## 3 Anaerobic Digestion of Lignocellulosic Biomass  
The AD process consists in the transformation of the organic matter contained in the waste into a gaseous effluent (biogas) together with a semisolid stabilized effluent named 'digestate'. The process is developed by a very complex microbial population operating in absence of molecular oxygen in the medium. Different stages can be distinguished in the overall AD process: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The microorganisms responsible of those stages should synchronize \begin{table}
\begin{tabular}{c c c c} \hline
**Lignocellulosic Biomass** & **Cellulose** & **Hemicellulose** & **Lignin** \\
**(per Group)** & **(\%)** & **(\%)** & **(\%)** \\ \hline Cellulose and paper wastes & 76–99 & 0–13 & 0–11 \\ \hline Energy crops and agriculture wastes & 16–48 & 19–36 & 8–25 \\ \hline Animal wastes & 5–28 & 12–21 & 2–14 \\ \hline Wood & 41–54 & 11–36 & 16–28 \\ \hline \end{tabular} Source of data from [19; 23; 24; 26; 28].  
\end{table} 
Table 1: Average composition of the main lignocellulosic biomass (LB) categories (expressed as percentage in dry basis). \begin{table}
\begin{tabular}{c c c c c} \hline
**Lignocellulosic Biomass** & **C** & **H** & **O** & **N** & **S** \\ \hline Sewage sludge & 26.8–32.2 & 3.8–5.1 & 18.2–20.1 & 3.7–5.4 & 1.6–2.0 \\ \hline Municipal solid wastes & 35.4–41.7 & 4.7–5.9 & 15.8–20.9 & 1.4–2.2 & 0.2–0.8 \\ \hline Animal manure & 28.7–44.2 & 3.3–6.1 & 2.4–43.2 & 0.8–3.8 & 1.9–3.1 \\ \hline Agricultural wastes & 38.3–54.1 & 5.4–6.7 & 1.0–45.1 & 0.2–5.5 & 0.6–2.9 \\ \hline Crop residues & 33.8–49.1 & 4.6–6.3 & 32.8–51.4 & 0.2–0.8 & 0.1–0.3 \\ \hline \end{tabular} Source of data from [30; 31; 32; 33; 34].  
\end{table} 
Table 2: Average elemental analysis of some organic wastes classified as LB (expressed as percentage in dry basis). their metabolic rates and work synergistically to break down the complex structures (i.e., lignocellulosic) in the organic matter and hence, to obtain a stable process with a high generation of biogas [35]. Regarding the availability of feedstock for the AD process, it should be taken into account that the amount of lignocellulosic agro-industrial wastes and byproducts produced is very high and, consequently, the exploitation of LB to obtain bioenergy or high-value bioproducts constitutes a priority. However, the complexity of the LB structure greatly hinders its AD due to the high level of crystallinity of cellulose, as well as the cross-linking of carbohydrates and lignin [1; 36]. Thus, lignocellulosic biomass is constituted mainly by cellulose, hemicellulose, and lignin and the above-mentioned interactions among these fractions lead to a very stable and recalcitrant structure. This complex structure of lignocellulosic material makes it resistant to enzymatic attack [37]. Solid material and macromolecules constituting organic matter should be hydrolyzed and solubilized in the medium in order to be used by the microorganisms. Therefore, this is a key step for the appropriate development of the overall AD process. Moreover, for lignocellulosic solid wastes, hydrolysis is normally the rate-limiting step of the global process [38]. This stage is performed by extracellular enzymes excreted by the acidogenic microorganisms. However, it should be taken into account that lignin is the most difficult biodegradable component in lignocellulosic materials and its cross-linking with the carbohydrates reduces the surface area available for enzyme attack [39]. In addition, [PERSON] et al. [40] reported that hemicellulose can be preferably attacked and, hence, broken down before than cellulose or lignin by the anaerobic microbiota. The structure of LB is weakened by the hemicellulose removal and, thus, the enzymatic solubilization of the cellulosic fraction is favored [41].