# Anaerobic Digestate from Biogas Plants--Nuisance Waste or Valuable Product?
## 3 Results and Discussion  
### Chemical Characteristics of Digestate and Products Obtained from Its Treatment  
All analyzed products obtained from DGs were characterized by alkaline reaction (pH above 7.0). Digestates not subject to processing (DG1, DG2) and liquid fractions (LF1, LF2) were characterized by low content of total solids (TS) (approximately 5.4 and 3.1%, respectively), and relatively low concentration of nutrients per 1 kg FM (fresh matter) (Tables 2 and 3). Due to higher content of TS, the remaining products contained higher amounts of nutrients (mean content of TS in SF1 and SF2 was approximately 16.0%, and in pellets more than 95%). DG1 and DG2 were characterized by low content of C (on average approximately 20 g C kg\({}^{-1}\) FM). The obtained data showed that during LF/SF separation, the SFs mainly become enriched in carbon (C) (Table 4). The highest C content was determined in pellets obtained from DGs and SFs (on average approximately 332 g C kg\({}^{-1}\) FM). N\({}_{\mathrm{T}}\) content in DG1 and DG2 was approximately 4.85 and 3.35 g N\({}_{\mathrm{T}}\) kg\({}^{-1}\) FM, respectively. This indicates that the type of organic materials used for anaerobic digestion affects the chemical composition of the resulting digestate [25]. More than 80% of nitrogen flows from DG to LF, while only 20% flows to the solid fraction. This shows that, in DG, the dominant form of nitrogen was soluble ionic forms (NH4\({}^{+}\)-N). The reported results are in agreement with other studies [26; 27]. In the analyzed digestates (DG1, DG2), the share of NH4\({}^{+}\)-N in N\({}_{\mathrm{T}}\) reached an average of approximately 70%. As confirmed by other authors [28], this suggests that digestate has a very high fertilizer potential with a high amount of plant-available N. Mechanical separation caused an increase in the NH4\({}^{+}\)-N/N\({}_{\mathrm{T}}\) ratio in the liquid fraction (Table 2), with an average increase of approximately 97%, and a decrease in the solid fraction that averaged approximately 56%. The lowest NH4\({}^{+}\)-N/Nratio was measured in the pellets, averaging approximately 6.7% (Table 2). The comparison of the content of NH\({}_{4}\)+-N in PDG and PSF with DG and SF indicate a decrease of about 96% in the PDG and PSF. This suggests intensive losses of ammonia with water vapor in the course of drying preceding the pelletizing process. The alkaline reaction of digestate-based products (DG and SF) could have favored the release of NH\({}_{3}\). The volatilization of NH\({}_{3}\) during the processing of digestate to pellets was addressed in the study by [PERSON] et al. [29]. Ammonia negatively affects human health and leads to air quality degradation. NH\({}_{3}\) emission and its further transformations constitute a significant indirect N\({}_{2}\)O emission pathway in agricultural systems [30]. Drying of DG and SF may thus contribute to the global climate warming. According to [PERSON] at al. [31], NH\({}_{3}\) emission results in 0.1\(-\)0.16 million tons of indirect N\({}_{2}\)O-N emission per year. A potential solution may be the application of scrubbers with acid that would bond with the released ammonia in the course of processing digestate-based products. Another practice to minimize NH\({}_{3}\) emissions is decreasing the pH of SF by adding acid. Such solutions have been successfully used to reduce NH\({}_{3}\) emissions from slurry [32]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Samples \({}^{*}\)** & **FM** & **C** & **N** & **P** & **K** & **Mg** & **Ca** & **Zn** & **Cu** & **Mn** \\ \hline LF 1 & 84.8 & 47.4 & 78.5 & 43.1 & 87.6 & 26.7 & 61.2 & 66.0 & 43.5 & 64.5 \\ LF 2 & 80.9 & 37.8 & 80.4 & 31.0 & 66.8 & 13.4 & 53.9 & 53.9 & 51.6 & 52.0 \\ SF 1 & 15.2 & 52.6 & 21.5 & 56.9 & 12.4 & 73.3 & 38.8 & 34.0 & 56.5 & 35.5 \\ SF 2 & 19.1 & 62.2 & 19.6 & 69.0 & 33.2 & 86.6 & 46.1 & 46.1 & 48.4 & 48.0 \\ \hline \hline \end{tabular} \({}^{*}\)Abbreviations as in Table 1.  
\end{table}