What Is Reverse Osmosis?

Reverse Osmosis SkidAlthough best known by consumers as a popular home water treatment technology, reverse osmosis actually is a naturally occurring process harnessed for a wide variety of purposes.

The Water Quality Association, an international trade association based in Illinois whose members are from the residential, commercial, and industrial water treatment industries, says reverse osmosis:

[…] allows people throughout the world to affordably convert undesirable water into water that is virtually free of health or aesthetic contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence to installations used in manned spacecraft.

Before continuing, it’s useful to understand one of the key words: osmosis.

Simply put, osmosis describes the physical process in which any solvent moves across a semi-permeable membrane using only kinetic energy. Jean-Antoine Nollet, a French scientist and cleric, first observed osmosis in 1748, and the word was coined based on the terms endosmose and exosmose, from the Greek as translated by the French. It is best known as the process cells use to transport water.

Then, reverse osmosis is… well, osmosis in reverse.

Pressure is used to force a pure solvent through a semi-permeable membrane. Water typically passes easily through membranes because its molecules are small. In a reverse osmosis water filtration system, water is present on both sides of the membrane. The difference between the water on each side of the membrane is the amount of dissolved minerals the water contains. Nature will seek to dilute the water with the greater concentration of contaminants. The membrane allows water to pass through, but excludes the contaminants.

Reverse osmosis (RO) is actually found in a wide range of processes including making ice, reclamation of car wash water, kidney and hemodialysis, and manufacturing of items from foods and pharmaceuticals to semiconductors.

“The most common uses of RO are for desalination of seawater and brackish water for potable and industrial applications,” writes Jane Kucera in “Reverse Osmosis: Design, Processes, and Applications for Engineers.” The technology is also increasingly being used for wastewater and water reuse.

NDSU Reverse OsmosisThese system are also known as ultrafiltration systems because the water must move or filter through a membrane. It is actually the membrane materials that have increased the process’s popularity. Until the 1960s, the available semi-permeable membranes were expensive and unreliable. This was solved by new synthetic materials, which made the membranes more efficient and durable.

One measure of the efficacy of a reverse osmosis unit is the rejection rate of common contaminants, such as dissolved solids or sodium, according to North Dakota State University. Jane Kucera, writing in “Reverse Osmosis: Design, Processes, and Applications for Engineers,” says the conventional ion rejection rate in industrial systems ranges between 96% and 99%.

For reverse osmosis systems to keep working efficiently, they must be flushed periodically. Without such maintenance, a scale can form on the membrane. Some smaller systems, such as home treatment units, have automatic membrane flushing. Membranes can also be damaged if exposed to too much chlorine or sediment. In industrial systems, pretreating is typically used to avoid these problems.

Kucera explains that the use of RO has increased since the 1990s, particularly as it was considered a greener technology. At the same time, she says:

Unfortunately, knowledge about RO has not kept pace with the growth in technology and use. Operators and others familiar with ion exchange technology are often faced with an RO system with little or no training. This has resulted in poor performance of RO systems and perpetuation of misconceptions about RO.

Image by sirdle, used under its Creative Commons license.
Graphic by North Dakota State University, used under its Creative Commons license.