Micro-analysis of nitrous oxide accumulation in denitrification under acidic conditions: The role of pH and free nitrous acid

Abstract

Nitrous oxide (N2O), as an energy gas, is of increasing interest for its recovery from wastewater treatment. Acidic pH is one of the important factors affecting N2O accumulation during denitrification; however, its generation mechanism still needs further clarification. In this study, the production efficiency of N2O increased from 1.0 to 65.0% with the decrease in pH from 6.5 to 4.5 during nitrate-based denitrification performed in batch tests, which was accompanied by an increase in the nitrite accumulation. To further explore the mechanism, the net volumetric reaction rates of N2O within denitrified sludge aggregates were quantified by application of a microelectrode. When using N2O as a nitrogen source, the consumption rate of N2O was negatively correlated with both the hydrogen ion and free nitrous acid (FNA) concentrations. The inhibition of nitrous oxide reductase (NOS) activity in the absence of FNA demonstrates that hydrogen ion is the true inhibitor. When using NO as nitrogen source, the net volumetric production rates of N2O increased with decreasing pH, and they were higher in the presence of FNA compared to those in the absence of FNA, revealing that the increase in nitrite substrate promotes N2O production. Overall, the inhibition of NOS activity by hydrogen ions and FNA and the increase of substrate from nitrite accumulation are the main reasons for N2O enrichment under a low-pH condition.