Reverse osmosis is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to 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. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

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 water 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 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) 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), but has also purified naturally occurring freshwater for medical, industrial process and rinsing applications since the early 1970s

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.

Applications

Drinking water purification

In the United States and Europe, 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 chlorine which will attack and degrade TFC reverse osmosis membranes
• a reverse osmosis (RO) filter which is 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 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 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 remove chlorine from the final product water.

Portable reverse osmosis (RO) 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 (40psi or over is the norm). Portable RO water processors 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 travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts, as the domestic water supply contains substances that are extremely toxic to most species of saltwater fish. In the production of bottled mineral water, the water passes through a 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.)

In the water treatment industry there is a chart of types of contaminants, their sizes and which ones pass through the various types of membranes] Membrane pore sizes can vary from 1 to 50,000 angstroms depending on filter type. "Particle filtration" removes particles of 10,000 angstroms or larger. Microfiltration removes particles of 500 angstroms or larger. "Ultrafiltration" removes particles of roughly 30 angstroms or larger. "Nanofiltration" removes particles of 10 angstroms or larger. Reverse osmosis is in the final category of membrane filtration, "Hyperfiltration," and removes particles larger than 1 angstrom.

Water and wastewater purification

Rain water collected from storm 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. The water is boiled and condensed over and over again and must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. 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.

Food industry

In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as 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.

Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for the concentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheese manufacture) is pre-concentrated with RO from 6% total solids to 10-20% total solids before UF (ultrafiltration) processing. The UF retentate can then be used to make various whey powders including WPI (whey protein isolate) used in bodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5% total solids to 18-22% total solids to reduce crystallization and drying costs of the lactose powder.

Although use of the process was once frowned upon in the wine industry, it is now widely understood and used. An estimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.

Reverse osmosis is used globally throughout the wine industry for many practices including wine and juice concentration, taint removal; such as acetic acid, smoke taint and brettanomyces taint; and alcohol removal. The patent holder for these processes, Vinovation, Inc., claims to have served over 1000 wineries worldwide, either directly or through one if its licensed partners, in the last 15 years. Its use has become so widely accepted that patent infringers have sprung up on several continents.

Car washing

Because of its lower mineral content, RO water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. RO water also enables the car wash operators to reduce the demands on the vehicle drying equipment.

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 75–80% 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, reverse osmosis is sometimes used to prevent formation of minerals on the surface of the electrodes and to remove organics and chlorine from drinking water.

Desalination

Areas that have no or limited surface water or groundwater may choose to desalinate seawater or brackish water to obtain drinking water. Reverse osmosis is the most common method of desalination, although 85 percent of desalinated water is produced in multistage flash plants. Large reverse osmosis and multistage flash desalination plants are used in the Middle East, especially Saudi Arabia. The energy requirements of the plants are large, but electricity can be produced relatively cheaply with the abundant oil reserves in the region. The desalination plants are often located adjacent to the power plants, which reduces energy losses in transmission and allows waste heat to be used in the desalination process of multistage flash plants, reducing the amount of energy needed to desalinate the water and providing cooling for the power plant.

Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s. Its first practical demonstration was done by Sidney Loeb and Srinivasa Sourirajan from UCLA in Coalinga, California. Because no heating or phase changes are needed, energy requirements are low in comparison to other processes of desalination, though still much higher than other forms of water supply (including reverse osmosis treatment of wastewater).

The typical single pass SWRO system consists of the following components:

• Intake
• Pre-treatment
• High-pressure pump
• Membrane assembly
• pH adjustment
• Disinfection

Pre-treatment

Pre-treatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion to allow only one way flow through the system. As such the spiral wound design doesn't allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material can not be removed from the membrane surface systems they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO system has four major components:

Screening of solids

Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth.

Screening of biologicals

Prefiltration pH adjustment

If the pH of upstream salinwater is above 5.8 in the acidic-alkaline measurement scale, sulfuric acid or other acidic soultion is used to adjust the pH of water at 5.5 to 5.8.

Cartridge filtration

High pressure pump

The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 lbf/in² (1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 lbf/in² (6 to 8 MPa).

Membrane assembly

The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and a hollow-fiber.

pH adjustment

Liming material is used in order to adjust pH at 6.8 to 8.1 to meet the potable water specifications.

Disinfection

Post-treatment consists of stabilizing the water and preparing for distribution. Post treatment drinking water is normally 6.8 (acidic). Deviation from the pH lower or higher can cause peptic cancer. Disinfection (sometimes called germicidal or bactericidal) is employed to kill the bacteria or other organisms in the products by means of ultraviolet radiation, using UV lamps directly on the product.

New developments

Prefiltration of high fouling waters with another, larger-pore membrane with less hydraulic energy requirement, has been evaluated and sometimes used since the 1970s. However, this means the water passes through two membranes and is often repressurized, requiring more energy input in the system, increasing the cost.

Other recent development work has focused on integrating RO with electrodialysis in order to improve recovery of valuable deionized products or miminize concentrate volume requiring discharge or disposal.

Sources

• Kramer, Matt. Making Sense of Wine. Philadelphia: Running Press, 2003.
• Emma Dadswell www.ro-man.com

Notes