Concentrating lactate-carbon flow on medium chain carboxylic acids production by hydrogen supply

Upgrading lactate/carbohydrate-rich waste biomass into medium-chain carboxylic acids (MCCAs) by chain elongation (CE) technology exhibits economic and environmental benefits. However, the largely dispersive lactate-carbon-flow decreases MCCAs yield. This work discovered appropriate H₂ supply could significantly reduce lactate-carbon-flow loss and improve MCCAs production (∼1.65 times) when the system is not operated according to well-defined operating conditions, and revealed corresponding mechanism. Hydrogen (H₂) supply largely enhanced electron efficiency and electron transfer capacity, and H₂ could reduce propionate (from competing acrylate pathway, which should be prevented, but when not possible, the carbon recovery from propionate is possible) to propanol, which was used as electron donor to elongate acetate and propionate. Moreover, H₂ could react with CO₂ (from CE process) to sequentially generate acetate and ethanol, which further contributed to caproate/caprylate generation. Comparing with non-H₂-supplemented test, the lactate-carbon-flow used for MCCAs production was enhanced by ∼28.4% after H₂ supply, and Clostridium spp. were the key discriminative microorganisms.