Steel slag enhanced simultaneous nitrogen and phosphorus removal from low C/N wastewater by sulfur-driven autotrophic denitrification

Abstract

Sulfur-driven autotrophic denitrification (SAD) has been widely applied for treating low C/N wastewater. However, it faces issues like high alkalinity consumption and poor phosphorus removal. Steel slag, an industrial waste characterized by high alkalinity, high hardness and large specific surface area, has the potential to combine with the SAD process to enhance the removal efficiency of nitrogen and phosphorus. In this study, a steel slag/sulfur-based autotrophic denitrification (SS-SAD) reactor with different sulfur/steel slag ratios was set up to investigate the effects of steel slag on the nitrogen and phosphorus removal. The addition of steel slag significantly enhanced the nitrogen and phosphorus removal performance of the SAD process. The average NO₃⁻-N removal efficiency of the SS-SAD reactor remained at over 99.0 % and the removal efficiency of PO₄³⁻-P could reach up to 85.2 %. The addition of steel slag promoted the secretion of extracellular polymeric substances, which was 34.2–51.3 mg/g VSS and the ratio of protein to polysaccharide was 6.0–18.9. The activity of relevant functional enzymes, nitrate reductase, nitrite reductase and sulfur oxidase, was also enhanced, which was 2.7–3.7, 0.8–1.1 and 3.3–5.4 U/g VSS, respectively. Autotrophic denitrifying bacteria, such as Ferritrophicum and Thiobacillus, could be effectively enriched after the addition of steel slag.