Mechanisms of nano magnetite-loaded biochar enhancing anaerobic digestion of food waste and effects of different loading methods and dosages on process performance and microbial characteristics

This study investigated effects of supplementing nano magnetite-loaded biochar (MBC) prepared by chemical coprecipitation (McpBC) and ball milling (MbmBC) at different dosages on anaerobic digestion (AD) of food waste (FW) and elucidated their strengthening mechanisms. McpBC and MbmBC supplementation greatly enhanced AD of FW, as indicated by higher methane yield and content, shorter lag phase and reaction period, more stable pH and faster volatile fatty acids (VFAs) degradation. The strengthening effect of MbmBC (methane yield increased by 25.71-38.74%) was greater than that of McpBC (methane yield increased by 22.54-29.69%) and 2.0 g/L was optimal dosage. McpBC and MbmBC with high electrical conductivity induced the establishment of efficient syntrophic community of Syntrophomonadaceae with Methanosarcina via mediating direct interspecies electron transfer therefore accelerated VFAs conversion and boosted methanogenesis. The dominant growth of Methanosarcina enhanced methanogenic pathway diversity and system robustness. Hydrogenotrophic Methanospirillum underwent functional degeneration, but another hydrogenotrophic Methanoculleus enriched, keeping CO2-reduction methanogenic pathway unobstructed. The alkaline pH, rough surface, polyporous structures, abundant functional groups of MbmBC and McpBC were also effective factors. The higher specific surface area, pore size, crystallinity and electrical conductivity of MbmBC than those of McpBC might be the reason for the greater enhancement of MbmBC on AD.