The Mechanism On The Coagulation-ultrafiltration Process And Regulation Of Flocs In Algal-laden System

With the gradual expansion of eutrophication,most water sources have suffered from varying degrees of algae pollution.Which not only increases the cost of water treatment,while extracellular organic matter secreted by algae also poses a serious threat to human health.Coagulation（sedimentation）-ultrafiltration process was widely used in algae removal process for its high water purification performance.The coagulation as pretreatment process of ultrafiltration process,its efficiency directly determines the subsequent process pressure.And coagulant as the core of coagulation process,and its performance determines the efficiency of coagulation process.In this paper,three coagulants with different aluminum speciation were used to study the coagulation-ultrafiltration processed of three algal-laden systems,through the investigation of turbidity,organic matter,floc characteristics in coagulation process and membrane fouling problem in ultrafiltration process,we obtained the mechanism of algal coagulation-ultrafiltration process.Furthermore,A series of composite coagulants were prepared according to the action mechanism of different coagulants,and through the analysis of the characteristics of the floc（floc size,strength factor,recovery factor and fractal dimension）formed by the compound coagulant,the mainly mechanism on the process of formation,breakage and re-growth process of algae floc and the mainly mechanism of synergistic optimization between the compound coagulant were obtained.The detailed conclusions are as follows:1.In the coagulation-ultrafiltration process of raw algal samples,the soluble microbial products in the extracellular organics matter secreted by algae was not only the main organic matter affecting coagulation process,but also for membrane fouling problem in ultrafiltration process.Al₁₃ and Al₃₀,which has a high positive charge,can play a role of electrostatic cluster effects well under low dosage,thus having higher turbidity and organic matter removal rate.The degree of mitigation of ultrafiltration membrane fouling is mainly affected by organic matter removal rate and floc characteristics formed in coagulation process,compared to Al₃₀ and Al₂（SO₄）₃,the floc size and breakage factor of floc formed by Al₁₃ in the coagulation process is the largest,thus the formation of the cake layer has the most loose structure.combined with the higher organic matter removal rate of Al₁₃,Al₁₃ can effectively alleviate the ultrafiltration membrane fouling.2.In the coagulation-ultrafiltration process of algal cells,The characteristics of flocs formed in coagulation process are the main factors influencing the membrane fouling.Compared to Al₂（SO₄）₃,the Al₁₃ and Al₃₀0 coagulants would partially act on the surface depression of Microcystis aeruginosa and Chlorella（irregular particles）cells,which made the coagulant deactivate and reduced the coagulation efficiency.But It can play a role of electrostatic cluster effects well on the surface of the Cyclotella cells（regular particles）,and the floc has a larger particle size and strength factor,thus it can effectively alleviate the membrane fouling.3.In the coagulation process of algal,The characteristics of flocs is the result of the organic matter and coagulant mechanism.The electrostatic cluster effects(Al₃₀)under the low dosage of Al₁₃ and the charge neutralization under high dosage have important effect on the destabilization of the floc,and the floc formed by Al₁₃ has a larger recovery factor.While the sweep flocculation of Al₃₀ plays an important role in floc formation under high dosage,and the floc formed by Al₃₀0 has larger breakage factor.For the composite coagulant with different aluminum species,the composite flocculants make up for the lack of sweep flocculation in Al₁₃ and the charge neutralization in Al₃₀when the ratio of Al₁₃ and Al₃₀0 is 7:3,the floc has the largest strength factor and size,at the same time the required dosage is the least.Finally,the role of PDADMAC coagulant-aid in coagulation process is closely related to its dosage.