Coagulation and membrane fouling mechanism of Al species in removing humic acid: Effect of pH and a dynamics process analysis

The pH conditions affected the binding sites and complexation ability of Al species and humic acid (HA), resulting in the difference of coagulation and membrane fouling mechanism. It is important to explore the dy-namic process of coagulation for the mechanisms of Al species and HA. In this study, three Al species (Ala/Alb/ Alc) were used in coagulation process to compare the pollution degree of ultrafiltration (UF). Then, the removal efficiency of Ala for DOC was better than that of Alb and Alc, that is, the removal performance of suspended and dissolved organic matter was better for Ala. Through the analysis of coagulation dynamic process and membrane resistances, the flocs formed were smaller size and grow slowly to the equilibrium stage at pH 4, which was easy to cause membrane fouling by concentration polarization. With the pH increasing to a neutral condition, the flocs had a large size and loose structure, and then the porous calce layer was easy to be removed by physical cleaning, leading to the membrane fouling mainly due to pore plugging. Under allcaline conditions, flocs with small size and dense structure were mainly formed by sweeping, which resulted in the calce layer as the main cause of membrane fouling. This study is helpful to optimize the process of HA removal by coagulation-UF, and provides technical support for the coagulation and membrane mechanism of Al species and HA under different pH conditions.

Automated summary

This study compared the pollution degree of ultrafiltration caused by three Al species (Ala/Alb/Alc) and found that Ala showed better efficiency in removing DOC than Alb and Alc, indicating a better removal performance for suspended and dissolved organic matter. Analysis of the coagulation dynamic process and membrane resistances revealed that at pH 4, smaller-sized flocs formed and grew slowly, causing membrane fouling due to concentration polarization. At neutral pH, larger-sized and loose-structured flocs formed, leading to membrane fouling mainly due to pore plugging. Under alkaline conditions, flocs with small size and dense structure formed, resulting in membrane fouling mainly caused by the formation of a calce layer.