Medium chain Fatty acids production from Food Waste via homolactic fermentation and lactate/ethanol elongation: Electron balance and thermodynamic assessment

This study explored the potential of Food Waste (FW) extract as a suited substrate for Medium Chain Fatty Acids (MCFAs) production, in a single phase reactor, were both fermentation and Chain Elongation (CE) processes simultaneously occurred. A continuous experiment was conducted at OLR = 20 grCOD L-1 d(-1) , fed in batch mode twice a week with pH = 6. In addition, four batch tests were performed, to assess the effects of caproate inhibition, hydrogen partial pressure (P-H2) and different lactate/acetate ratios. Thermodynamics and electron flux were calculated to gain insights into the process pathways. Due to the presence of aminoacids, fermentation was mostly homolactic and both lactate and ethanol were produced as Electron Donors (EDs); the average MCFAs efficiency was similar to 12 %, although past 4 weeks the elongation process was halted, resulting in EDs accumulation. This occurred regardless of inoculum selection and caproate inhibition, suggesting that EDs accumulation was due to the elongation process kinetics being slower than those of the fermentation step, thus calling for a longer HRT. It's worth noting that lactate was prevalently self elongated to butyrate, whereas ethanol elongation took only place after lactate depletion but was more efficient since it required other butyrate or propionate/valerate as Electron Acceptors (EAs). Moreover, the selected pH allowed to limit the acrylate pathway to a reasonable extent whereas the high P-H2 prevented both excessive ethanol oxydation and lactate oxydation to acetate.