## 2 Common Substrates Used in Anaerobic Digestion
### Lignocellulosic Biomass  
Other relevant substrates treated by anaerobic digestion are crop wastes, energy crops and any type of high cellulosic-containing material, such as cellulose pulp mill effluent. The material conforming plant cell walls (cellulose, hemicellulose and lignin) is a complex structure with different levels of heterogeneity based on the biological function, age and type of tissue [43]. Cellulose is a component suitable for valorization through anaerobic digestion. However, substrates containing cellulose may also have a fraction of the lignin structure linked to the cellulosic material, making its access difficult to the anaerobic microflora. Therefore, this is the reason for denoting this biomass as lignocellulosic material. Cellulose is an insoluble polymer with a high molecular weight having a main structure formed of D-glucopyranose units linked by \(\beta\)-1,4-glycosidic bonds and cellobiose repetitive units. Two forms of cellulose are generally considered, a crystalline and an amorphous structure, which are easier to degrade by enzyme complexes [44]. For cellulose to be assimilated by microorganisms, the degradation should be initiated excellularly, either completely extracellularly with the aid of specific enzymes or in association with the outer cell envelop layer [45]. Anaerobic bacteria possess cellulosome, which is an extracellular multi-enzyme complex. This complex attaches to the cell envelope and the substrate, starting the degradation of cellulose [46]. Hemicellulose is the other main component of lignocellulosic biomass, with a lower molecular weight than cellulose. Hemicellulose forms together with lignin in a covering structure of cellulose fibers. Hemicellulose is a polysaccharide containing different types of sugars linked by \(\beta\)-1,4- and, less frequently, by \(\beta\) 1,3-glycosidic bonds [47]. Hardwoods and straw contain xylans as the predominant hemicellulose constituent, whereas galactomanans are the largest hemicellulose fraction in softwoods [48]. Cellulose and hemicellulose can be degraded by anaerobic microflora resulting in the accumulation of recalcitrant aliphatic molecules [49]. The degradation of cellulose was studied by [PERSON] et al. [50] and [PERSON] et al. [51]. Its degradation produces short-chain components, such as acetic and propionic acid, which are mainly metabolized by clostidial species. However, this conversion takes a long time (40-50 days) compared with that of carbohydrates under the same anaerobic conditions [48]. This is the main reason for proposing the application of pre-treatments when biogas production is intended [52]. Thus, accelerating the initial stages of this degradation process leads to a significant reduction in digester volume and therefore in plant installation costs. Spectroscopic techniques have been used as a tool for evaluating the degradation of different substrates under anaerobic digestion [53; 54]. Techniques such as nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy allow for the evaluating of the fate of the process and characteristics of digestates in order to study its adequacy as an organic amendment and act as a soil improve when analyzing agronomic benefits [55; 56]. Under anaerobic conditions, a preferential degradation of carbohydrates, cellulose and hemicellulose takes place, thus concentrating chemically recalcitrant aliphatic structures [57]. Aromatic structures originally present in the substrate may be partially degraded, causing also the accumulation of this material. The previous features translate into large digester volumes and therefore high capital costs. The accumulation of recalcitrant materials affects the final amount of digestate to be disposed of and becomes a problem if there is not enough land nearby. Higher methane yields have been reported for cellulose, but a faster conversion was found for hemicellulose under mesophilic conditions [58; 59].