| Coagulation sedimentation process was widely applied in water and wastewater treatment because of its advantages:simple process, low cost, high coagulation removal and easy operation. Coagulant properties are the key factors which related to the coagulation efficiency and the operating cost. In order to improve the coagulation efficiency and reduce the operating cost, two or more inorganic and organic coagulants were used to prepare the new composite coagulant. In recent years, the inorganic-organic composite coagulant was the hot development area of water treatment agents.Synthesizing a number of references and literature, a composite flocculant, PAFC-PDMDAAC, was prepared by combining polymeric aluminum ferric chloride (PAFC) with polydimethyldiallylammonium chloride (PDMDAAC). Zeta potentials of PAFC-PDMDAAC were evaluated by modern advanced method. In synthetic surface water treatment, the effect of (Al+Fe)/PDMDAAC mass ratio (P), basicity (B) of PAFC, intrinsic viscosity (η) of PDMDAAC and pH value on the flocculation efficiency, flocculation mechanism and coagulation dynamics were investigated. Moreover, the floes properties, such as floe strength, the regrowth ability of broken floes and the fractal dimension of floes, were discussed. The correlation between inorganic and organic component in the composite flocculant was revealed. The research results provide theoretical guidance for the preparation and application of PAFC-PDMDAAC.The main conclusions are as follows:1. The zeta potential and charged characteristics of PAFC-PDMDAAC were determined using a Zetasizer3000HSA analyzer. The results showed that the surface charge, the zeta potential and the charge neutrality of PAFC-PDMDAAC were higher than that of PAFC. Moreover, the surface charge and the zeta potential of PAFC-PDMDAAC increased with the decrease of P and increase of η value. PAFC-PDMDAAC with B=1.0had the highest surface charge. 2.Coagulation experiments were conducted by treating synthetic surface water and disperse yellow dying wastewater using four dosing way,such as addition of composite coagulant PAFC-PDMDAAC,addition of PAFC,successive addition of PAFC and PDMDAAC.①In treating synthetic surface water,dosing way had little effect on the turbidity removal efficiency. Compared with PAFC, PAFC+PDMDAAC and PDMDAAC+PAFC, PAFC-PDMDAAC gave higher organic matter removal effi ciency and lower optimal dosage.Further,PAFC-PDMDAAC showed weaker charge neutralization ability but much stronger absorption/bridging and enmeshment ability than PAFC+PDMDAAC, and showed stronger charge neutralization, absorption/bridging and enmeshment ability than PDMDAAC+PAFC.②In treating disperse yellow dying wastewater, the order of the decolorization rate was PAFC-PDMDAAC>PDMDAAC+PAFC>PAFC+PDMDAAC>PAFC.Compared with PAFC,PAFC+PDMDAAC and PDMDAAC+PAFC,PAFC-PDMDAAC gave lower optimal dosage.The order of the charge neutralization was PAFC+PDMDAAC>PAFC-PDMDAAC>PDMDAAC+PAFC>PAFC.In coagulating disperse yellow, charge neutralization;bridging and enmeshment were the main coagulation mechanisms for PAFC-PDMDAAC.Consequently,PAFC-PDMDAAC achieved good performance by synergic effect of PAFC and PDMDAAC during surface water treatment.3. The flocculation efficiency of PAFC-PDMDAAC with different (Al+Fe)/PDMDAAC mass ratio(P),basicity(B)of PAFC and intrinsic viscosity(η)of PDMDAAC was studied in the treatment of synthetic surface water and disperse yellow dying wastewater,and the zeta potential of the flocs was also investigated,which explained the flocculation mechanism.①In the trealment of synthetic surface water, PAFC-PDMDAAC with intermediate P(P=10),intermediate B(B=1.0)and higher η (η=1.21dL/g)gave higher coagulation efficiency.②In the treatment of disperse yellow dying wastewater,PAFC-PDMDAAC with lower P(P=4),higher B(B=1.5) and higher η(η=1.21dL/g)gave higher decolorization efficiency.The charge neutralization,adsoyption/bridging and enmeshment ability of PAFC-PDMDAAC played a role for the coagulation mechanisms. The charge neutralization ability of PAFC-PDMDAAC was stronger with the decrease of P value and the increase of η value of PDMDAAC.4. The effect of pH on the flocculation efficiency of PAFC-PDMDAAC was studied in the treatment of surface water and disperses yellow dying wastewater.①In the treatment of synthetic surface water, pH value had lower effect on the turbidity removal and higher effect of the organic matter removal. The UV254and DOC removal efficiencies decreased with the increase of alkalinity. The organic matter removal efficiency is the highest when pH value of the raw water is5.5.②In the treatment of disperse yellow dying wastewater, the appropriate pH range of PAFC-PDMDAAC was wider. The decolorization efficiency is the highest when pH value of the raw water is6-9.5. The laser scatter analyzer PDA2000was used to on-line monitor the floe properties of PAFC-PDMDAAC. In the treatment of disperse yellow dying wastewater, the larger average floe size (Ratio) and lower Relative Standard Deviation (RSD) were achieved by PAFC-PDMDAAC compared with PDMDAAC and PAFC. Therefore, the dying removal efficiency of PAFC-PDMDAAC was better than that of PDMDAAC and PAFC. At lower dosage, the Ratio was increased and the RSD was decreased with the organic regent contents increasing. At higher dosage, the Ratio and the RSD was increased with the organic regent contents increasing. When pH was6-7.5, the Ratio of PDMDAAC,PAFC and PAFC-PDMDAAC were larger.6. In the treatment of synthetic surface water, faster floe growth velocity, shorter reaction time, larger average floe size, faster sedimentation velocity and better flocculation efficiency were achieved by PAF-DMDAAC compared with PAFC. Besides, the effect of (Al+Fe)/PDMDAAC mass ratio (P), B, η and pH values on the floe aggregation was investigated. Faster floe growth, larger average floe size and better flocculation efficiency were achieved by PAFC-PDMDAAC (P=4, B=1.5, η=1.21dL/g and pH=5.0). In breakage phase, the median equivalent particle diameter under all cases declined significantly and then followed by a gradual drop when the stirring speed was increased from40to200rpm. When the stirring rate was decreased to40rpm, the flocs began to regrow again. However, the flocs could not regrow to their previous sizes. The floe breakage degree decreased with the increase of the shear time and shear forces. Higher P, lower B, higher η value, shorter shear time and lower shear force could improve the floe strength. Lower P, lower B, higher η value, shorter shear time and higher shear force could increase regrowth ability in a certain extent. Moreover, PAFC-PDMDAAC gave higher floe strength and regrowth ability in the acid condition. Moreover, the floes formed by PAFC-PDMDAAC with lower P, higher-η and B value had higher fractal dimension and more compact structure. In addition, the floes formed in the acid condition were more compact.The above research studied the flocculation efficiency, flocculation behavior, coagulation mechanisms and floes properties of PAFC-PDMDAAC in the treatment of surface water, indicating that the composite flocculant was highly efficient and economical coagulant. This research provides fundamental and theoretical basis for the developing and practical application of PAFC-PDMDAAC...
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