Unravelling the critical mechanism by which pre-ozonation affects ultrafiltration membrane fouling via investigation of the relative size and interfacial forces

To date, the influence of pre-ozonation on membrane fouling behaviour remains vague, and contradictory phenomena have been reported. Hence, to better understand the essential mechanism by which pre-ozonation affects the ultrafiltration membrane fouling behaviour, the present experimental study was carried out on pre-ozonation of sodium alginate (SA) solutions prior to their filtration. The relative size of SA and membrane pores and the interfacial forces of SA at the membrane surface before and after pre-ozonation were systematically investigated, and their relationship with the membrane fouling rate was determined. The results showed that whether SA after pre-ozonation was larger than, smaller than, or similar in size to the membrane pores, the trend of variation of membrane fouling with an increasing ozone dosage remained consistent. The results suggested that the changes in the relative size between foulants and membrane pores after pre-ozonation were not associated with the membrane fouling rate. In contrast, the changes of the interfacial forces of SA at the membrane surface can well explain the membrane fouling behaviour after pre-ozonation. Ozonation was found to weaken hydrogen bonds and strengthen electrostatic repulsion between the membrane and SA and between SA molecules, which mitigated the deposition of SA onto the membrane surface and resulted in a loose fouling layer. This effect eventually caused the extent of SA fouling of the ultrafiltration membrane to gradually decrease with increasing ozone concentration.