Effects Of Coagulant Dosage And PAM Type On BF-UF Performance And Membrane Fouling Mitigation

Membrane separation technology is an emerging process that has been widely used in water treatment,and one of the most widely used membrane filtration techniques is Ultrafiltration(UF).With the increase of using time,ultrafiltration membrane will be blocked when adsorbing to remove pollutants,resulting in membrane pollution.Coagulation as a pretreatment process can effectively remove some pollutants,and the filter cake layer formed by coagulation process can be used as "a new membrane " to avoid direct contact between pollutants and membrane surface and slow down ultrafiltration membrane pollution.Ballasted Flocculation(BF)is produced by adding high density insoluble medium particles(BA,Ballasted Agent)and polymers as coagulant aids on the basis of a conventional Flocculation process to produce a Ballasted floc with a unique structure.In this paper,BF was used to replace the conventional coagulation process as a membrane pretreatment measure.The dosage of coagulant and the type of coagulant aid were selected as experimental conditions to explore the influence of ballasted flocculation conditions on the quality of BF-UF effluent,and the correlation between the structural characteristics of ballasted floc formed in the BF stage and the structure of ultrafiltration membrane cake layer.Based on the filtration and seepage mechanism,the steric-hindrance effect of cake layer was analyzed.Then the influence mechanism of ballasted flocculation pretreatment technology on membrane filtration efficiency and membrane fouling behavior in ultrafiltration stage was further explored.The dosage of aluminum sulfate(AS)coagulant in BF stage test was set AS1 mg/L,2.5 mg/L and 8 mg/L(calculated by aluminum),and different kinds of coagulant aids were selected: cationic polyacrylamide(CPAM),anionic polyacrylamide(APAM)and non-ionic polyacrylamide(NPAM).Microsand was taken as the carrier particle in BF,and the dosage was set at 3g/L to conduct BF-UF test.The results showed that under different BF test conditions,the effluent water quality of BF-UF combined process was good,and the turbidity removal rate was above 98 %,while the humic acid removal rate increased with the increase of AS dosage,reached 95.09 % under the optimal conditions.Analysis of BF stage ballasted floc size.When CPAM was used as coagulant aid and AS dosage was 8 mg/L,the average particle size of ballasted floc reached the maximum,which was 1100 μm.The ballasted floc average size of NPAM under 8mg/L coagulant dosage was almost similar to that under no coagulant dosage,and the value was only 95 μm.When the dosage of AS was2.5 mg/L and APAM was used as coagulant aid,the intensity of the ballasted floc was the highest,and its strength factor reached 90 %.Under all test conditions,the ballasted floc after breakage can basically recover to the particle size level before breakage in the re-formation stage.The fractal dimension distribution of the ballasted floc ranged from 1.2000 to 1.7000,and the maximum value(1.6498)was obtained when NPAM was selected as coagulant aid and AS dosage was 2.5 mg/L.The strength and fractal dimension of ballasted floc depend on the mechanism of BF.The effluent quality and the morphology of ballasted floc in BF stage have influence on membrane filtration efficiency and membrane fouling behavior in UF stage.At 8 mg/L AS dosage,the membrane specific flux of APAM and NPAM has the lowest attenuation amplitude,and the membrane specific flux of the end film is 0.9433 and 0.8554,respectively.The total resistance of the ultrafiltration membrane is also the lowest.For CPAM,the total membrane resistance was the highest at 2.5 mg/L AS dosage,and the specific flux of terminal membrane decreased sharply to 0.2247.Therefore,the effluent quality and floc characteristics under different coagulant aids and AS dosages in BF stage affect the membrane filtration efficiency and membrane resistance in ultrafiltration stage.In addition to the influence of appropriate ballasted floc size and fractal dimension on the pore structure of the filter cake layer,the effluent quality at BF stage and the formation mechanism of ballasted floc also affect the adsorption and retention of pollutants by the filter cake layer.Therefore,under the joint action of the two,different BF conditions have a significant impact on the purification efficiency of BF-UF process and the regulation of membrane pollution.