Tower Continuous Fermentation of Cellulosic Wastes

Abstract

The aim of this work was the study of the direct bioconversion of cellulose to biomass in a novel Tower fermentation system. Ball milled a-cellulose and newspaper were used as model substrates. Five cellulolytic fungi were selected from the literature for their bioconversion abilities. Their optimum pH and temperature for growth were determined on agar fermentation media by their linear growth rate. Batch cultures at their pH and temperature optima (controlled) were performed in a five litre Tower fermenter to assess their cellulose upgrading ability. The criteria were: substrate degradation, biomass productivity and protein content of biomass. A strain of Sporotrichum thermophile was selected and cultured continuously. The operational characteristics of the system were studied with respect to dilution rate, substrate concentration, substrate degradation, extracellular cellulase activity, biomass and protein productivity. The results confirmed the ability of the system to operate at dilution rates higher than the specific growth rate of the organism and demonstrated the differences in the retention properties of the Tower between soluble and insoluble carbohydrates fermentations. The fermentation was affected by the morphology of the organism and suspended solids concentration. Biomass concentration, cellulase activity and substrate degradation efficiency were decreasing with dilution rate whereas the total solids concentration increased. Maximum productivity was obtained well below the critical dilution rate. The most important factor which controlled the fermentation was the hydrolysis of the substrate. The biodegradability of the organism was not inhibited by the products of hydrolysis. The negative effect of lignin was also confirmed. The continuous Tower fermentation of cellulosic materials is biotechnically feasible. It is best suited for substrates with low lignin and low crystalline cellulose content as chemical waste fibres and waste water from fibre board mills.