Reverse Osmosis: How Does It Work?

In case you’re not yet aware, reverse osmosis or commonly referred to as RO, has demonstrated its efficiency and technology to generate water which is appropriate for numerous industrial applications that demand demineralized or deionized water.

In fact, conducting further treatment after the RO system can improve the quality of the RO permeate and enable it to suit the most challenging applications.

It’s essential to carry out a proper pre-treatment and monitoring of an RO system to eliminate the need for expensive repairs and unexpected maintenance. If you follow the correct maintenance program, your RO system could produce high purity water for years.

Enough with the introduction, how does reverse osmosis work?

How RO Works

Reverse Osmosis uses a high-pressure pump to escalate the pressure on the salt side of the RO system and push the feed water across the semi-permeable RO membrane. As it pushes the water, around 95 to 99 percent of the dissolved salts and other contaminants will be left behind in the reject stream. The contaminants either proceeds to the drain or returns to the feed water supply so they can be recycled through the system to conserve water. Depending on the salt concentration of the water used, the amount of pressure needed may vary. The more concentrated the water is, the more pressure is required to overpower the osmotic pressure.

Check out the diagram of the RO system process below.

RO system operation process

A – Applied pressure

B – Seawater

C – Contaminants

D – Semi-Permeable Membrane

E – Potable water out

F – Distribution

Watery Terms

The desalinated water, which has been demineralized or deionized, is referred to as permeate water. Sometimes the term “product water” is also used. The water stream which contains the concentrated contaminants which were not able to pass through the RO membrane is then called the reject stream, sometimes called the concentrate stream or the brine stream.

Filtration System

An RO System uses cross filtration, and not just standard filtration where the salts and other contaminants are stored up within the filter media. In the cross filtration, the solution will “cross” or pass through the filter with two outlets––the permeate water proceeds to one outlet and the reject stream goes through the other.

To prevent the building up of contaminants, cross filtration lets the water to brush away contaminant build up and also enables sufficient turbulence to maintain the cleanliness of the membrane surface.

Types of Contaminants Removed from Water

contaminantsAs mentioned earlier, RO can remove up to 99%+ of dissolved salts (ions), colloids, organics, particles, pyrogens, and bacteria from the feed water. However, we have to clarify that it cannot be expected to remove 100% of viruses and bacteria.

The RO membrane of an RO system discards contaminants according to the size and charge. Any contaminant which has a molecular weight of more than 200 will likely be rejected by an RO system that’s properly running. Moreover, the higher ionic charge the contaminant has, the more likely it will be hindered to go through the membrane. For instance, a sodium ion which only has one charge (monovalent) and calcium which contains two charges, won’t be turned down by the RO membrane.  This is the reason why an RO system cannot remove gases like CO2 efficiently because they aren’t highly charged while in solution and they have extremely low molecular weight.

Since a Reverse Osmosis system doesn’t remove gases, the pH level of the filtered water (permeate) can be a bit lower than normal, depending on the CO2 levels incorporated in the feed water since the CO2 is converted to carbonic acid.


iSpring RCC7 WQAIf you’re wondering why an RO system is an excellent choice, let me give you a hint: It’s proven to be effective in treating surface, brackish, and groundwater for both small and large flows applications. In fact, aside from the purification of drinking water, an RO system can also be used for metal finishing, semiconductor manufacturing, pharmaceutical, and even in the food industry!