# Digestate Management and Processing Practices: A Review
## 4 Digestate

Table 2: AD conducted with the aim to produce quality DG. According to previous studies, DG represents an organic matrix used more for its fertilization properties (high N and P content, low C:N ratio), while amendment properties were only visible at high dosages [50]. In contrast, compost may improve the physical and chemical properties of soils by increasing the nutrient and water holding capacity, organic content, pH, and cation exchange capacity [67]. However, [68] conducted research comparing the properties of liquid undigested and digested animal slurries and DG from dedicated energy crops. The results showed a wide range of nutrient contents: DG had higher NH\({}_{4}\)+:total N (TN) ratios, decreased OM contents, decreased total (TOC) and organic carbon (OC) contents, reduced biological oxygen demands (BOD) (factor 5-13), increased pH values, lower C:N ratios, and reduced viscosities than undigested animal manure. [PERSON] et al. [69] also highlighted the benefits of DG like malodor reduction, pathogen control, a more balanced nutrient mix, and higher nutrient bioavailability but still no increase in nutrient leaching losses. However, the scientific literature regarding the survival of pathogens, HMs, persistent organic pollutants, and inorganic contents of biogas DG is not so well determined and little is known about their fate during DG processing [17]. High-quality DG suitable for soil fertilization is characterized by some essential features related to its properties such as declared content of nutrients, pH value, the content of dry matter and OM, and homogeneity. Equally important are DG features related to health and safety such as purity (free of plastic, stones, glass, non-digestible matter, etc. ), hygiene or sanitization (free of pathogenic and other undesired biological content), and safety (safe for living organisms and the environment). These concerns demand strict controls and limitations on the undesirable content of both organic and inorganic chemical pollutants. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Substrates** & **AD Process Conditions** & **The Aim of the Study** & **Results** & **Ref.** \\ \hline \multirow{6}{*}{20 different DGs from different biogas plants-10} & \multirow{6}{*}{Biogas plants operate under mesophilic or thermophilic conditions.} & \multirow{6}{*}{Influence of chemical characteristics of DGs on soil microbial activities, i.e., PAO and soil respiration} & \multirow{6}{*}{DGs contained significantly higher NH\({}_{3}\) concentrations, but lower TC and VFA concentrations if compared to PS and COM. The DGs showed both stimulating and inhibiting effects on PAO, while all COM and PS except one showed inhibiting effects on PAO.} \\  & & & & High stability of the DG was obtained after AD of APW and DM. Single digestion of the APW and DM was incomplete & \\  & & & & compared with \\  & & & & the mixture thereby leading to the \\  & & & & lower stability of DG. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Unit** & **Values** & **References** \\ \hline EC & \(\mu\)S cm\({}^{-1}\) & 100–642 & [54; 61] \\ pH & - & 5.6–8.6 & [51; 54; 61; 62; 63; 64] \\ DM & \% & 0.7–90 & [54; 61; 62; 63; 64] \\ OM & \% DM & 15.6–98.0 & [54; 61] \\ Total C & \% DM & 10.4–58.7 & [51; 62; 63; 64; 65] \\ Total N & \% DM & 0.2–20.5 & [51; 61; 62; 63; 64; 65] \\ NH\({}_{4}\)+-N & g kg\({}^{-1}\) DM & 2.1–17.9 & [54; 61; 61; 66] \\ Ca & g kg\({}^{-1}\) DM & 0.6–98.5 & [54; 62; 63; 64; 65] \\ K & g kg\({}^{-1}\) DM & 0.9–110.5 & [54; 61; 62; 63; 65] \\ Mg & g kg\({}^{-1}\) DM & 0.1–14.1 & [54; 62; 63; 65] \\ P & g kg\({}^{-1}\) DM & 0.1–54.0 & [54; 62; 63; 64] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main DG characteristics obtained after AD of different feedstocks. Therefore, the only feasible way to ensure the production of high-quality DG is to use high-quality feedstock as a substrate for AD [70].