A combined chemical and biological process for the recycling of flue gas desulfurization (FGD) gypsum into calcium carbonate and elemental sulfur is demonstrated. In this process, a mixed culture of sulfate-reducing bacteria (SRB) utilizes sewage digest as its carbon source to reduce FGD gypsum to hydrogen sulfide. The sulfide is then oxidized to elemental sulfur via reaction with ferric sulfate, and accumulating calcium ions are precipitated to calcium carbonate using carbon dioxide. Employing anaerobically digested-municipal sewage sludge (AD-MSS) medium as a carbon source, SRB in serum bottles demonstrated an FGD gypsum reduction rate of 8 mg dm(-3) h(-1) (10(9) cells)(-1). A chemostat with continuous addition of both AD-MSS medium and gypsum exhibited sulfate reduction rates as high as 1.3 kg FGD gypsum m(-3) day(-1). The increased biocatalyst density afforded by cell immobilization in a columnar reactor allowed a productivity of 152 mg SO4 dm(-3) h(-1) or 6.6 kg FGD gypsum m(-3) day(-1). Both reactors demonstrated 100% conversion of sulfate, with 75-100% recovery of elemental sulfur and as high as 70% COD utilization. Calcium carbonate was recovered from the reactor effluent upon precipitation using carbon dioxide. The formation of two marketable products-elemental sulfur and calcium carbonate-from FGD gypsum sludge, combined with the use of a low-cost carbon source and further improvements in reactor design, promises to offer an attractive alternative to the landfilling of FGD gypsum.