Principle of reverse osmosis


Semi-permeable membranes allow certain substances through selectively, in this case water, but prevent others from crossing such as dissolved salts.

The reverse osmosis phenomenon was discovered by Abbè Nollet more than 200 years ago. He saw how water was transported through a pig’s stomach covering a jug containing vinegar.


When two solutions with different concentrations come into contact through a semi-permeable membrane (fig.1), natural forces make the pure water flow from the most diluted solution to the most concentrated, until the solution concentrations on either side of the membrane equal out (fig. 2). This is the natural phenomenon known as OSMOSIS.


Water passing through implies an increase in volume and therefore in the height of the container on the side of the concentrated solution and a drop on the more diluted side. This height difference between the two sides of the membrane becomes the pressure exerted by the liquid, the osmotic dissolution pressure (fig. 2).


By observing this phenomenon, the possibility of reversing it was determined by applying equal or greater pressure to the osmotic dissolving pressure on the side of the more concentrated solution, causing the water to pass in the reverse direction (fig. 3). This phenomenon is called REVERSE OSMOSIS.