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The effective retention of anaerobic ammonia oxidizing (anammox) bacteria and its high sensitivity to toxic substances and oxygen posed a major challenge to the application of partial nitrification combined with anammox (PN/A) in mature landfill leachate treatment, although it is a promising and efficient nitrogen removal process. In this study, a single-stage PN/A process based on expanded granular sludge bed was proposed to treat the mature landfill leachate. During the last phase, when the NH+ 4-N concentration of mature landfill leachate in influent was 1150.0 mg/L, the nitrogen removal efficiency (NRE) was 83.64% with 1.07 kg N/(m3·d) nitrogen removal rate (NRR). The activity of anammox bacteria (AnAOB) and ammonia oxidizing bacteria (AOB) was 9.21 ± 0.22 mg N/(gVSS·h) and 14.34 ± 0.65 mg N/(gVSS·h), respectively. The bacteria produced a high amount of tightly bound extracellular polymeric substance (TB-EPS) i.e., 4071.79 mg/(g·VSS). This helped to create granular sludge and provided favorable spatial conditions for the distribution of functional bacteria that were adapted to different environments. Due to the efficient retention of functional bacteria by the granular sludge, the relative abundance of Ca.Brocadia and Ca.Kuneneia was 1.71% and 0.31%, respectively. Redundancy analysis (RDA) and microbial correlation network diagram showed that the relative abundance of Ca. Kuenenia , Nitrosomonas and Truepera had a stronger positive correlation with the increase of the proportion of mature landfill leachate added to the influent. Overall, the PN/A process based on granular sludge provides an effective method for autotrophic biological nitrogen removal from mature landfill leachate. [Display omitted] • Mature landfill leachate was treated via one-stage AOB-AnAOB granular sludge. • Ca. Jettenia has shown strong tolerance to mature landfill leachate. • TIN removal efficiency of 83.64% was achieved when NLR was 1.23 kg N/(m3·d). [ABSTRACT FROM AUTHOR]