An Overview

Reverse Osmosis

The process of reverse osmosis through semi permeable membranes or a reverse osmosis membrane was first observed in 1748 by Jean Antoine Nollet. For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949 the University of California at Los Angeles (UCLA) first investigated desalination of seawater using semi permeable membranes. Researchers from both UCLA and the University of Florida successfully produced freshwater from seawater in the mid-1950s, but the flux was too low to be commercially viable. By the end of 2001, about 15,200 desalination plants using s reverse osmosis membrane were in operation or in the planning stages worldwide.

Reverse osmosis (RO) is a reverse  filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. In order to be ‘selective’, this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances around the H2O molecules. This is the reverse of the normal osmosis process, in which the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an external reverse pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis.

The process is similar to membrane filtration. However there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse Osmosis, however involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure and flux rate.

Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semi permeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure .

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only H2O to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 30–250 psi for fresh and brackish water, and 600–1000 psi for seawater. This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but since the early 1970s it has also been used to purify fresh and brackinh water for medical, industrial, and domestic applications.

Osmosis describes how solvent moves between two solutions separated by semi permeable membranes to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semi permeable membrane. The semi permeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential. Osmosis is an example of diffusion.

In reverse osmosis, in a similar setup as that in osmosis pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.

Domestic Reverse Osmosis Uses

Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and human consumption.

Such systems typically include a number of steps:

  • a sediment filter to trap particles including rust and calcium carbonate

  • optionally a second sediment filter with smaller pores

  • an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade TFC reverse osmosis membranes

  • a reverse osmosis (RO) filter which is a thin film composite membrane

  • optionally a second carbon filter to capture those chemicals not removed by the RO membrane

  • optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis membrane

In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to remove chlorine from the final product water.

Portable reverse osmosis water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should best be under some pressure (40 psi or greater is the norm). Portable RO purifiers can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (Saline water may need special membranes). Some travellers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use reverse osmosis water filters coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water, the water passes through an RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive) is not allowed under European law. (In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.)

Membrane pore sizes can vary from .1 to 5,000 nanometers (nm) depending on filter type. "Particle filtration" removes particles of 1,000 nm or larger. Microfiltration removes particles of 50 nm or larger. "Ultrafiltration" removes particles of roughly 3 nm or larger. "Nanofiltration" removes particles of 1 nm or larger. Reverse osmosis is in the final category of membrane filtration, "Hyperfiltration", and removes particles larger than .1 nm.

Used in: Under-the-sink units; often in combination with a carbon filter or UV disinfection unit or more recently portable reverse osmosis membrane purifiers.

Gets rid of: Most contaminants, including certain parasites such as Cryptosporidium and Giardia; heavy metals such as cadmium, copper, lead and mercury; and other pollutants, including arsenic, barium, nitrate/nitrite, perchlorate and selenium.

Sources on reverse osmosis membrane purification