# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Anaerobic Digestion and Co-Digestion  
Biogas as a renewable energy source is obtained through the anaerobic digestion of organic substances. Anaerobic digestion (AD) is the biochemical process of decomposing complex organic matter into simpler chemical components under anaerobic conditions [73]. Various waste materials are used in the AD process for sustainable conversion to biogas, including the organic components of municipal solid waste, industrial waste, wastewater production waste and solid waste from agriculture, and process water and agricultural waste from livestock and agricultural activities [74]. Some examples may be grass, leaves, cereal straw, hay, fruit, vegetables, mills, pruning, cells, fruit processing, livestock waste affecting poultry farms, stable washing water from cattle and pigs supporting biogas production [75; 76]. The biological dissolution of organic matter during anaerobic digestion is a multi-step chemical process involving the four steps described in Equations (1)-(5):  
\[\mathrm{CH_{3}COO^{-}+H_{2}O\to CH_{4}+HCO_{3}^{-}} \tag{1}\]  
\[\mathrm{HCO_{3}^{-}+H^{+}\to CH_{4}+3H_{2}O} \tag{2}\]  
\[\mathrm{4 CH_{3}OH\to CO_{2}+2H_{2}O} \tag{3}\]  
\[\mathrm{4 HCOO^{-}+2H^{+}\to CH_{4}+CO_{2}+2 HCO_{3}^{-}} \tag{4}\]  
\[\mathrm{4H_{2}+CO_{2}\to CH_{4}+2H_{2}O} \tag{5}\]  
Equations (1)-(3) are related to hydrolysis, acidogenesis (acid production) and the acetogenesis phase (acetic acid production), respectively. Equations (4) and (5) are related to the methanogenesis phase (methane production) [77]. Different groups of microorganisms are involved in each of these steps. The flow diagram of the AD process is shown in Figure 2.  
Each step in the process differs in the reaction pathways and metabolites of the microorganisms involved [78]. Biochemical steps occur simultaneously in an anaerobic context, generating the production of biogas and digestate [42]. Microorganisms, which play an essential role during this process, are distinguished among the four phases [79]. The AD process, in fact, depending on the operational phase, requires different associations  
Figure 2: Schematic diagram of the biogas production process through anaerobic digestion. of bacteria, namely, acetelastic and hydrogenotrophic methanogenic bacteria, sintrophic acetogens, and fermentative and homoacetogenic bacteria [80]. Therefore, the gradual decomposition of organic matter does not occur uniformly, since the different bacterial groups work at different rates [81]. The microbial decomposition of organic residues in an anaerobic environment is a process that occurs spontaneously in nature. However, knowledge of the factors involved in biological processes is necessary to optimize fermentation and subsequent biogas production. The disintegration of organic substances into components until they are converted into methane can only occur in the aquatic environment [81]. The anaerobic digestion process is considered advantageous for its economic availability, for the stabilization of waste, for the reduction in odors generated by farms, for the mitigation of biomethane emissions, for the recycling of nutrients using fertilizers and for the potential for local production of renewable energy [82]. Furthermore, anaerobic fermentation of the organic substrate allows for the reduction in the use of non-renewable sources, the reduction in greenhouse gas emissions and the increase in soil fertility thanks to the use of unconventional digestate-based fertilizers. In addition to improving waste management, AD can avoid potential environmental problems and provide clean energy [83]. AD plays an important role in reducing the use of fossil fuels in transport and industry, while managing organic waste and producing renewable fertilizers [84].