Influence of Granular Activated Carbon Addition on Methane Production in Dry and Semidry Anaerobic Digestion of Swine Manure

This work investigated the effect of granular activated carbon (GAC) on methane-rich biogas production in semidry (10% total solids, TS) and dry (15% TS) anaerobic digestion (AD) of swine manure. The conductive material GAC can improve methane production exponentially in AD, even without pretreatment. However, in semidry and dry conditions, there was no significant difference in methane production yield when using raw or pretreated swine manure with the addition of 20 g/L GAC. For this condition, when using the pretreated substrate, the main observed effect was related to the lag phase increase from 6.5 to 10.6 days and from 10.4 to 32.0 days at 10 and 15% TS, respectively. In semidry AD, with pretreated manure, 10 g/L GAC addition yielded 190 mL CH₄/gVS, a value 21% higher than with 20 and 30 g/L GAC. In dry AD, the best condition was with 30 g/L GAC (157 mL CH₄/gVS), followed by 20 and 10 g/L GAC (127 and 18 mL CH₄/gVS, respectively). Therefore, the TS content influences the ideal GAC concentration. About microbiology, the increase in the abundance of phyla <i>Acidobacteriota</i>, <i>Bacteroidota</i>, <i>Chloroflexi</i>, and <i>Firmicutes</i> was linked to greater methane production when using raw manure. The dominant genus of bacteria in most reactors was <i>Georgenia</i>, and the main abundances of archaea were <i>Methanobacterium</i> (slightly more favored with GAC addition) and <i>Methanosaeta</i>. Therefore, GAC increases methane production not only by favoring certain important species in AD but also mainly by promoting Direct Interspecies Electron Transfer (DIET) and adsorbing inhibitory compounds, thereby improving methane-rich biogas production.