Role of biochar in the granulation of anaerobic sludge and improvement of electron transfer characteristics

•Effect of biochar on sludge granulation and microbial community was investigated.•Biochar improves the integrity coefficient and conductivity of granular sludge.•Biochar favors the enrichment of functional microbes and DIET-based syntrophy. For improving the formation of anaerobic sludge granulation and its electron transfer characteristics, two up-flow anaerobic sludge blanket (UASB) reactors (R1 with 4 g/L biochar; R2 without biochar) were built in this study. With the addition of biochar, the lag time of methanogenesis was shortened by 28.6%, and the strengthening factor of COD removal rate reached 1.6. At the same time, the conductivity of granular sludge in R1 (23.29 ± 0.99 μS/cm) was 2-fold of that in R2, and the integrity coefficient and hydrophobicity of granular sludge were improved significantly. According to the results of microbial community succession and electron transfer characteristics, the added biochar not only favors the anaerobic sludge granulation working as an inert core, but also facilitates the selective enrichment of potential direct interspecies electron transfer (DIET) partners such as Methanothrix and Geobacter spp. for enhancing the DIET process.