Right, this post is actually being made because of my other post about Between TiO2 and UV254 to contribute fouling and performance. If you look at the paragraph 2, it tells everything about the problem of the permeability. Im gonna try again to define the problem.
Well, permeability as I already mention it in another post about will TiO2 coating cause denser pore or smaller MWCO? is representing the MWCO of membrane, because permeability is an indicator of the quantity of pure water passing through the membrane. Several conclusions about permeability based on my experiments are located below.
Higher permeability, same MWCO : higher flux in short term but same steady flux in long term
Lower permeability, same MWCO : lower flux in short term but same steady flux in long term
Higher permeability, same MWCO : more severe decline of flux, more fouling
Lower permeability, same MWCO : less severe decline of flux, less fouling
Higher permeability equals to higher MWCO
Lower permeability equals to lower MWCO
So, the hypothesis could be higher MWCO will have more severe decline of flux, more fouling, and vice versa, lower MWCO will result in less severe decline of flux, less fouling. I have not concluded what is the cause of fouling, is it internal pore blockage, or cake resistance, or concentration polarization. But I would think 90% that it wasn’t concentration polarization since all membranes were operated under the same cross-flow velocity rate.
Now lets see the result, again I will not display any image, as I haven’t published any of the image to the internation peer reviewed journals. I want to review from naked membrane perspective and TiO2 coated membrane perspective.
From perspective of naked membrane.. yes indeed, lower permeability, lower MWCO did give more fouling… Hypothesis for naked membrane is proven. Possible reason is that, higher MWCO (permeability) allow inner pore blockage. However, I would like to review MW distribution of humic acid first. >50 kD, 28%, 15-50 kD, 44%, 1-15 kD, 0%, <1 kD, 27%. It appears that 28% or rejected humic acid stay outside the membrane, and most probably is due to internal adsorption of 50 kD membrane caused by MW <50 kD. So the keyword here is internal pore adsorption. From the perspective of naked membrane, it can be seen that the lower MWCO used, the steeper slope was formed. What is proportional with that fact? Decreasing MWCO, steeper slope, fractional amount of humic acid. If we use 50 kD membrane, there are roughly 28% with 72% below 50 kD. 72% humic acid below 50 kD give severe decline of fouling. If we use 15 kD membrane, with 72% (28+44%) humic acid above 15 kD and ~27% below 15 kD, much steeper slope produced, perhaps because only 27% contribute to the fouling. And last, if we use 1 kD membrane filtering 27% below 1 kD humic acid fraction and 73% above 1 kD, we find that the less MWCO used, the less fouling occurred. My hypothesis is, higher MWCO, higher chance that high percentage of lower MW of humic acid fill up the pore (internal pore adsorption or pore blocking), while lower MWCO, small percentage of lower MWCO will also fill up the pore but with less degree because the quantity is not as abundance as higher MW distribution, and because not all lower MW humic acid will be attached and adsorbed thus they will generally form cake layer or concentration polarization. This cake layer in lower MWCO probably does not have a significant effect for fouling which make severe decline of flux. Now for TiO2 coated membrane, the permeability of them are lower than naked membrane. Regarding the fouling, it shows a bit different with naked membrane. If naked membrane shows that the lower MWCO used, the less fouling occured, in TiO2-UV membrane, it didn’t happen that way. 15 kD TiO2-UV membrane appears to produce more severe decline of flux, followed by 50 kD and 1 kD. In my opinion, there probably more lower MW distribution during the photocatalysis process. Just like as shown above, lower MW tends to cause more fouling. If the presence is much more than higher MW distribution, then more fouling occur. With TiO2-UV process, it seems that there are transformation for 15 kD membrane from above 15 kD MW humic acid distribution to below 15 kD, this results more fouling, which almost the same slope with 50 kD. The reason why the slope of 50 kD TiO2 is steeper than 15 kD, probably because there are transformation as well for humic acid MW distribution to become more fraction above 50 kD.
From above results it reminds me to check the real result of UF fractionation, and the result is……… voila.. my hypothesis is proven… Horay..