Reverse osmosis

== 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.