Effects Of Oxidation Based Pretreatments On Ultrafiltration Membrane Fouling And Water Purification Efficiency

Ultrafiltration?UF?has extensive application prospect in water treatment,but the problems of membrane fouling during long-term operation and poor removal of dissolved pollutants are still restricting its further popularization and application.Oxidation prior to UF membrane is an effective way to solve the above problems.Pre-oxidation could simultaneously mitigate membrane fouling and improve the permeate water quality by oxidative decomposition of pollutants in the water.In this paper,ozonation,membrane delivery catalytic ozonation,and ferrous ion activated peroxymonosulfate?Fe???/PMS?were chosen.Then the effects of oxidation based pretreatments on UF membrane fouling and water purification efficiency were systematically investigated,and the influence mechanisms were deeply discussed.Fristly,this paper investigated the effect of pre-ozonation on UF membrane fouling caused by different natural organic matter?NOM?fractions,including typical organic matter humic acid?HA?,sodium alginate?SA?and bovine serum albumin?BSA?as representatives of different components of membrane foulants in natural waters.The influence mechanism of different NOM fractions on membrane fouling control efficiency by pre-ozonation were studied through the effect of ozonation on the characteristics of NOM.The results showed that the species of NOM significantly affected the effect of pre-ozonation on membrane fouling conrol.Pre-ozonation could effectively mitigate reversible membrane fouling caused by HA through converting large and hydrophobic humic acid molecules into smaller and hydrophilic substances,but for irreversible fouling the mitigation effect was limited.Both reversible and irreversible membrane fouling caused by SA could be effectively mitigated by pre-ozonation due to the depolymerization of SA molecules.As for BSA,pre-ozonation exerted a minor influence on both reversible and irreversible fouling because the main chain of BSA molecule was difficult to be degraded.The effect of pre-ozonation on the removal of trace organics atrazine?ATZ?and sulfamethazine?SMT?was also investigated in this paper.The results showed that retention of ATZ and SMT by single UF was very limited?less than 5%?,whereas pre-ozonation piror to UF membrane could significantly decrease their concentration in the permeate.With a dose of 1.5 mg/L,the removal rates of ATZ and SMT were improved to 47.8% and 97.1%,respectively.Then,the surface of UF membrane was coated with Mn oxide catalysts,and a membrane delivery catalytic ozonation system was established to mitigate membrane fouling and improve water purification efficiency.The mechanism was further illustrated,and the optimization of catalytic membrane preparation method was also studied.The results showed that the preparation method of Mn oxide coating material significantly affected its particle size distribution,thus affecting the surface roughness of the Mn oxide catalytic membranes.The coating of Mn oxide particles increased membrane intrinsic resistance to some extent,whereas accelerated the decomposition of organic foulants SA on membrane surface with ozonation and improved the transmission of pollutants,thus alleviating the cake layer fouling on membrane surface.Compared with single ozonation,S-MnO₂ coating redcuced both reversible and irreversible fouling resistance with the removal rates of 23.0% and 39.2%,respectively.Membrane delivery catalytic oxidation could promote the production of ·OH,hence enhancing the removal effect of trace organic matter p-chloronitrobenzene?p-CNB?.After the surfaces of UF membranes were coated by C-MnO₂,M-MnO₂ and S-MnO₂,the removal efficiencies of p-CNB by membrane delivery catalytic oxidation were increased to varying degrees.The preparation method of Mn oxide coating material significantly affected the efficiency of membrane fouling control and p-CNB removal.Compared with C-MnO₂ and M-MnO₂,S-MnO₂ showed better performance for catalytic oxidation of ozone,and its preparation method was more suitable for surface modification of UF membrane.As a typical persulfate oxidation process,(Fe???/PMS was further employed in this paper.The effects of (Fe???/PMS pretreatment on controlling membrane fouling by NOM,and the removal of trace organics were investigated.The mechanisms for fouling mitigation and trace organics removal were clarified through the impact of pretreatment on the properties of organic compounds in the feed water.The results indicated that the pollution load was significantly reduced by coagulation with the in-situ generated Fe???in the (Fe???/PMS process.With an appropriate dose,(Fe???/PMS could effectively alleviate membrane fouling caused by HA,SA,BSA and their mixture,and both reversible and irreversible fouling resistance were significantly decreased.The generated SO₄^·-and ·OH during (Fe???/PMS process played a main role in the removal of trace organics,and the removal rate of ATZ was 98.2% with a dose of 50/50 m M.The molar ratio of (Fe??? and PMS had a significant effect on membrane fouling control and removal of trace organics.Under the experimental conditions,the optimum molar ratio of (Fe???and PMS was 1:1.A comparative study was conducted between (Fe???/PMS and ozonation as UF pretreatments for treating natural surface water.The results indicated that pre-ozonation only alleviated reversible membrane fouling,whereas the mitigation effect for irreversible fouling was limited.(Fe???/PMS pretreatment mitigated both reversible and irreversible membrane fouling.Pre-ozonation increased the concentration of organic matter to a certain degree,while (Fe???/PMS effectively reduced the pollutant load and improved the permeate quality.However,pre-ozonation exhibited better performance for the removal of trace organics than (Fe???/PMS with the same dose.In this paper,ozonation,membrane delivery catalytic ozonation,and (Fe???/PMS oxidation were chosen.The transformation of pollutants during oxidation,and the effect of these oxidation methods on UF membrane fouling and purification efficiency were systematically investigated.The research findings could provide new thoughts for the combined water purification process based on UF,which is expected to accelerate the development of UF in water treatment.