The suspended solids and the big molecule solutions are retained whereas the water and the small molecule solution go through the membrane thanks to the difference of pressure. This is the trans-membrane pressure (TMP).
The industry and the research fields use this separation process in order to purify and concentrate macromolecular solutions, especially the protein solutions.
Themolecular weight cutoff of an ultra-filtration membrane is set between 0.01 and 0.1 µ M. As a result, the pressure must be between 3 and 10 bars. This technique enables to separate the fluid particles that are smaller than the molecular weight cutoff.