The Functional Role of Coffee Husk–Derived Biochar Revealed by Repeated Use in Anaerobic Digestion of Water Hyacinth Juice

During anaerobic digestion, the addition of biochar is expected to enhance biogas production by adsorbing inhibitory substances, increasing the surface area available for microbial growth and the density of surface functional groups. Moreover, it promotes direct interspecies electron transfer (DIET) attributed to its high electrical conductivity. With repeated use, biochar develops a microbial biofilm on its surface, which reduces the effects of the surface area and functional groups while maintaining high electrical conductivity. Therefore, increasing the number of repeated uses allows for evaluating the effects of electrical conductivity separately from the other two factors. In this study, the pressed juice from water hyacinth was utilized as wastewater, and anaerobic digestion was performed through repeated-batch operations with the addition of biochar. In the first cycle, biochar addition improved the cumulative methane yield, with a maximum increase of 21.5%. However, regardless of biochar addition, an increase in the number of repeated cycles resulted in stabilization of the methane yield at approximately 170-mL/g-VS. This observation clearly demonstrated the minimal effect of electrical conductivity on biogas production. The subsequent evaluation of microbial community structure exhibited high similarity in microbial composition, except for Anaerolineae bacteria, irrespective of biochar addition. Although two distinct Anaerolineae groups emerged depending on the presence or absence of biochar, both possessed similar gene clusters related to acetate supply for acetoclastic methanogens. These findings are in line with the observation that biochar addition did not selectively enrich specific microorganisms involved in DIET, and its effect on electrical conductivity was negligible.