The present study aims at an optimal design of a model water treatment plant comprising of a Rapid mix unit, Clariflocculator and Rapid sand filter. The suspended solids are considered as the basic impurity whose volume is assumed to be reduced to 2 mg/lit in filtered water for different raw water qualities. Various parameters such as velocity gradient, detention time, surface overflow rate, filtration rate, diameter of sand grain, depth of sand bed and time of filter run are considered as design variables for the optimization process. A set of mathematical formulae comprising of the above design variables is used to arrive at optimal values. Algorithms developed based on dynamic programming are used to arrive at the stated optimal design variables. It can be observed from the study that the optimal design differs from conventional design mainly with reference to the two design variables, i.e., surface overflow rate in the clariflocculator and filtration rate of filtration unit. A comparison of the existing treatment plant of capacity 15 MGD locally with that of the optimal design of same capacity plant using the optimal design variables is presented.