Ammonium removal from landfill leachate by chemical precipitation

The landfill leachate in Hong Kong usually contains quite high NH4+-N conce ntration, which is well known to inhibit nitrification in biological treatm ent processes. A common pre-treatment for reducing high strength of ammoniu m (NH4+-N) is by an air-stripping process. However, there are some operatio nal problems such as carbonate scaling in the process of stripping. For thi s reason, some technical alternatives for NH4+-N removal from leachate need to be studied. In this study, a bench-scale experiment was initiated to in vestigate the feasibility of selectively precipitating NH4+-N in the leacha te collected from a local landfill in Hong Kong as magnesium ammonium phosp hate (MAP). In the experiment, three combinations of chemicals, MgCl2. 6H(2 )O+Na2HPO4. 12H(2)O, MgO + 85% H3PO4, and Ca(H2PO4)(2). H2O + MgSO4. 7H(2)O , were used with the different stoichiometric ratios to generate the MAP pr ecipitate effectively. The results indicated that NH4+-N contained in the l eachate could be quickly reduced from 5618 to 112 mg/l within 15 min, when MgCl2. 6H(2)O and Na2HPO4. 12H(2)O were applied with a Mg2+:NH4+:PO43- mol ratio of 1:1:1. The pH range of the minimum MAP solubility was discovered t o be between 8.5 and 9.0. Attention should be given to the high salinity fo rmed in the treated leachate by using MgCl2. 6H(2)O and Na2HPO4. 12H(2)O, w hich may affect microbial activity in the following biological treatment pr ocesses. The other two combinations of chemicals [MgO+85% H3PO4 and Ca(H2PO 4)(2). H2O + MgSO4. 7H(2)O] could minimise salinity after precipitation, bu t they were less efficient for NH4+-N removal, compared with MgCl2. 6H(2)O and Na2HPO4. 12H(2)O. COD had no significant reduction during this precipit ation. It was found that the sludge of MAP generated was easily settled wit hin 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 slu dge were also tested. The experimental results indicate that the settled sl udge is quite solid and can be directly dumped at a landfill site even with out any further dewatering treatment. (C) 1999 Elsevier Science Ltd. All ri ghts reserved.