Optimization of a depth filter structure requires information about the filter's performance during its loading with particles. In this paper a macroscopic model of filtration process was proposed. It was based on the description of the loading process of three different gradient fibrous filters produced in the melt-blown technology process. Due to the contamination deposition on fibers, a new method of determination of the filter porosity transition, was suggested. It was based on growth of the deposit mass accumulated in individual filter layers, which was defined upon the scanning electron microscope images, the clearance of layer samples, and the overall increase of the filter mass. Local phenomena, i.e. the agglomerate structure, its spatial distribution and resuspension, were exposed through a two-stage character of the process. The empirical correlations obtained for deposition kinetics and pressure drop development are helpful in filter design. By knowing the desired contamination removal, fiber diameter and porosity can be verified mathematically, so that the filter will work the most efficiently. The proposed description of the structural loading of the multilayer filter is simple and reliable. Further research on biotic and abiotic particle transport mechanisms should be performed. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.