The results of the study reported here suggest that promoting DIET with conductive materials could enhance syntrophic metabolism in the UASB reactors. The improvement did not appear to be merely related to providing a surface for microbial attachment because sludge attached on the each of three carbon materials was less than 5% of all the sludge in the reactor, much less than the 30–45% increased methane production (Fig. 2A). Yet, upon removal of the conductive materials, methane production and COD removal rates declined and effluent ethanol and acetate levels increased to values comparable to the control reactor (Fig. 3). These results suggest that the conductivity of the materials rather than IC87114 retention was the primary reason for the improved performance in the presence of the conductive carbon materials.
Further evidence that improved reactor performance could be attributed to improve the conductivity of the materials was the response at the lowest HRT. Reactors supplemented with conductive carbon materials, especially carbon cloth, metabolized ethanol and acetate much more effectively (Fig. 2). Considering that the conductive carbon materials could not promote interspecies H2 transfer (Chen et al., 2014b), this response can most reasonably be ascribed to improved DIET. Better syntrophic metabolism when DIET was promoted helped maintain a pH favorable for acetoclastic methanogens (Table. S1). Therefore, there were almost no ethanol or acetate detected in the reactors supplemented with carbon conductive materials (Fig. 2B and C) and the gap of methane production between the control and supplemented reactors increased further with the decrease in the HRT (P < 0.05) ( Fig. 2A).