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

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 = d¹⁵N^α - d¹⁵N^β) 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.