Removal Efficiency Of Combined Pollutants Containing Microplastics And Membrane Fouling Mechanisms During Coagulation And Ultrafiltration Process

Microplastics is a new type of pollutant which has a serious threat to the ecosystem and human health.A large number of microplastics have been detected in source water.The traditional coagulation sedimentation process has limited effect on the removal of microplastics.The coagulation ultrafiltration process can significantly improve the removal efficiency of microplastics and the effluent quality,but the membrane pollution is always an important reason limiting its wide application.In this paper,the combined process of coagulation and ultrafiltration was used to treat simulated water samples polluted by microplastic,humic acid,kaolin.The removal efficiency of various pollutants and the mechanism of membrane pollution control were analyzed from the perspective of flocculation morphology.The effect of microplastic particles on the relationship between cake structure and pre-filtration particle morphology was explained.Taking the dose of PFS,the concentration,particle size and species of microplastics in raw water as the investigation conditions,the removal effect of composite pollutants containing microplastics by the combined process was studied.The results show that the optimal coagulant dos e is 30mg/L,in the process of increasing PFS dose,the removal efficiency of microplastics increases first and then decreases,and the increase of the concentration of microplastics has a great impact on the remova efficiency of various pollutants,the removal efficiency of turbidity,humic acid and microplastics is the lowest when the concentration of microplastics in raw water is 100 mg/L.The size of microplastics has a great i mpact on the removal efficiency of microplastics.When the coagulation conditions are the same,the removal efficiency of large-size microplastics is very low.In addition,the removal efficiency of turbidity and humic acid increases as the size of the microplastic particles increases.The removal efficiency of PVC and PS microplastics after coagulation sedimentation process was significantly higher than that of PE and PP,but there was no significant difference among the four types of microplastics after coagulation ultrafiltration process.The effects of coagulant dose and microplastic properties on floc morphology,membrane flux attenuation and membrane fouling reversibility were studied by coagulation ultrafiltration test.The study shows that as the dose of PFS increases,the final floc size increases first and then decreases.When the dose of coagulant is30mg/L,the flocs formed are larger in size and loose in structure,the fractal dimension Df of the flocs is small,and the rate of decrease of membran e flux slows down.The presence of microplastic particles will adversely affect the pollution of ultrafiltration membranes.As the concentration of microplastics in the raw water increases,the degree of membrane flux decline increases,and the water perme ability of the filter cake layer becomes worse.However,low concentration microplastic can effectively slow down membrane pollution,forming loose filter cake layer on the surface of membrane,which is easy to be disturbed and dissipated by water,and reduce the irreversible pollution value.During the coagulation process,small-size microplastics are easily captured by floc and enter into the mesh space of flocs,and the size of flocs during this process is small.When a large number of small particles of plastic are adsorbed on the gel layer of the membrane surface,the porosity of the ultrafiltration membrane decreases,and the membrane separation performance decreases.The flocs formed by different kinds of microplastic coagulation are different,the flocs formed by coagulation of PVC microplastics are small in size and compact in structure,the cake layer deposited on the surface of the membrane is relatively dense,and the specific flux of the membrane decreases greatly.The distribution and migration of microplastic particles and the morphology of filter cake layer on the surface of ultrafiltration membrane were observed by digital microscope and scanning electron microscope respectively.The results showed that most microplastic particles in the mixed solution existed in water alone in the slow mixing stage,and microflocculus began to appear in the solution.With the extension of coagulation time,the flocs grow continuously,and the microplastic particles collide with flocs to form microplastic humic acid PFS flocs.The cake layer formed by the coagulation water sample without microplastic is fluffy and porous,and there are many pores on the surface.However,the cake layer formed by the water sample with microplastic on the membrane surface is very irregular,and there are many protruding microplastic particles on the surface,and most of the pore diameter on the membrane surface is blocked by pollutants,which is difficult to be removed by hydraulic backwashing.