Butanol production coupled with acidogenesis and CO2 conversion for improved carbon utilization

Abstract

Butanol production from lignocellulosic biomass has a low overall yield due to carbon loss in the form of xylose during the biomass pretreatment and CO2 during the fermentation processes. This research presents a cascading process for producing butanol coupling three bioprocesses. First, an indigenous microbial community performed the direct acidogenesis of raw corn stover producing an H2-CO2 gas stream and volatile fatty acids (VFA) using the most biodegradable fraction. The acidogenesis process had maximum hydrogen productivity of 87 mL/L day and VFA production of 8.5 g/L of acetic acid, 3.7 g/L of butyric acid, and 2.2 g/L of propionic acid. The acidogenesis process experienced a species succession with early colonizing bacteria dominated by Lactobacillus being replaced with more succeeding microbial groups dominated by Enterococcous, Prevotella, and Megasphaera. Second, the spent solids of corn stover were used for producing acetone-butanol-ethanol (ABE) using a mixed culture bioaugmented with Clostridium saccharobutylicum. A simplex centroid mixture design served to elucidate the effects of adding different mixtures of acetic, butyric, and propionic acids on butanol production. Pure butyric acid improved three times the butanol titer compared to the control treatment with no acid addition (610 mg/L versus 230 mg/L of butanol, respectively). Opposite, acetic and propionic acids inhibited the butanol production. Finally, additional butanol was produced using an H2-CO2 gas stream, where the type of inoculum and culture medium affected the process. An inoculum enriched with Spirochaetales, Pseudomonas, Enterobacter, and Proteiniphilum grown on a culture medium with trace metals reached the highest butanol titer of 889 mg/L. This cascading process improved the carbon utilization by producing butanol from VFA and CO2, and not only from cellulose.