# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Anaerobic Digestion and Co-Digestion
Mutual and syntrophic interactions among functionally distinct anaerobic microorganisms involved in each phase and the stability of the AD process are fundamental for the efficient conversion of organic matter; this can be obtained by preserving the delicate equilibrium between production and consumption of intermediate products [85]. Therefore, a detailed understanding of the properties and influence variables of the different degradation paths is of decisive importance for the process. As shown in Equations (1)-(5), biogas production through anaerobic digestion includes four phases. Each degradation steps simultaneously occurs in a single-stage continuous reactor. This results in narrow limits and high requirements under specific environmental and operating conditions for the degradation of complex substrates [78]. The development of the two-stage digestion process (i.e., separation of acid and methanogenic phases) is reported in the literature as an interesting alternative for increasing the stability and overall performance of the process [32]. Biomass generally contains complex polymers that must be broken down before microbial treatment. The initial stage of fermentation, i.e., hydrolysis, decomposes the complex species of the substrate (proteins, carbohydrates, lipids) into simple organic molecules, such as amino acids, fatty acids and sugars [86]. This stage of the process passes through extracellular enzymes of the hydrolase group (amylase, protease, lipase) produced by appropriate strains of hydrolyzing bacteria. It emerged that hydrolysis is supposed to be a limited-rate phase in anaerobic digestion. This is due to the limitation of the availability of soluble molecules to bacterial cells. Therefore, most reviews have focused on advances in improving hydrolysis using physical, thermochemical and biological pretreatments [87]. In the next stage of acidogenesis, acidifying bacteria absorb hydrolysis products to produce volatile fatty acids, such as acetic acid, propionic acid and butyric acid [88]. In the phase of acetogenesis, the volatile fatty acids produced in acidogenesis are converted into acetates, together with the production of hydrogen. Finally, in the final phase of the AD process, methanogenesis or biomethanation, there is the consumption of intermediates by methanogenic microorganisms [89], and there is the division into two processes of methane generation: reduction of CH\({}_{3}\)COO\({}^{-}\) and conversion of H\({}_{2}\) with CO\({}_{2}\). Acetotrophic methanogenic bacteria are responsible for reducing acetate (CH\({}_{3}\)COO\({}^{-}\)) to methane, while hydrogen methanogenic bacteria convert H\({}_{2}\) and CO\({}_{2}\) into CH\({}_{4}\), water and carbon dioxide. Acetogenesis and methanogenesis often occur simultaneously. Methane can also be formed through the reduction of carbon dioxide with formate or through the disproportion of methanol or various methylamines. The presence of CO\({}_{2}\), H\({}_{2}\)O, NO\({}_{\rm x}\) and N\({}_{2}\) leads to a reduction in calorific value [90]. Thus, these constituents could be separated or enhanced by increasing the CH\({}_{4}\) content. Process-critical components are H\({}_{2}\)S, NH\({}_{3}\)halogenated components and siloxanes. The methanogenic phase can also become the limiting step in the process, as methanogenic microorganisms are extremely sensitive to acid accumulation in the medium. The transformation of biomass into biogas requires a complex microbiological activity. Indeed, AD requires a number of microbial steps and leads to the formation of various intermediates. Digestion is performed by bacteria. Organic matter, such as cellulose, proteins and fats, as well as some insoluble organic compounds, is degraded by microorganisms [91]. In the process, there are distinct metabolic groups of microorganisms that differ both in substrates and in the products of their metabolism. Facultative anaerobic bacteria absorb dissolved oxygen in water, reducing the redox potential.