Fate of nitrogen during volume reduction of human urine using an on-site volume reduction system.

This study was carried to assess the effect of a mixture of salts, urea and creatinine on water evaporation from urine using an on-site volume reduction system in long-term experiments. Subsequently, the fate of nitrogen during volume reduction of urine was also assessed. The water evaporation rate, salt accumulation in the gauze sheet, concentrations of urea and ammonia-N, and pH of urine were measured periodically. Based on the results, a mass balance of nitrogen in concentrated urine was calculated for a moderate evaporating condition. The results revealed that steady-state evaporation was observed throughout the experiment period without any inhibition due to salt accumulation. Salt concentration in the gauze sheet reached steady-state illustrating the possibility of salt falling back to the tank from the sheet. No significant reduction of urea was observed for a moderate evaporating condition, which indicates inhibition of urea hydrolysis by the high concentration of the mixture of salts, urea and creatinine in the urine. In contrast, for a low evaporating condition, the pH of the urine increased to 8.9, which indicates early urea hydrolysis, causing an offensive odour and ammonia loss to the air. In simple storage experiments, a mixture of salts, urea and creatinine amounting to 227-334 g L(-1) in urine inhibited urea hydrolysis, even with faecal contamination, at 25 degrees C, while urine samples containing a mixture of salts, urea and creatinine at less than 227 g L(-1) did not provide strong inhibition of hydrolysis.

Performance evaluation of an on-site volume reduction system with synthetic urine using a water transport model.

The parameters of a model of the transport of water from a wet cloth sheet to the air, developed for deionized water, to establish design procedures of an on-site volume reduction system, were identified for high salt concentrations present in synthetic urine. The results showed that the water penetration was affected neither by the salts, urea or creatinine present in the synthetic urine nor by the salts accumulated on the surface of the vertical gauze sheet. However, the saturated vapour pressure decreased, leading to reduction in the evaporation rate, which occurred as a result of the salts accumulating on the surface of the vertical gauze sheet. Furthermore, a steady-state evaporation condition was established, illustrating salts falling back to the tank from the vertical gauze sheet. Accordingly, the existing design procedure was amended by incorporating the calculation procedure for the saturated vapour pressure using Raoult's law. Subsequently, the effective evaporation area of the vertical gauze sheet was estimated using the amended deign procedures to assess feasibility. This estimation showed that the arid, tropical, temperate and cold climates are suitable for the operation of this system, which require requires a small place at household level for 80% volume reduction of 10 L of urine per day for 12 hours' operation in the daytime.

Rational design of an on-site volume reduction system for source-separated urine.

Human urine contains nitrogen, phosphorus and potassium, which can be applied as fertilizer in agriculture, replacing commercial fertilizer. However, owing to the low nutrient content of the urine, huge quantities must be transported to farmland to meet the nutrient demand of crops. This highly increases the transportation cost for the farmers. To address the transportation issue, a new on-site volume reduction system was tested at the laboratory scale based on water evaporation from vertical gauze sheets. A mathematical water transport model was proposed to evaluate the performance of the system. The mass transfer coefficient and the resistance of water flow through the sheet in the water transport model were obtained from the experiments. The results agreed with the simulated data, thereby confirming the proposed model. The model was then applied to the dry climate of southern Pakistan, having an air temperature of 30-40 degrees C and air humidity of 20-40%, for an 80% volume reduction of 10 L urine per day, which corresponds to a family of 10 members (average for a household in Pakistan). The findings revealed that the estimated size of the vertical sheet is 440-2060 cm2, which is only a small area for setting up the system at a household level.