A contribution to analysis and modeling of NO and N2O emissions during nitritation. (Une contribution à l'analyse et la modélisation des émissions de NO et N2O pendant nitritation)

Nitrous oxide (N2O) is a key greenhouse gas (GHG) with a global warming potential 300 times stronger than carbon dioxide. Nitric oxide (NO) is toxic to micro-organisms and has a negative impact on the environment with contribution to ozone layer depletion. N2O emissions from wastewater treatment have been reported to constitute 0.22% of the total anthropogenic N2O emissions in 2010 and have increased for almost 25% in 20 years. It is thus important to understand the biological mechanisms involved in these emissions in order to control and reduce the environmental impacts of wastewater treatment systems. In wastewater treatments plants, both NO and N2O are produced during the nitrogen removal mostly by nitrification performed by ammonium oxidizing bacteria (AOB). Two production pathways are known to be responsible of these emissions by AOB: (1) the NN pathway which corresponds to the NO and N2O production during the oxidation of ammonia to nitrite and (2) the ND pathway which corresponds to the reduction of nitrite to NO and N2O. The influence of operating conditions on both NN and ND pathways is actually not fully elucidated. In addition, several N2O models based on the NN or the ND pathway have been proposed and the development of a generic N2O model is yet to be proposed. In this context, this work aims at investigating NO and N2O emissions during the nitrification performed in Sequencing Batch Reactors (SBR) treating ammonium rich effluents by nitrification/denitrification. The general objective of this thesis is to improve knowledge and understanding of the biological mechanisms involved in NO and N2O production by AOB using mechanistic models coupled to designed experiments. The general strategy has consisted to confront experimental data to N2O models based on a single pathway in order to identify AOB N2O production pathways responsible of NO and N2O emissions in response to environmental conditions. The analysis of SBR cycles has highlighted the effect of operating conditions on NO and N2O emissions during nitrification by AOB. The N2O emission factor (N2O-EF), which represents the fraction of nitrogen converted to N2O during ammonium removal, appears to be correlated to the HNO2 concentration. SBR cycles with the highest HNO2 accumulation around 0.9 μgN. L-1 are those with the highest N2O-EF (from 4 to 11%). At a HNO2 concentration lower than 0.5 μgN. L-1 the N2O-EF remained relatively low and below 1%. The relation between DO and N2O EF indicates that the lowest N2O emission factors were obtained at high DO (from 2.5 to 6.0 mgO2.L-1) whereas a large variation from 1% to 11% was observed at DO between 1 and 2 mgO2.L-1 depending on the nitrite level. In this range, the highest N2O emissions have been observed for cycles with the highest HNO2 concentration. In contrast, no clear tendencies were observed for the variation of NO emission factor (NO-EF, calculated similarly to the N2O-EF) neither with the DO nor the HNO2 concentration. The NO-EF remained between 0.004% and 0.078% for all SBR cycles. The NO-EF/N2O-EF ratio decreased with the increase of the HNO2 whereas the highest value was observed after hydroxylamine injections. From these observations it comes that the decrease of this ratio reflects the increase of the ND pathway contribution on N2O emissions with the increase of HNO2 concentration (exacerbated at a low DO concentration). On the contrary, the contribution of the NN pathway increases for cycles with a high NO/N2O ratio associated to a low HNO2 accumulation. The confrontation of these experimental observations to N2O models based on a single pathway indicates that N2O emissions are better described by models based on the ND pathway. These models are able to predict the relation between the N2O-EF and the HNO2 concentration. On the over hand, these models have difficulties to predict NO emissions observed in the SBR. These emissions are better described by the model based on the NN pathway. This suggests that the N2O emissions are more related to the ND pathway and NO emissions to the NN pathway in the studied system. Based on these observations, a new model is proposed in this work. This model considers both the NN pathway and the ND pathway and is able to describe both NO and N2O emissions. Moreover, this new model can catch the variation of the NO-EF/N2O-EF ratio in relation to environmental conditions, reflecting the contribution of both NN and ND pathways on N2O emissions.