Relative importance of drag force and adhesion forces in the fouling of ultrafiltration membranes by secondary effluent organic matter after pre-ozonation

Abstract

Pre-ozonation prior to ultrafiltration is widely used, but its influencing mechanism on membrane fouling is still debated. Herein, the root influencing factors of pre-ozonation on membrane fouling by secondary effluent organic matter (EfOM) were analyzed from the perspectives of drag force, adhesion forces and the properties of EfOM and membranes. The results showed that EfOM fouling could be either exacerbated or mitigated by pre-ozonation, which depended on the relative magnitudes of the drag force and the adhesion forces. For membranes with a relatively low molecular weight cutoff (MWCO), fouling was exacerbated by pre-ozonation. This was attributed to the fact that pre-ozonation weakened electrostatic repulsion and enhanced hydrogen bonding, causing the adhesion forces of EfOM at the membrane surface to increase, which eventually promoted the deposition of foulants and resulted in a denser fouling layer. Nevertheless, when the MWCO reached a certain degree, the drag force increased substantially, which could push greater amounts of smaller ozone-treated EfOM into or through the membrane pores, thus reducing the deposition of foulants and increasing the probability of pore blocking. Therefore, after pre-ozonation, the fouling rate decreased, but irreversible fouling increased. However, membrane structure-related steric effects could weaken the influence of the drag force on membrane fouling.