**2.5 Biogas production process**

Different types of organic matter can be digested anaerobically to produce biogas. The final step in the chain of chemical and biological processes used to break down organic matter for the purposes of producing biogas and managing garbage is called anaerobic digestion [73]. The production of biogas involves the controlled breakdown of organic material into smaller molecules by several anaerobic bacteria [73, 77].

Chemical reactions such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis occur during anaerobic digestion, which produces biogas from biomass [35, 73].

Biogas is produced by the microbial action in the digester soon after biomass is prepared and fed after undergoing preparation and a gradual fermentation process; the biomass is introduced into the reactor, where microbial activity within the digester rapidly generates biogas. Consequently, the process is a consequence of bacterial consumption of organic matter, specifically proteins, carbohydrates, and lipids/fats. This microbial digestion gives rise to the production of gases, predominantly methane and carbon dioxide. The process of biogas production encompasses several distinct stages, namely pretreatment, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Prior to introducing feedstock into the digester for the purpose of anaerobic degradation, the initial stage of biogas production involves feedstock processing or pretreatment. The pretreatment of feedstock is an essential and pivotal stage to mitigate instances of failure, enhance the production and enhance the quality of digestate, among other advantageous outcomes.

The bio digestion process for *biogas* production involves several key stages, namely the *pretreatment* stage of raw materials, anerobic digestion, purification of raw biogas, final biogas utilization, and *post-treatment* of *digestate*. Impurities like sand are, stones, plastics, etc. removed from the feedstock at the pretreatment stage, and the total solids (TS) concentration and temperature of the *feedstock* moderated for optimum bio-digestion [93]. The digester constitutes the core or the main element of the biogas production plant whose size and performance characteristics should be selected based on specific process and output conditions. At the terminal stage of the biogas production, the residue and slurry are further treated for intended application e.g. biofertilizer [21, 23, 94]. The process flowchart of a biogas plant process is shown in **Figure 2**.

From **Figure 2**, it is noted that the process of biogas production starts with feedstock collection and pretreatment before it is fed into the anerobic digester. The main products of the anaerobic digestion process are raw biogas and digestate. Biogas can be purified and stored ready for distribution and consumption. The digestate in slurry form is prepared or conditioned and applied in irrigation or controlled discharge as an organic fertilizer. Digested can also be used as a solid fertilizer in addition to slurry form. The various stages of biogas production are discussed as follows.

**Figure 2.** *Biogas plant operation process and products.*

#### *2.5.1 Pretreatment stage*

Pretreatment is implemented as a strategy to enhance the breakdown of substrates, thereby enhancing the overall efficacy of the process. The employed pretreatment methods can be classified into four primary categories: chemical, mechanical, thermal, and enzymatic processes. The aforementioned techniques are designed to expedite the process of decomposition, although it is crucial to acknowledge that they do not necessarily lead to augmented biogas generation [10]. First, the feedstock is washed, then it is macerated, screened, and finally pressed (however, these stages can change depending on the feedstock). Mechanical parts can survive longer if they are free of contaminants like plastic, and magnetic traps are used to effectively eliminate magnetic pollutants. Glass, eggshells, pottery, bones, and sand are among nonmetallic impurities that must be removed during the preparation step. These pollutants do not decompose during digestion and hence accumulate as deposits at the digestor's base [72].

Enzymes are required for hydrolyzing lignocellulose, which is necessary for digestion. An economic and technical barrier to producing biogas from lignocellulosic waste is its complicated structure. Strong and compact molecular bonds are formed in lignocellulose due to the presence of cellulose, hemicellulose, and lignin. The effectiveness of pretreatment is critical for maximizing lignocellulose's potential as a biogas source. Pretreatment, in general, hastens reactions, boosts biogas yields, and creates a plethora of novel substrates [10, 95].

Technological advancements in pretreatment have enhanced biogas yields from lignocellulosic feedstocks, resulting in lower methane emissions into the atmosphere. Pretreatment is crucial because it increases biomass digestion and provides more biogas than without it by facilitating microbial breakdown of lignocellulose and its polymers, especially cellulose and hemicellulos. It has been shown that autoclaving and microwave heating can cause the hydrolysis of several non-biodegradable chemicals found in municipal garbage. Biomass pretreatment achieves this by partially or completely breaking down the feedstock to yield fermentable sugars, reducing the lignin barrier, and smoothing out the crystalline structure of cellulose [10, 30, 88, 89].

**Table 1** demonstrates the wide range of pretreatment options available, each with its own set of benefits and drawbacks, and cost benefit implications. While pretreatment enhances the efficacy of the process and the biogas yield, it also introduces difficulties such as higher energy input, increased operating and maintenance expenses, and the introduction of inhibitory chemicals.

AD have become increasingly popular in producing renewable and sustainable energy from waste. Although synergies do exist between various technologies, the configurations to couple need further development [96].

#### *2.5.2 Processes in anaerobic digestion*

#### i.Hydrolysis

Hydrolysis is a chemical reaction in which water is broken down into OH− anions and H+ cations. In the presence of an acidic catalyst, hydrolysis breaks down the massive biomass polymers present in the substrate [33]. The proteins, carbohydrates, and lipids, all of which are massive organic polymers, are broken down into their component simple sugars, fatty acids, and amino acids to make up biomass [33, 73, 77]. During the process of hydrolysis, fermenting bacteria, such as Bactericides, Clostridia, and Bifidobacterial, efficiently decompose biopolymers, including carbohydrates,


#### **Table 1.**

*Pretreatment methods and applications [10].*

proteins, and lipids, into soluble forms such as sugars, fatty acids, and amino acids [97]. Hydrolysis produces acetate and hydrogen, which are put to good use by methanogens in the final stages of anaerobic digestion. Methane formation by acidogenesis necessitates further hydrolysis product breakdown, as these are still quite large molecules [45, 98].
