This paper describes the development of a mathematical model and its application for predicting the dimensions of an industrial wastewater pretreatment system, and its comparison with alternative solutions. The model is based on pseudo first order kinetics for both the municipal and the industrial wastewater, and also on the Monod kinetics for the industrial wastewater. In the combined treatment process the biomass is taken as a mixture of specialist microorganisms capable of degrading target compounds. The kinetic rate of the combined stream is taken as the weighted average of the reciprocal K values. The model was used to compare the alternative treatment schemes and the main conclusions were: (1) pretreatment of industrial wastewater is always the most efficient alternative, and (2) an optimal VSS concentration exists in the pretreated effluent, leading to the most efficient treatment system. Experimental studies aimed at validation of this model are strongly recommended.