Feasibility of Sulfite-Based Autotrophic Denitrification in Wastewater Treatment

Autotrophic sulfur-based denitrification has gained significant interest in recent years as an alternative to conventional heterotrophic denitrification using expensive supplementary electron donors such as methanol. Past research has addressed the potential of elemental sulfur (S⁰) and reduced sulfur compounds such as thiosulfate (S₂O₃^2-) and sulfide (S^2-) to serve as electron donors for denitrification in wastewater treatment systems. However, little is known about the efficacy of sulfite (SO₃^2-) as an electron donor. Unlike S⁰, SO₃^2- is soluble in water and could be applied to existing suspended growth systems, while avoiding the drawbacks of elevated costs and handling risks associated with other reduced sulfur compounds.

This thesis tested the hypothesis that sulfur-oxidizing denitrifiers such as Thiobacillus denitrificans can use SO₃^2- as an electron donor in an activated sludge system. It also explored whether SO₃^2-, known to have some antimicrobial properties, might inhibit nitrification. In experiments with four-stage, nitrifying and denitrifying sequencing batch reactors, SO₃^2- was found to effectively serve as an electron donor for autotrophic denitrification. However, SO₃^2- had a chronic inhibitory effect on nitrification. This may preclude its use for processes including nitrification, such as the Modified Ludzack-Ettinger (MLE) process. SO₃^2- may be more suitable for processes without nitrification, such as denitrifying biological filters.