Porous and polymeric membranes have a thin layer of semi-permeable material that is used for solute separation as transmembrane pressure is applied across the membrane. The degree of selectivity is largely based on the membrane charge and porosity. Membranes with symmetric pores are more uniform, while asymmetric pores have variable pore diameters.

Porous
Porous membranes are mainly used for microfiltration and ultrafiltration. The membrane contains pores ranging from 0.1 to 10 μm for microfiltration and 0.001 to 0.1μm for ultrafiltration. The separation is based on particle size. In order to achieve high selectivity, pores on the membrane need to be relatively smaller than the particles in the mixture. One disadvantage of using a porous membrane is membrane fouling that causes flux decline over time. Chemical and thermal stability are also significant factors to consider when selecting porous materials because temperature and concentration affects selectivity and flux of the membrane.

Non-porous
Non-porous membranes are mainly used for reverse osmosis, nanofiltration, or molecular separation in the gas phase. The membrane is a dense film where permeate diffuse through by pressure, concentration, or electrical potential gradient. The polymeric material affects the permeability and selectivity of the membrane. The separation process occurs from differences in solubility and diffusivity. One disadvantage of non-porous membranes is low flux; therefore, the dense film is usually made extremely thin and is deposited on top of asymmetric membranes.