Impact of gel layer formation on colloid retention in membrane filtration processes

Colloidal particles in the feed streams of membrane filtration processes control membrane fouling rate in many instances. In this study, the non-gelling colloidal Na-alginate and the gelling colloidal Ca-alginate are employed to investigate the significance of gel layer formation in membrane filtration processes in terms of contribution to membrane fouling and supplementary impurity removal. The results show that contribution of colloidal particles to membrane fouling depends on the gelling propensity of the colloids and the operational mode (constant pressure or constant flux) implemented. A small dose of either Na-alginate or Ca-alginate was found to greatly increase membrane fouling rate during constant pressure filtration. Both the resistance to removal by application of shear and the lower susceptibility of the concentration polarization layer to shear resulted in more severe fouling during constant flux filtration in the presence of Ca-alginate assemblages than in the presence of Na-alginate. Apparent channel sizes of the Ca-alginate gel layer were calculated from the material properties of the fouling layer. Incomplete catalase retention highlighted the likely heterogeneity in size of liquid transport pathways. Adsorption also contributed to the trapping of colloidal particles according to the retention behaviour of BSA by the Ca-alginate gel layer. Gel layer formation propensity should be seriously considered for the operation of membrane filtration processes. Two simple methods based on (i) a permeability recovery experiment and (ii) comparison of dead-end filtration behaviour with and without shear application are proposed for evaluation of the gelling propensity of colloidal dispersions.     

Simulation applied to the gel filtration of surfactants

Summary In this paper is described a simulation technique applicable to the gel filtration of a mixture of two ionic surfactants. The applicability was tested by using a mixture of sodium decyl and docecyl sulfates. The theoretical elution curves showed good agreement with the experimental ones. It can be shown not only experimentally but also theoretically that the gel filtration is a powerful and convenient method to isolate the component which has a lower CMC from the mixture. The separation of components is achieved more effectively in a mixture of ionic surfactants than in a mixture of nonionic surfactants. The difference is due to ionic effect, on which a brief discussion is presented.

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Zusammenfassung In dieser Mitteilung wird eine Simulationstechnik für die Gelfiltration eines Gemisches von zwei ionischen oberflächenaktiven Substanzen beschrieben und mit einem Gemisch von Natrium-Decyl- und Dodecylsulfat geprüft. Die theoretischen Elutionskurven zeigten gute Übereinstimmung mit den experimentellen Kurven. Es wird gezeigt, daß die Gelfiltration eine zweckmäßige und bequeme Methode zur Isolierung der Komponente mit niedrigeren KMK aus dem Gemisch ist. Die Trennung zweier Komponenten gelingt besser aus einem Gemisch von ionischen oberflächenaktiven Substanzen als aus einem solchen von nichtionischen.

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With 7 figures     

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 which exert an osmotic pressure but do not form a ‘gel’; (b) solutes which do not exert an osmotic pressure but form a ‘gel’ and (c) solutes which exert an osmotic pressure and also form a ‘gel’. It is also shown that for stirred cell UF, changes in the bulk feed solution properties (concentration, volume) are negligible on the time scale needed to attain a stable permeate flux. Unsteady-state permeate flux measurements could therefore be made at short filtration times so that the results would not be masked by changes in bulk properties.