## 3 Processes for Bioenergy Production
### Biochemical Methods
#### 3.2.4 Anaerobic Digestion
Considering the energy requirement of the process (input) and the energy produced (output), the output/input ratio reveals a high efficiency for crop residues such as potatoes, fodder beet, and maize (4.6-5.1), with low efficiency for rye and rapeseed oil (1.4 and 2.1, respectively) [5]. More recently, lignocellulosic biomasses, namely agro-residues and energy crops, have been gaining much attention as candidate feedstocks for the generation of bioenergy and biobased products [5; 43; 44]. Unlike conventional bioreenewable feedstocks (i.e., sugar- and starch-rich crops), lignocellulosic biomass does not directly compete with food or feed production [29]. The composition of lignocellulosic biomass, however, consists primarily of cellulose, hemicellulose, and lignin, and the combination of these components creates a highly resistant and recalcitrant biomass structure. Consequently, the hydrolysis of lignocellulosic often becomes the rate-limiting step during traditional AD [45]. In any case, lignocellulosic biomass has a great potential to serve as a feedstock for CH\({}_{4}\) production. The complexity of the biomass structure and the lack of appropriate digesters designed for efficient handling of the high-solids biomass are major challenges in the digestion of lignocellulosic biomass. However, harvesting of the biomass at an appropriate stage of maturity and co-digesting it with other feedstocks significantly enhances the CH\({}_{4}\) yield [29]. A common alternative biological pretreatment of feedstocks is treating them with rumen microorganisms (RM), the microflora found in ruminant stomachs, which create an in situ cellulolytic ecosystem that has a high potential to degrade the complex carbohydrate structures of lignocellulosic biomass [46]. Anaerobic degradation using RM is a pH-dependent process. The highest efficiency in terms of substrate digestion was found at a pH between 6.8 and 7.3 [47]. To evaluate a specific organic substrate for biogas production through AD, the biochemical methane potential (BMP) test is commonly used [48]. The general principle is the anaerobic incubation of the organic substrate in an inoculum containing a suitable medium (water and minerals) with a variety of anaerobic microorganisms, at neutral pH and with specific temperature conditions (mesophilic or thermophilic). The substrate acts as a source of carbon and energy for the microorganisms. During the incubation, the production and composition of the biogas and the degradation rate of the substrate are evaluated and the conversion rate determined [37]. The determination of the BMP allows the definition of the potential of a material to produce biogas by AD on an industrial scale. However, the efficiency of the industrial process must be proven. The factors that affect the biodegradability and, therefore, the production of biogas include: the characteristics of the input materials (composition of the organic matter, moisture, total solids and volatile solids contents, biodegradability, and even the crystallinity of the cellulose) and the inoculation and anaerobic conditions [37; 43; 44; 49; 50; 51]. In general, AD is classified into three important groups based on the operating total solids (TS) content: liquid (L-AD), with TS \(<\) 10%; semi-solid (S-AD), with 10%-20% TS; and solid-state (SS-AD), with TS \(>\) 20% [52; 53]. However, [PERSON] et al. [54] referred to liquid and solid-state AD with TS content \(<\) or \(>\)15%, respectively. SS-AD is ideal for high-solids organic feedstocks, which typically have a TS content between 10% and 50%, like energy crops, food wastes, livestock manure, agro-residues, etc. [49]. In recent years, there has been growing interest in SS-AD with agricultural and forest residues, and also with energy crops.