Editing Desalination (section)

====Reverse osmosis====
[[File:PlantaSchemaNotional.png|thumb|upright=1.5|Schematic representation of a typical desalination plant using [[reverse osmosis]]. Hybrid desalination plants using [[#Freeze–thaw|liquid nitrogen freeze thaw]] in conjunction with reverse osmosis have been found to improve efficiency.<ref>{{cite journal |last1=Najim |first1=Abdul |title=A review of advances in freeze desalination and future prospects |journal=npj Clean Water |publisher=[[Nature (journal)|Nature]] |language=en |doi=10.1038/s41545-022-00158-1 |date=19 April 2022|volume=5 |issue=1 |page=15 |s2cid=248231737 |doi-access=free |bibcode=2022npjCW...5...15N }}</ref> ]]
The leading process for desalination in terms of installed capacity and yearly growth is [[reverse osmosis]] (RO).<ref>{{cite journal|title=State-of-the-art of reverse osmosis desalination|year=2007|last1=Fritzmann|first1=C|last2=Lowenberg|first2=J|last3=Wintgens|first3=T|last4=Melin|first4=T|journal=Desalination|volume=216|issue=1–3|pages=1–76|doi=10.1016/j.desal.2006.12.009|bibcode=2007Desal.216....1F }}</ref> The RO membrane processes use semipermeable membranes and applied  pressure (on the membrane feed side) to preferentially induce water permeation through the membrane while rejecting salts. [[Reverse osmosis plant]] membrane systems typically use less energy than thermal desalination processes.<ref name=WarsingerEntropy /> Energy cost in desalination processes varies considerably depending on water salinity, plant size and process type. At present the cost of seawater desalination, for example, is higher than traditional water sources, but it is expected that costs will continue to decrease with technology improvements that include, but are not limited to, improved efficiency,<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|date=2016|bibcode=2016WatRe.106..272W |url=https://dspace.mit.edu/bitstream/1721.1/105441/4/CCRO%20with%20tank%20journal%20paper%20v116%20Preprint.pdf|hdl=1721.1/105441|doi-access=free}}</ref> reduction in plant footprint, improvements to plant operation and optimization, more effective feed pretreatment, and lower cost energy sources.<ref>{{Cite journal|title = Salty solutions |journal = Physics Today|date = 2015-06-01|issn = 0031-9228|pages = 66–67|volume = 68|issue = 6|doi = 10.1063/PT.3.2828|first = Gregory P.|last = Thiel|bibcode = 2015PhT....68f..66T |doi-access = free}}</ref>

Reverse osmosis uses a thin-film composite membrane, which comprises an ultra-thin, aromatic polyamide thin-film. This polyamide film gives the membrane its transport properties, whereas the remainder of the thin-film composite membrane provides mechanical support. The polyamide film is a dense, void-free polymer with a high surface area, allowing for its high water permeability.<ref>{{cite journal|doi=10.1073/pnas.1804708115|pmid=30104388|title=Electron tomography reveals details of the internal microstructure of desalination membranes|year=2018|last1=Culp|first1=T.E.|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=115|issue=35|pages=8694–8699|pmc=6126755|bibcode=2018PNAS..115.8694C|doi-access=free}}</ref> A 2021 study found that the water permeability is primarily governed by the internal nanoscale mass distribution of the polyamide active layer.<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.|date=2021-01-01|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|issn=0036-8075|pmid=33384374|bibcode=2021Sci...371...72C|s2cid=229935140}}</ref>

The reverse osmosis process requires maintenance. Various factors interfere with efficiency: ionic contamination (calcium, magnesium etc.); [[dissolved organic carbon]] (DOC); bacteria; viruses; [[colloid]]s and insoluble particulates; [[biofouling]] and [[Fouling|scaling]]. In extreme cases, the RO membranes are destroyed. To mitigate damage, various pretreatment stages are introduced.  Anti-scaling inhibitors include acids and other agents such as the organic polymers [[polyacrylamide]] and [[polymaleic acid]], [[phosphonate]]s and [[polyphosphate]]s. Inhibitors for fouling are [[biocide]]s (as oxidants against bacteria and viruses), such as chlorine, ozone, sodium or calcium hypochlorite. At regular intervals, depending on the membrane contamination; fluctuating seawater conditions; or when prompted by monitoring processes, the membranes need to be cleaned, known as emergency or shock-flushing. Flushing is done with inhibitors in a fresh water solution and the system must go offline. This procedure is environmentally risky, since contaminated water is diverted into the ocean without treatment. Sensitive [[marine habitats]] can be irreversibly damaged.<ref>{{Cite book|title=Membranverfahren – Grundlagen der Modul und Anlagenauslegung|last=Rautenbach|first=Melin|publisher=Springer Verlag Berlin|year=2007|isbn=978-3540000716|location=Germany}}</ref><ref>{{Cite book|title=Seawater Desalination – Impacts of Brine and Chemical Discharge on the Marine Environment|publisher=Sabine Lattemann, Thomas Höppner|isbn=978-0866890625|date=2003-01-01}}</ref>

Off-grid [[solar-powered desalination unit]]s use solar energy to fill a buffer tank on a hill with seawater.<ref>{{cite web |title=Access to sustainable water by unlimited resources {{!}} Climate innovation window |url=https://climateinnovationwindow.eu/innovations/access-sustainable-water-unlimited-resources |website=climateinnovationwindow.eu |access-date=2019-02-22 |archive-date=2023-08-04 |archive-url=https://web.archive.org/web/20230804105115/https://climateinnovationwindow.eu/innovations/access-sustainable-water-unlimited-resources |url-status=dead }}</ref>  The reverse osmosis process receives its pressurized seawater feed in non-sunlight hours by gravity, resulting in sustainable drinking water production without the need for fossil fuels, an electricity grid or batteries.<ref>{{cite web |title=Solving fresh water scarcity, using only the sea, sun, earth & wind |url=http://www.glispa.org/glispa-bright-spots/27-emerging-bright-spots/206-elemental |website=www.glispa.org|date=7 March 2023 }}</ref><ref>{{cite web |title=From Plentiful Seawater to Precious Drinking Water |url=https://sidsgbn.org/2018/03/20/tackling-water-scarcity-on-islands/ |website=SIDS Global Business Network|date=March 20, 2018 }}</ref><ref>{{cite web |title=HH Sheikh Maktoum bin Mohammed bin Rashid Al Maktoum honours 10 winners from 8 countries at Mohammed bin Rashid Al Maktoum Global Water Award |url=http://www.suqia.ae/en/media-center/news/112-2017-04-27 |website=Suqia |language=en-gb}}</ref> Nano-tubes are also used for the same function (i.e., Reverse Osmosis).

Deep sea reverse osmosis (DSRO) installs equipment on the [[seabed]] to force water through RO membranes using the ocean's naturally occurring water pressure.<ref name=":0">{{Cite web |last=Chant |first=Tim De |date=2024-12-10 |title=Exclusive: A new wave of desalination startups argues that deeper is better |url=https://techcrunch.com/2024/12/10/a-new-wave-of-desalination-startups-argues-that-deeper-is-better/ |access-date=2025-05-14 |website=TechCrunch |language=en-US}}</ref> A 2021 study suggested DSRO could improve energy efficiency compared to standard RO by up to 50%.<ref>{{Cite journal |last=Fasano |first=Matteo |last2=Morciano |first2=Matteo |last3=Bergamasco |first3=Luca |last4=Chiavazzo |first4=Eliodoro |last5=Zampato |first5=Massimo |last6=Carminati |first6=Stefano |last7=Asinari |first7=Pietro |date=2021-12-15 |title=Deep-sea reverse osmosis desalination for energy efficient low salinity enhanced oil recovery |url=https://www.sciencedirect.com/science/article/abs/pii/S0306261921010242 |journal=Applied Energy |volume=304 |pages=117661 |doi=10.1016/j.apenergy.2021.117661 |issn=0306-2619|hdl=11696/75400 |hdl-access=free }}</ref> The concept of DSRO has long been known, but has only recently become feasible due to technological advances from the deep sea oil and gas industry, drawing early-stage investments in DSRO startups.<ref name=":0" />