Reverse osmosis

=== 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>