In Reverse Osmosis (RO) systems, the water to be purified is pushed against a semipermeable membrane and comes cleaner the other side, the path of water is the reverse of regular osmosis, hence the name. However, it seems to me that this process is pretty similar to a regular filtration.

So, is RO just a fancy term for a fancy filtration? What is it that justifies it not being called filtration, ultrafiltration or some variation of that?

  • 3
    $\begingroup$ The wikipedia article on Reverse osmosis discusses this in the paragraph before the “History” subsection of the article. $\endgroup$
    – Ed V
    Jun 27, 2020 at 23:52
  • $\begingroup$ Size of the holes. $\endgroup$ Jun 28, 2020 at 3:52
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    $\begingroup$ Do you prefer to call osmosis "reverse filtration" ? $\endgroup$
    – Poutnik
    Jun 28, 2020 at 6:01
  • $\begingroup$ If the size of the holes are the important factor, could it be called ultrafiltration? If the pressure difference is the important factor, could vacuum filtration be called reverse osmosis? I find it hard to understand the wikipedia text."[RO] differs from filtration in that the mechanism of fluid flow is by osmosis across a membrane", this doesnt seem right as osmosis is not that. $\endgroup$
    – peruca3d
    Jun 28, 2020 at 13:04
  • $\begingroup$ @LocoAsker There is another difference in that in vacuum filtration, the pores are usually big enough that only precipitated particles are separated from the solution, whose dissolved composition (unlike with RO) remains unchanged. "Filtering" away the disolved portion of a solution is a very specific kind of "filtration." $\endgroup$
    – Hans
    Jun 28, 2020 at 13:10

1 Answer 1


Ultrafiltration is ongoing at any pressure, even if slowly and liquid always goes to the side with lower pressure.

Reverse osmosis is ongoing only at pressure higher then osmotic pressure. At pressure lower than osmotic pressure, normal osmosis occurs and liquid flows to the side of higher pressure.


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