A three-dimensional simulation was performed to predict the flow field and the separation efficiency for particles in a hydrocyclone to be used for the sludge separation in water purifying plants. The Reynolds averaged Navier-Stokes and Reynolds averaged continuity equations employing renormalization group k-epsilon model were solved to calculate the turbulent flow field in a hydrocyclone. The particle trajectories were computed by integrating the force balance equations on particles based on the predicted flow field. The separation efficiency defined as the fraction of particles recovered to underflow was obtained by using the calculated particle trajectories. The separation efficiency of a hydrocyclone for the sludge separation in water purifying plants was measured experimentally as a function of particle size. Simulation results agreed favorably with experimental data. (C) 2004 Elsevier Ltd. All rights reserved.