Effect of initial pH and pH-adjusted acid on nutrient recovery from hydrolysis urine by combining acidification with...

Nutrient recovery from human urine is a promising pretreatment of domestic wastewater and provides a sustainable recyclability of nitrogen (N), phosphorus (P), and potassium (K). In this study, we present a method to recover all nutrients from hydrolysis urine (HU) in the form of solid products by combining acidification with evaporation-crystallization. The effect of initial pH (pH_init.) on N retention was investigated, and the optimal pH_init. was further determined by analysis of N retention efficiency. Additionally, crystallization process and product composition based on different pH-adjusted acids were also compared. The results revealed that pH_init. of HU was the key factor for N retention, and the optimal pH_init. was 4. In addition, compared with HCl and H₂SO₄, acidification by H₃PO₄ could effectively reduce energy consumption and improve nutrient content in urine-derived solid products (UDSPs) but increase the acid consumption. The major compositions (mass percentage, %) in UDSPs-Cl, UDSPs-S, and UDSPs-P were salammoniac (80%), lecontite (41%) and ammonium nitrate sulfate (30%), and biphosphammite (84%), respectively. The results also demonstrated that the method of mixing of UDSPs-Cl/UDSPs-S and UDSPs-P and addition of fillers such as calcite, ground limestone, or ground dolomite into them is suitable for improving nutrient balance and fertilizer efficiency.