Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemical species as well as biological substances (principally bacteria), and is used in industrial processes and the production of potable water. In developing nations like Pakistan, industrial reverse osmosis plants are widely adopted across textile, pharmaceutical, and manufacturing sectors to overcome groundwater contamination and ensure compliant process water.
RO retains the solute on the pressurized side of the membrane and the purified solvent passes to the other side. The relative sizes of the various molecules determines what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules (such as solvent molecules, e.g., water).
Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules. As of 2013 the world's largest RO desalination plant was in Sorek, Israel, outputting . RO systems for private use are also available for purifying municipal tap water or pre-treated well water.
History A process of osmosis through semi-permeable membranes was first observed in 1748 by Jean-Antoine Nollet. For the following 200 years, osmosis was only a laboratory phenomenon. In 1950, the University of California at Los Angeles (UCLA) first investigated osmotic desalination. Researchers at both UCLA and University of Florida desalinated seawater in the mid-1950s, but the flux was too low to be commercially viable. Sidney Loeb at UCLA and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, found techniques for making asymmetric membranes characterized by an effectively thin "skin" layer supported atop a highly porous and much thicker substrate region. John Cadotte, of Filmtec corporation, discovered that membranes with particularly high flux and low salt passage could be made by interfacial polymerization of m-phenylene diamine and trimesoyl chloride. Cadotte's patent on this processCadotte, John E. (1981) "Interfacially synthesized reverse osmosis membrane" was the subject of litigation and expired. Almost all commercial RO membrane is now made by this method. By 2019, approximately 16,000 desalination plants operated around the world, producing around . Around half of this capacity was in the Middle East and North Africa region.
In 1977 Cape Coral, Florida became the first US municipality to use RO at scale, with an initial operating capacity of 11.35 million liters (3 million US gal) per day. By 1985, rapid growth led the city to operate the world's largest low-pressure RO plant, producing 56.8 million liters (15 million US gal) per day (MGD).2012 Annual Consumer Report on the Quality of Tap Water . City of Cape Coral
Osmosis In (forward) osmosis, the solvent moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the Gibbs free energy of the system in which the difference in solvent concentration between the sides of a membrane is reduced. This is called osmotic pressure. It reduces as the solvent moves into the more concentrated solution. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications.
RO differs from filtration in that the mechanism of fluid flow is reversed, as the solvent crosses membrane, leaving the solute behind. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution's pressure and concentration. RO instead involves solvent diffusion across a membrane that is either nonporous or uses nanofiltration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.
RO requires pressure between 2–17 bar (30–250 psi) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater. Seawater has around 27 bar (390 psi) natural osmotic pressure that must be overcome. As for their energy consumption, seawater RO systems typically require 2.9-5.5 kWh/m3, although state-of-the-art systems are around 2.3 kWh/m3.
Membrane pore sizes vary from 0.1 to 5,000 nm. Particle filtration removes particles of 1 μm 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. RO is in the final category of membrane filtration, hyperfiltration, and removes particles larger than ~0.2 nm.
Fresh water applications Reverse osmosis is used for household and municipal drinking-water treatment, water reuse, industrial process-water production, food and beverage processing, and other applications requiring water with a reduced concentration of dissolved substances.
Drinking water purification
Point-of-use RO systems are used to treat water for drinking and cooking. Systems may include additional filters before or after the RO membrane. According to the Centers for Disease Control and Prevention (CDC), RO systems can remove parasites, bacteria and viruses, as well as some dissolved chemicals, including lead, copper, chromium, chloride and sodium. They may also reduce the concentrations of substances including arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium and nitrate. The contaminants removed vary between systems, so individual contaminant-reduction claims must be checked for the particular unit.
At municipal scale, water-treatment plants use RO when treating saline water or recycled water for drinking. RO is normally used as one stage of a larger treatment system that may also include pretreatment and disinfection.
Solar-powered reverse osmosis
Electricity generated by photovoltaic panels can be used to operate RO desalination systems. Renewable-energy-powered desalination is particularly applicable to remote or off-grid locations, although intermittent energy production and the need for water or energy storage can complicate system design.
Military and emergency use
Mobile reverse-osmosis water-purification systems are used by military organizations to produce potable water in the field. The United States Army operates several types of mobile systems, including the Reverse Osmosis Water Purification Unit (ROWPU), for military operations and for humanitarian and disaster-relief missions. Depending on the system, source water may include fresh, brackish or seawater.
Water reuse and industrial process water
RO is used as an advanced treatment stage in some systems that convert treated wastewater into high-quality reclaimed water. One example is Singapore's NEWater program. Its treatment process generally combines microfiltration or ultrafiltration, reverse osmosis and ultraviolet disinfection. The resulting water is used mainly for industrial and cooling purposes, while some is added to reservoirs for indirect potable reuse.
Industrial uses of RO include the demineralisation of boiler feedwater and the production of low-conductivity process water.
Food and beverage processing
RO can concentrate liquid foods without the prolonged heating required by evaporation, helping preserve heat-sensitive components. Applications include concentrating fruit and vegetable juices, preconcentrating milk and whey, improving grape must, and reducing the alcohol content of wine.
Maple syrup production
Commercial maple syrup producers use RO to remove part of the water from maple sap before it is boiled in an evaporator. Concentrating the sap before boiling reduces the time and energy required for evaporation.
Low-alcohol beer
RO is one of several industrial methods used to remove some or all of the ethanol from conventionally brewed beer. It is classified as a membrane-based physical method of producing low-alcohol or alcohol-free beer.
During RO dealcoholisation, water and ethanol pass through the membrane more readily than many non-volatile components of the beer. The retained concentrate can subsequently be diluted to the desired alcohol concentration.
Hydrogen production
RO can be used as a desalination stage in the preparation of feedwater for electrolytic hydrogen production. Electrolysers require high-purity water, so RO permeate generally requires further purification, such as ion exchange or electrodeionisation, before it is supplied to the electrolyser.
Aquariums
RO water is used in marine and reef aquariums to prepare artificial seawater and to replace water lost through evaporation. Because RO water contains low concentrations of dissolved substances, its use can limit the introduction of compounds such as silicates that can contribute to unwanted growth in aquariums. Aquarium shops may sell either freshwater produced by RO or prepared RO saltwater.
Window cleaning
Some professional window cleaners use purified water delivered through a telescoping water-fed pole fitted with a brush. RO, deionisation or a combination of the two can be used to produce the purified water. Because most dissolved minerals have been removed, the rinse water can be allowed to dry on the glass without leaving the mineral deposits associated with untreated water. Landfill leachate purification
Treatment with RO is limited, resulting in low recoveries on high concentration (measured with electrical conductivity) and membrane fouling. RO applicability is limited by conductivity, organics, and scaling inorganic elements such as CaSO4, Si, Fe and Ba. Low organic scaling can use two different technologies: spiral wound membrane, and (for high organic scaling, high conductivity and higher pressure (up to 90 bars)), disc tube modules with RO membranes can be used. Disc tube modules were redesigned for landfill leachate purification that is usually contaminated with organic material. Due to the cross-flow, it is given a flow booster pump that recirculates the flow over the membrane between 1.5 and 3 times before it is released as a concentrate. High velocity protects against membrane scaling and allows membrane cleaning.
Power consumption for a disc tube module system
Energy consumption per m3 leachatename of module1-stage up to 75 bar2-stage up to 75 bar3-stage up to 120 bardisc tube module6.1–8.1 kWh/m38.1–9.8 kWh/m311.2–14.3 kWh/m3
Desalination Areas that have limited surface water or groundwater may choose to desalinate. RO is an increasingly common method, because of its relatively low energy consumption.
Energy consumption is around , with the development of more efficient energy recovery devices and improved membrane materials. According to the International Desalination Association, for 2011, RO was used in 66% of installed desalination capacity (0.0445 of 0.0674 km3/day), and nearly all new plants.International Desalination Association Yearbook 2012–13 Other plants use thermal distillation methods: multiple-effect distillation, and multi-stage flash.
Sea-water RO (SWRO) desalination requires around 3 kWh/m3, much higher than those required for other forms of water supply, including RO treatment of wastewater, at 0.1 to 1 kWh/m3. Up to 50% of the seawater input can be recovered as fresh water, though lower recovery rates may reduce membrane fouling and energy consumption.