Safe Nutrient Recovery from Human Urine - System and Hygiene Evaluation of Alkaline Urine Dehydration
Abstract: Only 7% of the world’s wastewater receives tertiary treatment, while the rest is causing eutrophication, hypoxia and climate change through the plant nutrients found in excreta, particularly urine. If managed adequately, urine can be used as a fertiliser because it contains the same nutrients as the fertilisers used to produce food world-wide. To replace the nutrients removed from fields during harvesting, vast amounts of fertiliser are manufactured and applied, and ultimately more plant nutrients are being released into the environment.Use of human urine as a fertiliser is limited by its low nutrient concentration compared with commercial fertilisers. This thesis describes efforts to increase the nitrogen (N) concentration from 0.6% to >6% through dehydration to produce a dry urine-based fertiliser, so that no liquid urine disposal from toilets is required. The overall aim of the work was to develop and evaluate alkaline dehydration as a nutrient recycling urine treatment technology. Fresh human urine was added at various intervals to wood ash and biochar to alkalise the material and thus inhibit the enzyme urease, which catalyses hydrolysis of urea. The urine was then dehydrated at temperatures between 35 and 65 °C. A hygiene evaluation was undertaken to monitor inactivation of five microorganisms during and after alkaline urine dehydration.Urine mass was reduced by 95% during dehydration, while preserving up to 90% of nitrogen and all phosphorus (P) and potassium (K). Inactivation data for a persistent pathogen, Ascaris eggs, were fitted to a non-linear regression model, which estimated that 325 days of open storage would be required for a 3 log10 reduction at 20 °C, compared with 58-110 days in a sealed container. Bacteria and bacteriophages showed a 6 log10 reduction within four days at 20 °C. Simply keeping urine separate from faeces will result in a 4.3 log10 lower pathogen concentration than collecting urine together with faeces.A truly innovative finding was the final product, a dry powder with 7.8% N, 2.5% P and 10.9% K by dry weight, i.e. equivalent to commercial fertiliser. After only four days of storage, the dehydrated medium met world guidelines for unrestricted fertiliser use on non-processed crops in areas not prone to soil-transmitted helminths. By connecting the alkaline dehydration system to new or existing urine-diverting toilets, urine drying can be performed in-house, minimising handling of liquid urine and potentially decreasing the transportation costs of urine-based fertiliser.
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