Optimization of process parameters for mature landfill leachate pretreatment using MAP precipitation

Abstract

Chemical precipitation is a useful technology as a pretreatment to treat mature landfill leachate with high concentrations of ammonium-nitrogen (\mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N}) and refractory organic compounds. Orthogonal experiments and factorial experiments were carried out to determine the optimal conditions enhancing the magnesium ammonium phosphate (MAP) precipitation process, and the experimental results demonstrated that the removal rate of \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N} was more than 85% when MgO and NaH₂PO₄·2H₂O were applied as external sources of magnesium and phosphorous under the optimal conditions that molar ratio n(Mg)∶n(N)∶n(P) = 1.4∶1∶0.8, reaction time 60 min, original pH of leachate and settling time 30 min. In the precipitation process, pH could be maintained at the optimal range of 8–9.5 because MgO could release hydroxide ions to consume hydrogen ions. Calcium ions and carbonate ions existed in the leachate could affect the precipitation process, which resulted in the decrease of \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N} removal efficiency. The residues of MAP sediments decomposed by heating under alkaline conditions can be reused as the sources of phosphorous and magnesium for the removal of high concentrations of \mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N}, and up to 90% of ammonium could be released under molar ratio of n[OH]∶n[MAP] = 2.5∶1, heating temperature 90°C and heating time 2h.

Keywords

magnesium ammonium phosphate precipitation / mature landfill leachate / optimization / ammonium-nitrogen

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