Enhanced direct interspecies electron transfer in anaerobic digestion of fat, oil, and grease by pre-attachment of microbes to granular activated carbon: the essential role of inoculum

Long chain fatty acids (LCFAs) adsorption onto granular activated carbon (GAC) surface competitively inhibits the establishment of direct interspecies electron transfer (DIET) during anaerobic digestion of fat, oil, and grease (FOG). Although pre-attachment of inoculum to GAC can prevent the adsorption of LCFAs onto GAC, the assembly of microbial community on GAC driven by pre-attachment remains poorly understood. This study investigated the effect of pre-attachment using different inoculum types (varied in microbial composition and aggregation) on GAC-induced DIET during FOG digestion. The results show that pre-attachment of distinct inocula to GAC significantly altered methane yield in FOG digestion (549.6-776.7 mL/gVSadded), varying from 24.2% lower to 73.6% higher than the treatment without pre-attachment. The methane yield improvement from pre-attachment is most effectively achieved by promoting LCFAs degradation via GAC. The most efficient LCFAs degradation via GAC was accomplished with a large-size acclimated inoculum, enriching Geobacter (36.2% relative abundance in bacteria) and Methanosaeta (71.4% in archaea) and promoting the cooperative interactions among microbes involved in acetate/propionate degradation and nitrogen conversion on GAC, leading to a maximum increase of 63.4% in methane yield compared to the control. Overall, this work provides an in-depth understanding of GAC-induced DIET via inoculum pre-attachment during FOG digestion.