Conference Dates

March 8-13, 2009

Abstract

The economic scenario established at the beginning of the third millennium has revived the interest in Acetone-Butanol-Ethanol (ABE) fermentations. Recent developments in molecular techniques applied to solventogenic microrganisms in combination with recent advances in fermentation systems and downstream processing have contributed to improve ABE fermentation processes feasibility and competitiveness. The challenges raised over the last years as regards ABE production may be synthesized in: i) use of renewable resources as substrate; ii) selection of strains characterized by high ABE productivity; iii) development of new fermentation systems; iv) development of new downstream strategies for enhanced solvent recovery. The selection of unconventional substrates is favoured by the ability of clostridia strains to metabolize a wide range of carbohydrates like glucose, lactose, etc...., typically present in wastewater streams e.g. from food industries. Even though clostridia have been proven successful to produce ABE, information about kinetics of substrate conversion, cell growth and butanol production is still lacking. Studies available in literature most typically regard batch tests whose results do not apply easily to continuous processes. The strong interaction between the growth/acidogenesis phase and the solventogenesis phase should be taken into account.

The reactor systems investigated for the ABE fermentation belong to the batch and fed-batch typologies. Some attempts are reported in literature regarding continuous fermentation by means of clostridia strains confined in the reactor by immobilization or cell-recycling.

The present study reports the preliminary results of a research activity aiming at investigating the feasibility of the ABE production by Clostridium acetobutylicum ATCC824 in a continuous biofilm reactor adopting cheese whey as feedstock. The contribution regards the characterization of the kinetics related to the ABE production process by free C. acetobutylicum ATCC824 adopting as medium lactose solutions, in order to emulate the cheese whey. The conversion process is characterized in terms of cells, acids, solvents, pH, gas composition and total organic compounds as a function of time. Results are worked out to assess the kinetics of the cells growth and of the ABE production. The yields of the carbon source in cells, acids and solvents are also assessed. The investigation is carried out adopting both batch reactors and two continuous reactors. In particular, the continuous reactors are equipped to operate under controlled conversion regimes, acidogenesis or solventogenesis.

Tests carried out under batch conditions show that: i) cells growth follows the Monod kinetics for lactose concentration (CL) smaller than 100 g/L; ii) the butanol specific production rate increases linearly with CL; iii) the lactose conversion - measured at the end of the solventogenesis phase - decreases with CL; iv) the selectivity of butanol with respect to total solvents increases with CL and stabilizes at about 72%W for CL larger than 30 g/L. Preliminary tests carried out with the continuous reactor operated under solventogenesis regime show that steady state establishes with respect to cells and metabolites concentration at dilution rate of about 0.04 h-1.

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