Desalination Technologies:
Reverse Osmosis (RO) Explained
What is Reverse Osmosis
Reverse osmosis (RO) represents the latest technology in water treatment and is the most convenient and thorough water filtration method. Reverse osmosis membranes generally act as a barrier to dissolved salts and organic or inorganic molecules with a molecular weight greater than 100.
On the other hand, water molecules pass unhindered through the membrane and produce a purified product stream. Dissolved salt rejection is typically 95% to over 99%, depending on the following main factors: type of membrane, feedwater composition, temperature and system design.
Some of the applications of RO include:
- seawater or brackish water desalination ,
- Wastewater Recovery,
- Food and Beverage Processing,
- Biomedical Separation,
- Home Beverage Purification Water and
- Industrial process water.
- RO is also commonly used in the production of ultrapure water, used in the semiconductor industry, power industry (boiler water supply) and medical/lab applications.
How Reverse Osmosis (RO) Works
Osmosis occurs when clean water passes across a semipermeable membrane from a dilute saline solution to a more concentrated saline solution. The term "semipermeable" refers to a membrane that is permeable to some species but not to others.
That said, assume that this membrane allows water to pass through but not salt. Then, place a salt solution in one compartment and pure water in the other compartment. The membrane will allow water to permeate through it to either side. But salt cannot pass through the membrane.
Overtime, the system above will attempt to achieve equilibrium - with the solution on either side of the semipermeable membrane having the same concentration. For equilibrium to be achieved, water molecules must move from the compartment containing the dilute solution to the one that has concentrated solution.
Inevitably, this will result in a rise in the height of the salt solution as shown in the image above. This height will rise until the pressure of the water column (salt solution) becomes so great that the force of the water column blocks the flow of water.
The equilibrium point of this water column height in terms of water pressure against the membrane is called osmotic pressure.
Reverse Osmosis Process
A high-pressure pump delivers high-pressure feedwater to the membrane system where it gets separated into a low-saline and/or purified product known as permeate and a high-saline or concentrated brine known as concentrate or reject.
The percentage of feedwater that goes to the concentrate stream and the permeate that will be recovered from the feed are controlled by a flow regulating valve known as a concentrate valve.
The following are the definitions of the important terminology used in the reverse osmosis process:
- Recovery - Percentage of feedwater that emerges as product water or "permeate" from a membrane system.
- Rejection - the percentage of solute concentration removed from system feedwater by the membrane.
- Passage - the percentage of dissolved elements (contaminants) in the feedwater that is allowed to flow through the membrane
- Permeate - the purified product water produced by a membrane system.
- Flow - the rate of feedwater introduced to the membrane element or membrane system.
- Flux - the rate of permeate transported per unit of membrane area.
Factors affecting the performance of a RO System
The key performance criteria of a reverse osmosis process are permeate flux and salt rejection. Flux and rejection are intrinsic features of membrane performance under specified reference conditions.
Variable parameters such as pressure, temperature, recovery, and feedwater salt concentration all influence the flux and rejection of a membrane system. Several important elements that cannot be observed directly in membrane performance should not be overlooked.
These include plant maintenance and operation, as well as optimal pretreatment design. Each OEM (original equipment manufacturer) and end-user of a reverse osmosis system must consider these three 'parameters,' which have a significant impact on the system's performance.
consider these three 'parameters,' which have a significant impact on the system's performance.
- Pressure - Increasing the effective feed pressure lowers the permeate TDS while increasing the permeate flux.
- Temperature - if the temperature rises while the rest of the parameters remain constant, the permeate flux and salt passage will rise.
- Recovery - this is the ratio of permeate flow to feed flow. If the salt concentration reaches a level where the osmotic pressure of the concentrate is equal to the applied feed pressure, the permeate flux will diminish and eventually stop. With improved recovery, salt rejection will decrease.
Interested in Superior Quality RO Membranes?
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Featuring innovative Thin Film Nanocomposite (TFN) technology, LG BWRO membranes are available in industry standard configurations for easy installation into existing and new RO systems.
