Ammonium Removal from Landfill Leachate by Chemical Precipitation

The landfill leachate in The Khajod Solid Waste Disposal Site, Surat usually contains quite high NH₄⁺-N concentration, which is well known to inhibit nitrification in biological treatment processes. A common pre-treatment for reducing high strength of ammonium (NH₄⁺-N) is by an air-stripping process. However, there are some operational problems such as carbonate scaling in the process of stripping. For this reason, some technical alternatives for NH₄⁺-N removal from leachate need to be studied. In this study, a bench-scale experiment was initiated to investigate the feasibility of selectively precipitating NH₄⁺-N in the leachate collected from a local landfill in Surat as magnesium ammonium phosphate (MAP). In the experiment, three combinations of chemicals, MgCl₂.6H₂O + Na₂HPO₄.12H₂O, MgO + 85% H₃PO₄, and MgO + Na₅P₃O₁₀, were used with the different stoichiometric ratios to generate the MAP precipitate effectively. The results indicated that NH₄⁺-N contained in the leachate could be quickly reduced from 1108 mg/l to 40 mg/l within short period of time, when MgCl₂.6H₂O + Na₂HPO₄.12H₂O were applied with a Mg²⁺:NH₄⁺:PO₄^3- mole ratio of 1:1:1. The pH range of the minimum MAP solubility was discovered to be between 8.5 and 9.0. Attention should be given to the high salinity formed in the treated leachate by using MgCl₂.6H₂O + Na₂HPO₄.12H₂O, which may affect microbial activity in the following biological treatment processes. The other two combinations of chemicals [MgO + 85% H₃PO4 and MgO + Na₅P₃O₁₀] could minimize salinity after precipitation, but they were less efficient for NH₄⁺-N removal, compared with MgCl₂.6H₂O + Na₂HPO₄.12H₂O. COD had reduced up to 50% during this precipitation. It was found that the sludge of MAP generated was easily settled within 30 minutes to reach its solids content up to 50%.