## 3 Pretreatment Methods in the Chain of Biogas Production
### Biological Pretreatments
[153] evaluated fungal pretreatment of two forest residues, viz., hazel and acacia branches and as well as two agricultural residues, viz., barley straw and sugarcane bagasse using _Ceriporiopsis subvermispora_ in order to improve the methane production. They reported that manganese peroxidase and lacase produced by the fungus increased the methane yield by 100% in comparison to untreated hazel. Although fungal pretreatment of lignocellulosic biomass are environmentally and economically friendly, it is a relatively time-consuming process and [PERSON] et al. [153] observed that a special bioreactor should be designed to create aerobic and aseptic conditions for fungal pretreatments. It is also reported that selection of substrate specific and as well as effective strain and culture conditions can reduce treatment time and carbohydrate losses [154]. [PERSON] et al. [155] reported that biogas production from lignocellulosic biomass can be accelerated after pretreatment with _Neocallimusti frontalis_, an anaerobic fungus isolated from rumen fluid of a cow and of a chamois. The byproducts formed after biological pretreatments normally do not inhibit subsequent hydrolysis, since the pretreatment is carried out at moderate reaction conditions, and also has other environmental benefits like low inputs (energy, chemicals) and outputs (inhibitors and wastes) [156]. However, as mentioned earlier, its process time is long compared to the others [148; 157]. As a result, several studies select different microbial populations for hydrolysis of their specific substrates [158]. [PERSON] and [PERSON] [39] selected strains producing lignin peroxididases and lacases and decreased 48.2% of lignin from wheat straw within 7 days of batch incubation; and observed 407.1% increase of the methane yield in comparison with the untreated wheat straw. Microbial pretreatment, when compared to enzymatic pretreatment, demonstrates much better outcome in anaerobic digestion process due to their higher functional diversity and tolerance to environmental factors--i.e., temperature and pH. [PERSON] et al. [38] isolated bacterial strains from soil (_Bordetla muralis_ VKVVG5), the gut of silverfish (_Citrobacter werkmanii_ VKVVG4), and millipede (_Paenibacillus_ sp. VKVVG1); and employed these bacterial strains to accelerate hydrolysis of water hyacinth. _Citrobacter werkmanii_ VKVVG4 pretreatment of water hyacinth at a density of 109 CFU/mL for 4 days increased the solubilization by 33.3%. On the other hand, several others report the use of anaerobic lignocellulosicdegrading consortia. [PERSON] et al. [159] reported that anaerobic consortium TC-5 resulted in 45.7% degradation of wheat straw and showed an increase of 22.2% and 36.6% of methane yield under mesophilic and thermophilic conditions, respectively. Rumen fluid was found to be effective in increasing the yield from rapeseed stems and leaves by 47% within 24 h of treatment time [160]. [PERSON] et al. [19] used a thermophilic microbial consortium MC1 to pretreat the cotton stalk. The results indicated that the concentrations of soluble chemical oxygen demand and volatile organic products increased significantly in the early stages of the pretreatment, reaching the maximum methane yield of 128 mL/g VS after 6 days of MC1 pretreatment. However, with 122 mL/g VS, the methane yield was not significantly different. [PERSON] et al. [33] observed a 72.6% increased higher methane yield over control from saw dust treated with a microbial consortium LCDC isolated from rotten sawdust. They also reported a significant reduction of lignocellulosic compounds within 10 days of pretreatment. Enzymatic pretreatment by using oxidative and hydrolytic enzymes produced by bacteria and fungi such as exo-, endo- glucanases, cellobiase, xylanase, pectinase; ligninolytic enzymes such as lactose, manganese, and versatile peroxididases; as well as \(\alpha\)-amylases and protease is advantageous in comparison to bacterial/fungal pretreatments [161].