Anaerobic production of valeric acid from crude glycerol via chain elongation

Abstract

Glycerol from biodiesel production was used as a substrate for valerate production in a 13.1-L anaerobic filter with an open microbiome. Ethanol at 15% of the influent chemical oxygen demand was supplemented as an additional electron donor source. The 114-day experimental period was divided in three phases which included initial adaptation phase, external sludge addition from a caproic acid producing reactor and valerate extraction using a pertraction system. Propionate (0.63–1.75 g COD L⁻¹ day⁻¹) and valerate (0.86–1.81 g COD L⁻¹ day⁻¹) were the main carboxylates formed throughout all operational phases. An increase in production rates of butyrate (0.26–0.31 g COD L⁻¹ day⁻¹), caproate (0.01–0.08 g COD L⁻¹ day⁻¹), and 1,3-propanediol (0.43–0.52 g COD L⁻¹ day⁻¹) was observed with addition of external caproic acid producing sludge rich in Clostridium members. In the operational phase with pertraction, a sudden decrease in 1,3-propanediol and concomitant increase in acid production were verified. Propionate and valerate reached higher production rates compared with the other two phases, suggesting that the pertraction system favored the oxidative pathway of glycerol fermentation. Valerate extraction reached a maximum of 30 g COD m⁻² day⁻¹. The filter microbiome was highly diverse with Simpson index values close to 0.1 for the four sludge samples collected, and a concomitant increase in Megasphaera elsdenii and an increase in valerate production rates were observed. Hence, high valerate production and extraction rates through the carboxylate platform are a feasible alternative for crude glycerol valorization with a great potential for improvement in future research.