A novel configured fluidized bed reactor (FBR) with granular rubber as the fluidized media was operated without internal recirculation to achieve denitrification. This FBR could operate under a low hydraulic retention time (HRT) of 50 min due to the low rubber media density and absence of recirculation. Synthetic nitrate-rich wastewater with a fixed nitrate (NO3-N) concentration and varying COD concentrations was fed into the FBR. The nitrate removal profile showed a rapid nitrate reduction at the bottom of the reactor with a high performance under the low HRT. Different microbial communities were identified using Illumina Miseq sequencing. The dominant microorganisms belonged to the Beta-and Gamma-proteobacteria classes and played important roles in nitrate reduction. Acidovorax was abundant at low COD: NO3-N ratios, while Rhizobium and Zoogloea were dominant at high COD: NO3-N ratios. The COD: NO3-N ratio strongly influenced the composition of the microbial community including the dominant species. (C) 2016 Elsevier Ltd. All rights reserved.