Resistance-in-series analysis in cross-flow ultrafiltration of fermentation broths of Bacillus subtilis culture

Quantitative analysis of various resistances that lead to flux decline during cross-flow ultrafiltration (UF) of the fermentation broth of Bacillus subtilis ATCC (American Type Culture Collection) 21332 culture was studied. Polyethersulfone membrane with a molecular weight cut-off (MWCO) of 100 kDa was used. Prior to cross-flow UF, the broth was treated by acid precipitation (pH 4.0) and centrifugation, and the precipitate was re-dissolved in NaOH solution. Experiments were performed at a feed pH of 7.0, a feed surfactin concentration of 1.48 g L−1, and a cross-flow velocity of 0.32 m s−1 but at different transmembrane pressures (ΔP, 20–100 kPa). The resistance-in-series model was used to analyze the flux behavior, which involves the resistances of membrane itself and cake as well as those due to adsorption and solute concentration polarization. It was shown that the resistance due to solute concentration polarization and of membrane dominated under the conditions examined. The resistances due to cake formation and solute adsorption were comparable, and their sum contributed below 20% of the overall resistance.