Basic parameters and separation mechanisms

The mass transport across the membrane is caused by one or more driving forces. A gradient in chemical or electrical potential is the usual basis for these driving forces. Pressure or concentration differences are the most common driving forces. The total flux (Ni) of a species is related to the driving force (AP or AC) by 

The driving force is (AP - All) when there is an osmotic pressure difference (AO) across the membrane. Q is the permeability and Q/l is the permeance. Permeance is used when the actual membrane thickness is unknown. The separation factor Uij is calculated as the ratio of permeabilities or permeances. 

Liquid separations, which typically involve filtration, use a term called rejection (R). This term, with values between 0 and 1, measures the degree of rejection (lack of permeation) for a given solute. It is defined as  

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