Unsteady flows resulting from time-dependent inflow and variable aerator speed in a full-scale oxidation ditch and implications on residence time are explored via computational fluid dynamics (CFD). Flow in the ditch is driven (forced) by one inflow and two surface aerators. It is found that varying aerator speed is the primary factor inducing unsteady flow dynamics in the ditch while the time-dependent inflow induces negligible unsteady effects. In order to avoid expensive unsteady flow calculations, residence time analysis is performed for several steady flow simulations representative of the various stages during aerator operation. Significant differences between the mean residence time computed in the cases simulated suggests that unsteady flow dynamics throughout the operational cycle may have a significant impact on residence time characteristics in the ditch. Traditionally, residence time analysis is conducted based on a single steady flow representative of the average flow throughout the operation cycle of the system. Instead, following the results obtained here, it is recommended that residence time analysis be conducted under unsteady flow conditions or be based on a series of steady flow simulations each representative of the various operational stages of the system, in order to obtain a more realistic prediction of the hydraulic efficiency.