Effects of Feedstocks and Inoculum Sources on Mixed-Acid and Hydrogen Fermentations
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With increasing energy demand, decreasing oil supply, and continuously accumulating waste in landfills, the interest in converting lignocellulosic biomass to liquid fuels has grown. The MixAlco™ process requires no exogenous enzymes, no sterility, can be adapted to any biodegradable feedstock, and converts lignocellulosic biomass into valuable chemicals and transportation fuels. This work focuses on the effects different feedstocks and inocula have on mixed-acid/hydrogen fermentations. When volatile solids (VS) are digested, mixed-acid fermentations produce hydrogen gas as a secondary byproduct. Hydrogen is only produced when there is an excess of NADH within the cell and when the energy selectivity (gamma) of the system has not been met. Continuous fermentations of paper produced 16.7 g carboxylic acid/L and 15.7 mL H2/g VS digested. Continuous fermentations of pretreated bagasse produced 17.1 g carboxylic acid/L and 41.1 mL H2/g VS digested. Both fermentations produced a fraction of the theoretical amount of hydrogen. The paper fermentation had a hydrogen percent yield of 6.9 percent, whereas the bagasse fermentation had a hydrogen percent yield of 22.6 percent. Hydrogen production was capped at this level because gamma had been met for these systems. The Bioscreening Project, a joint project between three departments, sought to improve the MixAlco™ process by finding natural cultures containing high biomass converters and high acid producers. A total of 505 inoculum samples were collected from 19 sites and screened using paper and yeast extract fermentations. The best converters were analyzed with Continuum Particle Distribution Modeling (CPDM). Nine inocula were run in paper and yeast extract countercurrent fermentations in which the overall performance varied less than 13 percent. Comparisons between six countercurrent train cultures showed an average culture similarity of 0.40 (Yue-Clayton similarity). With the dissimilar microbial cultures and the very similar fermentation performance, the performance of the MixAlco™ process depends on fermentation conditions, not on the microorganisms. Batch fermentations of office paper wastes, pineapple residue, Aloe vera rinds, wood molasses, sugar molasses, extracted algae, non-extracted algae, crude glycerol, obtained from the biodiesel process, and pretreated water hyacinths produced sufficient carboxylic acids and had sufficiently high conversions to be viable substrates for the MixAlco™ process.
Forrest, Andrea Kelly (2010). Effects of Feedstocks and Inoculum Sources on Mixed-Acid and Hydrogen Fermentations. Doctoral dissertation, Texas A&M University. Available electronically from