Between TiO2 and UV254 which one contributes more towards fouling? This question that I would like to discuss in this post. The answer at this time will solely based on my own experience. From what I have before about permeability, it is important factor to be considered. Because as I said already in that post, permeability means the size, the MWCO of the membrane. Higher permeability tends to give higher flux in the short term, depending on the MWCO of the membrane. Higher permeability membranes will give more severe decline of flux, means more fouling. Lower permeability membranes in initial operation produce lower flux, but at least the fouling is not as severe as higher permeability membranes. Another advantage for lower permeability membranes is higher rejection, stable as a function of operation time.
So, let’s review a bit about permeability before I move on discussing TiO2 and U254. Permeability which reflects MWCO, will only affect flux in the short term and the rejection. Since higher permeability means higher fouling, sharp decline of flux it will give more fouling. My question is how come? Why higher permeability higher fouling? I will discuss that in another post, What is the effect of MWCO towards fouling?. Back to the main topic about which one is contributing towards fouling for TiO2-UV membrane.
For 15 kD membrane the order is as follows: naked membrane, UV, TiO2-UV and TiO2. Steady state flux for TiO2-UV was actually the same with only TiO2, the difference is more fouling with TiO2 membrane. So in this case, seems TiO2 cause the fouling. It seems reasonable when permeability after coating is lower or decreasing. So, TiO2 cause more fouling but UV tries to prevent it. So the combination between TiO2 and UV produce flux in between only TiO2 and only UV membrane. The mechanism in which UV prevent fouling is probably by organic pollutant removal by conversion or transformation to smaller MW distribution.
Now how about rejection, in this case DOC rejection. The order is UV, naked membrane, TiO2 and TiO2-UV membrane. Strange a bit though. UV is higher, than naked membrane. Well the permeability of UV is lower than naked membrane, not surprising if UV produces higher rejection. But naked membrane produces higher flux, again probably its not surprising since naked membrane has higher permeability. Whats interesting is, the degree of slope of flux between naked membrane and UV membrane is the same. This mean, the removal of organic by UV did not change the molecular weight distribution of humic acid. Simply remove it via the attack of OH radicals. The rejection for TiO2 membrane is slightly better than TiO2-UV membrane. This is probably because of massive adsorption occur for TiO2 membrane, while on the other hand for TiO2-Uv membrane, adsorption do occur along with the change of MW distribution of humic acid, so that TiO2-UV combination give lower rejection.
Let’s now talk about 1 kD membrane. I would expect the result is similar with those above for 15 kD membrane, since the treatment was same. In terms of flux starting from higher flux to lower: UV, naked membrane, TiO2 membrane and TiO2-UV membrane. In terms of DOC rejection: naked membrane, UV, TiO2-UV and the last is TiO2 membrane. The permeability of all membranes were nearly the same. So I think this is pure because of the process, both photocatalysis and membrane screening process.
Why UV produces highest (steady state) flux? The fouling is even less, the slope is steeper, same with UV 15 kD membrane. It seems that UV really works to remove organic by oxidation process. If my assumption correct, comparison between UV 1 and 15 kD membrane, 15 kD membrane should have produced deeper slope. Yap, it’s true, with 15 kD UV membrane produce more flux. Okay, back to the topic, why UV? Possible reason is oxidation just like what I have said above. But what is the evidence? Compare to naked membrane, UV produces more flux but a little bit lower rejection (similar). Another explanation would be, compare with others, theres possibility that UV remove a larger portion of organic below 1 kD, but I think this reason is not valid since the slope is almost similar with naked membrane, showing that no MW alteration or portion removal happen. Well, I think its because the permeability. At 15 kD membrane, UV membrane had lower permeabilility, lower flux, same fouling degree and higher rejection compare to naked membrane. For 1 kD membrane, UV had higher permeability, higher flux, lower rejection, same fouling degree and lower DOC rejection compare to naked membrane. So, until this point is because the physical structure of the membrane (permeability).
Now why in 1 kD, the TiO2 membrane had better flux compare to TiO2-UV membrane. The permeability of TiO2 membrane is a bit higher than TiO2-UV membrane. Perhaps thats the reason, but still its not strong enough. What Im thinking is, in TiO2-UV membrane, during the the process, photocatalysis take action to transform MW distribution of humic acid, as a result, more adsorption of organic below 1 kD into the inner pore size of TiO2-UV membrane. While on the other hand, for only TiO2 membrane, there is no change of MW distribution, which means the fraction of below 1 kD is smaller than in TiO2-UV membrane, so the fouling (adsorption) is less than TiO2-UV membrane.
Now, what about in 15 kD? My thinking is, possibly in 15 kD membrane, TiO2-UV membrane had better (higher) flux than TiO2 membrane because of transformation of MW distribution of organic above 50 kD. Remember about the UF fractionation when approximately humic acid between 1-50 kD was either transformed into below 1 kD or above 50 kD. So, it is possible that during TiO2-UV membrane, organic fraction below 15 kD is less than organic fraction in TiO2 membrane.
Now, between TiO2 and UV, what can be inferred? Definitely TiO2 will cause more fouling due to its surface characteristics, void between particles, valley, irregular shape, dimension and thickness, make organic to be adsorped. On the other hand, UV did very well in removing NOM, I think UV mechanism is different with photocatalysis under the presence of catalyst. UV can attack and remove organic, while if there present catalyst, they will provide adsorption (more fouling) and transformation of MW distribution of humic acid.