Nitrous oxide (N₂O) production pathways in a single stage, continuously fed partial nitritation-anammox reactor were investigated using online isotopic analysis of offgas N₂O with quantum cascade laser absorption spectroscopy (QCLAS). N₂O emissions increased when reactor operating conditions were not optimal, for example, high dissolved oxygen concentration. SP measurements indicated that the increase in N₂O was due to enhanced nitrifier denitrification, generally related to nitrite build-up in the reactor. The results of this study confirm that process control via online N₂O monitoring is an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N₂O emissions.
Under normal operating conditions, the N₂O isotopic site preference (SP) was much higher than expected – up to 40‰ – which could not be explained within the current understanding of N₂O production pathways. Various targeted experiments were conducted to investigate the characteristics of N₂O formation in the reactor. The high SP measurements during both normal operating and experimental conditions could potentially be explained by a number of hypotheses: i) unexpectedly strong heterotrophic N₂O reduction, ii) unknown inorganic or anammox-associated N₂O production pathway, iii) previous underestimation of SP fractionation during N₂O production from NH₂OH, or strong variations in SP from this pathway depending on reactor conditions. The second hypothesis – an unknown or incompletely characterised production pathway – was most consistent with results, however the other possibilities cannot be discounted. Further experiments are needed to distinguish between these hypotheses and fully resolve N₂O production pathways in PN-anammox systems.