Impact of applied voltage on methane production and microbial activity in anaerobic digesters in the presence of granular activated carbon (GAC)
Suspended conductive materials (e.g., GAC) and bioelectrochemical systems (BES) have independently been shown to improve methane production by enhancing anaerobic digestion (AD). Incidentally, both of these approaches also have also been shown to mitigate membrane fouling in anaerobic membrane bioreactors (AnMBRs), an emerging alternative mainstream wastewater treatment technology. Yet, little is known regarding the potential effect of their combined application in AD systems. Here, we investigated the effect of a combined BES-AD system with and without GAC supplementation. Results indicated that the BES-AD system with GAC could achieve accelerated and robust methane production (> 300 mL CH4/gCOD) at applied voltages of up to 2.75V, whereas the BES-AD system alone could not sustain equivalent applied voltage rates due to water electrolysis. Additionally, the BES-AD system with GAC maintained its methane production capacity at elevated organic loading rates (20 g COD/L) during the treatment of food waste, while control AD and BES-AD systems showed decreases in their specific methane production rates. Microbial analysis revealed that improved performance of the BES-AD system with GAC coincided with significantly higher relative activity of exoelectrogenic bacteria of the genus Geobacter on both anodic biofilms and GAC-developed biofilms. Furthermore, this advantageous application of GAC to the digestion system also saw the emergence of Methanospirillum as a dominant contributor to methanogenic community relative activity. These observations suggest that the presence of conductive materials in suspended form can serve to enhance the already beneficial coupling of BES systems with anaerobic digesters through significant alteration of key microbial groups.