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The effects of urine storage conditions on struvite recovery Tilley, Elizabeth Anne

Abstract

Phosphorus, like oil, is a non-renewable resource that must be harvested from finite resources in the earth’s crust. An essential element for life, phosphorus is becoming increasingly scarce, contaminated, and difficult to extract. Struvite, or magnesium ammonium phosphate (MgNH₄P0₄.6H₂0) is a white, crystalline phosphate mineral that can be used as a bioavailable fertilizer and can be recovered from aqueous solutions such as digestor supernant. In response to diminishing water resources, increasing nutrient pollution, and largely unaffordable centralized treatment, a paradigm of Ecological Sanitation (EcoSan) has emerged. A central tenant of EcoSan technology is nutrient recovery; by separating urine from feces in the absence of water, urine can be used as a clean, concentrated nutrient source. Urine harvested in this manner is used as a liquid fertilizer with varying degrees of success and acceptance. This research examines the potential of urine to be a feedstock for struvite recovery. By recovering a sustainable source of phosphorus from urine, the prospect of appropriate sanitation and closed-loop nutrient systems, may move closer to reality. In laboratory experiments using synthetic and real human urine, different methods of preparing urine to be used as a feedstock for struvite recovery, were examined. The effect of temperature, faecal contamination, dilution, and headspace on stored nutrient levels was examined. The effect of adding different quantities of magnesium, at different times, on the amount of phosphorus that could be removed from solution, was also examined. An average of 70% of phosphorus could be removed from real urine in the form of struvite when magnesium was added to the urine solution after ureolysis had forced the precipitation of calcium and magnesium minerals; magnesium added before ureolysis began retarded the process. Dilution and the presence of wastewater were found to affect the rate of ureolysis but not the purity of the struvite recovered; recovered struvite was approximately 99% pure regardless of dilution or contamination. Based on a comparison of the results, synthetic urine was found to be representative of the general behaviour of real urine during struvite formation.

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