The aim of this study is to investigate the stability of partial nitritation in a lab-scale sequencing batch reactor (PN-SBR) fed with high ammonium strength old municipal landfill leachate without pH adjustment, bicarbonate addition and low dissolved oxygen control. Long sludge retention of over 120 days was applied for the PN-SBR. The PN-SBR, which run over 250 days used the old urban landfill leachate containing 3096 mg NH₄⁺-N l⁻1, 12960 mg HCO₃ l⁻1, and influent HCO₃⁻:NH₄⁺ molar ratio of 1.06. After 75 days of operation, the partial nitritation achieved stability when the effluent nitrite to ammonium molar ratio obtained in the range of 1.0–1.32, which were suitable for feeding to an anammox reactor. The effluent NO₃⁻-N concentration was always below 20 mg N l⁻1. The maximum FA concentration reached 506 mg NH3-N l−1 at hour 26th during the cycle of 73 h. Even though old land fill leachate mainly contained refractory COD, COD removal efficiency achieved about 11%. The OUR tests show that the maximum growth rate μmax, Ko, Ks and yield coefficient YX/N of AOB were 0.12 day−1, 1.35 mg O2 l−1, 53 mg NH4+-N l−1 and 0.26 mgVSS mg−1 NH4+-N, respectively.
