Adsorption or direct interspecies electron transfer? A comprehensive investigation of the role of biochar in anaerobic digestion of hydrothermal liquefaction aqueous phase

[Display omitted] •Adsorption capacity and pore volume of biochar are the dominant parameters.•Adding biochar enhanced digestion by affecting microbial enrichment and metabolism.•Biochar addition may suppress methane production and organic conversion.•Mediated interspecies electron transfer was the primary mechanism rather than DIET.•A DIET prediction model was developed with an accuracy of 87%. Hydrothermal liquefaction (HTL) is promising for the conversion of biowaste into biofuels, but the energy recovery from the HTL aqueous phase (HTL-AP) by anaerobic digestion is limited due to its degradability resistance. Adding biochar was reported to facilitate digestion, but its role has not been explicitly determined. Direct interspecies electron transfer (DIET) was reported to participate and dominate the digestion process; however, the adsorption and detoxification effects of biochar cannot be ignored. This study is conducted to confirm the exact role of biochar and its primary mechanism on the digestion process. Results showed that the total pore volume and adsorption capacity of biochar played the most influential role. In comparison, DIET was very likely not dominant due to the limited electrical conductivity and electron-donating/accepting capacities of biochar. The microbial analysis further indicated that mediated interspecies electron transfer remained the primary mechanism rather than DIET. The addition of facilitative biochar promoted the enrichment of Thermovirga and Methanosaeta, whereas a suppressive biochar addition shifted the dominant microbes to Asaccharospora, Clostridium, and Methanobacterium. Furthermore, a Random Forest prediction model was developed, with an accuracy of 87%, to forecast whether DIET dominantly influenced methane generation with biochar addition. This study proved that the effect of biochar on anaerobic digestion of HTL-AP relied mainly on adsorption, mediated interspecies electron transfer was more effectively enhanced rather than DIET, and a modeling approach was developed to verify the presence of DIET.