A purification method by mixing and diffusion of a closed water area comprises the steps of sucking bottom layer water and surface layer water, preparing a mixed water by mixing them before discharging, and discharging the mixed water into a water layer having substantially the same temperature as that of the mixed water to form and diffuse a density current of the mixed water. Therefore, convection currents in a vertical direction can be prevented, and the mixed water can be quickly diffused across a broad range of the closed water zone.
展开▼