Reverse Osmosis (RO)

Overcoming osmotic pressure

In ordinary osmosis, when a semipermeable membrane separates solutions of differing solute concentrations, the lower-concentration solution flows into the higher-concentration solution in attempt to reach equilibrium: an equal degree of solute concentration on both sides of the membrane. As the amount of solution on the higher-concentration side increases, pressure on that water column rises until it is high enough to hinder the flow of the lower-concentration solution across the membrane. This is the action of osmotic pressure.

In reverse osmosis (RO), pressure that exceeds a system’s osmotic pressure is applied to that system. The pressure forces the higher-concentration solution back across the semipermeable membrane, leaving solutes that are blocked by the semipermeable membrane behind.

Typically, reverse osmosis water treatment results in a rejection of dissolved salts that is 95 – 99 percent or greater, depending on membrane type, feed composition, temperature, and system design.

Reverse osmosis water treatment can provide finer filtration than either nanofiltration or ultrafiltration. Using RO as a pretreatment process for ion exchange (IX)  can substantially reduce the operating costs and regeneration frequency of the IX system.

Typical applications include:

• Purification of home drinking water.
• Desalination of seawater or brackish water to produce drinking water.
• Wastewater recovery.
• Food and beverage processing.*
• Biomedical separation.
• Industrial process water treatment.

Reverse osmosis water treatment is also often used to produce ultrapure water for the semiconductor industry , in boiler water treatment for the power industry, and for applications in the health care and bioprocessing industry.

Maintaining flow and desired results

Precipitate salts and other impurities that a pressurized flow of feedwater forces against a semipermeable RO membrane can clog or “foul” that membrane. This, in turn, can decrease the performance of a reverse osmosis water treatment system overall.

To significantly reduce the rate of membrane fouling, RO elements use crossflow filtration. This process forces lower-concentration water through the RO membrane, while the separated flow of higher-concentration water moves across the surface of the membrane, carrying away the rejected salts and impurities.

In essence, crossflow filtration happens like this: A high-pressure pump continuously pumps feedwater into the element of the reverse osmosis water treatment system. The pressure forces some water to cross the semipermeable RO membrane, resulting in a low-saline or purified product called permeate on one side, and a high-saline or concentrated brine, called concentrate or reject, on the other. A concentrate valve controls the percentage of feedwater that goes to the concentrate stream and the permeate. In the system, the low-saline or purified permeate — the feedwater that has passed through the membrane — remains isolated from the concentrate flow. The concentrate stream removes the concentrate that cannot permeate the membrane and sweeps them out of the system.

Reverse osmosis systems can thus:

• Produce purified water (or permeate) from a feed stream or brine.
• Remove a concentrate (or concentrated brine or reject) from a feed stream.

Depending on the need and application, either the permeate or the concentrate may be the desired product.

Reverse osmosis water treatment can also be used to selectively separate certain ions and molecules, although to a lesser degree than can ion exchange  systems.

Influencing factors

RO membrane performance (improvement or degradation) is affected by a number of different factors, including aspects of the feedwater, such as:

• Temperature.
• pH balance.
• Salt concentration.

Normalization calculation tools  can help distinguish between the normal, predictable performance changes caused by factors such as those listed above, and deteriorating performance caused by membrane fouling or similar issues.

Other factors influencing RO membrane performance include:

• Operations parameters such as system recovery.
• Concentration polarization.

Find more details in our tech fact sheet: FILMTEC™ Membranes: Factors Affecting RO Membrane Performance