Reverse osmosis

{{short description|Water purification process}}
{{pp|small=yes}}
{{Use dmy dates|date=May 2022}}

{{Desalination}}
'''Reverse [[osmosis]]''' ('''RO''') is a [[water purification]] process that uses a [[partially permeable membrane|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]]. RO retains the [[solute]] on the pressurized side of the membrane and the purified [[solvent]] passes to the other side. It relies on the relative sizes of the various molecules to decide what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules (such as solvent molecules, e.g., water).<ref name=WarsingerBatch>{{Cite journal|last1=Warsinger|first1=David M.|last2=Tow|first2= Emily W.|last3=Nayar|first3=Kishor G.|last4=Maswadeh|first4=Laith A.|last5=Lienhard V|first5=John H.|title=Energy efficiency of batch and semi-batch (CCRO) reverse osmosis desalination|journal=Water Research|volume=106|pages=272–282|doi=10.1016/j.watres.2016.09.029|pmid=27728821|year=2016|bibcode=2016WatRe.106..272W |hdl=1721.1/105441|doi-access=free}}</ref>

RO is most commonly known for its use in drinking [[water purification]] from [[seawater]], removing the salt and other [[effluent]] materials from the water molecules.<ref>{{Cite journal|last1=Panagopoulos|first1=Argyris|last2=Haralambous|first2=Katherine-Joanne|last3=Loizidou|first3=Maria|date=25 November 2019|title=Desalination brine disposal methods and treatment technologies – A review|journal=Science of the Total Environment|volume=693|pages=133545|doi=10.1016/j.scitotenv.2019.07.351|pmid=31374511|issn=0048-9697|bibcode=2019ScTEn.693m3545P|s2cid=199387639 }}</ref>

As of 2013 the world's largest RO desalination plant was in [[Nahal Sorek|Sorek, Israel]], outputting {{convert|624|e3m3/day|e6usgal/day|abbr=off}}.<ref>{{cite web |last=Wang |first=Brian |date=19 February 2015 |title=Next Big Future: Israel scales up Reverse Osmosis Desalination to slash costs with a fourth of the piping |url=http://nextbigfuture.com/2015/02/isreal-scales-up-reverse-osmosis.html |publisher=nextbigfuture.com}}</ref>

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== 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.<ref>{{cite journal |title=The early history of reverse osmosis membrane development|doi=10.1016/S0011-9164(98)00122-2 |author=Glater, J. |year=1998 |journal=Desalination |volume=117 |issue=1–3 |pages=297–309 }}</ref> [[Sidney Loeb]] at UCLA and Srinivasa Sourirajan<ref>{{cite journal|author=Weintraub, Bob|url=https://drive.google.com/file/d/1hpgY6dd0Qtb4M6xnNXhutP4pMxidq_jqG962VzWt_W7-hssGnSxSzjTY8RvW/edit?usp=sharing |title=Sidney Loeb, Co-Inventor of Practical Reverse Osmosis|journal=Bulletin of the Israel Chemical Society|date=December 2001| issue =8|pages= 8–9}}</ref> 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-Phenylenediamine|''m''-phenylene diamine]] and trimesoyl chloride. Cadotte's patent on this process<ref>Cadotte, John E. (1981) "Interfacially synthesized reverse osmosis membrane" {{US Patent|4277344}}</ref> 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 {{convert|95|e6m3/day|e9usgal/day|abbr=off}}. Around half of this capacity was in the Middle East and North Africa region.<ref>{{cite journal |last1=Jones |display-authors=etal |first1=Edward |title=The state of desalination and brine production: A global outlook |journal=Science of the Total Environment |date=20 March 2019 |volume=657 |pages=1343–1356 |doi=10.1016/j.scitotenv.2018.12.076|pmid=30677901 |bibcode=2019ScTEn.657.1343J |s2cid=59250859 }}</ref>

[[File:Northcapecoral-RO.jpg|thumb|RO production train, North Cape Coral Reverse Osmosis Plant]]In 1977 [[Cape Coral]], Florida became the first US municipality to use RO at scale, with an initial operating capacity of 11.35&nbsp;million liters (3&nbsp;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&nbsp;million liters (15&nbsp;million US gal) per day (MGD).<ref>[http://www.capecoral.net/department/utilities_department/docs/2012_Citywide_CCR.pdf 2012 Annual Consumer Report on the Quality of Tap Water] {{Webarchive|url=https://web.archive.org/web/20160304001506/http://www.capecoral.net/department/utilities_department/docs/2012_Citywide_CCR.pdf |date=4 March 2016 }}. City of Cape Coral</ref>

== 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 [[micrometre|micrometer]]s 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 (biology)|diffusivity]], and the process is dependent on [[pressure]], solute concentration, and other conditions.<ref name="water">Crittenden, John; Trussell, Rhodes; Hand, David; Howe, Kerry and Tchobanoglous, George (2005). ''Water Treatment Principles and Design'', 2nd ed. John Wiley and Sons. New Jersey. {{ISBN|0-471-11018-3}}</ref>

RO requires pressure between 2–17 [[Bar (unit)|bar]] (30–250 [[Pound-force per square inch|psi]]) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater. Seawater has around 27 bar (390 psi)<ref>{{cite web |last=Lachish |first=Uri |title=Optimizing the Efficiency of Reverse Osmosis Seawater Desalination |url=http://urila.tripod.com/Seawater.htm |publisher=guma science}}</ref> natural [[osmotic pressure]] that must be overcome.

Membrane pore sizes vary from 0.1 to 5,000&nbsp;nm. Particle filtration removes particles of 1 [[micrometre|µm]] or larger. [[Microfiltration]] removes particles of 50&nbsp;nm or larger. [[Ultrafiltration]] removes particles of roughly 3&nbsp;nm or larger. Nanofiltration removes particles of 1&nbsp;nm or larger. RO is in the final category of membrane filtration, hyperfiltration, and removes particles larger than 0.1&nbsp;nm.<ref>"[https://www.researchgate.net/file.PostFileLoader.html?id=56b22df55e9d97048e8b45da&assetKey=AS%3A325085152989184%401454517748678 Purification of Contaminated Water with Reverse Osmosis]" ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 12, December 2013</ref>

== Fresh water applications ==
{{More citations needed section|date=May 2023}}[[File:Yen Sun Technology YS-8103RWT 20201101.jpg|thumb|Countertop RO system]]

=== Drinking water purification ===
Around the world, household [[drinking water]] [[water purification|purification]] systems, including a RO step, are commonly used for improving water for drinking and cooking.

Such systems typically include these steps:
* a [[sediment]] filter to trap particles, including rust and [[calcium carbonate]]
* a second sediment filter with smaller pores
* an [[activated carbon]] filter to trap [[Organic compound|organic chemicals]] and [[chlorine]], which degrades certain types of [[thin-film composite membrane]]
* an RO thin-film composite membrane
* an [[ultraviolet]] lamp for sterilizing any [[Microorganism|microbes]] that survive RO
* a second carbon filter to capture chemicals that survive RO
In some systems, the carbon prefilter is replaced by a [[cellulose triacetate]] (CTA) membrane. CTA is a paper by-product membrane bonded to a synthetic layer that allows contact with chlorine in the water. These require a small amount of chlorine in the water source to prevent bacteria from forming on it. The typical rejection rate for CTA membranes is 85–95%.

The cellulose triacetate membrane rots unless protected by [[Water chlorination|chlorinated water]], while the thin-film composite membrane breaks down in the presence of chlorine. The [[thin-film composite]] (TFC) membrane is made of synthetic material, and requires the chlorine to be removed before the water enters the membrane. To protect the TFC membrane elements from chlorine damage, [[carbon filter]]s are used as pre-treatment. TFC membranes have a higher rejection rate of 95–98% and a longer life than CTA membranes.

Portable RO water processors are sold for personal water available. To work effectively, the water feeding to these units should be under pressure (typically 280 kPa (40 psi) or greater).<ref>{{cite book|last1=Knorr|first1=Erik Voigt, Henry Jaeger, Dietrich|title=Securing Safe Water Supplies : comparison of applicable technologies|date=2012|publisher=[[Academic Press]]|location=[[Oxford]]|isbn=978-0124058866|page=33|edition=Online-Ausg.|url=https://books.google.com/books?id=fWGZLmhpxvgC&pg=PA33}}</ref> These processors can be used in areas lacking clean water.

US mineral water production uses RO. In Europe such processing of natural [[mineral water]] (as defined by a European directive)<ref>[http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0777:19961213:EN:PDF Council Directive of 15 July 1980 on the approximation of the laws of the Member States relating to the exploitation and marketing of natural mineral waters]. eur-lex.europa.eu</ref> is not allowed. In practice, a fraction of the living bacteria pass through RO through membrane imperfections or bypass the membrane entirely through leaks in seals.

For household purification absent the need to remove dissolved minerals (soften the water), the alternative to RO is an activated carbon filter with a microfiltration membrane.

====Solar-powered RO====
A [[solar-powered desalination unit]] produces [[potable water]] from [[saline water]] by using a [[photovoltaic]] system to supply the energy. Solar power works well for water purification in settings lacking grid electricity and can reduce operating costs and [[greenhouse emissions]]. For example, a solar-powered desalination unit designed passed tests in [[Australia's]] [[Northern Territory]].<ref name=uow>{{cite web|url=http://media.uow.edu.au/news/2005/1104c/ |title=Award-winning Solar Powered Desalination Unit aims to solve Central Australian water problems |date=4 November 2005 |publisher=University of Wollongong |access-date=19 July 2017}}</ref>

Sunlight's intermittent nature makes output prediction difficult without an energy storage capability. However batteries or [[thermal energy storage]] systems can provide power when the sun does not.<ref name="TES">[http://ac.els-cdn.com/S0306261911006696/1-s2.0-S0306261911006696-main.pdf?_tid=1afa88d0641c9bdee6495e5eec5cbc9c&acdnat=1335424299_1b6be246f1cf3e2adbbc38a26e965813 Low temperature desalination using solar collectors augmented by thermal energy storage]</ref>

==== Military ====
Larger scale reverse osmosis water purification units (ROWPU) exist for military use. These have been adopted by the [[United States armed forces]] and the [[Canadian Forces]]. Some models are [[Containerization|containerized]], some are trailers, and some are themselves vehicles.{{citation needed|date=March 2015}}

The water is treated with a [[polymer]] to initiate [[coagulation]]. Next, it is run through a multi-media filter where it undergoes primary treatment, removing [[turbidity]]. It is then pumped through a cartridge filter which is usually spiral-wound cotton. This process strips any particles larger than 5 [[micrometre|µm]] and eliminates almost all turbidity.

The clarified water is then fed through a high-pressure piston pump into a series of RO vessels. 90.00–99.98% of the raw water's [[total dissolved solids]] are removed and military standards require that the result have no more than 1000–1500 [[parts per million]] by measure of [[electrical conductivity]]. It is then disinfected with [[chlorine]].{{citation needed|date=February 2016}}

=== Water and wastewater purification ===
RO-purified rainwater collected from storm drains is used for landscape irrigation and industrial cooling in Los Angeles and other cities.

In industry, RO removes minerals from [[boiler water]] at [[power plant]]s.<ref>{{cite book |editor-last1=Shah |editor-first1=Vishal |title=Emerging Environmental Technologies |date=2008 |publisher=[[Springer Science]] |location=[[Dordrecht]] |isbn=978-1402087868 |page=108 |url=https://books.google.com/books?id=ldeJ713eKPAC&pg=PA108}}</ref> The water is [[distilled]] multiple times to ensure that it does not leave deposits on the machinery or cause corrosion.

RO is used to clean effluent and [[Brackish water|brackish groundwater]]. The effluent in larger volumes (more than 500 m<sup>3</sup>/day) is treated in a [[water treatment plant]] first, and then the effluent runs through RO. This hybrid process reduces treatment cost significantly and lengthens membrane life.

RO can be used for the production of [[deionized water]].<ref>{{cite book|last1=Grabowski|first1=Andrej|title=Electromembrane desalination processes for production of low conductivity water|date=2010|publisher=Logos-Verl.|location=Berlin|isbn=978-3832527143|url=https://books.google.com/books?id=ORVFoMq6HroC&pg=PA2}}</ref>

In 2002, Singapore announced that a process named [[NEWater]] would be a significant part of its water plans. RO would be used to treat wastewater before discharging the effluent into reservoirs.

=== Food industry ===
Reverse osmosis is a more economical way to concentrate liquids (such as fruit juices) than conventional heat-treatment. Concentration of orange and tomato juice has advantages including a lower operating cost and the ability to avoid heat-treatment, which makes it suitable for heat-sensitive substances such as [[protein]] and [[enzyme]]s.

RO is used in the dairy industry to produce [[whey protein]] powders and concentrate milk. The [[whey]] (liquid remaining after cheese manufacture) is concentrated with RO from 6% solids to 10–20% solids before [[ultrafiltration]] processing. The retentate can then be used to make whey powders, including [[whey protein isolate]]. Additionally, the permeate, which contains [[lactose]], is concentrated by RO from 5% solids to 18–total solids to reduce crystallization and drying costs.

Although RO was once avoided in the wine industry, it is now widespread. An estimated 60 RO machines were in use in [[Bordeaux]], France, in 2002. Known users include many of elite firms, such as [[Château Léoville-Las Cases]].

=== Maple syrup production ===
In 1946, some [[maple syrup]] producers started using RO to remove water from [[plant sap|sap]] before boiling the sap to [[syrup]]. RO allows about 75–90% of the water to be removed, reducing energy consumption and exposure of the syrup to high temperatures.

===Low-alcohol beer===
{{main|Low-alcohol beer}}
When beer at typical concentration is subjected to reverse osmosis, both water and alcohol pass across the membrane more readily than other components, leaving a "beer concentrate". The concentrate is then diluted with fresh water to restore the non-volatile components to their original intensity.<ref>{{cite book |last1=Lewis |first1=Michael J |last2=Young |first2=Tom W |title=Brewing |date=6 December 2012 |publisher=Kluwer |location=New York |isbn=978-1-4615-0729-1 |page=110 |edition=2}}</ref>

=== Hydrogen production ===
For small-scale [[hydrogen production]], RO is sometimes used to prevent formation of mineral deposits on the surface of [[electrode]]s.

=== Aquariums ===
Many [[reef aquarium]] keepers use RO systems to make fish-friendly seawater. Ordinary tap water can contain excessive [[chlorine]], [[chloramines]], [[copper]], [[nitrate]]s, [[nitrite]]s, [[phosphate]]s, [[silicate]]s, or other chemicals detrimental to marine organisms. Contaminants such as [[nitrogen]] and phosphates can lead to unwanted algae growth. An effective combination of both RO and [[deionization]] is popular among reef aquarium keepers, and is preferred above other water purification processes due to the low cost of ownership and operating costs. Where [[chlorine]] and [[chloramine]]s are found in the water, carbon filtration is needed before RO, as common residential membranes do not address these compounds.

Freshwater aquarists also use RO to duplicate the soft waters found in many tropical waters. While many tropical fish can survive in treated tap water, breeding can be impossible. Many aquatic shops sell containers of RO water for this purpose.

===Window cleaning===
An increasingly popular method of cleaning windows is the "water-fed pole" system. Instead of washing windows with conventional detergent, they are scrubbed with purified water, typically containing less than 10 ppm dissolved solids, using a brush on the end of a pole wielded from ground level. RO is commonly used to purify the water.

== Landfill leachate purification ==
{{Unreferenced section|date=May 2023}}
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 CaSO<sub>4</sub>, 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===
[[File:Disc tube module and Spiral wound module.png|alt=Disc tube module and Spiral wound module|thumb|Disc tube module with RO membrane cushion and Spiral wound module with RO membrane]]

{| class="wikitable"
! colspan="4" |Energy consumption per m<sup>3</sup> leachate
|-
|name of module
|1-stage up to 75 bar
|2-stage up to 75 bar
|3-stage up to 120 bar
|-
|disc tube module
|6.1–8.1 kWh/m<sup>3</sup>
|8.1–9.8 kWh/m<sup>3</sup>
|11.2–14.3 kWh/m<sup>3</sup>
|}

== Desalination ==
Areas that have limited surface water or groundwater may choose to [[desalination|desalinate]]. RO is an increasingly common method, because of its relatively low energy consumption.<ref name=WarsingerEntropy>{{Cite journal|last1=Warsinger|first1=David M.|last2=Mistry|first2= Karan H.|last3=Nayar|first3=Kishor G.|last4=Chung|first4=Hyung Won|last5=Lienhard V|first5=John H.|title=Entropy Generation of Desalination Powered by Variable Temperature Waste Heat|journal=Entropy|volume=17|issue=11|pages=7530–7566|doi=10.3390/e17117530|year=2015|bibcode=2015Entrp..17.7530W|doi-access=free}}</ref>

Energy consumption is around {{Convert|3|kWh/m3|J/L|abbr=on}}, 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&nbsp;km<sup>3</sup>/day), and nearly all new plants.<ref>[http://www.globalwaterintel.com/advertise/ida-desalination-yearbook/ International Desalination Association] Yearbook 2012–13</ref> Other plants use thermal distillation methods: [[multiple-effect distillation]], and [[Multi-Stage Flash|multi-stage flash]].

Sea-water RO (SWRO) desalination requires around 3 kWh/m<sup>3</sup>, much higher than those required for other forms of water supply, including RO treatment of wastewater, at 0.1 to 1 kWh/m<sup>3</sup>. Up to 50% of the seawater input can be recovered as fresh water, though lower recovery rates may reduce membrane fouling and energy consumption.

Brackish water reverse osmosis (BWRO) is the desalination of water with less salt than seawater, usually from river estuaries or saline wells. The process is substantially the same as SWRO, but requires lower pressures and less energy.<ref name=WarsingerBatch /> Up to 80% of the feed water input can be recovered as fresh water, depending on feed salinity.

The [[Ashkelon]] desalination plant in Israel is the world's largest.<ref>[http://www.israel21c.org/briefs/israel-is-no-5-on-top-10-cleantech-list Israel is No. 5 on Top 10 Cleantech List] in [http://www.israel21c.org/technology/archive Israel 21c A Focus Beyond] {{webarchive|url=https://web.archive.org/web/20101016013525/http://www.israel21c.org/technology/archive |date=16 October 2010 }} Retrieved 21 December 2009</ref><ref>[http://www.water-technology.net/projects/israel/ Desalination Plant Seawater Reverse Osmosis (SWRO) Plant]. Water-technology.net</ref><ref>{{cite journal|doi=10.1016/j.desal.2006.03.525|title=Ashkelon desalination plant&nbsp;— A successful challenge|year=2007|last1=Sauvetgoichon|first1=B|journal=Desalination|volume=203|issue=1–3|pages=75–81}}</ref>

The typical single-pass SWRO system consists of:
* Intake
* Pretreatment
* High-pressure pump (if not combined with energy recovery)
* Membrane assembly
* Energy recovery (if used)
* [[Remineralisation]] and pH adjustment
* Disinfection
* Alarm/control panel

=== Pretreatment ===
Pretreatment is important when working nanofiltration membranes due to their spiral-wound design. The material is engineered to allow one-way flow. The design does not allow for backpulsing with water or air agitation to scour its surface and remove accumulated solids. Since material cannot be removed from the membrane surface, it is susceptible to [[fouling]] (loss of production capacity). Therefore, pretreatment is a necessity for any RO or nanofiltration system. Pretreatment has four major components:
* Screening solids: Solids must be removed and the water treated to prevent membrane fouling by particle or biological growth, and reduce the risk of damage to high-pressure components.
* Cartridge filtration: String-wound [[polypropylene]] filters are typically used to remove particles of 1–5&nbsp;[[micrometre|µm]] diameter.
* [[Dosing]]: Oxidizing [[biocide]]s, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivate the chlorine that can destroy a thin-film composite membrane. [[Biofouling]] inhibitors do not kill bacteria, while preventing them from growing slime on the membrane surface and plant walls.
* Prefiltration [[pH]] adjustment: If the pH, [[hardness]] and the [[alkalinity]] in the feedwater result in scaling while concentrated in the reject stream, acid is dosed to maintain [[carbonate]]s in their soluble [[carbonic acid]] form.
:CO<sub>3</sub><sup>2−</sup> + H<sub>3</sub>O<sup>+</sup> = HCO<sub>3</sub><sup>−</sup> + H<sub>2</sub>O
:HCO<sub>3</sub><sup>−</sup> + H<sub>3</sub>O<sup>+</sup> = H<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O
* Carbonic acid cannot combine with calcium to form [[calcium carbonate]] scale. Calcium carbonate scaling tendency is estimated using the [[Hard water|Langelier saturation index]]. Adding too much [[sulfuric acid]] to control carbonate scales may result in [[calcium sulfate]], [[barium sulfate]], or [[strontium sulfate]] scale formation on the membrane.
* Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of more scales than acid, which can only prevent formation of calcium carbonate and [[calcium phosphate]] scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales and disperse [[colloid]]s and metal oxides. Despite claims that antiscalants can inhibit [[Silica Formation|silica formation]], no concrete evidence proves that silica [[polymerization]] is inhibited by antiscalants. Antiscalants can control acid-soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid.<ref>{{cite journal|author=Malki, M.|title=Optimizing scale inhibition costs in reverse osmosis desalination plants|journal=International Desalination and Water Reuse Quarterly|year=2008|volume=17|issue=4|pages=28–29 |url=https://www.membranechemicals.com/conference-papers/}}</ref>
* Some small-scale desalination units use 'beach wells'. These are usually drilled on the seashore. These intake facilities are relatively simple to build and the seawater they collect is pretreated via slow filtration through subsurface sand/seabed formations. Raw seawater collected using beach wells is often of better quality in terms of solids, silt, oil, grease, organic contamination, and microorganisms, compared to open seawater intakes. Beach intakes may also yield source water of lower salinity.

=== High pressure pump ===
The high pressure [[pump]] pushes water through the membrane. Typical pressures for [[brackish water]] range from 1.6 to 2.6 MPa (225 to 376 psi). In the case of seawater, they range from 5.5 to 8 MPa (800 to 1,180 psi). This requires substantial energy. Where energy recovery is used, part of the high pressure pump's work is done by the energy recovery device, reducing energy inputs.

=== Membrane assembly ===
[[File:Reverse osmosis membrane element layers.jpg|thumb|The layers of a membrane]]
[[File:Cutaway of a 16" reverse osmosis tube.jpg|thumb|Cutaway of a 16" RO tube]]
The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pushed against it. The membrane must be strong enough to withstand the pressure. RO membranes are made in a variety of configurations. The two most common are spiral-wound and [[Hollow fiber membrane|hollow-fiber]].

Only part of the water pumped onto the membrane passes through. The left-behind "concentrate" passes along the saline side of the membrane and flushes away the salt and other remnants. The percentage of desalinated water is the "recovery ratio". This varies with salinity and system design parameters: typically 20% for small seawater systems, 40% – 50% for larger seawater systems, and 80% – 85% for brackish water. The concentrate flow is typically 3 bar/50 psi less than the feed pressure, and thus retains much of the input energy.

The desalinated water purity is a function of the feed water salinity, membrane selection and recovery ratio. To achieve higher purity a second pass can be added which generally requires another pumping cycle. Purity expressed as [[total dissolved solids]] typically varies from 100 to 400 parts per million (ppm or mg/litre) on a seawater feed. A level of 500 ppm is generally the upper limit for drinking water, while the [[Food and Drug Administration|US Food and Drug Administration]] classifies [[mineral water]] as water containing at least 250 ppm.

===Energy recovery===
[[File:ReverseOsmosis with PressureExchanger.svg|thumb|upright=1.3|Schematics of a RO desalination system using a [[pressure exchanger]].<br>''1'': Sea water inflow,<br>''2'': Fresh water flow (40%),<br>''3'': Concentrate flow (60%),<br>''4'': Sea water flow (60%),<br>''5'': Concentrate (drain),<br>''A: Pump flow (40%),<br>''B'': Circulation pump,<br>''C'': Osmosis unit with membrane,<br>''D'': Pressure exchanger]]
[[File:Reverse Osmosis with Pressure Recovery Pump.jpg|thumb|upright=1.3|Schematic of a RO desalination system using an energy recovery pump.<br>''1'': Sea water inflow (100%, 1 bar),<br>''2'': Sea water flow (100%, 50 bar),<br>''3'': Concentrate flow (60%, 48 bar),<br>''4'': Fresh water flow (40%, 1 bar),<br>''5'': Concentrate to drain (60%,1 bar),<br>''A: [[Pressure exchanger|Pressure recovery pump]],<br>''B'': Osmosis unit with membrane]]

Energy recovery can reduce energy consumption by 50% or more. Much of the input energy can be recovered from the concentrate flow, and the increasing efficiency of energy recovery devices greatly reduces energy requirements. Devices used, in order of invention, are:
* [[Water turbine|Turbine]] or [[Pelton wheel]]: a water turbine driven by the concentrate flow, connected to the pump drive shaft provides part of the input power. Positive displacement axial piston motors have been used in place of turbines on smaller systems.
* Turbocharger: a water turbine driven by concentrate flow, directly connected to a [[centrifugal pump]] that boosts the output pressure, reducing the pressure needed from the pump and thereby its energy input,<ref name="Yu Jenne p=132">{{cite journal | last1=Yu | first1=Yi-Hsiang | last2=Jenne | first2=Dale | title=Numerical Modeling and Dynamic Analysis of a Wave-Powered Reverse-Osmosis System | journal=Journal of Marine Science and Engineering | publisher=MDPI AG | volume=6 | issue=4 | date=8 November 2018 | issn=2077-1312 | doi=10.3390/jmse6040132 | page=132| doi-access=free }}</ref> similar in construction principle to car engine [[turbocharger]]s.
* [[Pressure exchanger]]: using the pressurized concentrate flow, via direct contact or a piston, to pressurize part of the membrane feed flow to near concentrate flow pressure.<ref name="Stover 2007 pp. 168–175">{{cite journal | last=Stover | first=Richard L. | title=Seawater reverse osmosis with isobaric energy recovery devices | journal=Desalination | publisher=Elsevier BV | volume=203 | issue=1–3 | year=2007 | issn=0011-9164 | doi=10.1016/j.desal.2006.03.528 | pages=168–175}}</ref> A boost pump then raises this pressure by typically 3&nbsp;bar&nbsp;/&nbsp;50&nbsp;psi to the membrane feed pressure. This reduces flow needed from the high-pressure pump by an amount equal to the concentrate flow, typically 60%, and thereby its energy input. These are widely used on larger low-energy systems. They are capable of 3 kWh/m<sup>3</sup> or less energy consumption.
* [[Energy recovery|Energy-recovery]] pump: a reciprocating [[piston pump]]. The pressurized concentrate flow is applied to one side of each piston to help drive the membrane feed flow from the opposite side. These are the simplest energy recovery devices to apply, combining the high pressure pump and energy recovery in a single self-regulating unit. These are widely used on smaller low-energy systems. They are capable of 3&nbsp;kWh/m<sup>3</sup> or less energy consumption.
* Batch operation: RO systems run with a fixed volume of fluid (thermodynamically a [[closed system]]) do not suffer from wasted energy in the brine stream, as the energy to pressurize a virtually incompressible fluid (water) is negligible. Such systems have the potential to reach second-law efficiencies of 60%.<ref name=WarsingerBatch /><ref name="Cordoba Das Leon Garcia 2021 p=114959">{{cite journal | last1=Cordoba | first1=Sandra | last2=Das | first2=Abhimanyu | last3=Leon | first3=Jorge | last4=Garcia | first4=Jose M | last5=Warsinger | first5=David M | title=Double-acting batch reverse osmosis configuration for best-in-class efficiency and low downtime | journal=Desalination | publisher=Elsevier BV | volume=506 | year=2021 | issn=0011-9164 | doi=10.1016/j.desal.2021.114959 | page=114959| s2cid=233553757 }}</ref><ref name="Wei Tucker Wu Trueworthy 2020 p=114177">{{cite journal | last1=Wei | first1=Quantum J. | last2=Tucker | first2=Carson I. | last3=Wu | first3=Priscilla J. | last4=Trueworthy | first4=Ali M. | last5=Tow | first5=Emily W. | last6=Lienhard | first6=John H. | title=Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation | journal=Desalination | publisher=Elsevier BV | volume=479 | year=2020 | issn=0011-9164 | doi=10.1016/j.desal.2019.114177 | page=114177| hdl=1721.1/124221 | s2cid=213654912 | hdl-access=free }}</ref>

=== Remineralisation and pH adjustment ===
The desalinated water is stabilized to protect downstream pipelines and storage, usually by adding [[Lime (material)|lime]] or [[Sodium hydroxide|caustic soda]] to prevent corrosion of concrete-lined surfaces. Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control. Remineralisation may be needed to replace minerals removed from the water by desalination, although this process has proved to be costly and inconvenient in order to meet mineral demand by humans and plants as found in typical freshwater. For instance water from Israel's national water carrier typically contains dissolved magnesium levels of 20 to 25&nbsp;mg/liter, while water from the [[Ashkelon]] plant has no magnesium. Ashkelon water created [[Magnesium deficiency|magnesium-deficiency]] symptoms in crops, including tomatoes, basil, and flowers, and had to be remedied by fertilization. Israeli drinking water standards require a minimum calcium level of 20&nbsp;mg/liter. Askelon's post-desalination treatment uses sulfuric acid to dissolve calcite (limestone), resulting in calcium concentrations of 40 to 46&nbsp;mg/liter, lower than the 45 to 60&nbsp;mg/liter found in typical Israeli fresh water.

=== Disinfection ===
Post-treatment disinfection provides secondary protection against compromised membranes and downstream problems. Disinfection by means of [[Ultraviolet radiation|ultraviolet]] (UV) lamps (sometimes called germicidal or bactericidal) may be employed to sterilize pathogens that evade the RO process. [[Water chlorination|Chlorination]] or [[chloramination]] (chlorine and ammonia) protects against pathogens that may have lodged in the distribution system downstream.<ref>{{cite journal|last1=Sekar|first1=Chandru|title=IEEE R10 HTA Portable Autonomous Water Purification System|journal=[[IEEE]]|url=https://www.academia.edu/4350783|access-date=4 March 2015}}</ref>

==Disadvantages==
Large-scale industrial/municipal systems recover typically 75% to 80% of the feed water, or as high as 90%, because they can generate the required higher pressure.

=== Wastewater ===
Household RO units use a lot of water because they have low back pressure. Household RO water purifiers typically produce one liter of usable water and 3-25 liters of [[wastewater]].<ref>{{cite web|url=https://www.forbes.com/home-improvement/home/reverse-osmosis-water-pros-cons/|title=Learn The Pros And Cons Of Reverse Osmosis Water Filtration Systems|language=en-US |magazine=Forbes|access-date=2023-10-08}}</ref> The remainder is discharged, usually into the drain. Because wastewater carries the rejected contaminants, recovering this water is not practical for household systems. Wastewater is typically delivered to house drains. A RO unit delivering {{convert|20|liter|usgal}} of treated water per day also discharge between {{convert|50 and 80|liter|usgal}}. This led India's [[National Green Tribunal]] to propose a ban on RO water purification systems in areas where the [[total dissolved solids]] (TDS) measure in water is less than 500&nbsp;mg/liter.{{citation needed|date=May 2023}} In [[Delhi]], large-scale use of household RO devices has increased the total water demand of the already water-parched [[National Capital Territory of India]].<ref>{{Cite journal|last=Singh|first=Govind|year=2017|title=Implication of Household Use of R.O. Devices for Delhi's Urban Water Scenario|url=http://jiid.in/2017/02/implication-household-use-r-o-devices-delhis-urban-water-scenario/|journal=Journal of Innovation for Inclusive Development|volume=2|issue=1|pages=24–29|access-date=15 April 2017|archive-date=17 May 2017|archive-url=https://web.archive.org/web/20170517005500/http://jiid.in/2017/02/implication-household-use-r-o-devices-delhis-urban-water-scenario/|url-status=dead}}</ref>

=== Health ===
RO removes both harmful contaminants and desirable minerals. Some studies report some relation between long-term health effects and consumption of water low on [[calcium]] and [[magnesium]], although these studies are of low quality.<ref>{{cite web|last=Kozisek|first=Frantisek|url=https://www.who.int/water_sanitation_health/dwq/nutrientschap12.pdf|archive-date=7 February 2022|archive-url=https://web.archive.org/web/20220207222904/https://www.who.int/water_sanitation_health/dwq/nutrientschap12.pdf|title=Health risks from drinking demineralised water|publisher=[[National Institute of Public Health]]|location=[[Czech Republic]]}}</ref>

=== Waste-stream considerations ===
Depending upon the desired product, either the solvent or solute stream of RO will be waste. For food concentration applications, the concentrated solute stream is the product and the solvent stream is waste. For water treatment applications, the solvent stream is purified water and the solute stream is concentrated waste.<ref>{{cite book|last=Weber|first=Walter J.|title =Physicochemical Processes for Water Quality Control|publisher =John Wiley & Sons|year =1972|oclc=1086963937|url=https://books.google.com/books?id=2rS0AAAAIAAJ|location =New York|isbn =9780471924357|page=320}}</ref> The solvent waste stream from food processing may be used as [[reclaimed water]], but there may be fewer options for disposal of a concentrated waste solute stream. Ships may use [[marine dumping]] and coastal desalination plants typically use [[marine outfall]]s. Landlocked RO plants may require [[evaporation pond]]s or [[injection well]]s to avoid polluting [[groundwater]] or [[surface runoff]].<ref>{{cite book |last=Hammer |first=Mark J.|url=https://books.google.com/books?id=qgVSAAAAMAAJ|title =Water and Waste-Water Technology |publisher =John Wiley & Sons |date =1975 |location=New York|isbn =9780471347262|page=266}}</ref>

== Research ==

{{update section|date=March 2021}}

=== Improving Current Membranes ===
Current RO membranes, thin-film composite (TFC) polyamide membranes, are being studied to find ways of improving their permeability. Through new imaging methods, researchers were able to make 3D models of membranes and examine how water flowed through them. They found that TFC membranes with areas of low flow significantly decreased water permeability.<ref>{{Cite journal |last1=Culp |first1=Tyler E. |last2=Khara |first2=Biswajit |last3=Brickey |first3=Kaitlyn P. |last4=Geitner |first4=Michael |last5=Zimudzi |first5=Tawanda J. |last6=Wilbur |first6=Jeffrey D. |last7=Jons |first7=Steven D. |last8=Roy |first8=Abhishek |last9=Paul |first9=Mou |last10=Ganapathysubramanian |first10=Baskar |last11=Zydney |first11=Andrew L. |last12=Kumar |first12=Manish |last13=Gomez |first13=Enrique D. |date=January 2021 |title=Nanoscale control of internal inhomogeneity enhances water transport in desalination membranes |url=https://www.science.org/doi/10.1126/science.abb8518 |journal=Science |language=en |volume=371 |issue=6524 |pages=72–75 |doi=10.1126/science.abb8518 |pmid=33384374 |bibcode=2021Sci...371...72C |s2cid=229935140 |issn=0036-8075}}</ref> By ensuring uniformity of the membranes and allowing water to flow continuously without slowing down, membrane permeability could be improved by 30%-40%.<ref>{{Cite web |title=Desalination breakthrough could lead to cheaper water filtration |url=https://www.sciencedaily.com/releases/2020/12/201231141511.htm |access-date=2023-05-26 |website=ScienceDaily |language=en}}</ref>

=== Electrodialysis ===
Research has examined integrating RO with [[electrodialysis]] to improve recovery of valuable deionized products, or to reduce concentrate volumes.

=== Low-pressure High-recovery (LPHR) ===
Another approach is low-pressure high-recovery multistage RO (LPHR). It produces concentrated [[brine]] and freshwater by cycling the output repeatedly through a relatively porous membrane at relatively low pressure. Each cycle removes additional impurities. Once the output is relatively pure, it is sent through a conventional RO membrane at conventional pressure to complete the filtration step. LPHR was found to be economically feasible, recovering more than 70% with an OPD between 58 and 65 bar and leaving no more than 350 ppm TDS from a seawater feed with 35,000 ppm TDS.

=== Carbon Nanotubes (CNTs) ===
Carbon nanotubes are meant to potentially solve the typical tradeoff between the permeability and the selectivity of RO membranes. CNTs present many ideal characteristics including: mechanical strength, electron affinity, and also exhibiting flexibility during modification. By restructuring carbon nanotubes and coating or impregnating them with other chemical compounds, scientists can manufacture these membranes to have all of the most desirable traits. The hope with CNT membranes is to find a combination of high water permeability while also decreasing the amount of neutral solutes taken out of the water. This would help decrease energy costs and the cost of remineralization after purification through the membrane.<ref>{{Cite journal |last1=Ali |first1=Sharafat |last2=Rehman |first2=Syed Aziz Ur |last3=Luan |first3=Hong-Yan |last4=Farid |first4=Muhammad Usman |last5=Huang |first5=Haiou |date=2019-01-01 |title=Challenges and opportunities in functional carbon nanotubes for membrane-based water treatment and desalination |url=https://www.sciencedirect.com/science/article/pii/S0048969718328432 |journal=Science of the Total Environment |language=en |volume=646 |pages=1126–1139 |doi=10.1016/j.scitotenv.2018.07.348 |pmid=30235599 |bibcode=2019ScTEn.646.1126A |s2cid=52311560 |issn=0048-9697}}</ref>

=== Graphene ===
Graphene membranes are meant to take advantage of their thinness to increase efficiency. Graphene is a singular layer of carbon atoms, so it is about 1000 times thinner than existing membranes. Graphene membranes are around 100&nbsp;nm thick while current membranes are about 100&nbsp;µm. Many researchers were concerned with the durability of graphene and if it would be able to handle RO pressures. New research finds that depending on the substrate (a supporting layer that does no filtration and only provides structural support), graphene membranes can withstand 57MPa of pressure which is about 10 times the typical pressures for seawater RO.<ref>{{Cite journal |last1=Cohen-Tanugi |first1=David |last2=Grossman |first2=Jeffrey C. |date=2014-11-12 |title=Mechanical Strength of Nanoporous Graphene as a Desalination Membrane |url=https://pubs.acs.org/doi/10.1021/nl502399y |journal=Nano Letters |language=en |volume=14 |issue=11 |pages=6171–6178 |doi=10.1021/nl502399y |pmid=25357231 |bibcode=2014NanoL..14.6171C |issn=1530-6984}}</ref>

Batch RO may offer increased [[Energy efficiency (physics)|energy efficiency]], more durable equipment and higher salinity limits.

The conventional approach claimed that molecules cross the membrane individually. A research team devised a "solution-friction" theory, claiming that molecules in groups through transient pores. Characterizing that process could guide membrane development. The accepted theory is that individual water molecules diffuse through the membrane, termed the "solution-diffusion" model.<ref>{{Cite magazine |last=Levy |first=Max G. |title=Everyone Was Wrong About Reverse Osmosis—Until Now |language=en-US |magazine=Wired |url=https://www.wired.com/story/everyone-was-wrong-about-reverse-osmosis-until-now/ |access-date=2023-05-20 |issn=1059-1028}}</ref>

== See also ==
{{colbegin}}
* [[Electrodeionization]]
* [[ERDLator]]
* [[Forward osmosis]]
* [[Microfiltration]]
* [[Reverse osmosis plant]]
** [[Richard Stover]], pioneered the development of an energy-recovery device currently in use in most seawater reverse-osmosis desalination plants
* [[Silt density index]]
* [[Salinity gradient]]
* [[Milli-Q|Milli-Q water]]
* [[Water pollution]]
* [[Water quality]]
{{colend}}

== References ==
{{reflist}}

== Sources ==
* {{cite book |last1=Metcalf |last2=Eddy |title =Wastewater Engineering |publisher =McGraw-Hill Book Company |date =1972 |location =New York |isbn =978-0-070-49539-5}}

{{Separation processes}}
{{Wastewater}}

{{DEFAULTSORT:Reverse Osmosis}}
[[Category:Food processing]]
[[Category:Water desalination]]
[[Category:Filters]]
[[Category:Water technology]]
[[Category:Membrane technology]]
[[Category:Separation processes]]
[[Category:Industrial water treatment]]