Biotechnology for Biofuels | 2021
Enhanced methane production from cellulose using a two-stage process involving a bioelectrochemical system and a fixed film reactor
Abstract
Background It is desirable to improve the anaerobic digestion processes of recalcitrant materials, such as cellulose. Enhancement of methane (CH 4 ) production from organic molecules was previously accomplished through coupling a bioelectrochemical system (BES); however, scaling-up BES-based production is difficult. Here, we developed a two-stage process consisting of a BES using low-cost and low-reactive carbon sheets as the cathode and anode, and a fixed film reactor (FFR) containing conductive material, i.e., carbon fiber textiles (CFTs) (:BES\u2009→\u2009FFR). By controlling the cathodic current at 2.7 μA/cm 2 without abiotic H 2 production, the three-electrode BES system was operated to mimic a microbial electrolysis cell. Results The thermophilic BES (inlet pH: 6.1) and FFR (inlet pH: 7.5) were operated using hydraulic retention times (HRTs) of 2.5 and 4.2\xa0days, respectively, corresponding to a cellulose load of 3555.6\xa0mg-carbon (C)/(L\xa0day). The BES\u2009→\u2009FFR process achieved a higher CH 4 yield (37.5%) with 52.8 vol% CH 4 in the product gas compared to the non-bioelectrochemical system (NBES)\u2009→\u2009FFR process, which showed a CH 4 yield of 22.1% with 46.8 vol% CH 4 . The CH 4 production rate (67.5\xa0mM/day) obtained with the BER\u2009→\u2009FFR process was much higher than that obtained using electrochemical methanogenesis (0.27\xa0mM/day). Application of the electrochemical system or CFTs improved the yields of CH 4 with the NBES\u2009→\u2009FFR or BES\u2009→\u2009non-fixed film reactor process, respectively. Meta 16S rRNA sequencing revealed that putative cellulolytic bacteria (identified as Clostridium species) were present in the BES and NBES, and followed (BES→\u2009and NBES→) FFR. Notably, H 2 -consuming methanogens, Methanobacterium sp. and Methanosarcina sp., showed increased relative abundances in the suspended fraction and attached fraction of (BES→) FFR, respectively, compared to that of (NBES→) FFR, although these methanogens were observed at trace levels in the BES and NBES. Conclusions These results indicate that bioelectrochemical preprocessing at a low current effectively induces interspecies H 2 transfer in the FFR with conductive material. Sufficient electrochemical preprocessing was observed using a relatively short HRT. This type of two-stage process, BES\u2009→\u2009FFR, is useful for stabilization and improvement of the biogas (CH 4 ) production from cellulosic material, and our results imply that the two-stage system developed here may be useful with other recalcitrant materials.