Upgrading liquor-making wastewater into medium chain fatty acid: Insights into co-electron donors, key microflora, and energy harvest

Ethanol and lactate are considered suitable electron donors (EDs) for chain elongation (CE); however, their respective shortcomings still limit the substrate conversion ratio and medium chain fatty acid (MCFA) production. To address this limitation, different EDs and electron acceptors (EAs) were combined to compare their CE performances, and to investigate whether the combination of ethanol and lactate could further enhance the MCFA production based on the complementary characteristics of ethanol and lactate. The results verified, for the first time, ethanol and lactate as the co-EDs formed a cooperative relationship to largely promote the conversion of substrates into MCFA. The co-EDs of ethanol and lactate stimulated the transformation of dispersive lactate-carbon flux from the competing acrylate pathway into n-heptylate. Additionally, the coexisting by-products (H₂ and CO₂) from ethanol and lactate also contributed to the supererogatory MCFA generation. The key microbial taxa that distinguished the co-EDs from their single action were the preponderant species from class Negativicutes and family Ruminococcaceae. In addition, the co-EAs of acetate, n-butyrate, and n-caproate also promoted MCFA generation. Low concentration of n-caproate could be directly elongated into n-caprylate, while n-caproate concentration exceeding the toxic limit was unsuitable as an EA. This research provided a guide for substrate selection and collocation for CE technology. Chinese liquor-making wastewater (CLMW) was subsequently used as a substrate for MCFA production since it contains abundant lactate, ethanol, and short-chain fatty acids. In this study, a MCFA selectivity of 80.34 ± 5.26%, a slightly higher selectivity which is in the range of previously reported ones, was obtained. This study paves a way for the sustainable development of Chinese liquor industry by recycling the high-output CLMW into MCFA.