Electrical resistivity tomography used to characterize bubble distribution in complex aerated reactors: Development of the method and application to a semi-industrial MBR in operation

Membrane bioreactors (MBRs) are widely used in wastewater treatment processes. However, membrane fouling mitigation remains challenging. Several strategies have been developed industrially to enhance MBR productivity, including coarse bubble aeration. The way such aeration participates in hydrodynamic patterns is an important research topic given its major contribution to the energy costs of such facilities. The methods currently used for hydrodynamic characterization suffer from several drawbacks, mainly due to the system’s complexity. Consequently, there is a need for a nonintrusive method that could be employed in reactors with complex internal geometry and in the presence of activated sludge. This article presents the evaluation and adaptation of the electrical resistivity tomography (ERT) to gain insights into hydrodynamic conditions and to determine how bubbles are distributed within membrane bioreactors in different aeration conditions. An approach used by geophysicists was adapted to a semi-industrial MBR: a numerical procedure was used to validate ERT’s ability to recover precise information in a complex geometry such as MBR membrane tank. Experiments were conducted in a semi-industrial membrane bioreactor with clear water and activated sludge. The resulting images were analyzed in terms of bubble dispersion over a section of the pilot. Heterogeneities were detected in all configurations studied in numerical simulations, although the results also emphasize the diffuse character of gas distribution obtained with the ERT method. Experimental results highlight how gas distribution is mainly localized inside membrane modules and its homogeneity over the module depends on activated sludge rheological properties and air flow rate. MBR operation could be optimized by considering the operating conditions which provide efficient gas distribution over the membrane module obtained at a scale representative of industrial reactors

Modelling gas-liquid mass transfer in wastewater treatment : when current knowledge needs to encounter engineering practice and vice versa

Abstract 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

Pantropical variability in tree crown allometry

Aim Tree crowns determine light interception, carbon and water exchange. Thus, understanding the factors causing tree crown allometry to vary at the tree and stand level matters greatly for the development of future vegetation modelling and for the calibration of remote sensing products. Nevertheless, we know little about large‐scale variation and determinants in tropical tree crown allometry. In this study, we explored the continental variation in scaling exponents of site‐specific crown allometry and assessed their relationships with environmental and stand‐level variables in the tropics. Location Global tropics. Time period Early 21st century. Major taxa studied Woody plants. Methods Using a dataset of 87,737 trees distributed among 245 forest and savanna sites across the tropics, we fitted site‐specific allometric relationships between crown dimensions (crown depth, diameter and volume) and stem diameter using power‐law models. Stand‐level and environmental drivers of crown allometric relationships were assessed at pantropical and continental scales. Results The scaling exponents of allometric relationships between stem diameter and crown dimensions were higher in savannas than in forests. We identified that continental crown models were better than pantropical crown models and that continental differences in crown allometric relationships were driven by both stand‐level (wood density) and environmental (precipitation, cation exchange capacity and soil texture) variables for both tropical biomes. For a given diameter, forest trees from Asia and savanna trees from Australia had smaller crown dimensions than trees in Africa and America, with crown volumes for some Asian forest trees being smaller than those of trees in African forests. Main conclusions Our results provide new insight into geographical variability, with large continental differences in tropical tree crown allometry that were driven by stand‐level and environmental variables. They have implications for the assessment of ecosystem function and for the monitoring of woody biomass by remote sensing techniques in the global tropics

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

La mécanique des fluides numérique appliquée à l'optimisation du transfert d'oxygène dans les bassins d'aération

L’optimisation des dispositifs d’aération équipant les bassins aérobies des stations d’épuration est essentielle pour garantir la qualité du traitement des eaux résiduaires, mais aussi pour limiter les coûts de construction et énergétiques des installations. À l’aide de la mécanique des fluides numérique, Irstea développe des modèles pour améliorer la conception et le fonctionnement de ces dispositifs

Modélisation de l'hydrodynamique et du transfert d'oxygène dans les chenaux d'aération

L’objectif général du travail présenté était de développer un outil d'analyse et de simulation\ud des écoulements et du transfert d'oxygène en bassins d'aération équipés de diffuseurs fines\ud bulles et d'agitateurs lents à grandes pales. A terme, cet outil permettra de simuler et\ud d'interpréter l'impact de paramètres de conception, de dimensionnement et de gestion\ud technique des installations sur les capacités d'oxygénation des systèmes d'aération.\ud Dans un premier temps, des méthodes de mesure ont été développées pour mesurer in situ les\ud paramètres locaux caractéristiques des écoulements et du transfert d'oxygène en eau claire sur\ud site réel (vitesse de circulation horizontale de l'eau, rétention gazeuse, taille des bulles). La\ud faible influence de la mise en circulation de l'eau sur la taille des bulles d'air (de 4,6 à 4,4 mm\ud pour une augmentation de la vitesse horizontale de l'eau de 0 à 0,42 m.s-1) a été mise en\ud évidence. L'augmentation du transfert d'oxygène (+29%) par imposition d'une vitesse de\ud circulation horizontale de l'eau est principalement due à une augmentation de la rétention\ud gazeuse globale, par neutralisation des spiral flows.\ud Le deuxième objectif a été de développer un outil numérique permettant de simuler les\ud écoulements gaz/liquide dans les bassins d'aération. Cet outil, validé expérimentalement sur\ud un bassin pilote (1 m3) et sur un site réel (1493 m3), permet de reproduire précisément le\ud transfert d'oxygène.\ud Dans un troisième temps, cet outil numérique a permis d'analyser localement les phénomènes\ud de transfert d'oxygène. L'évolution de la concentration en oxygène à saturation instantanée\ud CL\ud * et son influence sur la détermination du coefficient de transfert d'oxygène ont été mises en\ud évidence. Une adaptation de la méthode de détermination du coefficient de transfert\ud d'oxygène réel prenant en compte cette évolution a été proposée.\ud L'impact de la disposition des rampes de diffuseurs, du débit d'air surfacique et de la vitesse\ud de circulation de l'eau sur le transfert d'oxygène ont été analysé globalement et localement. - The objective of this work was to develop a tool in order to analyse and to model\ud hydrodynamics and oxygen mass transfer in aeration tanks equipped with EDPM fine bubble\ud membrane diffusers and slow speed mixers. This tool will be used to simulate and interpret\ud the impact of design and operating parameters on oxygen transfer in aeration tanks.\ud Measurement methods were firstly developed to determine in situ local parameters that\ud govern hydrodynamics and oxygen mass transfer (horizontal liquid velocity, gas hold-up,\ud bubble size), in clean water on real sites. The bubble size slightly decreases with the\ud horizontal liquid velocity (from 4.6 to 4.4 mm for an increase in horizontal liquid velocity\ud from 0 to 0.42 m.s-1). The obtained enhancement of the oxygen transfer coefficient (+29%)\ud with the horizontal liquid velocity is mainly due to the increase in the global gas hold-up, by\ud neutralization of the spiral flows.\ud The second objective was to develop a computerised tool in order to simulate gas/liquid flows\ud in aeration tanks. This tool, validated on a pilot unit (1 m3) and on a real site (1493 m3), can\ud precisely reproduce the oxygen transfer.\ud Finally, this numerical tool was used to analyze the local oxygen transfer phenomena. The\ud evolution of the instantaneous saturation oxygen concentration CL* and its influence on the\ud determination of the oxygen transfer coefficient were highlighted. An adaptation of the\ud method to determine the oxygen transfer coefficient taking into account this evolution was\ud proposed.\ud The impact of the arrangement of diffuser grids, of the surface air flow rate (air flow divided\ud by the total area covered by the diffuser grids) and of the horizontal liquid velocity on oxygen\ud transfer was globally and locally analyzed

Modélisation de l' Hydrodynamique et du transfert d'oxygène dans les chenaux d'aération

TOULOUSE-INSA (315552106) / SudocSudocFranceF

La mécanique des fluides numérique appliquée à l'optimisation du transfert d'oxygène dans les bassins d'aération

L’optimisation des dispositifs d’aération équipant les bassins aérobies des stations d’épuration est essentielle pour garantir la qualité du traitement des eaux résiduaires, mais aussi pour limiter les coûts de construction et énergétiques des installations. À l’aide de la mécanique des fluides numérique, Irstea développe des modèles pour améliorer la conception et le fonctionnement de ces dispositifs