The systems approach is employed in the present work for synthesizing an optimal mechanical aeration system for a stream receiving multiple thermal and organic waste discharges. It is assumed that a modified version of the well-known Stroolor-Pholps model is adequate for describing the stream water quality as characterized by the variations of the temperature, biochemical oxygen demand (BOD) and dissolved oxygen concentration. According to the illustrated example, it is found that the total system cost is very critical to the BOD constraint imposed at the end of the reach of stream under consideration. It is also observed that the effect of the waste discharge structure on the total system cost is very significant.