An unsteady-state model is developed to predict the feed surface and permea
te concentration for ultrafiltration of macromolecules. A priori knowledge
of the diffusion coefficient through the membrane is however necessary. It
is shown that the model is capable of predicting the feed surface concentra
tion once the operating conditions are fixed and does not make an implicit
assumption of constant feed surface concentration. It is further shown that
for a certain range of system parameters, a negative rejection (based on t
he feed concentration) is possible at steady state. (C) 2000 Elsevier Scien
ce Ltd. All rights reserved.