# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Recovery of Digestate
Due to the high plant nutrient content, digestate can be effectively used as a fertilizer in plants and can be complementary or alternative to mineral fertilization, providing those essential plant macronutrients needed to sustain the food supply of a constantly growing population. Its use can alter the soil ecosystem or have a direct impact on plant growth. Evidence has been documented regarding digestate as a high-quality biological fertilizer, rich in key components and nutrients (N, P, K, Cu and Zn). In addition, organic matter, amino acids, vitamins and some favorable microorganisms can increase the amount of humic substance in the soil and improve its fertility, providing an adequate defense [155]. Its use for the fertilization of plants allows for the diversification of and increase in the economic efficiency of agricultural activities [156]. However, its impact on soil characteristics and biomass yield has not yet been investigated [157]. Its use as an organic fertilizer closes the carbon and nutrient cycles and seeks to reduce the dependence on inorganic fertilizers; besides arousing interest in this regard, it also contributes to improving soil structure and fertility. Digestate turns out to be a clean, easily available and convenient fertilizer that can be reintroduced into crops to improve agriculture. It plays a better role than directly using animal manure as a fertilizer, because it has higher homogeneity and nutrient availability, and better C/N ratio, and it significantly reduces odor [158]. However, digestate contains odors, pathogens and other pollutants. Therefore, it is necessary to implement technical and effective strategies for the treatment of biodgestate and for its use in the environment [159]. It also contains contaminants such as heavy metals, including Cd, Cr, Cu, Pb, Ni, Zn and As [160], and the greenhouse gas (GHG) emissions derived from digestate have underlined the need for appropriate management of digestate before its safe discharge into \begin{table}
\begin{tabular}{c c} \hline \hline
**Positive Aspects** & **Negative Aspects** \\ \hline High-quality organic fertilizer for plants, rich in nutrient components. & Digestate contains odors, pathogens and other pollutants, and contaminants. \\ \hline Reduces dependence on inorganic fertilizers. & Greenhouse gas emissions resulting from digestate have underscored the importance of appropriate digestate management. \\ \hline Increases the economic efficiency of agricultural activities. & The use of digestate at high doses may cause a harmful effect on agriculture. \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Positive and negative aspects of using digestate from biogas. the receiving environment [161]. Prior to the use of digestate, the content of heavy metals and the presence of pathogens should be checked [150]. Depending on the type of raw material and the operating conditions (thermophilic/mesophilic temperature), the digestate may require sanitization, deodorization and neutralization phases [153]. According to the sanitation standard of the European Union, for the safe use of biogas digestate, it should be treated in a continuous mixing tank at 70 \({}^{\circ}\)C for 1 h. The disadvantage of the sanitization process is that it is removed from the N fertilizer in ammonia at the applied temperature. This loss can be compensated for by adding ammonium sulphate to biogas digestate [162]. The fertilizer can be used after enrichment. It also can be used as feedstock for further nutrient applications in agriculture and the chemical industry, but this usually requires additional treatment [163]. An essential aspect of the agricultural applications of digestate is the need to inactivate pathogens present in digestate and reduce the risk of them entering the soil environment. Before foliar or soil application, conditioning (e.g., with inorganic acids) and neutralization (e.g., with potassium hydroxide (KOH)) are required to eliminate odorous compounds and microorganisms [163].