Combined effects of liquid digestate recirculation and biochar on methane yield, enzyme activity, and microbial community during semi-continuous anaerobic digestion

The combined effects of liquid digestate recirculation (LDR) and biochar on methanogenesis and microbial communities were studied in semi-continuous anaerobic reactors fed with wheat straw and swine manure. The tolerated organic loading rate (OLR) was expanded from 5 g- volatile solids (VS)∙L^-1∙d^-1 in the control to higher than 6 g-VS∙L^-1∙d^-1 in the LDR. At the OLR of 5.0 g-VS∙L^-1∙d^-1, average special methane yield in LDR with biochar was 0.234 L∙g-VS^-1, which was 5.4 % higher than that of the LDR alone. Moreover, enzyme activity and microbial community analysis indicated that LDR with biochar enhanced the processes of hydrolysis and methanogenesis, and balanced the pathway between hydrogenotrophic and acetoclastic methanogenesis. The co-application of LDR and biochar synergistically enhanced the degradation pathways of substrates and the loading shock resistance of anaerobic digestion system. This study could offer strategies for developing sustainable applications of full and continuous LDR in industrial biogas projects.