A comprehensive mathematical model for describing flow, turbulence and gas hold-up distribution in a bubble column reactor is developed. For large diameter bubble columns (> 0.1 m), operated with the zero or low liquid throughput, sparged gas tends to pass preferentially through the central portion of the column. This non-uniform gas hold-up distribution leads to macroscale circulation and turbulence in the reactor. Two new mechanisms are proposed and modelled to explain such a non-uniform gas hold-up distribution in bubble columns. The influence of accompanying wakes and of the column wall on the motion of gas bubbles is accounted for for the first time. Turbulent, dispersed gas-liquid flow is described by the time averaged two phase momentum equations. The turbulent stresses are simulated using a k-epsilon model. Extra terms arising from the gas hold-up fluctuations and pressure gradients are included in the model. The predicted flow characteristics of the bubble column reactor are verified by comparison with the published experimental data over a wide range.