Potential use of sulfite as a supplemental electron donor for wastewater denitrification

Abstract

Biological denitrification typically requires the addition of a supplemental electron donor, which can add a significant operating expense to wastewater treatment facilities. Most common electron donors are organic, but reduced inorganic sulfur compounds (RISCs), such as sulfide (HS⁻) and elemental sulfur (S⁰), may be more cost-effective. S⁰ is an inexpensive and well characterized electron donor, but it provides slow denitrification rates due to its low solubility. A lesser-known RISC is sulfite (${\text{SO}}_{3}^{2 - }$), which can be easily produced from S⁰ by a simple combustion process. Unlike S⁰, ${\text{SO}}_{3}^{2 - }$ is highly soluble, and therefore may provide higher denitrification rates. However, very little is known about microbial denitrification with ${\text{SO}}_{3}^{2 - }$. Also, ${\text{SO}}_{3}^{2 - }$ is a strong reductant that reacts abiotically with oxygen and has toxic effects on microorganisms. This paper reviews ${\text{SO}}_{3}^{2 - }$ in the environment, ${\text{SO}}_{3}^{2 - }$ chemistry, microbiology, toxicity, and its potential use for denitrification. Since ${\text{SO}}_{3}^{2 - }$ is an intermediate in the sulfur oxidation pathway of most sulfur-oxidizing microorganisms, it is an energetic electron donor and it should select for a ${\text{SO}}_{3}^{2 - }$-oxidizing community. Our review of the literature, as well as our own lab experience, suggests that ${\text{SO}}_{3}^{2 - }$ can effectively serve as an electron donor for denitrification. Further research is needed to determine the kinetics of ${\text{SO}}_{3}^{2 - }$-based denitrification, its toxic threshold for sulfur-oxidizing microorganisms, and its potential inhibition of sensitive species such as nitrifying microorganisms and potential formation of nitrous oxide. Its effect on sludge settling efficiency also should be explored.