Comparison of different cleaning strategies on fouling mitigation in hollow fiber nanofiltration membranes for river water treatment

In this study, a novel hollow fiber nanofiltration (HFNF) was investigated for treating real Cijengkol river water in Indonesia. Ultrafiltration (UF) pretreatment was performed to evaluate the effect of UF on the HFNF performance. The water quality of the raw water and permeate of the UF, HFNF, and UF-HFNF hybrid was first characterized. UF could remove only solids, while both nanofiltration (NF) and the UF-NF hybrid achieved a high removal efficiency for impurities in river water, meeting the standards for drinking water quality. However, severe organic fouling occurred in the HFNF membranes. To effectively control organic fouling, various cleaning methods were investigated, and then the analyses of gel permeation chromatography (GPC) and fluorescence excitation emission matrix (FEEM) were conducted on backwashing wastewater to identify the cleaning mechanism. First, hydraulic flushing and backwashing were evaluated, and both methods achieved higher than 90% efficiency particularly for high-molecular-weight organic foulants (>1000 Da). However, the cleaning efficiency decreased during long-term operation because of the low removal efficiency of low-molecular-weight (LMW) organics. Hence, chemically enhanced backwashing (CEBW) (i.e., 5 ppm and 10 ppm NaOCl) was employed to enhance the cleaning efficiency. The results showed that membrane fouling was easily controlled by CEBW with 95% removal efficiency. To reduce the usage of chemicals, CO 2 scouring, and CO 2 nucleation backwashing were also examined. CO 2 nucleation backwashing exhibited a better efficiency than CO 2 scouring because of the additional effect of CO 2 nucleation on the HFNF membrane surface. Interestingly, a relatively high removal efficiency for fulvic acid-like organic fouling was observed in both CEBW and CO 2 nucleation backwashing, whereas tryptophan and tyrosine-like proteins, soluble microbial products (SMPs), and humic-like organics were hardly removed by acidic condition (pH < 6.5) or high shear force, leading to a low cleaning efficiency of CO 2 nucleation backwashing. • Indonesia river water was treated by novel hollow fiber nanofiltration membrane. • The dominant membrane fouling was due to low molecular weight organic fouling. • Periodical hydraulic backwashing and CO 2 scouring can replace pretreatment. • Hydraulic backwashing removes only high molecular weight organic foulants. • Chemical and CO 2 backwashing can remove low molecular weight organic fouling.