A mechanistic model for activated sludge sewage treatment was developed to predict exploitative competition of six aerobic heterotrophic bacterial species competing for three complementary growth limiting substrates using the noninteractive Monod equation. The central hypothesis of the model is that in a multispecies/substrate system the number of coexisting bacterial species, N, exceeds the number of limiting resources, K, available for them. The model was used to investigate the effect of bioreactor conditions on the diversity of bacterial species. Preliminary results of model simulations showed that for a certain range of solids retention times (2.28-5.66 days) the competition of six bacterial species for three growth limiting substrates produces oscillations within the structure of the bacterial community allowing for the sustained growth of more than three species on the three substrates.