Reverse osmosis

== Research ==

{{update section|date=March 2021}}

=== Improving Current Membranes ===
Current RO membranes, thin-film composite (TFC) polyamide membranes, are being studied to find ways of improving their permeability. Through new imaging methods, researchers were able to make 3D models of membranes and examine how water flowed through them. They found that TFC membranes with areas of low flow significantly decreased water permeability.<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. |last12=Kumar |first12=Manish |last13=Gomez |first13=Enrique D. |date=January 2021 |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 |pmid=33384374 |bibcode=2021Sci...371...72C |s2cid=229935140 |issn=0036-8075}}</ref> By ensuring uniformity of the membranes and allowing water to flow continuously without slowing down, membrane permeability could be improved by 30%-40%.<ref>{{Cite web |title=Desalination breakthrough could lead to cheaper water filtration |url=https://www.sciencedaily.com/releases/2020/12/201231141511.htm |access-date=2023-05-26 |website=ScienceDaily |language=en}}</ref>

=== Electrodialysis ===
Research has examined integrating RO with [[electrodialysis]] to improve recovery of valuable deionized products, or to reduce concentrate volumes.

=== Low-pressure High-recovery (LPHR) ===
Another approach is low-pressure high-recovery multistage RO (LPHR). It produces concentrated [[brine]] and freshwater by cycling the output repeatedly through a relatively porous membrane at relatively low pressure. Each cycle removes additional impurities. Once the output is relatively pure, it is sent through a conventional RO membrane at conventional pressure to complete the filtration step. LPHR was found to be economically feasible, recovering more than 70% with an OPD between 58 and 65 bar and leaving no more than 350 ppm TDS from a seawater feed with 35,000 ppm TDS.

=== Carbon Nanotubes (CNTs) ===
Carbon nanotubes are meant to potentially solve the typical tradeoff between the permeability and the selectivity of RO membranes. CNTs present many ideal characteristics including: mechanical strength, electron affinity, and also exhibiting flexibility during modification. By restructuring carbon nanotubes and coating or impregnating them with other chemical compounds, scientists can manufacture these membranes to have all of the most desirable traits. The hope with CNT membranes is to find a combination of high water permeability while also decreasing the amount of neutral solutes taken out of the water. This would help decrease energy costs and the cost of remineralization after purification through the membrane.<ref>{{Cite journal |last1=Ali |first1=Sharafat |last2=Rehman |first2=Syed Aziz Ur |last3=Luan |first3=Hong-Yan |last4=Farid |first4=Muhammad Usman |last5=Huang |first5=Haiou |date=2019-01-01 |title=Challenges and opportunities in functional carbon nanotubes for membrane-based water treatment and desalination |url=https://www.sciencedirect.com/science/article/pii/S0048969718328432 |journal=Science of the Total Environment |language=en |volume=646 |pages=1126–1139 |doi=10.1016/j.scitotenv.2018.07.348 |pmid=30235599 |bibcode=2019ScTEn.646.1126A |s2cid=52311560 |issn=0048-9697}}</ref>

=== Graphene ===
Graphene membranes are meant to take advantage of their thinness to increase efficiency. Graphene is a singular layer of carbon atoms, so it is about 1000 times thinner than existing membranes. Graphene membranes are around 100&nbsp;nm thick while current membranes are about 100&nbsp;µm. Many researchers were concerned with the durability of graphene and if it would be able to handle RO pressures. New research finds that depending on the substrate (a supporting layer that does no filtration and only provides structural support), graphene membranes can withstand 57MPa of pressure which is about 10 times the typical pressures for seawater RO.<ref>{{Cite journal |last1=Cohen-Tanugi |first1=David |last2=Grossman |first2=Jeffrey C. |date=2014-11-12 |title=Mechanical Strength of Nanoporous Graphene as a Desalination Membrane |url=https://pubs.acs.org/doi/10.1021/nl502399y |journal=Nano Letters |language=en |volume=14 |issue=11 |pages=6171–6178 |doi=10.1021/nl502399y |pmid=25357231 |bibcode=2014NanoL..14.6171C |issn=1530-6984}}</ref>

Batch RO may offer increased [[Energy efficiency (physics)|energy efficiency]], more durable equipment and higher salinity limits.

The conventional approach claimed that molecules cross the membrane individually. A research team devised a "solution-friction" theory, claiming that molecules in groups through transient pores. Characterizing that process could guide membrane development. The accepted theory is that individual water molecules diffuse through the membrane, termed the "solution-diffusion" model.<ref>{{Cite magazine |last=Levy |first=Max G. |title=Everyone Was Wrong About Reverse Osmosis—Until Now |language=en-US |magazine=Wired |url=https://www.wired.com/story/everyone-was-wrong-about-reverse-osmosis-until-now/ |access-date=2023-05-20 |issn=1059-1028}}</ref>