**2.3 Biochemical conversion processes**

Biochemical conversion mechanisms disintegrate biomass using enzymes produced by bacteria and other microbes. Microbes are employed to carry out the biomass transformation operation in most cases. Biochemical conversion is one of the few methods for extracting energy from biomass that is environmentally friendly.

**Figure 3.** *Sequence for derivation of syngas from biomass adapted from [20].*

#### *2.3.1 Fermentation mechanism*

Fermentation is a biochemical technique applied for bioethanol production after biomass pretreament (makes the cellulose accessible) and hydrolysis (breaks the polysaccharide in feedstock to free sugar molecules). Fermentation is a metabolic operation that uses enzymes to induce chemical reactions in organic feedstocks. There are three fermentation processes that are frequently employed in bioethanol synthesis: separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and simultaneous saccharification and co-fermentation (SSCF). Separate hydrolysis and fermentation (SHF) is the most popular approach utilized in bioethanol synthesis. The hydrolysis of lignocellulosic biomass is excluded from the ethanol fermentation process in SHF. It is possible to deploy enzymes at elevated temperatures for improved efficiency while fermenting microbes can be utilized at mild temperatures for optimal sugar consumption. SSF and SSCF have a brief entire operation since the enzymatic hydrolysis and fermentation processes take place concurrently to keep the level of glucose as minimal as possible during the operation. In SSF, the fermentation of glucose is segregated from the fermentation of pentoses, but in SSCF, the fermentation of glucose and pentoses are carried out in the same facility [34]. SSF and SSCF are preferable over SHF because the procedure can be completed in the same vessel. The advantages of both procedures are cheaper costs, larger ethanol yields, and reduced operating times [35].

Fermentation of bioethanol can be done in a batch, fed-batch, repeated batch, or continuous mode, depending on the process. In a **batch method**, the feedstock is delivered at the start of the operation and the media is not added or removed throughout the operation [36]. This mode of fermentation is beneficial due to the absence of labour skills and ease of biomass management [37]. **Continuous method** is accomplished by continuously introducing feedstock, culture medium, and nutrients to a bioreactor comprising functional microbes [38]. The merits of continuous systems over batch and fed-batch systems include increased yield, smaller bioreactor volumes, and lower capital and operating expense [37]. **Fed-batch fermentation** is an integration of batch and continuous modes of fermentation that involves the input of feedstock into the fermentor without withdrawing the medium from the fermentor. It has been successfully employed to mitigate the issue of biomass inhibition in batch operations. Comparing this procedure to other modes of fermentation, it accounts for an increased efficiency, produces more dissolved oxygen in the medium, requires less fermentation duration, and has a less harmful impact on the medium constituents [39].

#### *2.3.2 Anaerobic digestion*

Anaerobic digestion (AD) is a mechanism in which microbes disintegrate organic matter in the absence of oxygen, including animal dung, wastewater biosolids, and food residues. In order to produce biogas (biomethane), anaerobic digestion must occur in an airtight vessel known as a bioreactor, which can be built in a variety of forms and dimensions to accommodate the site's and biomass requirements. These bioreactors include diverse microbial populations that decompose (or digest) the residue and generate biogas and digestate (the solid and liquid substance end-products of the AD operation), which are released from the digester once the waste has been broken down [40]. However, **anaerobic co-digestion** is the method of combining different organic substances in a single digester. Co-digested resources comprise manure, food wastes (including processing, distribution, and consumer generated

materials), energy crops, crop wastes, and fats, oils, and greases (FOG) from restaurant grease traps. Co-digestion can raise the quantity of biogas produced from organic residue that is low yielding or challenging to digest.

The mechanism of anaerobic digestion is divided into four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In single-stage batch bioreactors, all residues are fed at the same time, and all four approaches as shown in **Figure 4** are permitted to take place in the same reactor consecutively; the compost is then discharged after a specified retention interval or after the termination of biogas generation [39]. **Hydrolysis** is employed to break down organic macromolecules into their constituent parts, which can then be used by acidogenic bacteria [41]. During **acidogenesis**, acidogenic microbes are capable to manufacture intermediate volatile fatty acids (VFAs) and other compounds by accumulating the products of hydrolysis via their cell membranes and converting them into other products [42]. **Acetogenesis** is the mechanism by which these high VFAs and other intermediates are transformed into acetate, with H2 and/or CO2 being generated during the operation [42]. **Methanogenesis** is the ultimate step of anaerobic digestion, during which readily available intermediates are consumed by methanogenic microbes, resulting in the production of methane. The environmental requirements of methanogenesis are as follows: greater pH than earlier phases of anaerobic digestion, as well as lower redox potential [43].
