Closed-loop air recirculation architecture for alkaline urine dehydration and water recovery using regenerable superabsorbent polymers

We evaluated a novel closed-loop evaporative system designed to concentrate alkalised human urine while simultaneously recovering water using regenerable superabsorbent polymers (SAPs). This architecture recirculates air and physically isolates urine from atmospheric CO₂, thereby maintaining high alkalinity and preventing enzymatic urea hydrolysis. The system was operated at ∼30 °C using sodium polyacrylate, potassium polyacrylate, or a 1:1 ( w /w) blend of both SAPs across eight absorption–desorption cycles. All treatments exhibited high initial water uptake (>1.4 kg m −2 day −1 ) and gravimetric absorption (>0.8 g g −1 ), with performance declining due to polymer fatigue after repeated use. FT-IR spectra revealed the depolymerisation of the acrylate backbone leading to the formation of acrylic acid residues, confirming chemical deterioration during the thermal regeneration of the SAPs. Notably, the closed-loop design eliminated the need for supersaturating urine with Ca(OH)₂, which is required in open evaporative systems to buffer against CO₂-induced acidification. Colorimetric and targeted metabolomic analyses confirmed complete nitrogen retention and > 99 % recovery of the 30 most abundant endogenous organic solutes in urine, including urea, creatinine, and hippuric acid. These results demonstrate that low-temperature evaporation can preserve the full biochemical complexity of urine, producing a dry, sanitised fertiliser as well as water with extremely low organic content. • Closed-loop air recirculation system for low-temperature urine dehydration • Regenerable SAP-based desiccants enable cyclic air–vapour moisture absorption. • Eliminates CO₂-driven acidification and nitrogen loss without excess Ca(OH)₂ • >99 % retention of major endogenous organics confirmed by targeted metabolomics • Produces a dry fertiliser and clean water suitable for various reuse applications