An ecology-based competition model was developed to evaluate the impact of solid retention time (SRT) on aerobic heterotrophic bacterial species diversity in activated sludge systems. Model simulations were conducted using different configurations of activated sludge system including continuous stirred tank reactors (CSTRs) in series and selector activated sludge (SAS) system. The results of model simulations showed that the outcome of diversity in activated sludge systems is a function of both SRT and reactor configuration where biodiversity was higher in bioreactors behaving as a completely mixed activated sludge (CMAS) system compared to bioreactors behaving as a conventional activated sludge (CAS) system. The significance of these findings is that the model could be used as a design tool to predict which engineering control strategies (i.e., SRT and reactor configuration) to apply for enhancing species diversity in activated sludge systems.