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<p>Denitrifying polyphosphate accumulating organisms (DNPAOs) are very promising for simultaneous nitrogen and phosphorus removal. While during denitrification, emission of a greenhouse gas, nitrous oxide (N<sub>2</sub>O), may occur. In this study, DNPAOs were enriched in a lab-scale reactor, and N<sub>2</sub>O emission was examined under different electron acceptor conditions. During the anoxic phase, with the uptake of phosphorus, denitrification of nitrate nitrogen (NO<sub>3</sub>-N) was observed without the accumulation of nitrite nitrogen (NO<sub>2</sub>-N). In general, a very low amount of N<sub>2</sub>O 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 N<sub>2</sub>O emission occurred. The N<sub>2</sub>O emission factor to the denitrified NO<sub>2</sub>-N was 6.2%, 5.3% and 4.9% at the initial NO<sub>2</sub>-N concentration of 10, 20 and 40 mg l<sup>-1</sup>, respectively. In addition, a much higher N<sub>2</sub>O emission occurred with the co-existence of NO<sub>3</sub>-N and NO<sub>2</sub>-N. The initial organic carbon concentration had no significant effect on N<sub>2</sub>O emission with NO<sub>3</sub>-N as the electron acceptor. When stored organic carbon by DNPAOs was used as the electron donor, N<sub>2</sub>O emission was mainly dependent on the electron acceptor. </p>
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Nitrous oxide emission depending on the type of electron acceptor by a denitrifying phosphorus removal sludge
