Quality assurance of excreta-derived fertilizers: insights from applying DIN SPEC 91421 and pathways for standardization in circular sanitation

When applying recycling fertilizers derived from human excreta in horti- or agriculture, quality assurance is crucial for health protection, environmental safety, and societal acceptance. The German product specification DIN SPEC 91421 (2020) represents the first standard regulating such recycling products for horticultural use. As its further development towards a European Norm (EN) is under consideration, it is essential to assess its practical applicability and identify areas for improvement. The applicability of DIN SPEC 91421 (2020) was evaluated through field trials, analytical testing, and expert discussions within two research projects, the German research project REGION. innovativ–zirkulierBAR and the Horizon Europe innovation project P2GreeN . Practical challenges, analytical feasibility, and stakeholder feedback were systematically documented and compared against regulatory requirements and scientific evidence. Overall, the DIN SPEC standard proved suitable for applied quality assurance, but showed certain limitations in scope, methodological flexibility, and adaptation to diverse pathways such as the given varieties of urine treatment processes. With regards to hygiene parameters, most microbiological indicators ( Salmonella spp. , E. coli, Enterococcaceae ) were widely accepted as relevant, though further specification (e.g., Enterococcus faecalis ) and revision of parameters such as Clostridium perfringens were recommended. Chemical criteria, particularly for pharmaceutical residues and hormones, require an updated prioritization based on toxicological relevance. Annexes of DIN SPEC on sampling frequency, analytical ranges, and process deviations were identified to require substantial revision based on the latest knowledge generated since its first issue. Recommendations include (i) expanding the application beyond horticulture, (ii) adjusting differentiated criteria by feedstock and process, (iii) revising hygienic and chemical indicators, and (iv) integrating protocols for process validation alongside product testing. Based on the harmonized, risk-based, and application-oriented approach of DIN SPEC 91421, an updated issue—developed in collaboration with authorities, science, and industry—is capable to facilitate safe and accepted commercialization of excreta-based fertilizers within the framework of future European regulation.