An improving example for traditional rapid mix system was studied in the base of mechanisms of alum coagulation. Local status of the major water treatment plants was also investigated and evaluated for upgrading these plants. A new design and operating criterium for rapid mix system was proposed to velocity gradient, G of 1,000-1,500 sec^-1 and detention time, t of 1 sec from the results of experiments and literature reviews. Comparing the present rapid mix system to this criterium, apparent difference existed between them. In this study, for improving Seongnam water treatment plant, a design criterium of velocity gradient, and detention time was set to 1,100 sec^-1, 1 sec, respectively. A new rapid mix system adopted the nozzle injection countcurrently cross the inlet pipe to the whole area. The injection velocity was 17m/s, nozzle diameter was 1.0mm, and number of nozzle was 70. The new modified system without running present four 75 HP agitators was able to improve water quality(based on sedimentation effluent) by 15-35% and to reduce electrical energy by 98%.