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Theses

Analyse and modelling of nitrous oxide emissions from full-scale tertiary nitrifying biological aerated filters

Abstract : Nitrous oxide (N2O) is a powerful greenhouse gas (GHG), playing a major role in the ozone layer depletion. Mainly from natural origin, it is also emitted by wastewater treatment processes, during biological nitrogen removal through nitrification and denitrification. Because of its high global warming potential, about 300 times the one of carbon dioxide, N2O contributes significantly to the carbon footprint of wastewater resource recovery facilities (WRRF). For the last decade, considerable efforts have been made to understand the mechanisms of N2O production and evaluate in situ emissions, which led to the development of mechanistic models. The latter have been mainly applied to suspended biomass systems, and rarely to fixed biomass processes. Yet, recent measurements performed on biologically active filters (BAF) of Seine Aval WRRF (~ 5 million people equivalents) indicated high N2O emissions, much higher than those measured on conventional activated sludge systems. The objective of this PhD thesis was to increase knowledge on the comprehension of N2O production mechanisms in tertiary nitrifying BAFs. To this end, a BAF model describing the functioning of Seine Aval tertiary nitrification units was extended to include the main biological N2O production pathways. Studying the influence of the gas/liquid transfer representation on the prediction of nitrification performances and the gas/liquid partition of N2O fluxes showed that considering a mass balance on the gas phase did not significantly affect oxygen transfer. In contrast, including a mass balance was found essential to represent gas/liquid exchanges of nitric oxide (NO) and N2O. To investigate the triggers of N2O production, the biokinetic model was calibrated on a dataset including two years of functioning of the nitrification stage and two periods during which N2O fluxes were measured. A sensitivity analysis highlighted the major effect of parameters controlling the accumulation of nitrite, a precursor to N2O production, on the prediction of N2O concentrations. By modifying 7 parameters only (on over 90), the model was able to predict nitrification performances and the order of magnitude and main dynamics of N2O fluxes measured during both measuring campaigns. The calibrated model was then used to extrapolate the predictions on the entire period of study, and analyze the effect of operating conditions on N2O production mechanisms. The N2O emissions factor (EF), which corresponds to the proportion of influent ammonium emitted as N2O, was on average 2.2%, which is over 60 times the factor generally applied to estimate the GES balance of WRRFs. This factor fluctuates from 0.3 to 4.4%, mainly in correlation to the applied ammonium load, airflow rates, and temperature. Based on these results, mitigation levers were identified, and a statistical model was proposed as an alternative methodology to quantify N2O emissions.
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Submitted on : Monday, May 9, 2022 - 1:12:13 PM
Last modification on : Tuesday, May 10, 2022 - 3:51:08 AM

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2019JustineFiat.pdf
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  • HAL Id : tel-03662491, version 1

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Justine Fiat. Analyse and modelling of nitrous oxide emissions from full-scale tertiary nitrifying biological aerated filters. Chemical and Process Engineering. INSA de Toulouse, 2019. English. ⟨NNT : 2019ISAT0049⟩. ⟨tel-03662491⟩

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