Effects Of Composite Coagulant On Water Treatment Efficiency And Membrane Fouling Of Coagulation-ultrafiltration Process

As an efficient water treatment technology,ultrafiltration(UF)has been widely and effectively used in water treatment.However,it usually needs to be combied with other technologies due to its membrane fouling.Coagulation-ultrafiltration(C-UF)hybrid process can significantly improve the NOM removal and alleviate membrane fouling.The combination of poly dimethyldiallylammonium chloride(PDMDAAC)and polyaluminium chloride(PAC)can improve the quality of effluent.However,in C-UF process,the coagualtin mechanism of PAC-PDMDAAC and the impact of coagulation performance and floc characteristic on membrane fouling have not been systematically studied.Based on the current research background,in order to clarify the influence of bacisity of PAC and viscosity of PDMDAAC on the coagulation mechanism,a series of PAC-PDMDAAC composites with different parameters are prepared in this paper,and its mechanism was also studied in depth.In the treatment of different water sample,the influence of coagulation behavior of PAC-PDMDAAC and floc formation on OF membrane fouling were discussed combined with the NOM characteristics and floc formation.The C-UF process and C-sedimentation-UF process were constructed to analyze the coagulation behavior and floc characteristics of PAC-PDMDAAC on membrane fouling.In addition,PAC will hydrolyze to form different forms of Al,including Ala,Alb and Alc after adding to water.Different A1 species of PAC were composited with PDMDAAC and applied to the treatment of different water sample by C-UF process.The mechanism and application prospect of Al species-PDMDAAC were clarified.The main results are summarized as follows:(1)Reserach of the structure and characteristics of composite coagulant:In this paper,PAC was synthesized by micro drop alkali method with aluminum chloride hexahydrate and anhydrous sodium carbonate,and then compounded with PDMDAAC to obtain PAC-PDMDAAC composite coagulant.The structure,morphology and charge characteristics of PAC-PDMDAAC were studied.The results showed that PDMDAAC can improve the charge density and molecular weight of PAC.Compared with PAC,PAC-PDMDAAC has more branches and wider distribution of particle size.The FT-IR of PAC-PDMDAAC showed that most of the absorption peaks were as the same as PAC,but some of them changed or even disappeared.It indicated that the introduction of PDMDAAC into PAC was not a simple physical mixing,but a certain interaction between them.(2)Research of the coagulation performance and mechanism of PAC-PDMDAAC:PAC with different basicity(B)and PDMDAAC with different viscosity(?)are combined with different mass ratio.The results showed that the coagulation mechanism is significantly different with the change of coagulant parameters.When B of PAC was low,PAC-PDMDAAC mainly played the role of charge neutralization at low dosage,but with the increase of dosage,colloidal particles tended to stabilize again.When the B value was high,the main mechanism was charge neutralization and adsorption bridging.When the ? of PDMDAAC was high,the main mechanism of PAC-PDMDAAC wass adsorption bridging at low dosage,and the charge neutralization gradually improved with the increase of dosage.When PAC-PDMDAAC was used to treat HA and HA-BSA,the mechanism of charge neutralization and adsorption bridging is the main mechanism.When treating HA-SA and HA-BSA-SA,the gel layer formed by Al3+ and SA could also play the role of adsorption bridging,net trapping and sweeping to enhance coagulation effect.PAC could achieve a good coagulation performance when the NOM concentration is low,but with the increase of NOM concentration,the coagulation performance of PAC obviously decreased.In contrast,due to the strong neutralization and adsorption bridging effect,PAC-PDMDAAC could still achieve a better coagulation performance when the concentration of organic matter was high,and had a wider range of application of NOM concentration.(3)Research on floc characteristics of PAC-PDMDAAC composite coagulant:Compared with PAC,the flocs fonned by PAC-PDMDAAC had larger size and more branch structure.With the increase of PDMDAAC content,the adsorption bridging effect of PAC-PDMDAAC was enhanced,and the amorphous branching structure of flocs was more.compared with HA and HA-BSA water samples,the flocs formed in HA-SA and HA-BSA-SA water samples had faster growth rate,larger particle size and better recovery under weak shear force.However,when the NOM concentration was too low,low density of colloidal particles resulted in low collision probability.When the NOM concentration was too high,high colloidal density made electrostatic repulsion too strong.Coagulant could enhance the particle size and structure characteristics of flocs and reduce the influence of NOM concentration on flocs after PDMDAAC was introduced.The flocs formed by PAC-PDMDAAC still had better regeneration ability when the organic matter concentration is 20 mg·L-1 and 50 mg·L-1.(4)Research of membrane fouling mechanism of PAC-PDMDAAC composite coagulant:For different materials of UF membrane,the effect of coagulant on fouling alleviation was different.For the regenerated cellulose(RC)membrane with relatively large surface energy,PAC-PDMDAAC could effectively improve membrane flux becaused of the better hydrophilicity and the lower self-resistance of RC membrane.PAC-PDMDAAC could mitigate membrane fouling by forming loose cake layer and reduce the resistance of cake layer.The main mechanism of membrane fouling induced by wate samples with different NOM was obvious different.The main mechanism of HA fouling and HA-BSA fouling was pore plugging and surface adsorption,and the irreversible fouling that could not be recovered after backwashing was relatively serious.PAC-PDMDAAC mainly alleviated membrane fouling by reducing the resistance of cake layer;the mechanism of HA-SA fouling and HA-BSA-SA fouling was the formation of cake layer,and PAC-PDMDAAC could significantly mitigating membrane fouling through the formation of cake layer with loose and porous structure With the increase of the NOM concentration in the water sample,membrane flux gradually decreased,while PAC-PDMDAAC could significantly alleviate membrane fouling and weaken the influence of NOM concentration on membrane fouling through improving the structure of cake layer and reducing NOM concentration.(5)Research of the coagulation and ultrafiltration mechanism of Al species-PDMDAAC:The main coagulation mechanism of Ala was hydrolysis-adsorption-sedimentation and charge neutralization,the main coagulation mechanism of Alb was charge neutralization and adsorption bridging,and for Alc it was adsorption bridging and net-trapping and sweeping.PDMDAAC,as a cationic polymer could enhance the adsorption bridging,electroneutralization and electroneutralization of Ala/Alb/Alc,respectively.Compared with Alb-PDMDAAC and Alc-PDMDAAC,Ala-PDMDAAC had an insignificant removal effeciency on dissolved organic matter,but a significant coaugaltion performance on aromatic organic matter.The residual aluminum in the coagulated effluent mainly existed in unstable granular aluminum and could not meet the concentration requirements of China's water quality standards(National Standard of the People's Republic of China GB/t5750.6-2006).Ala-PDMDAAC could effectively control the residual aluminum concentration of ultrafiltration effluent and alleviate membrane fouling by forming a porous cake layer on the membrane surface.For water samples with different SUVA values,the coagulation performance and fouling mechanisms were different.Water samples with higher SUVA value were easy to be adsorbed on the membrane surface and result in increase of resistance of cake layer and reversible fouling due to the existence of more hydrophobic organics.In water samples with small SUVA value,more small molecular hydrophilic organics were difficult to be removed by coagulation,and easy to enter membrane pore,resulting in membrane pore plugging and irreversible fouling.Ala-PDMDAAC could effectively capture macromolecular hydrophobic organics and further capture micromolecule hydrophilic organics through the effects of neutralization and adsorption bridging,then reduce the concentration of organics and form loose cake layer on the membrane surface,effectively alleviate reversible fouling and irreversible fouling.