Acidic, alkali, and oxidative pretreatment approaches have been applied to enhance the biological degradation of biomass. Pretreatment by acid reagents is commonly used for solubilization of hemicellulose and lignin compounds that make other organic materials more available for enzymatic attacks. Also, during acidic pretreat­ment, the hemicellulose is hydrolyzed to sugar monomers, furfural, hydroxymeth- ylfurfural, and other products [240]. Acidic pretreatment has several possible drawbacks. First, some of the solubilized compounds can be toxic to methanogens. Second, the production of hydrogen sulfide or ammonia nitrogen instead of methane can be enhanced.

Addition of a strong alkali reagent causes solvation, saponification, alkali hydro­lysis, and degradation of polymeric organic compounds. Alkali pretreatment (62.0 mEq Ca(OH)2/L for 6.0 h) of the organic fraction of municipal solid waste increased the COD solubilization and enhanced methane yields up to 172% [241]. The influence of different alkaline reagents NaOH, KOH, Mg(OH)2, and Ca(OH)2 on solubilization of WAS has also been examined. The following levels of COD solubilization at pH 12 with the four alkaline reagents have been reported—39.8%, 36.6%, 10.8%, 15.3% [201] and 60.4%, 58.2%, 29.1%, 30.7% [225], respectively.

The effect of NaOH concentration was studied in the range from 0 to 26 g/L. COD solubilization rose significantly as the NaOH dose increased up to 5 g/L [225] or up to 7 g/L [201] . The largest biogas production or biodegradability was reported at NaOH concentrations between 4 and 10 g/L [225]. Additional amounts of NaOH led to decreasing amounts of biogas production. Based on the results from biotoxic­ity tests, the decrease in biodegradability was not caused by sodium toxicity, but by the formation of refractory compounds under extreme alkali conditions.

Several oxidative reagents have been tested for biomass pretreatment: ozone, oxygen, hydrogen peroxide, and peracetic acid. Hemicellulose and lignin are com­mon targets of oxidative pretreatment. The oxidative chemical reactions include electrophilic substitution; side chain displacement; radical reactions; and cleavage of alkyl, aryl, ether, and ester linkages [242]. Disadvantages of most oxidative meth­ods are losses of organic materials (e. g., sugars) due to the nonselective oxidation and formation of inhibitors.

The oxidative pretreatment of a wastewater sludge mixture with ozone resulted in COD solubilization from 1.3 to 29% for an ozone dose of 0.05 g/g COD and 40% at an ozone dose of 0.1 g/g COD [212]. The methane yield increased by a factor of

1.5 and 1.8, and the methane production rate increased by a factor of 1.7 and 2.2, respectively. Increasing the ozone dose (0.2 g/g COD) resulted in significant oxida­tion of the sludge organic fraction (about 30%), therefore decreasing the methane yield. The reactions of ozone with some chemicals, such as phenol and LCFA, can form products toxic to methanogens [243] .