The permeation of an oil-in-water (O/W) emulsion through a coalescing filter was numerically studied using the lattice Boltzmann method (LBM). A numerical simulation model for the coalescing phenomena was developed based on the free-energy LBM. We investigated the effects of the wettability of fibers, filter porosity, and fiber diameter on the coalescing behaviors by performing two-dimensional permeation simulations for the O/W emulsions through modeled fibrous filters. We mainly focused on hydrophilic filters because they did not generate small secondary droplets during oil droplet detachment from the filter, and this is preferred for precise separation of oil and water. Our simulations demonstrated that filters with larger pore spacings enable formation of larger droplets but allow more droplets to pass without coalescing. To solve this problem, we designed bi-layered filters composed of a small-pore filter to accurately catch the droplets and a large-pore filter to enlarge the droplets; we demonstrated the effectiveness of the bilayer structure for membrane coalescence. (C) 2017 Elsevier Ltd. All rights reserved.