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Study of operational conditions for sulfur recovery in a bioreactor with sulfide oxidizing bacteria
Abstract
Biological desulfurization has been studied during the last decades as a good option to treat sulfide rich streams as those generated for example from biogas desulfurization, anaerobic digestion of sulfate-rich wastewater or wastewater from sewer systems. These streams cause negative environmental impacts and it is necessary to implement efficient removal methods. Partial oxidation of sulfide, in microaerated reactors with suspended sulfide oxidizing bacteria (SOB) is a good alternative to remove sulfide. Partial oxidation allows recovering sulfide as elemental sulfur, which is a value added product; SOB are suitable for the low nutritional requirement, low oxygen demand (as sulfur is the end product), high sulfide affinity and removal, among other advantages. In this work, the biological sulfide oxidation process was studied to find the optimal operational conditions for sulfur recovery.
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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.
The investigations described show that the formation of elemental sulfur from the biological oxidation of sulfide can be optimized by controling the redox state of the solution. The nonsoluble sulfur can be removed by gravity sedimentation and re-used as a raw material, i.e., in bioleaching processes. It was shown that, by supplying an almost stoichiometrical amount of oxygen to the recirculated gas phase, the formation of sulfate is minimized. The redox potential is mainly determined by the sulfide concentration because this compound has a high standard exchange current density with the platinum electrode surface. By maintaining a particular redox setpoint value, in fact, the reactor becomes a "sulfide-stat." It was shown that in a sulfide-oxidizing bioreactor the measured redox potential, using a polished redox electrode, is kinetically determined rather than thermodynamically. The optimal redox value for sulfur formation is between -147 and -137 mV (H2 reference electrode, 30 degrees C, pH 8). The presented results are currently used for controling several full-scale installations, which desulfurize biogas and high-pressure natural gas. Copyright 1998 John Wiley & Sons, Inc.