ii.Acidogenesis

Anaerobic activity of acetogens produces mostly organic acids, alcohols, hydrogen gas, and hydrogen sulfide throughout this process [33, 97]. In an acidic environment generated by the fermentative bacteria, hydrolysis products are degraded by acidogenic microorganisms to produce ammonia, carbon dioxide, hydrogen sulfide, carbonic acid, shorter-volatility fatty acids, alcohol, and other trace products based on the substrate composition and products of hydrolysis [97]. Acidogenesis byproducts are too large to be ideal for methane production. Thus, they engage in acetogenesis [67].

#### iii.Acetogenesis

The process of acetogenesis produces acetate from acetic acid [99]. Through anaerobic digestion, acetic acid, hydrogen, and carbon dioxide are generated during acidogenesis. The results of acetogenesis and other processes are digested to the point where methanogens can produce methane [67].

#### iv.Methanogenesis

Methane and carbon dioxide are created in the final stage, called methanogenesis, thanks to the activity of acetoclastic methanogens (AM) and carbon dioxide (CO2) reducing methanogens (CM), respectively. Finally, anaerobic bacteria called methanogens like *Methanosarcina barkeri*, *Methanosaeta concilii*, and *Methanococcus mazei* complete the methanogenesis process by producing methane from the byproducts of acetogenesis and other intermediate products of acidogenesis and hydrolysis [33, 97, 100]. The first three stages of methanogenesis (as shown in Eqs. (1) and (2)) involve the utilization of CO2 and acetic acid (CH3 COOH):

$$\text{CH}\_2 + 4\text{ H}\_2 \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{1}$$

$$\text{CH}\_3\text{COOH} \rightarrow \text{CH}\_4 + \text{CO}\_2 \tag{2}$$

While (CO2) might theoretically be turned into water and methane, at this time the acetic acid pathway is the primary mechanism for methane generation. Therefore, (CO2) and (CH4) are produced as the primary products of anaerobic digestion via the acetic acid pathway [67]. H2 and CO2 gas combinations are transformed into biogas, which is composed of 60–70% CH4 and 40–60% CO2 [16]. The four steps of biogas production, together with associated activities and microorganisms, are depicted in **Figure 3**.

**Figure 3.** *Summary of the biogas formation process.*

**Figure 3** shows the main stages in biogas production being hydrolysis, acidogenesis, acetogenesis, and methanogenesis as the four primary stages in anaerobic digestion. Maximal biogas production requires careful management of the four main stages.
