Studies on continuous tower fermentation

Abstract

A Tower fermenter was constructed for the aerobic continuous cultivation of filamentous fungi using as test organism a strain of Aspergillus niger. Normally the mould grew as flocs of biomasss, the formation of which was promoted, as in the batch system, by the low shear and agitation resulting from the absence of mechanical stirring. The fermenter biomass concentration gradually decreased with increased dilution rate; because of the flocculent morphology, the biomass concentration was determined physically by sedimentation of the flocs due to gravity, and their fluidation by the flow of liquid medium and air. This may be referred to as an internal recycle of biomass. When excess carbohydrate was supplied the organism specific growth rate equalled the dilution rate, but, when the carbohydrate was limiting, only the biomass concentration in the effluent stream decreased. Steady-states were thus realised when the organism specific growth rate was less than the dilution rate; the fermenter was consequently not a true Chemostat. Complete organism washout occurred only when the dilution rate was so great that the fermenter contents became heterogeneous, behaving instead as a tubular reactor with biomass recycle. Synchronous growth was observed, biomass being produced periodically. During the phases of negligible growth non-growth associated with organic acids were formed. The maximum organism specific growth rate was measured at steady-state by controlling the culture broth pH and maintaining a sufficiently high dilution rate. Smaller particles, including yeasts and bacteria, were not governed by the internal biomass recycle but selectively washed from the fermenter; this allowed the use of non-autoclaved nutrient medium. Similarly, when the mould was in a filamentous form the fermenter behaved similarly to a conventional Stirred Tank Reactor. The characteristics of the Continuous Tower Fermenter suggest its use for the treatment of non-aseptic foods processing effluents, with the recovery of a biomass byproduct.