# A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants
## 2 Anaerobic Digestion Process and Microbial Communities  
Anaerobic digestion offers a valuable option for converting biodegradable feedstock into renewable energy. In this process, the conversion occurs by bacteria and archaea in the absence of oxygen, wherein a series of complex biochemical reactions determine the product output [12]. Generally, the organic part of the biomass is utilized by the microorganism consortia and yielded to methane and carbon dioxide, while the rest is transformed into other minorities [13]. The complete digestion process takes place in four different stages called hydrolysis, acidogenesis, acetogenesis, and methanogenesis [14], which are briefly overviewed in Figure 2.  
Hydrolysis involves depolymerization of insoluble complex organic hydrocarbons into soluble monomers where the principle substrate compounds (i.e., carbohydrates, lipids, and proteins) are broken down into corresponding low molecular weight monosaccharides, long chain fatty acids, and amino acids that are favorable for bacterial degradation. Hydrolysis is a complex multistep process mediated by extracellular enzymes. The enzymes required for hydrolysis can either be attached to microbial cells or secreted to the solution [15]. Several groups of hydrolytic microorganisms are involved in the degradation of several substrate compositions, where the bacteria Bacteriods, Clostridium and Staphylococcus are significant drivers [16] (see Table 1). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Primary Substrate** & **Hydrolyzed Products** & **Bacterial Group** \\
**Components** & **Hydrolyzed Products** & **Bacterial Group** \\ \hline Carbohydrates & Soluble sugars & _Clostritium_, _Aticovibrio cellulilicus_, _Staphylococcus_, _Bacteroides_ \\ Lipids & Higher fatty acids or alcohols and glycerol & _Clostritium_, _Staphylococcus_, _Microoccus_ \\ Proteins & Soluble peptides and amino acids & _Clostritium_, _Proteins vulgaris_, _Peptococcus_, _Bacteroids_, _Bacillus_, _Vibrio_ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Bacterial groups involved in hydrolysis of different substrate components [16]. Figure 2: Simple schematic representation of anaerobic digestion steps. In the acidogenesis stage, acidogenic bacteria such as Lactobacillus, Streptococcus, and Clostridium [16] (Table 1) transform hydrolysis products (amino acids and sugars) into volatile fatty acids (VFAs) (acetic acid, butyric acid, and propionic acid), organic acids (succinic acid and lactic acid), ammonia (NH\({}_{3}\)), hydrogen gas (H\({}_{2}\)), carbon dioxide (CO\({}_{2}\)), hydrogen sulfide (H\({}_{2}\)S), and low alcohols [17]. The concentration of hydrogen produced at this stage affects the final product after digestion and the resulting organic matters such as VFAs are not suited for direct conversion to methane by the methanogens. Hence, the third stage, namely acetogenesis, converts the VFAs, especially acetic acids and butyric acids, into acetate, H\({}_{2}\) and CO\({}_{2}\). Among the VFAs, 65-95% of methane is directly produced from acetic acid, while propionic acid remains mainly unconverted because its degradation is thermodynamically less favorable (based on the relationship between hydrogen partial pressure and VFA degradation), in comparison to butyric acid [18]. In the final stage, methane is generated by the function of three groups of methanogens, namely acetotrophic, hydrogenotrophic, and methylotrophic [19]. The majority of the methane is produced by acetotrophic methanogens, which transform the acetate (resulting from acetogenesis) into CH\({}_{4}\) and CO\({}_{2}\)[20]. In this process, the principle reaction can be interpreted as shown by Equation (1). \[\text{CH}_{3}\text{COOH}\rightarrow\text{CH}_{4}+\text{CO}_{2} \tag{1}\]  
The hydrogenotrophic group converts hydrogen and carbon dioxide into methane through the reactions stated in Equations (2) and (3) [20]. From this route, around 30% of methane may be produced.