# A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions
## 3 Pretreatment of Feedback to Enhance Biogas Production
### Chemical Pretreatment  
This method depends mainly on breakdown or destruction of organic compounds using acids, alkalis, and oxidants, and its efficiency is highly affected by the substrate characteristics [61]. Importantly, this type of pretreatment is less recommended for easily biodegradable substrates, as it can be associated with accumulation of VFAs, which ultimately leads to inhibition of the methanogenesis step [57].  
During acid pretreatment, hemicellulose is hydrolyzed into monosaccharides, while the lignin condensates and precipitates [55; 61; 62]. The process is effective for substrates with high lignocellulosic content as it breaks down the lignin, and also because the hydrolytic microbes are capable of acclimating to acidic conditions [61]. Strong acidic pretreatment may result in the production of inhibitory by-products, such as furfural and hydroxymethylfurfural (HMF) [62; 63; 64]. The use of diluted acids combined with thermal methods is normally recommended to avoid the formation of such inhibitory products [57]. In this regard, and with mesophilic anaerobic digestion of wheat plant, diluted sulfuric acid pretreatment at 121 \({}^{\circ}\)C confirmed that methane yield could be increased by 15.5% higher than that of the untreated wheat plant after pretreatment for 120 min [65]. Also, in semi-continuous digestion experiments conducted for 12 days at 35 \({}^{\circ}\)C using HCl at pH 2 for subsequent digestion of waste-activated sludge (WAS), an increase of 14.3% in methane yield was achieved compared to untreated substrates [66].  
The aim of alkali pretreatment is to enhance the enlargement of the solid particles, thus increasing the surface area, meaning the substrate becomes more accessible for microbial activities. Solvation and saponification are the main reactions during this process [55; 57; 62; 63; 67]. This process is considered more effective for materials with low lignin contents, such as agricultural residues, herbaceous crops, and hardwoods, than on softwood, which has high lignin contents [68]. Basic compounds, such as calcium oxide, ammonia, and sodium hydroxide, are used to solubilize lignin in this process [62]. The effect of alkaline (NaOH) pretreatment on ensiled sorghum forage was investigated in semi-continuous digesters. The researchers observed that pretreatment with 10 g NaOH/100 g TS increased the methane yield by 25% compared to untreated sorghum, without experiencing any inhibition of the process [69]. Additionally, the effect of trace element (TE) addition and NaOH pretreatment on the AD of rice straw was investigated in batch tests. Co, Ni, and Se were added to the raw rice straw at different dosages. The NaOH pretreatment was applied to the rice straw, both alone and in combination with the addition of TEs, in order to evaluate potential synergistic effects of the pretreatment and the TE supplementation on the biogas production yields. The obtained results showed that the alkaline pretreatment was more effective than the TE addition in increasing the cumulative biogas production, causing a 21.4% enhancement of the final biomethane yield, whereas the increase due to TE dosing was not statistically significant. The analysis of volatile fatty acids (VFAs) confirmed that the NaOH pretreatment resulted in a higher production of VFAs, indicating increased hydrolysis, while TE addition did not cause significant changes in the VFA concentrations [70].