Slow colloidal aggregation and membrane fouling

We observed that the concept of critical flux, although established on physical bases, does not describe all typical fouling situations found in membrane filtration. We especially focus on the slow flux decline that is observed in many industrial membrane applications, and that has found several types of explanations that we briefly discuss. In order to get a better understanding of this situation, we have considered the orders of magnitude of the slow aggregation kinetics that are expected to happen within a boundary layer, on an ultrafiltration or microfiltration membrane in operation. The results help to understand that whereas the critical flux is limiting cross-flow filtration of stable colloids, it should be combined to kinetics aspects of slow aggregation in cases of colloids of intermediate stability (metastable). We discuss some consequences on the design and operation of processes using membrane filtration

Calibration of ultrafiltration membranes against size exclusion chromatography columns

Using the extension of the concept of universal calibration parameter, yielding a relation between the hydrodynamic volume of molecules and the elution volume in sizeexclusionchromatography (SEC), to retention coefficients in ultrafiltration (UF), we propose a direct calibration of UF membranes against chromatography columns. Plotting the retention coefficient by one given UF membrane of a series of probe molecules versus their elution volume in SEC chromatography provides a calibration curve for this membrane. For a wide range of retentions, such calibration can be directly used to predict the retention of any molecule: one only needs to measure its exclusion volume by the SEC column, and read the retention by the calibrated membrane on the calibration curves. The method has been tested with dextran and PEG for the calibration, and milk proteins as test molecules, for three different membranes. The predicted values of the retention are in rather good agreement with those experimentally measured in a UF cell

Adsorption of MS2 bacteriophage on ultrafiltration membrane laboratory equipments

Virus adsorption has been quantified (i) on different materials including various containers made of glass, plastic or stainless steel and hollow fiber membranes made of polyethersulfone, polysulfone, PVDF or cellulose acetate (with two configurations: in/out and out/in and various molecular weight cut-offs); and (ii) on the filtration equipment. The selected virus is MS2 bacteriophage used as a model to estimate viral survival in water or to quantify virus elimination by membrane filtration. A series of experiments have been conducted with suspensions of MS2 at different concentrations prepared in a sterilized saline solution (8 g/L of KCl) or in sterilized distilled water. This study has shown that the most appropriate material to be used as a filtration test tank is Pyrex glass. We show that an addition of a virus solution 15 min after the beginning of the experiment allows keeping the virus concentration at a high level in the system (including tank, tubing,and pump). No adsorption was observed on membrane materials tested during soaking

Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

This paper discusses the distribution of critical flux (DCF) in cross flow filtration. These distributions are described here by a normal function with a mean critical flux and its standard deviation. The DCF model allows the description, through an analytical relationship, of the variation in steady state permeate flux with trans-membrane pressure. Both strong and weak forms of critical flux, which can be observed on a membrane operating in cross-flow mode, are depicted. A simple graphical method to determine the mean critical flux and its standard deviation from experimental results is derived from the theoretical model. The theoretical trends are compared to experimental data and show good agreement for cross flow filtration of latex and BSA suspensions. The distribution function parameters obtained by fitting the DCF model to experiments are compared to critical flux measured via a pressure step method. We thus propose a tool to analyse filtration results and to determine new global parameters for critical conditions (mean value and its standard deviation), which appears to be a good way to account for fouling complexity

Experimental study of the effects of hypochlorite on polysulfone membrane properties

Although chemical solutions are widely employed to clean membranes, they can also be responsible for changes in membrane properties. The two major drawbacks are then either the functional properties of the membrane gradually change, so the production can no longer meet requirements in terms of volume or quality, or the membrane simply breaks up (hollow fibers), and the system has to be shut down during maintenance. The aim of this experimental study was to gain a better understanding of the effect of hypochlorite cleaning solutions in different conditions (various pHs and temperatures) on the changes observed on an ultrafiltration hollow fiber membrane made from polysulfone (PSf) and polyvinylpyrrolidone (PVP). A wide range of methods characterizing the material have been used, from the atomic scale (ESCA) up to the module scale by force measurements on fibers. Exposure to sodium hypochlorite at rather high concentration seems to lead to chain breaking in the PSf molecules (gel permeation chromatography results). The consequences are changes in the membrane texture (scanning electron microscopy images), which are closely related to changes in the mechanical properties of the membrane. Membrane permeability appears to be poorly sensitive to such changes, which can be related to the fact that the permeability relies on the skin properties mainly

Removal of bisphenol A by a nanofiltration membrane in view of drinking water production

The efficiency with which a nanofiltration membrane (Desal 5 DK) removes bisphenol A(BPA) was investigated, together with the mechanisms involved. Whereas high retention (490%) was obtained at the beginning of the filtration, the observed retention coefficient(Robs) decreased to around 50% when the membrane became saturated, due to adsorption of BPA onto the membrane structure. The presence of ions (Na+, Cl) affects the Robs, this effect being attributed to a change in BPA hydrodynamic radius. Moreover, in our operating conditions, the presence of natural organic matter (1 mg/L) in the feed solution does not lead to variation in BPA retention at steady state

Ageing of polysulfone membranes in contact with bleach solution: Role of radical oxidation and of some dissolved metal ions

In water production plants using membrane processes, contact with chemicals such as chlorine plays an important role in membrane ageing. This experimental study was aimed at gaining a better understanding of the effect of hypochlorite cleaning solutions on the properties of a polysulfone ultrafiltration membrane. Accelerated ageing of the membrane was simulated by soaking it in chlorine solutions and the mechanical properties of the membrane were monitored versus soaking time. An oxidation mechanism is validated which involves the catalytic effect of dissolved metal ions and the inhibitor effect of an antioxidant when these are present in soaking solutions

Dichloroaniline retention by nanofiltration membranes

This study evaluates the performance of two nanofiltration membranes in removing a herbicide: dichloroaniline. The membranes, one polyamide and one cellulose acetate, have a cut-off in the range 150–300 g/mol (manufacturers’ data). The experiments were carried out with solutions of dichloroaniline in demineralized water, with concentrations from 1 to 10 ppb. For each membrane, the amount of herbicide retained and adsorbed by the membrane was determined as a function of feed concentration and transmembrane pressure. The two membranes, made of different materials but having the same nominal cut-off, retained dichloroaniline to very different extents and by different mechanisms

Critical and sustainable fluxes: theory, experiments and applications

Over the last ten years, numerous membrane filtration data have been viewed in the light of the concept of critical flux. This concept, used in a number of different ways often without explicit redefinition, is here clarified both from a theoretical and from an experimental viewpoint. Also, a link is make with the sustainable fluxes. Also covered are the various methods of measurement and the influence of membrane and suspension properties on the critical flux. Over the same period of time, models have been developed to explain the observed behaviour. Those for stable colloidal suspensions are based on the existence of repulsive interactions between soft matter constituents. The assumptions and usefulness of various models are discussed. The concept of a critical concentration for phase transition is introduced into the theoretical discussion. For theoreticians and experimentalist, this and the clarified concept of a small set of critical fluxes will continue to provide a valuable framework. For membrane users dealing with most industrial process streams (mixtures and complex fluid) the concept of a sustainable flux (shown as being derived from critical flux) is of a great utility; above a certain key flux (dependent on hydrodynamics, feed conditions and process time) the rate of fouling is economically and environmentally unsustainable. For many, knowledge of the point below which no major irreversible fouling occurs (the critical flux) in a membrane separation will always be of greatest utility

Role of the cell-wall structure in the retention of bacteria by microfiltration membranes

This experimental study investigates the retention of bacteria by porous membranes. The transfer of bacteria larger than the nominal pore size of microfiltration track-etched membranes has been studied for several kinds of bacterial strains. This unexpected transfer does not correlate to the hydrophobicity,neither to the surface charge of the microorganism, as suggested in previous reports. We conclude that,in our conditions, the kind of bacteria (Gram-positive or Gram-negative) is finally the most important parameter. As the distinction between those two types of bacteria is related to the cell-wall structure, we provide an experimental evidence, via the action of an antibiotic, that the cell-wall flexibility triggers the transfer of the bacteria through artificial membranes, when the pores are smaller in size than the cell