Isotopic evidence for nitrous oxide production pathways in a partial nitritation-anammox reactor. (15th October 2015)

• Record Type: Journal Article
• Authors: Harris, Eliza; Joss, Adriano; Emmenegger, Lukas; Kipf, Marco; Wolf, Benjamin; Mohn, Joachim; Wunderlin, Pascal
• Is Part Of: Water research. Volume 83(2015)
• Page Start: 258
• Page End: 270
• Publication Date: 2015-10-15
• Subjects: Nitrous oxide -- Isotopic composition -- Laser spectroscopy -- Nitrifier denitrification -- Process control; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2015.06.040
• Languages: English
• ISSNs: 0043-1354

Abstract

Abstract: Nitrous oxide (N2 O) production pathways in a single stage, continuously fed partial nitritation-anammox reactor were investigated using online isotopic analysis of offgas N2 O with quantum cascade laser absorption spectroscopy (QCLAS). N2 O emissions increased when reactor operating conditions were not optimal, for example, high dissolved oxygen concentration. SP measurements indicated that the increase in N2 O 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 N2 O monitoring is an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N2 O emissions. Under normal operating conditions, the N2 O isotopic site preference (SP) was much higher than expected – up to 40‰ – which could not be explained within the current understanding of N2 O production pathways. Various targeted experiments were conducted to investigate the characteristics of N2 O 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 N2 O reduction, ii) unknown inorganic or anammox-associated N2 O production pathway, iii) previous underestimation of SP fractionation during N2 O production from NH2 OH, 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 N2 O production pathways in PN-anammox systems. Graphical abstract: Highlights: Offgas N2 O monitoring is ideal to maintain process stability in PN-anammox reactors. Online isotopic measurements can elucidate N2 O production mechanisms. Peak N2 O production during reactor imbalances is due to nitrifier denitrification. High isotopic SP during optimal operation may show an unknown N2 O production pathway. This pathway, which appears to be associated with anammox, dominates baseline N2 O emissions.