Filtration characteristics of hollow fiber microfiltration membranes used in a specific double membrane bioreactor

The performance of the microfiltration in a specifically designed membrane bioreactor operating under various transmembrane pressures with periodic backwashing was investigated for model media. These media were representative of some usual components of a fermentation medium: BSA solution (2 g L−1), yeast suspension (8 g L−1, dry mass) and a mixture of BSA/yeast (2 g L−1/8 g L−1). In this system, the separation was provided by a 0.1 μm polysulfone hollow fiber membrane. The net permeate fluxes observed for yeast/BSA mixture were proportional to the transmembrane pressure applied (ΔP) but were less than those obtained with water osmosis, showing that, in spite of the periodic backwash, a small amount of irreversible fouling remained. This fouling can be assumed to be due to internal fouling by protein and/or external fouling by a residual yeast cake. Moreover, the net permeate flux obtained with the yeast/BSA mixture was higher than that obtained with the BSA alone, showing that a thin yeast cake probably acted as a primary filtration layer that could protect the polysulfone membrane against protein fouling. These experiments enable operating recommendations to be made for the use of this specific bioreactor concerning the transmembrane pressure value and the possible addition of inert particles

A numerical approach to study the impact of packing density on fluid flow distribution in hollow fiber module.

The aim of this study was to analyze the influence of hollow fiber module design, specially packing density, and filtration operating mode on the filtration performance. In order to perform this analysis, a model based on the finite element method was used to simulate numerically the flow and filtration velocity along the fiber. An annular region of fluid surrounding the fiber was considered in order to account for the packing density Φ of the module. The originality of this approach lies in the study of fiber density effect on the hydrodynamic conditions, both for inside/out (IO) and outside/in (OI) filtration modes. The numerical simulations of fluid flow have shown a modification of the axial filtration velocity profile with packing density. When the density of fibers was high, filtration took place preferentially in the bottom of the fiber. In contrast, when the packing density was low, permeate flow was higher at the top of the fiber, i.e. the filtration module. Two experimental hollow fiber modules with two packing densities were tested and showed good agreement with the numerical data. These results underline the variations of filtration velocity along the fiber that will allow some predictions on fouling deposit to be done

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

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

Optimisation of pressurized liquid extraction using a multivariate chemometric approach for the determination of anticancer drugs in sludge by ultra high performance liquid chromatography–tandem mass spectrometry

The present paper describes an analytical method for the determination of 2 widely administered anticancer drugs, ifosfamide and cyclophosphamide, contained in sewage sludge. The method relies on the extraction from the solid matrix by pressurized liquid extraction, sample purification by solid-phase extraction and analysis by ultra high performance liquid chromatography coupled with tandem mass spectrometry. The different parameters affecting the extraction efficiency were optimized using an experimental design. Solvent nature was the most decisive factor for the extraction but interactions between some parameters also appeared very influent. The method was applied to seven different types of sludge for validation. The performances of the analytical method displayed high variability between sludges with limits of detection spanning more than one order of magnitude and confirming the relevance of multi-sample validation. Matrix effect has been determined as the most limiting analytical step for quantification with different extent depending on analyte and sludge nature. For each analyte, the use of deuterated standard spiked at the very beginning ensured the complete compensation of losses regardless of the sample nature. The suitability of the method between freshly spiked and aged samples has also been verified. The optimized method was applied to different sludge samples to determine the environmental levels of anticancer drugs. The compounds were detected in some samples reaching 42.5 μg/kgDM in ifosfamide for the most contaminated sample

Rheological characterization of mixed liquor in a submerged membrane bioreactor: Interest for process management

Rheological analyses of a submerged membrane bioreactor mixed liquor were performed in the aim of characterizing the mixed liquor present in the bioreactor and thus proposing a process management. These analyses pointed out that the mixed liquor was characterized by its viscoplastic property, which leads to a possible restructuring ability when a shear stress lower than the yield stress is applied. As the shear stress in the bioreactor is essentially generated by coarse bubbles, specific experiments were carried out in which coarse bubbles were injected in an intermittent way. The results of these experiments showed that this method could avoid damage to the mixed liquor. So working with intermittent coarse bubbles is useful to prevent fouling, keep good flocculation and minimize the energy cost

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

Aeration mode, shear stress and sludge rheology in a submerged membrane bioreactor: some keys of energy saving

Aeration mode, shear stress and sludge rheology in a submerged membrane bioreactor: some keys of energy savin

Analysis of membrane fouling during cross-flow microfiltration of wine

The aim of this study was to investigate the individual impact on wine molecules as tannins, pectins and mannoproteins on multichannel ceramic membrane fouling during wine cross-flow microfiltration. The characterization of fouling mechanisms involved in the previous filtrations was realized by using the classical fouling models and the analysis of the total resistance curves. It was shown that the obtained initial fluxes are dependant of the nature of the studied molecules and their concentration. According to their increasing effect on permeate flux decline, the studied wine components could be ranked as: mannoproteins < tannins < pectins. During the filtration of wine added with tannins, it was found that the filtrations were governed by the cake layer formation mechanism. The presence of pectins caused the formation of gel-type layer which is found to be compressible under high pressures. For wines added with mannoprotein filtrations, it was shown that there is a threshold concentration above which a plateau value of permeate flux is obtained. Industrial relevance: The cross-flow microfiltration applied to wine filtration has become a legitimate alternative to conventional filtration processes. However, membrane fouling which affects the operating costs and the plant maintenance, limits the widespread application of this technique. To avoid or reduce membrane fouling, it is extremely important to identify the fouling elements and the mechanisms that govern the process. A better understanding of the mechanisms whereby fouling is formed during wine microfiltration may lead to be in position to control fouling or reduce it, to improve cleaning procedures and to adapt the process to the product to be filtered. The results presented in this paper concern the investigation and the understanding of fouling mechanisms by wine colloids (tannins, pectins and mannoproteins). We found that wine colloids had a strong impact on membrane fouling. Independently of their concentrations found in wine, they can be ranked according to their increasing effect on permeate flux as: mannoproteins b tannins b pectins. Such result provides important information and a better vision on the methods which can be used to limit membrane fouling for example the use of pectinolytic enzymes before filtration in order to hydrolyze pectin chains or precipitation of unstable tannins by finning the wine with bentonite. By elucidating fouling mechanisms such as cake layer and gel type layer, we can adapt the hydrodynamic process to control membrane fouling