How Reverse Osmosis System Works

Reverse Osmosis System is an innovation that is utilized to expel an extensive amount of contaminants from water by pushing the water under pressure through a semi-penetrable membrane.

Understanding Osmosis and Reverse Osmosis

Osmosis is a natural procedure and is perhaps one of the most important phenomena. During osmosis, the weaker saline solutions tend to move towards the region where a strong saline solution is present. Osmosis mainly depends on upon the concentration of solutions. The solution, which has a lower concentration of solute, tends to move in the region where the solution has a higher concentration of solute.

The general movement of the solution is from lower concentration to higher concentration. The reverse osmosis is the procedure opposite to osmosis. It means that the solution moves from a region of higher concentration to an area of lower concentration by applying pressure or some external force.

The reverse osmosis is a process whereby the solvent is passed through a porous membrane in the direction that is opposed to the natural osmotic procedure. The reverse osmosis process is carried out when the solutions are subjected to hydrostatic pressure, which is greater than the osmotic pressure. Reverse osmosis (RO) procedure is integrated into the water filtration systems, which purifies water and makes it fit for drinking while retaining its original taste and texture. The RO process uses a semi-permeable membrane to work best.

Working of the Reverse Osmosis

The reverse osmosis utilizes a high-pressure pump to increase the pressure on the side where the solute concentration is high. It is also called as the feed pressure. Moreover, it also integrates a semi-permeable membrane. This layer is installed in between the higher concentration solutions and low concentration solutions.

The pressure passing through this membrane is called as the permeate pressure. When the hydrostatic pressure is applied, the solution with higher concentration of salts and other dissolved material passes through the semi-permeable to the other side where density is lower leaving behind 95% - 99% of the dissolved salts and other material. Thus, the procedure purifies water and makes it free of all the dissolved and undissolved contaminants.

Contaminants that Reverse Osmosis removes from water

The water has many components that are dissolved in it. These may be salts, ions, pyrogens, colloids, particles, etc. the RO procedure removes 95 to 99% of these dissolved substances. But the RO systems in the water filtration systems are unable to remove bacteria and viruses up to 100%. The semi-membrane utilized in the RO procedure expels out the contaminants by the charge and size.

For instance, the water molecules have a molecular weight of 18 Amu. Likewise, components having higher charges are not allowed to pass through the RO membrane. For instance, molecules of lower charges such as sodium having one charge and calcium having two charges, go through the RO membrane, but as the charge increase e.g. the carbon dioxide has a higher charge than sodium or calcium, so it is not allowed to pass through the RO membrane.

Reverse Osmosis Performance

Reverse osmosis doesn’t just occur. It is more pronounced in some systems than others. This is particularly due to the condition of the system with regards to certain parameters. It is based on the degree of influence of each of these factors on the system that we can measure the performance of reverse osmosis in that environment. It is through the measurement of the impact of each of these parameters that we can obtain an accurate data on the efficiency of the RO system. This also enables us to determine the optimal working condition of the system.

Two things that can also be checked to evaluate the performance of the RO system at any time are the permeate pressure and the feed pressure. Both pressures often give an indication of the state of the system when computing performance. Apart from both pressures, the flow of liquid through the system can also aid our calculation. The flows of the feed and permeate are the two important aspect of the system dynamics that also needs to be considered when determining the performance because they help to indicate how much have been added or taken away by the RO system.

Some other things that are given attention while calculating the efficiency of an RO system include the percentage salt rejection of the system, the salt passage and the recovery of the system. The salt rejection percentage helps to determine the amount of salt that is rejected by the RO membrane during reverse osmosis while the salt passage percentage indicates the amount of salt that has passed through the membrane. A higher salt rejection percentage is an indication that the system is functioning properly. The desired percentage is usually between 95 to 99%. The recovery percentage tells how much water that has passed through the membrane. This figure obtained here is highly dependent on where the RO system is used.

Concentration Factor

The concentration factor is used to detect the scaling on the RO membrane. This is somewhat related to the recovery %. The concentration factors help to know the concentration of the slats that has been deposited over the RO membrane. If the percentage is higher, then it shows that the RO membrane has a higher concentration of salts.


The flux measures the water that passes through the each square foot of the RO membrane per day. The flux depends on the type of feed water contained in the RO system and the design of system i.e. either it is used for homes, or it is used for industries. It also determines the type of membrane is used and keeping the membrane consistent through the system.

Mass Balance

The mass balance equation takes into consideration the feed flow and conductivity which is equal tot the permeate flow and the permeate conductivity, and the concertation flow and conductivity. By acquiring data about these parameters, the equation must equalize. If the equation does not equalize than the RO systems faces some problems.

Understanding the difference between passes and stages in a Reverse Osmosis System

1 stage vs. 2 stage Reverse Osmosis System

The 1 stage and the 3 stage procedures are a bit different from each other, during the 1 stage procedure the feed water enters the system as once and then comes out as either concentrate water or as the permeate water.

In the 2 stage, RO procedure is similar to the 1 stage; the difference arises when the concentrate water is again utilized to produce the permeate water, where as in 1 stage the concentrate water is not utilized again.


Array describes the physical arrangement of the RO membranes. Each stage has its pressure vessels that are set according to their usage. The stage 2 vessels are arranged differently as compared to the stage 1 pressure vessels.

Reverse Osmosis (RO) System with a concentrate recycle

The reverse osmosis system that incorporates the concentrate cycle works a bit differently. The feed water that is left in the concentration panel is recovered again to the feed water and again sent to the RO membranes for filtration.

Single Pass vs. Double Pass Reverse Osmosis (RO) Systems

The single-pass RO system follows the stage 1 procedure where the feed water is passed through the RO membrane only once. On the other hand, the double pass system follows the stage 2 procedure where the feed water is passed twice through the RO membrane; the concentrate water is used twice.

RO Pre-treatment

The chemical and mechanical treatments are essential for the RO system to work properly. The RO membranes require frequent cleaning as it safeguards it from some problems.


Fouling occurs when the impurities in the water accumulate on the surface of the RO membrane. This decreases the effectiveness of the membrane. When the contaminants accumulate on the surface if the membrane then the permeate pressure decreases and the quality of water also decreases.


The solubility limit of some substances increases then the scaling occurs. There are some major effects of the scaling. The pressure drops to a significant level; the permeate water quality is low because of the lower permeate flow. For instance, when CaCO3 is deposited on the RO membrane the scaling decreases the pressure of the system.

Chemical Attack

The RO systems with thin frames are not suitable for usage in homes or the industries because they get damages because of certain chemicals. The chemical s like chloramines or chlorine or other oxidisers make whole in the thinner RO membranes making it unfit for drinking purposes. Chemical attack can also initiate microorganism growth on the RO membrane.

Mechanical Damage

The mechanical damage is also incurred on the RO membrane. If the back pressure of the RO system is too high, then it can damage the membrane. The frequency drive of the motors can be altered to control the pressure of water across the RO system.

Pre-treatment Solutions for Reverse Osmosis (RO)

Multimedia Filtration

The multimedia filtration is used to prevent fouling in Reverse Osmosis System. The multi-media filter that is used as a pre-treatment solution has three layers. The first layer is the anthracite coal layer; the second layer is of the sand and the third layer comprises of garnet, which has a supportive layer of gravels and stones at the bottom. The multi-media filtration allows the water to pass through it. It removes the dirt particles and all other dissolved contaminants in the water. The multi-media filtration treatment is essential for the RO systems. This prevents the fouling of the RO system.


The microfiltration deals in removing the microbial contaminants that are dissolved in water. These contaminants mostly comprise of the colloidal particles and bacteria present in water. The micro filters are very useful in filtering the microbial contaminants. The water after being filtered by the micro filters has the best quality and low turbidity.

Anti-scalants and Scale Inhibitors

The RO system faces the problem of scaling, so to prevent it, the scale inhibitors are utilized. There are certain chemicals that can be added to the feed water. These avoid the scaling procedure of the RO system. The choice of the scale inhibitors and the antiscalants depend on upon the usage and chemistry of water and also upon the design of RO system.

Softening by Ion Exchange

Instead of using the scale inhibitors, the water softeners can also be used to prevent the scaling. Along with a 5-micron multimedia filter, the water softener acts to prevent scaling at every level. There are many advantages of using the water softeners.

Sodium bisulphate injection

The water contains chlorines and chloramines that can oxidize the water, however, by adding the reducer the oxidizing property can be diminished. Sodium bisulfate acts as a reducer for the feed water. It reduces the oxidizing capability of water and prevents the chloramines from damaging the thin RO membranes.

Granular Activated Carbon

To remove the dissolved organic components from the water, the granular activated carbon is used. This removes all the organic residuals, disinfectants, constituents, from the water. The GAC removes the chlorine immediately from the water and forms a bed. The bed acts a feed for the bacteria and might help the bacteria to grow over the GAC bed. It may also produce some foul of carbons too.

RO Data Trending and Normalization

The data of the Ro can be collected by the system pressure, quality, flows, and temperature. The water temperature is directly related to the pressure of the water. If the water temperature decreases the water becomes viscous, and the permeate flow drops. The normal data about the RO system should be collected initially when the RO system is installed and then it should be taken as a reference to check whenever the RO membranes are replaced or cleaned.

RO Membrane Cleaning

The Ro membrane cleaning is the most critical event. The periodic cleaning procedure must be followed to clean the RO system. The Ro system must be cleaned 2 to 4 a year depending upon its usage. You may only replace the RO membrane or cleanse the system as a whole. You can also hire cleaning companies to the job for you. The cleaning is very effective.


The reverse osmosis is the procedure opposite to osmosis.Reverse osmosis technique is very useful in filtering the water. It can be used in homes and in the industries too. It also provides several years of purified water, if it is periodically cleaned.

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