Influence of Floc Charge and Related Distribution Mechanisms of Humic Substances on Ultrafiltration Membrane Behavior

The integration of flocs with ultrafiltration (UF) membranes is a promising method to alleviate membrane fouling and reduce land use; however, the floc charge and related pollutant distribution mechanism in the floc-based layer, which are key issues in further improving membrane performance, have been given little attention up to this point. Herein, the UF membrane performance in the presence of differently charged floc layers and the corresponding humic acids (HA) distribution were investigated. The results indicated that the floc charge played an important role in UF membrane behavior due to the different HA distribution mechanisms. Owing to the electrostatic adsorption, most negatively charged HA molecules were relatively uniformly adsorbed inside the positively charged floc layer (pH < 8.5), and fewer HA molecules reached the membrane surface. The HA removal efficiency was as high as 92.5 ± 1.5% at a pH of 6, and the final J/J0 was 0.73. It has been shown that pore adsorption or blocking was mainly alleviated, and the proportion of the irreversible fouling was only 6.0 ± 2.4% at a pH of 6. In contrast, fewer HA molecules were intercepted inside the negatively charged floc layer (pH > 8.5), and they were mostly distributed at the top and bottom of the cake layer due to electrostatic repulsion. The HA removal efficiency significantly decreased to 32.4 ± 1.2%, while the final J/J0 increased to 0.78 at a pH of 9.5. Further experiments demonstrated that the cake-layer resistance was considerably alleviated, and the proportion of the reversible fouling was remarkably reduced to 49.5 ± 3.2% at a pH of 9.5. In addition, membrane fouling caused by large–molecular weight (MW) HA was easily alleviated by the positively charged floc layer, while that caused by small-MW HA was easily alleviated by the negatively charged one. These findings have significant potential applications in practical membrane-based treatments of drinking water.