Recovery of ammonium-nitrogen from landfill leachate as a multi-nutrient fertilizer

Leachates generated in Hong Kong landfill sites contain high strength of ammonium-nitrogen (NH₄⁺-N) in the range of 2000-5000 mg 1^-1, which could be used to produce nitrogen-fertilizer. To recover the NH₄⁺-N from the leachate, a lab-scale study was performed to investigate the efficiency of magnesium ammonium phosphate (MAP) precipitation using three combinations of chemicals: MgCl₂·6H₂O+Na ₂HPO₄·12H₂O, MgO+85% H₃PO₄ and MgSO₄·7H₂O+ Ca(H₂PO₄)·H₂O. The results indicated that the NH₄⁺-N was recovered by 92, 36 and 70%, respectively, using the selected chemicals at pH 9.0 and a molar ratio of Mg:N:P = 1:1:1. The MAP precipitate (MgNH₄PO₄·6H₂O) as struvite was examined by scanning electron microscopy and the results showed that the MAP precipitate had a typical morphology of elongated tubular and short prismatic crystals. The MAP precipitate was also analyzed by energy dispersive spectroscopy and the analytical results indicated its composition of 4.9% N, 8.6% Mg and 16% P, which was similar to the standard struvite composition of 5.7% N, 9.9% Mg and 12.6% P. To investigate the fertility of the MAP precipitate, a set of pot trial tests was conducted. Four quick-growth vegetables, Chinese flowering cabbage (Brassica parachinensis), Chinese chard (Brassica rapa var. chinensis), water spinach (Ipomoea aquatica) and water convolvulus (Ipomea aquatica, I. reptans), were planted in sandy clay (Red Earth) and the MAP precipitate was applied as a fertilizer. The germination and growth of the selected vegetables in the pots with MAP showed significantly greater rates than those in the pots without MAP as control tests. The experiment also demonstrated that over-dosing of about 2-8 times of MAP in the soil did not cause any problems with water spinach (Ipomoea aquatica) growth, due to its limited solubility in water. MAP application as a fertilizer did not result in more heavy metals in the vegetables than those from soil and model fertilizer, whereas the higher levels of P and Mg in the vegetables would stimulate vegetables' growth. This study confirmed the feasibility of recovering NH₄⁺-N from landfill leachate effectively by chemical precipitation, and also the feasibility of applying the MAP precipitate as a multi-nutrient fertilizer for vegetable growth.