Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: "small" and "large" bubbles. For the air-water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag-, coefficients, which differ for the "small" and "large" bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k-epsilon model. For a 0.1 m diameter column operating with the air-water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006-0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.