Study On Membrane Fouling In The Coagulation-Ultrafiltration Treatment Of Algae-Laden Water

At present,the surface water source is still facing the problem of eutrophication.The eruption and growth of algae in the high algal stage aggravate the ultrafiltration membrane pollution of the water plant,which not only increases the cost of water treatment but also affects the quality of the effluent water.As an effective pretreatment technology for high algal water,coagulation can obviously alleviate the pollution of ultrafiltration membrane and improve the effluent quality.In this study,we investigated the effect of interspecific cell morphology differences of unicellular M.aeruginosa,multicellular M.aeruginosa and filamentous C.raciborskii on ultrafiltration membrane pollution in the combined coagulation ultrafiltration process,and discu ssed the mechanism of the influence of dissolved（dEOM）and attached（bEOM）extracellular algal organic matter on membrane pollution in the combined coagulation ultrafiltration process.The effect and mechanism of three coagulation methods on membrane pollution control of algal water of different algae species were studied.The main conclusions are as follows:1.The morphology of algal cells had significant influence on membrane fouling.Under direct ultrafiltiation,the total membrane fouling resistance of unicellular M.aeruginosa（spheroid）was the highest,the filamentous C.raciborskii was the second,and the multicellular M.aeruginosa was the lowest.After coagulation treatment,the total membrane fouling resistance caused by unicellular M.aeruginosa was reduced by 95%,the total membrane fouling resistance of multicellular M.aeruginosa was reduced by 71%,and the total membrane fouling resistance of C.raciborskii was only reduced by about 40%.Based on the Hermia model,the membrane fouling mechanism of unicellular M.aeruginosa and C.raciborskii in direct ultrafiltration was filter cake clog,intermediate clog and standard clog（R²>0.95）,while that of multicellular M.aeruginosa was mainly filter cake clog.After coagulation treatment,the membrane fouling mechanism of unicellular M.aeruginosa and C.raciborskii was mainly filter cake clog,while the membrane fouling mechanism of multicellular M.aeruginosa was intermediate clog and filter cake clog.The zeta potential results showed that the electronegativity of C.raciborskii as weak,reflecting the small electrostatic repulsion between cells and membranes.The results of scanning electron microscopy（SEM）further proved that the filaments of C.raciborskii on the ultrafiltration membrane were interspersed and interspersed to form relatively dense cake layer more easily.2.dEOM can aggravate the ultrafiltration membrane pollution of Microcystis aeruginosa.In direct ultrafiltration,the total membrane fouling resistance caused by algal cell,algal cell+bEOM,algal cell+bEOM+dEOM solution was 5.43×10¹² m^-1,7.05×10¹² m^-1 and 9.16×10¹² m^-1,respectively.The contribution of bEOM and dEOM to membrane fouling was 17.6%and 23.0%,respectively.After coagulation-ultrafiltration treatment,the membrane pollution resistance was reduced to 1.54×10¹¹ m^-1,2.71×10¹¹ m^-1 and 5.05×10¹¹ m^-1,respectively.The contribution of bEOM and dEOM to ultrafiltration membrane pollution was 23.2%and 46.3%.The results of atomic force microscopy（AFM）and scanning electron microscopy（SEM）show that the presence of bEOM and dEOM can reduce the roughness of the polluted layer on the surface of ultrafiltration membrane and make the surface more dense,which is an important reason for the pollution of ultrafiltration membrane.In direct ultrafiltration,dissolved microbial metabolites in dEOM are important substances causing membrane fouling.In the process of coagulationultrafiltration,bEOM can promote the increase of floc particle size,while dEOM can reduce the size of floc.Humus in dEOM is the key organic component of membrane fouling.3.The choice of coagulation method depends on the morphology of algal cells and the nature of organic matter.For unicellular M.aeruginosa,flocculation-precipitation pretreatment should be used to control the ultrafiltration membrane pollution,while for filamentous C.raciborskii,online flocculation pretreatment has the best effect on the membrane pollution control.Different coagulation methods have little difference on the membrane pollution control effect of multicellular M.aeruginosa.The Hermia model showed that the membrane fouling mechanism of unicellular M.aeruginosa changed from the co-existing blocking mechanisms to that of filter cake layer after coagulation-flock-precipitation treatment,and the scanning electron microscope results showed that the fouling layer formed by unicellular M.aeruginosa on the surface of ultrafiltration membrane was more loose after coagulation-flock-precipitation treatment.The fouling mechanism of filamentous C.raciborskii was mainly cake clog after different coagulation pretreatment.