# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Anaerobic Digestion and Co-Digestion
Complex organic compounds, converted into the simplest form, can be further transformed into volatile fatty acids, such as acetic, propionic acid, butyric acid, formic acid and lactic acid. In addition, the concentration of the generated H\({}^{+}\) ions directly affects fermentation, since the high concentration hinders the production of acetate. In this fermentation stage, monomeric sugars, fats and amino acids are transformed into organic acids and alcohol. Simultaneously, intermediate substrates are transformed into H\({}_{2}\), CO\({}_{2}\) and acetate [92]. During the AD process, acetogenic bacteria grow together with methanogenic bacteria; methane is produced in complete absence of O\({}_{2}\), and the reaction is highly exothermic. With the advent of high-throughput sequencing, several microbial groups involved in biogas production have emerged [93]. Several factors influence the AD process, among which are temperature, pH, C/N ratio and substrate particle size. Smaller particles can be expected to decompose more rapidly, resulting in faster generation of biogas. The identified multiple factors that intervene in the anaerobic digestion process and that allow for the control of its efficiency are the fundamental elements for the proper functioning of biogas plants. Digestion processes can take place under very different operating conditions, which are reported in Table 5.  
AD can be conducted in three different temperature ranges. The temperature should be kept as constant as possible to ensure process stability. Mesophilic and thermophilic temperatures are the most used and range from 20 to 40 \({}^{\circ}\)C (usually 35 \({}^{\circ}\)C) and from 50 to 65 \({}^{\circ}\)C (usually 45 \({}^{\circ}\)C), respectively. Lower temperatures, in fact, reduce biogas production. pH is another factor that affects the stability and performance of the AD process. pH, as well as temperature, affects the metabolic activity of different microbial populations. Since microorganisms are sensitive to pH, the ideal pH range for the process is about 6.0 to 7.5 [73]. The mentioned parameters have an impact on the performance of the bacteria in the anaerobic digester, influencing the efficiency of the AD process and the final biogas production. Anaerobic digestion can be applied to semi-solid, liquid and gaseous wastes. It can be carried out on single materials (mono-digestion) or mixtures of numerous materials (mixed digestion or co-digestion). In the past, only one substrate was used in the process, but today, the tendency is to prefer a combination of different substrates (co-digestion). Anaerobic co-digestion offers the opportunity to overcome the disadvantages of mono \begin{table}
\begin{tabular}{c c c} \hline \hline
**Thermal Conditions of** & **Solid Content** & **Biological Phase** \\ Reaction & & \\ \hline Psychophilia (\(<\)25 \({}^{\circ}\)C) & \begin{tabular}{c} Wet process \\ (solid fraction 5–8\%) \\ \end{tabular} & A single reactor \\ Mesophilia (20–40 \({}^{\circ}\)C) & \begin{tabular}{c} Semi-dry process \\ (solid fraction 8–20\%) \\ \end{tabular} & \
\begin{tabular}{c} Separate hydrolytic and \\ methanogenic phases \\ \end{tabular} \\ Thermophilia (50–65 \({}^{\circ}\)C) &
\begin{tabular}{c} Dry process \\ (solid fraction \(>\)20\%) \\ \end{tabular} & In separate reactors \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Process parameters of anaerobic digestion [73]. digestion by simultaneously digesting two or more raw materials, improves digestion and is often used in larger biogas plants that treat organic waste of various origins. Co-digestion of various substrates improved biogas production potentials compared with individual substrates [94,95], leading to higher gas production per tons of substrate. The combination of these substrates is an excellent method for the coincident management of solid and liquid organic wastes. This helps to solve the problem of nutrient imbalance linked to the anaerobic use of individual substrates and the consequences of the accumulation of dangerous compounds during digestion.