**5. Conclusions**

Solid-state anaerobic microbial ensilage pretreatment is one of the effective means to improve the biogas yield of raw materials. Other pretreatment methods only change the composition and structure. Biological pretreatment can not only change the composition and structure but also have the ability to ferment and metabolize decomposed substrates into other small molecules, and change the composition and structure. It takes a certain time for fermentation to metabolize small molecule nutrients; so, exploring the optimal pretreatment time can improve the efficiency. Because of the particularity of biological pretreatment, it is also of great significance to explore its impact on the structure, activity, and functional expression of microbial community in the subsequent AD. Understanding the impact of biological pretreatment from multiple angles can better optimize the biological pretreatment conditions, thereby improving the efficiency and enhancing the gas production efficiency for biogas engineering.

The high efficiency of physical pretreatment is accompanied by high energy consumption, and chemical pretreatment is also facing environmental pressure. Biological pretreatment also needs to constantly improve its pretreatment efficiency. Therefore, more and more studies began to focus on mixed pretreatment, that is, to absorb the advantages of different pretreatments to make up for the shortcomings, to achieve a higher hydrolysis rate while reducing energy consumption and reaction time, and to reduce pretreatment costs and environmental hazards. For mixing pretreatment with different materials, screening suitable microbial communities will be a future study direction to ensure a controllable and stable treatment process.
