Fractal Analysis Of The Membrane Fouling During Purifying Raw Water Containing Microplastics By Coagulation And Ultrafiltration Processs

As a new type of pollutant,microplastics are widely distributed in water bodies,which have received widespread attention because they may enter the human circulatory system and cause serious harm.Different amounts of microplastics are detected in water treatment,which will affect the water treatment process.In recent years,the problem of lead pollution has become more prominent with the widespread using of lead（Pb）,coagulation-ultrafiltration（CF-UF）process is a common method to remove heavy metal pollution,but the rapid growth of the membrane cakes leads to a rapid decrease in membrane flux,which is still an obstacle to the application of CF-UF in water plants.The structure of membrane cakes directly determines its filtering resistance.With the development of scanning electron microscopy（SEM）and professional image processing software,the test of the membrane cakes’structure becomes possible.At the same time,the fractal dimension theory provides a new method for describing the structure of membrane cakes.In this paper,starting with the treatment of"microplastics+humic acid+kaolin+Pb（II）"combined pollution simulated water samples by coagulation-ultrafiltration combined process,we examine the influences of the dosage of coagulant,the p H value of the water sample and the content of microplastics in the raw water on the treatment effect of the combined process were examined.The test results shows that the optimum coagulation conditions for the coagulation-ultrafiltration process are PFS 30.0mg/L,p H=7.0,while turbidity removal rate at 99.7%,HA removal rate at96.2%,and remaining lead concentration after ultrafiltration at 0.008mg/L.When the dosage of PFS is 10-70mg/L,the removal efficiency of turbidity,UV₂₅₄and Pb（II）will increase first and then decrease.When the p H value of the water sample is in the range of 5-10,the increase of p H is not conducive to the removal of humic acid,but the removal rate of turbidity and Pb（II）first increases and then decreases with the increase of p H.It was also found that the presence of microplastics with a particle size of 13μm is not conducive to the removal of pollutants.The higher the content,the worse the effluent quality.With the help of scanning electron microscope（SEM）and image processing software,we analyzed the changes in the pore structure of the filter cake at each typical time during the filtration process,and the results show that the filter cake layer structure has good fractal characteristics.During the ultrafiltration process,the average pore size,porosity,and pore perimeter gradually decrease,the pore size fractal dimension gradually increases,and the structure is gradually compacted,and the filter cake layer pore size fractal dimension and the average pore size and porosity are in a relationship Linear negative correlation,each parameter can well reflect the growth process of the filter cake layer.Generally speaking,The larger the particle size of the floc,the larger the porosity of the filter cake layer,the smaller the fractal dimension of the pore size,and the looser the structure.The more serious the reversible pollution of the membrane,the smaller the corresponding filter cake’s porosity,average pore size,and average pore perimeter,the larger the pore size fractal dimension,and the denser the filter cake structure.The values of irreversible pollution,reversible pollution and total pollution of the ultrafiltration membrane in the coagulation-ultrafiltration process first decrease and then increase with the increase of the PFS dosage and the p H value of the water sample.Alkalic cleaning agent can effectively remove the irreversible fouling of the membrane,and its cleaning effect is obviously better than that of acid and oxidant.When the dosage of PFS is low,the content of the agent in the water body is insufficient,the flocs produced are small in size and large in fractal dimension,which in turn causes the pore size and fractal dimension of the filter cake layer to be large,the structure is relatively dense,and the membrane is reversible.The pollution value is high,and the humic acid cannot be removed efficiently in the coagulation stage,and the irreversible pollution of the membrane is more serious.When the dosage of PFS is too large,the excessive hydrolysis of PFS will cause excessive positive charge in the water body,resulting in a stabilization phenomenon.At this time,the particle size of the flocs is small,and the small particle size suspended particles enter the filter cake layer to make its pore size fractal dimension.As the number increases,the structure is dense and the water permeability decreases,and the reversible pollution of the membrane increases.At the same time,the removal effect of humic acid and other pollutants in the coagulation stage becomes worse,and the irreversible pollution of the membrane increases.When the p H value of the water sample is low,although the hydrolysate forms more adsorption sites,due to the presence of excessive positive charges,the excessive electrostatic repulsion has a compressive effect on the outer surface of the flocs,making the particle size smaller,The fractal dimension is large and the structure is dense,which further causes the filter cake layer to have a smaller porosity,a larger pore size and a larger fractal dimension,and a larger reversible pollution of the membrane.At the same time,electrostatic repulsion will cause poor coagulation effect and high irreversible pollution of the membrane.When the p H of the water sample is too high,the negative charge content in the solution is too high,which also causes the flocs to have a small particle size and poor sedimentation performance,forming a filter cake layer with a larger pore size and fractal dimension,and a larger reversible membrane pollution.Microplastics mainly affect the reversible pollution of the membrane by changing the pore structure characteristics of the filter cake layer.When the content of microplastics with a particle size of 13μm is higher,more microplastic particles will be brought into the filter cake layer with the flocs.Or embedded on the surface of the filter cake layer,the more severe the squeezing effect on the water passage of the filter cake layer,the greater the porosity of the filter cake layer,the larger the fractal dimension of the pore size,and the significantly higher membrane reversible pollution value.