Biological reactions within the sewer play an important role in establishing the water quality characteristics of combined sewer overflow and of the sewage entering a wastewater treatment plant. A simulation model that shows the effects of reactions in the collection system on wastewater treatment plant performance has been developed. A series of numerical experiments was conducted to illustrate the features of the model, and the potential benefits of the approach. The study also serves to identify areas in which the models need to be improved.