## 3 Pretreatment Methods in the Chain of Biogas Production
### Chemical Pretreatments
#### 3.2.2 Alkaline
Organic solvents--such as methanol, ethanol, acetone, acetic acid, peracetic acid, and so on--are used with or without the addition of a catalyst reagent [100]. The catalysts used include mineral acids (hydrochloric, sulfuric, and phosphoric acids) and organic acids (oxalic, acetylsalicylic, and salicylic acids) [101]. Although catalyst addition aids in higher pretreatment efficiency, use of a catalyst could have a negative impact on the environment [102], i.e., chemical catalyst as acid causes acid-catalyzed degradation of the monosaccharides into furfural and 5-hydroxymethyl furfural followed by condensation reactions between lignin and the reactive aldehydes [103]. Acid catalysts are added in order to increase the rate of lignin removal and decrease pretreatment temperature, because acid catalysts cleave acid-labile bonds (\(\alpha\)-aryl ether and arylglycerol-\(\beta\)-aryl ether bonds), which help stabilization of lignin fragments [101]. However, the use of chemical catalysts involves issues, such as equipment corrosion and the need of processing downstream effluents, resulting in high water consumption [103]. Formic acid is a promising catalyst as it is believed to have fewer corrosive effects than stronger mineral acids and further to avoid production of inhibitors that are normally formed when acetic acid is employed [104]. The organosolv process causes hydrolysis of the internal bonds in lignin and between lignin and hemicellulose. The organic solvents also cause hydrolysis of the glycosidic bonds in hemicelluloses and to a smaller extent in cellulose [103]. The preferred conditions of organosolv process is generally in the following ranges: a cooking temperature of 180-195 \({}^{\circ}\)C, a cooking time of 30-90 min and liquor to solid ratio ranging from 4:1 to 10:1 [105]. As mentioned previously, organic acids have been suggested as an alternative to inorganic ones, to avoid corrosion and decrease the energy demand during acid recovery. [PERSON] et al. [106] pretreated the rice straw with organic acids--such as acetic acid, citric acid, and oxal acid--and make a comparison with hydrochloric acid as inorganic acid. It was found that pretreatment with oxal acid reagent yielded 2.68 and 1.25 times higher reducing sugars than untreated and HCI-treated rice straw, respectively. Further, the accumulated methane yields obtained from pretreatments of organic acids were 2.47-2.53 times higher than the inorganic acid one. A critical issue with organosolv pretreatment is the risk of operating at high pressure and the solvents volatility due to their low boiling point of the organic solvents. A system of solvent recovery is necessary to reduce the cost of operation and to avoid their residual inhibitory effect on microorganisms. [PERSON] et al. [107] recovered more than 98% of organic solvent N-methylmorpholine-N-oxide (NMMO or NMO), which was used for pretreatment of softwood spruce, rice straw, and tritical straw at 130 \({}^{\circ}\)C for 1-15 h. Further, they reported an increase in methane production between 400% and 1200%. [PERSON] et al. [108] studied the use of recycled NMMO for pretreatment of barley straw and forest residues and observed similar performance as the fresh NMMO on barley straw. However, pretreatment of forest residues with recycled NMMO resulted in 55% reduction of methane yield. Among solvents, low boiling point alcohols, especially ethanol, have attracted more attention because of their high effectiveness on lignocellulosic biomass, as well as the simplicity of their recovery by distillation [109]. Other advantages are lower cost and lower toxicity to humans compared to solvents as methanol [104,110].