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"Regular" filtration with reverse osmosis filter?

  1. Jun 28, 2017 #1
    A nanofiltration filter has a pore size around 0.001 micron. Reverse osmosis filters have a pore size around 0.0001 micron.

    Additionaly, reverse osmosis occurs when a semi-permeable membrane separates solutions of different concentrations so osmotic pressure has to be applied to move water from the more concentrated solution to the less concentrated solution.

    Why reverse osmosis filter (0.0001 micron) isn't used to just filter out the water in regular way as in case of nanofiltration, where two solutions aren't separated with a semi-permeable membrane, so no osmotic pressure is applied?
    What's the point of "contancting" two liquids if it requires the additional - osmotic pressure to be applied if both processes have a large pressure drop due to the pore size anway?
  2. jcsd
  3. Jun 28, 2017 #2


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    The pressure applied in an RO machine is more like reverse osmotic pressure. It is working against the osmotic pressure that would be trying to equalized the concentrations of the two different solutions. This would be in addition to overcoming the resistance of the small pores to fluid flow.

    Generally speaking, RO machines are used to remove molecular impurities from a solution (like dissolved ions).
    Presumably, a nanoscale filter with 10 x larger pores would not do this.

    The point of using an RO machines is that it can do things that a nanoscale filter could not do, separate molecules a at a smaller scale. A very similar principle is used in dialysis bags (with only the pressure of a pressurized closed bag opposing any osmotic pressure) and artificial kidney machines.
  4. Jun 28, 2017 #3
    But neither could RO machine do that if it had 10× larger pores than it already has, so why should I be using reverse osmosis if it requires overcoming osmotic pressure in addition to pressure drop due to the resistance of small pores?
    What's wrong about using filter "nanofilter machine" and switching its 0.001 micron pores with 0.0001 micron pores?
  5. Jun 28, 2017 #4


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    If didn't use an RO machine, you would not get the function of the RO machine, which is to remove smaller molecules from the filtered water.
    If you don't want to do that, then you don't need the RO machine.
  6. Jun 28, 2017 #5
    What about this theoretical situation? Replace solid with salt. There is no osmotic pressure to counter since filtrate can't diffuse. However, filtration should be possible if pore sizes are smaller than ions of salt (which is true for R. O. membrane).
    This way, i only counter pressure caused by resistance to flow, i.e. pressure drop. Wouldn't this be cheaper? Ofcourse, if we imagine industrial-sized analogous version of this filtration equipment.
  7. Jun 28, 2017 #6
    If this is correct, then I guess I get it:
    Microfiltration, ultrafiltration, nanofiltration and reverse osmosis all include reverse osmosis mechanism strictly speaking since all of them are designed to have contact between feed and filtrate throught the semi-permeable membrane. The difference are particles that contribute to their osmotic pressure.
  8. Jun 28, 2017 #7


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    I think that as the water went through the filter, a film of filtered solution would form on the downstream side of the filter paper (RO membrane).
    Osmotic pressure could arise from that film of water on one side of the membrane and the unfiltered water on the other.
    I am not sure how well a non-wet RO membrane would work.

    I think this is correct.
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