# Anaerobic Digestate from Biogas Plants--Nuisance Waste or Valuable Product?
## 3 Results and Discussion
### Chemical Characteristics of Digestate and Products Obtained from Its Treatment
Due to the lower P content, fertilization with the remaining forms, i.e., SFs, PSFs, and PDGs, did not increase the risk of phosphorus losses from the soil. In the analyzed digestates DG1 and DG2, P content reached 0.44 and 0.81 g kg\({}^{-1}\) FM, respectively (Table 3). Thus, during separation of SF/LF, more phosphorus was supplied to SFs (approximately 57% from DG1 and 69% from DG2, Table 4). Literature data [12] indicate that, during SF/LF separation, only 30% of total amount of phosphorus flows to SF. Our results further show a high content of phosphorus in pellets obtained from DGs (9.35 and 12.52 g kg\({}^{-1}\) FM for PDG1 and PDG2, respectively). Due to the partitioning of P between SF and LF, pellets obtained from SFs contained less P than pellets from DGs (PDG). Pellets PSF1 contained approximately 8% less P than PDG1. In PSF2, P content was approximately 15% lower than in PDG2. Differences in potassium (K) content in the analyzed pellets were even more evident than differences in phosphorus content. K content in PSF1 was more than 44% lower than in PDG1, and in PFS2 approximately 34% lower than in PDG2 (Table 3). In digestate, potassium primarily occurs in an unbound ionic form that during separation mainly flows to the liquid fraction. In our study, 87% of K contained in DG1 flows to LF1 (Table 4). Such large flows of K to LF may disqualify the possibility of returning LF to the fermenter in order to dilute the solid substrates. Such an engineering solution is proposed [37], but as the conducted research shows, it may lead to excessive salinity and inhibition of methane fermentation. Potassium recovery from LF is difficult, because this nutrient forms soluble salts that cannot be precipitated from solution. Moreover, membrane technologies can be used to a limited extent [38]. More advanced treatment methods are too expensive considering the amount of LF produced in the biogas plant [37]. Hence, it can be concluded that LF, as a nitrogen- and potassium-rich, liquid, fast-acting fertilizer, is the best eco-friendly and cost-effective solution. Mg and Ca content in the analyzed digestate-based products was lower than the content of the remaining macroelements (Table 3). Magnesium content was the lowest in LF (averaging 0.07 g kg\({}^{-1}\) FM) and, as expected, the highest in pellets (from 5.1 g kg\({}^{-1}\) FM in PDG1 to 10.7 g kg\({}^{-1}\) FM in PDG2). During LF/SF separation, more magnesium flows to SF (approximately 73% from DG1 and 87% from DG2). The opposite dependency was observed in the case of Ca. During separation, more Ca flows from DGs to LFs (Table 4). Due to this, no considerable differences were recorded between Ca content in DGs and LFs. Results indicate that the various forms of digestate may be a valuable source of nutrients for plants. The potential of digestate to harm the environment and human health, however, is a matter of concern [29]. An important indicator used to assess the agronomic quality of digestates is the content of heavy metals. Contents of heavy metals (HMs) in the analyzed digestate-based products were low (Table 3). The lowest content of HMs was determined in LFs and DGs. Mean content of HMs in the LFs (averaging LF1 and LF2) was approximately 15.2 mg Zn; 3.7 mg Cu; and 6.6 mg Mn kg\({}^{-1}\) FM. Mean content of HMs in the DGs (averaging DG1 and DG2) was 22.2 mg Zn; 6.4 mg Cu; and 9.5 mg Mn kg\({}^{-1}\) FM. Content of Zn and Mn in SFs was more than twice as much, and Cu more than three times higher, than in DGs. The obtained results correspond with the scientific literature. Exemplarily, [PERSON] et al. [12] report for DG: Zn content of 13.5 mg kg\({}^{-1}\) FM, Cu 4.2 mg kg\({}^{-1}\) FM; for the LF: 10.1 mg Zn kg\({}^{-1}\) FM and 3.0 mg Cu kg\({}^{-1}\) FM; and for SF: 69.9 mg Zn kg\({}^{-1}\) FM and 22.1 mg Cu kg\({}^{-1}\) FM. In our study, during SF/LF separation, more Zn and Mn flowed from DG to LF (Table 4). The distribution of Cu to LF and SF was divided evenly between SF and LF. The highest content of HMs was observed in pellets of both SFs and DGs.