RECYCLING OF FGD GYPSUM TO CALCIUM-CARBONATE AND ELEMENTAL SULFUR USING MIXED SULFATE-REDUCING BACTERIA WITH SEWAGE DIGEST AS A CARBON SOURCE

A combined chemical and biological process for the recycling of flue g as desulfurization (FGD) gypsum into calcium carbonate and elemental s ulfur is demonstrated. In this process, a mixed culture of sulfate-red ucing bacteria (SRB) utilizes sewage digest as its carbon source to re duce FGD gypsum to hydrogen sulfide. The sulfide is then oxidized to e lemental sulfur via reaction with ferric sulfate, and accumulating cal cium ions are precipitated to calcium carbonate using carbon dioxide. Employing anaerobically digested-municipal sewage sludge (AD-MSS) medi um 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 wi th continuous addition of both AD-MSS medium and gypsum exhibited sulf ate reduction rates as high as 1.3 kg FGD gypsum m(-3) day(-1). The in creased biocatalyst density afforded by cell immobilization in a colum nar reactor allowed a productivity of 152 mg SO4 dm(-3) h(-1) or 6.6 k g 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 g ypsum sludge, combined with the use of a low-cost carbon source and fu rther improvements in reactor design, promises to offer an attractive alternative to the landfilling of FGD gypsum.