Granular activated carbon stimulated caproate production through chain elongation in fluidized cathode electro-fermentation systems

Abstract

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. 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. • GAC was feasible as fluidized cathode for enhanced CE in EF. • GAC with a filling ratio of 8% mostly stimulated electro-bioactivity and conductivity. • Clostridia , Oscillibacter and Caproiciproducens were putative contributors to MCFA production.