# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Recovery of Digestate  
Another relevant issue related to biogas production concerns the by-product of the process, i.e., digestate. Digestate is the residue of the anaerobic digestion process. It can result from the digestion of livestock manure, plant biomass (waste or dedicated), animal by-products, sewage sludge or organic fraction of municipal solid waste. Promising applications for digestate have been found in the agricultural sector, where it is used both as a soil amendment and a fertilizer [148]. The content of biogas digestate has been studied by many researchers. As a result of this study, it was found that the content of digestate is rich in nutrients useful for the plant. In fact, digestate is a potential biofertilizer and source for several biogas plants all around the world [130]. Therefore, the increase in the demand for biogas will result in a significant increase in the amount of digestate produced each year [149]. Given the many different substances and wide range of compounds contained in digestate, the main recommended use for its management is its direct use as a fertilizer [150]. The resulting digestate has higher moisture content than the starting biomass, because \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Advantages** & **Disadvantages** \\ \hline \multirow{3}{*}{Solvent scrubbing} & \multirow{3}{*}{High efficiency and low losses} & Expensive investment, high consumption of water and biomethane contamination from N\({}_{2}\) and O\({}_{2}\) desorbed from water, high amount of heat and foaming problems \\ \cline{2-3}  & & Highly efficient technology, lower energy \\ \cline{2-3}  & requirement and no chemicals required \\ \hline \multirow{2}{*}{Hydrogenation processes} & Highly selective technology and easy \\  & transportation of methane produced \\ \hline \multirow{2}{*}{Cryogenic separation} & Efficient treatment, lower methane losses \\  & and high methane purity \\ \hline \multirow{2}{*}{Membrane separation} & Low operational requirements and \\  & non-chemical process \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Advantages and disadvantages of techniques for removal of CO\({}_{2}\)[133; 134; 142; 147]. some of the dry matter is biologically degraded for biogas production. The residual organic matter is more stable and contains nutrients such as nitrogen, phosphorus and potassium, which can return to the soil to provide nutrients for crops. Digestate is, in fact, an excellent fertilizer for plants, rich in both organic matter and macro- and micro-nutrients. The physicochemical characteristics of digestate vary considerably depending on the nature and composition of the digested substrates as well as the operating parameters of the biogas processes. The general components of digestate are dry matter, organic matter, total and ammoniacal nitrogen, diphosphorus pentoxide and potassium oxide and vary depending on the origin. In digestate derived from livestock manure, the highest rate of nitrogen is in ammonia form [151]. Precisely because the natural composition of the raw materials of the process can be varied, the digestate produced also has different physicochemical properties. Other elements that may affect the final composition of digestate include the fragmentation of the substrate and the technologies used in the fermentation process [151, 152]. All the factors influencing the biogas system, directly and indirectly, affect the quality of biogas digestate. The use of digestate as a fertilizer offers significant advantages for agriculture. However, to guarantee a safe use, abuse and inefficient uses should be avoided. Table 9 summarizes the advantages and disadvantages of using digestate from biogas, which will be described in detail below. Digestate consists of a solid fraction in suspension and a liquid fraction containing soluble nutrients [153]. It is made up of organic and inorganic nutrients, enriching the soil and improving its structure and absorption capacity [154].