Microfiltration critical flux

Figure 7.17 Experiments showing the rate of fouling of 0.22-p.m microfiltration membranes used to treat dilute biomass solutions. The membranes were operated at the fluxes shown, by increasing transmembrane pressure over time to maintain this flux as the membranes fouled [12]. Reprinted from J. Membr. Sci. 209, B.D. Cho and A.G. Fane, Fouling Transients in Nominally Sub-critical Flux Operation of a Membrane Bioreactor, p. 391, Copyright 2002, with permission from Elsevier...

Microfiltration membranes are commonly used in MBRs to separate sohds from water. The fluxes are very low, often below critical flux, and at low pressures when hollow fibers are used, backflushing is added to prevent or reduce flux decrease. MBRs are discussed in detail in Section 35.6.3.2, tubular modules with MF membranes have also been tested in the pulp and paper industry. 

Field RW, Wu D, Howell JA, and Gupta BB. Critical flux concept for microfiltration fouhng. J Memb Sci. 1995 100 259-272. 

The flux decline is larger than 40% for CA-UF and TFC-SR at a pressure of 10 bar. This may indicate the presence of a critical flux phenomenon, as previously explained for microfiltration. This is defined by Howell et at. (1995) as the flux below which there is no deposition of colloids on the membrane. For NF, this critical flux may be the flux below which concentration polarisation does not reach the solubility limit of the solute species or it could be the flux at which the deposition of nanometer siae... 

Flowell J.A. (1995), Sub-critical flux operation of microfiltration. Journal of Membrane Science, 107, 165-171. 

Eield, R., Wu, D., Howell, J., Critical flux concept for microfiltration fouling. Journal of Membrane Science 1995, 100, 259-272. 

Recent research into the reduction of filtration flux that occurs as soon as the microfiltration process is started (Figure 7.2.7(a)) indicates that it is very much dependent on the level of the flux and therefore the applied AP. Beyond a critical flux, Vs,a, deposits form on the membrane since the forces dragging the colloidal particles in suspension toward the membrane are larger than the forces causing the particles to move away from the membrane. If this critical flux value is reached at the end of the membrane channel, where the boundary layer is the thickest, then the... 

Membrane Characterization MF membranes are rated by flux and pore size. Microfiltration membranes are uniquely testable by direct examination, but since the number of pores that may be observed directly by microscope is so small, microscopic pore size determination is mainly useful for membrane research and verification of other pore-size-determining methods. Furthermore, the most critical dimension may not be observable from the surface. Few MF membranes have neat, cylindrical pores. Indirect means of measurement are generally superior. Accurate characterization of MF membranes is a continuing research topic for which interested parties should consult the current literature. 

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