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Research article - Peer-reviewed, 2022

Drying fresh human urine in magnesium-doped alkaline substrates: Capture of free ammonia, inhibition of enzymatic urea hydrolysis & minimisation of chemical urea hydrolysis

Vasiljev, Anastasija; Simha, Prithvi; Demisse, Natnael; Karlsson, Caroline; Randall, Dyllon G.; Vinneras, Bjorn;

Abstract

Recycling urine can reduce the flux of reactive nitrogen in the environment. This paper presents a novel approach to recover all N (Ntot) from urine, including ammonia (TAN; about 5% of Ntot), which is usually volatilised when alkalised urine is dehydrated. As analytical methods for measuring N have a standard deviation of at least 5%, real fresh urine was fortified with ammonia (urineN) or ammonia and phosphate (urineNP) so that TAN comprised 10% of Ntot. The urine was then added to different magnesium-based alkaline substrates (MgO, Mg(OH)2, MgCl2 + Mg(OH)2) and dried at 38 C. Chemical speciation modelling suggested that, irrespective of the substrate, >98% of Ntot in urineNP was recovered and 86% of TAN was precipitated as struvite. Experimental results showed that < 90% of Ntot was recovered when urineNP was dried in MgO and Mg(OH)2, suggesting that no TAN was captured. However, all phosphorus and potassium and 93% (+/- 5%) of Ntot and 30% of TAN were recovered when urineNP was dried in MgCl2 + Mg(OH)2, as the [Mg]:[NH4]:[PO4] molar ratio of 1.69:1.14:1.0 in urine favoured formation of struvite. Overall, this study demonstrated that all ammonia excreted in real fresh urine (unfortified, TAN < 5% Ntot) can be captured if urine is dried in substrates containing 3.7 g MgCl2.6H2O L-1 or 2.2 g MgSO4 L-1, but no calcium. Ammonia can also be captured if fresh urine is saturated with MgO or Mg(OH)2 with high reactivity (<60 s citric acid test). If the drying substrate has pH > 10 throughout the treatment, urease enzyme-catalysed degradation of urea to ammonia is prevented, resulting in complete recovery of all nutrients. The end-product is a solid fertiliser containing 10-11% nitrogen, 1-2% phosphorus and 2-3% potassium.

Keywords

Alkaline Dehydration; Ammonia; Fertiliser; Resource recovery; Source separating Sanitation; Wastewater treatment

Published in

Chemical Engineering Journal

2022, volume: 428, article number: 131026
Publisher: ELSEVIER SCIENCE SA

Authors' information

Swedish University of Agricultural Sciences, Department of Energy and Technology
Swedish University of Agricultural Sciences, Department of Energy and Technology
Swedish University of Agricultural Sciences, Department of Energy and Technology
Addis Ababa University
Swedish University of Agricultural Sciences, Department of Energy and Technology
Randall, Dyllon G.
University of Cape Town
Swedish University of Agricultural Sciences, Department of Energy and Technology

Sustainable Development Goals

SDG6 Clean water
SDG12 Ensure sustainable consumption and production patterns

UKÄ Subject classification

Water Treatment

Publication Identifiers

DOI: https://doi.org/10.1016/j.cej.2021.131026

URI (permanent link to this page)

https://res.slu.se/id/publ/115052