The influence of nitrite and pH (nitrous acid) on aerobic-phase, autotrophic N₂O generation in a wastewater treatment bioreactor

Abstract

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 ...

Nitrous Oxide – a Powerful Greenhouse Gas with a Wastewater Treatment Connection

“…Correlation itself, of course, does not imply causation. However, the data are consistent with the findings of Anderson et al (1993), Kester et al (1997) and Shiskowski and Mavinic (2006), where the presence of nitrite, or elevated nitrite levels, were associated with an increased Nitrosomonas N 2 O generation rate. Of particular interest, in the current study, was the fact that the nitrite levels were relatively low.…”

“…As a result, between days 20 and 35, the SBR was "autowasting" biomass from the effluent decant port, reducing reactor MLSS levels and causing the biomass SRT to be erratic and as low as 7 d, compared to the normal 10 d value. Under the unstable operating condition, if the AOB had a faster growth rate than the NOB it could result in elevated nitrite levels and increased N 2 O generation (Shiskowski and Mavinic, 2006). Then, once the system and SRT became stable, the AOB and NOB would eventually reach a more balanced growth state, thus resulting in lower nitrite concentrations and less N 2 O generation.…”

Nitrous oxide emissions from wastewater treatment processes

“…In addition, the optimum pH for the production of NO 2 2 and N 2 O is approximately 8.5, in the investigated pH range of 5.4 -9.5, further suggesting that a high free nitrous acid (HNO 2 ) concentration, the true substrate for NO 2 2 reduction [76], is not required for higher N 2 O production. As the aerobic N 2 O production is completely inhibited by acetylene (C 2 H 2 ), the authors suggest that N 2 O is predominantly produced through degradation of NOH under aerobic conditions [75].…”

Nitrous oxide monitoring for nitrifying activated sludge aeration control: A simulation study

“…It was thus necessary to develop a correlation to generate N 2 O profiles as a result of existing liquid phase conditions. While insights into the specific N 2 O production mechanisms have been provided in recent years (Colliver and Stephenson, 2000;Shiskowski and Mavinic, 2006), it remains an area of ongoing research and no thorough investigations of specific biokinetic parameters have been conducted. Thus, at the current state of knowledge with regards to this process, a strict mechanistic modeling of N 2 O generation was not feasible.…”