Biogas Plant with Ammonia Recovery for Biobased Fertilisers

This study evaluates a full-scale biogas refinery plant in Germany that recovers ammonium sulphate and liming substrate from nitrogen-rich digestate using vacuum NH3 stripping and FGD-gypsum scrubbing. The system achieves 57% ammonium nitrogen recovery, produces 12 t d⁻¹ of 22% ammonium sulphate solution, and operates with a production cost of 5.8 € per ton of digestate. Energy use is highest in the stripping unit (7.5 kWhel t⁻¹), and the process recovers 36% of total nitrogen. The resulting biobased fertilisers meet EU fertiliser regulations and support circular economy goals by replacing synthetic mineral fertilisers. Economic analysis shows a net benefit of approximately 7 € t⁻¹ digestate when used on-farm, with market values of 67 € t⁻¹ for ammonium sulphate and 47 € t⁻¹ for liming substrate. The study confirms the technical and economic viability of nutrient recovery from digestate, highlighting the role of FGD-gypsum as a sustainable scrubbing agent.