Partitioning behavior and crystallization of urea, salts and water during stepwise dehydration of acidified human urine by evaporation

This study evaluated the stepwise evaporative concentration of acidified human urine, focusing on dehydration dynamics, crystallization sequence, and the partitioning of major constituents between liquid and solid phases. Unhydrolyzed urine, stabilized with sulfuric acid, was concentrated by convective evaporation, achieving concentration factors (CF) of up to 30 at 20 °C and 43 at 70 °C. Water activity (a_w) was a reliable indicator for monitoring urine dehydration progress, with a critical threshold of 0.3 a_w identified at CF 43, indicating tightly bound residual moisture which prevented further urine dehydration. No detectable losses of urea, major cations, or anions were observed throughout the treatment, suggesting that previous findings on nutrient loss were likely affected by analytical limitations as opposed to chemical degradation. Reconstitution to CF 1, followed by colorimetric analysis, was effective in determining nutrient content in viscous and solidified urine samples. At CFs above 30, over 80 % of urea, ammonium, and orthophosphate remained in the liquid phase, whereas sulphate, chloride, and potassium preferentially crystallized, forming compounds such as gypsum, syngenite, and biphosphammite. These findings demonstrate that adjusting dehydration conditions enables the nutrient profile of urine-derived fertilizers to be tailored to meet specific agricultural or environmental requirements. Overall, this study shows that stepwise evaporation-crystallization of acidified human urine can effectively produce multi-nutrient fertilizers with customizable nutrient compositions, thus enhancing targeted nutrient recirculation in agriculture.