The safety of water quality at several water treatment plant(WTP) in Gwang-ju was investigated when the backwashing water was reused. Concentrations of turbidity, E. coli, NH₃-N, metals in recycled water of Y-WTP were generally low under the standard limit of drinking water. However, turbidity and color of the backwashing water at N-WTP were so high that affected to the water treatment process as coagulation/sedimentation and also increased the amount of sludge in sedimentation basin. When the ratio of recycled water to raw water was increased, much alum was needed and residual turbidity in effluent of sedimentation was also increased. 15% NaCl solution was used to chase the stream characteristics of processes at N-WTP. A drift of one side was occurred because the flow rate of streams and the amount of effluents were different at two sedimentation basins.