How Reverse Osmosis Works?

Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from drinking water. Reverse osmosis can remove many types of dissolved and suspended species from water, including bacteria, and is used in both industrial processes and the production of potable water (Source: Wikipedia ) It is worthy to note that 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. Humans have worked this process for decades until they finally cracked the process!

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 is a complicated process which uses a membrane under pressure to separate relatively pure water (or other solvents) 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 most 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.

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). An 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. (Source: Freshly Squeezed Water )

This technology is one among the effective processes that makes desalination possible. Desalination is the process of removal of salt from sea water. Many water treatment plants now started using this Reverse Osmosis technique to deal with many water related issues.

We are quite used to the concept of diffusion – which is the movement of a fluid from an area of higher concentration to an area of lower concentration. Diffusion is a result of the kinetic properties of particles of matter. The particles will mix until they are evenly distributed. So what is the difference between osmosis and diffusion?

About Education offers the best answer for this question: Both diffusion and osmosis are passive transport processes, which means they do not require any input of extra energy to occur. In both diffusion and osmosis, particles move from an area of higher concentration to one of lower concentration. Diffusion can occur in any mixture, including one which includes a semipermeable membrane, while osmosis always occurs across a semipermeable membrane.

One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but when we talk about osmosis, only the solvent molecules (water molecules) cross the membrane. This can be confusing to understand because while the solvent particles are moving from higher to lower solvent concentration across the membrane, they are moving from lower to higher solute concentration (from a more dilute solution to a region of more concentrated solution). This occurs naturally because the system seeks balance or equilibrium. If the solute particles can’t cross a barrier, the only way to equalize concentration on both sides of the membrane is for the solvent particles to move in. You can consider osmosis to be a special case of diffusion in which diffusion occurs across a semipermeable membrane and only the water or other solvent moves.


While we have gone at great length to explain what an RO system does, it is highly important to use such RO filters at home for water purification purposes. A good RO system can remove contaminants such as arsenic, nitrates, sodium, copper and lead, some organic chemicals, and the municipal additive fluoride.

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