**Abstract**

Pretreatment technology has become the main bottleneck restricting the development of biogas. This chapter provides an overview of recent studies on solid-state microbial ensilage pretreatment for the production of biogas with wastes. The goal of microbial ensilage pretreatment is to maximize the production of lactic acid, thereby reducing the pH value and establishing an environment that is not suitable for the growth of harmful organisms. The use of various additives, especially lactic acid bacteria, is the main factor to ensure the success of anaerobic pretreatment. Sensory evaluation is carried out by observing the smell, structure, and color of silage to judge the quality of silage. The pH values, ammonia nitrogen, and organic acids (lactic-, acetic-, propionic-, and butyric acid) are used as reference values to determine the fermentation quality of silage. An overall comparison of the effectiveness of microbial ensilage with aerobic microbial pretreatment for biogas production is also discussed. Finally, the research on solid-state anaerobic microbial silage pretreatment in biogas conversion is summarized. The combined anaerobic digestion method with different pretreated materials will be the future development direction due to its advantages.

**Keywords:** biogas, biomass, ensiling, pretreatment, wet storage
