Study On Membrane Fouling Behavior Of Microplastics/Organic Matter System Under Different Coagulation-ultrafiltration Processes

Microplastics is an emerging contaminant and have been detected in fresh water,which pose a certain threat to human and ecology.Ultrafiltration technology is an effective method to remove microplastic pollutants from water,among these,membrane fouling is the main problem.Among many membrane pollution prevention and control strategies,coagulation is one of the most widely used strategies to prevent membrane fouling.However,there are few reports on the influence mechanism of microplastic pollutants on membrane fouling in coagulation-ultrafiltration process Therefore,this study selected typical coagulants FeCl₃·6H₂O and AlCl₃·H₂O,mainly for the coexistence of HA/PS humic acid and microplastics,under the conditions of traditional coagulation-ultrafiltration process,short-process coagulation-ultrafiltration process and integrated coagulation-ultrafiltration process,The influence mechanism and key factors of microplastics system on polyvinylidene fluoride（PVDF）membrane fouling behavior were investigated.（1）The traditional coagulation-ultrafiltration process,Firstly,the microplastics system was pretreated by coagulation,and the supernatant was filtered by ultrafiltration membrane after static precipitation.The results showed that for different kinds of microplastics（PS and PA66）,both ferric salt and aluminum salt could effectively capture and remove pollutants in the microplastics system,which could reduce the membrane fouling load of the ultrafiltration system and greatly reduce the membrane fouling.The ability of aluminum salt to capture and remove microplastics system was better than that of ferric salt,and the ability to slow down the fouling of ultrafiltration membrane was better.The p H and microplastic size were the key factors affecting the removal of microplastics by ferric and aluminum salts.Fe Cl₃·6H₂O was easier to remove microplastics and organic pollutants under acidic conditions,while AlCl₃·H₂O was easier to remove microplastics and organic pollutants under alkaline conditions.Therefore,aluminum salt pretreatment was more suitable to control the membrane fouling of microplastics system for the traditional coagulation-ultrafiltration process.（2）The short-flow flocculation-ultrafiltration process.Ultrafiltration membrane filtration experiment was carried out directly after the coagulation reaction,and all pollutants and flocs entered the ultrafiltration system.The results showed that in the short process,both ferrite and aluminum pretreatment could effectively mitigate membrane fouling,and aluminum salt had a stronger ability to mitigate membrane fouling,which was similar to that of the traditional coagulation ultrafiltration process.However,different from the traditional coagulation-ultrafiltration process,in the short-flow flocculation-ultrafiltration process,only after the coagulant concentration reached a certain value,the pretreatment process could form a larger size of flocs,thus slowing down the membrane fouling.In addition,all flocs would enter the ultrafiltration system under the short-flow process,so the floc structure formed during the coagulation process became one of the key factors affecting membrane fouling behavior.The ability of ferric salt to control membrane fouling was better under neutral and acidic conditions,while the ability of aluminum salt to slow down the membrane fouling was little affected by p H.The presence of large-size microplastic particles on the surface of the membrane reduced the effective filtration area,so membrane fouling was+aggravated with the increase of microplastic particles.（3）Integrated coagulation-ultrafiltration process:firstly,pure flocs are deposited on the membrane surface to form a"floc layer",and then the polluted liquid of the microplastic system is filtered.The results showed that the control effect of two kinds of coagulants on membrane fouling was different.High concentration of ferric salt could slow down the membrane fouling,while low concentration of aluminum salt could form a protective layer on the membrane surface to achieve effective control of membrane fouling,which was caused by the different structure of flocs formed by the two coagulants.When aluminum salt was used as coagulant,the membrane surface formed a symmetrical protective layer,which made it difficult for the microplastics system to enter the membrane pores or fouling layer,thus greatly reducing the membrane fouling.Research on the influence of p H conditions and microplastic properties on membrane fouling in the integrated coagulation-ultrafiltration process showed that in actual operation,various factors such as floc layer structure characteristics,effective filtration area and pollutant size would affect the final membrane fouling,and under different conditions,the leading influencing factors of membrane fouling were not the same.