Utilisation de la spectroscopie acoustique pour la caractérisation de suspensions denses

L’objet de cet article est de présenter les avantages et les limitations de la spectroscopie acoustique pour l’analyse de systèmes dispersés denses de particules de taille micronique ou colloïdale. L’étude expérimentale, menée sur deux matériaux différents, est basée sur la mesure de l’atténuation acoustique d’une onde ultrasonore à différentes fréquences à travers la suspension. Dans le cas de suspensions denses de calcite de quelques dizaines de microns, l’exploitation des spectres d’atténuation acoustique permet d’accéder aisément à la distribution de taille des particules en suspension. L’utilisation d’une plage de fréquence réduite peut être envisagée pour réduire le temps d’analyse sans limiter pour autant la précision des résultats. Au contraire, les interactions particulaires existant au sein de suspensions denses de silice colloidale perturbent la réponse acoustique du milieu dispersé et ne permettent pas une exploitation directe des spectres d’atténuation. Des méthodes de correction sont proposées pour parvenir à une détermination correcte des distributions de taille. Enfin, en se fondant sur des mesures effectuées avec des mélanges de produits, l’utilisation de la spectroscopie acoustique pour caractériser les propriétés de suspensions de particules polydisperses est discutée

Effect of cytostatic drugs on microbial behaviour in membrane bioreactor system

The aim of this work is to evaluate the influence of cyclophosphamide and its principal metabolites (CPs) on microbial behaviour in a membrane bioreactor system. Two laboratory-scale membrane bioreactors (MBR) were run in parallel with a sludge retention time of 70 days (one with the cytostatic drugs, MBR-CPs, the second without, MBR-control). The microbial activity was measured by respirometric analysis. The endogenous and exogenous respirations of heterotrophic micro-organisms were evaluated. Micro-organisms exposed to CPs showed higher endogenous respiration rates and lower exogenous respiration rates than micro-organisms present in MBR-control. The effects were observed several days after adding the cocktail. Reduced sludge production was observed in MBR-CPs compared to MBR-control. This reduction of sludge production and the increase in the endogenous respiration rate in relation to MBR-control suggest that the chemical stress caused by CPs led to a diversion of carbon and/or energy from growth to adaptive responses and protection. In addition, the inhibitory effect on the assimilation of exogenous substrate (reduced exogenous respiration rate) suggests an inhibition of catabolism and anabolism despite the low CPs concentration studied (μg/L). However, this inhibitory effect can be offset by the biomass still active under low ratio (substrate/biomass) conditions in the bioreactor (due to complete retention of biomass and high sludge age), which helped to maintain high overall performance in the removal of conventional pollution

Organic pollution selective degradation by adsorption on high silica zeolites and regeneration by ozone

This study concerns the treatment of industrial airstreams polluted with volatile organic compounds (VOCs), and focuses on highlighting the experimental feasibility of a new hybrid process combining VOC adsorption on hydrophobic zeolites and regeneration by an ozonated airstream in a single reactor. A preliminary study of high-silica zeolites has shown the different kinds of interactions between gaseous ozone and two mineral zeolitic adsorbents, a faujasite Y (FauY) and a ZSM-5 type silicalite (SilZ), and the conservation of the solids adsorption properties (capacities and selectivity) after their contact with ozone. Here, results concerning the contact of gaseous ozone and zeolites saturated with adsorbed VOC are presented. Using temperature measurements inside the fixed bed, oxidation of two models VOC, methyl ethyl ketone (MEK) and toluene (TOL), was observed on both zeolites. Adsorbents were fully regenerated, and the adsorption capacities of the zeolites after their contact with ozone were not affected when used in a discontinuous adsorption / oxidation mode. In addition, the adsorbed VOC were totally mineralized. Detection and identification of oxidation sub-products traces using mass spectrometer - gas chromatography (formic, oxalic, acetic acids, and acetaldehyde for TOL and 2,3-butanedione and acetic acid for MEK) makes possible the indentification of oxidative species. Moreover, a mass balance on oxygen showed that all the produced ozone was used for organic compounds oxidation. There was no or very little ozone lost via side-effect decomposition/deactivation mechanisms. At last, using the zeolites selectivity determined in a previous study toward a binary MEK-TOL mixture, the adsorption / ozonation cyclic process enables the recovery of the minor compound and the selective oxidation of the other component. The great interest of this new process is clearly highlighted, and the technology transfer to industrial operating plants is now studied

Role and variations of supernatant compounds in submerged membrane bioreactor fouling

Many studies have been performed to analyze the influence of extracellular polymeric substances (EPS) in membrane fouling. Most of these works deal with the impact of solid contents in the fouling, and some of them have studied the role ofsupematant compounds. The aim of this work was to clarify the role of the different sludge fractions in the context of membrane bioreactor fouling. The laboratory-scale reactor used for experiments consists of a submerged membrane bioreactor for the treatment of synthetic wastewater. For the same organic load (0.4 g COD/g MLSS.d), several samples of sludge were taken off and divided intohree fractions (solid contents, soluble and colloids). COD and extractable EPS were quantified (carbohydrates and proteins). Dead-end filtration tests for each fraction were also carried out. According to these experiments, no correlation between EPS concentration in the solid part of the sludge and filtration resistance was found. Instead, a change of the filtration resistance was explained as a function of COD in the supematant, and more especially as a function of proteins concentration. Indeed, when the value of proteins concentration in the supernatant changes from 30 to 100 mg/1, the value of specific resistance increased by a factor of 10. Finally, the characterization of the supematant was shown as a key parameter for the MBR operating control. When the COD and proteins concentration in the supematant remained low, the transmembrane pressure in the reactor remained even lower. Moreover, with the biomass growth rate analysis, our results suggest that the EPS production was linked to growth of microorganisms. The faster he growth, the less EPS production

Aeration and hydrodynamics in submerged membrane bioreactors

Membrane bioreactor (MBR) is already a well-developed wastewater treatment process for both municipal and industrial applications. Nonetheless, membrane fouling remains a significant problem for its wider development. In the case of submerged membrane bioreactors (SMBRs), one of the most efficient strategies to limit fouling is the use of a gas/liquid two-phase flow to enhance the mass transfer. However, the effect of aeration still remains incompletely understood. The complexity of flows and of the nature of activated sludge makes a theoretical approach difficult. Aeration is the source of a large part of the operating costs in most industrial scale plants and its optimization is a necessity to make the process really efficient. This paper first deals with hydrodynamics in MBRs, then it reviews the parameters of aeration and their impact on filtration performance. Finally, the effects of aeration mechanisms on biological media are described

Agrégation de dispersions concentrées de silice colloïdale sous écoulement

L’objectif de cette étude est d’analyser le comportement de suspensions denses de nanoparticules soumises à des forces hydrodynamiques lors de procédés d’agrégation. Le phénomène d’agrégation des particules, obtenu par déstabilisation de dispersions stables de silice colloïdale a été observé ici en l’absence de tout autre phénomène d’apparition ou de croissance des particules. La déstabilisation des suspensions colloïdales, mise en oeuvre dans une cuve agitée discontinue, est provoquée par ajout de sel, modifiant ainsi la force ionique du système. L’utilisation de différentes techniques de caractérisation de la granulométrie des particules (spectroscopie acoustique, spectroscopie par corrélation de photons) et de leurs propriétés physico-chimiques (stabilité, pH, potentiel zêta) a permis d’appréhender les propriétés des suspensions initiales. La technique de spectroscopie acoustique a ensuite été utilisée lors d’expériences d’agrégation réalisées sous différentes conditions de concentration en solide pour analyser l’évolution au cours du temps de la distribution de taille des agrégats en milieu concentré et définir les mécanismes d’agrégation de la silice colloïdale sous agitation

Dynamical Modelling and Simulation of Waste water Filtration Process by Submerged Membrane Bioreactors

A mathematical model was developed for the filtration process and the influence of aeration on Submerged Membrane Bioreactors. The dynamics of sludge attachment to and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The influence on the membrane fouling of intermittent aeration injected on the membrane surface, and its synchronization with intermittent filtration, were studied numerically and experimentally. For the evaluation of filtration cake development, the assumption of the presence of two cake layers (one dynamic and the other stable) was considered. The model development and simulation focused on the description of existing relationships among important system variables like mixed liquor suspended solids concentration, aeration, temperature of the sludge suspension, transmembrane pressure, and the fouling increase during the filtration process. The model obtained offers the possibility of improving the design configuration and operation strategies of Submerged Membrane Bioreactors in wastewater treatment, and it allows the of aeration-filtration cycles to be optimized

Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics, aeration and fouling process

A mathematical model was developed to simulate filtration process and aeration influence on Submerged Membrane Bioreactor (SMBR) in aerobic conditions. The biological kinetics and the dynamic effect of the sludge attachment and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The model was established considering soluble microbial products (SMP) formation-degradation. The fouling components responsible of pore clogging, sludge cake growth, and temporal sludge film coverage were considered during calculation of the total membrane fouling resistance. The influence of SMP, trans-membrane pressure, and mixed liquor suspended solids on specific filtration resistance of the sludge cake was also included. With this model, the membrane fouling under different SMBR operational conditions can be simulated. The influence of a larger number of very important process variables on fouling development can be well quantified. The model was developed for evaluating the influence on fouling control of an intermittent aeration of bubbles synchronized or not with the filtration cycles, taking into account the effects of shear intensity on sludge cake removal

Analyse morphologique des boues biologiques d'épuration. Descriptifs les plus pertinents et propriétés physico-chimiques des boues d’épuration

Analyse morphologique des boues biologiques d'épuration. Descriptifs les plus pertinents et propriétés physico-chimiques des boues d’épuratio

Effect of cytostatic drugs on the sludge and on the mixed liquor characteristics of a cross-flow membrane bioreactor: Consequence on the process

The influence of cyclophosphamide and its principal metabolites (CPs) on the physicochemical properties of the mixed liquor of a cross-flow membrane bioreactor and the consequences for membrane fouling were investigated. The influence of CPs was determined by comparing the performance of two bioreactors running in parallel, MBR-CPs (with CPs) and MBR-control (without CPs). The physicochemical properties of the mixed liquor were characterized by soluble extracellular polymeric substances (soluble EPS content), particle size distribution and specific cake resistance. Results suggested that the CP toxicity altered the characteristics of the biological matrix of the activated sludge. Micro-organisms exposed to CPs showed higher endogenous respiration rates than MBR-control micro-organisms. The accumulation of soluble EPS and the formation of small particles (in MBR-CPs after cross-flow velocity was raised) increased the resistance to filtration. The fouling potential of the supernatant seemed to be linked more closely to the concentration of polysaccharides than of proteins and humic substances. Modifications of the membrane performance were observed. Under operating conditions, membrane fouling was faster in MBR-CPs than MBR-control. Moreover, the membrane played an important role in the permeate quality and the overall performance of the process, making possible the biological treatment of such an effluen