Partial nitrification and denitrification of mature landfill leachate using a pilot-scale continuous activated sludge process at low dissolved oxygen

•Mature landfill leachate nitrogen removal via nitrite pathway at low DO was achieved.•Excellent NH4+-N removal and nitrite accumulation was obtained at DO of 0.3–0.5mg/L.•Actual HRT of the first oxic reactor mainly decided the operational limit.•High-throughput sequencing analysis was used for bacterial community variation.•Genus Nitrosomonas was responsible for NH4+-N removal and nitrite accumulation. Controlling of low dissolved oxygen (DO) levels (0.1–0.5mg/L), a cost-effective strategy, was applied to a pilot-scale anoxic-oxic-oxic-anoxic process for partial nitrification and denitrification of mature landfill leachate. High ammonium removal efficiency, stable nitrite accumulation rate and total nitrogen removal efficiency was higher than 95.0%, 90.0% and 66.4%, respectively, implying potential application of this process for nitrogen removal of mature landfill leachate. Efficient nitrite accumulation in the first oxic reactor depended on low DO conditions and sufficient alkalinity. However, operational limit was mainly decided by actual hydraulic retention time (AHRT) of the first oxic reactor and appeared with AHRT less than 13.9h under DO of 0.3–0.5mg/L. High-throughput sequencing analysis demonstrated significant change of bacterial diversity in the first oxic reactor after a long-term operation and dominant bacteria genus Nitrosomonas was shown to be responsible for NH4+-N removal and nitrite accumulation under low DO levels.