Complete treatment of sulfur dioxide (SO2) from flue gases in a two-stage process consisting of a biotrickling filter followed by biological post-treatment unit was investigated. The biotrickling filter could remove 100% of influent S02from simulated flue gas at an empty bed residence time of 6 s for a concentration range of 300-1000 ppmv. All the absorbed SO2 was recovered in the biotrickling filter liquid effluent as sulfite (a product of chemical reaction of SO2) and sulfate (product of biological oxidation of sulfite). The biotrickling filter liquid effluent was further processed biologically in a single post-treatment unit consisting of a combined anaerobic and microaerophilic reactor for the simultaneous reduction of sulfate and sulfite to sulfide and oxidation of sulfide to elemental sulfur. The post-treatment unit could effectively treat the biotrickling filter effluent and produce elemental sulfur. The sulfur production efficiency of the reactor reached about 80% of the SO2 treated. This new biological treatment system seems to be a promising alternative for flue gas desulfurization.