## 1 Introduction
### Lignocellulosic Substances  
Presence of lignocellulosic substances in substrates is one of the main reasons for the low conversion efficiency of particle-rich substrates. Lignocellulosic substances consist of three biopolymers called cellulose, hemicellulose and lignin that are present in the cell walls of plant matter. The relative composition of the polymers differs from plant to plant. Hardwoods and softwoods contain a relatively high amount of cellulose whereas straws and grass contain higher hemicellulose content [2]. Glucose molecules are linked through beta-(1,4) glycosidic bonds to form a disaccharide that is polymerized into cellulose (Figure 1). Cellulose is homogenous because it is formed from a single monosaccharide. Hemicellulose, on the other hand, is formed from several monosaccharides including xylose and glucose. This results in a heterogeneous polymer that is more amorphous and has a more hydrolysable structure than that of cellulose. Lignin is made up of phenol-based monomers that are cross-linked to form a large and complex chemical structure that is chemically and biologically resistant to degradation. Particle-rich substrates such as manure slurry contain a significant amount of lignocellulosic substances [3]. The source of such lignocellulosic substances is plant matter that is fed to the animals and used as bedding material for the animals. Readily biodegradable material in the animal feed is absorbed in the intestine and the leftover manure is composed of a substantial amount of lignocellulosic matter that is difficult to biodegrade. Up to 40-50% of the total solids in manure are lignocellulosic substances [3]. Lignocellulosic substances are difficult to biodegrade because their composite structure limits the accessibility of substrates by hydrolyzing enzymes [4]. A total of 20-300 monomers of cellulose are bound together by hydrogen and Van der Waals forces to make packed cellulosic microfibrils. The microfibrils are mostly in crystalline form and their outer layer is covered with hemicellulose \begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Raw Feed (RF)** & **Centrifuged Feed (RF)** \\ \hline TS (g/L) & 21.5 & 12.2 \\ VS (g/L) & 13.8 & 5.9 \\ TSS (g/L) & 14.2 & 2.5 \\ VSS (g/L) & 12.0 & 2.3 \\ TDS (g/L) & 7.3 & 9.7 \\ VDS (g/L) & 1.7 & 3.6 \\ COD\({}_{\text{total}}\) (g/L) & 33.2 & 19.7 \\ COD\({}_{\text{soluble}}\) (g/L) & 16.6 & 11.4 \\ NH\({}_{4}\)\({}^{*}\)(g/L) & 1.8 & 1.3 \\ pH & 7.0 & 7.0 \\ \hline \hline \end{tabular}  
* TS: Total Solids; VS: Volatile Solids; TSS: Total Suspended Solids; VSS: Volatile Suspended Solids; TDS: Total Dissolved Solids; VDS: Volatile Dissolved Solids; COD: Chemical Oxygen Demand.  
\end{table} 
Table 1: Feed sample characteristics. chains. Lignin polymer binds the cellulosic microfibrils and hemicellulose together and acts like a \"glue\" to form a rigid macromolecular structure that is inaccessible for enzymatic attack. Due to this reinforced concrete-like structure, disintegration and hydrolysis are difficult. By some estimates, up to 80% of lignocellulosic substances remain undegraded in biogas reactors [5]. The composition of the lignocellulosic content of manure differs from animal to animal as well as the age of the animal. For swine manure, typical cellulose, hemicellulose and lignin contents are 30-50%, 20-30% and 10-20%, respectively (Figure 2). Despite favorable qualities such as abundance, easy availability and being renewable, lignocellulosic substances have not been efficiently used for biogas production due to their strong resistance to biodegradation. Various physical, chemical and biological methods were tried to unlock the biogas potential of lignocellulosic substances with various degrees of success. One of these methods involves the addition of enzymes to facilitate the breakdown of lignocellulosic components into their monosaccharides. Bacteria naturally secrete enzymes such as cellulase and hemicellulose that facilitate hydrolysis of lignocellulosic substances.