Development of an enhanced chain elongation process for caproic acid production from waste-derived lactic acid and butyric acid

Medium chain carboxylic acids (MCCA), such as caproic acid, are high-value chemicals with many industrial applications. The development of a biological mixed culture chain elongation (CE) process, using waste-derived lactic acid offers an opportunity to sustainably produce MCCA, such as caproic acid. However, the nature of the biological conversions in mixed communities is still poorly understood. In particular, inhibition of methanogens and MCCA competitors for the development of a specialised inoculum and optimisation of lactic acid conversion and selectivity toward caproic acid are required. Here, the impacts of physical and chemical treatments of the inoculum (anaerobic granular sludge) on the efficiency of CE were evaluated. Additionally, the effects of lactic acid/butyric acid concentrations and ratio (r LA/BA) on selectivity and concentration of caproic acid were investigated. Blended and acidified (pH 3) granular sludge yielded the highest caproic acid concentrations. Moreover, using pH 3 acidified and blended granular sludge, substrate concentrations of 250 mM (r LA/BA = 1.5:1) and 300 mM (r LA/BA = 1:1) were optimal for efficient caproic acid accumulation with a selectivity ≥90 and a mole of lactic to mole of butyric acid consumption (C r LA/BA) of 2:1. However, a similar outcome was achieved when using an enriched caproic acid producing culture with a lower substrate concentration of 200 mM (r LA/BA = 1:1). We report a lactic acid to butyric acid threshold concentration, below and above which the selectivity toward caproic acid is reduced. With these optimum substrate concentrations and ratios, the feasibility for efficient caproic acid synthesis using lactic acid as electron donor and butyric acid as electron acceptor was established. Caproic acid synthesis was completely inhibited when the substrate concentrations were increased to 400 mM (r LA/BA = 1:1). Finally, species affiliated with Ruminococcaceae were likely involved in the caproic acid synthesis. These findings have strong potential for process-design applications in continuous reactor systems, thus allowing for continuous, efficient and sustainable caproic acid production from biomass-derived lactic acid and butyric acid.