Reverse osmosis

==Disadvantages==
Large-scale industrial/municipal systems recover typically 75% to 80% of the feed water, or as high as 90%, because they can generate the required higher pressure.

=== Wastewater ===
Household RO units use a lot of water because they have low back pressure. Household RO water purifiers typically produce one liter of usable water and 3-25 liters of [[wastewater]].<ref>{{cite web|url=https://www.forbes.com/home-improvement/home/reverse-osmosis-water-pros-cons/|title=Learn The Pros And Cons Of Reverse Osmosis Water Filtration Systems|language=en-US |magazine=Forbes|access-date=2023-10-08}}</ref> The remainder is discharged, usually into the drain. Because wastewater carries the rejected contaminants, recovering this water is not practical for household systems. Wastewater is typically delivered to house drains. A RO unit delivering {{convert|20|liter|usgal}} of treated water per day also discharge between {{convert|50 and 80|liter|usgal}}. This led India's [[National Green Tribunal]] to propose a ban on RO water purification systems in areas where the [[total dissolved solids]] (TDS) measure in water is less than 500&nbsp;mg/liter.{{citation needed|date=May 2023}} In [[Delhi]], large-scale use of household RO devices has increased the total water demand of the already water-parched [[National Capital Territory of India]].<ref>{{Cite journal|last=Singh|first=Govind|year=2017|title=Implication of Household Use of R.O. Devices for Delhi's Urban Water Scenario|url=http://jiid.in/2017/02/implication-household-use-r-o-devices-delhis-urban-water-scenario/|journal=Journal of Innovation for Inclusive Development|volume=2|issue=1|pages=24–29|access-date=15 April 2017|archive-date=17 May 2017|archive-url=https://web.archive.org/web/20170517005500/http://jiid.in/2017/02/implication-household-use-r-o-devices-delhis-urban-water-scenario/|url-status=dead}}</ref>

=== Health ===
RO removes both harmful contaminants and desirable minerals. Some studies report some relation between long-term health effects and consumption of water low on [[calcium]] and [[magnesium]], although these studies are of low quality.<ref>{{cite web|last=Kozisek|first=Frantisek|url=https://www.who.int/water_sanitation_health/dwq/nutrientschap12.pdf|archive-date=7 February 2022|archive-url=https://web.archive.org/web/20220207222904/https://www.who.int/water_sanitation_health/dwq/nutrientschap12.pdf|title=Health risks from drinking demineralised water|publisher=[[National Institute of Public Health]]|location=[[Czech Republic]]}}</ref>

=== Waste-stream considerations ===
Depending upon the desired product, either the solvent or solute stream of RO will be waste. For food concentration applications, the concentrated solute stream is the product and the solvent stream is waste. For water treatment applications, the solvent stream is purified water and the solute stream is concentrated waste.<ref>{{cite book|last=Weber|first=Walter J.|title =Physicochemical Processes for Water Quality Control|publisher =John Wiley & Sons|year =1972|oclc=1086963937|url=https://books.google.com/books?id=2rS0AAAAIAAJ|location =New York|isbn =9780471924357|page=320}}</ref> The solvent waste stream from food processing may be used as [[reclaimed water]], but there may be fewer options for disposal of a concentrated waste solute stream. Ships may use [[marine dumping]] and coastal desalination plants typically use [[marine outfall]]s. Landlocked RO plants may require [[evaporation pond]]s or [[injection well]]s to avoid polluting [[groundwater]] or [[surface runoff]].<ref>{{cite book |last=Hammer |first=Mark J.|url=https://books.google.com/books?id=qgVSAAAAMAAJ|title =Water and Waste-Water Technology |publisher =John Wiley & Sons |date =1975 |location=New York|isbn =9780471347262|page=266}}</ref>