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

Among processes developed to increase biological performances, membrane bio reactors have provided the best results. The membrane bioreactor combines a continuous fermenter and a crossflow filtration module enabling separation of cells from liquid media. Very high biomass concentrations have thus bee n reached and important bioconversion yields obtained. However the potentia lity of this process is mainly limited by the rapid decline in permeate flu x due to membrane fouling. In our laboratory, various technological solutio ns: 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 membra ne 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 lo wer efficiency for microfiltration due to pore blocking by cell debris. The impact of an unsteady jet generated by a pneumatically controlled valve wa s also evaluated. Although the strong physico-chemical affinity between the membrane material and the culture medium, a flux enhancement of 1.3 was ac hieved at the end of fermentation. It was also pointed our that when the fo rmation 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 environment al problems were carried out. The interest of a gas/liquid slug flow as a m eans to increase both the permeate flux and the oxygen transfer rate was de monstrated 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 biologica l treatment process combining a gas/liquid contactor ('aero-ejector') and a membrane bioreactor was developed in order to ensure total microbial degra dation of pollutants which were initially present in industrial gaseous eff luents. The 'aero-ejector' technology allowed the solubilisation of gaseous compounds then ethanol) in a liquid phase before their degradation in the bioreactor itself. During aerobic cultures of Candida utilis, almost all in jected ethanol was transferred and degraded over 350 h of culture. (C) 2001 Elsevier Science B.V. All rights reserved.