Solid/liquid separation (SLS) is a fundamental operation in the chemical and allied industries and urban wastewater treatment plants. Usually the first step in SLS involves slurry pretreatment for the purpose of increasing settling velocities and cake permeability. The Capillary Suction Apparatus (CSA) developed by Gale and Baskerville at Stevenage, UK in 1967 provides a simple, rapid, and inexpensive method for comparing the effects of different agents and dosages. Slurry is poured into a small open tube resting on a piece of filter paper. Filtrate is sucked out by capillary suction, and cake is formed at the bottom of the tube. The rate at which filtrate is sucked out depends on the resistance of the cake. By measuring the distance the filtrate traveling along the paper as a function of time, the cake resistance can be determined. One problem in CSA test is the effect of sedimentation which can not be neglected for a majority of flocculated slurries we have investigated. Most existing theory does not take the effect of sedimentation into account, and leads to overestimation of cake resistance. In this work, a modified capillary suction theory for rectangular cells including sedimentation is presented. Calculated values of resistance based on the new model indicated the existence of large errors in the former model which omitted sedimentation.