What is Reverse Osmosis?


Reverse Osmosis

Reverse osmosis is a filtration process typically used for water. It works by using pressure to force a solution through a membrane, retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.


Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.

This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but it has also been used to purify fresh water for medical, industrial and domestic applications since the early 1970s.
When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure. For more information..

Effluent from RO contains all natural minerals with effluent normally around 200-500 ppm TDS. That is normal good drinking water or about same as home tap water. Only with Distillation / Vaporization you get flat water as in rainwater. RO produces very good drinking water. Viruses and microorganisms (bacteria's) are all removed, they all too big to pass through a membrane so it is clean safe drinking water,

All plants vary depending on the TDS in the water and that can vary from polluted well water with 200 ppm TDS (in case a UF system). If Brackish water, 1000 ppm and up to 10.000 ppm TDS, that would require Brackish Water membranes. Or, a 35,000 - 45,000 ppm TDS seawater which would require desalination RO membranes. In each of these cases pre and post treatment may vary.

It is preferable if a water analysis done prior to all estimates, even for clean seawater which can contain chemicals and pollutants that will determine how we construct the system. RO plants in general are all customized to applications. If a client doesn't have a water analysis, he should get one.

To quote well water, we need the capacity they require and a water analysis. Well water normally is fresh water but could also be brackish, we must always know constituents and TDS, as this will determine what system (UF or R.O.) Filter or Membranes to select. We have information forms on our website. Go to Application Form Reverse Osmosis



In this case we may be able to use Ultra Filtration. With UF you have 100% recovery of water. Water analysis will tell us how to configure the plant, as well water can have some nasty pollutions.


You have Brackish water starting at 1000 ppm TDS to 5000 ppm TDS. All R.O. membrane systems will have to considerthe exact TDS to configure these plants correctly. We can have 80% recovery plus. Brackish water is close to shore and/or on-shore (wells) hence are suspected to contain pollutants other than salt, therefore a water analysis is required.


Sea water contains 35000 to 45000 ppm TDS depending on location. We can achieve up to 45% R.O. recovery. It is not important to have water analysis here other than TDS, so we know what pressure and membrane selection.

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