Ammonium removal from landfill leachate by chemical precipitation

The landfill leachate in Hong Kong 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 Hong Kong as magnesium ammonium phosphate (MAP). In the experiment, three combinations of chemicals, MgCl2•6H2O+Na2HPO4•12H2O, MgO+85% H3PO4, and Ca(H2PO4)2•H2O+MgSO4•7H2O, 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 5618 to 112 mg/l within 15 min, when MgCl2•6H2O and Na2HPO4•12H2O were applied with a Mg2+:NH4+:PO43− mol 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 and Na2HPO4•12H2O, which may affect microbial activity in the following biological treatment processes. The other two combinations of chemicals [MgO+85% H3PO4 and Ca(H2PO4)2•H2O+MgSO4•7H2O] could minimise salinity after precipitation, but they were less efficient for NH4+–N removal, compared with MgCl2•6H2O and Na2HPO4•12H2O. COD had no significant reduction during this precipitation. It was found that the sludge of MAP generated was easily settled within 10 min to reach its solids content up to 27%. The other characteristics including capillary suction time (CST) and dry density (DD) of the MAP sludge were also tested. The experimental results indicate that the settled sludge is quite solid and can be directly dumped at a landfill site even without any further dewatering treatment.