# Digestate Management and Processing Practices: A Review
## 1 Introduction  
The main role of the agricultural sector is to provide sufficient and high-quality food for an ever-growing population [1], which is expected to reach 9.7 billion people in 2050 [2]. This implies that a significant increase in agricultural production must be achieved. Due to its importance and relevance, agriculture is at the center of policy agendas worldwide [1]. However, it is also a major contributor to climate change through greenhouse gas emissions and air, water, and soil pollution. Intensive soil fertilization with mineral fertilizers, which is carried out to make up the gap between the nutrients required for optimal crop development and those provided by the soil and from available organic sources, has led to several ecosystem and environmental problems, such as decreases in organic matter (OM) content and soil fertility, and increases in environmental degradation [3, 4]. Environmental degradation, which includes biodiversity loss, deforestation, erosion, water use, and human interference with the nitrogen (N) and phosphorus (P) cycles has intensified especially in recent decades. On the other hand, intensive livestock farming can also have seriousnegative environmental impacts [5]. Livestock farms produce large amounts of animal manure, which can pollute nearby underground and surface waters if not handled properly. However, when properly handled and distributed, manure can be applied to fields as organic fertilizer, mitigating potential contamination of soil and water [1]. The application of intensive mineral fertilizers represents an anthropogenic impact on the soil where humans have the greatest control. Therefore, appropriate soil management practices should be ensured to reverse the trend of soil quality decline [4]. Fertilization with organically derived fertilizer represents an alternative for sustainable agriculture that can provide sufficient nutritious food for all while reducing environmental risk [6]. Manure is the most popular organic fertilizer used in crop production which contains complex molecules as nutrients and diverse microorganisms with the ability to decompose OM. Long-term use of manure can improve physical, chemical, and microbiological soil properties, and can also improve crop nutrition [7]. However, manure may contain pathogens and/or components that compromise food safety, limiting its direct use as a fertilizer. The solution lies in processes such as recycling and recovery of manure. For this purpose, anaerobic digestion (AD) and composting are used to stabilize manure so that it can be safely used for land application. AD, which stabilizes organic waste and reduces pathogens and odors, is a process that converts organic waste into methane-rich biogas and digestate (DG). Biogas is used for energy while DG is usually used as fertilizer in agriculture [8]. DG is a rich source of N, P, potassium (K) and sulfur (S), various micronutrients, and OM, the addition of which to the soil can stimulate microbial biomass and soil metabolic activities and accordingly improve soil ecosystem function [9]. However, the optimal fertilization properties of DG may be lost if it is neither fully stabilized nor contains biodegradable materials. To overcome such a problem, composting has been proposed as a post-treatment of DG. The composting process can transform the DG into mature, stable, safe, humus- and nutrient-rich compost, which offers the relevant advantage over the DG of maintaining and improving environmental quality and conserving resources. Compost is considered an important source of OM and nutrients for agriculture and plays an essential role in maintaining soil biodiversity and horticultural production when used as a component in the preparation of pot substrates. Moreover, the microflora of compost has significant antagonistic effects against various soil-borne phytopathogenic microorganisms, which may play an important role in their control. However, physico-chemical analysis of compost is important as it can provide real information about its composition [10].