Ferric iron amendment increases Fe(III)-reducing microbial diversity and carbon oxidation in on-site wastewater systems

Onsite wastewater systems, or septic tanks, serve approximately 25% of the United States population; they are therefore a critical component of the total carbon balance for natural water bodies. Septic tanks operate under strictly anaerobic conditions, and fermentation is the dominant process driving carbon transformation. Nitrate, Fe(III), and sulfate reduction may be operating to a limited extent in any given septic tank. Electron acceptor amendments will increase carbon oxidation, but nitrate is toxic and sulfate generates corrosive sulfides, which may damage septic system infrastructure. Fe(III) reducing microorganisms transform all major classes of organic carbon that are dominant in septic wastewater: low molecular weight organic acids, carbohydrate monomers and polymers, and lipids. Fe(III) is not toxic, and the reduction product Fe(II) is minimally disruptive if the starting Fe(III) is added at 50–150 mg L^?1. We used ¹⁴C radiolabeled acetate, lactate, propionate, butyrate, glucose, starch, and oleic acid to demonstrate that short and long-term carbon oxidation is increased when different forms of Fe(III) are amended to septic wastewater. The rates of carbon mineralization to ¹⁴CO₂ increased 2–5 times (relative to unamended systems) in the presence of Fe(III). The extent of mineralization reached 90% for some carbon compounds when Fe(III) was present, compared to levels of 50–60% in the absence of Fe(III). ¹⁴CH₄ was not generated when Fe(III) was added, demonstrating that this strategy can limit methane emissions from septic systems. Amplified 16S rDNA restriction analysis indicated that unique Fe(III)-reducing microbial communities increased significantly in Fe(III)-amended incubations, with Fe(III)-reducers becoming the dominant microbial community in several incubations. The form of Fe(III) added had a significant impact on the rate and extent of mineralization; ferrihydrite and lepidocrocite were favored as solid phase Fe(III) and chelated Fe(III) (with nitrilotriacetic acid or EDTA) as soluble Fe(III) forms.