The low fuel-water separation efficiency and premature failure of coalescer media in ultra-low sulfur diesel and biodiesel applications pose significant challenges. This study investigates the influence of surfactants under low interfacial tension (IFT = 11 mN/m) on coalescer media failure mechanisms, and systematically conducts fiber screening, structural design, and synergistic evaluation of separation and filtration performance. By observing the interaction between five types of single fibers with different wettabilities and water droplets, the mechanisms of droplet capture and coalescence were elucidated, leading to the selection of polyethylene terephthalate (PET) fibers and glass fibers for material construction. Performance evaluation according to ISO 16332:2018 (IFT = 11 mN/m, D₅₀ = 10 µm) shows that...