Critical flux concept – Strong form and weak form critical fluxes

No particles deposit at sub-critical flux by definition. However, this does not necessarily mean membrane surface is perfectly clean. In the very early stage of filtration, some particles/macromolecules can adsorb on membrane surface due to electrostatic interaction, van der Waals force, etc., if feed water contains complex mixture of particles and macromolecules. Once the membrane surface is coated, further adsorption may not occur as long as the operating flux is below the critical level. Under this condition, membrane permeability would be somewhat lower than clean membrane’s.

The critical flux obtained with initial fouling is defined as “weak form” critical flux, which contrasts to the “strong form” critical flux without any membrane foulant (Field, 1995).

True critical flux rarely exists in practical filtration whether it is strong form or weak form because feed water contains various foulants that can interact with membrane surface even before water permeation begins. For instance, the mixed liquor of MBR contains various macromolecules with various charges, functional groups, sizes, etc., which can interact with membrane surface as soon as the mixed liquor contacts with membrane. In addition, macromolecules with very low back transport velocity continue to deposit at any flux condition (Zhang, 2006). As a result, true critical flux does not exist in MBR whether it is a strong form or a weak form as discussed here. The term, “Sustainable flux”, is often used instead of critical flux to indicate the flux that can last considerable amount of time without causing excessive membrane fouling (Fane, 2002).


© Seong Hoon Yoon