Reverse osmosis

== 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>