Permeate Flux and Transmembrane Pressure

Fouling propensity is commonly assessed by monitoring permeate flux and transmembrane pressure (TMP). Since membrane processes are generally operated either under constant TMP or constant permeate flux, a decrease in permeate flow rate or an increase of TMP is observed, respectively, once the fouling forms on the membrane. From these two parameters, calculation of hydraulic resistance (m ), membrane permeability (Lm h bar ) or specific cake resistance (mkg ) is also possible and allows further assessment of fouling conditions. The hydraulic resistance of the filtration system can be quantified by correlating TMP and permeate flux during a clean water test. This correlation is described as the Darcy s law  

With permeate flux J, ms ) as a function of TMP (Pa), total resistance (Jltotai. m ) and permeate viscosity ( , Pa s) [31]. Rtotsi consists of the intrinsic membrane and fouling, both reversible and irreversible resistance. Furthermore, membrane permeability (K) is a function of flux on TMP and it also can be monitored to assess fouling. 

Specific cake resistance can be calculated as a function of flux, TMP and deposited mass [15, 32]. ft is generally used to characterize the fouling cake structures, either in dead-end [32] or crossflow filtration [33]. Table 14.1 provides a list of specific cake resistance values for activated sludge, which was studied with different MBR operating conditions. 

Varied operating conditions Specific cake resistance (x10 mkg ) Ref. 

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For pure water or other solvents, there is a linear relationship between permeate flux and transmembrane pressure up to a certain limit. With a solvent-solute model as feed, there is the tendency of the flow curve reach an asymptotic behavior with increasing pressure. This is due to several phenomena, including the concentration polarization, gel layer formation, fouling, and osmotic effects. Flow values are also dependent on temperature that is, these values generally increase with increasing temperature as a result of the decrease in the viscosity and diffusivity of the fluid supply [1]. 

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