Limitation of the theories

As described in this section, there are many different factors affecting membrane fouling. However, it appears that no clear consensus exits in professional society regarding dominant factor on membrane fouling. Perhaps the only consensus is that there is no single membrane property dominating membrane fouling.

Although a large group of people had advocated the theories involving interactions between particles and membrane surface, e.g. charge interaction, hydrophobic interaction, etc., it is apparent that once cake layer is formed on membrane surface the original membrane surface property hardly affect the subsequent particle deposition. More recently, these theories are losing ground to some extent due to the fuzzy relations between membrane surface properties and membrane fouling tendency especially for MF and UF.

Rather than membrane surface properties, cake layer growth rate is highly affected by the hydrodynamic conditions around membrane surface such as air/liquid velocity, direct air bubble contact, irregular hollow fiber movements that cause pulses in effective liquid velocity on the membrane surface, etc. This will be discussed in other section.

It can be concluded that hydrodynamic conditions around membrane are arguably more dominant factor affecting membrane fouling than membrane surface properties especially in MF and UF. One indirect evidence supporting this argument is that the operating flux of MBR has marginally increased during the last decade in spite of the improvement in membrane surface properties including surface roughness, hydrophilicity, permeability, etc (Lesjean, 2011). As a matter of fact, design fluxes of all membrane types such as hollow fiber, flat sheet, and tubular stay nearly identical for more than a decade. Instead of improving flux, membrane manufacturers are more focusing on reducing souring air demand by modifying module/frame design, improving module handling properties, improving durability of membrane, etc.

 

© Seong Hoon Yoon