Autotrophic ammonia oxidizing bacteria (AOB) are capable of generating nitrous oxide (N 2 O), via nitrite reduction, in oxygen-limited environments. The recognition of the environmental fate and effects of N 2 O, as a "greenhouse gas" has prompted researchers to study N 2 O generation and emission control in wastewater treatment systems. Oxygen, often expressed in terms of the bioreactor liquid dissolved oxygen concentration, is generally viewed as the most important variable with respect to influencing N 2 O generation. However, some literature data suggest that the nitrite concentration may also influence AOB N 2 O generation under oxygen-limited conditions, although there are contradictions in the reported information. This paper presents the findings of an investigation that specifically examined the sensitivity of aerobic-phase biomass N 2 O generation to changes in bioreactor nitrite concentration, via supply of exogenous nitrite, as well as changes in nitrous acid concentration through mixed liquor pH manipulation, in a bench-scale wastewater treatment bioreactor. The data demonstrate the significant influence of nitrite availability on biomass N 2 O generation in the studied system. Most of the collected data suggest that nitrous acid, as opposed to nitrite proper, was the actual AOB nitrite reductase (NiR) electron acceptor, based on observed N 2 O generation. In this case, the bioreactor mixed liquor pH, as well as nitrite concentration, would be important with respect to AOB N 2 O generation and greenhouse gas emissions.Résumé : Les bactéries autotrophes oxydant l'ammoniac (AOB) peuvent produire de l'oxyde de diazote (N 2 O) par la réduction des nitrites dans les environnements à faible teneur en oxygène. La reconnaissance de l'évolution dans l'environnement et les effets du N 2 O en tant que « gaz à effet de serre » ont amené les chercheurs à étudier la génération et le contrôle des émissions de N 2 O dans les systèmes de traitement des eaux usées. L'oxygène, souvent exprimé en termes de concentration d'oxygène dissous dans le liquide du bioréacteur, est généralement considéré comme étant la variable la plus importante pour la production de N 2 O. Toutefois, certaines données dans la littérature suggèrent que la concentration de nitrites peut également influencer la production de N 2 O par les AOB dans des conditions de faible teneur en oxygène, bien qu'il y ait des contradictions dans l'information rapportée. Cet article présente les conclusions d'une étude qui a spécifiquement examiné la sensibilité de la production de N 2 O dans la biomasse en phase aérobie aux changements dans la concentration en nitrites dans le bioréacteur, par l'apport de nitrites exogènes, ainsi que les changements dans la concentration en acide nitreux par la manipulation du pH de la liqueur mixte dans un bioréacteur de traitement des eaux usées à l'échelle du laboratoire. Les données démontrent l'influence significative de la disponibilité des nitrites sur la production de N 2 O par la biomasse dans le système à l'étude. La ...

This paper describes a relatively simple experimental procedure used to assess the relative contribution of autotrophic and heterotrophic organisms in the generation of aerobic-phase nitrous oxide from a mixed microbial population in a bench-scale, anoxic-aerobic, sequencing batch reactor (SBR) wastewater treatment system. The procedure eliminated ammonia oxidation from a "perturbation" cycle, while maintaining pH and dissolved oxygen levels similar to the "baseline" cycle. Nitrite and carbon oxidation were maintained during the perturbation cycle. Compared to more complex methods such as stable isotope labelling techniques, the described procedure is logistically simple and requires equipment readily available in most environmental engineering laboratories. When applied to an SBR system treating synthetic wastewater (influent ammonia N concentration = 160 mg/L), the experimental results strongly suggest that essentially all of the nitrous oxide produced during the aerobic phase of the cycle was generated by autotrophic, ammonia oxidizing organisms. Little, if any, nitrous oxide appeared to be generated by heterotrophic denitrification during the aerobic phase of the cycle.Resume: Le present article d6crit une procedure experimentale relativement simple utilizee pour evaluer la contribution relative des organismes autotrophes et heterotrophes dans la generation de I'oxyde d'azote en phase a6robie a partir d'une population microbienne mixte anoxique/aerobie, a echelle laboratoire, dans un systeme de traitement des eaux usees par reacteur discontinu sequentiel (RDS). La procedure a elimine I'oxydation de I'ammoniaque d'un cycle de « perturbation » tout en maintenant les niveaux de pH et d'oxygene dissous similaires a ceux du cycle de « reference ». L'oxydation des nitrites et du carbone a ete maintenue durant le cycle de perturbation. Par rapport a des methodes plus complexes, telles que les techniques d'etiquetage par isotopes stables, la procedure decrite est simple du point de vue logistique et demande des equipements aisement disponibles dans presque tous les laboratoires de genie environnemental. Lorsqu'ils sont appliques a un systeme RDS pour le traitement des eaux usees synthetiques (concentration en ammoniaque de I'influent = 160 mg N/L), les resultats experimentaux suggerent fortement que presque tout I'oxyde d'azote produit durant la phase aerobie du cycle ait ete genere par des organismes autotrophes oxydant I'ammoniaque. Peu ou pas d'oxyde d'azote semblait etre genere par denitrification heterotrophe durant la phase aerobie du cycle.

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Several biochemical pathways can induce nitrogen loss from aerated, aerobic wastewater treatment bioreactors. These pathways include "traditional" simultaneous nitrification-denitrification (SND) (i.e. autotrophic nitrification - heterotrophic denitrification), autotrophic denitrification, and anaerobic ammonia oxidation. An oxygen limitation, often expressed in terms of low dissolved oxygen (DO) concentration, is a common element of these pathways. The presented research investigated the effect of mixed liquor DO concentration and biomass slowly degradable carbon (SDC) utilization rate on the heterotrophic nitrous oxide (N2O) reduction rate, for biomass cultured in an anoxic/aerobic wastewater treatment bioreactor. Biomass oxygen and SDC availability-limitation, expressed in terms of DO concentration and SDC ultilization rate, respectively, were found to significantly impact the observed heterotrophic N2O reduction rate. The findings support the hypothesis that nitrogen lost from the mixed liquor of an aerobic bioreactor could result from simultaneous autotrophic N2O generation (i.e. autotrophic denitrification) and heterotrophic N2O reduction. The results also support the idea that autotrophic N2O generation could be occurring in a bioreactor, although N2O may not be measurable in the reactor off-gas. Therefore, this autotrophic N2O generation - heterotrophic N2O reduction mechanism provides an alternative explanation to nitrogen loss, when compared to "conventional" SND, where heterotrophic organisms are assumed to reduce autotrophically generated nitrite and nitrate to dinitrogen (N2). In addition, nitrogen loss speculatively attributed to N2 formation via anaerobic ammonia oxidation in oxygen-limited environments, again because of the absence of measurable N2O, may in fact be due to the autotrophic N2O generation - heterotrophic N2O reduction mechanism.

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