Passive solar-thermal concentration of human urine for decentralised sanitation using an air-closet evaporation architecture

Human urine is composed largely of water, making volume reduction a key bottleneck in its decentralised treatment and resource recovery. Here, we present an air-closet evaporation architecture for concentrating source-separated acidified urine under natural climatic forcing without external energy input. We evaluated the evaporator during a 142-day field campaign under arid conditions spanning austral autumn to winter, using continuous gravimetric monitoring and concurrent meteorological observations to resolve evaporation dynamics across diurnal, seasonal, and concentration-dependent timescales. Evaporation was strongly diurnal, with median rates of 276 g h −1 m −2 by day and 50 g h −1 m −2 by night, peaking near local solar noon at 400–450 g h −1 m −2 and reaching maxima of 950 g h −1 m −2 . Mean evaporation rates declined from 4.6 to 2.8 kg d −1 m −2 as radiative forcing weakened through the season. The system achieved substantial mass reduction, reaching up to 30-fold concentration and producing a readily recoverable solid product upon complete dehydration. Progressive concentration, however, did not consistently suppress evaporation, indicating that short-term performance was governed more strongly by environmental forcing than by increasing solute concentration. Shortwave downward radiation was the dominant predictor of daytime evaporation, followed by air temperature, whereas relative humidity had less additional explanatory power. Using these predictors, we developed an empirical model and a global map to identify where passive solar-thermal evaporation of urine is most viable, providing an initial basis for climate-responsive siting and adaptation of the air-closet concept.