The Science Behind Reverse Osmosis

Osmosis is a special case of diffusion (molecule movement) in which the molecules are water and the concentration gradient occurs across a semi-permeable membrane. To illustrate, imagine a semi-permeable membrane with fresh water on one side and a concentrated aqueous solution like sea water on the other side. If normal osmosis takes place, the fresh water will wish to cross the membrane to dilute the concentrated solution until equilibrium is reached.

Reverse osmosis is the opposite of the natural osmotic process, where water from a solution with a low concentration of dissolved solids travels through a membrane seeking to dilute a higher concentration solution. A reverse osmosis water system applies pressure with a pump to a solution with a high concentration of dissolved solids, causing water from the concentrated solution to pass through the membrane to the fresh water side.

The first practical reverse osmosis (RO) membrane was invented in 1959 by Loeb and Sourirajan at the University of California, Los Angeles.

Now there are at least seven major manufacturers of RO membranes in the world, even more that make their own nanofiltraton and ultrafiltration membranes, and numerous other companies that buy membrane sheet for the purpose of fabricating commodity-type membrane elements.

The driving force for the development and use of RO membranes is the advantages that these have over traditional separation processes such as distillation, extraction, ion exchange, and adsorption.

Reverse osmosis is a pressure-driven process so no energy-intensive phase changes or potentially expensive solvents or adsorbents are needed for RO separations.

Reverse osmosis is a process that is inherently simple to design and operate compared to many traditional separation processes.  Also, simultaneous separation and concentration of both inorganic and organic compounds is possible with the RO process.

Depending on the membrane specifications, reverse osmosis water treatment systems have a very high effectiveness in removing protozoa (ie. Cryptosporidium, Giardia); bacteria (ie. Campylobacter, Salmonella, Shigella, E. coli); viruses (ie. Enteric, Hepatitis A, Norovirus, Rotavirus); and will remove common chemical contaminants (metal ions, aqueous salts), including sodium, chloride, copper, chromium, and lead. They may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorous.

Reverse osmosis is typically used for boiler feed/makeup water, ice making, laboratories, manufacturing process water, metal finishing, pharmaceutics, chemical process water, cosmetics, drinking water, electronics, food and beverage processing, horticulture, humidification, photographic processing, printing, and vehicle washes.

Finally, reverse osmosis technology can also be combined with ultrafiltration, pervaporation, distillation, and other separation techniques to produce hybrid processes that result in highly efficient and selective separations (Bhattacharyya et al., 1992).

With nanofiltration ("loose RO") membranes selective solute separations based on charge and molecular weight/size differences are possible.

Semi-permeable refers to a membrane that selectively allows certain things to pass through it while retaining others. The membrane allows the passage of water, but not ions (ie. Na+, Ca2+, Cl-) or larger molecules (ie. glucose, urea, bacteria). Unwanted particles generally pass through the membrane slower than the water. In salt water RO, for example, the solvent (water) passes through the membrane at a much faster rate than the dissolved solids (salts). The net effect is that a solute-solvent separation occurs, with pure water being the product.