Nitrate Removal from Waters Containing Sulfate Using Autotrophic Denitrification with Sulfide Return

In this study, a process has been proposed whereby the sulfide required for autotrophic denitrification is supplied by reducing the sulfate of influent water without the need to add an external sulfide source. The molar ratio of nitrate-to-sulfide was maintained at 1.6. The proposed system was operated continuously for 6 months, including two anoxic and anaerobic reactors with upward flow. The results indicate that the average amount of nitrate declined by 74%. The pH of 7-8 was more effective than a pH of 6 in removing the nitrate. As the hydraulic retention time was prolonged from 1.5 to 3 and was further prolonged to 5 h, the system efficiency was enhanced by removing the nitrate. An alkalinity consumption rate of 1.15 mg (as CaCO3) per mg of removed NO3--N was achieved. In the effluent water, the increased sulfate was 6.7 mg per mg of removed NO3--N, while the hardness was diminished by 2.85 mg (as CaCO3).