TOWARDS A ZERO-DISCHARGE DRY TOILET: EVAPORATION OF SOURCE-SEPARATED URINE
We used passive solar evaporation to remove water from source-separated human urine. The resulting solid product was sterile, had almost no odor, and was mostly comprised of salt (Na and Cl) and nutrients (N, P and K). Nitrogen loss, primarily by ammonia volatilization, significantly decreased the amount of N relative to P and K in the solid product.
The urine evaporation unit, which consisted of vertically stacked plastic cafeteria-style trays, was designed to create a large evaporation surface within a small footprint. A Plexiglas® enclosure permitted passive solar heating, while passive heating of a black metal chimney creates upward airflow through the unit. The unit was field-tested in a semi-arid temperate climate (Calgary, Canada) from August to November, 2013, with an average daily evaporation rate of about three litres per day (ranging from 8.8 to 0.4 L/day). The evaporation rate was related to temperature, wind speed and solar radiation, allowing for prediction of evaporation potential in other climates.