A bio-trickling filter (BTF) packed with polyhedral spheres was used to remove nitrogen oxides (NOx) from the flue gas of a coal-fired power plant. The BTF system consistently removed 64-95% of the NOx after start-up and acclimation under dynamic conditions (e.g., 120-240 m(3)/h flue gas flow rate and inlet 300-900 mg NOx/m(3)). Scanning electron microscopy of the biofilms that were formed showed a shift in the predominating bacteria. Analyses by PCR-denaturing gradient gel electrophoresis showed that the naturally-selected mixed cultures in the biofilm under a flue gas environment were mainly Klebsiella sp. and Pseudomonas sp.