Coagulation efficiency, floc properties and membrane fouling of polyaluminum chloride in coagulation-ultrafiltration system: THE role of magnesium

The influence of magnesium on coagulation-ultrafiltration process was investigated regarding coagulation efficiency, floc properties and the subsequent impact on membrane fouling. PACl with different basicity values (B, OH/Al³⁺molar ratio) were used as coagulants. Fulvic acid (FA) water and high hardness FA water (containing magnesium) were used as original water in this study. Floc characteristics were investigated using a laser diffraction particle sizing device while membrane fouling was evaluated by a dead-end ultrafiltration batch unit. The results showed that PACl with higher OH/Al³⁺ value was more efficient for FA removal. Moreover, the addition of magnesium could improve its coagulation efficiency. Flocs generated in FA-magnesium water was smaller than that of FA water and showed a lower D_f value, which indicated that these flocs had open structures. Results of ultrafiltration test showed that membrane fouling became severe as OH/Al³⁺ value increasing in FA water, but could be reduced apparently when magnesium was added. In addition, the permeate pH increased as OH/Al³⁺ values growing whereas the permeate pH of FA-magnesium solution was higher than that of FA solution regardless of the original pH and coagulants.