Both ultrafiltration and reverse osmosis depend on pressure as the driving force and require a membrane that is permeable to some components and impermeable to others.

The difference between the two processes is that, while ultrafiltration is usually used to separate solutes above a molecular weight of 500-2000, which implies a relatively small osmotic pressure, reverse osmosis is used to remove material of low molecular weight, which causes a high osmotic pressure. The polarization is, however, usually greater by ultrafiltration than by reverse osmosis, because the diffusion constant is two or three orders of magnitudes smaller for the macro-molecules than for inorganic ions.

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