Reverse osmosis is based on the principle of osmosis, which can be observed, e.g., in burst cherries after a summer rain. Inside each cherry there is a concentrated solution: cherry juice, while raindrops are the dilute solution. The concentrations are gradually balanced through the skin of the cherry. The solvent passes through the cherry skin, the semipermeable membrane, into the cherry. The system seeks to achieve a balance between the two concentrations, but because the volume of the cherry is unable to increase, there is an increase in pressure. This pressure is the osmotic pressure. In nature, this increase in pressure causes the cherry to burst. Inverting this effect results in reverse osmosis.
It works by exerting pressure on the concentrated solution in order to overcome the osmotic pressure of this solution. The solvent passes back through the membrane and all of the soluble substances contained in water are removed as they remain on the concentrate side.
On an industrial scale, however, reverse osmosis systems are not dead-end filtration systems, but rather use cross-flow filtration, where the raw water inflow is split into the pure water, the permeate, with a percentage of 75 – 80 % and a residual salt content of approx. 2 %, and the contentrate that is constantly flushed out of the system with all of the other substances in the water.