Effect Of Coagulated Water Containing Microplastics On Membrane Filtration Efficiency During Shortened Ultrafiltration Process

Microplastics（MPs）refer to plastic fragments or particles with a particle size less than 5 mm.They have been widely detected in the aquatic environment in recent years and are regarded as one of the emerging pollutants because they are not included in routine monitoring and have known or potentially negative impacts on ecology and humans.Ultrafiltration is an efficient physical separation technology,which can effectively reduce the content of microplastics in water.But this technology gives rise to a quite low removal efficiency of dissolved organic matter in water.The coagulation/ultrafiltration combined process can improve it,alleviate membrane fouling and prolong the membrane filtration cycle.In this paper,coagulation experiment of water samples containing microplastics with different particle sizes and concentrations was carried out.The coagulated water containing microplastics was associated with the cake layer formed during ultrafiltration process.Then the water purification efficiency and removal mechanism of the ultrafiltration water containing microplastics were investigated by the shorten process of coagulation and ultrafiltration as the core.The research results showed that the effect of microplastic size on coagulation/ultrafiltration was manifested in the following aspects:the fractal dimension of flocs formed by 0μm PE microplastics was 1.5763,the floc size was60.21μm after breakage and reformation,and showed the best settling performance.The high density flocs were deposited on the membrane surface,resulting in a low porosity of the filter cake layer,so the membrane resistance was large.At the end of the filtration,the specific flux of the membrane was 0.4879,the ability to return to the original flux after cleaning was limited;the floc formed by the 13μm PE microplastic had the smallest average particle size,fractal dimension and intensity factor,small particle size and low compactness led to poor sedimentation performance,loose structure of the floc deposited on the membrane surface,filtration resistance was small,membrane flux decay was the slowest;and flocs formed by 150μm microplastic had larger particle size,the structure is relatively looser,after breakage and reformation of the floc particle size was 85.76μm,the recovery factor was215.38%,the sedimentation performance was better,it can still maintain a large terminal membrane specific flux after ultrafiltration.It had strong ability to return to high membrane flux after physical cleaning.As the concentration of microplastics increased,the particle size and fractal dimension of the flocs decreased.Although the loose structure was not conducive to the settlement of flocs,it can improve the porosity of the filter cake layer on the surface of the ultrafiltration membrane.The flux decay rate decreased,and the membrane resistance decreased.The SEM results of the membrane surface after filtration also showed that the filter cake layer structure formed by ultrafiltration of coagulated water containing higher concentrations of microplastics was looser and more porous.When the volume of pre-deposition water was 800 m L,the specific flux of the terminal membrane was 0.7052 and 0.7780,but it showed an excessively long filtration period,and the total membrane resistance reached 0.43×10¹⁰m^-1and 0.33×10¹⁰m^-1;when the pre-deposition water was 200 m L,it not only slowed down the decay of membrane flux,but also reduced the resistances of filtration.By controlling whether the flocs undergo breakage and reformation to obtain pre-deposition layers with different compactness,the research results showed that the process of breaking and reforming was not conducive to slowing down the membrane fouling;Membrane fouling was less in the water sample system of plastic.The membrane filtration efficiency and membrane fouling characteristics of ultrafiltration,coagulation/ultrafiltration,andcoagulation/pre-deposition ultrafiltration were compared in water with and without microplastics systems,and it was found that the advantages and disadvantages of ultrafiltration performance were followed the order of:coagulation/pre-deposition ultrafiltration>coagulation/ultrafiltration>ultrafiltration,and microplastics could mitigate membrane fouling.The coagulation/ultrafiltration combined process could effectively alleviate the membrane fouling,mainly because the coagulation could effectively remove the dissolved organic substances in the water,and the formed flocs played a secondary interception and adsorption effect on the pollutants,reducing the filter cake layer resistance,thereby increasing the water permeability of the membrane;coagulation/pre-deposition ultrafiltration was to deposit a cake layer on the membrane surface in advance,intercepting the pollutants that first reach the membrane surface,further reducing membrane fouling;The existence of microplastics could relieve the membrane fouling in the short-flow ultrafiltration process.On the one hand,the specific surface area of microplastics was large and had strong ability to adsorb pollutants in water.Besides,it had considerable strength.As a supporting structure,it improved the structure of filter cake layer,so that the filter cake layer on the membrane surface was loose and more porous,the membrane resistance was reduced and the water permeability was increased.