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 (SO2−3), which can be easily produced from S0 by a simple combustion process. Unlike S0, SO2−3 is highly soluble, and therefore may provide higher denitrification rates. However, very little is known about microbial denitrification with SO2−3. Also, SO2−3 is a strong reductant that reacts abiotically with oxygen and has toxic effects on microorganisms. This paper reviews SO2−3 in the environment, SO2−3 chemistry, microbiology, toxicity, and its potential use for denitrification. Since SO2−3 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 SO2−3-oxidizing community. Our review of the literature, as well as our own lab experience, suggests that SO2−3 can effectively serve as an electron donor for denitrification. Further research is needed to determine the kinetics of SO2−3-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.