Life cycle performance of urine-to-fertilizer pathways: Critical review of technologies and products.

Human urine is a concentrated nutrient stream, but its recovery is often evaluated separately in terms of treatment, fertilizer-product performance, and environmental impacts. This review integrates these dimensions from source separation to fertilizer use. Stabilization, concentration, and element-specific recovery pathways are synthesized to show how process design determines nutrient retention, product form, composition, handling, and fertilizer properties. Product performance is evaluated through mineral-fertilizer substitution, nutrient use efficiency, and soil responses, together with constraints related to salinity, pathogens, pharmaceutical residues, and antimicrobial resistance. Process-based and product-based life cycle assessment perspectives are distinguished to clarify recovery burdens, avoided wastewater treatment, direct emissions, transport, and mineral-fertilizer substitution. Reported studies indicate that urine-derived products can support mineral-fertilizer substitution under suitable nutrient balance and application conditions, although outcomes depend on product characteristics and application context. Environmental performance is similarly context dependent. Dense urban systems may favor low-dilution collection and nutrient-dense products, whereas rural and peri-urban systems may favor the local use of safely stabilized liquid urine. Lower-complexity options may be more appropriate for household and sparsely populated settings with limited maintenance, energy, chemical supply, and transport capacity. This review provides a pathway-oriented framework linking recovery technologies, fertilizer-product usability, and environmental assessment to support context-specific selection of urine nutrient recovery strategies. Overall, effective urine nutrient recovery requires matching recovery technologies and fertilizer products with local agricultural demand, infrastructure, and environmental conditions to achieve practical and sustainable nutrient substitution.