Sustainable flux

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 

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Brookes, A., Jefferson, B., Guglielmi, G. and Judd, S.J. (2006) Sustainable flux fouling in a membrane bioreactor ... 

Eor mixed feed, the limiting critical flux will be that of the component with lowest critical flux in practice, this may be impracticaUy low and a sustainable flux, requiring infrequent cleaning, may be adopted. For soluble species and fine colloids, the critical flux can be considered as the flux below which the wall concentration does not initiate fouling. 

The critical flux or sustainable flux can be used as the design flux for the processes, which is to be enhanced by the hydrodynamic techniques applied. 

Sustainable flux The flux at which TMP increases gradually at an acceptable rate and chemical cleaning required is minimal. 

Critical and sustainable fluxes theory, experiments and applications (Review). 

The characteristic TMP history for a submerged membrane with cross flow (bubbling) is depicted in Figure 10.2a. Due to deposition TMP rises slowly, and the rate of rise is less with more imposed shear or lower solids. In an idealized situation TMP would remain unchanged at subcritical flux conditions. However, for various reasons (see Section 10.5.1) some degree of TMP rise tends to occur at aU fluxes so the interest is in the acceptable rate of rise and the sustainable flux. Figure 10.2a also includes the potential for a sudden TMP jump that can be observed in prolonged operation at constant flux (discussed further in Section 10.5.1). 

Values of SAD range from 1.0 down to about 0.2 Nm/h, and data are plotted in Figure 10.13 versus membrane permeability (L/m h bar). SAD ranges from about 60 down to as low as 10, with typical values from 20 to 15. The permeabilities in Figure 10.13 are based on long-term operation and can be considered as sustainable flux... 

In practical applications the feed tends to be a mixture in which each component (particle, colloid, solute) has a specific critical flux determined by back transport (dependent on size, charge, etc.). MBR mixed liquor is a classic example of this type of feed, and as a result dTMP/dt tends to be nonzero even at very low flux. The approach to this is to define a sustainable flux, which is a more subjective concept and depends on an acceptable value of dTMP/dt and the cleaning frequency. 

In summary, the spatial and temporal features of submerged membranes in bubbly flow complicate the definition of critical flux in these systems. However, in practice it is found that sustainable flux can be achieved and that it is improved by bubbling. 

Sustainable Flux Operation Balances the capital cost of lowering flux with the benefits of less fouling and cleaning. 

Sustainable Flux Operation Typical fluxes are chosen in the modest range of 10-30 L/m h to avoid cake formation of biomass. 

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