Effects Of The Ultraviolet Based Oxidation Pretreatment On Ultrafiltration Membrane Fouling

Ultrafiltration(UF)membrane is one of the most promising and reliable technology for drinking water treatment because of the effectively rejection of suspended particles,colloids and micro-organisms.Besides,the application of the UF is convenient and its cost is relatively low.However,membrane fouling is one of the major barriers slowing down its further application.Feed water pretreatment prior to membrane filtration could change the structure of the organics and decrease concentration of foulant,thereby reducing the membrane fouling.In this study,three commonly used technologies in recent years including ultraviolet/persulfate(UV/PS),ultraviolet/chlorine(UV/chlorine)and ultraviolet/hydrogen peroxide(UV/H₂O₂)were applied as the pretreatment alternatives,the mechanisms of membrane fouling mitigation were systemically evaluated.Firstly,the model organic matters including humic acid(HA),sodium alginate(SA)and bovine serum albumin(BSA)were chosen as representatives of humic substances,polysaccharides and proteins,respectively,in natural waters.The effects of UV/PS pretreatment on UF membrane fouling caused by typical natural organic matter(NOM)fractions were investigated.The fouling mitigation mechanisms were investigated through the characterization of feed water and fouled membranes.The results show that UV/PS pretreatment significantly alleviated HA fouling by the destruction of unsaturated bonds and fluorescent substances,and the SA fouling could be controlled though the depolymerization of SA molecules.As for BSA,UV radiation alone could significantly aggravate the membrane fouling,the specific membrane flux at the end of the first filtration cycle was only 8%.This fouling could not be mitigated when UV/PS pretreatment was applied with0.04 m M,where the specific membrane flux was 15%.Modeling result indicates that the mitigation of membrane fouling derived from SA and mixed organic fractions was primarily ascribed to the control of cake filtration,while the mitigation of HA fouling was attributed to the declined contribution of standard blocking.Then,the effects of UV/chlorine pretreatment on the membrane fouling caused by HA,SA,BSA and mixture were studied and compared.The dissolved organic carbon(DOC)and fluorescent substances in the feed water was measured,and the morphologies of membrane surface fouled by NOM with or without UV/chlorine pretreatment were characterized.The mechanism of fouling alleviation was further analyzed by model fitting.The results show that membrane fouling induced from HA,SA and HA-SA-BSA mixture could be effectively mitigated by UV/chlorine pretreatment,which could be further improved by increasing the chlorine dose.As for SA,membrane fouling cloud be controlled under relatively low chlorine dose,and the specific membrane flux was improved by 58%when the chlorine dose elevated from 0 to 0.0625 m M.Although UV irradiation alone severely aggravated BSA fouling,which could be avoided by the addition of chlorine(0.25 m M)to the pretreatment process.UV/chlorine pretreatment significantly reduced the UV₂₅₄ value and fluorescent substances in feed water and increased the DOC concentration of feed water,indicating the reduction of molecular weight(MW)of organic compounds,and the decomposition of unsaturated organic species after pretreatment,thereby reducing the accumulation of organics on the membrane surface and membrane pores.Membrane fouling during the filtration of untreated HA,SA,and HA-SA-BSA mixture was occupied by cake filtration and intermediate pore blocking,while the UV/chlorine pretreatment led to the exacerbation of pore blocking at the initial filtration stage.The initial fouling mechanism of untreated BSA was mainly governed by complete blocking,which was shifted to intermediate pore blocking after UV/chlorine pretreatment.Based on the above findings,ultraviolet based advanced oxidation processes(UV-AOPs)including UV/PS,UV/chlorine,and UV/H₂O₂ were employed to alleviate ultrafiltration membrane fouling during the treatment of algae-laden water.The effect of different pretreatments on the DOC,fluorescent substances,the concentration of algae cells and debris as well as hydrophilicity were measured.In addition,the variation of functional groups and the morphologies on the surface were also characterized.The results show that UV/PS pretreatment exhibited the best performance on fouling control,followed by the UV/H₂O₂ pretreatment.The fouling mitigation performance improved with the increase of oxidant dose.However,UV/chlorine pretreatment aggravated membrane fouling,and the irreversible fouling resistance increased by five times compared with that of raw water.The DOC in the algae-laden solution was reduced after UV/PS pretreatment,while both UV/chlorine or UV/H₂O₂ pretreatment had little influence on it.UV/PS and UV/H₂O₂pretreatments were effective in the degradation of fluorescent compounds,thus reducing the deposition of organic matter on the membrane surface.Additionally,the decreased concentration of hydrophobic organics,algal cells,and debris in feed water after UV/PS pretreatment was also contributed to the fouling alleviation.The aggravated irreversible fouling after UV/chlorine pretreatment was probably ascribed to the increased accumulation of hydrophobic fractions in the membrane pores.Modeling result indicates that membrane fouling during the filtration of raw algae-laden water was dominated by intermediate blocking and cake filtration mechanisms.Both UV/PS and UV/H₂O₂ pretreatments transformed the combined fouling mechanism into standard blocking,while UV/chlorine pretreatment aggravated the pore blocking in the initial filtration period.