Membrane fouling critical flux operation

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...

Cho, B.D. and Fane, A.G. (2002) Fouling transients in nominally cub-critical flux operation of a membrane bioreactor. Journal of Membrane Science, 209 (2), 391 103. 

The concept of critical flux ( Jcrit) was introduced by Field et al. [3] and is based on the notion that foulants experience convection and back-transport mechanisms and that there is a flux below which the net transport to the membrane, and the fouling, is negligible. As the back transport depends on particle size and crossflow conditions the Jcrit is species and operation dependent. It is a useful concept as it highlights the... 

Membrane fouling is a complex process where the physicochemical properties of the membrane, the type of cells, the quality of the feed water, the type of solute molecules, and the operating conditions all play a role. The end result of most membrane separations is a fouled surface that the operator will not be able to clean to its original state. To reduce the tendency to irreversible fouling it is essential to operate the plant/unit below the critical flux. This must go hand-in-hand with reliable feed water pretreatment schemes. 

Below a certain permeation rate, the TMP varies linearly with flux, and above this transition flux, a sharp increase in TMP is observed concomitant with a permeate flux decline. A time-dependent flux decline is also observed. The critical flux hypothesis is that upon start-up, there exists a flux below which a time-dependent flux decline does not occur, whereas fouling takes place above this critical value [44]. It is known that the critical flux in MBRs depends on hydrodynamics, particle size, and membrane surface characteristics. Typically, MBRs are operated below the critical flux in order to minimize fouling. However, the validity of the critical flux concept in MBR operation has been questioned since after prolonged operation, irreversible fouling has been observed at subcritical fluxes [45]. 

Critical flux Membrane flux below which flux decline is minimal. Operation at below critical flux condition results in minimal fouling. See also Flirx-step. 

Finally, they concluded that the technique is able to observe the deposition of particles as a cake inside the lumen of a hollow fiber membrane and also to detect deposition and fouling within the membrane structure and they suggested that it can also be applied to the evaluation of critical flux in cross-flow operation by observing the flux at which deposition begins. 

The concept of critical flux was introduced in Section 10.2.2, and in simple terms it refers to a flux beyond which deposition and/or fouling occurs and dTMP/dt [Eq. (10.4)] becomes nonzero. Critical flux can be viewed as a condition where the convection and back transport of foulant are in balance so the net flux of solids to the membrane [Eq. (10.5)] is zero. The situation can be observed in Figure 10.14, which shows the deposits on hollow fibers operated at different fluxes in a bubbled yeast suspension. The flux at lOL/m h is clearly subcritical and > 15 L/m h is supercritical. 

One of the most useful practical operating concepts for membrane processes is that of a critical filtration flux or critical operating pressure. These critical parameters are such that below such critical values rejection will occur and fouling will be minimum, while above these critical values both transmission and fouling may take place. For colloidal particles, the critical values may arise as a balance between the hydrodynamic force driving solutes toward a membrane pore and an electrostatic (electrical double layer) force opposing this motion. 

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