**1. Introduction**

At present, a large number of organic wastes (such as straw, animal manure, excess sludge, and other wastes) are produced in industry, agriculture, and aquaculture every year. How to deal with organic wastes sustainably has become a global challenge. Anaerobic digestion (AD) technology for the stable utilization of organic wastes (mineralizing volatile solids and reducing pathogens) has been applied all over the world. AD is a biological process in which organic matter is decomposed by an assortment of microbes under oxygen-free conditions and produces biogas (about 50–60% CH4 and 25–30% CO2) [1]. Physical approaches, mainly compression and liquefaction, have been commercially applied to upgrade biogas to bio-compressed natural gas (CNG) and liquefied biogas (LBG) [2]. Meanwhile, the nutrient-rich biogas slurry is commonly used as an organic fertilizer [3]. At present, AD with various wastes to produce biogas is the main method to solve the problems of energy shortage and environmental pollution [4]. During the development of AD engineering, the difficulty faced is how to improve the conversion efficiency of waste. Various wastes have unstable AD and low conversion efficiency due to structural composition and nutritional imbalance. The agricultural waste is closely bound together by the covalent bonds among its

main components cellulose, hemicellulose, and lignin, which greatly hinders the degradation of carbohydrates [5]. Especially for corn stover, it found that the index changed after the straw was dried, which was very unfavorable for biogas fermentation and other bioenergy conversion methods. For example, the lignin content is almost doubled, which seriously hinders the degradation of cellulose and hemicellulose in the process of biochemical transformation. AD will lead to problems such as slow start of fermentation, long fermentation time, and low gas production rate. Therefore, pretreatment becomes a necessary step for AD of lignocellulose. In the field of biomass transformation, the digestibility of cellulose is affected by many factors, such as hemicellulose content, lignin characteristics (content and distribution), matrix-specific surface area and porosity, cellulose crystallinity, and cell wall thickness [6]. The purpose of pretreatment is to remove or destroy the complex structure between cellulose-hemicellulose-lignin, improve the effective contact between cellulase and biomass, and then increase the rate of enzymatic hydrolysis [7, 8]. So far, scientific researchers in various countries have developed many promising pretreatment technologies, but most of them are chemical methods that require the addition of chemical reagents, such as acid, alkali, ammonia, organic solvents, or ionic liquids, and require certain high-temperature conditions and corresponding special reaction equipment [9–12]. The effect of different pretreatment methods is different, resulting in a great difference in the final gas production situation.

Biological pretreatment is to destroy the cell wall structure of biomass by the metabolic activity of microorganisms, which has the characteristics of mild use conditions, low cost, and has great potential in the field of biomass pretreatment [13]. Compared with chemical methods, biological methods do not need to consume a large amount of energy and recover chemical reagents, nor produce harmful inhibitors in the reaction system [14]. In parallel, microbes have evolved mechanisms, including cellulose-degrading enzymes, to degrade plant cell walls to access the plants' nutritious sugars. Fungi (aerobic pretreatment) use degradative enzymes called cellulases, whereas in bacteria (anaerobic pretreatment), multiple enzymes self-assemble into a complex called the cellulosome [15]. Both biological pretreatment methods have their advantages and disadvantages. Therefore, the main purpose of the chapter was to provide an overview of recent studies on solidstate microbial pretreatment for the production of biogas with wastes, focusing on the steps involved in the anaerobic ensilage formation with microbial ensilage pretreatment, additives, and quality evaluation of the silage.
