What Is Reverse Osmosis and How Does It Work? An examination of the process of reverse osmosis by Freshly Squeezed Water.

What Is Reverse Osmosis and How Does It Work?


Seabirds use reverse osmosis to desalinate seawater. They possess a membrane in their throats which allows water molecules to get through and stops the salt. This enables them to drink fresh, unsalted water, and they spit out the salty waste.

Reverse Osmosis and Humans

Humans worked out how to copy this process around 40 years ago for the same reason - to desalinate seawater.

A Reverse Osmosis Desalination Plant
Schematic diagram of a Reverse Osmosis Desalination Plant

Reverse osmosis is now used in medicine and industry as a means of purifying or separating water and other solvents from other components. In recent years, it has been used increasingly for making pure water for dialysis in hospitals and for producing Water for Injection.

Reverse Osmosis: A Scientific Explanation

Reverse osmosis is a complicated process which uses a membrane under pressure to separate relatively pure water (or other solvent) from a less pure solution. When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, water passes through the membrane in the direction of the more concentrated solution as a result of osmotic pressure. If enough counter pressure is applied to the concentrated solution to overcome the osmotic pressure, the flow of water will be reversed.

Water molecules can form hydrogen bonds in the reverse osmosis membrane and fit into the membrane matrix. The water molecules that enter the membrane by hydrogen bonding can be pushed through under pressure. Most organic substances with a molecular weight over 100 are sieved out, i.e., oils, pyrogens and particulates including bacteria and viruses.

Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Ions are repelled by dielectric interactions; ions with higher charges are repelled to a greater distance from the membrane surface. The nominal rejection ratio of common ionic salts is 85 - 98%.


The majority of the commercially manufactured Reverse O smosis (RO) membranes are usually made from cellulose acetate, polysulfonate, and polyamide. The membrane consists of a skin about 0.25 microns and a support layer about 100 microns. The skin is the active barrier and primarily allows water to pass through.

Quality of Reverse Osmosis Product Water

The amount of dissolved solids in water produced by reverse osmosis is approximately a constant percentage of those in the feed water. For example, when the feed water contains 300 ppm total dissolved solids (TDS), the product water may have 15 to 30 ppm (95% and 90% rejection ratio respectively). A RO system design is based on a certain range of feed water TDS, the percentage of rejection and percentage of recovery desired. For a given system, the higher the percentage of recovery or the lower the percentage of rejection, the poorer the quality of product water becomes. The video below shows a Reverse Osmosis Electrolysis Demonstation.


For more information on the Freshly Squeezed Water Reverse Osmosis System, click here or see our full range of water systems here.



1. Gregor, H.P., and Gregor, C.D., "Synthetic Membrane Technology," Scientific American, July, 1978.
2. Sourirajan, S., Reverse Osmosis, Logos Press, London; Academic Press, New York, N.Y., 1970.
3. Karger, Barry L. et al. An Introduction to Separation Science. New York: Wiley, 1965.
4. Londale, H. and Podall, T., Ed. Reverse Osmosis Membrane Research. New York: Plenum Press, 1972.
5. Elias, S., "Membrane Processing," Food Engineering. Oct. 1979.
6. Lacey, R. E., "Membrane Separation Process," Chem. Eng., Sept. 1972.
7. Datta, R. et al. "Concentration of Antibiotics by Reverse Osmosis," Biotechnology and Bioengineering XIX, 1419-1429, 1977.
8. Favero, M. S., Peterson, N. J., et al. "Gram-Negative Water Bacteria in Hemodialysis Systems," Health Laboratory Science, Vol. 12. No. 4. 1975.
9. Klumb, G. H., "Reverse Osmosis - A Process in the Purification of Water for Parenteral Administration," Bulletin of the Parenteral Drug Association, Vol. 29. No. 5. 1975.
10. Frith, C. F., Dawson, F. W. and Sampson, R. L., "Water for Injection USP XIX By Reverse Osmosis," Bulletin of the Parenteral Drug Association, Vol. 30, No. 2, 1976.
11. Hoag, Selwyn B. and William F. Albern. "Reverse Osmosis: The Economical Production of Quality Water," Plumbing Engineer, May-June, 1977.
12. Juberg, Donald L. "Application of Reverse Osmosis for the Generation of Water for Injection," Bulletin of the Parenteral Drug Association 31: 70-78, March-April, 1977.
13. Belfort, G., Rotem, Y., and Katzenelson, E., "Virus Concentration Using Hollow Fiber Membranes," Water Research, Vol. 9, 1975.
14. Osol, Arthur, Ed. Remington's Pharmaceutical Sciences, 16th Edition, Easton, Penn: Mack, 1980.
15. Peterson, N. J., et al. Quarterly Reports, April-June, 1976, July-Sept. 1977. HHS, PHS, CDC, Phoenix, Lab. Div., Phoenix, Arizona.

The information in this section is based on the Inspector’s Technical Guide of the Food and Drug Administration (FDA) in the USA.


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Freshly Squeezed Water offers superb Reverse Osmosis Filtration Systems with a number of different financial options to make our system affordable for all. You can find more information on our Reverse Osmosis Water systems throughout this site.


What Is Reverse Osmosis and How Does It Work? An examination of the process of reverse osmosis by Freshly Squeezed Water.