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Abstract

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.

Keywords

Water reuse; Safe nutrient recycling; Decentralised sanitation; Source separation; Wastewater treatment; Fertiliser production

Published in

Desalination
2026, volume: 622, article number: 119786

SLU Authors

UKÄ Subject classification

Environmental Management
Agricultural Science

Publication identifier

  • DOI: https://doi.org/10.1016/j.desal.2025.119786

Permanent link to this page (URI)

https://res.slu.se/id/publ/145375