Medium-chain carboxylic acids production from brewer’s spent grains by mixed microbial consortium : influence of biomass pre-treatments and H2/CO2 availability on biomass fermentability and medium-chain carboxylic acids production

Lignocellulosic biomass is considered as a promising renewable and low cost resource to produced energy, chemicals and other bio-products through biochemical conversion processes. Challenges in lignocellulosic biomass conversion reside in overcoming its natural recalcitrance to depolymerization and improving the selectivity and yields towards high-value products. In this perspective, this master’s thesis focused on the production of carboxylates, especially medium-chain carboxylates, from brewer’s spent grains by batch-mode fermentations thanks to mixed microbial consortium. The aims this master’s thesis were to assess the influence of ensiling and dilute alkaline hydrothermal pre-treatments of the biomass on its further hydrolysis and fermentation and the influence of H2/CO2 availability on the fermentation efficiency and medium-chain carboxylates production. To achieve this, we tested ensiled brewer’s spent grains, solid fraction of brewer’s spent after a dilute alkaline hydrothermal pre-treatment with a high-pressure reactor with KOH 0.25M at 120°C for 20’ and at 80°C for 20’ and brewer’s spent grains after a dilute alkaline hydrothermal pre-treatment with a water bath with different KOH concentrations (0M, 0.25M and 0.5M), and different pre-treatment times (40’ and 120’) and with reference conditions at room temperature and a reference without pre-treatment. We also tested three gas supply method, namely, flushing the gas three times a week, flushing the gas every day and a continuous sparging of the gas. The results showed that ensiling did not contribute to improve carboxylates production. However, ensiling showed a heptanoate yield of 2.0 ± 0.0 kgCOD_C7/kgCOD_IN after a fermentation launched over 32 days with an initial concentration in chemical oxygen demand of 141.2 ± 0.2 gCOD/LML in the bioreactor. Therefore, ensiling may promote odd-chain carboxylates and thus, heptanoate productions. Among the dilute alkaline hydrothermal and non-thermal pre-treatments tested, the highest productions of short- and medium-chain carboxylates were obtained at room temperature with KOH 0.25M for 120 minutes, with a caproate yield of 2.0 ± 0.1 kgCOD_C6/ kgCOD_Substrate and a heptanoate yield of 0.71 ± 0.04 kgCOD_C7/ kgCOD_Substrate. The results suggested that decreasing the severity of the pre-treatments may be promising to improve the biomass conversion towards carboxylates production. In average, among the three gas supply methods tested there were no obvious or significant differences between the productions of short- and medium-chain carboxylates.