Thermophilic anaerobic digestion of kitchen waste driven by microbial electrolysis cell (MEC): Digestion performances, anodic microorganisms’ distribution and current utilization efficiency

Abstract

The aims of present study were to investigate the performances of kitchen waste thermophilic anaerobic digestion driven by microbial electrolysis cell (MEC), and to clarify the methanogenic metabolism pathways on the electrodes (especially on the anodic biofilm). The results showed that both 0.6 V and 1.0 V voltages supplementation could enhance methane yields and production rates, while efficient methanogenesis failed to be sustained after power outages under high organic loading rate (OLR, 4.22 gVS/L/day) condition. Voltages supplementation could also enhance VS removal efficiencies from 83.24–83.35% to 87.01–87.46%, and strengthened the hydrolysis and acidogenesis of dissolved organic matters (DOMs) on the outer layer of anodic biofilm. Voltages supplementation accelerated the β-oxidation of volatile organic acids (VFAs) on the inner layer of anodic biofilm and facilitated acetoclastic methanogenesis on the outer layer of anodic biofilm. The generated electrons on the anode were transferred to the cathode for producing hydrogen as a substrate of hydrogenotrophic methanogenesis, rather than directly generating methane by reducing carbon dioxide through direct interspecies electron transfer (DIET) pathway. On the anodic biofilm, the syntrophic relationship between Syntrophomonas and Methanosarcina was confirmed as the main contributor of methanogenesis. Analysis of current utilization efficiency proved that a little amount of energy input could lead several folds of current utilization efficiencies in return, especially under high OLRs condition, which was anticipated to be applied into practical engineering of kitchen waste thermophilic digestion for higher economic benefits.