Membrane clogging

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

On-line dialysis also separates the analyte from tissue matrix based upon molecular size, but in this case, the sample extract is passed over a membrane filter through which the analyte (and other low molecular weight compounds) is diffused into a second solvent on the other side of the membrane filter. Usually, the second solvent is then concentrated on to an SPE column to minimize the dilution effect that is caused by the dialysis process. Agasoester used on-line dialysis to separate oxytetracycline from muscle, liver, milk, and egg tissue matrix components. A problem encountered with on-line dialysis is the inability of analyte molecules that are bound to proteins in the sample extract to pass through the membrane filter. Problems with membrane clogging are reduced with on-line dialysis compared with ultrafiltration because no external force is being applied to bring the analyte across the membrane filter. 

One drawback relating to this filtration technique is its susceptibility to rapid membrane clogging. Viscous solutions also lead to rapid decreases in flow rates and prolonged processing times. 

A common problem of this technique is the gradual decrease in permeate flux associated with membrane clogging or fouling, caused by adsorption or physical deposition of particles and/or macromolecules on membrane pores. Fouling can be minimized by prior clarification (particulate removal) of the feed solution, by the selection of operational conditions that minimize interactions between membranes and macromolecules, by the use of tangential flow, or by performing intermittent back-flushing operations. 

If in stationary operation conditions, membrane clogging does not occur or is negligible, then the modelling case becomes banal. Nevertheless, vhen surface clogging cannot be eliminated by the tangential flow rate, we must introduce a continuous increase in the hydrodynamic resistance of the membrane [3.16-3.18],... 

Referring to the dynamics of the exits of concentrated suspension and filtrate it is interesting to observe (Figs. 3.21 and 3.22) that the cases with slow membrane clogging reproduce almost identically the corresponding cases where a rapid... 

Figure 3-3. Pressure ultrafiltration. The solution to be concentrated is placed in the ultrafiltration chamber which is fitted with a semi-permeable membrane on the lower surface, and filtered under pressure. Membrane clogging is prevented by continuous stirring of the solution.

In several instances, in order to avoid membrane clogging and to guarantee filtrate quality, ubiafibration membranes are used for what would normally be regarded as a microfibration duty [Gutman and Knibbs, 1989]. 

The cross-flow filtration device (Figure 14.6(b)) aims to avoid the deposition of particles on the upstream side of the membrane and the associated resistance to filtration. Parallel to the surface of the filter, a flow is maintained, whose velocity is much higher than the velocity of the permeate flow induced by the pressure difference through the filter. The fluid and particles essentially just travel along the surface of the filter, and a small fraction of the fluid flow rate passes through the membrane. The friction of the tangential flow on the filter wall is used to re-entrain the particles that are deposited on the filter, in order to prevent the formation of a cake which would diminish the filtration flow rate. Cross-flow filtration reduces the phenomenon of membrane clogging. 

A hollow fiber membrane module is the most commonly used membrane bioreactor (Figure 7.7). The hollow fiber membrane module contains hundreds to thousands of hollow fibers in an assembly similar to a shell and tube heat exchanger. The feed can be applied to either the fiber (lumen) side or to the shell side. However, the feed is usually applied to the shell side because the flow path from the lumen side to shell side is too narrow, which inaeases the risk for membrane clogging. The permeate stream from the membrane module usually contains the product and the retentate contains the concentrated feed stream. 

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