Research was undertaken in order to simulate and optimize an Aquatair Model P-3 package sewage treatment plant for which operating data from the National Sanitation Foundation was available. The treatment plant consists essentially of a plastic medium trickling filter mounted overtop of a complete-mix activated sludge (CMAS) tank; the unit also has a primary sludge holding tank and a secondary clarifier. From consideration of reaction kinetics, reactor type, and geometry, a mechanistic model was developed to explain the phenomena of substrate removal in the Aquatair Model P-3. The two-factor SIMPLEX optimization procedure was used to determine the best set of biological reaction constants K1 and K2 which would characterize the substrate removal behavior of the Aquatair biological oxidation tower and recirculation chamber, respectively, based on actual plant data. Once the behavior of the plant was accurately simulated, cost functions were developed in order to relate the various design variables to cost. An n-factor PERPLEX optimization routine was then used to optimize the various plant variables for a given flow, organic load, and efficiency. PERPLEX, a modification of the Box COMPLEX method, was shown to be a useful tool for evaluating the factor space defined by a set of mathematical assumptions. The cost benefit of aerobic digestion occurring in the Aquatair bio-oxidation tower was predicted by the model and confirmed by nitrification data and sludge production calculations. / Master of Science