Membrane fouling by turbidity constituents of beer and wine: characterization and prevention by means of infrasonic pulsing

During microfiltration of fermented beverages such as beer and wine, colloi dal turbidity constituents play an important role in membrane fouling and t herefore hinder the permeate flux. This paper studies polymeric membrane fo uling by turbidity constituents of beer and wine and proposes a fouling pre vention method to improve permeate flux. The model turbidity suspensions, based on gelatin and tannic acid, were mic rofiltered in a stirred cell using 0.2 mum cellulose acetate (CA), polyviny lidene di-fluoride (PVDF) and polycarbonate (PC) membranes under constant p ressure (0.79 bar) and agitation (1350 rpm). Fouling caused by either a sep arate protein solution or a separate polyphenol solution was considerably l ower than the fouling caused by a solution of both at the same concentratio n, which suggests that fouling is mainly caused by light scattering complex es formed by polyphenols and proteins. In all cases, during the microfiltra tion of model turbidity suspensions, the internal fouling mechanism dominat ed first and this was later followed by a period of external fouling. It wa s shown that membrane fouling depends strongly on the initial turbidity of the solution. A lower turbidity of the model solution resulted in a lower f inal total resistance. The initial turbidity also influenced the duration o f the initial period of internal fouling. For the lower turbidity model sol utions internal fouling lasted longer. An infrasonic pulsing was applied to remove foulant cake and improve permea te flux during microfiltration of model turbidity suspensions, and wine and beer samples. In infrasonic pulsing, high frequency pulses of permeate are periodically sent back in the direction of the membrane at a lower pressur e than the transmembrane pressure. This causes the membrane to vibrate rapi dly and removes a portion of the foulant cake. The model turbidity suspensi on (33 +/- 2 NTU) was filtered through a flat-sheet PVDF membrane at a tran smembrane pressure of 0.65 bar. In the experiments with infrasonic pulsing a 0.2 duty cycle (Phi) was used. The improvement in net permeate flux depen ded on the infrasonic pulse frequency. For the highest frequency (6.67 Hz) net permeate flux improved fourfold in comparison with long-term permeate f lux in normal cross-flow. These data fitted very well to the theoretical mo del first published by Czekaj [Reduccion del Ensuciamiento de Membranas Pol imericas en Procesos de Microfiltracion Tangencial de Suspensiones Biologic as Mediante Utilizacion de Infrasonidos y de la Tecnica de Flujo Transversa l, PhD Thesis, Universitat Rovira i Virgili, Spain] Experiments with wine a nd beer samples filtered through a PS hollow fiber membrane yielded approxi mately 2.4 and 2.1 times larger permeate fluxes with infrasonic pulsing tha n with normal cross-flow. (C) 2001 Elsevier Science Ltd. All rights reserve d.