, the passage of molecules, but rather exploiting the different chemical affinity of species with the membrane, allowing the passage of hydrophilic molecules (water-like). That is to say chemically similar to water such as short-chain alcohols. From the point of view of the facilities, 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 the treatment of water, both for desalination and for removing traces of phosphates, calcium and heavy metals, pesticides, radioactive materials and almost all pollutants.

In recent years, “zero liquid discharge" systems have been constructed in which the reverse osmosis section increases the concentration of chemical species present in the wastewater to values close to or higher than their solubility (supersaturated solutions).

In the reverse osmosis process, thin film composite membranes (TFC or TFM, Thin Film Composite Membrane) are used. These membranes are semi-permeable and are manufactured primarily for use in water purification or desalination systems. They are used even in chemical applications such as batteries and fuel cells.

A TFC material is essentially a molecular sieve constructed in the form of a film of two or more laminated materials.

The membranes used in osmosis are generally polyamide, a substance chosen primarily for its water permeability and impermeability to various dissolved impurities, including saline ions and other small molecules that cannot be filtered. Another example of a semi-permeable membrane is that used in dialysis.

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