In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H₂S) is removed from sour gas and oxidized to elemental sulfur (S₈) by sulfide-oxidizing bacteria. Besides S₈, the byproducts sulfate (SO₄^2-) and thiosulfate (S₂O₃^2-) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S₈ by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S₈ formation was 96.6% on a molar H₂S supply basis; selectivity for SO₄^2- and S₂O₃^2- were 1.4 and 2.0% respectively. The selectivity for S₈ formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO₄^2- and S₂O₃^2- formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO₄^2- formation further decreased. In addition, chemical formation of S₂O₃^2- was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S₈ formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.

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