A two-phase (liquid-solid) computational fluid dynamics (CFD) model for simulating the flow field and sludge settling in a full-scale Carrousel oxidation ditch (OD) has been proposed. The Takacs double exponential sedimentation velocity function was applied to simulate the two-phase flow. The flow field simulation results are comparable with the data obtained from the field. Compared to the single-phase simulation, the relative error between the simulation results and field data in this two-phase model is reduced from 8% to 5%. Based on the simulation results of the flow field and sludge settling by using this two-phase CFD model, an optimized operation scheme of the OD was proposed. Compared with the existing one, the volume fraction of solid phase at the bottom of the OD in the optimized operation scheme decreases from 0.260 to 0.258, which seems to be insignificant, but the distribution of sludge becomes more uniform. (C) 2014 Elsevier Ltd. All rights reserved