Mechanistic evaluation of water removal dynamics in stabilised human urine for circular fertiliser production

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Drying source-separated, stabilised human urine offers a route to produce circular fertilisers with low environmental impact, with potential to reduce dependence on synthetic fertilisers and close nutrient loops. This work examines the behaviour of water during the drying stage of the urine-to-solid fertiliser value chain. Specifically, it evaluated how stabilisation chemistry influences water activity ( a w ) and moisture transport dynamics. Freshly excreted urine was treated with citric acid, calcium hydroxide, or wood ash, and subjected to controlled isothermal drying at 50 °C, with continuous mass loss data fitted to ten thin-layer drying kinetics models. The Two-term model best described wood ash–stabilised urine ( k 1 = 8.7 × 10 −5 s -1 , k 2 = 1.2 × 10 −4 s -1 ), suggesting water removal was initially rapid, followed by a slower drying phase. In contrast, water removal in calcium hydroxide- and citric-acid-stabilised urine slowed progressively as drying advanced and was best described by the Page model, with k values of 7.3 × 10 −6 and 3.3 × 10 −8 s -1 , respectively. Peleg isotherm fits revealed a non-linear relationship between a w and moisture content, with water activity declining sharply only after substantial water removal. Fresh and acid-stabilised urine showed Type II-like desorption behaviour, whereas alkalised urine showed Type III-like behaviour, with a w remaining >0.7 even after majority of the water had been removed. Considering water activity thresholds for microbial inhibition reported in the literature, and recent evidence that urine concentration supresses urease activity, we propose a five-fold mass concentration, corresponding to a w <0.6, as a design target for limiting microbial growth and ureolysis in urine concentrating systems.

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