The macroscopic design of granular filtration is based on phenomenological equations for the prediction of the overall filtration efficiency and pressure drop across the length of a typical filter. Despite the fact that a variety of empirical models have already been proposed in the past, an analytical expression for the prediction of the performance of the porous medium during filtration process is not yet broadly accepted. In the particular case of horizontal flow filtration, a non-uniform distribution of particle and cluster deposits along the main direction of flow (width of the horizontal filter) is obtained, which necessitates a three-dimensional phenomenological description. To evade the assumption of uniform deposition of particles within a certain zone, differential mass balance expressions are introduced in the present work, and filtration rates are expressed as functions of the width and the depth of the filter. Results for the filter coefficient correction factors entered in the macroscopic equations are presented for the case of horizontal flow and are compared with results from downward filtration. (C) 2011 Elsevier B.V. All rights reserved.