Characteristics Of N₂O Emission During Biological Removal Of Nitrogen And Phosphorus From Wastewater

The characteristics of N₂O emission from different wastewater treatment processesor different stages of one process are still unclear. It varies with environmental factorsand process parameters significantly. Two types of wastewater treatment processes wereacclimated, with one of biological nitrogen removal process and the other of biologicalnitrogen and phosphorus removal process. Characteristics of N₂O emission in differentprocess stages and under various environmental conditions （i.e. C/N ratio, initial NH₄-Nconcentrations, initial NO₂-N concentrations） were examined. In addition, the effect ofmicrobial competition on N₂O emission was investigated, with focusing on the effect ofheterotrophic bacteria activity on N₂O emission by nitrifiers under different DOconcentrations.In the acclimated two wastewater treatment processes, N₂O emission was mainlyfrom the nitrification stage rather than the denitrification stage. The amount of N₂Oemission during denitrification stage was only1/2042and1/970of the total amount ofN₂O emission during the whole operation cycle.N₂O emission increased obviously at a low C/N ratio. The reason was due to that alow efficient denitrification occurred with intracellular organic carbon as the electrondonor during denitrification. When the C/N ratio was3:4, the conversion ratio of nitrateto N₂O in these two processes was42.6and31.3times of those at the C/N ratio of3:2.The C/N ratio should be controlled to above3:2during denitrification.N₂O emission increased with increasing initial NH₄-N or NO₂-Nconcentrations during nitrification. With the same concentration of NH₄-N, theconversion ratio of N₂O from ammonium at the initial NO₂-N concentration of5mg/L were1.6and1.3times of that at the initial NO₂-N concentration of0mg/L.Heterotrophic activities had a high effect on N₂O emission duringnitrification, with a higher effect using external organic carbon than internalorganic carbon as the substrate. DO had a important impact on the N₂O emissionduring nitrification with the competition of oxygen by heterotrophs.