What is RO
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 concentrations.
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 others. 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 Membrane.
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 osmotic pressure. 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 contaminants.
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 flows applications.
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 reject streams.
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.