Study On Control Mechanism Of Membrane Fouling During Coagulation/Ultrafiltration Process Based On Effect Of Aluminum Species

With more stringent requirement regarding safety and reliability of water supply,ultrafiltration has been increasingly applied in drinking water treatment.However,membrane fouling inhibited the application of this technology.Coagulation could effectively lower membrane fouling.Aluminum?Al?salt coagulants have been widely applied at this stage.Al speciation of Al salt coagulants have significant influence on the performance of coagulation/ultrafiltration process.In this research,influence of the composition of raw water on membrane fouling was investigated.Based on the research above,the influence of different Al species on coagulantion/ultrafiltration process were investigated under various conditions,and effect of Al species on cake characteristics were also explored.In addition,machanical coagulation and varialble velocity sedimentation technology was used to investigate the effect of Al species on membrane fouling.Inorganic particle led to lower membrane fouling and most of the membrane fouling were reversible fouling.Under the most serious membrane fouling condition,the normalized flux at third cycle end was 0.876.With similar particle amount,the decrease of particle size fractal dimension resulted in more serious membrane fouling.While organic matter resulted in severe membrane fouling and most of the membrane fouling were irreversible fouling.Under the slightest membrane fouling condition,the normalized flux was 0.418.For organic matter with MW 30-50kDa,membrane fouling was the most severe.While MW fraction of<10kDa led to the slightest membrane fouling.When both inorganic particle and organic matter were existed in raw water,the adsorption of organic matter on particles resulted in lower organic matter concentration in raw water,and the restriction of organic matter on self-aggregation of particles contributed to lower particle size fractal dimension,generating larger reversible fouling and total fouling.Reversible fouling increased from 0.376 to 0.470.The role of different Al species on membrane fouling for different solution conditions were explored,Results found that different Al species have different optimal solution conditions to mitigate membrane fouling,indicating that various Al species could be used in different solution conditions.In addition,floc breakage and re-growth was used to optimize particle characteristics for further controlling membrane fouling.Results showed that larger velocity gradient at each stage inhibited the formation of flocs with large size and loose structure for all Al species.However,under medium polymer Al species?Al_b?and colloidal Al species?Al_c?conditions,floc breakage and re-growth still effectively alleviate membrane fouling due to higher removal of organic matter.Comparing the role of different Al species,Al_b contributed to the slightest membrane fouling before floc breakage and after floc re-growth.For Al_b,the normalized flux at the third cycle end was 0.770 before floc breakage,while the value was 0.791 after floc re-growth.This was because organic matter with MW smaller than 50kDa was best removed for Al_b although small flocs with compact structure were formation during coagulation process.Considering cake is a significant factor to generate membrane fouling.The establishment of relation between particle size fractal dimension and two-dimension fractal dimension during coagulation process and pore fractal dimension of cake is very important to explore the influence Al species on coagulation/ultrafiltration process.First,analysis method of cake was estabilished by using scanning electron microscope?SEM?image.Results showed that SEM image with magnification×5000 was suitable to calculate cake characteristics.The relative error was lower than5%.Then cake porosity calculated by Carman-Kozeny equation was compared with that calculated by SEM images,finding that cake porosity calcualted by SEM images are reasonalbe.Then,pore fractal dimension of cake calcualted by SEM images was applied to vertify the mode about the influence of floc properties on cake.Based on the research above,the effect of Al species on cake was analyzed.Different cake characteristics under various Al species were due to the role of particle size fractal dimension and two-dimension fractal dimension during coagulation process.Under floc breakage and re-growth condition,the flocs with smallest two-dimension fractal dimension?1.803?and largest particle size fractal dimension?1.005?contributed to the loosest cake which porosity was 7.183%.In the study on the influence of pre-coagulation on membrane fouling,results showed that coagulation/ultrafiltration generated lowest irreversible fouling,indicating coagulation/ultrafiltration was optimal combination mode.However,coagulation/sedimentation/ultrafiltration also has some advantages due to lower total fouling.Furthermore,a machanical coagulation and varialble velocity sedimentation reactor was used to applied in this research.Under optimal shear rate combination of coagulation pocess(59-48-17s^-1)condition,irreversible fouling for coagulation/ultrafiltration process was lower than those for micro-coagulation/ultrafiltration and coagulation/sedimentation/ultrafitration with various Al species,indicating that coagulation/ultrafiltration was also optimal combination mode for continuous flow device,which was in accordance with jar-tests.However,with the formation of suspended layer in varialble velocity sedimentation tank,coagulation/sedimentation/ultrafitration generated lower membrane fouling compared with coagulation/ultrafiltration.Exploring the role of Al species on the formation of suspended layer,Al_c was the most benefical Al specie to form suspended layer.However,the interception ability of suspended layer formed with Al_c addition was weak due to good initial effluent water quality for sedimentation tank.The particle number was only 1013 in 1ml.Compared with Al_c,Al_b has weaker abiltiy to form suspended layer.But suspended layer formed with Al_b addition has optimal interception ability since initial effluent water for sedimentation tank contained more particles.The particle number was 1862 in 1ml.In addition,sludge reflux was applied to enhance dynamic suspended layer.Results showed that sludge reflux with one cycle could improve the quality of settled water,whereas sludge reflux with two cycles would lead to the deterioration of settled water quality,particularly dissolved residual Al higher than 0.2mg/L.Analysis on the accumulation mechanism of suspended layer which enhanced coagulation/ultrafiltration process,orthokinetic flocculation was main accumulation mechanism for suspended layer.