Skip to main content
Research article - Peer-reviewed, 2022

Thermodynamics and Kinetics of pH-dependent Dissolution of Sparingly Soluble Alkaline Earth Hydroxides in Source-Separated Human Urine Collected in Decentralised Sanitation Systems

Simha, Prithvi; Deb, Chinmoy Kanti; Randall, Dyllon G.; Vinneras, Bjorn;

Abstract

Alkaline earth hydroxides are widely used in water and wastewater treatment. Within the emerging niche of source-separating sanitation, these chemicals have found a new application-to prevent urease-catalysed degradation of urea present in freshly excreted human urine. However, little is known about the dissolution behaviour of these hydroxides in biological fluids like human urine. Herein, we investigate the solubility of Mg(OH)(2) and examine factors that govern its dissolution in different types of urine (real fresh urine, synthetic fresh urine, synthetic dephosphatised fresh urine and real fresh urine concentrated by CO2-free drying). We report experimentally determined as well as thermodynamically simulated data on Mg(OH)(2) solubility, dissolution kinetics, and chemical speciation in urine. We find that it takes between 6 and 16 min for Mg(OH)(2) to dissolve and the average solubility in real fresh urine at 25 degrees C to be 650 mg L-1. We show that solubility is influenced mainly by concentration of organic compounds, soluble phosphate, and magnesium excreted in fresh urine. When fresh urine is supersaturated with Mg(OH)(2), the pH increases to >10.5 and urease-catalysed degradation of urea is inhibited for >14 days. Removing 95% water present in urine increases the solubility of Mg(OH)(2) to 16,240 mg L-1 but reduces pH to < 10. Because relative increase in Mg(OH)(2) solubility decreases as more water is removed and the solubility is retrograde with respect to temperature, to increase the urine pH to >10 and prevent enzymatic ureolysis, the temperature must be kept < 29 degrees C at 75% water removal and < 22 degrees C at 95% water removal. We find this dissolution behaviour of Mg(OH)(2) in concentrated urine solutions to be unlike other alkaline earth hydroxides. These findings have significant implications for the design of new sanitation systems that separately collect and recycle plant-essential nutrients present in human urine.

Keywords

free ammonia; nutrient recycling; solubility; source separating sanitation; urease; wastewater

Published in

Frontiers in Environmental Science

2022, volume: 10, article number: 889119
Publisher: FRONTIERS MEDIA SA

Authors' information

Swedish University of Agricultural Sciences, Department of Energy and Technology
Deb, Chinmoy Kanti
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

UKÄ Subject classification

Water Treatment
Environmental Sciences

Publication Identifiers

DOI: https://doi.org/10.3389/fenvs.2022.889119

URI (permanent link to this page)

https://res.slu.se/id/publ/117169