Shaping biofilm microbiomes by changing GAC location during wastewater anaerobic digestion

The addition of granular activated carbon (GAC) to up-flow anaerobic sludge blanket (UASB) reactors treating synthetic wastewater enhanced methane production by stimulating direct interspecies electron transfer (DIET). A modified UASB reactor with GAC packed in plastic carriers that allowed the GAC to float in the upper reactor zone achieved enhanced performance compared to a UASB reactor with GAC settled at the bottom of the reactor. Microbial communities in the biofilms developed on settled or floated GAC were compared. Methanosarcina (56.3-73.3%) dominated the floated-GAC biofilm whereas Methanobacterium (84.9-85.1%) was greatly enriched in the settled-GAC biofilm. Methanospirillum and Methanocorpusculum were enriched in the floated-GAC biofilm (8.8-19.8% and 5.1-9.5%, respectively), but only existed in low abundances in the settled-GAC biofilm (3.4-3.6% and 0-0.4%, respectively). The floated GAC developed bacterial communities with higher diversity and more syntrophic bacteria enrichments on its surface, including Geobacter, Smithella, and Syntrophomonas, than the settled-GAC biofilm. Common hydrogen-donating syntrophs and hydrogenotrophic archaea, Methanospirillum and Methanoregula, were identified as potential electro-active microorganisms related to DIET.