Two-stage solid-state anaerobic digestion of highland barley straw with liquid fraction of digestate and calcium oxide pretreatment: Performance and microbial community analysis

Solid-stage anaerobic digestion (AD) was commonly limited due to the long lag phase and slow startup, resulting in poor methane production efficiency and AD performance. To solve these problems, a combined pretreatment method of chemical and biological using solid alkali (CaO) and the liquid fraction of digestate (LFD) was applied to HBS. AD performances, microbial community, and metabolic mechanisms were investigated in two-stage solid-state AD system under 20%, 25%, and 30% total solid (TS) content. The results showed that the LFD-CaO pretreatment improved the acetic acid concentration in acidogenic stage, which provided more substrate for methane production. The LFD-CaO pretreatment mitigated system acidification under high TS conditions, leading to a more stable and higher pH in the anaerobic digestion system. In methanogenic stage, the cumulative methane yield increased by 60.87%, 42.87%, and 17.23% for LFD-8%CaO pretreatment groups under the 20%, 25%, and 30% TS content conditions, compared with the control group (CG), respectively. Microbial community analysis revealed that the abundance of Ruminofilibacter in the LFD-CaO pretreatment group (7.61%-41.88%) was higher than that in CG (4.05%-4.81%), indicating an enrichment of cellulolytic and hemicellulolytic microorganisms. Besides, the mechanism analysis demonstrated that the LFD-CaO pretreatment could alter the functional pathways and key enzymes related to the acidification and methanogenesis process, ultimately improving the performance of two-phase AD and providing technical support for the straw pretreatment.