Additionally, the high pH kills pathogens and prevents biological processes that produce malodor. When calcium hydroxide is added to the urine only about the amount dissolves that is needed to reach the necessary high pH value. This allows providing a depot of the reagent in the stabilization reactor; thus, no expensive and complicated dosage mechanisms are required. Moreover, calcium hydroxide, also known as hydrated lime, is a cheap reagent and readily available worldwide.
The direct application of human urine as fertilizer is a common practice in many rural areas around the globe. However, the high water content of urine – no matter if stabilized or not – requires significant storage capacity and can make the collection and transport to the agricultural fields very costly. Volume reduction does not only reduce costs for storage and transport, but could also facilitate field application of the concentrated fertilizer. Standard volume reduction techniques are most often energy intensive processes, because they require high temperature (distillation) or pressure (reverse osmosis).
Offering an alternative to these processes, our approach uses forced convection to reduce the volume of the urine. The evaporation reactor consists of a stacked tray system to create a large surface area. Fans generating a high air stream accelerate the evaporation of the the water from the incoming urine. The offgas is filtered through an activated carbon filter, preventing the emission of organic contamination or malodour.
Once a month, the system needs to be serviced. Tasks are to refill the calcium hydroxide depot and to harvest the trays. The remaining end-product is a concentrate of inorganic and organic nutrients that can be used as fertilizer in agriculture.