A gravity-driven membrane (GDM) system is a cleaning-free ultrafiltration (UF) process for decentralized water purification. However, GDM has a poor permeate quality and low stable flux when the feed water contains high levels of particulates, organic matter, and micropollutants. To address these challenges, this study used riverbank filtration (BF) as a pretreatment for GDM. The experimental results showed that BF could effectively reduce turbidity and particulate organic matter, and preferentially remove biopolymers and protein-like fluorescent components from natural organic matter. The removal efficiencies of micropollutants (diclofenac, carbamazepine, acetamidophenol, and bisphenol A) increased by 15.2%–65.3% in the presence of BF. Moreover, BF-GDM improved the removal of assimilable organic carbon (AOC) by 42%, thereby enhancing the biological stability of the permeate. Despite a modest increase of approximately 20% in the removal of dissolved organic matter, the BF significantly improved the stable flux from 2.8 to 7.3 L·m⁻²·h⁻¹. This remarkable improvement is attributed to the effective removal of key foulants, including particulate substances, biopolymers, and protein-like fluorescent substances, which leads to a thinner bio-cake layer with a higher density of microorganisms. Additionally, because of the high microbial diversity of the soil, BF pretreatment enriched the microbial diversity of the bio-cake layer, thereby enriching functional microorganisms capable of degrading pollutants in BF-GDM, such as Nitrospirota and Ascomycota. Overall, BF is a highly effective pretreatment for GDM, which potentially broadens its application to polluted source water.

• BF pretreatment increased the stable flux of GDM from 2.8 to 7.3 L·m⁻²·h⁻¹.

• BF pretreatment effectively remove key foulants including POC, biopolymers, and protein-like fluorescent component.

• BF effectively enhanced the micropollutants removal and the permeate bio-stability.

• BF enriched functional microorganisms in the bio-cake layer of GDM.