Reverse osmosis

The term reverse osmosis comes from the process of osmosis, the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration if no external pressure is applied.

In simple terms, reverse osmosis is the process of pushing a solution through a filter that traps the solute on one side and allows the pure solvent to be obtained from the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis have no pores, the separation takes place in a dense polymer layer of only microscopic thickness. In most cases the membrane is designed to only allow water to pass through. The water goes into solution in the polymer of which the membrane is manufactured, and crosses it by diffusion. This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2 - 14 bar (30 - 200 pounds per square inch) for fresh and brackish water, and 40 - 70 bar [(600 - 1000 psig)] for seawater, which has around 24 Bar (350 psi) natural osmotic pressure which must be overcome.

This process is best known for its use in desalination (removing the salt from sea water to get fresh water) and has been used in this way since the early 1970s. Its first demonstration was done by Sidney Loeb and Srinivasa Sourirajan from UCLA in the California town of Coalinga.

Method

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. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable 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.

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. In the same way as in conventional osmosis, the solute cannot move from areas of high pressure to areas of low pressure because the membrane is not permeable to it; only the solvent can pass through the membrane. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.

Reverse osmosis in use

Drinking water purification

In the United States, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.

Such systems typically include four or five stages:

• 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 chlorination
• a reverse osmosis filter with a thin film composite membrane (TFM or TFC)
• optionally a second carbon filter to capture those chemicals not removed by the RO membrane.
• optionally an ultra-violet lamp is used for disinfection of the remaining microbes.

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 the 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 eliminate the chlorine.

Portable reverse osmosis (RO) water processors are sold for personal water purification in the home. These units are gravity powered (they need no water pump), and need no electricity. The pressure of gravity pushes/drains the water though the filters, much like a coffee-maker filter. A filter lasts for about seven years before replacement is needed. RO water processors are 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. Some travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use RO water processors. In production of bottled mineral water, the water passes through a RO water processor to remove pollutants and microorganisms, including the smallest microbe known, archaeobacteria. (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.) In developing nations where people die from drinking bacteria-infested well water, water from mud holes, or from ruptured water pipes, RO water processors may save lives. In the deserts of Middle Eastern nations, fresh water is rare, but salt water from the ocean is commonplace. This ocean water can be changed into fresh water with RO water processors, to be used for drinking, bathing, farming, etc. without the high energy cost of water desalination.

Water & Wastewater Purification

Rain water collected from sewer drains is purified with reverse osmosis water processors and used as tap water in Los Angeles and other cities, as a solution to the problem of water shortages.

In industry, reverse osmosis removes minerals from boiler water at power plants. It is also used to clean effluent and brackish groundwater.

Reverse osmosis product can be used for the production of deionized water.

In July 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs. [1]

Application of reverse osmosis in food industry

In addition to desalination, reverse osmosis is a more economical unit operation for concentrating food liquids, e.g. fruit juices, than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a low operating cost and the ability to avoid heat treatment processes, which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.

Although use of the process is frowned upon in the wine industry, it is reportedly widely used. Château Léoville-Las Cases in Bordeaux, France, is one of the few producers to acknowledge use of the process. However, an estimated sixty reverse osmosis machines were is use in Bordeaux in 2002. Known users include many of the elite classed growths (Kramer).

Maple syrup production

Starting in the 1970s, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 75 to 80 percent of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination and degradation of the membranes has to be monitored.

Hydrogen production

For small scale production of hydrogen water purifiers are installed to prevent formation of minerals on the surface of the electrodes and to remove organics and chlorine from utility water. First the water passes through a 20 micron interference ( mesh or screen filter) filter to remove sand and dust particles, second, a charcoal filter to remove organics and chlorine, third stage, a de-ionizing filter to remove metallic ions. A test can be done before and after the filter for proper functioning on barium, calcium, potassium, magnesium, sodium and silicon.

Another used method is reverse osmosis.

See also

• Osmosis
• Desalination
• RO Desalination
• Hydrogen production
• Microfiltration

Source

• Kramer, Matt. Making Sense of Wine. Philadelphia: Running Press, 2003.

External links

• First Demonstration Of Reverse Osmosis
• Sidney Loeb - Co-Inventor of Practical Reverse Osmosis
• Membrane Separation and Treatment
• Chapter 7: North American Maple Syrup Producers Manual - discusses the use of reverse osmosis in maple syrup production.