Nitrite effect on nitrous oxide emission from denitrifying activated sludge

Laboratory-scale experiments were conducted to examine the N₂O emission during the denitrification process. For each of the 6 runs carried out, synthetic effluent was fed in a 10 l batch mixed liquor to investigate the effect of nitrite on N₂O emission and Helium was continuously bubbled through the reactor at constant rate (0.12 l/min) to favour N₂O transfer and detection. An increasing COD/NO₃^?-N influent ratio from 3 to 7 was firstly applied (runs 1–3). Secondly, NO₂^? pulse additions were performed during run 4 and 5 (10 and 20 mg N/l, respectively). Finally, the reactor was fed with influent containing both NO₂^? and NO₃^?. We showed that N₂O emission was detected shortly after NO₂^? accumulation, few minutes after the substrate feeding. The highest emission occurred at the lower COD/NO₃^?-N ratio (=3) and at the higher NO₂^? addition (20 mg N/l). In addition, the higher nitrogen conversion to N₂O gas (14.4%) was obtained with an influent containing initially both NO₂^? and NO₃^?. Our results suggest a direct effect of the NO₂^? concentration on the N₂O emission. We have also confirmed the inhibitory effect of NO₂^? concentration on N₂O reduction.