Effects of the COD/NO3(-)-N ratio and pH on the accumulation of denitrification intermediates with available pyridine as a sole electron donor and carbon source.

Batch experiments were conducted to investigate the effects of COD/NO3(-)-N ratio and pH on the accumulation of the denitrification intermediates - nitrite and nitrous oxide - with pyridine as the sole electron donor and carbon source. Significant transient accumulation of nitrite was observed at COD/NO3(-)-N ratios in the range of 2-13 and pH in the range of 6.5-9.5. The optimal pH was 7.5 and the minimum COD/NO3(-)-N ratio was 4 for complete denitrification. The pH had a significant effect on denitrification. At a lower pH of 6.5, the reaction was severely inhibited whereas at a higher pH of 8.5 and 9.5, nitrate was completely reduced, but a high concentration of nitrite remained in the reactors while pyridine was almost completely degraded after one hour, even at the COD/NO3(-)-N ratio of 4. While the specific nitrate reduction rate and nitrite reduction rate in the absence of nitrate remained constant at different COD/NO3(-)-N ratios, they increased with the increase in pH. The nitrite reduction was inhibited in the presence of nitrate because of the faster rate of nitrate reduction over nitrite reduction. Nitrous oxide accumulation was also observed with a COD/NO3(-)-N ratio below 3 in the pH range of 7.5-8.5. At a COD/NO3(-)-N ratio of 4 and pH of 6, the production of nitrous oxide was much higher than that at pH 7 and 8.