with the volume of each bubble \(V_{Bubble,\ \ i}\) and the volume of one impeller compartment \(V_{\text{Compartment}}\) (Figure 1). To distinguish between the ratio of the actual gas passing the impeller to the gas circulating in the compartment and bypassing the impeller, the gas hold-up in the volume of each impeller was calculated. Eq.2 was applied replacing\(\ V_{\text{Compartment}}\) by\(V_{\text{Impeller}}\), the volume of the cylinder around each impeller, with the dimensions of the impeller diameter and the impeller blades height (Figure 1). To examine the local gas load of each impeller, the effective aeration rate was introduced. This rate is equal to the radial gas flow leaving the impeller region as dispersed gas. The flow of gas dispersed by each impeller was estimated by multiplying the velocities of the bubbles in the lateral surface of the impellers cylinder with the void fraction of the respective cells. Thus, this key figure reflects the axial aeration at the impeller by the gas bubbles recirculating from top and below of the impeller, and the radial gas dispersion process.