Reverse Osmosis (RO) can be understood in a better way if the naturally
occurring process of osmosis is understood clearly.
Osmosis is a naturally occurring phenomenon. It is a process where water of a weaker saline solution
will tend to migrate to a strong saline solution. Examples of osmosis are when plant roots absorb
water from the soil and movement of water from blood to tissues and back to blood depending on
The process of osmosis is shown in the following diagram. If two solutions, one with high
concentration and another with less concentration (or pure water), are separated by a semi-permeable
membrane, then the water with the lower salt concentration will migrate towards the water with the
higher salt concentration.
A semi-permeable membrane is a membrane that allows some atoms or molecules to pass but not
It depends on size of pores of the membrane. An example is tea strainer which allows liquid tea but
does not allow tea leaves.
Reverse Osmosis, commonly referred to as RO, is a process where water is
demineralized or deionized by pushing it under pressure through a semi-permeable Reverse Osmosis
Reverse Osmosis is the process of Osmosis in reverse. Osmosis occurs naturally without any energy
requirement. To reverse the process of osmosis energy is required to move water from concentrated
side to the opposite side. A reverse osmosis membrane is a semi-permeable membrane that allows the
passage of water molecules but not the majority of dissolved salts, organics and bacteria. Energy is
required to ‘push’ water in reverse direction using a pressure that is more than naturally occurring
With the process of RO, pure water is collected without any dissolved particles. Solutes are then
added as per requirements which are different for drinking and industrial usage water.
The process of RO is shown in a simplified diagram. When pressure is applied to the concentrated
solution, the water molecules are forced through the semi-permeable membrane holding back
Terms used in RO Plant:
Feed Water: Concentrated water which is fed to RO plant.
Permeate Water: Low concentrated or pure water as end product.
Reject Stream: Highly concentrated waste water with almost all solutes.
RO Membrane: Semi-permeable membrane.
In the process of reverse osmosis (RO), pressure is applied to the feed water using a high
pressure pump. This is to push water from the feed side of the RO across the semi-permeable RO
membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream.
The amount of pressure required depends on the salt concentration of the feed water. The more
concentrated the feed water, the more pressure is required to overcome the osmotic pressure. RO
membrane is also according to concentration of feed water.
As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic
pressure) the water molecules pass through the semi-permeable membrane holding back salts and other
contaminants which are discharged through the reject stream. The reject stream water either goes to
drain or it can be recycled and fed back into the feed water supply in some circumstances through
the RO system to save water. The water that makes it through the RO membrane is called permeate or
product water and usually has around 95% to 99% of the dissolved salts removed from it.
Reverse Osmosis is capable of removing up to 99%+ of the dissolved salts (ions), particles,
colloids, organics, bacteria & pyrogens from the feed water.
However an RO system should not be
relied upon to remove 100% of bacteria and viruses. An RO membrane rejects contaminants based on
their size and charge.
Any contaminant that has a molecular weight greater than 200 is likely
rejected by a properly running RO system (for comparison a water molecule has a MW of 18). Reverse
Osmosis is very effective in treating brackish, surface and ground water for both large and small
Some examples of industries that use RO water include pharmaceutical, boiler
feed water, food and beverage, metal finishing and semiconductor manufacturing to name a few.
One Stage RO: Feed water passes through single system of membrane, resulting in permeate and
Two Stage RO: Feed water once passes through system of membrane, resulting in permeate and
reject streams. The reject stream water passes through another membrane resulting in final reject
stream and permeate water, which is added to the permeate water of first membrane.
The terms stage and pass are often mistaken for the same thing in an RO system and can be confusing
terminology for an RO operator. It is important to understand the difference between a 1 and 2 stage
RO and a 1 and 2 pass RO.
In a one stage RO system, the feed water enters the RO system as one stream and exits the RO as
either concentrate or permeate water.
In a two-stage system the concentrate (or reject) from the first stage then becomes the feed water
to the second stage. The permeate water is collected from the first stage is combined with permeate
water from the second stage. Additional stages increase the recovery from the system.
In a Reverse Osmosis System an array describes the physical arrangement of the pressure vessels in a
2 stage system. Pressure vessels contain RO membranes (usually from 1 to 6 RO membranes are in a
pressure vessel). Each stage can have a certain amount of pressure vessels with RO membranes. The
reject of each stage then becomes the feed stream for the next successive stage. The 2 stage RO
system displayed on the previous page is a 2:1 array which means that the concentrate (or reject) of
the first 2 RO vessels is fed to the next 1 vessel.
RO system with concentrate recovery:
With an RO system that can't be properly staged and the feed water chemistry allows for it, a
concentrate recycle setup can be utilized where a portion of the concentrate stream is fed back to
the feed water to the first stage to help increase the system recovery.
Think of a pass as a stand alone RO system. With this in mind, the difference between a single pass
RO system and a double pass RO system is that with a double pass RO, the permeate from the first
pass becomes the feed water to the second pass (or second RO) which ends up producing a much higher
quality permeate because it has essentially gone through two RO systems.
Besides producing a much higher quality permeate, a double pass system also allows the opportunity
to remove carbon dioxide gas from the permeate by injecting caustic between the first and second
pass. C02 is undesirable when you have mixed bed ion exchange resin beds after the RO. By adding
caustic after the first pass, you increase the pH of the first pass permeate water and convert C02
to bicarbonate (HCO3-) and carbonate (CO3-2) for better rejection by the RO membranes in the second
pass. This can't be done with a single pass RO because injecting caustic and forming carbonate
(CO3-2) in the presence of cat-ions such as calcium will cause scaling of the RO membranes.