Membrane can be described as a thin layer of material that is capable of separating
materials as a function of their physical and chemical properties when a driving force is applied
across the membranes. Physically membrane could be solid or liquid. In membrane separation
processes, the feed is separated into a stream that goes through the membrane, i.e., the permeate
and a fraction of feed that does not go through the membrane, i.e., the retentate or the concentrate.
A membrane process then allows selective and controlled transfer of one species from one bulk
phase to another bulk phase separated by the membrane.