title: Source identification of nitrous oxide emission pathways from a single-stage nitritation-anammox granular reactor.
abstract: Nitrous oxide (N2O) production pathway in a signal-stage nitritation-anammox sequencing batch reactor (SBR) was investigated based on a multilateral approach including real-time N2O monitoring, N2O isotopic composition analysis, and in-situ analyses of spatial distribution of N2O production rate and microbial populations in granular biomass. N2O emission rate was high in the initial phase of the operation cycle and gradually decreased with decreasing NH4(+) concentration. The average emission of N2O was 0.98 ± 0.42% and 1.35 ± 0.72% of the incoming nitrogen load and removed nitrogen, respectively. The N2O isotopic composition analysis revealed that N2O was produced via NH2OH oxidation and NO2(-) reduction pathways equally, although there is an unknown influence from N2O reduction and/or anammox N2O production. However, the N2O isotopomer analysis could not discriminate the relative contribution of nitrifier denitrification and heterotrophic denitrification in the NO2(-) reduction pathway. Various in-situ techniques (e.g. microsensor measurements and FISH (fluorescent in-situ hybridization) analysis) were therefore applied to further identify N2O producers. Microsensor measurements revealed that approximately 70% of N2O was produced in the oxic surface zone, where nitrifiers were predominantly localized. Thus, NH2OH oxidation and NO2 reduction by nitrifiers (nitrifier-denitrification) could be responsible for the N2O production in the oxic zone. The rest of N2O (ca. 30%) was produced in the anammox bacteria-dominated anoxic zone, probably suggesting that NO2(-) reduction by coexisting putative heterotrophic denitrifiers and some other unknown pathway(s) including the possibility of anammox process account for the anaerobic N2O production. Further study is required to identify the anaerobic N2O production pathways. Our multilateral approach can be useful to quantitatively examine the relative contributions of N2O production pathways. Good understanding of the key N2O production pathways is essential to establish a strategy to mitigate N2O emission from biological nitrogen removal processes.
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan; Water Desalination and Reuse Center (WDRC), Biological and Environmental Science and Engineering (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
authors.forename: Muhammad
authors.initials: M
authors.lastname: Ali
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan; Department of Civil Engineering, Faculty of Engineering, University of Peradeniya, Peradeniya 20400, Sri Lanka.
authors.forename: Rathnayake M L D
authors.initials: RMLD
authors.lastname: Rathnayake
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan; Department of Soil, Water and Climate, University of Minnesota, 258 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, USA.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Ishii
authors.affiliation: Department of Civil and Environmental Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan.
authors.forename: Tomonori
authors.initials: T
authors.lastname: Kindaichi
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan.
authors.forename: Hisashi
authors.initials: H
authors.lastname: Satoh
authors.affiliation: Department of Environmental Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
authors.forename: Sakae
authors.initials: S
authors.lastname: Toyoda
authors.affiliation: Department of Environmental Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
authors.forename: Naohiro
authors.initials: N
authors.lastname: Yoshida
authors.affiliation: Division of Environmental Engineering, Faculty of Engineering, Hokkaido University, North-13, West-8, Sapporo, Hokkaido 060-8628, Japan. Electronic address: sokabe@eng.hokudai.ac.jp.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Okabe
mesh_terms: D019149:Bioreactors; D058440:Denitrification; D052836:Heterotrophic Processes; D009584:Nitrogen / Q000378:metabolism; D009609:Nitrous Oxide; D010084:Oxidation-Reduction
chemical_list: D009609:Nitrous Oxide; D009584:Nitrogen
keywords: FISH; Microsensors; N(2)O isotopomer analysis; N(2)O production pathway; Nitritation-anammox reactor