Concentrating stabilized urine with reverse osmosis: How does stabilization method and pre-treatment affect nutrient recovery, flux, and scaling?

Abstract

Human urine can be used as a fertilizer, however, due to the high water content (97%), concentration is required to make transportation economically feasible. Reverse osmosis (RO) has been identified as an energy efficient concentration method. Furthermore, to maximize nitrogen recovery from source-separated urine it should be stabilized with an acid or base to prevent urea hydrolysis. However, the method of stabilization will have an impact on the downstream RO process. Calcium hydroxide is often used as a base stabilization method for human urine but would require pre-treatment to remove excess calcium and subsequent membrane scaling. Three pre-treatment methods such as air bubbling, NaHCO3 addition, and NH4HCO3 addition, were investigated in this study. Each method successfully reduced the scaling potential and air bubbling was determined to be the most effective method as it resulted in the highest nutrient recovery during concentration and did not require the addition of any chemicals. Base stabilization with air bubbling pre-treatment was then compared to urine stabilized with citric acid. Acid stabilized urine had a higher nitrogen recovery (7.6% higher). However, solids formed in the real acidified urine and during concentration a brown organic compound formed on the membrane surface. The crystals were identified as uric acid dihydrate and the brown organic fouling resulted in a significant decrease in permeate flux as compared to the base stabilized urine with air bubbling pre-treatment. At a 60% water recovery, 85.5% of the urea and 99.2% of the potassium was recovered in the brine stream and more than 99.9% of the phosphorus was recovered as a separate solid calcium phosphate fertilizer. Whilst nutrient recovery was higher with acid stabilization, the membrane fouling that occurred with this stabilization method meant that base stabilization with air bubbling pre-treatment was the preferred treatment option. It is recommended that acid stabilized urine be concentrated using evaporation processes instead.