This study investigated the denitrification potential of an optimized solid-phase filter with activated carbon fiber (ACF) setting at the top and bottom under hydraulic retention time (HRT) shorter than 60 min from the synthetic secondary effluent. The results showed that, high nitrate removal efficiencies (>99 %) were obtained in reactors filled with poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) 3020 at initial stage of start-up. During stable operation under either HRT 40 min or 30 min, the reactor with PHBV 3020 as carrier achieved 90 %-100 % nitrate removal without nitrite accumulation. Moreover, it reached the maximum denitrification rates of 41.8 mg-N/Lh at HRT of 30 min which was significantly higher than that of the reactors filled by ACF/quartz grains and quartz grains at top and bottom. Microbial community in this reactor was dominated by the phylum Proteobacteria as well as Bacteroidetes. The ACF worked as favorable habitat for denitrifying bacteria Hydrogenophaga, Thiothrix, Desulfomicrobium and Sulfurimonas. The genera Ottowia, Limnohabitans, and Propioniyibrio attached to ACF at top tended to degrade remaining organics. The optimized solid-phase denitrification filter with ACF showed good application potential for nitrate removal from the secondary effluent. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.