Effect of Substrate to Inoculum Ratio on Biogas Production and Microbial Community During Hemi-Solid-State Batch Anaerobic Co-digestion of Rape Straw and Dairy Manure

The substrate to inoculum (S/I) ratio is crucial for the rapid start-up of solid-state anaerobic digestion (SS-AD) systems. In this study, the performance of methane production and microbial community structure were evaluated during co-digestion of rape straw (RS) and dairy manure (DM) at different S/I ratios (2:3, 1:1, 2:1, 3:1, and 4:1) in batch hemi-solid-state anaerobic digestion (HSS-AD) tests. The highest methane yield of 209.1 mL/g VSadded and highest volumetric methane production of 0.4 L/(L center dot d) were achieved at S/I ratios of 2:3 and 2:1, respectively. Lower S/I ratios (1:2, 1:1, and 2:1) steadily produced biogas throughout the AD period, while higher S/I ratios (3:1 and 4:1) failed to produce biogas during the initial stage of AD because of excess accumulation of volatile fatty acids and low pH. The predominant bacteria and archaea in stable digesters were Firmicutes and acetoclastic Methanosaeta, respectively, while Bacteroidetes predominated and the relative abundance of hydrogenotrophic Methanobacterium increased significantly in acidic digesters. Amounts of bacteria and archaea were inhibited in acidic digesters. Our results provide useful information for enhancing efficient methane production and advancing the understanding of the microbiome in HSS-AD of RS and DM at different S/I ratios.