Micro-nanobubbles for removing membrane foulants from the surface of nanofiltration membranes in drinking water applications

Abstract

Mitigating membrane fouling is an inevitable phenomenon in nanofiltration (NF) membrane treatments. In particular, direct treatment of surface waters using submerged NF membranes can induce higher membrane fouling owing to the lack of a pre-filtration process. This study aimed to optimize membrane cleaning conditions using micro-nanobubbles (MNBs) during submerged NF membrane treatment. The submerged NF system using NF270 membranes was operated at a constant flux of 6 L/m²h with weekly MNB cleaning intervals. The results showed that MNB cleaning for over 90 min achieved complete recovery of water permeance. During long-term operation (49 days), weekly MNB cleaning restored the transmembrane pressure to its initial value within the first four filtration cycles. The rejection of dissolved organic carbon remained high at > 74 %, indicating that MNB cleaning did not compromise the quality of treated water. The cleaning efficiency decreased in subsequent filtration cycles. The cleaning effect of MNBs was attributed to the shear force and collapse phenomenon, which reduced the formation of a cake layer comprising organic matter (e.g., protein-like substances) and bacterial cells on the NF membrane surface. These results demonstrate the effectiveness of MNB cleaning in mitigating fouling during submerged NF treatment.