Passive evaporation of source-separated urine from dry toilets: prototype design and field testing using municipal water

A prototype urine evaporation unit (UEU) that removes water from human urine produced from a urine-diverting dry toilet using passive solar evaporation was designed and field-tested at a meteorological station. Municipal water was evaporated on vertically stacked plastic cafeteria-style trays that create a large evaporation surface with a small land-area footprint. The trays were located inside a Plexiglas^® enclosure exposed to UV light while passively heating the UEU like a solar oven. A metal black chimney also heated up in the sun, causing air to enter the UEU at the front of the UEU through a louvered vent, flow across each tray, and then exit at the back up through the chimney. The UEU was field-tested in a semi-arid temperate climate (Calgary, Canada) from 22 August to 5 November 2013. The average UEU evaporation rate was 3.2 L/day (0.66 mm), varying from 0.4 L/day (0.08 mm/day) on a cloudy day to 8.8 L/day (1.82 mm) on a sunny day. A multiple-regression analysis indicates that 63% of the UEU evaporation rate can be explained by changes in air temperature, wind speed and incoming solar radiation, thus allowing for predictions of the UEU's relative evaporation potential in other climates.