Design of a packed-bed fungal bioreactor : the application of enzymes in the bioremediation of organo-pollutants present in soils and industrial effluent

Abstract

Certain fungi have been shown to excrete extracellular enzymes, including peroxidases, laccases, etc. These enzymes are useful for bioremediation of aromatic pollutants present in industrial effluents (Leukes, 1999; Navotny et aI, 1999). Leukes (1999) made recent significant development in the form of a capillary membrane gradostat (fungal) bioreactor that offers optimal conditions for the production of these enzymes in high concentrations. This system also offers the possibility for the polluted effluent to be treated directly in the bioreactor. Some operating problems relating to continuous production of the enzymes and scale-up of the capillary modules, were, however, indentified. In an attempt to solve the above-mentioned identified problems the research group at Peninsula Technikon considered a number of alternative bioreactor configurations. A pulsed packed bed bioreactor concept suggested by Moreira et at. (1997) was selected for further study. Their reactor used polyurethane pellets as the support medium for the fungal biofilm and relied upon pulsing of the oxygen supply and recycle of nutrient solution in order to control biomass accumulation. These authors reported accumulation due to the recycle of proteases that were believed to destroy the desired ligninases. We experimented with a similar concept without recycle to avoid backrnixing and thereby overcome protease accumulation. In our work, a maximum enzyme productivity of 456 Units.L1day·1 was attained. Since this was significantly greater than the maximum reported by Moreira et aI, 1997 (202 Units.L-1day-I) it appeared that the elimination of recycle had significant benefits. In addition to eliminating recycle we also used a length / diameter (L / D) ratio of 14: 1 (compared with 2.5: 1 used by Moreira et aI, 1997) in order to further reduce backrnixing. Residence time distributions were investigated to gain insight into mechanisms of dispersion in the reactor. It was found that the pulsed packed bed concept presented problems with regard to blockage by excess biomass. This led us to consider the advantages of a fluidized bed using resin beads. Accordingly, growth of fungi on resin beads in shake flasks was investigated with favorable results. An experimental program is proposed to further investigate the fluidized bed concept with a view to extending the operation time of the bioreactor. From our literature survey to date, packed bed fungal bioreactors are still the best reactor configuration for continuous production ofligninolytic enzymes. An interesting study of the application of laccases to the degradation of naphthalene and MTBE is described in an addendum to this thesis.