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Abstract
Sulfate-rich wastewaters are generated by many industrial processes that use sulfuric acid or sulfate-rich feed stocks (e.g., fermentation or sea food processing industry). Also, the use of reduced sulfur compounds in industry, that is, sulfide (tanneries, kraft pulping), sulfite (sulfite pulping), or thiosulfate (pulp bleaching, fixing of photographs), contaminates wastewaters with sulfate. A major problem for the biological treatment of sulfate-rich wastewaters is the production of H2S. Gaseous and dissolved sulfides cause physical (corrosion, odor, increased effluent COD) or biological (toxicity) constraints that may lead to process failure. H2S is generated by sulfate-reducing bacteria, in both anaerobic and aerobic (anoxic microenvironments) wastewater treatment systems. No practical methods exist to prevent sulfate reduction. Selective inhibition of SRB by molybdate, transition elements, or antibiotics is unsuccessful at full scale. Selection of a treatment strategy for a sulfate-rich wastewater depends on the aim of the treatment. This can be (1) removal of organic matter, (2) removal of sulfate, or (3) removal of both. Theoretically, wastewaters with a COD/sulfate ratio of 0.67 or higher contain enough COD (electron donor) to remove all sulfate by sulfate-reducing bacteria. If the ratio is lower, addition of extra COD, for example, as ethanol or synthesis gas (a mixture of H2, CO2, and CO) is required. Complete COD removal in wastewaters with a COD/sulfate ratio of above 0.67 also requires methanogenic COD degradation. Methods to reduce sulfide toxicity and to allow optimal COD removal are presented. Sulfate can be removed from the wastestream by the coupling of a sulfide oxidation step to the sulfate reduction step. Sulfur can be recovered from the wastewater in case H2S is partially oxidized to insoluble elemental sulfur.