Digestate as a Source of Nutrients: Nitrogen and Its Fractions

Due to fossil-fuel-limitation constraints, new energy sources are being sought. On the other hand, organic fertilizers that can be used in agriculture are increasingly being sought. One of the renewable energy sources is biogas produced from substrates large in organic matter. Apart from biogas, the product of the anaerobic digestion process is digestate. Due to the high content of nutrients, mainly nitrogen, this product can be successfully used as a fertilizer. This study aims to determine the content of total nitrogen (N_tot) and its selected fractions in the raw and processed digestate from agricultural biogas plants. The nitrogen fractions included N-NH₄, N-NO₃, and Norganic. The total nitrogen content (N_tot) and its fraction in raw digestate were determined. Samples used for the research came from five agricultural biogas plants. Separation into liquid and solid fractions is one of the methods for digestate management. The nitrogen content in selected samples obtained after separation of digestate in a biogas plant and on a laboratory scale was also checked. The obtained results show that digestate from agricultural biogas plants is a nitrogen-rich fertilizer. The content of N_tot in the tested samples ranged from 1.63 g∙kg⁻¹ to 13.22 g∙kg⁻¹ FM. The N-NH₄ content in the analyzed material ranged from 0.75 to 4.75 g∙kg⁻¹ FM. The determined physical and chemical properties confirm that the raw and processed digestate is characterized by appropriate fertilization properties, with particular emphasis on the content of N_tot and the share of its mineral forms. Based on the chemical composition, digestate from agricultural biogas plants can be considered a multi-component fertilizer.