MAP Precipitation from Landfill Leachate and Seawater Bittern Waste

Abstract

The leachates generated at Hong Kong landfill sites contain high strength of ammonium-nitrogen (NH4 +-N) over 3,000 mg l−1 and are generally inhibitive to most biological treatment processes. To remove the NH4 +-N from the landfill leachates and also recover the nitrogen as a struvite fertilizer, a lab-scale study was performed to investigate the feasibility of NH4 +-N removal and struvite crystallization using different magnesium sources. Three combinations of chemicals, MgCl2·6H2O+Na2HPO4·12H2O, MgO+H3PO4 and MgSO4·7H2O+Ca(H2PO4)·H2O, were first used at different molar ratios to precipitate NH4 +-N from the leachate. The experimental results indicated that NH4 +-N was removed by 92%, 36% and 70% respectively at pH 9.0 and a molar ratio of Mg:N:P−=1:1:1. Two synthetic seawater bittern wastes containing Mg2+ at 9,220 mg l−1 and 24,900 mg l−1 respectively were then used as a magnesium source, while 85% H3PO4 chemical was used as a phosphorus source. The results revealed that NH4 +-N was removed by 80% and 72% respectively, while a molar ratio of Mg:N:P=1:1:1 was applied. In the final stage of experiments, the magnesium-ammonium-phosphate (MAP) precipitates were examined by SEM, EDS and XRD. The SEM micrographs of the MAP precipitates showed a typical morphology of elongated tubular and short prismatic crystals. The EDS and XRD results indicated that the chemical composition of the MAP precipitates depended on the chemicals used and experimental conditions. The study confirmed that the recovery of NH4 +-N from landfill leachate and seawater bittern wastes could be effectively achieved by MAP precipitation to obtain struvite crystals with a composition of 5.1%N, 10.3%Mg and 16.5%P.