Nitrate and nitrite inhibition of methanogenesis during denitrification in granular biofilms and digested domestic sludges

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.