Assessment of the fouling mechanisms of an ultrafiltration membrane bioreactor during synthesis of galacto-oligosaccharides: Effect of the operational variables

The flux decay of an ultrafiltration membrane bioreactor for the synthesis of galacto-oligosaccharides was modeled, by varying the processing conditions: temperature (40–60 °C), transmembrane pressure (2.5–4 bar) and cross-flow velocity (3.5–7 m/s) according to a 2^kdesign. Fouling mechanisms varied according to the operational condition, showing that the predominant mechanism were the intermediate fouling, which was associated to the higher flux decay due to the partial adsorption of the enzyme on the membrane, and the cake fouling mechanism, which was associated to those runs where the flux declined by around 20% because of the high concentration of substrate used (40% w/w). Strong statistical analysis allowed validation or rejection of initial adjustments given by a simple R-square statistical test, showing that misinterpretation of the fouling mechanism can be done under particular conditions, but also revealed that fouling equations have some limitations when drastic flux decay occurs or when this remains virtually unchanged. The experimental design also showed that temperature was the variable having the main positive effect on flux stability by decreasing the solution viscosity; however, the interaction between cross-flow velocity and transmembrane pressure had the main effect on flux decay.