Granular activated carbon stimulated microbial extracellular secretions in anaerobic digesters: An egalitarian act and beyond

Abstract

Granular activated carbon (GAC) has been widely studied for its roles in improving anaerobic digestion (AD) through its conductive properties in stimulating direct interspecies electron transfer or its surface functional groups in altering redox conditions in AD. In the present study, the impacts of GAC on the biosynthesis of extracellular microbial secretions (e.g., amino acids and vitamins), and the roles of these secretions in AD were investigated. Four continuous laboratory reactors were operated under ambient temperature (20 ± 0.5 °C) conditions, including two up-flow anaerobic sludge blanket (UASB) bioreactors with GAC addition to one of the UASBs; and two anaerobic sequencing batch reactors (ASBRs) each fed with the centrifuged supernatants (to remove suspended biomass and GAC debris) of UASB effluents supplemented with additional carbon source (glucose). Our finding shows that GAC addition stimulated extracellular microbial secretions in UASB, and these secretions, which can work as essential nutrients, enhanced COD removal (from 48 ± 3 % to 60 ± 1 %) and methane production (31 ± 3 % to 47 ± 5 %) in its downstream ASBR. Interestingly, microbial kinetics test showed that extracellular microbial secretions from the GAC-amended UASB improved methane production rate from 72.5 ± 1.5 to 105.6 ± 1.2 mg CH4-COD/g VSS/d without dramatic microbial community shift. We conclude that the impact of GAC on AD does not only rely on the direct contact of microorganisms and GAC materials, and the GAC-stimulated microbial extracellular secretions represent an important mechanism in conductive material amended AD.