## 3 Pretreatment Methods in the Chain of Biogas Production
### Biological Pretreatments  
Biological pretreatment offers an alternative choice to replace chemical pretreatment; however, its very slow reaction rate is unattractive from commercial point of view [148]. Different microorganisms \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Substrates** & **Pretreatment Conditions** & **Methane Yield (Untreated)** & **Methane Yield (After Treatment)** & **Reaction System** & **References** \\ \hline Cocoa pods residues & Hydrothermal 150 \({}^{\circ}\)C & 196.3 mL/g VS & 289.3 mL/g VS & Bottle 0.5 L & [144] \\ \hline Harvested hay & Steam explosion 175 \({}^{\circ}\)C; 10 min & 243 mL/g VS & 281 mL/g VS & Digester 0.25 L & [134] \\ \hline  & Steam explosion 0.5 MPa; 153 \({}^{\circ}\)C & 181.2 mL/g VS & 192.3 mL/g VS & Reactor 0.5 L & [136] \\  & 5 min & 181.2 mL/g VS & 217.9 mL/g VS & Reactor 0.5 L & [136] \\ Miscanthus lutarioriparrius & Steam explosion 1.5 MPa; 198 \({}^{\circ}\)C & 181.2 mL/g VS & 274.1 mL/g VS & Reactor 0.5 L & [136] \\  & 3 min & 181.2 mL/g VS & 274.1 mL/g VS & Reactor 0.5 L & [136] \\  & Steam explosion 1.5 MPa; 198 \({}^{\circ}\)C & 181.2 mL/g VS & 272.0 mL/g VS & Reactor 0.5 L & [136] \\  & Steam explosion 1.5 MPa; 198 \({}^{\circ}\)C & 181.2 mL/g VS & 272.2 mL/g VS & Reactor 0.5 L & [136] \\  & 10 min & & & & \\ \hline Reed biomass & Steam explosion 160 \({}^{\circ}\)C; 5 min & 188 mL/g VS & 226 mL/g VS & Digester 0.25 L & [147] \\  & Steam explosion 200 \({}^{\circ}\)C; 15 min & 188 mL/g VS & 355 mL/g VS & Digester 0.25 L & [147] \\ \hline Rice straw & Hydrothermal 100 \({}^{\circ}\)C; 10 min & 92 mL/g VS & 280 mL/g VS & Glass Bottle 1 L & [143] \\ \hline Safflower straw & Hydrothermal 120 \({}^{\circ}\)C; 60 min & 96.5 mL/g VS & 191.4 mL/g VS & Glass bottle 0.118 L & [142] \\ \hline Sugarcane & Ammonia & 105.6 mL/g VS & 299.3 mL/g VS & Bioreactor 0.118 L & [146] \\ \hline  & Hydrothermal 120 \({}^{\circ}\)C; 60 min & 388.9 mL/g VS & 483.3 mL/g VS & Serum bottle 0.3 L & [141] \\  & Hydrothermal 140 \({}^{\circ}\)C; 60 min & 388.9 mL/g VS & 511.2 mL/g VS & Serum bottle 0.3 L & [141] \\  & Hydrothermal 160 \({}^{\circ}\)C; 60 min & 388.9 mL/g VS & 552.7 mL/g VS & Serum bottle 0.3 L & [141] \\ Wheat straw & Hydrothermal 180 \({}^{\circ}\)C; 60 min & 388.9 mL/g VS & 611.7 mL/g VS & Serum bottle 0.3 L & [141] \\  & Ammonia & 0.7\%; 55 \({}^{\circ}\)C & 407.8 mL/g VS & 491.7 mL/g VS & Reactor 0.25 L & [145] \\  & Ammonia & 407.8 mL/g VS & 538.1 mL/g VS & Reactor 0.25 L & [145] \\  & 0.7\%; 105 \({}^{\circ}\)C & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Effect of different physicochemical pretreatments on methane yield from different lignocellulosic biomass. are used for lignocellulosic pretreatment, such as; white, brown, and soft rot fungi. Although, brown rot fungi participates in lignin decomposition [149], white rot fungi are well known and most effective microorganism for delignification process [150]. In general, most of the fungi degrading lignocellulosic secrete accessory enzymes like cellobiose dehydrogenase, aryl alcohol oxidase, glyoxyl oxidase, copper oxidase, and hydrolytic enzymes, which cause the simultaneous or selective degradation of cellulose and hemicellulose along with lignin. These enzymes provide free radicals and intermediates that help in lignin and polysaccharide degradation [151]. [PERSON] et al. [152] evaluated the impact of biological pretreatment with different enzymes such as lacase, manganese peroxidase, and versatile peroxidase, at different incubation times--0, 6, and 24 h. Pretreatments did not yield high concentrations of phenolic compounds, inhibitors of methane production. The lacase enzyme showed 25% increase in biomethane production after 24 h of incubation, meanwhile, pretreatment with peroxidase enzymes increased biomethane production by 17%, but aided in reducing the incubation time from 24 to 6 h. [PERSON] et al.