# Anaerobic Digestate from Biogas Plants--Nuisance Waste or Valuable Product?
## 3 Results and Discussion
### Chemical Characteristics of Digestate and Products Obtained from Its Treatment

Table 4: Distribution (%) of FM and nutrients in SF and LF after mechanical separation of digestate. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Samples \({}^{*}\)** & **pH** & **TS \%** & \multicolumn{2}{c}{**C**} & \(\mathbf{N_{T}}\) & \(\mathbf{N\text{-}NH_{4}}\) & **C**N \\ \cline{3-6}  & & & & \multicolumn{4}{c}{**g kg\({}^{-1}\) FM**} \\ \hline DG 1 & 7.74 & 5.11 & 18.32 & 4.85 & 3.67 & 3.8 \\ DG 2 & 7.34 & 5.69 & 21.78 & 3.35 & 2.17 & 6.5 \\ LF 1 & 7.71 & 3.07 & 10.82 & 4.30 & 4.12 & 2.5 \\ LF 2 & 7.40 & 3.12 & 11.58 & 3.23 & 3.19 & 3.6 \\ SF 1 & 7.66 & 16.5 & 67.04 & 6.58 & 3.74 & 10.2 \\ SF 2 & 7.32 & 16.56 & 80.47 & 3.33 & 1.85 & 24.2 \\ PSF 1 & 7.33 & 95.6 & 344.87 & 19.17 & 1.30 & 18.0 \\ PSF 2 & 7.23 & 96.69 & 317.70 & 16.80 & 0.99 & 18.9 \\ PDG 1 & 7.35 & 96.58 & 333.35 & 23.15 & 1.83 & 14.4 \\ PDG 2 & 7.28 & 94.63 & 334.28 & 20.58 & 1.31 & 16.2 \\ \hline \hline \end{tabular} \({}^{*}\)Abbreviations as in Table 1.  
\end{table} 
Table 2: Chemical characteristics of the samples (TS = Total Solids). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Samples \({}^{*}\)** & **P** & **K** & **Mg** & **Ca** & **Zn** & **Cu** & **Mn** \\ \cline{2-9}  & & & \multicolumn{4}{c}{**g kg\({}^{-1}\) FM**} \\ \hline DG 1 & 0.44 & 4.94 & 0.22 & 0.34 & 23.52 & 6.25 & 8.73 \\ DG 2 & 0.81 & 2.28 & 0.61 & 0.61 & 20.98 & 6.56 & 10.28 \\ LF 1 & 0.22 & 4.27 & 0.06 & 0.33 & 15.86 & 3.28 & 6.14 \\ LF 2 & 0.31 & 1.90 & 0.08 & 0.46 & 14.46 & 4.20 & 7.05 \\ SF 1 & 1.62 & 3.38 & 0.92 & 1.17 & 45.52 & 23.74 & 18.88 \\ SF 2 & 2.92 & 4.00 & 2.18 & 1.24 & 52.35 & 16.65 & 27.52 \\ PSF 1 & 8.63 & 19.23 & 5.25 & 6.05 & 310.27 & 172.13 & 112.18 \\ PSF 2 & 10.64 & 21.79 & 8.26 & 7.09 & 333.92 & 95.53 & 175.85 \\ PDG 1 & 9.35 & 34.75 & 5.07 & 8.63 & 461.90 & 157.17 & 162.87 \\ PDG 2 & 12.52 & 32.89 & 10.71 & 9.35 & 431.23 & 179.42 & 156.01 \\ \hline \hline \end{tabular} \({}^{*}\)Abbreviations as in Table 1.  
\end{table} 
Table 3: Content of macronutrients and heavy metals in the samples. The rate of nutrients released from organic matter depends on its susceptibility to the processes of mineralization, determined by the ratio of carbon to nitrogen compounds (C:N). Soil N immobilization after anaerobic digestate application has been previously reported for products with the C:N ratio exceeding 25\(-\)30 [27]. All the analyzed digestate-based products were characterized by a narrow C:N ratio (ranging from 2.5 for LF 1 to 24.2 for SF 2) (Table 2). Thus, their mineralization in the soil and release of plant nutrients was fast. However, [PERSON] et al. [29] reported that the C:N ratio is not an accurate indicator to predict N mineralization in soil treated with anaerobic digestates. According to the authors, that kind of products contain organic matter with lower biodegradability in the soil. In the case of a very narrow C:N ratio, however, higher nitrogen losses are probable, as previously observed by [PERSON] and [PERSON] [33]. According to [PERSON] at al. [34], the magnitude of nitrogen losses through leaching corresponds with the fertilization system, and was highest in the mineral-organic system. The application of mineral forms of nitrogen decreased the C:N ratio, increasing leaching of nitrogen. Phosphorus is a depleting resource. Therefore, great attention is paid to the search for alternative sources [35]. Compared with P contents of wastewater or urine [36], the P content in digestate is relatively high. The N:P ratio is an important indicator for the assessment of the fertilizing properties of digestate-based products. Low N:P ratios (i.e., \(\leq\)2) in digestate may indicate P deficiencies that should be supplemented with mineral fertilizers containing P [29]. A high excess of P in relation to N may lead to an increased risk of run-off or leaching of P from soil to surface water bodies. Among the analyzed forms of digestate, in DGs and FCs, the N:P ratio was considerably higher than 2.