Balance of total mass and nitrogen fluxes through consecutive digestate processing steps: Two application cases

The high water content and low nutrient concentration of digestate complicate its storage, transportation, and utilization. Subsequent digestate processing can effectively remove water and influence nutrient partitioning among digestate fractions and final products. The current study was carried out to evaluate the performance of two typical digestate processing chains, solid and liquid ones, respectively, and to give practical recommendations for optimization. Two fully operating biogas plants with advanced heat utilization were considered as data sources. The digestate mass flow balance of dry matter (DM), water, total N (TN), and ammonium-N mass flows was performed and the efficiency of the examined processing units was calculated. It was found that solid-liquid separation of raw digestate shifted 73-87% of TN and 60-93% of NH₄-N to the liquid phase. Subsequent drying of separated solid fraction removed about 6% of the initial water and required 84% less thermal energy per kg N recovered than the processing of separated liquid. The final product, pellets, contained 14% of initial TN, but only 2% of initial NH₄-N as a result of microbial conversion of inorganic N during drying. Vacuum evaporation of separated liquid fraction removed 34% of the initial water and left a DM-rich concentrate. At the same time, an ammonium sulfate solution (ASS) containing 21% of initial TN and 34% of initial NH₄-N was produced. Both evaluated processing chains showed specific advantages and challenges. Solid products were characterized by a high share of recalcitrant organic compounds and could serve as a soil improver. Liquid processing concentrated plant-available N in ASS, which could be used as valuable inorganic fertilizer.