Abstract
In order to avoid the loss of ammonia during solar drying of stored urine, low-tech stripping is suggested as a pretreatment process for ammonia recovery. The mass transfer of ammonia from stored urine with an initial pH of about 9 was tested in a simple closed vessel operated at 72°C, 74°C and 85°C. The specific urine/gas interface was 16.97 m-1. For ammonia absorption, a beaker with sulfuric acid was positioned in the gas phase of the container. After keeping the stored urine for 73 h at 85°C, the concentration of free ammonia (NH3) was reduced by more than 99%, and the pH of the stored urine decreased to 6.4 due to ammonia volatilization. Total ammonia (NH3+ NH4+) concentration was reduced by only 83% in the same period. At lower temperatures, the process was slower. Required treatment time can be reduced when specific gas/liquid interface is increased. Because it is known that water can be heated in solar boxes to temperatures above 90°C, this simple stripping apparatus is feasible to be operated with solar energy in remote areas with suitable climatic conditions where no electric power is available. As the area demand for solar “low-tech stripping” is less than 1 m2 per capita, this process can be looked at as a suitable pretreatment of stored urine prior to solar evaporation.
Highlights
Human urine separately collected in urine-diverting toilets or waterless urinals has been demonstrated to be a valuable fertilizer in agriculture [1]-[4] and may contribute to food security especially for poor farmers who cannot afford synthetic fertilizers [5]
Due to mass transfer of NH3 from the stored urine to the gas phase and subsequent absorption into the sulfuric acid, the TAN concentration as well as the pH in the stored urine was decreasing with time
At elevated temperatures (74 ̊C - 80 ̊C), ammonia can be recovered from stored human urine in a simple closed mass transfer apparatus without the requirement of devices which depend on electric power
Summary
Human urine separately collected in urine-diverting toilets or waterless urinals has been demonstrated to be a valuable fertilizer in agriculture [1]-[4] and may contribute to food security especially for poor farmers who cannot afford synthetic fertilizers [5]. A six months storage of urine at 20 ̊C is considered to make it a hygienically safe fertilizer for any crop, as there is even virucidal action of the ureolyzed urine due to its elevated pH of around 9 [6]. Vinnerås et al [7] have emphasized that the urine should not be diluted, because hygienization during storage was found to depend on ammonia concentration. Using human urine for fertilization requires separate urine collection, i.e. so-called “ecological sanitation” [5] or “resources-oriented sanitation” systems. The urine (or yellow water)—consisting mainly of water—has to be transported from the collection sites to the fields by trucks causing considerable energy consumption when this distance is long. A feasible concentration process for enrichment of the nutrients and transporting only the concentrate to the fields would help to save energy
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