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Transitioning Towards a Circular Economy in Québec: An Integrated Process for First-, Second- and Third-Generation Ethanol from Sweet Sorghum and Chlorella vulgaris Biomass

Boboescu, Iulian-Zoltan, Beigbeder, Jean-Baptiste, Damay, Jérémie, Duret, Xavier, Lalonde, Olivier, Lavoie, Jean-Michel
Industrial biotechnology 2019 v.15 no.3 pp. 169-178
Chlorella vulgaris, Saccharomyces cerevisiae, antibiotics, biomass, carbon dioxide, circular economy, commercialization, economic sustainability, effluents, ethanol, ethanol fermentation, ethanol production, feedstocks, hydrolysates, lignocellulose, liquids, microalgae, streams, sugars, sweet sorghum, urea, yeast extract, Quebec
Full feedstock potential needs to be tapped to make lignocellulosic ethanol an economically viable reality. This work focuses on the Saccharomyces cerevisiae ethanol fermentation of fresh sorghum carbohydrates extracted through a mild steam-treatment process, and the subsequent Chlorella vulgaris cultivation using the generated liquid and gaseous fermentation effluents. The first section of the manuscript focuses on the effect of nutrient addition (fermentation effluent, yeast extract and urea) on the conversion efficiency of the sorghum carbohydrates to ethanol. Overall, the fermentation time was reduced to half when yeast extract and urea were supplemented to the free and hemicellulosic carbohydrate stream, accelerating the total sugar consumption time from 24 h to under 12 h. However, regarding the cellulosic carbohydrate hydrolysate, the sole addition of urea resulted in a slight improvement of the fermentation kinetics. The second half of the manuscript presents the impact of these different fermentation effluents and various process parameters (addition of yeast extract, antibiotic and CO₂) on the microalgal cultivation and composition. The cellulosic hydrolysate yielded the highest concentrations of microalgal carbohydrates (507 mg/L) under a CO₂-rich environment. Further cultivation scale-up assays confirmed these observations in the presence of 10% CO₂ using the mixed fermentation effluents of the free and constitutive sorghum carbohydrates. Thus, an integrated sorghum-based first- (free carbohydrates), second- (constitutive carbohydrates) and third-generation (microalgal carbohydrates) ethanol production process was thoroughly investigated. This work could represent a step towards bridging the gap leading to full-scale commercialization of these advanced-biofuel technologies.