A Novel Urine Evaporation and Collection System for Dry Toilets

A Urine Evaporation System (UES) that evaporates human urine and collects the nutrient-rich dry product from a urine-diverting dry toilet at backcountry or rural locations was designed and tested in the laboratory and field. The UES consists of vertically stacked plastic cafeteria-style trays with a thin layer of sand on the bottom of each tray. The trays provide an evaporation surface and allow collection of the solid product remaining after evaporation. The sand prevents the solid product from sticking to the bottom of the trays. The trays are held on tracks cut into the inside of a wood or Plexiglas box. The initial laboratory experiment (Chapter 2) consisted of evaporating urine supplied at a constant rate by a peristaltic pump. The evaporation process produced a dark, highly saline, brine solution before drying to a solid product. The solid product had low odour and was mostly comprised of K, Na, Cl, P and N. About 90% of the ammonia was lost during the evaporation process (primarily by volatilization). In the second laboratory experiment (Chapter 3), the UES was improved to enhance airflow across the surface of each tray by constructing a wooden box with a fan. Five litres per day (output of approx. 4 persons per day) was input to the UES at the same time each day until the first trays were entirely full of solid product (65 days). During the experiment, increasingly oxidizing conditions were measured in the evaporating urine brine. The solid products in the stack of trays had a physical and chemical zonation characterized by leaching of solids in the upper trays and accumulation in the lower trays. The majority of N remained in the stable, organic form associated with organic carbon and approximately 35% of the ammonia nitrogen was lost due to volatilization. Oxidizing conditions favoured more stable mineralized forms of nitrogen and sulphur. A prototype passive field UES was designed and constructed and field-tested in a semi-arid temperate climate (Chapter 4). The UES consisted of a Plexiglas® box vented by a black metal chimney. The average evaporation rate over the study period was 3.2 L/day (0.66 mm) with monthly amounts of 117 L (24.3 mm) for August and 76 L (15.8 mm) for September. A multiple-regression analysis indicates that continuously running of the UES from April 1 to Oct 31 (when daytime temperatures in Calgary are consistently above freezing) would have evaporated a total of almost 1000 L (204 mm) of municipal water. A multiple-regression analysis indicated that the evaporation rate could be explained by changes in air temperature, wind speed and incoming solar radiation. The UES design and operation can be optimized according to the user’s desire to maximize evaporation, minimize O&M, minimize costs or maximize fertilizer collection.

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