The 5th International Conference on Advances in Civil and Ecological Engineering Research (ACEER 2023)

Abstract: The rapid consumption of fossil energy has caused increasingly serious energy shortage and environmental pollution problems worldwide, which also poses extra burden to carbon neutrality, and led to a frontier and hot issue aiming to realize the sustainable development of the environment and energy demand. A growing number of researchers have focused on the production of high-value biofuels from organic waste, such as ethanol and fatty acids, which are also precursors of renewable fuels and chemicals and deserve wider applications. Medium chain fatty acids (MCFA) production via chain elongation (CE) of carboxylic acids through a reversed β-oxidation pathway has been a great interest recently. MCFA have more excellent physical properties than volatile fatty acids (VFA) and methane with 6–12 C atoms, and the high hydrophobicity contribute to further processing and separation. Electro-fermentation (EF) is the application of bio-electrochemical system in traditional anaerobic fermentation, which can use inexhaustible and cheap cathode instead of hydrogen/ethanol/lactate as electron donor. Caproate production from volatile fatty acids (VFAs) through chain elongation processes in electro-fermentation is a sustainable and promising technique for organic waste recycling and resources recovery. However, the small surface area of working electrode is one of the main factors impeding the improvement of performance. Therefore, fluidized cathode via granular activated carbon (GAC) dosage was employed in this study to stimulate the electro-fermentation for enhanced caproate generation, and the relevant mechanism was revealed. Herein, the effects of different filling ratios of GAC (0, 3, 8, 13, and 18%) on electro-fermentation with three different types of electron donors (lactate, lactate and cathode, and cathode) were investigated. It proved that the cathode could serve as the sole electron donor in the fluidized cathode EF systems, which outcompeted lactate, and lactate and cathode with easier availability and higher cost effectiveness. Results showed that the fluidized cathode electro-fermentation systems achieved the best performance at a GAC filling ratio of 8% with cathode as the sole electron donor. The yield of caproate, carbon recovery rate and electron recovery rate were 2.1, 1.8 and 1.6 times higher than those without GAC, respectively. The electrochemical analysis also verified the highest electrochemical activity of the cathode biofilm and the relatively small internal resistance of the system. It was noted that the dominant bacteria on the cathode biofilm shifted from Lactobacillus to Clostridia, Oscillibacter and Caproiciproducens, which probably contributed to the caproate production via chain elongation process. This work would provide some insights into application of electro-fermentation for high value added caproate production.