Osmosis is a phenomenon in which a solvent flows between two solutions separated by a semipermeable membrane. This process is generally driven by concentration differences but can also result from electrical potential differences between two electrodes immersed in the solutions (electroosmosis) or temperature differences (thermo-osmosis).

The less concentrated fluid flows toward the one with a higher concentration of dissolved solids until equilibrium is reached between the two solutions. This phenomenon is known as osmotic pressure.

Reverse osmosis, used in water treatment, artificially reverses this process. It is a method of separating a solute from a solvent by applying external pressure to the membrane, reversing the osmotic flow. This causes the solvent to move from the more concentrated solution to the less concentrated one.

In practice, water with a higher concentration of dissolved solids flows toward the less concentrated solution. The result is the separation into two distinct fluids: one highly concentrated, containing all the residues, and the other consisting of demineralized water.

Reverse osmosis is the most refined water filtration technique and is used to remove a wide range of contaminants from water, including salts, pathogens, synthetic organic compounds, pesticides, and most other pollutants.