# "Reverse Osmosis"? **URL:** \ **Category:** Factual Questions\ **Created:** [June 23, 2004, 7:38pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858 "2004-06-23T19:38:33Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Cisco](https://avatars.discourse-cdn.com/v4/letter/c/ccd318/32.png) [@Cisco](https://boards.straightdope.com/u/Cisco)\ **Post date:** [June 23, 2004, 7:38pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/1 "2004-06-23T19:38:33Z")
I’m going off of 9th grade biology here (almost a decade ago) but isn’t there a word for this? Isn’t it diffusion? Why do I keep hearing the term reverse osmosis? ---
**Author:** ![NinjaChick](https://avatars.discourse-cdn.com/v4/letter/n/71c47a/32.png) [@NinjaChick](https://boards.straightdope.com/u/NinjaChick)\ **Post date:** [June 23, 2004, 7:49pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/2 "2004-06-23T19:49:20Z")
Osmosis is a type of diffusion. Diffusion is (IIRC) simply one substance passing through a permiable or semi-permiable membrane. Osmosis is diffusion specifically with water. [Reverse Osmosis](http://www.gewater.com/library/tp/833_What_Is.jsp) is a process used in water purification to filter salt and stuff out of water. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [June 23, 2004, 8:04pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/3 "2004-06-23T20:04:05Z")
> [@NinjaChick](#): > > Osmosis is a type of diffusion. Diffusion is (IIRC) simply one substance passing through a permiable or semi-permiable membrane. Osmosis is diffusion specifically with water. Erm, no. Diifusion is simply the scattering of one substance throughout another due to random thermal motion. (Think a drop of dye in water). It has nothing to do with a semi-permeable membrane. Osmosis is a solvent (it doesn’t have to be water) moving across a semipermeable membrane from the side with a low concentration of solutes (such as salts) to the side with the high concentration. This motion will happen without any outside energy applied until both sides contain the same solute concentrations. Osmosis works because the semi-permeable membrane will allow some molecules to pass through, but not others. “Reverse Osmosis” is applying enough pressure to the side with a high concentration of solutes. If you apply enough pressure, you can use the semi-permeable membrane as a filter to filter out salts and the like. more here: [HowStuffWorks.com : Reverse Osmosis](http://www.howstuffworks.com/question29.htm) ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [June 23, 2004, 8:13pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/4 "2004-06-23T20:13:14Z")
Osmosis is diffusion across a semipermiable membrane. A semipermiable membrane is a membrane that some things e.g. water, but not others e.g. salt, through. The normal direction of diffusion of water across a semipermiable membrane is towards the side with _less_ water, so with a membrane between pure water, and salty water, the water will move from the less salty side to the more salty side. This is the wrong direction, if you’re interested in removing salt from water. You can reverse the direction of this osmotic flow by putting the salty water under pressure; hence reverse osmosis. ---
**Author:** ![NinjaChick](https://avatars.discourse-cdn.com/v4/letter/n/71c47a/32.png) [@NinjaChick](https://boards.straightdope.com/u/NinjaChick)\ **Post date:** [June 23, 2004, 8:25pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/5 "2004-06-23T20:25:46Z")
> [@A Monkey With a Gun](#): > > Erm, no. Diifusion is simply the scattering of one substance throughout another due to random thermal motion. (Think a drop of dye in water). It has nothing to do with a semi-permeable membrane. Osmosis is a solvent (it doesn’t have to be water) moving across a semipermeable membrane from the side with a low concentration of solutes (such as salts) to the side with the high concentration. This motion will happen without any outside energy applied until both sides contain the same solute concentrations. > > Osmosis works because the semi-permeable membrane will allow some molecules to pass through, but not others. “Reverse Osmosis” is applying enough pressure to the side with a high concentration of solutes. If you apply enough pressure, you can use the semi-permeable membrane as a filter to filter out salts and the like. > > more here: [HowStuffWorks.com : Reverse Osmosis](http://www.howstuffworks.com/question29.htm) D’oh. I stand corrected, and say to my biology teacher: I was right, I did forget everything after I took the exam. ---
**Author:** ![mmmiiikkkeee](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mmmiiikkkeee](https://boards.straightdope.com/u/mmmiiikkkeee)\ **Post date:** [June 23, 2004, 11:15pm UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/6 "2004-06-23T23:15:55Z")
So is reverse osmosis not just simple mechanical filtration through a _ **VERY** _ fine mesh (molecular-sized)? Seems like they’ve bastardized the name a little since there really isn’t any such thing as “reverse” osmosis… sorta like calling an elevator a “reverse gravity” device. You’re not _reversing_ gravity at all, you’re just opposing it. Ah, maybe the name’s supposed to refer to the water flow through the membrane as being opposite to what would naturally occur… ---
**Author:** ![mischievous](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mischievous](https://boards.straightdope.com/u/mischievous)\ **Post date:** [June 24, 2004, 12:07am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/7 "2004-06-24T00:07:52Z")
> [@mmmiiikkkeee](#): > > So is reverse osmosis not just simple mechanical filtration through a _ **VERY** _ fine mesh (molecular-sized)? Seems like they’ve bastardized the name a little since there really isn’t any such thing as “reverse” osmosis… sorta like calling an elevator a “reverse gravity” device. You’re not _reversing_ gravity at all, you’re just opposing it. Ah, maybe the name’s supposed to refer to the water flow through the membrane as being opposite to what would naturally occur… Yes, I think that it’s an absolutely moronic name. Filtration is just fine, and totally accurate. Use something like “ultra-filtration” if you want to get across the point that you are also filtering out salts. I used to think that “reverse osmosis” was a bullshit faux-geek word used to sell purified water to geek places like laboratories, but now I see it on purified supermarket water. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 24, 2004, 12:09am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/8 "2004-06-24T00:09:23Z")
> [@A Monkey With a Gun](#): > > Erm, no. Diifusion is simply the scattering of one substance throughout another due to random thermal motion. (Think a drop of dye in water). It has nothing to do with a semi-permeable membrane. Osmosis is a solvent (it doesn’t have to be water) moving across a semipermeable membrane from the side with a low concentration of solutes (such as salts) to the side with the high concentration. This motion will happen without any outside energy applied until both sides contain the same solute concentrations. [slight and possibly bogus nitpick]I _think_ I’m right in saying that the solutes won’t actually reach equal strength, but will _tend towards_ equal osmotic pressure. As the solvent moves from the weaker to the stronger solution, so the level rises in the stronger solution and drops in the weaker, increasing the mechanical pressure opposing the direction of flow; so the point of equal concentration might never be reached. Actually, I suspect equal concentration would never \*actually be reached in any case, only approached asymptotically. ---
**Author:** ![Cisco](https://avatars.discourse-cdn.com/v4/letter/c/ccd318/32.png) [@Cisco](https://boards.straightdope.com/u/Cisco)\ **Post date:** [June 24, 2004, 12:17am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/9 "2004-06-24T00:17:23Z")
> [@mmmiiikkkeee](#): > > Seems like they’ve bastardized the name a little… sorta like calling an elevator a “reverse gravity” device. That was what I was trying to get at in my OP. Is reverse osmosis a contrived term used to sell more water? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 24, 2004, 12:25am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/10 "2004-06-24T00:25:18Z")
> [@Cisco](#): > > That was what I was trying to get at in my OP. Is reverse osmosis a contrived term used to sell more water? I don’t think it is; the term (regardless of its technical accuracy\* has been in use for many years; certainly it is used\* as a marketing gimminck, but it wasn’t contrived for that purpose. ---
**Author:** ![Cervaise](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cervaise/32/16693_2.png) [@Cervaise](https://boards.straightdope.com/u/Cervaise)\ **Post date:** [June 24, 2004, 12:38am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/11 "2004-06-24T00:38:43Z")
_—pointing and laughing at **Mangetout** ’s coding—_ 😉 ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [June 24, 2004, 12:39am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/12 "2004-06-24T00:39:04Z")
When did this become a viable method for desalination? When I was in school in the 70s & 80s the only method of desalination was (very energy intensive) distillation. So my science teachers would always talk about a ‘filtering’ method as being some kind of holy grail that would revolutionize the world. I was amazed to recently find out that its actually being used and has been for some time. Was there some breakthrough that made it possible? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [June 24, 2004, 1:08am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/13 "2004-06-24T01:08:08Z")
> [@Hail Ants](#): > > When did this become a viable method for desalination? The method really started coming on in the 80’s: > [@](#): > > The concepts of “osmosis” and “reverse osmosis” have been known for many years. In fact, studies on osmosis were carried out as early as 1748 by the French scientist Nollet, and many researchers investigated these phenomena over the next two centuries (Reid, 1966; Mason, 1991). However, the use of reverse osmosis (RO) as a feasible separation process is a relatively young technology. In fact, only in the late 1950’s did the work of Reid show that cellulose acetate RO membranes were capable of separating salt from water, even though the water fluxes obtained were too small to be practical (Reid and Breton, 1959; Ferguson, 1980; Lonsdale, 1982; Applegate, 1984). Then, in the early 1960’s, Loeb and Sourirajan developed a method for making asymmetric cellulose acetate membranes with relatively high water fluxes and separations, thus making RO separations both possible and practical (Loeb and Sourirajan, 1962; Loeb, 1981; Sourirajan and Matsuura, 1985). > Since then, the development of new-generation membranes such as the thin-film, composite membrane that can tolerate wide pH ranges, higher temperatures… [A Brief Review of Reverse Osmosis Membrane Technology](http://64.233.161.104/search?q=cache:tM2UqZc2kg0J:www.wescinc.com/files/RO_Review.pdf+desalination+%22osmotic+membranes%22+history&hl=en&ie=UTF-8) ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 20, 2018, 5:50am UTC](https://boards.straightdope.com/t/reverse-osmosis/251858/15 "2018-02-20T05:50:48Z")
Reported.