Unsteady state flux response: a method to determine the nature of the solute and gel layer in membrane filtration

The unsteady-state permeate flux response to a step change in transmembrane pressure is shown to result in unique flux-pressure profiles for the three types of solutes common in membrane ultrafiltration (UF): (a) solutes whic h exert an osmotic pressure but do not form a 'gel'; (b) solutes which do n ot exert an osmotic pressure but form a 'gel' and (c) solutes which exert a n osmotic pressure and also form a 'gel'. It is also shown that for stirred cell UF, changes in the bull; feed solution properties (concentration, vol ume) are negligible on the time scale needed to attain a stable permeate fl ux. Unsteady-state permeate flux measurements could therefore be made at sh ort filtration times so that the results would not be mashed by changes in bulk properties. (C) 2001 Elsevier Science B.V. All rights reserved.