Sulfur-based Autotrophic Denitrification

Sulfur-based autotrophic denitrification processes utilize autotrophic denitrifiers, such as Thiobacillus denitrificans and Thiomicrospira denitrificans, to reduce nitrate to nitrogen gas. Bench-scale experiments were conducted to evaluate the sulfur-based autotrophic denitrification process. This paper addresses three issues: (1) start-up and operation of a CSTR to provide an adequate seed source of autotrophic denitrifiers, (2) optimum ratios of elemental sulfur to limestone for various hydraulic loading rates to determine effectiveness of above-ground treatment of nitrate-contaminated ground water, and (3) feasibility studies on in situ remediation of nitrate-contaminated surface water using pond sulfur-based systems. Thiosulfate in a CSTR system was chosen over elemental sulfur in a batch system to provide a sufficient seed source of autotrophic denitrifiers. Four individual upflow fixed-bed reactors with sulfur:limestone ratios of 1:3, 1:1, 3:1, and sulfur only were operated at empty bed hydraulic loading rates of 17.0 hours, 9.6 hours, and 3.1 hours. Initial concentrations of nitrate-nitrogen were 35 mg/l. Results indicate that optimal conditions occurred with a sulfur:limestone ratio of 3:1 and that limestone buffering is critical. A nitrate-nitrogen removal rate of 17.8 mg/l/hr was achieved. Studies on batch pond reactors with complex sediment media reveal denitrification occurring under both aerobic and anaerobic conditions. The addition of sulfur and limestone enhances the denitrification rates in the system and further addition of a seed of autotrophic denitrifiers accelerates the processes even more.