Reverse osmosis

=== 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}}