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 co llected from a local landfill site in Hong Kong. Two experiments were carri ed out to study (1) the inhibition of microbial activity of activated sludg e by NH4+-N and (2) the chemical precipitation of NH4+-N from leachate as a preliminary treatment prior to the activated sludge process. The experimen tal results demonstrated that the efficiency of COD removal decreased from 97.7% to 78.1%, and the dehydrogenase activity of activated sludge decrease d from 9.29 to 4.93 mug TF/mg MLSS, respectively, when the NH+4-N concentra tion increased from 53 to 800 mg/l. The experiment also demonstrated that t he NH4+-N in the leachate can be quickly precipitated as MgNH,P04 6H201 aft er addition of MgCI, 6H20 f NaZHP04 12H20. The NH4+-N concentration was red uced from 5618 to 112 mg/l within 15 min when a molar ratio of Mg2+:NH4+:PO 43- = 1 : 1 : 1 was used. The optimum pH to reach the minimum solubility of MgNH4PO4 . 6H(2)O down arrow was found to be in the range of 8.5-9.0. Atte ntion should be given to the high salinity formed in the treated leachate b y using MgCl2 . 6H(2)O + Na2HPO4 . 12H(2)O, which may affect microbial acti vity in the following biological treatment processes. Using two other combi nations of chemicals [MgO + 85%H3PO4 and Ca(H2PO4)(2) . H2O + MgSO4 . 7H(2) O] could minimise salinity generation after precipitation, while they were less efficient for NH4+-N removal. (C) 2001 Elsevier Science Ltd. All right s reserved.