Continuity of biochar-associated biofilm in anaerobic digestion

Several single dose studies found that biochar was an effective additive promoting methanogenesis in anaerobic digestion. However, little attention was paid to the continuity of the biofilm formed on biochar and the subsequent evolution of microorganisms. In this study, the continuous performance and changes were quantified when biochar-biofilm consortia were applied as the recyclable inoculant to four cycles of acetate methanogenesis with increasing acetate loading. Biochar-biofilm consortia as the inoculant were found to consistently realize stable methane production, despite the removal of planktonic microorganisms in the reactor, and no extra inoculum and biochar were added. Consortia with biochar particles smaller than 5 μm promoted initiation of methanogenesis more rapidly than those with biochar particles larger than 1 mm, especially when the activity of microorganism was low. Moreover, the microorganisms were enriched throughout the continuous cycles. Biochar < 5 μm was found to accumulate 6.6–7.1 E + 11 16S copies per gram in the 3rd and 4th cycles, which was about 27–51 times that of biochar > 1 mm and also comparable to a fresh sludge inoculum. In addition, specific functional methanogens proliferated during continuous application. Methanosarcina was dominant in biochar > 1 mm, while the syntrophic acetate oxidizing bacteria Thermovirga and Mesotoga accounted for the majority of microorganisms in biochar < 5 μm. Therefore, with the competitive total cell count dominated by functional microorganisms, biochar-biofilm consortia demonstrated feasible recycling and reuse for bioaugmentation purposes or in the economical long-term application of anaerobic digestion for waste or wastewater treatment.