Denitrifying polyphosphate accumulating organisms (DNPAOs) are very promising for simultaneous nitrogen and phosphorus removal. While during denitrification, emission of a greenhouse gas, nitrous oxide (N2O), may occur. In this study, DNPAOs were enriched in a lab-scale reactor, and N2O emission was examined under different electron acceptor conditions. During the anoxic phase, with the uptake of phosphorus, denitrification of nitrate nitrogen (NO3-N) was observed without the accumulation of nitrite nitrogen (NO2-N). In general, a very low amount of N2O was produced with nitrate as the electron acceptor, independent of the applied different nitrate concentrations. However, with nitrite as the electron acceptor, a much higher N2O emission occurred. The N2O emission factor to the denitrified NO2-N was 6.2%, 5.3% and 4.9% at the initial NO2-N concentration of 10, 20 and 40 mg l-1, respectively. In addition, a much higher N2O emission occurred with the co-existence of NO3-N and NO2-N. The initial organic carbon concentration had no significant effect on N2O emission with NO3-N as the electron acceptor. When stored organic carbon by DNPAOs was used as the electron donor, N2O emission was mainly dependent on the electron acceptor.
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