Effect of Heterotrophic Activities on Nitrous Oxide Emission during Nitrification under Different Aeration Rates

EXTENDED ABSTRACT Nowadays, biological nutrient removal (BNR) processes have been applied widely for wastewater treatment. However, one type of greenhouse gases, nitrous oxide (N2O), can be released from both nitrification and denitrification during BNR. In our previous study, we found that N2O emission during nitrification could be affected by heterotrophic activities significantly in a BNR system. Therefore, it is necessary to examine N2O emission during nitrification from the viewpoint of “ecology of activated sludge”, which has not been received much attention in the research area of N2O emission during nitrification. In this study, effect of heterotrophic activities on N2O emission under different aeration rates was investigated in batch experiments for activated sludge taken from a BNR system. The BNR system was acclimated to remove nitrogen and phosphorus from synthetic wastewater. N2O emission under three aerobic conditions were examined, one without heterotrophic effect, one with heterotrophic activities by using internal stored organic carbon (polyhydroxybutyrate, PHB), and the other with heterotrophic activities by using external organic carbon (acetate). Under each condition, the applied aeration rates were 100 ml/min, 250 ml/min and 500 ml/min, respectively. Under the condition with PHB as the organic carbon, the released N2O-N to the produced oxidized nitrogen (NOx-N) was 10.0% at 100 ml/min, 3.6% at 250 ml/min, and 0.6% at 500 ml/min. Under the condition with acetate as the organic carbon, the released N2O-N to the produced NOx-N was 14.5% at 100 ml/min, 4.1% at 250 ml/min, and 0.7% at 500 ml/min. Under the condition without organic carbon, the released N2O-N to the produced NOx-N was 0.18% at 100 ml/min, 0.20% at 250 ml/min, and 0.41% at 500 ml/min. These results showed that (i) heterotrophic activities affected N2O emission during nitrification significantly; (ii) there was no significant difference in N2O emission during nitrification affected by heterotrophic activities by using internal or external organic carbon; (iii) a high aeration rate with a high dissolved oxygen (DO) concentration reduced N2O emission significantly under aerobic conditions. For controlling N2O emission during nitrification concurrent with heterotrophic activities, DO concentrations should be carefully controlled.