Reverse osmosis

== 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>