Electron donors for biological sulfate reduction

Biological sulfate reduction is widely used for treating sulfate-containing wastewaters from industries such as mining, tannery, pulp and paper, and textiles. In biological reduction, sulfate is converted to hydrogen sulfide as the end product. The process is, therefore, ideally suited for treating metal-containing wastewater from which heavy metals are simultaneously removed through the formation of metal sulfides. Metal sulfide precipitates are more stable than metal hydroxides that are sensitive to pH change. Theoretically, conversion of 1 mol of sulfate requires 0.67 mol of chemical oxygen demand or electron donors. Sulfate rich wastewaters are usually deficient in electron donors and require external addition of electron donors in order to achieve complete sulfate reduction. This paper reviews various electron donors employed in biological sulfate reduction. Widely used electron donors include hydrogen, methanol, ethanol, acetate, lactate, propionate, butyrate, sugar, and molasses. The selection criteria for suitable electron donors are discussed.