## 3 Pretreatment Methods in the Chain of Biogas Production
### Chemical Pretreatments
#### 3.2.1 Acid  
The main objective of dilute acid treatment is to achieve greater access to carbohydrate fractions of cellulose by hydrolyzing the hemicellulose [80]. Low acid concentration at 0.2% to 2.5% _v/v_ and temperatures between 130 \({}^{\circ}\)C and 210 \({}^{\circ}\)C are employed [81], a common practice is the use of autoclave at 121 \({}^{\circ}\)C for 1 h [82; 83]. On the contrary, low temperature (room temperature) is used while using high reagent acid concentration (above 30% _v/v_). High acid concentrations could cause corrosion problems and as a result high maintenance costs [84] and further, generate inhibitor compounds at high rate and therefore increase the purification costs. Many studies confirmed that the dilute acid treatment is the most promising technology for lignocellulosic pretreatment [85]. According to [PERSON] et al. [86], pretreatment of _Salvinia molesta_ pretreatment by 2%, 4%, and 6% _v/v_ of sulfuric acid (H\({}_{2}\)SO\({}_{4}\) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Substrates** & **Particle Size** & \begin{tabular}{c} **Methane Yield** \\ **(Untreated)** \\ \end{tabular} & \begin{tabular}{c} **Methane Yield** \\ **(After Treatment)** \\ \end{tabular} & **Reaction System** & **References** \\ \hline \multirow{3}{*}{Barley Straw} & 5 mm & 240 mL/g VS & 370 mL/g VS & Glass reactor 2 L & [73] \\  & 20 mm & 240 mL/g VS & 339 mL/g VS & Glass reactor 2 L & [73] \\  & 50 mm & 240 mL/g VS & 286 mL/g VS & Glass reactor 2 L & [73] \\ \hline \multirow{3}{*}{Crop feedstocks (winter rye, sorghum, forage rye, maize, triplicate)} & 6–33 mm & 278 mL/g ODM & 403 mL/g ODM & \begin{tabular}{c} Stirred tank \\ reactor 3 L \\ \end{tabular} & [70] \\ \hline \multirow{3}{*}{ Maize stalks} & 2 mm & 246 mL/g VS & 272 mL/g VS & Glass reactor 2 L & [73] \\  & 20 mm & 246 mL/g VS & 254 mL/g VS & Glass reactor 2 L & [73] \\ \hline \multirow{3}{*}{Meadow grass} & 2 mm & 297 mL/g VS & 376 mL/g VS & Bottle 0.5 L & [25] \\  & \textless{}200 mm & - & 347 mL/g VS & \begin{tabular}{c} Bottle 0.5 L \\ (increase 20\%) \\ \end{tabular} & [72] \\  & 0–200 mm & 303 mL/g VS & 372 mL/g VS & Bottle 0.5 L & [69] \\ \hline \multirow{3}{*}{Rice straw} & 0.3 mm & 58.1 mL/g VS & 62.7 mL/g VS & - & [68] \\  & 0.75 mm & 58.1 mL/g VS & 65.7 mL/g VS & - & [68] \\  & 50 mm & 197 mL/g VS & 203 mL/g VS & Glass reactor 2 L & [73] \\ \hline Switchgrass & 2-10 mm & 127.4 mL/g VS & 170.7 mL/g VS & Bottle 0.5 L & [75] \\ \hline \multirow{3}{*}{Water hyacinth} & 0.001 mm & - & \begin{tabular}{c} Increase 20\% \\ (from 50 to 70\%) \\ \end{tabular} & Digester 0.45 L & [76] \\  & 0.05 mm & - & \begin{tabular}{c} Increase 16\% \\ (from 50 to 66\%) \\ \end{tabular} & Digester 0.45 L & [76] \\  & 1.0 mm & - & \begin{tabular}{c} Increase 10\% \\ (from 50 to 60\%) \\ \end{tabular} & Digester 0.45 L & [76] \\  & 2.5 mm & - & \
\begin{tabular}{c} Increase 5\% \\ (from 50 to 55\%) \\ \end{tabular} & Digester 0.45 L & [76] \\ \hline \multirow{3}{*}{Wheat straw} & 0.3 mm & 167.8 mL/g VS & 245.6 mL/g VS & Reactor 2 L & [74] \\  & 1.2 mm & 167.8 mL/g VS & 264.7 mL/g VS & Reactor 2 L & [74] \\  & 0.3 mm & 67.1 mL/g VS & 70.3 mL/g VS & - & [68] \\  & 0.75 mm & 67.1 mL/g VS & 93.1 mL/g VS & - & [68] \\  & 0.088-0.759 mm & 183.4 mL/g VS & 252.8 mL/g VS & Lab Flask & [65] \\  & 2 mm & 182 mL/g VS & 334 mL/g VS & Glass reactor 2 L & [73] \\  & 50 mm & 182 mL/g VS & 285 mL/g VS & Glass reactor 2 L & [73] \\ \hline \multirow{3}{*}{Waste paper} & 60 min of &
\begin{tabular}{c} 132 mL/g VS \\ beating time \\ \end{tabular} & 132 mL/g VS & 215 mL/g VS & Flask 0.5 L & [63] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Effect of mechanical pretreatments on methane yield from lignocellulosic biomass. increased methane yield around 100% in comparison to untreated, however no yield differences were reported between the tested concentrations. Similarly, [PERSON] et al.