Ballasted flocculation pretreatment for mitigating ultrafiltration membrane fouling: Role of differently charged polyacrylamides at typical coagulant dosages

Abstract

In this work, ballasted flocculation (without sedimentation) was suggested as a potential pretreatment for ultrafiltration to improve cake layer microstructure and mitigate membrane fouling. First, three representative aluminum sulfate (AS) coagulant dosages (including 1.0, 2.5 and 8.0 mg Al/L) were determined during preliminary testing to produce microfloc aggregates with negative, zero and positive zeta potentials at the coagulation stage of ballasted flocculation. Regardless of the applied coagulant dosage, not only ballasted floc size but also their strength and recovery abilities (following breakage) were dependent upon the selection of differently charged polyacrylamides (PAMs), likely due to the effect of PAM selection on polymer bridging and associated flocculation mechanisms. Second, comparisons of membrane flux decline and resistance distribution were performed to evaluate the importance of coagulant dosage and PAM type to cake layer formation and membrane fouling potential in the ballasted flocculation and ultrafiltration integrated process. The results demonstrated that the ballasted floc aggregates formed by ballasted flocculation pretreatment contributed to diverse structural characteristics of the fouled layer during their accumulation on the membrane surface, thereby affecting the degree of membrane fouling for various combinations of AS dosage and PAM type. Besides, the microsand particles involved in the formed layer could act as a skeleton of the cake layer structure, so their spatial distribution within the layer interior played a critical role in cake layer microstructure improvement, ultrafiltration efficiency enhancement and membrane fouling mitigation. In other words, the extent of steric-hindrance effects by ballasted floc aggregates should be emphasized in the formation of cake layer.