Dual-layer granular beds consist of a lower layer of fine granules and an upper layer of coarse granules. In order to study the individual functions of the two granular layers with respect to filtration, a small granular bed filtering device (inner diameter of 100 mm) was employed. The two granular layers were separated using a wire mesh such that the upper layer was laid over the wire mesh. A welas (R) 3000 aerosol spectrometer was used to determine the dust concentrations and size distributions of the inlets and outlets of the upper and lower layers. According to the results, the upper layer was able to remove 96.08-98.78% of the dust. Moreover, owing to the features of the upper bed, including the coarse nature of its granules, its large voidage, and its deep-bed filtration ability, the pressure drop through the granular layer increased slowly with the amount of dust deposited. The lower layer, which had fine granules, was able to effectively remove the fine particulates that passed through the upper layer, reducing the dust concentration at the outlet to 3-7.5 mg/m(3). The experimental results confirmed that the dual-layer granular bed filter exhibits unique synergistic properties, namely, a low pressure drop because of the upper layer and a high filtration efficiency because of the lower layer. (C) 2016 Elsevier B.V. All rights reserved.