Study On The Influencing Factors Of The Efficiency Of Ozone Pre-oxidation In Mitigating Organic Fouling Of Ultrafiltration Membrane

Ultrafiltration technology has an excellent removal effect on particulates and microorganisms,and has relatively low operating pressure and a small footprint.Therefore,it is more and more used in drinking water treatment,sewage reuse,and nanofiltration/reverse osmosis pretreatment.However,the pollution of ultrafiltration membranes by the ubiquitous natural organic matter in the water is the main obstacle restricting the large-scale application of ultrafiltration technology.Ozone pre-oxidation can change the structure and properties of organic matter,and has received widespread attention in the control of organic pollution of ultrafiltration membranes,but there are still some disputes about its combination and control efficiency.Based on investigating the tolerance of commonly used polymer ultrafiltration membranes to ozone,this paper studies the influence of factors such as the composition of raw water ions,the natural organic matter,and the molecular weight cut-off of the membrane on the effectiveness of ozone pre-oxidation to control the organic pollution of ultrafiltration membranes.The main conclusions of the paper are as follows:（1）Polyethersulfone（PES）and polyvinylidene fluoride（PVDF）ultrafiltration membranes have significant differences in ozone tolerance.After being exposed to saturated ozone water（19.2±0.5 mg/L）for 2 hours,the water permeability of the PES membrane sharply increased to more than 3 times that of the new membrane,and the rejection rate of humic acid（HA）dropped to about 0.The membrane flux did not decrease when filtering HA.Combined with the analysis of the membrane surface composition and properties,ozone decomposed PES molecules while oxidizing the hydrophilic additive polyvinylpyrrolidone（PVP）,destroying the structure and function of the PES separation layer.For the PVDF membrane,the water permeability of the membrane increased to about 1.4 times that of the new membrane after being in contact with saturated ozone water for 2 hours,and the retention rate of HA decreased from about 60%of the new membrane to about 40%,However,when the contact time with ozone was further extended,the film performance did not decline further.The analysis of the membrane surface composition and properties showed that the damage of ozone to the PVDF membrane was mainly caused by the oxidation and loss of PVP,and the PVDF itself did not decompose.Therefore,in order to avoid the damage of low concentration residual ozone to the ultrafiltration membrane,the PVDF membrane with better ozone tolerance should be used in the ozone pre-oxidation-ultrafiltration combined process.（2）The effects of different doses of ozone pre-oxidation on HA induced ultrafiltration membrane fouling were studied without p H buffer（NaCl solution）,carbonate buffer（NaCl+Na HCO₃）,phosphate buffer（PBS solution）and calcium+carbonate buffer（NaCl+Na HCO₃+Ca Cl₂）.Under the four ion composition conditions,ozone pre-oxidation can reduce membrane fouling to varying degrees,and the effect of membrane fouling control increases with the increase of ozone dose;The ozone pre-oxidation in NaCl solution reduced the p H of the solution,and the membrane fouling control effect was partially offset by the decrease of p H.The effect of membrane fouling control under carbonate buffering conditions is slightly better than that of phosphate buffering,which should be due to the fact that carbonate inhibits the formation of free radicals and promotes the direct oxidation of ozone molecules to HA.The direct oxidation of ozone molecules makes more HA change from large molecules to small molecules,which is more conducive to the mitigation of membrane pollution;Ca²⁺significantly aggravated the deposition of HA on the membrane and the degree of membrane fouling through the"bridging effect".Ozone pre-oxidation reduced total fouling index（TFI）and hydraulically irreversible fouling index（HIFI）by 74.6%and 65.6%,respectively.The efficiency of ozone pre-oxidation in mitigating organic fouling of ultrafiltration membrane is greatly affected by the ion composition of raw water,and therefore special attention should be paid to the ion composition of raw water when analyzing and summarizing relevant literature and evaluating the efficiency of ozone pre-oxidation.（3）The difference in the natural organic matter makes the membrane fouling behavior different.Both HA and sodium alginate（SA）cause a significant decrease in membrane flux.Ozone pre-oxidation has a obvious effect on the control of membrane fouling caused by HA and SA.The membrane surface is analyzed by SEM and FTIR.It also shows that the irreversible accumulation of HA and SA on the membrane is significantly reduced after ozone pre-oxidation;the molecular size of bovine serum albumin（BSA）is small,and the degree of membrane fouling caused by it is very light but mainly irreversible.The ozone pre-oxidation reduces the BSA pollution to some extent.（4）Ultrafiltration membrane fouling increases as the membrane pore size decreases.For HA without ozone pretreatment,the TFI and HIFI of a 50 k Da ultrafiltration membrane are 1.48 and 1.19 times that of a 150 k Da ultrafiltration membrane,respectively;the effect of ozone pre-oxidation on the pollution control of the ultrafiltration membrane with larger pore size is better than that of the ultrafiltration membrane with smaller pore size.This is because the amount of organic matter trapped on the membranes with two different pore sizes is reduced by ozone pre-oxidation,and the decrease of organic matter trapped on the ultrafiltration membrane with larger pore size is more obvious.