Fouling in Membrane Processes

J.V. Lepore and R.C. Ahlert, Fouling in Membrane Processes, in Reverse Osmosis Technology, B.S. Parekh (ed.), Marcel Dekker, New York, pp. 141-184 (1988). 

As a result of these improvements in membrane performance, the major factors determining system performance have become concentration polarization and membrane fouling. All membrane processes are affected by these problems, so... 

Fat or lipid materials and calcium-lipid complexes also contribute to fouling and flux decline in membrane processing of milk or whey. The transport properties of the feed stream and the changes they undergo as the concentration process proceeds also affect the rate of permeation. At high concentrations, the increased fluid viscosity near the membrane surface limits back-diffusion of solids from the polarized layer to the bulk phase, thereby, depressing flux rate [46]. 

R. Sheikholeslami, Fouling mitigation in membrane processes. Desalination 123, 45-53 (1999). 

Several novel techniques for mass transfer enhancement reported in the literature have been discussed in detail. The present study is focused on the mass transfer enhancement in the rate-controlled separation processes using flow instabilities. There are a large number of examples about the success of flow instabilities produced by Dean vortices in improving the performance by increasing flux and reducing fouling in membrane separation processes. Several curved modules... 

Of particular interest in this study is the fouling of membrane processes by natural organics. Fouling depends on the characteristics of the natural organics. A detailed review of the characteristics of interest is required to highlight the factors that may influence membrane fouling. Membrane filtration... 

Colloids seemed to play an important role in MF fouling. Some can pass through the MF and may cause fouling in further processes, such as NF. The effect of colloids needs to be further examined and the effect of using a tighter membrane such as UF as a pretreatment step needs to be investigated. These issues are addressed in Chapter 5. 

While advances will continue in membrane manufacture, concentration polarization and fouling remain significant problems in membrane processes used to produce drinking water from saline or fresh water sources [91-92]. To control fouling, a variety of pretreatment, material design, and module design strategies have been studied. 

The main problem in membrane processes, especially for UF and MF separations, is the decrease of permeate flux caused by concentration polarization and fouling, whereas other membrane processes such as gas separation and pervaporation are less affected. Different approaches have been studied to reduce fouling. Hybrid systems using different types of membrane operations (e.g., distillation, dialysis, NF, pervaporation, and osmosis) prevent microbial fouling, offering a strong potential for the use of new types of thin-film composite membranes. 

An overview of mass transfer correlalion.s in membrane processes can be found in ref. J In microflltration and ulirailllration. the diffusion coefficients of the retained macromolecuies, or suspended panicles are small relative to those which apply to the retained components in reverse osmosis, gas separation and pervaporation. In addition, the fluxes in microflltration and ultrafiltration are large relative to those in pervaporation and gas separation. Hence, the consequences of concentradon polarisation in the case of microfiltradon and ultrafiltration are very severe. The consequences of fouling will be discussed later. 

In membrane processing the application of surface shear is required to control concentration polarization and fouling for high solids feeds or to assist cake removal for batch membrane filtration of low solids feeds. For submerged membranes the common practice is to use two-phase bubbly flow to induce surface shear. This section deals with the role of bubbles as well as other hydrodynamic aspects of submerged membranes. 

The build-up of a fouUng layer on the surface of a membrane is one of the most serious problems in membrane processes. The term fouling, rather than the more familiar filter cake, arose from the origins of membrane processes in molecular separations, where macromolecular proteins would separate on to the membrane surface as a slimy. 

It was reported that the morphology and surface properties of membrane affect its performance in separation processes [49,50]. Surface properties such as surface porosity, pore size, surface roughness, surface charge, and hydrophobicity affect the performance of membrane in separation processes such as ultraflltration, nanofiltration, membrane distillation, and membrane contactors. For example, it was reported that the surface hydrophobicity of membrane had great effect on membrane fouling in ultraflltration processes used to separate humic acids from water [51]. Furthermore,... 

Folding is defined in Background and Definitions and is a significant problem in most process applications, and somewhat of a problem in most water applications. RO membranes may be fouled by sparingly soluble sealants which supersaturate at the membrane. 

Fouling is the term used to describe the loss of throughput of a membrane device as it becomes chemically or physically changed by the process fluid (often by a minor component or a contaminant). A manifestation of fouling in cross-flow UF is that the membrane becomes unresponsive to the hydrodynamic mass transfer which is rate-controlling for most UF. Fouling is different from concentration polarization. Both reduce output, and their resistances are additive. Raising the flow rate in a cross-flow UF will increase flux, as in Eq. 

Fouling Fouling affec ts MF as it affects all membrane processes. One difference is that the fouling effect caused by deposition of a foulant in the pores or on the surface of the membrane can be confounded by a rearrangement or compression of the sohds cake which may form on the membrane surface. Also, the high, open space found in tortuous-pore membranes makes them slower to foiil and harder to clean. 

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