This paper describes an interactive microcomputer program which was developed for the use in the synthesis of wastewater treatment systems. With on screen instructions and menus, the user can select several techniques to generate different wastewater treatment system design alternative for evaluation. These various design alternatives provide useful information for the decision-maker, and hence more understanding and insight of the design problem can be obtained and the best system design can be located. The feature of interactive data input coupled with screen instruction of this program also ensure that the correct information and data are given to the optimization model to reduce the misuse of the model. Because of the interactive nature of this program, it can be utilized by users who have little experience in computer programming. Thus, the use of optimization models in the preliminary design of wastewater treatment systems can be greatly enhanced.