# Digestate Management and Processing Practices: A Review
## 5 Digestate Processing  
About 180 million tons of DG are produced annually in the EU. Most of the DG produced (120 million tons) is of agricultural origin (typically a mixture of manure and plants, particularly energy crops). About 46 million tons are produced from the organic fraction of mixed municipal solid waste, at least 7 million tons from source-separated biowaste, while the smaller quantities (about 1.7 million tons each) come from sewage sludge and agro/food industry by-products [89]. DG remaining after AD is stored before utilization. Several factors affect the required storage period of DG, such as environmental restrictions for application, DG stabilization, geographic location, soil and crop type, and DG demand. The storage of DG requires a significant amount of cost and land occupation [88]. The most common utilization of DG is a direct application to agricultural land which is a well-known and proven technology. However, to protect groundwater resources, the balance of nutrients in the soil needs to be considered and the legal limits of maximum allowable nutrients loads have to be complied with [90]. In addition, some factors such as non-uniformity (which is determined by properties of the biomass which was used for AD), highly variable nutrient content, high volume of DG, and the impossibility of spreading DG near the biogas plant impact the economics of DG marketing sometimes making its use difficult to realize [88]. For these reasons, there is an increasing focus on adequate DG treatment options.  
Figure 2: Distribution models of DG (according to [88]).  
The major goals of DG treatment are (i) volume reduction to improve the manageability and reduce transportation costs, and (ii) recovery of nutrients in concentrated form. In general, DG treatment processes can be classified into two different approaches: (a) Partial treatment--this aims to reduce the volume or separate it into solid and liquid fractions that can be more easily handled or stored. It is usually the first step in the DG treatment and is less energy demanding and cheaper if compared to (b) complete purification--where the valuable ingredients are separated and concentrated while the remaining liquid fraction is purified, allowing reuse in the AD process or direct discharge to a water body [88; 90].  
Different processing technologies (Figure 3) result in different physical characteristics and fertilizer values of the final product [90]. Although significant capital costs are required due to adequate machinery, high energy, and reagent consumption [88; 90], the lower operating costs derived from the decrease in water, energy, and feedstock consumption make these technologies a profitable investment in the medium and long-term. Since DG treatment has only recently (about the last 10 to 15 years), become a focus of interest, there is no standard procedure and rather a wide variety of treatment schemes exists.  
Except for specific cases, such as some advanced technologies and membrane filtration technologies, all processes described below have already been applied at a large scale [91]. Most of the DG treatment technologies that are currently available on the market work on volume reduction and concentration of nutrients [92].