Dead-end filtration of natural organic matter: experimental evidence of critical conditions

The development of membrane technology has reached a state whereby operational optimisation is becoming the major issue to both researchers and industrial operators. A key focus is towards sustainable operation where fouling is limited and cleaning is greatly reduced. The paper presents an example of such an approach for the filtration of natural organic matter (NOM) in a dead-end ultrafiltration cell. Sustainable operation has been assesse d in relation tothe application of a cessation period followed by a gentle rinse. The work demonstrates the existence of a critical filtered volume below which the mass accumulated at the membrane’s surface is reversible and above which significant fouling occurs. Further, appropriate selection of operating conditions (filtered volume and applied pressure) makes it possible to avoid the formation of an irreversible fouling layer

Low fouling conditions in dead-end filtration: Evidence for a critical filtered volume and interpretation using critical osmotic pressure

This paper presents experiments showing the existence of a critical filtered volume (CFV) when operating colloid dead-end filtration. The CFV is here defined as the filtered volume below which there is no irreversible (with respect to a break in the filtration) fouling on the membrane surface: it has thus the same meaning as cross-flow critical flux but applied to a dead-end process. The existence of the CFV is demonstrated when filtering stable latex or clay suspensions in constant-flux filtration experiments with alternating rinses: in contradiction to the current view, an irreversible deposit is not formed as soon as dead-end filtration begins. This critical filtered volume is shown to be dependent on the suspension stability and to be fully linked to the permeate flux: for permeate fluxes of 80 and 110 l h−1 m−2 the CFV is, respectively, 82 and 65 l m−2 for latex particles. Analyses of results are made by depicting the transition between concentration polarisation and deposit formation considering a critical osmotic pressure, which appears to be a characteristic of the fouling potential of a suspension. The results are discussed in the light of how this concept could lead to an interesting way to control and develop a strategy to operate filtration in dead-end mode

Effect of hydrophilic / hydrophobic fractions of Natural Organic Matter on irreversible fouling of membranes

Natural organic matter (NOM) has been identified as a major factor affecting membrane processes performances, but its impact is difficult to quantify from global parameters such as organic carbon content. The extent of fouling due to the different fractions of NOM from surface water has been examined in dead-end ultrafiltration using criteria such as flux decline and irreversibility in regard with organic matter rejection. The most important flux decline was observed during the filtration of the hydrophilic acids fraction whereas fulvic acids led to the most irreversible fouling. Furthermore, the hydrophilic fraction lost its fouling character when mixed with other fractions underlining that interactions between numerous components are possibly more important than the composition itself

Numerical simulation of colloid dead-end filtration: effect of membrane characteristics and operating conditions on matter accumulation

The aim of this work is to develop a simulation capability applicable to dead-end filtration of colloidal dispersions in order to investigate the effect of process conditions, such as membrane configuration and operating parameters, on filtration efficiency through the analysis of the appearance of a deposit on the membrane. To reach this goal, a model describing the transport behaviour of a concentrated colloidal dispersion is implemented in a commercial CFD code (ANSYS-CFX). The collective diffusion induced by inter-particle interactions is accounted for from knowledge of the variation of the osmotic pressure with the particle volume fraction. Coupled with a transient, two dimensional hydrodynamic solution, such a model allows description of the mass transport properties both in the dispersed (concentration polarization) and the condensed (deposit) forms of accumulation. Two-dimensional concentration profiles along the membrane are obtained. Simulations are used to understand the role of operating parameters and membrane characteristics on the appearance of a deposit at the membrane surface. This formation is controlled by the hollow fibre configuration, where there are zones working in both cross-flow and dead-end mode due to the particular hydrodynamic conditions

Stability and performance of two GSBR operated in alternating anoxic/aerobic or anaerobic/aerobic conditions for nutrient removal

Two granular sludge sequencing batch reactors (GSBR) with alternating anoxic/aerobic (R1) and anaerobic/aerobic (R2) conditions were operated with a 4-carbon-source synthetic influent. The physical properties of the granular sludge were very good (SVI≈20 mL g−1) and high solid concentrations (up to 35 g L−1) were obtained in the bioreactor operated with a pre-anoxic phase with additional nitrate (R1). In contrast, performance and granule settleability were lower in R2 due to the development of filamentous heterotrophic bacteria on the surface of granules. These disturbances were linked to the fact that a fraction of COD remained during the aerobic phase, which was not stored during the anaerobic period. To stabilize a GSBR with a mixture of organic carbon sources, it is thus necessary to maximize the amount of substrate used during the non-aerated, anaerobic or anoxic, phase. Comparable phosphate removal efficiency was observed in both systems; enhanced biological P removal being greater in anaerobic/aerobic conditions, while the contribution of precipitation (Ca–P) was more significant in anoxic/aerobic conditions

Analyse et modélisation du traitement de l'azote dans un procédé de granulation aérobie hydride

Dans ce travail, le traitement des nutriments et plus particulièrement celui de l azote en procédé de granulation aérobie a été étudié. L approche expérimentale a dans un premier temps eu pour objet de comparer la stabilité des caractéristiques physiques et microbiologiques d agrégats développés dans deux réacteurs fonctionnant en alternances de phases anaérobie / aérobie ou anoxie / aérobie. L opération d un procédé de granulation aérobie en alternance de phases anoxie / aérobie a favorisé la stabilisation des performances de traitement de l azote et notamment celle de la nitrification. Le développement d une boue hybride comprenant une fraction de flocs et de granules a été observé. Dans le but d évaluer comment le caractère hybride de la boue obtenue en conditions anoxie/aérobie oriente les performances et vitesses de transformation de l azote, une caractérisation ex-situ des limitations au transfert d oxygène au sein des flocs, granules et boue hybride a été réalisée par respirométrie. En complément, la localisation des espèces nitrifiantes par la technique d hybridation fluorescente in situ (FISH) a été réalisée. Les résultats obtenus indiquent que la présence de flocs au sein d une boue granulaire permet d augmenter la vitesse de la nitrification, en particulier pour de faibles concentrations en oxygène dissous. D autre part, il est ici mis en évidence la nécessité de contrôler simultanément le ratio floc/granules et la taille des granules en vue de l optimisation du traitement de l azote.Enfin un outil mathématique permettant de décrire les phénomènes réactionnels ayant lieu au sein d une biomasse hybride a été développé. Celui-ci a été employé dans le but d optimiser le fonctionnement du procédé hybride par l évaluation de l effet des propriétés physiques de la biomasse, et plus particulièrement de la proportion de granules en présence, pouvant conduire à une élimination efficace de l azote et à l augmentation de la robustesse du procédé vis-à-vis de diminutions ponctuelles de l oxygène dissousIn this work, the treatment of nutrients, especially that of nitrogen in aerobic granulation process was studied.The experimental approach has initially been intended to compare the stability and the physical and microbiological characteristics of aggregates developed in two reactors operating in alternating anaerobic / aerobic or anoxic / aerobic conditions. The presence of a pre-anoxic phase promoted the stabilization of nitrogen removal performances and especially those of nitrification. The development of a hybrid sludge process containing a fraction of flocs and granules was observed.In order to evaluate and quantify the influence of the simultaneous presence of flocs and granules in the nitrifying activity of the hybrid sludge developed in the alternating anoxic / aerobic conditions, the nitrification rate and oxygen limitation of flocs, granules and hybrid sludge was assessed using respirometric assays at different dissolved oxygen concentrations. The spatial distribution of nitrifying bacteria was investigated using fluorescence in situ hybridization (FISH). Results indicated that the presence of flocs with granules could increase the rate of nitrification to transitory reductions of aeration. On the other hand, the optimization of nitrogen removal requires the simultaneous control of the floc to granule ratio and granule size.Finally, a mathematical model to describe the reaction phenomena taking place in the hybrid biomass was developed. It was used in order to optimize the operation of the hybrid process through the evaluation of the effect of physical properties of biomass, specifically the ratio of granules and flocs in the reactor that can lead to efficient removal of nitrogen and increase the robustness of the processTOULOUSE-INSA-Bib. electronique (315559905) / SudocSudocFranceF

Numerical simulation of colloidal dispersion filtration: description of critical flux and comparison with experimental results

During filtration via membrane processes, colloids accumulate at the porous surface leading to fouling phenomena. In this study, a rigorous simulation of momentum and mass transfer using CFD modelling has been developed to describe such an accumulation during cross flow filtration. These simulations integrate detailed modeling of physicochemical properties specific to colloidal dispersions (because of the surface interactions (repulsive and attractive) occurring between the colloids particles). These interactions are accounted for via the experimental variation of the colloidal osmotic pressure with volume fraction (associated with a variation in the diffusion coefficient) which are fitted by a relationship integrated into the CFD code. It contains a description of the colloidal phase transition leading to the formation of a condensed phase (deposit or gel layer) from the accumulated dispersed phase (concentration polarization). It is then possible to determine the critical flux which separates filtration conditions below which mass accumulation is reversible (in the dispersed phase) and above which it is irreversible (in the condensed phase). The computed value of critical flux is compared with that determined experimentally for a dispersion of latex particles

Filtration frontale sur membrane : mise en évidence du volume filtré critique pour l'anticipation et le contrôle du colmatage

Membrane processes running in dead-end mode are limited by fouling. Control and anticipation of fouling are thus essential in improving process management: they are the focus of this study where the application in view is drinking-water production. It has been found experimentally that the transition between a reversible accumulation of matter (dispersed phase) and an irreversible deposit (condensed phase) can be characterized by a critical filtered volume; its dependence on filtration flux has been investigated with different kinds of dispersion. By analysing and simulating filtration, using models for the properties of the suspended colloids, the critical filtered volume was found to be related to osmotic pressure. These results have been used to improve the operating mode for filtering natural waters: when rinsing steps are performed before the filtered volume reaches its critical value, there is a significant decrease in fouling and an improvement in energy efficiency.Les procédés à membranes fonctionnant en mode frontal sont limités par le colmatage. La maîtrise et l'anticipation du colmatage, alors indispensables à la bonne gestion de l'opération, font l'objet de cette étude appliquée à la potabilisation d'eau. Un volume filtré critique, caractérisant la transition entre une accumulation réversible de matière (phase dispersée) et un dépôt irréversible (phase condensée), a été mis en évidence expérimentalement et ses variations avec le flux de filtration et le type de dispersion étudiées. L'analyse et la simulation théorique de la filtration, couplées à la modélisation des propriétés colloïdales, ont permis de relier la pression osmotique au volume filtré critique. Ces résultats ont été utilisés pour améliorer la conduite des opérations de filtration d'une eau de surface : des rinçages réalisés après un volume filtré inférieur à la valeur critique ont conduit à une réduction significative du colmatage et de la consommation énergétique associée

Filtration frontale sur membrane (mise en évidence du volume filtre critique pour l'anticipation et le contrôle du colmatage)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Simulation de la filtration frontale de suspensions colloïdales

Les procédés de filtration par membrane se sont beaucoup développés ces dernières années, notamment dans la production d'eau potable. Leur principale limitation dans ce secteur d'activité réside dans le colmatage, c'est-à-dire l'accumulation de matière à la surface de la membrane. Un des enjeux dans ce domaine est de pouvoir proposer des outils prédictifs de façon à anticiper la conduite des procédés. Dans ce cadre, une étude théorique et expérimentale est menée de façon à décrire l'accumulation de matière lors de filtrations frontales menées à flux constant. Lors de cette simulation transitoire, la pression osmotique est utilisée pour décrire le comportement spécifique des colloïdes dans toute la gamme de concentration rencontrée au cours de la filtration – depuis une phase dispersée jusqu'à la formation et la croissance d'un dépôt-. Si les simulations s'avèrent extrêmement sensibles à la taille des particules considérées, elles permettent cependant une bonne description qualitative et quantitative de l'accumulation de matière en mode frontal et de ses conséquences directe sur l'opération de filtration