B23B-0204:
Nitrous oxide production pathways in a partial nitritation-anammox reactor: Isotopic evidence for nitrous oxide production associated anaerobic ammonium oxidation?

Tuesday, 16 December 2014
Eliza Jean Harris1, Adriano Joss2, Lukas Emmenegger1, Marco Kipf2, Joachim Mohn1, Hansruedi Siegrist2 and Pascal Wunderlin2, (1)Swiss Federal Institute for Materials Science and Technology, Dubendorf, Switzerland, (2)EAWAG Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland
Abstract:
Nitrous oxide (N2O) is a strong greenhouse gas and a major sink for stratospheric ozone. In biological wastewater treatment N2O can be produced via several pathways. This study investigates the dynamics of N2O emissions from a nitritation-anammox reactor, and links its interpretation to the nitrogen and oxygen isotopic signature of the emitted N2O. A 400-litre single-stage nitritation-anammox reactor was operated and continuously fed with digester liquid. The isotopic composition of N2O emissions was monitored online with quantum cascade laser absorption spectroscopy (QCLAS; Aerodyne Research, Inc.; Waechter et al., 2008). Dissolved ammonium and nitrate were monitored online (ISEmax, Endress + Hauser), while nitrite was measured with test strips (Nitrite-test 0-24mgN/l, Merck).

Table 1. Summary of experiments conducted to understand N2O emissions

Experimental conditions

O2

[mgO2/L]

NO2-

[mgN/L]

NH4+

[mgN/L]

N2O/NH4+

[%]

Normal operation

<0.1

<0.5

10

0.6

Normal operation, high NH4+

<0.1

<0.5

100

6.1

High aeration

0.5 to 1.5

up to 50

10 and 50

4.9

NO2- addition (oxic)

<0.1

<0.5 to 4

10

5.8

NO2- addition (anoxic)

0

<0.5 to 4

10

3.2

NH2OH addition

<0.1

<0.5

10

2.5

Results showed that under normal operating conditions, the N2O isotopic site preference (SP = d15Nα - d15Nβ) was much higher than expected - up to 41‰ – strongly suggesting an unknown N2O production pathway, which is hypothesized to be mediated by anammox activity (Figure 1). A less likely explanation is that the SP of N2O was increased by partial N2O reduction by heterotrophic denitrification. Various experiments were conducted to further investigate N2O formation pathways in the reactor. Our data reveal that N2O emissions increased when reactor operation was not ideal, for example when dissolved oxygen was too high (Table 1). SP measurements confirmed that these N2O peaks were due to enhanced nitrifier denitrification, generally related to nitrite build-up in the reactor (Figure 1; Table 1). Overall, process control via online N2O monitoring was confirmed to be an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N2O emissions.

References

Waechter H. et al. (2008) Optics Express, 16: 9239-9244.

Wunderlin, P et al. (2013) Environmental Science & Technology 47: 1339-1348.