Study On Process Mechanism Of Vacuum Ultraviolet/Ozone For Algal Organic Matter Treatment To Alleviate Ultrafiltration Membrane Fouling

During the algal blooms in the surface water source,the algal organic matter（AOM）that was difficult to remove by conventional water purification processes poses big challenges to drinking water treatment.The residual AOM in the water treatment system could not only aggravate the membrane fouling propensity of the ultrafiltration process,but also the taste and odour compounds,algal toxins,and the secondary disinfection byproducts of AOM affect the quality of supplied water significantly,threatening human health.Therefore,it is urgent to remove AOM effectively and control the ultrafiltration membrane fouling caused by AOM to ensure the effect of drinking water treatment during the algal blooms.Ozone（O₃）oxidization technology has been used to treat AOM in water,but the ozonation process had a low mineralization rate for AOM and could not control membrane fouling caused by AOM effectively.Vacuum ultraviolet（VUV,100-200 nm UV）radiation can dissociate water to form reactive oxygen species（ROS）such as hydroxyl radicals（·OH）in situ to remove the organic pollutants.In this study,the oxidation process vacuum ultraviolet/ozone（VUV/O₃）was adopted to mineralize AOM effectively and mitigate the ultrafiltration membrane fouling caused by AOM,and the process mechanism was elaborated.Firstly,the efficiency and mechanism of AOM removal by the produced ROS during the VUV irradiation of water were investigated.The produced·OH and superoxide anion radical(O₂^·-,conjugate acid HO2·)in situ during VUV irradiation of water were identified using the characteristic fluorescence probe method.The effects mechanisms of dissolved oxygen on the formation pathway and the generation law of ROS(·OH,O₂^·-,and H2O2)were explored.The removal efficiency,characteristics variation,and action mechanism of VUV irradiation for algal extracellular organic matter（EOM）in water were studied.Results showed that dissolved oxygen promoted the production of ROS.The dissolved oxygen increased the formation rate and reduced the decomposition rate of H₂O₂.The production and transformation mechanisms of ROS included:·OH was formed during VUV irradiation of water;H₂O₂ was produced by the recombination of·OH in the oxygen-free condition,while via the reactions related to O₂^·-/HO₂·and the formed intermediate O₃ besides the recombination of·OH in the oxygen-containing condition;the formed H₂O₂ was decomposed simultaneously by the VUV irradiation and radicals attack to generate·OH and O₂^·-/HO₂·;the production of H₂O₂ tended to steady-state when the formation and the decomposition were balanced,and various ROS transformed mutually and kept a dynamic balance.Besides,the removal of dissolved organic carbon（DOC）and UV₂₅₄ in EOM by VUV irradiation was significantly higher than that by UV irradiation.The removal rate of DOC in EOM was 20.0%after VUV irradiation for 120min.VUV irradiation decreased the fluorescence characteristics of EOM and decomposed the organics with higher molecular weight（MW）into organics with lower MW.The dissolved oxygen enhanced the removal of EOM by VUV irradiation,and the lower the initial concentration of EOM,the higher the removal efficiency of EOM was.The formed ROS in situ during VUV irradiation promoted the removal of EOM.To promote the mineralization effect of EOM,the process of VUV/O₃ was used to remove EOM.Results showed that compared with the processes of UV/O₃,O₃,VUV,and UV,VUV/O₃（O₃ dosage of 3.0 mg/min,irradiation time of 50 min）with the synergistic effect of VUV and O₃ showed the best removal of EOM,and the removal efficiencies of DOC and UV₂₅₄ were 75.6%and 80.8%,respectively.Besides,VUV/O₃ process could remove the characteristic components of EOM effectively.The removal rate of fluorescence components was≥92.8%（at 10 min）.The removal rates of protein and polysaccharide were 80.1%and 78.0%,and the yield of trichloromethane during chlorination decreased by 70.5%.The typical parameters including O3 dosage,solution p H,and coexisting ions and organic matter affected the removal of EOM by influencing the form and concentration of ROS as well as the photon absorption distribution during the VUV/O3 process.The ROS in VUV/O3 process were confirmed comprehensively based on the fluorescence molecular probe,characteristic probe compound of·OH,and variations of O₃ and H2O2.Combined with the results of the performance comparison of different processes,it was found that EOM was effectively removed by the combination of VUV photolysis,direct O₃ oxidation,and ROS(·OH and O₂^·-)oxidation in the VUV/O₃ process.Moreover,VUV/O₃ removed DOC,UV₂₅₄,and characteristic components of algal intracellular organic matter（IOM）effectively,and VUV/O₃ process was universally applicable to the removal of these two kinds of AOM.To mitigate the ultrafiltration membrane fouling caused by AOM,VUV/O₃ process was adopted as the pretreatment technology for the ultrafiltration membrane.The effects and mechanisms of VUV/O₃ for alleviation of ultrafiltration membrane fouling caused by EOM and IOM were investigated systematically,and the discrepancies between EOM and IOM were compared.Results showed that the contents of the organics in ultrafiltration feed were reduced after EOM and IOM pretreatment by VUV/O₃ process.When EOM and IOM were pretreated by VUV/O₃ process,the content of DOC in the filtrate decreased for EOM but increased for IOM.Without pretreatment,IOM caused more severe membrane fouling than EOM.Compared with the processes of UV/O₃,O₃,VUV,and UV,VUV/O₃ pretreatment showed the best performance on membrane fouling mitigation.The fluxes of membranes increased obviously after EOM and IOM pretreatment by VUV/O₃,and VUV/O₃ achieved 89.5%and 97.2%mitigation of reversible fouling caused by EOM and IOM,respectively.Moreover,pore blockage and cake filtration were the major fouling mechanisms during the filtration of EOM and IOM.The correlation between influent characteristics and membrane fouling suggested that pretreatment changed the characteristics of AOM and was beneficial to membrane fouling mitigation.The deposited foulants on the membrane surface and the membrane surface roughness were reduced after VUV/O3 pretreatment.With the analysis of AOM characteristics changes and ROS,and analysis of interaction free energy by XDLVO theory,it was found that VUV/O₃ changed characteristics of fluorescence,MW distribution,hydrophilicity,and interaction free energy of AOM via various actions and decreased the organics load,mitigating the membrane fouling.Factors such as O3 dosage,operating pressure,and the MW cut-off of the ultrafiltration membrane influenced the membrane fouling mitigation by VUV/O3 pretreatment to varying degrees.VUV/O3pretreatment alleviated the membrane fouling caused by AOM in natural algae-rich water effectively,implying the considerable practical application potential of VUV/O3.In conclusion,the action mechanism of the formed ROS in situ during VUV irradiation of water on AOM removal was investigated in this paper.Besides,the VUV/O₃process was adopted to achieve the efficient mineralization of AOM and VUV/O₃ could be used as a pretreatment technology to mitigate ultrafiltration membrane fouling caused by AOM significantly.The research findings could provide theoretical guidance for the application of the VUV-based advanced oxidation processes in water treatment,and it was of great significance for abating the adverse effects to drinking water treatment caused by AOM during algal blooms.