Effects of additives on shifting phosphorus to solid phase during Solid-Liquid separation of digestate in full-scale biogas plant

Abstract

Phosphorus (P) is critical for plant growth, but global reserves are exhausting within 250–300 years, therefore enhancing phosphate recycling is crucial for the future. Biogas digestate, rich with nutrients is a promising resource for nutrient recovery. Conventional solid–liquid separation shifts approximately 35 % of the total P in the digestate to the solid phase. Separation trials with additive treatment using a screw press with a 0.75  mm sieve were performed at University of Hohenheim’s full-scale biogas plant. After 22  h, 67.41 % (kieserite treatment) and 52.35 % (straw flour treatment) of total P shifted to the solid phase. Treatment with kieserite enhanced P shift into the solid phase by forming non-labile fractions through a chemical bond between P and Mg²⁺ ions. Kieserite treatment for 22 h effectively increases the share of total P in the separated solid phase, it also ensures a sustainable nutrient supply and mitigates the risk of nutrient runoff.