Microbial succession analysis reveals the significance of restoring functional microorganisms during rescue of failed anaerobic digesters by bioaugmentation of nano-biochar-amended digestate

[Display omitted] •Nano-biochar application was studied for anaerobic digestion of orange peel waste.•Sewage sludge derived biochar had the highest performance among different biochars.•Nano-biochar facilitated electron transfer, hydrogen transfer and nutrition supply.•Successfully rescued failed digester relied on restore of microbial communities.•Restored key microbes included phylum Hydrogenispora and genus Methanosarcina. Nano-biochar application was investigated for anaerobic digestion of orange peel waste. The application for methane production focused on the optimization of biochar feedstock, rescue of failed digesters, and microbial succession analysis. It showed that sewage sludge (SS) derived biochar had the highest performance enhancement among the different feedstocks, which could be ascribed to the improvement of electron transfer, interspecies hydrogen transfer, and supply of trace elements. Subsequently, nano SS biochar-amended digestate was evaluated for rescuing failed digesters, and the experimental results indicated its positive roles through gradual bioaugmentation operation. The dynamic analysis of microbial succession indicated the successful application was through the mechanism of restoring partially the functional microbial communities. The major reconstruction of functional microorganisms included bacteria phyla Hydrogenispora (24.5%) and Defluviitoga (18.8%) as well as methanogenic genera of Methanosarcina (41.5%) and Methanobacterium (27.3%). These findings would contribute to rescuing failed anaerobic digesters by bioaugmentation with biochar-amended digestate.