Gas–liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades from both academia and industry. Indeed, improvements in modelling gas–liquid mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid–base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas–liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas–liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas–liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas–liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners.
Abstract. Nitrous oxide (N2O) emissions from a nitrifying
biofilm reactor were investigated with N2O isotopocules. The nitrogen
isotopomer site preference of N2O (15N-SP) indicated the
contribution of producing and consuming pathways in response to changes in
oxygenation level (from 0 % to 21 % O2 in the gas mix), temperature
(from 13.5 to 22.3 ∘C) and ammonium concentrations (from 6.2 to
62.1 mg N L−1). Nitrite reduction, either nitrifier denitrification or
heterotrophic denitrification, was the main N2O-producing pathway under
the tested conditions. Difference between oxidative and reductive rates of
nitrite consumption was discussed in relation to NO2-
concentrations and N2O emissions. Hence, nitrite oxidation rates seem
to decrease as compared to ammonium oxidation rates at temperatures above 20 ∘C and under oxygen-depleted atmosphere, increasing N2O
production by the nitrite reduction pathway. Below 20 ∘C, a
difference in temperature sensitivity between hydroxylamine and ammonium
oxidation rates is most likely responsible for an increase in N2O
production via the hydroxylamine oxidation pathway (nitrification). A
negative correlation between the reaction kinetics and the apparent isotope
fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium.
The approach and results obtained here, for a nitrifying biofilm reactor
under variable environmental conditions, should allow for application and
extrapolation of N2O emissions from other systems such as lakes, soils
and sediments.
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