*1.4. Pretreatment of Agricultural Wastes*

The breakdown of complex molecular structures of agro-waste into simpler monomers is generally considered essential during the pretreatment process. Thus, it contributes to a high output after the conversion process [17]. Different technological approaches can be employed for biomass treatment; this includes physical (grinding, milling), thermal (e.g., steam explosion), biological (e.g., enzymatic), chemical (e.g., use of acids, alkalis) methods, and a combination of treatments such as thermochemical treatments [18]. These methods provide ease of accessibility to enzymes for hydrolysis, increasing the surface area while minimizing operational costs. For instance, the physical treatment of the waste enhances the surface area as it provides easy accessibility for microbial populations and enzymes during hydrolysis. On the other hand, the thermochemical method of pretreatment increases the rate of heat and mass transfer and facilitates the rate of uniform temperature distribution within the agro-waste particles, thus high efficiency for hydrolysate constituents' recovery during liquefaction. Similarly, thermochemical conversion involves two essential methods:

drying and torrefaction [19]. The former involves moisture removal from the waste, which increases the efficiency of the process, while the latter involves the thermal treatment of waste at a temperature of 200–300 ◦C, where sufficient oxygen is removed from the waste, including water.

For the biological conversion process, the waste can be treated at a temperature of 50–250 ◦C. This provides an efficient treatment process in terms of pathogen removal and biodegradability. However, in the biological pretreatment method, different types of enzymes and fungi are utilized; hence, it is considered the less energy-consuming method as it can be operated at both milder and economical temperature. However, it seems to be a very slow process as several days are required for the process to be completed. Therefore, various fungi are required for delignification of the agro-waste. It is carried out by inoculating the agro-waste with fungal spores or hydrolysis by a cocktail of enzymes [20]. In essence, ligninolytic enzymes play a role in the hydrolysis of the recalcitrant lignin. Simultaneously, the fungi (white-rot fungi) participate in lignin degradation with minimal holocellulose consumption [21].

The chemical pretreatment method involves using various chemicals such as acids and alkalis that contribute to the breakdown of organic components present in the agrowaste. This pretreatment method will break down the lignin-carbohydrate bond and crystalline cellulosic structure (Figure 1). Examples of the acid used during pretreatment include H3PO4, H2SO4, HNO3, HCl, etc., and alkalis such as NaOH and KOH. Many researchers have considered the use of liquid-ammonia-water mixture to treat the recalcitrant lignocellulosic constituents in the agro-waste.

**Figure 1.** Effect of pretreatment on the biomass component.
