Key syntrophic partnerships identified in a granular activated carbon amended UASB treating municipal sewage under low temperature conditions.

Two laboratory-scale up-flow anaerobic sludge blankets (UASB) reactors, one with and one without granular activated carbon (GAC), were operated for municipal sewage treatment at low temperatures (16.5 ± 2.0 °C). During the 120-day operation, the GAC-amended reactor significantly enhanced COD removal (from 62% to 75%, P < 0.05) and methane production (from 87 to 218 mg CH4-COD/reactor/d) than the non-GAC reactor. Bacterial communities were significantly different between the two reactors (P < 0.05). Geobacter, a key indicator for direct interspecies electron transfer (DIET), had the highest differential score (LEfSe analysis), showing significantly higher abundances in the GAC-amended reactor (3.7-8.8%) than the non-GAC reactor (0.9-4.0%). GAC also enriched syntrophic bacteria, Syntrophomonas, Syntrophus and sulfate reducing bacteria. Methanobacterium dominated the archaeal community in the GAC-amended reactor sludge (35.7%) and GAC-biofilm (75.3%), and was less abundant in the non-GAC reactor (9.9%). It indicates that GAC enriched microbial syntrophic partners with potential electro-activities in the anaerobic digestion process.