N₂O production by ammonia oxidizing bacteria in an enriched nitrifying sludge linearly depends on inorganic carbon concentration

The effect of inorganic carbon (IC) on nitrous oxide (N₂O) production by ammonia oxidizing bacteria (AOB) was investigated over a concentration range of 0-12mmolC/L, encompassing typical IC levels in a wastewater treatment reactors. The AOB culture was enriched along with nitrite-oxidizing bacteria (NOB) in a sequencing batch reactor (SBR) to perform complete nitrification. Batch experiments were conducted with continuous carbon dioxide (CO₂) stripping or at controlled IC concentrations. The results revealed a linear relationship between N₂O production rate (N₂OR) and IC concentration (R²=0.97) within the IC range studied, suggesting a substantial effect of IC on N₂O production by AOB. Similar results were also obtained with an AOB culture treating anaerobic sludge digestion liquor. The fundamental mechanism responsible for this dependency is unclear; however, in agreement with previous studies, it was observed that the ammonia oxidation rate (AOR) was also influenced by the IC concentration, which could be well described by the Monod kinetics. These resulted in an exponential relationship between N₂OR and AOR, as previously observed in experiments where AOR was altered by varying dissolved oxygen and ammonia concentrations. It is therefore possible that IC indirectly affected N₂OR by causing a change in AOR. The observation in this study indicates that alkalinity (mostly contributed by IC) could be a significant factor influencing N₂O production and should be taken into consideration in estimating and mitigating N₂O emissions in wastewater treatment systems. © 2015 Elsevier Ltd.