Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration process is to apply controlled transmembrane pressures to the retentate side of the membrane, with the permeate side open ended. Often the minimum transmembrane pressure available is sufficient to cause membrane fouling in a given system. A membrane rig has been built which monitors transmembrane pressure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detection of trace fouling. Under a variety of low flux conditions fouling was not observed and it was found to be useful to produce an experimentally related definition of two types of critical flux. In the first definition a `strong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the second definition a `weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line differs from that for clean water. This paper describes how the use of this operating mode led to the successful experimental measurements of critical fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experimental evidence in support of a `strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membrane pore size on critical flux measurements for the three types of feed fluids.     

Analysis of humic acid fouling during microfiltration using a pore blockage–cake filtration model

Fouling by natural organic matter, such as humic substances, is a major factor limiting the use of microfiltration for water purification. The objective of this study was to develop a fundamental understanding of the underlying mechanisms governing humic acid fouling during microfiltration using a combined pore blockage–cake filtration model. Data were obtained over a range of humic acid concentrations, transmembrane pressures, and stirring speeds. The initial flux decline was due to pore blockage caused by the deposition of large humic acid aggregates on the membrane surface, with a humic acid deposit developing over those regions of the membrane that have first been blocked by an aggregate. The rate of cake growth approaches zero at a finite filtrate flux, similar to the critical flux concept developed for colloidal filtration. The data were in good agreement with model calculations, with the parameter values providing important insights into the mechanisms governing humic acid fouling during microfiltration. In addition, the basic approach provides a framework that can be used to analyze humic acid fouling under different conditions.     

A Combined Pore Blockage and Cake Filtration Model for Protein Fouling during Microfiltration

Previous studies of protein fouling during microfiltration have shown significant discrepancies between filtrate flux data and predictions of the classical pore blockage, pore constriction, and cake filtration models. A new mathematical model was developed for the filtrate flux which accounts for initial fouling due to pore blockage and subsequent fouling due to the growth of a protein cake or deposit over these initially blocked regions. The model explicitly accounts for the inhomogeneity in the cake layer thickness over different regions of the membrane arising from the time-dependent blockage of the pore surface. The model was shown to be in excellent agreement with experimental data obtained during the stirred cell filtration of bovine serum albumin solutions through polycarbonate track-etched microfiltration membranes over the entire course of the filtration. The model provides a smooth transition from the pore blockage to cake filtration regimes, eliminating the need to use different mathematical formulations to describe these two phenomena. In addition, the model provides the first quantitative explanation for some of the unusual observations reported previously in investigations of protein microfiltration. The results provide important insights into the underlying mechanisms of protein fouling during microfiltration. Copyright 2000 Academic Press.     

Yeast cells,beer composition and mean pore diameter impacts on fouling and retention during cross-flow filtration of beer with ceramic membranes

Clarification of rough beer (RB) and pasteurisation of clarified beer (CB) by cross-flow microfiltration (CFMF) stand as potential applications of membranes in the food industry. This study is based on the comparison of the resistances to mass transfer obtained during the filtration of RB and CB. Empirical correlations made it possible to calculate the CB resistances for the same conditions of transmembrane pressure and cross-flow velocity as for the RB filtration runs. The 1.4 μm membrane led to much lower resistances than the other membranes because of the lower retention rates of the beer compounds such as proteins and polyphenols. In this case, the predominant fouling mechanism was due to the yeast cell layer which was very sensitive to cross-flow velocity. On the other hand, with the 0.1, 0.45 and 0.8 μm membranes, the retention or adsorption of the CB compounds, such as proteins and polyphenols was the predominant mechanism. The resistances obtained with the rough and CB were fairly similar. We assume that the presence of yeast cells lead to less compact proteins and polyphenols fouling.     

Reversibility of fouling formed in activated sludge filtration

This paper investigates the reversibility of membrane fouling by activated sludge in a membrane bioreactor equipped with a 0.1 μm pore ceramic membrane. The membrane was submitted to a series of tests in which the permeate flux, the transmembrane pressure (TMP) or the circulation velocity were successively varied in cycles by step increments or decreases. When the permeate flux is set below the critical flux, the TMP remains stable and fouling is reversible. On the contrary, when the critical flux is exceeded, the TMP increases and does not stabilize, as in dead-end filtration. The fouling formed is partly irreversible when the flux is lowered again. When the TMP is first increased up to 400 kPa and then decreased back at constant velocity, no hysteresis is found on the flux–TMP graph, showing that fouling is reversible in this case. Velocity cycles were performed by first lowering the velocity from 5 to 1 m/s and raising it again to 5 m/s. In this case again, the fouling induced by reducing the velocity was found to be reversible. However, when the same pressure and velocity cycle tests were performed with activated sludge collected in the aeration tank of a classical wastewater treatment plant, fouling was found to be partly irreversible, showing that the cake formed in the absence of shearing is much more cohesive. In the final part of the paper, we tested a hydrodynamic method of fouling control consisting in alternating short periods of filtration (1–4 s) and short periods of washing (1 or 2 s) at low TMP and high velocity. This method yielded to a 20% permeate flux increase with a 10% reduction in hydraulic energy consumption for classical plant activated sludge.     

Humic acid fouling during microfiltration

A major factor limiting the use of microfiltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during microfiltration have been examined using stirred cell filtration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large flux decline observed during constant pressure microfiltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Prefiltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids.     

The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration

Although macromolecular fouling of microfiltration membranes is one of the critical factors governing the performance of these filtration processes, there is still little fundamental understanding of the underlying phenomena that influence the initiation, rate, and extent of fouling. We have obtained experimental data for the flux decline during the stirred cell filtration of different commercial preparations of bovine serum albumin (BSA) through asymmetric polyethersulfone microfiltration membranes. The fouling characteristics of these commercial solutions varied substantially, with the flux decline directly related to the technique utilized to initially precipitate and prepare the BSA. Prefiltration of BSA solutions prior to microfiltration substantially reduced their fouling tendency, with the degree of improvement increasing as the prefiltration was performed through smaller molecular weight cut-off membranes. The protein solutions were also characterized using gel permeation chromatography (GPC), with the fouling tendency of the different BSA preparations highly correlated with the concentration of BSA dimers and other high molecular weight species present in these BSA solutions. These results suggest that BSA fouling of these microfiltration membranes is associated with the deposition of trace quantities of aggregated and/or denatured BSA, with these fouling species serving as initiation sites for the continued deposition of bulk protein.     

Influence of permeate flux on fouling during the microfiltration of β-lactoglobulin solutions under cross-flow conditions

Experiments to investigate the microfiltration fouling behaviour of a β-lactoglobulin solution were performed on a constant-flux, computer-controlled, cross-flow membrane rig equipped with zirconium oxide membranes. Fouling was dependent upon the permeate flux, being light at low flux (50 l/m² h) and severe at high flux (200 l/m² h). The fouling increased in severity as the flux was increased from 50 to 200 l/m² h. At 50 l/m² h, protein transmissions of>90% were observed. At higher fluxes, the protein transmission decreased with increasing fouling resistance. The relationship between fouling resistance and protein transmission was similar for 50 and 100 nm membranes and was independent of the starting permeate flux. Standard poreplugging and pore-narrowing models did not describe the observed behaviour. Development of a model to predict protein transmission from the fouling resistance indicated that fouling occurred only in a small part of the membrane pore, most likely at the pore entrance. It is proposed that the microfiltration pore acts in a way similar to a pressure-relief valve where shear-induced protein denaturation has been observed. Shear forces on the protein perhaps lead to protein denaturation and aggregation, and narrowing of the pore in the immediate vicinity of the pore entrance.     

Modelling the clarification of lactic acid fermentation broths by cross-flow microfiltration

This work is focused on modelling microfiltration for clarifying fermentation broths for the production of lactic acid. The hydraulic resistance-in-series model was used with membrane resistance, bacterial cell cake resistance, adsorption resistance and solute concentration polarisation resistance. Most of the model parameters were determined from independent experiments. This model was applied for microfiltrations operated either under constant transmembrane pressure or under constant permeate flux. Resistances due to adsorption and to solute concentration polarisation dominated. Bacterial cake resistance was found to be very low or equal to zero when microfiltration was below the critical flux.     

Controlled backwashing in a membrane sequencing batch reactor used for toxic wastewater treatment

A new control algorithm for performing filtration in a membrane sequencing batch reactor (MSBR) to prevent fouling is presented. Based on continuous measurements of the transmembrane pressure (TMP) and the permeate flux, the algorithm decides when to initiate backwashing. The algorithm was tested on a laboratory scale bioreactor treating synthetic wastewater containing 4-chlorophenol (4CP) as model toxic compound and filtration was carried out using a submerged tubular membrane module and a diaphragm pump. Several controller configurations were tested for different MSBR cycles. The results showed that the proposed algorithm was robust against the highly varying mixed liquor characteristics and was able to keep the TMP below critical values and maintain the flux at a maximum for most of the filtration time. Therefore, despite possible frequent backwashes, the total filtration time was minimized.