TOWARDS A ZERO-DISCHARGE DRY TOILET: EVAPORATION OF SOURCE-SEPARATED URINE

More than 2.5 billion people in the world lack improved sanitation. While dry toilets offer ‘improved’ sanitation and hygiene, they can contaminate groundwater and cause surface water eutrophication. The urine-diverting toilet is being increasingly adopted with the advantage that source-separated solids and urine are more easily managed separately. A ‘zero-discharge’ toilet would be highly valued.

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.