Environmental analytical methods

Urea hydrolysis and recovery of nitrogen and phosphorous as MAPfrom stale human urine

LIU Zhigang , ZHAO Qingliang , WANG Kun , LEE Duujong , QIU Wei , WANG Jianfang


Received September 18, 2007,Revised November 12, 2007, Accepted , Available online

Volume 20,2008,Pages 1018-1024

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Laboratory-scale tests for magnesium ammonium phosphate (MAP) precipitation following urea hydrolysis of human urine were conducted using orthogonal experiment design. The effects of initial pH, temperature and the volumetric ratios of stale urine to fresh urine, on urea hydrolysis in urine were studied to determine the final hydrolysis time to recover most nitrogen from separated human urine by MAP. With a volumetric ratio of stale to fresh urine >10% and at temperature >20°C, urea hydrolysis could be completed in two days. Alkaline pH inhibited urea hydrolysis progress. The final pH values were all around 9.0 following urine hydrolysis, while the suspension pH might act as an indicator to detect the start and extent of urea hydrolysis. Over 95% of ammonium nitrogen and over 85% of phosphorus from hydrolyzed urine as MAP precipitate were obtained using MgCl2·6H2O and Na2HPO4·12H2O as precipitation agents at pH 8.5, molar ratio of Mg2+:NH4 +-N:PO4 3?-P at (1.2–1.3):1:1, mixing speed of 120 r/min, and precipitation time and reaction time of 3 h and 15 min, respectively. The precipitate has a structure resembling pure MAP crystal.

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