How unsteady filtration conditions can improve the process efficiency during cell cultures in membrane bioreactors

Abstract

Abstract Among processes developed to increase biological performances, membrane bioreactors have provided the best results. The membrane bioreactor combines a continuous fermentor and a crossflow filtration module enabling separation of cells from liquid media. Very high biomass concentrations have thus been reached and important bioconversion yields obtained. However the potentiality of this process is mainly limited by the rapid decline in permeate flux due to membrane fouling. In our laboratory, various technological solutions, based on unsteady hydrodynamics inside the tubular filters to limit the external fouling, have been developed and applied during cell cultures in membrane bioreactors. The biological model was alcoholic fermentation. The first kind of flow unsteadiness was based on an air injection at the membrane inlet to create a gas/liquid slug flow. For the same energy consumption, this process enabled a mean twofold gain in ultrafiltration flux with a lower efficiency for microfiltration due to pore blocking by cell debris. The impact of an unsteady jet generated by a pneumatically controlled valve was also evaluated. Although the strong physico–chemical affinity between the membrane material and the culture medium, a flux enhancement of 1.3 was achieved at the end of fermentation. It was also pointed out that when the formation of a cell cake layer was expected to be the main mechanism for flux decline, flow unsteadiness failed to disrupt a previously built-up deposit and for a maximal efficiency it had to be started at the very beginning of the filtration operation. After these feasibility studies on a relatively simple and well-known biological model, further applications on environmental problems were carried out. The interest of a gas/liquid slug flow as a means to increase both the permeate flux and the oxygen transfer rate was demonstrated during continuous phenol degradation by Ralstonia eutropha . The active biomass could be doubled without encountering oxygen depletion while the permeate flux was 75% higher. This led to the complete degradation of a high phenol load higher than 70 kg m −3 day −1 . Finally, a new biological treatment process combining a gas/liquid contactor (‘aero-ejector’) and a membrane bioreactor was developed in order to ensure total microbial degradation of pollutants which were initially present in industrial gaseous effluents. The ‘aero-ejector’ technology allowed the solubilisation of gaseous compounds (here ethanol) in a liquid phase before their degradation in the bioreactor itself. During aerobic cultures of Candida utilis , almost all injected ethanol was transferred and degraded over 350 h of culture.