With a growing number of landfills reaching the mature stage, finding feasible alternatives for effective treatment of mature landfill leachate has become an imminent challenge for landfill management worldwide. Due to its special characteristics of low levels of biodegradable organic matters but extremely high levels of ammonia, removing nitrogen from mature landfill leachate is especially challenging. Focusing on the nitrification process, this study examined the nitrification efficiency of the moving bed biofilm reactor (MBBR) inoculated with biocarriers from a municipal wastewater treatment plant (MWTP). The MBBR was operated to treat mature landfill leachate over a period of 126 days to evaluate its nitrification efficiency in five phases with variable operation conditions in terms of running mode, hydraulic retention time, carbon source, duration of aeration and agitation, pH, and influent ammonia concentration. Our study results have indicated that the MBBR could tolerate the large fluctuations in ammonium concentrations at high levels and maintained a nitrification efficiency of above 60% under various operation conditions. The abundance of ammonia-oxidizing bacteria such as beta-Proteobacteria and nitrite-oxidizing bacteria such as alpha-Proteobacteria in the biofilm sludge facilitated the nitrification process in the MBBR. As more MWTPs are expected to install MBBRs in the near future, inoculating the MBBR with biocarriers from MWTPs for mature landfill leachate treatment may provide a viable solution to the imminent risk of secondary landfill leachate pollution in China, and possibly other countries around the world. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.