Abstract
Anaerobic digestion (AD) at low temperature (20 °C) for low-strength municipal sewage (COD of 500 mg/L) treatment was evaluated in two laboratory-scale up-flow anaerobic sludge blankets (UASBs), one with and one without granular activated carbon (GAC). During the 120-day operation, the addition of GAC significantly improved average COD removal (from 56% to 82%) and methane production (from 132 to 264 mL CH4/g feed-COD), allowing for a reduced hydraulic retention time (from 1 d to 0.25 d) and an increased organic loading rate (from 500 to 2000 mg COD/L/d). There was a significant (p < 0.01) improvement in specific methanogenic activity (SMA) from 20 mL CH4/g VSS/d in non-GAC UASB sludge to 58 mL CH4/g VSS/d in GAC-amended UASB sludge, which may explain the enhanced performance with GAC. Direct interspecies electron transfer (DIET) was proposed as the mechanism and sludge properties were compared. Surprisingly, the archaeal and bacterial communities were similar between the two reactors. Further studies revealed significant sludge physiological changes in the GAC-amended reactor, including enhanced electric conductivity (from 1.52 to 8.37 μS/cm), increased functional gene pilA expression as confirmed through RT-qPCR (36-fold increase), as well as reduced inhibition by high H2 partial pressure (0.17 atm), all implying the development of DIET through the cellular electro-conductive structure e-pili.