Separation processes are often based on size difference. Larger molecules than the pore size of the membrane cannot go through the membrane and  will be sieved by the membrane. Generally, the feed side is pressurized and the pressure difference across the membrane is used as the driving force of the permeation.

It is a continuous process and the separation unit can have a compact and simple design. Molecules in the feed can make a cake layer on the membrane surface and plug the pores. A cleaning process is sometimes required during a long term operation.

Membrane filtration can be divided into microfiltration (MF, pores of 100nm – 1µm), ultrafiltration (UF, pores of 2 – 100 nm), nanofiltration (NF, pores of < 2 nm) and reverse osmosis (RO). Various membranes are commercially available.