The effect of re-aggregated floc by additional coagulant on membrane

The removal of turbidity, organic and inorganic contaminants in drinking water treatment plants is well established by using the hybrid membrane system combined with conventional coagulation and filtration techniques. It is well known that the aggregation of fine particles and colloids into larger particles can improve membrane filtration performance. However, flocs are easily broken by many factors such as when the pump has an impeller. Alternatively, in this study, the additional coagulant dose was applied for effective regrowth of the floc. The flocs formed by poly aluminum chloride (PACl) are artificially broken down by increasing the paddle speed such as the impeller of a feed pump and the effects of re-aggregation are compared both with additional coagulant and without it. The re-growth flocculation size index (FI) with additional coagulant and without it was 0.20 and 0.15, respectively, when the total coagulant dosage was 20 mg/L. The flux decline rates according to additional coagulant dosage also differed. The flux decline rate of the flocs without additional dosage was 7.5 times larger than that of the flocs with additional dosage. Namely, the additional dosage process was more effective than the conventional process when the total amount of coagulant was the same.

► Additional coagulant process was evaluated in the coagulation-membrane hybrid process.
► With additional coagulant dosage, the membrane performance could be improved.
► The floc strength is more important than the floc size in the membrane performance.