High n-caprylate productivities and specificities from dilute ethanol and acetate: chain elongation with microbiomes to upgrade products from syngas fermentation†
Past studies on chain elongation by reactor microbiomes have shown promising productivities and specificities of n-caproate, which is a six-carbon (C6) medium-chain carboxylate (MCC). The production of small quantities of n-caprylate (C8) was also reported, but merely as a by-product of n-caproate production. Since n-caprylate is considerably more valuable than n-caproate by weight, the question was raised whether n-caprylate could be produced at a high specificity (product vs. all other carboxylate products). Here, we optimized the operating conditions of an anaerobic upflow bioreactor during a period of 186 days to accomplish this goal. We achieved an n-caprylate productivity of 19.4 g chemical oxygen demand (COD) L−1 d−1 (0.33 g L−1 h−1), which is the highest ever reported. The product ratio of n-caprylate to n-caproate was 11 g COD per g COD, which was also greater than past studies. This ratio was even 25 g COD per g COD during an earlier operating period with a lower productivity, resulting in an n-caprylate specificity of 96% (based on COD). We accomplished this high n-caprylate productivity and specificity by: (1) feeding a substrate with a high substrate ratio of ethanol to acetate; (2) extracting the product from the bioreactor broth; and (3) acclimating an efficient chain-elongating microbiome. Because syngas-fermentation effluent contains a high ratio of ethanol to acetate, we propose coupling syngas fermentation with chain elongation to produce n-caprylate and to increase product value.