Graphene Addition to Digestion of Thin Stillage Can Alleviate Acidic Shock and Improve Biomethane Production

Production of biomethane from distillery byproducts (such as stillage) in a circular economy system may facilitate a climate neutral alcohol industry. Anaerobic digestion (AD) of easily degradable substrates can lead to rapid acidification and accumulation of intermediate volatile fatty acids, reducing microbial activity and biomethane production. Carbonaceous materials may function as an abiotic conductive conduit to stimulate microbial electron transfer and resist adverse impacts on AD. Herein, nanomaterial graphene and more cost-effective pyrochar were comparatively assessed in their ability to recover AD performance after acidic shock (pH 5.5). Results showed that graphene addition (1.0 g/L) could lead to a biomethane yield of 250 mL/g chemical oxygen demand; this is an 11.0% increase compared to that of the control. The recovered process was accompanied by faster propionate degradation (CH3CH2COO- + 2H(2)O -> CH3COO- + CO2 + 6H(+) + 6e(-)). The enhanced performance was possibly ascribed to the high electrical conductivity of graphene. In comparison, pyrochar addition (1.0 and 10.0 g/L) did not enhance the biomethane yield, though it reduced the digestion lag-phase time by 18.1% and 12.2% compared to the control, respectively. Microbial taxonomy analysis suggested that Methanosarcina (81.5% in abundance) with diverse metabolic pathways and OTU in the order DTU014 (6.4% in abundance) might participate in direct interspecies electron transfer contributing to an effective recovery from acidic shock.