title: Isotopic evidence for nitrous oxide production pathways in a partial nitritation-anammox reactor.
abstract: 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.
authors.affiliation: Laboratory for Air Pollution and Environmental Technology, Empa, Überlandstrasse 129, 8600 Dübendorf, Switzerland. Electronic address: eliza.harris@empa.ch.
authors.forename: Eliza
authors.initials: E
authors.lastname: Harris
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
authors.forename: Adriano
authors.initials: A
authors.lastname: Joss
authors.affiliation: Laboratory for Air Pollution and Environmental Technology, Empa, Überlandstrasse 129, 8600 Dübendorf, Switzerland.
authors.forename: Lukas
authors.initials: L
authors.lastname: Emmenegger
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
authors.forename: Marco
authors.initials: M
authors.lastname: Kipf
authors.affiliation: Laboratory for Air Pollution and Environmental Technology, Empa, Überlandstrasse 129, 8600 Dübendorf, Switzerland; Institute of Meteorology and Climate Research (IMK-IFU), Karlsruhe Institute of Technology, Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Wolf
authors.affiliation: Laboratory for Air Pollution and Environmental Technology, Empa, Überlandstrasse 129, 8600 Dübendorf, Switzerland.
authors.forename: Joachim
authors.initials: J
authors.lastname: Mohn
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
authors.forename: Pascal
authors.initials: P
authors.lastname: Wunderlin
mesh_terms: D000641:Ammonia / Q000378:metabolism; D000693:Anaerobiosis; D019149:Bioreactors; D054023:Lasers, Semiconductor; D009587:Nitrogen Isotopes / Q000032:analysis; D009609:Nitrous Oxide / Q000032:analysis*; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D014865:Waste Disposal, Fluid; D018508:Water Purification
chemical_list: D009587:Nitrogen Isotopes; D000641:Ammonia; D009609:Nitrous Oxide
keywords: Isotopic composition; Laser spectroscopy; Nitrifier denitrification; Nitrous oxide; Process control