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.

An instantaneous increase in reactor free ammonia concentration resulted in a gradual decrease in nitrite oxidizer activity that was coincidental with an increase in the nitrous oxide emissions from the reactor. Microbial community analysis using RNA slot blotting techniques indicated that Nitrospira, and not Nitrobacter, populations appeared to be most affected by this induced perturbation. It is hypothesized that competition for nitrite and possibly oxygen between ammonia oxidizers and Nitrospira, and not free ammonia stress, was the true cause of impaired nitrite oxidizer activity.

This paper reports on the use of fatty acid analysis for tracking the relative proportion of nitrifiers in reactors treating high strength ammonia wastewater with low carbon to nitrogen ratio. These results suggest the ratio of cis-9-hexadecenoic acid to cis-11-hexadecenoic acid may be useful for a number of acclimation responses including proliferation of small populations and appearance of new genotypes. This method is proposed as a supplement to molecular methods. The fact that fatty acid analysis has a much lower cost per sample and much higher sample throughput relative to most molecular methods makes it particularly suited as a prescreening tool when choosing samples to be submitted for the more costly analysis. Key words: nitrification, fatty acid methyl esters, Nitrospira, free ammonia, nitrite, ammonia inhibition.

Batch tests were carried out with Nitrospira in mixed and pure cultures using concentrations of free ammonia widely believed to be inhibitory to nitrite-oxidizing organisms. The mixed culture batch tests were conducted with mixed liquor from a bench scale completely stirred tank reactor (CSTR) treating a synthetic wastewater having a low C:N ratio. Nitrospira were confirmed as the dominant nitrite oxidizers via RNA slot blotting. Nitrospira moscoviensis were used for the pure culture trials. The results from this study suggest that free ammonia (NH3-N) concentrations of up to 10 mg/L were not inhibitory to Nitrospira either in situ or in pure culture. Key words: fatty acid methyl esters, free ammonia inhibition, molecular methods, nitrification, nitrite, Nitrobacter, Nitrospira, nitrous acid, nitrous oxide, RNA slot blotting, wastewater treatment.

Selenium contaminated waters are produced by various industrial activities such as mining (coal, hard rock, uranium and phosphate), refineries (metal and oil), power generation (coalfired power plants), and agriculture (irrigation waters and selenium fortification) (Chapman et al., 2010). Selenium can also be present in municipal wastewater, due to inflow and infiltration of groundwater into sewers emanating from alluvium or sedimentary deposits that are high in selenium (Pontarolo et al., 2017).Background concentrations of Se in uncontaminated surface waters are generally below 1 μg/L (Seiler et al., 1965), but in areas where the weathering and erosion of seleniferous soils is augmented by anthropogenic activities, environmental Se concentrations have been shown to be significantly elevated. Elevated intake of selenium, particularly in oviparous animals such as predatory fish and waterfowl, can result in teratogenic effects on entire populations in contaminated ecosystems (Janz et al., 2010). The maximum permissible concentration of Se in the aquatic environment is, on average, 10 times lower than that in drinking water, at values of between 1 to 5 μg/L and 10 to 50 μg/L, respectively (Lemly, 2007). While overall, guidelines for Se in freshwater for aquatic life are very low level, they do vary by country.An excellent review of global selenium regulations by Kumkrong et al.