MEMBRANE FOULING DURING CONSTANT FLUX CROSS-FLOW MICROFILTRATION OF DILUTE SUSPENSIONS OF ACTIVE DRY YEAST

Fouling of microporous and ultrafiltration membranes during crossflow microfiltration of rehydrated active dry yeast (ADY) was investigated using measurements of the transmembrane pressure as a function of time at constant flux. By centrifuging the suspensions and comparing the increase in transmembrane pressure produced by both the original suspensions and the supernatant alone, it was determined that this increase was mainly caused by soluble components in the supernatant. This finding is consistent with previous observations that considerable quantities of intracellular matter leak from cells of ADY when they are rehydrated. The increase in transmembrane pressure caused by the supernatant alone was found to be independent of tangential now rate, suggesting that the underlying mechanism was one of internal membrane fouling. Fouling was found to be enhanced by increasing the transmembrane flux and reducing the membrane pore size. Membrane fouling by the supernatant was modeled as a process involving simultaneous deposition and removal of foulant, onto and from, the walls of membrane pores. In contrast to the standard blocking model, but in agreement with experimental observations, the new model does not predict that the transmembrane pressure will increase indefinitely, but will reach a constant value which will depend on the flux and the pore diameter.