Nitrate and nitrite inhibition of methanogenesis during denitrification in granular biofilms and digested domestic sludges
2009
Banihani, Qais | Sierra-Alvarez, Reyes | Field, James A
Anaerobic bioreactors that can support simultaneous microbial processes of denitrification and methanogenesis are of interest to nutrient nitrogen removal. However, an important concern is the potential toxicity of nitrate (NO₃ ⁻) and nitrite (NO₂ ⁻) to methanogenesis. The methanogenic toxicity of the NO x ⁻ compounds to anaerobic granular biofilms and municipal anaerobic digested sludge with two types of substrates, acetate and hydrogen, was studied. The inhibition was the severest when the NO x ⁻ compounds were still present in the media (exposure period). During this period, 95% or greater inhibition of methanogenesis was evident at the lowest concentrations of added NO₂ ⁻ tested (7.6-10.2 mg NO₂ ⁻-N l⁻¹) or 8.3-121 mg NO₃ ⁻-N l⁻¹ of added NO₃ ⁻, depending on substrate and inoculum source. The inhibition imparted by NO₃ ⁻ was not due directly to NO₃ ⁻ itself, but instead due to reduced intermediates (e.g., NO₂ ⁻) formed during the denitrification process. The toxicity of NO x ⁻ was found to be reversible after the exposure period. The recovery of activity was nearly complete at low added NO x ⁻ concentrations; whereas the recovery was only partial at high added NO x ⁻ concentrations. The recovery is attributed to the metabolism of the NO x ⁻ compounds. The assay substrate had a large impact on the rate of NO₂ ⁻ metabolism. Hydrogen reduced NO₂ ⁻ slowly such that NO₂ ⁻ accumulated more and as a result, the toxicity was greater compared to acetate as a substrate. The final methane yield was inversely proportional to the amount of NO x ⁻ compounds added indicating that they were the preferred electron acceptors compared to methanogenesis.
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