# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Recovery of Digestate
Digestate can be used directly or separated into its fractions, i.e., solid and liquid fractions; in the first case, high investment and operating costs are required, but costs related to storage and transportation tariffs are greatly reduced [157], increasing the resource recovery potential with the use of advanced technologies. Both fractions can be applied to agricultural land. In any case, the liquid fraction includes higher volatile total ammonia nitrogen (TAN) concentrations; this could result in possible negative consequences related to ammonia emissions. The direct application of digestate to soil causes nitrogen loss through ammonia volatilization. Volatilized ammonia nitrogen enters the atmosphere, causing an increase in nitrogen content in natural soil and water, resulting in eutrophication [164]. Recently, digestate has gained importance, in part due to the rising cost of mineral fertilizers and the problems related to their availability and purchasing. The many benefits of using digestate are gaining increasing recognition and are linked to the promotion of sustainable development and the circular economy. Scientific research indicates that biogas digestate can also be used in animal husbandry, aquaculture and algae production. The production and recycling of digestate as fertilizer requires quality, management and quality control throughout the life cycle of AD, from the production of feedstocks to the final use of digestate as fertilizer [165]. Waste Framework Directive 2008/98/EC introduced the \"end of waste\" criteria, indicating that waste ceases to be waste when the required criteria are met. Digestate, conventionally recognized as waste, can be legally used in agriculture and other land applications if it meets legal requirements. Despite the high potential for closing nutrient loops in agriculture and reducing external inputs of mineral fertilizers, the application of digestate on agricultural land has some potential drawbacks [166]. In fact, its uncontrolled disposal can cause nutrient runoff; the anaerobic degradation of protein-rich biomass enriches the concentration of total ammonia nitrogen (TAN) in digestate, limiting its use in areas classified as vulnerable to nutrient pollution of underground and surface waters. Similarly, the content of phosphorus (P) is of concern in digestate management, being responsible for the phenomenon of eutrophication (Water Framework Directive, 2000/60/EC). Traditional methods of digestate application, such as spray irrigation, can cause emissions of ammonia, carbon dioxide and nitrous oxide [167]. In Europe, national legislation establishes the maximum nitrogen (N) loads to be applied to fields, the minimum required digestate storage capacity and the obligatory spreading season to adequately fertilize agricultural fields without posing environmental risks (Directive 91/676/EEC). In Europe, environmental regulations regarding pathogens (EU Commission Regulation No. 142/2011) and heavy metals (Directive 86/278/EEC) also apply to the quality of digestate. Due to the profligate utilization of biogas digestate, there is evidence of a harmful effect on agriculture. In fact, despite the fact that its use promotes plant growth at small doses, at high doses, instead, it can produce a harmful effect [168]. This negative effect can be resolved by controlling the dilution or planting time in the fertilizer concentration [169; 170]. Biogas digestate is an ecological and advantageous product for sustainable production due to climate change and limited fossil resources. In any case, the advantages of the product can only be achieved with the appropriate use of the product and proper control before its use. Another aspect related to its use concerns the economic evaluation. Transport distances have a crucial impact on the economic feasibility of using anaerobic digestate for land application; therefore, the correspondence between the location of existing digesters and neighboring agricultural land should always be checked.