Effects of hydrochar and biogas slurry reflux on methane production by mixed anaerobic digestion of cow manure and corn straw

This study aimed to enhance methane production from mixed anaerobic digestion of cow manure and corn straw by adding hydrochar and biogas slurry reflux. The hydrochar characterization revealed that it can provide attachment for microbial growth, and abundant surface functional groups (such as C–O, C[dbnd]O, C–OH, and C–N) for adsorption. Direct interspecies electron transfer (DIET) mediated by surface oxygen-containing functional groups on hydrochar increased the methane yield. The experimental group added with hydrochar and biogas slurry reflux had the highest methane and biogas production (34.40% and 36.98% higher than the control group, respectively). Results demonstrate hydrochar and biogas slurry reflux can improve microorganism species richness in anaerobic digestion systems, in which hydrochar can also improve microorganism species uniformity. Distance-based redundancy analysis showed that the VFAs, and pH had the greatest effects on the composition of the microbial community. The dominant microorganism at the phylum level in AD system were Bacteroidetes, Firmicutes, and Proteobacteria. The addition of hydrochar and biogas slurry reflux can significantly increase the species abundance of Methanobacterium. These results indicate that the addition of hydrochar and biogas slurry reflux can improve the corresponding microbial abundance, in which hydrochar can enhance the redox characteristics and DIET between microorganism, biogas slurry reflux can also increase nutrient content of anaerobic digestion system, and collectively promote the methane yield.