Biochar establishing syntrophic partnership between exoelectrogens to facilitate extracellular electron transfer.

Biochar was regarded as a promising accelerator for extracellular electron transfer (EET), while the mechanism of biochar facilitating electricity harvest in bioelectrochemical system (BES) was in debates. In this study, sawdust-based biochar with low conductivity but strong redox-based electron exchange capacity was added into BES with two forms, including a suspended form (S-BC) added in anode chamber and a fixed form closely wrapping up the anode (F-BC). Compared with the control group, S-BC and F-BC addition dramatically increased accumulated electricity output by 2.0 and 5.1 times. However, electrochemical analysis characterized the lowest electrochemical property on anode surface in F-BC modified group. A 2nd period conducted by separating F-BC modified group with "aged F-BC + new anode" group and "single aged anode" group demonstrated that F-BC contributed >95 % to the current generation of F-BC modified group, while the anode almost acted as a conductor to transfer the generated electrons to cathode. Microbial community analysis revealed that both heterotrophic and autotrophic exoelectrogens contributed to current generation. The presence of biochar upregulated functional genes encoding cytochrome-c and type IV pilus, thereby boosting electricity harvest efficiency. Interestingly, the heterotrophic exoelectrogens of Geobacter/Desulfovibrio tended to attach on fixed surfaces of both biochar and anode, and the autotrophic exelectrogen of Hydrogenophaga was selectively enriched on biochar surfaces whatever fixed or suspended form. Consequently, a syntrophic partnership between Geobacter/Desulfovibrio and Hydrogenophaga was potentially establishment on F-BC surface for highly-efficient electricity harvest. In this syntrophic EET model, biochar potentially acted as the redox-active mediator, which temporarily accepted electron released by Geobacter/Desulfovibrio via acetate oxidation, and then donated them to Hydrogenophaga attached on biochar surfaces for autotrophic EET. This was distinct from a regular EET conducted by heterotrophic exoelectrogens. These findings provided new insights to understand the mechanisms of biochar facilitating EET by syntrophic metabolism pathway.

Biochar-assisted anaerobic membrane bioreactor towards high-efficient energy recovery from swine wastewater: Performances and the potential mechanisms.

A high-efficient energy recovery system of biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was established for swine wastewater treatment. Comparing with a conventional AnMBR, biochar addition accelerated volatile fatty acids (VFA) degradation during start-up stage, thereby shortened start-up duration by 44.0 %. Under a high organic loading rate (OLR) of 21.1 gCOD/L/d, BC-AnMBR promoted COD removal efficiency from 90.1 % to 95.2 %, and maintained a high methane production rate of 4.8L CH4/L/d. The relative abundance of Methanosaeta declined from 53.9 % in conventional AnMBR to 21.0 % in BC-AnMBR, whereas that of Methanobrevibacter dramatically increased from 10.3 % to 70.9 %, respectively. Metabolic pathway analysis revealed that biochar not only strengthened hydrogenotrophic methanogenesis pathway, but also upregulated the genes encoding electron transfer carriers and riboflavin metabolism, suggesting the role of biochar facilitating direct interspecies electron transfer for syntrophic methanogenesis. The excellent energy yield performances under high OLR confirmed BC-AnMBR as an advanced system for high-strength swine wastewater treatment.