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

Un protocole pour la modélisation du fonctionnement des stations d'épuration à boues activées GMP Protocol for activated sludge models

Les stations d’épuration sont des systèmes dynamiques, soumis à d’importantes variations temporelles. La modélisation de leur fonctionnement, qui consiste à représenter mathématiquement l’évolution dans le temps de variables d’intérêt représentant les phénomènes biologiques, physiques et chimiques qui se déroulent dans les ouvrages, est utilisée pour mieux comprendre les processus mis en jeu et optimiser le traitement des eaux résiduaires.. Focus sur les activités du groupe de travail Good Modelling Practice de l’IWA, auquel participe activement les équipes d’Irstea, notamment dans l’élaboration d’un protocole pour l’utilisation des modèles de boues activées en pratique.<br>The IWA Task Group on Good Modelling Practice (GMP) has analysed current practice and experience on the use of activated sludge models. The group developed a framework to make modelling more straightforward and that helps structuring the interaction between modellers and the project stakeholders. The GMP protocol is a synthesis of existing procedures, and contains five major steps: (i) Project definition, (ii) Data collection and reconciliation, (iii) Plant model set-up, (iv) Calibration and validation, and (v) Simulation and result interpretation. The Unified Protocol forms the basis of a Scientific and Technical Report that will be published in late 2012

Oxygen transfer under process conditions: comparison of measurements methods

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Modèles biocinétiques de boues activées de type ASM (analyse théorique et fonctionnelle, vers un jeu de paramètres par défaut)

La modélisation du fonctionnement des stations d'épuration est un outil largement utilisé notamment pour l'optimisation et la réhabilitation des ouvrages existants et le dimensionnement de nouvelles installations. S'assurer d'une bonne qualité des modèles est donc primordial. Or, d'après les résultats d'une enquête internationale effectuée auprès de 96 utilisateurs potentiels de modèles, deux étapes dans l'utilisation des modèles sont considérées comme particulièrement délicates: le choix du modèle à utiliser parmi les modèles disponibles et l'étape de calage de ces modèles. Le travail présenté visait à fournir des éléments pour lever des obstacles à une utilisation plus généralisée des modèles biocinétiques à boues activées. Il a porté sur sept des modèles publiés : (1) ASM1, (2) ASM2d, (3) ASM3, (4) ASM3+BioP, (5) ASM2d+TUD, (6) Barker & Dold et (7) UCTPHO+. Dans un premier temps, une analyse des connaissances pratiques des modèles a été effectuée afin d'améliorer le transfert des connaissances en modélisation. Une base de données de jeux de paramètres a été créée à partir d'études publiées et d'un questionnaire adressé aux utilisateurs de modèles. Cette base de données a notamment permis d'établir des fourchettes de valeurs utilisées pour l'ASM1 et l'ASM2d. Puis, une analyse connaissances théoriques ayant pour but d'aider les utilisateurs à mieux comprendre les sept modèles et à choisir le modèle adapté à leur projet a été réalisée. Les modèles étudiés ont d'abord été vérifiés et les erreurs de frappe et incohérences ont été corrigées. Les concepts de modélisation ont été comparés entre eux grâce à une nouvelle représentation graphique, et confrontés aux connaissances sur le fonctionnement biologique des boues activées afin de mettre en évidence les limites théoriques des modèles. En dernier lieu, une méthodologie a été développée pour l'obtention de jeux de paramètres par défaut qui pourraient être utilisés comme valeurs initiales lors du calage des modèles. Pour cela, une procédure de calage multi-jeux de données a été élaborée. Cela nécessite au préalable le développement d'une procédure de calage automatisée et l'utilisation d'un critère de qualité permettant de définir l'arrêt de la procédure de calage. Une analyse est effectuée sur les critères de qualité utilisés en sciences de l'environnementMathematical modelling of activated sludge systems has become a widely accepted tool and is used in particular for optimization and rehabilitation of existing plants and for new facilities design. Ensuring the adequate quality of modelling results is therefore essential. However, an international survey conducted among 96 potential users of activated sludge models (ASM) points out two main obstacles in the use of modelling: the choice of the model to use among the available models and the model calibration step. The objective of this work is to provide elements to overcome these obstacles and to promote wider use of biokinetic models for activated sludge systems. It focused on seven published models: (1) ASM1, (2) ASM2d, (3) ASM3, (4) ASM3+BioP, (5) ASM2d+TUD, (6) Barker & Dold and (7) UCTPHO+. First, an analysis of practical knowledge on the models was performed to improve the transfer of modelling knowledge. A database of practical modelling applications from published case studies and from the answers of a questionnaire sent to model users was created. This database enables to establish ranges of parameter values commonly used for the ASM1 and ASM2d. Then the theoretical knowledge on ASMs was analysed to help users to better understand the seven studied models and to choose the model appropriate to their project. The studied models were first verified and typing errors and inconsistencies have been corrected. The modelling concepts were compared to each others through a new graphical representation, and confronted with knowledge about the biology of activated sludge, in order to highlight the theoretical limits of each model. Finally, a methodology has been developed to obtain default parameter sets that could be used as initial values for model calibration. To this aim, an automated calibration procedure that allows calibration on multiple data sets was proposed. Then, the quality criteria used in environmental sciences have been synthesised. These criteria are required to determine the best set of parameters based on the goodness-of-fit of the model and to compare results from different models.PARIS-AgroParisTech Centre Paris (751052302) / SudocSudocFranceCanadaFRC