Reverse osmosis Reverse osmosis (RO) is a 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. 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 from the water 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 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 water flux rate.[1].

Schematics of a reverse osmosis system (desalination) using a pressure exchanger. 1:Sea water inflow, 2: Fresh water flow (40%), 3:Concentrate Flow (60%), 4:Sea water flow (60%), 5: Concentrate (drain), A: High pressure pump flow (40%), B: Circulation pump, C:Osmosis unit with membrane, D: Pressure exchanger


reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This process requires that a high pressure be exerted on the high concentration side of the membrane. By the end of 2001. usually 2–17 bar (30–250 psi) for fresh and brackish water. and 40–70 bar (600–1000 psi) for seawater. The semipermeable membrane does not allow the solutes to move from one compartment to the other.The process of osmosis through semipermeable membranes was first observed in 1748 by Jean Antoine Nollet. but the flux was too low to be commercially viable[2]. In 1949. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration. Researchers from both UCLA and the University of Florida successfully produced fresh water from seawater in the mid-1950s. but allows the solvent to move. When two solutions with different concentrations of a solute are mixed. which has around 24 bar (350 psi) natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt from sea water to get fresh water). osmosis was only a phenomenon observed in the laboratory. In most cases. For the following 200 years. about 15. and domestic applications. industrial. it is instead achieved by the . but since the early 1970s it has also been used to purify fresh water for medical. Formally.200 desalination plants were in operation or in the planning stages worldwide. The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. the University of California at Los Angeles (UCLA) first investigated desalination of seawater using semipermeable membranes. Instead of mixing the two solutions together. Osmosis describes how solvent moves between two solutions separated by a semipermeable membrane to reduce concentration differences between the solutions. the membrane is designed to allow only water to pass through this dense layer.[1] Process A semipermeable membrane coil used in desalinization. while preventing the passage of solutes (such as salt ions). they can be put in two compartments where they are separated from each other by a semipermeable membrane.

[edit] Applications [edit] Drinking water purification Around the world. In the production of bottled mineral water. To work effectively. the water passes through an RO water processor to remove pollutants and microorganisms. In reverse osmosis. Such systems typically include a number of steps: • • • • • • a sediment filter to trap particles. as the device is easy to use (saline water may need special membranes). In CTA systems. In this case. which will attack and degrade TFC reverse osmosis membranes a reverse osmosis (RO) filter. 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. 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 tendency for solvent to flow through the membrane can be expressed as "osmotic pressure". and cellulose triacetate membrane (CTA) is used. Portable RO water processors can be used by people who live in rural areas without clean water. Some travelers on long boating. since it is analogous to flow caused by a pressure differential. a carbon postfilter is needed to remove chlorine from the final product water. RO systems are also now extensively used by marine aquarium enthusiasts. including a reverse osmosis step. including rust and calcium carbonate optionally. On the other side. it causes these areas to become more concentrated. household drinking water purification systems. In European countries. This process is termed osmosis. pressure is applied to the compartment with high concentration.movement of the solvent from areas of low solute concentration to areas of high solute concentration. a fraction of the living bacteria can and do pass through RO membranes through minor imperfections. are commonly used for improving water for drinking and cooking. When the solvent moves away from low concentration areas. such processing of Natural Mineral Water (as defined by a European Directive citation required) is not allowed under European law. Osmosis is an example of diffusion. which is a thin film composite membrane (TFM or TFC) optionally. or in countries where the local water supply is polluted or substandard. fishing. while the TFC membrane is prone to breaking down under the influence of chlorine. a second sediment filter with smaller pores an activated carbon filter to trap organic chemicals and chlorine. The CTA membrane is prone to rotting unless protected by chlorinated water. Rural people filter river or ocean water themselves. though. or bypass the membrane entirely through tiny . In practice. far away from the city's water pipes. in a similar setup as that in osmosis. use RO water processors coupled with one or more UV sterilizers. Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. or island camping trips. the carbon prefilter is omitted. solute concentration will decrease. the water feeding to these units should best be under some pressure (40 psi or greater is the norm). when the solvent moves into areas of high concentration.

In the United States military. meter per day) should be treated in an effluent treatment plant first. and then the clear effluent is subjected to reverse osmosis system.000 soldiers.000 to 60. as a solution to the problem of water shortages. Microfiltration removes particles of 50 nm or larger. Capacities range from 1500 gallons per day (GPD) to 150. Food Industry In addition to desalination. with a required 4-hour maintenance window to check systems. During a normal 24-hour period. reverse osmosis water purification units (ROWPUs). The effluent in larger volumes (more than 500 cu. reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. A single ROWPU can sustain a force the size of a battalion.leaks in surrounding seals.000 GPD.000 nm or larger. "hyperfiltration".000 to 6. It is also used to clean effluent and brackish groundwater. The process of reverse osmosis can be used for the production of deionized water. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs. depending on the need. one unit can produce 12. "Ultrafiltration" removes particles of roughly 3 nm or larger. or roughly 1. are used on the battlefield and in training. Treatment cost is reduced significantly and membrane life of the RO system is increased. RO elements and the engine generator. Both are able to purify salt water and water contaminated with nuclear/biological/chemical (NBC) contamination from the water. It must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. "Particle filtration" removes particles of 1. complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.1 to 5. Reverse osmosis is in the final category of membrane filtration.000 nanometers (nm) depending on filter type. "Nanofiltration" removes particles of 1 nm or larger. hence poor power production at turbine. Water and wastewater purification Rain water collected from storm drains is purified with reverse osmosis water processors and used for landscape irrigation and industrial cooling in Los Angeles and other cities. Singapore announced that a process named NEWater would be a significant part of its future water plans. . Its advantages include a lower operating cost and the ability to avoid heat-treatment processes. In 2002. The most common of these are the 600 gallons per hour (GPH) and the 3. Membrane pore sizes can vary from 0. which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.000 gallons of water. and removes particles larger than 0. Research has been done on concentration of orange juice and tomato juice. reverse osmosis removes minerals from boiler water at power plants. Thus. The deposits inside or outside the boiler tubes may result in under-performance of the boiler.1 nm. The water is boiled and condensed repeatedly.000 GPH units. bringing down its efficiency and resulting in poor steam production. In industry. pumps.

the whey (liquid remaining after cheese manufacture) is concentrated with RO from 6% total solids to 10-20% total solids before UF (ultrafiltration) processing. it is now widely understood and used. such as air blowers. some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. the UF permeate. reverse osmosis is sometimes used to prevent formation of minerals on the surface of electrodes. including whey protein isolate used in bodybuilding formulations. [edit] Reef aquariums Typical RO/DI unit used for an aquarium . [edit] Maple Syrup Production In 1946. especially in drought stricken areas where water conservation is important. The use of reverse osmosis allows approximately 5442% of the water to be removed from the sap. 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. reverse osmosis water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. [edit] Hydrogen production For small scale production of hydrogen. Additionally. reducing energy consumption and exposure of the syrup to high temperatures. An estimated 60 reverse osmosis machines were in use in Bordeaux. Reverse osmosis is often used to conserve and recycle water within the wash/pre-rinse cycles. In whey applications. France in 2002. [edit] Car Washing Because of its lower mineral content. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux. The UF retentate can then be used to make various whey powders. Microbial contamination and degradation of the membranes has to be monitored.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. Although use of the process was once avoided in the wine industry. Reverse osmosis water also enables the car wash operators to reduce the demands on the vehicle drying equipment.

Many reef aquarium keepers use reverse osmosis systems for their artificial mixture of seawater. which reduces energy losses in transmission and allows waste heat to be used in the desalination process of multistage flash plants. [edit] Desalination Areas that have either no or limited surface water or groundwater may choose to desalinate seawater or brackish water to obtain drinking water. energy requirements are low in comparison to other processes of desalination.[6] The typical single-pass SWRO system consists of the following components: • • • • • • • Intake Pretreatment High pressure pump Membrane assembly Remineralisation and pH adjustment Disinfection Alarm/control panel [edit] Pretreatment Pretreatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. although 85 percent of desalinated water is produced in multistage flash plants.[4][5] The project was developed as a BOT (Build-Operate-Transfer) by a consortium of three international companies: Veolia water. and is preferred above other water purification processes due to the low cost of ownership and minimal operating costs. Because no heating or phase changes are needed. Contaminants such as nitrogen compounds and phosphates can lead to excessive. 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. especially Saudi Arabia. California. as the common residential membrane used by reef keepers does not cope with these compounds. silicates. IDE Technologies and Elran. copper. but are still much higher than those required for other forms of water supply (including reverse osmosis treatment of wastewater). The energy requirements of the plants are large. Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s.[citation needed] The Ashkelon seawater reverse osmosis (SWRO) desalination plant in Israel is the largest in the world. or many other chemicals detrimental to the sensitive organisms in a reef environment. chloramines. and unwanted. Its first practical use was demonstrated by Sidney Loeb and Srinivasa Sourirajan from UCLA in Coalinga. Reverse osmosis is the most common method of desalination. nitrogen. reducing the amount of energy needed to desalinate the water and providing cooling for the power plant. algae growth. An effective combination of both reverse osmosis and deionization (RO/DI) is the most popular among reef aquarium keepers. phosphates. Ordinary tap water can often contain excessive chlorine.[3] Large reverse osmosis and multistage flash desalination plants are used in the Middle East. carbon filtration is needed before the membrane. Where chlorine and chloramines are found in the water. The material is engineered in such a fashion as to allow only .

antiscalants inhibit sulfate and fluoride scales. the spiral wound design does not allow for backpulsing with water or air agitation to scour its surface and remove solids.6 to 2. [edit] Membrane assembly . followed by bisulfite dosing to deactivate the chlorine.5 flow through the system. they are highly susceptible to fouling (loss of production capacity). even as the membrane rejects the passage of salt through it. such as chlorine. or 1. and specialty products can be to inhibit silica formation. Dosing: Oxidizing biocides. This requires a large amount of energy. Therefore. and reduce the risk of damage to high-pressure pump components.+ H2O HCO3. barium sulfate or strontium sulfate scale formation on the RO membrane. which kill bacteria. Since accumulated material cannot be removed from the membrane surface systems. acid is dosed to maintain carbonates in their soluble carbonic acid form. string-wound polypropylene filters are used to remove particles between 1 . Typical pressures for brackish water range from 225 to 375 psi (15.6 MPa).+ H3O+ = H2CO3 + H2O • • • • Carbonic acid cannot combine with calcium to form calcium carbonate scale. In addition to inhibiting carbonate and phosphate scales. disperse colloids and metal oxides. In the case of seawater. Pretreatment in SWRO systems 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. Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to acid.5 to 26 bar. • [edit] High pressure pump The pump supplies the pressure needed to push water through the membrane. pretreatment is a necessity for any RO or NF system.180 psi (55 to 81. There are also biofouling inhibitors. Cartridge filtration: Generally. but simply prevent them from growing slime on the membrane surface and plant walls. they range from 800 to 1. which can destroy a thin-film composite membrane. Calcium carbonate scaling tendency is estimated using the Langelier saturation index. are added to kill bacteria. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate. which can only prevent formation of calcium carbonate and calcium phosphate scales.5 bar or 6 to 8 MPa). Prefiltration pH adjustment: If the pH. As such. hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream. CO3-2 + H3O+ = HCO3.

The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. using UV lamps directly on the product. [edit] Remineralisation and pH adjustment The desalinated water is very corrosive and is "stabilized" to protect downstream pipelines and storages. The remainder is discharged as waste water. Wastewater is typically connected to the house drains and will add to the load on the household septic system. The membrane must be strong enough to withstand whatever pressure is applied against it. primarily for effective disinfection and for corrosion control. either chlorination or chloramination is used to provide a "residual" disinfection agent in the water supply system to protect against infection of the water supply by contamination entering the system. Disinfection may be by means of ultraviolet radiation. because they can generate the high pressure needed for more efficient RO filtration. usually by adding lime or caustic to prevent corrosion of concrete lined surfaces. In many countries. with the two most common configurations being spiral-wound and hollow-fiber. RO membranes are made in a variety of configurations. they recover only 5 to 15 percent of the water entering the system. protozoa and viruses that have bypassed the desalination process into the product water.[7] Large-scale industrial/municipal systems have a production efficiency closer to 48%. however. An RO unit delivering 5 gallons of treated water per day may discharge 40 to 90 gallons of wastewater per day to the septic system. Liming material is used to adjust pH between 6. [edit] Disadvantages Household reverse osmosis units use a lot of water because they have low back pressure. Disinfection (sometimes called germicidal or bactericidal) is employed to sterilise any bacteria.1 to meet the potable water specifications. Desalination processes are very effective barriers to pathogenic organisms. methods to recover this water are not practical for household systems. As a result.8 and 8.The layers of a membrane. [edit] Disinfection Posttreatment consists of stabilizing the water and preparing it for distribution. Because waste water carries with it the rejected contaminants. disinfection is used to ensure a "safe" water supply. . or by chlorination or chloramination (chlorine and ammonia).

this means the water passes through two membranes and is often repressurized. . has been evaluated and sometimes used. However. since the 1970s. increasing the cost. larger-pore membrane with less hydraulic energy requirement. requiring more energy input in the system. Other recent development work has focused on integrating RO with electrodialysis to improve recovery of valuable deionized products or minimize concentrate volume requiring discharge or disposal.[edit] New developments Prefiltration of high fouling waters with another.

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