Multi-stage recovery of ammonia–potassium liquid fertilizer and phosphate mineral from real human urine

Abstract

This study explores a novel multi-stage process for recovering valuable nutrients–nitrogen, phosphorus, and potassium–from real hydrolyzed urine as value-added products. The approach utilizes a combination of membrane contactor, zeolite ion exchange, and mineral precipitation techniques. A closed-loop system was established by reusing the acid regeneration solution from ion exchange as the acid-stripping solution in the hollow fiber membrane contactor (HFMC), thereby minimizing chemical usage. Ammonia recovery using the HFMC achieved over 90 % removal across three cycles from hydrolyzed urine. Zeolite columns of chabazite and clinoptilolite demonstrated consistent potassium recovery from HFMC-treated urine, with slightly higher uptake by chabazite compared with clinoptilolite. This suggests zeolite selection can be based on cost and availability. The regeneration of the potassium-saturated zeolite columns using sulfuric acid exhibited rapid and substantial amounts of potassium desorption. Potassium regeneration remained stable over two cycles, with potassium concentrations reaching up to 14 g/L. The release of other ions, such as sodium, was minor compared with potassium, highlighting the minimal impact of sodium interference. The combined ammonia–potassium liquid fertilizer exhibited a favorable N:K mass ratio (3.6 % N and 0.7 % K), with negligible amounts of other ions, making it suitable for facilitating plant growth. Iron phosphate precipitation, a promising alternative resource for fertilizer or lithium iron phosphate batteries, was successfully achieved. Iron doses were more effective in precipitating phosphate at neutral pH than basic pH, reaching over 90 % phosphate removal. This study provides a promising approach for recovering valuable resources from human urine, promoting a more sustainable approach to wastewater management and nutrient recycling.