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?