The underlying mechanism of enhanced methane production from microbial electrolysis cells assisted anaerobic digestion (MEC-AD) of protein

Herein, the mechanisms of MEC-AD affecting protein digestion were investigated using bovine serum albumin (BSA) as the substrates. Experimental results showed that the maximal methanogenesis efficiency increased by 45.6% to 225% of BSA (800 mg/L, 5 g/L, and 15 g/L) with the applied voltage of 0.6 V compared to that with open circuit. Mechanism explorations revealed that the applied voltage significantly strengthened the acidogenesis and methanogenesis processes rather than hydrolysis in anaerobic digestion of protein. Microbial community characterization showed that fermentative bacteria was increased by 46% in anode, meanwhile, the abundance of Methanobacterium in cathode increased from 10.4 to 84.3% with applied voltage, indicating the methanogenesis pathway was transferred from acetoclastic to hydrogenotrophic. Further external circuit electron transfer calculation demonstrated only 10% methane produced could be attributed to direct interspecies electron transfer (DIET). From the perspective of thermodynamics, the applying external voltage led to the cathodic potential reduced to -0.9 V, which is beneficial for enhanced methane production via mediated interspecies electron transfer (MIET) by enrichment of hydrogenotrophic methanogen. The findings reported here revealed the actually existing, yet previously unrecognized contributor to MEC-AD of protein, which help engineers better understand the role of applied voltage in MEC-AD.