Bubble passage effect

The effect of bubbles passages on the fouling formation was studied with a DO setup [66], The system did not capture the images of bubble passages onto the membrane surface due to camera speed limitation, but the consequences of the bubbles on the deposited cake were recorded. As expected, the introduction of bubbles into the system led to a limitation of the cake layer formation by enhancing particle back-transport and reducing the particle adhesion to the membrane surface. 

In its passage through a water column, a bubble acts as an interface between the liquid and vapour phases, and as such collects surface-active dissolved materials as well as colloidal micelles on its surface. Thus in a well-aerated layer of water, the upper levels will become progressively enriched in-surface-active materials. In the open ocean, an equilibrium undoubtedly exists between the materials carried downward by bubble injection from breaking waves and those carried upward by rising bubbles. In the laboratory, however, this effect will enrich the surface layer with organic materials. 

Figure 3.22 shows the bubble point measured with isopropanol (IPA) on polyvinylidene difluoride UF membranes with MWCO s between 10,000 and 1,000,000. A 300,000 MWCO membrane (F300) should have an estimated effective pore size of 0.02 ju yet the bubble point indicates a maximum pore size in the skin over 0.4 ju. This is one reason why UF membranes can be less retentive for bacteria than MF membranes. However, Figure 3.22 also indicates that a 10,000 MWCO membrane can have a (I.P.A.) bubble point of 100 psi. Equation 3 of Chapter 2 may be used to calculate a maximum pore diameter of 0.12 jtt which should be retentive for all bacteria. Indeed, small laboratory discs of these membranes can be subjected to high challenge levels of bacteria with absolute retention (zero passage). However, industrial scale UF modules often employ 10 to 100 square feet of membrane area it is difficult to manufacture a pinhole-free module with this much area. Broken fibers, bubbles in glue-line seals, and other defects provide leak paths for bacteria. 

S. Q. Zhou, R. K. Shah, and K. A. Tagavi, Advances in Film Condensation including Surface Tension Effect in Extended Surface Passages, in Fundamentals of Bubble and Droplet Dynamics Phase Change and Two-Phase Flow, E. Ulucakli (ed.), ASME HTD-Vol. 342, pp. 173-185,1997. 

---