A good understanding of the hydraulic performance of aerated lagoons is required for their design and operation. A comprehensive numerical procedure has been developed for the three-dimensional computational modelling of the flow in large lagoons including high-speed floating mechanical surface aerators. This paper describes the procedure that consists of separate aerator modelling, then applying the obtained results as boundary data for a full lagoon model. A model application to an industrial aerated lagoon serves as an example of flow analysis. Post processing of the results by calculating the local average residence time (age of fluid) provides a powerful and intuitive technique to visualize and analyse the lagoon performance. The model has been verified by comparing the local average residence time predictions with measurements from a dye study. It is shown that the numerical modelling proposed is feasible and constitutes an effective new tool in improving the performance and design of industrial lagoons.