Emission and control of nitrous oxide from a biological wastewater treatment system with intermittent aeration

Nitrous oxide (N2O) can be emitted as a by-product of the process of nitrog en removal from wastewater. Two methods of complete denitrification and med ia application were studied in lab-scale intermittent aeration reactors fed with domestic wastewater to refine methods of controlling the N2O emission rate. A study on cyclic patterns showed that the highest N2O emission rate was at the beginning of the aerobic phase rather than the anoxic: phase. T his was probably because the nitrifying bacteria had accumulated nitrite ni trogen (NO2-) under low DO conditions. Methanol as an external carbon sourc e was added during the anoxic phase to reduce nitrate nitrogen (NO3-) when denitrification was completed. The N2O emission rates in both the aerobic a nd anoxic phases were significantly influenced by residual NO3-, increasing monotonically as the concentration of NO3- in the reactor increased. Over 95% of average N2O emissions in both the aerobic and anoxic phases were pre vented when methanol was added. The biofilm reactor showed similar patterns to those of the non-biofilm reactor in track behavior, but the former was more effective in the reduction of N2O emissions.