Osmosis is a natural phenomenon in which a liquid [ in this case water ] passes through a semi-permeable membrane from a relatively dilute solution toward a more concentrated solution. This flow produces a measurable pressure, osmotic pressure.

If pressure is applied to the more concentrated solution, and if that pressure exceeds the osmotic pressure, water flows through the membrane from the more concentrated solution towards the dilute solution.

This process, called reverse osmosis, or RO, removes up to 98% of dissolved salts, and 100% of colloidal and suspended matter.

The pores in the membrane are about .0001 microns. To give you some perspective, the smallest known bacteria is .02 microns, a human hair is about 50 microns.

Water passes through the membrane while the dissolved and particulate materials are left behind. As pressure is applied to the concentrated solution, the flow is reversed and water is forced through the membrane from the concentrated side to the dilute side.

Water molecules penetrate the thin cellulose acetate layer of the membrane and diffuse through it molecule by molecule. Dissolved salt ions would also diffuse through this layer, except that the solubility of the salt ions in the acetate is much less that that of the water.

Thus, the water moves through more rapidly with the result that a separation occurs.

The driving force is furnished by both the pressure and the concentration differentials across the thin layer.

For water, the pressure effect is the most important. Therefore, increases in pressure increase the water flux without a corresponding increase in salt flux.

Reverse Osmosis is the optimal way to rid water of impurities.

A water treatment process whereby dissolved salts, such as sodium, chloride, calcium carbonate, and calcium sulfate, etc. may be separated from water by forcing the water through a semi-permeable membrane under high pressure. The water diffuses through the membrane and the dissolved salts remain behind on the surface of the membrane to be dumped into the drain.