Stabilization of source-separated urine by heat-activated peroxydisulfate

Source-separated urine is an attractive fertilizer due to its high nutrient content, but the rapidly hydrolysis of urea leads to ammonia volatilization and other environmental problems. Urine stabilization, which meanly means preventing enzymatic urea hydrolysis, receives increasing attention. Accordingly, this study developed a technique to stabilize fresh urine by heat-activated peroxydisulfate (PDS). The effect of three crucial parameters, including temperature (55, 62.5, and 70 °C), heat-activated time (1, 2, and 3 h), and PDS concentration (10, 30, and 50 mM) that affect the activation of PDS in urine stabilization were investigated. Nitrogen in fresh urine treated with 50 mM PDS at 62.5 °C for 3 h existed mainly in the form of urea for more than 22 days at 25 °C. Moreover, the stabilized urine could remain stable and resist second contamination by continuous and slow pH decrease due to PDS decomposition during storage. Less than 8% of nitrogen loss in stabilized urine was detected during the experiment. The investigation of nitrogen transformation pathway demonstrated that urea was decomposed into NH₄⁺ by heat-activated PDS and further oxidized to NO₂^- and NO₃^-. The nitrogen loss during treatment occurred via heat-driven ammonia volatilization and N₂ emission produced by synproportionation of NO₂^- and NH₄⁺ under acid and thermal conditions. Overall, this study investigated an efficient approach of urine stabilization to improve urine utilization in terms of nutrient recovery.