Passive evaporation of source-separated urine from dry toilets: a lab study

A methodology for evaporating human urine from dry toilets using gravity-drainage through vertically stacked plastic ‘cafeteria-type’ trays was tested. A thin layer (500 g) of medium-grained sand on the bottom of each tray enhanced evaporation, ammonia stabilization and solid product removal. A prototype laboratory unit initially evaporated up to 8.5 L m⁻² d⁻¹ but decreased to 1.5 L m⁻² d⁻¹ over time as salinity increased. The evaporation process produces a dark, highly saline, brine solution before drying to a solid product. The solid product has almost no odor and is mostly comprised of Na, Cl, N, P and K. Nitrogen loss, primarily by ammonia volatilization, significantly decreased the amount of N relative to P and K in the brine and solid product. About 90% of the NH₄/NH₃ initially present in the input urine was lost in the evaporator system.