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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 a-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 a-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 FROM AUTHOR]

Abstract: The objective of this experimental study was to collect and to interpret data in order to better understand the oxygen mass transfer phenomena occurring in full-scale aeration tanks equipped with fine bubble diffusers and slow speed mixers (inducing horizontal liquid flows). Bubble size, local depth and oxygen mass transfer coefficient were measured in situ for a given air flow rate (1555Nm3 h−1) and for two different axial liquid velocities. The increase in the global oxygen transfer coefficient is of 29% when the mean axial liquid velocity varies from 0 to 0.42ms−1. The small influence of the liquid velocity on the local bubble Sauter diameter (about −4%) cannot explain the increase in k L a 20. This increase in k L a 20 with the axial liquid velocity is mainly due to the attenuation of the vertical liquid circulation induced by the gas injection. [Copyright &y& Elsevier]

[Display omitted] • Hydrodynamics and liquid/gas transfer are described using a comprehensive 1-D model. • The axial 1-D model is applied to 3 datasets obtained on bubble columns with low ɛ g. • Gas hold-up and mass transfer are affected by contamination and pressure effects. • Global apparent mass transfer coefficient K L a is lower than axial averaged . • The depletion effect in the gas is emphasized in tall bubble columns and with low d b. This study proposed to develop a model coupling hydrodynamics and mass transfer in order to gain an understanding of measurements taken on air–water bubble columns with low gas hold-up. Three experimental datasets with various operating conditions (water quality, liquid height, air flow range) were chosen. The model analyzes and interprets the significant impact of the local hydrostatic pressure and the effects of contamination on hydrodynamic and mass transfer parameters. The oxygen concentration in gas significantly depletes with the distance from diffusers, which explains the difference between the calculated mean of the local and global K L a coefficients. This difference is highly significant for a high bubble column and/or systems with a low mean bubble size. The impact of water quality on mass transfer can be characterized by the contamination angle using comprehensive 1-D modeling and highlights a differentiated impact on the hydrodynamic or mass transfer parameters. [ABSTRACT FROM AUTHOR]

In order to optimize aeration in the activated sludge processes, an experimentally validated numerical tool, based on computational fluid dynamics and able to predict flow and oxygen transfer characteristics in aeration tanks equipped with fine bubble diffusers and axial slow speed mixers, is proposed. For four different aeration tanks (1;1493;8191 and ), this tool allows to precisely reproduce experimental results in terms of axial liquid velocities, local gas hold-ups. Predicted oxygen transfer coefficients are within 5% of experimental results for different operating conditions (varying pumping flow rates of the mixers and air flow rates). The actual bubble size must be known with precision in order to have a reliable estimation of the oxygen transfer coefficients. [Copyright &y& Elsevier]

Nitrifying biologically active filters (BAFs) have been found to be high emitters of nitrous oxide (N 2 O), a powerful greenhouse gas contributing to ozone layer depletion. While recent models have greatly improved our understanding of the triggers of N 2 O emissions from suspended-growth processes, less is known about N 2 O emissions from full-scale biofilm processes. Tertiary nitrifying BAFs have been modeled at some occasions but considering strong simplifications on the description of gas-liquid exchanges which are not appropriate for N 2 O prediction. In this work, a tertiary nitrifying BAF model including the main N 2 O biological pathways was developed and confronted to full-scale data from Seine Aval, the largest wastewater resource recovery facility in Europe. A mass balance on the gaseous compounds was included in order to correctly describe the N 2 O gas-liquid partition, thus N 2 O emissions. Preliminary modifications of the model structure were made to include the gas phase as a compartment of the model, which significantly affected the prediction of nitrification. In particular, considering gas hold-up influenced the prediction of the hydraulic retention time, thus nitrification performances: a 3.5% gas fraction reduced ammonium removal by 13%, as the liquid volume, small in such systems, is highly sensitive to the gas presence. Finally, the value of the volumetric oxygen transfer coefficient was adjusted to successfully predict both nitrification and N 2 O emissions. Image 1 • A nitrifying BAF model was extended to describe full-scale N 2 O emissions. • Inclusion of a mass balance for the gas phase is essential to describe N 2 O emissions. • Gas phase assumptions heavily impacted the prediction of nitrifying performances. • The calibrated model successfully described nitrification and the mass transfer of O 2 and N 2 O. [ABSTRACT FROM AUTHOR]

A full-scale nitrifying biofilter was continuously monitored during two measurement periods (September 2014; February 2015) during which both gaseous and liquid N 2 O fluxes were monitored on-line. The results showed diurnal and seasonal variations of N 2 O emissions. A statistical model was run to determine the main operational parameters governing N 2 O emissions. Modification of the distribution between the gas phase and the liquid phase was observed related to the effects of temperature and aeration flow on the volumetric mass transfer coefficient (k L a). With similar nitrification performance values, the N 2 O emission factor was twice as high during the winter campaign. The increase in N 2 O emissions in winter was correlated to higher effluent nitrite concentrations and suspected increased biofilm thickness. [ABSTRACT FROM AUTHOR]