In this paper, the unit processes in the typical water treatment plant, which need to be expanded because the water demand is over the existing water treatment capacity in the near future, were carefully examined to upgrade the water treatment plant. The models were installed in the fields as a distorted model based upon the hydraulic similitudes. The models having the constant discharge ratio in the unit processes between the model and the prototype were installed as two units to compare the treatment efficiencies. The capacity of the individual unit, which is a model of the prototype of 250,000m^3/day capacity, was 24m^3/day. In the mixing and flocculation experiments, the mixing intensity of flocculators G was selected as the main experimental item. The optimal mixing intensities G, which are 65/sec for experimental discharge of 1m^3/hr and 85/sec for experimental discharge of 1.3m^3/hr, are identified based upon the comparison the relative turbidity removal efficiencies. Also, the outlet weir loading was selected as the main experimental item in the sedimentation process. Through the continuous experiments with the main experimental item of the mixing intensity of flocculators G and the outlet loading of the weir in the sedimentation basin, about 20% upgrading compared to the existing water treatment capacity was obtained.