Efficiency of biological treatment affected by high strength of ammonium-nitrogen in leachate and chemical precipitation of ammonium-nitrogen as pretreatment

Leachate samples with a high strength of ammonium-nitrogen (NH4+-N) were collected from a local landfill site in Hong Kong. Two experiments were carried out to study (1) the inhibition of microbial activity of activated sludge by NH4+-N and (2) the chemical precipitation of NH4+-N from leachate as a preliminary treatment prior to the activated sludge process. The experimental results demonstrated that the efficiency of COD removal decreased from 97.7% to 78.1%, and the dehydrogenase activity of activated sludge decreased from 9.29 to 4.93 μg TF/mg MLSS, respectively, when the NH4+-N concentration increased from 53 to 800 mg/l. The experiment also demonstrated that the NH4+-N in the leachate can be quickly precipitated as MgNH4PO4·6H2O↓ after addition of MgCl2·6H2O+Na2HPO4·12H2O. The NH4+-N concentration was reduced from 5 618 to 112 mg/l within 15 min when a molar ratio of Mg2+:NH4+:PO43−=1:1:1 was used. The optimum pH to reach the minimum solubility of MgNH4PO4·6H2O↓ was found to be in the range of 8.5–9.0. 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. Using two other combinations of chemicals [MgO+85%H3PO4 and Ca(H2PO4)2·H2O+MgSO4·7H2O] could minimise salinity generation after precipitation, while they were less efficient for NH4+-N removal.