Effect of COD on mainstream anammox: Evaluation of process performance, granule morphology and nitrous oxide production.

The effect of COD addition into an anammox reactor was assessed during short and long term exposure. The short term exposure was assessed via batch tests and lasted 48 h. Results indicated the presence of a very active denitrifying community able to consume COD using nitrate and nitrite as electron acceptors. However, the presence of COD did not result in an increase of the ammonium concentration at the end of the tests indicating that anammox activity was not suppressed by the addition of COD. Different COD concentrations (125, 225 and 175 mg COD/L) were also added in the reactor during 3 periods within its operation (period II, III and V respectively). Long term COD addition (up to 102 d of continuous addition during period II and III) caused a decrease of the anammox activity and a shift on the microbial community, with a decrease on the anammox fraction. However, the anammox process was never lost and it fully recovered as soon as COD addition stopped. Finally, dissolved N2O was monitored under periods with and without COD addition, showing higher concentrations during transient periods from COD addition to no addition. The results of this paper provide evidence of how a long term COD exposure into an anammox reactor affect the overall nitrogen removal process, the granular structure of the anammox biomass, its microbial composition and for the first time, its N2O emissions.

The effect of temperature shifts on N2O and NO emissions from a partial nitritation reactor treating reject wastewater.

Temperature has a known effect on ammonia oxidizing bacteria (AOB) activities, reducing its ammonia oxidizing rate (AOR) when temperature is lowered. However, little is known concerning its effect on N2O and NO emissions which are produced during ammonia oxidation having a greenhouse effect. To study this, an AOB enriched partial nitrification sequencing batch reactor (PN-SBR) was operated within a two step-wise feed under 5 different temperatures (30-25-20-15-10 °C). A decrease on the specific AOR (sAOR) was detected when decreasing the temperature. N2O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle, when AOBs shifted from a period of low activity to a period of high activity. N2O emission factors (%) detected during the second aerobic phase were similar among all temperatures tested and lower than the emissions detected during the first aerated phase. The average N2O emission factor was in the range of 0.15-0.70% N2O-N/NH4+-N oxidized in the first aeration phase and 0.14-0.15% N2O-N/NH4+-N-oxidized in the second aeration phase at 10 to 30 °C, respectively. On the other hand, NO emissions were very similar under all temperatures resulting in 0.03-0.06% of NH4+-N oxidized.

Denitrifying capabilities of Tetrasphaera and their contribution towards nitrous oxide production in enhanced biological phosphorus removal processes.

Denitrifying enhanced biological phosphorus removal (EBPR) systems can be an efficient means of removing phosphate (P) and nitrate (NO3-) with low carbon source and oxygen requirements. Tetrasphaera is one of the most abundant polyphosphate accumulating organisms present in EBPR systems, but their capacity to achieve denitrifying EBPR has not previously been determined. An enriched Tetrasphaera culture, comprising over 80% of the bacterial biovolume was obtained in this work. Despite the denitrification capacity of Tetrasphaera, this culture achieved only low levels of anoxic P-uptake. Batch tests with different combinations of NO3-, nitrite (NO2-) and nitrous oxide (N2O) revealed lower N2O accumulation by Tetrasphaera as compared to Accumulibacter and Competibacter when multiple electron acceptors were added. Electron competition was observed during the addition of multiple nitrogen electron acceptors species, where P uptake appeared to be slightly favoured over glycogen production in these situations. This study increases our understanding of the role of Tetrasphaera-related organisms in denitrifying EBPR systems.

Distinctive denitrifying capabilities lead to differences in N2O production by denitrifying polyphosphate accumulating organisms and denitrifying glycogen accumulating organisms.

This study aims at investigating the denitrification kinetics in two separate enriched cultures of denitrifying polyphosphate accumulating organisms (dPAO) and denitrifying glycogen accumulating organisms (dGAO) and compare their N2O accumulation potential under different conditions. Two sequencing batch reactors were inoculated to develop dPAO and dGAO enriched microbial communities separately. Seven batch tests with different combinations of electron acceptors (nitrate, nitrite and/or nitrous oxide) were carried out with the enriched biomass from both reactors. Results indicate that in almost all batch tests, N2O accumulated for both cultures, however dPAOs showed a higher denitrification capacity compared to dGAOs due to their higher nitrogen oxides reduction rates. Additionally, the effect of the simultaneous presence of several electron acceptors in the reduction rates of the different nitrogen oxides was also assessed in dPAOs and dGAOs.