Steady-state sieving across membranes.

The constraint of steady-state operation for sieving or ultrafiltration across membranes greatly restricts possible theoretical mechanisms. Effective sieving in the steady state requires the coexistence of a removal mechanism with the rejection mechanism. These points are illustrated without elaborate mathematics by a model of membranes in a series array with intervening compartments. This model also shows that in certain regimes the structure of the first membrane alone determines the overall sieving characteristics of the array.