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 5618 to 112 mg/l within 15 min when a molar ratio of Mg²⁺: NH₄⁺: PO₄^3- = 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.