Characterizing, understanding, and reducing biofouling on pervaporation membranes

Membrane pervaporation represents a promising approach for the low energy, in situ recovery of biofuel alcohols such as ethanol.  The practicality and scalability of this technology, however, is challenged by occurrences of biofouling which can reduce flux and separation factor, causing process disruptions necessitating costly process down time for cleaning and membrane module servicing. To better understand the factors influencing membrane biofouling and its resultant effects, a series of model studies were performed. The behavior of ethanologenic microorganisms including Escherichia coli KO11 and strains of Saccharomyces cerevisiae in the presence of both silicalite and polydimethylsiloxane membranes has been characterized using scanning electron and atomic force microscopy. The focal microbes were found to foul membrane surfaces differently, with E. coli in general forming the more robust biofilms.  Meanwhile, the deletion of key genes associated with flocculation in S. cerevisiae was found to reduce biofilm formation.  Separation studies were performed on virgin and fouled membranes using both binary model solutions and complex spent fermentation media to understand fouled membrane separation performance. Membrane swelling, adhesion of biomolecules and biofilm formation each greatly increased the mass transport resistance. Fouled membranes displayed more hydrophilic surface properties while separation factors decreased and subsequently, permeates became water enriched. Furthermore, fouled membranes displayed increased surface wetting and a ~10-fold increase in surface roughness; factors which increased surface energies, consequently acting as promoters of further cellular adhesion. To address these issues, our group is now exploring the application of tunable electric surface potentials to reduce surface biofouling of membranes.