# Anaerobic Digestate from Biogas Plants--Nuisance Waste or Valuable Product?
Biogas plants attempt to solve the problem through the application of different techniques of DG treatment. DG is usually subject to mechanical separation, resulting in the liquid and solid fraction (LF and SF, respectively) [12]. LF can be applied to fields or in cultivation of aquatic plants and algae [13;14]. Nitrogen and phosphorus recovery from LF is also practiced through struvite precipitation and ammonia stripping [15]. Although many authors confirmed a high fertilizer value of struvite and ammonium sulphate [16; 17; 18], the recovery methods have limited practical application. This results from the fact that the alkaline effluent remaining after these processes is characterized by considerable salinity. Study results by [PERSON] et al. [13] show that the effluent (EFL) after struvite precipitation and ammonia stripping from LF had a pH value of 12.2, and potassium and sodium content of 3.8 and 9.2 g kg\({}^{-1}\) FM (fresh matter), respectively. Management of this effluent as a medium for cultivating _Lenna minuta_ required very high dilution of EFL with water to create conditions suitable for duckweed growth. SF resulting from separation can be applied to fields [11], composted [19], dried by heat energy generated by the biogas plant, or pelletized. Pelletized SF (PSF) can be applied to fields or burned in energy production [20]. Research is also being conducted on the pyrolysis of SF leading to the production of gas and biochar [21]. As a very durable carbon compound, biochar is considered to be a CO\({}_{2}\) sink [22]. In summary, manifold technologies for processing DG into products with added value currently exist. However, most emerging technologies require significant financial investments and are thus not always economically viable. Due to these and other technical challenges, the predominant treatment of DG is still its use as fertilizer. The use of DG for crop nutrition gains from the rapid increase in mineral fertilizer prices. These lead to an increase in the cost of cultivation and, consequently, food prices. Farmers are dependent on alternative sources of nutrients for their crops. This increases the demand for DG and the products resulting from DG treatment. Accordingly, there is still a need for research on the fertilizer efficiency of the different products obtained from DG. Most of the publications on biogas production focus on the evaluation of the effectiveness of anaerobic digestion, ignoring the issue of digestate management. Furthermore, the search for methods for the rational use of digestate is as important as the search for methods to increase the efficiency of biogas production. Due to the considerable amount of available digestate, its treatment must be optimized to avoid negative effects on the environment, such as excessive ammonia emission, leaching of N and P to waterbodies, or an increase in the content of heavy metals in plants. This study therefore aimed to determine the effect of different methods of DG treatment on the chemical composition of the resulting products, and their resulting effect on the yields and chemical composition of plants. Additionally, the article presents a methodology for calculating the flow of nutrients to the liquid and solid fractions during the separation of raw digestate, which should be considered as a significant added value of the work. This is especially important from an engineering point of view. Estimating the chemical composition of the LF of the DG is important at the stage of selecting the technology and designing the biogas plant. The LF can be returned to the fermenter in order to dilute the solid substrates. The inflow of an excessive amount of potassium ions contained in the LF of the DG may lead to salinity and inhibition of the biological process of methane fermentation. The calculation formulas proposed in the work will allow the assessment of the chemical composition of the LF without its laboratory analysis.