Biological Sulfur Recovery Unit (Bio-SRU) with Thiobacillus as biocatalysts is believed to be suitable for handling hydrocarbon feed gas with moderate to low-H2S concentration. The purpose of this study is to evaluate the effect of SO4 2-, PO4 3-, and Fe2+ concentration on Bio-SRU performance particularly on the conversion of sulfide to elemental sulfur. Biological pathway of sulfur recovery from natural gas-treating facility wastewater was then studied by measuring the pH and redox potential of the sample. It shows that the variation of SO4 2- concentration does not significantly affect the conversion process, while a small increment of PO4 3- (KH2PO4 1.7 g/L and K2HPO4 1.36 g/L) results in a higher yield of elemental sulfur production. Fe2+also gave a significant effect on sulfur formation and conversion rate.