The polymeric membrane is a semi-permeable barrier, which separates two phases, and filters various components of different sizes selectively. The porous membrane is a filter that separates components by a sieving mechanism or diffusion kinetics determined by the component size, the pore size, and overall porosity.

The transportation through this asymmetric membrane is made by, either concentration, pressure, temperature gradient, an electric field or diffusion kinetics.

The influent stream is separated, with the membrane filtration system, into two effluent streams called permeate and concentrate. The permeate is the part of the fluid that has been filtered by the permeable membrane, and the concentrate is composed of the elements that have been rejected.