PURPOSE:To appropriately control the injection of ozone in accordance with the change in water quality, control a water purifying plant at a lower running cost and obtain high-quality water by using this device. CONSTITUTION:A TV camera is arranged at plural positions vertical to the water surface of an ozone contact basin to take the image of ozone gas bubble. The diameter and volume of the bubble are measured in a real time by image processing in a bubble diameter measuring means 21, and the ozone absorption amt. of the water to be treated is obtained from the decrease in the actual volume of the ascending bubble based on the bubble volume in the respective positions with an ozone absorption amt. calculating means 22. The injection of ozone from an ozonizer is controlled based on the absorption amt. so that the amt. of remaining ozone is decreased to 0. Meanwhile, the optimum bubble diameter is controlled in a real time with an objective bubble diameter estimating means 25 in accordance with the water quality, etc., so that the ozone absorption efficiency is maximized, and the actual bubble diameter is controlled by adjusting the gas amt. (pressure) in a diffuser pipe by a blower control command output means 27 to the optimum bubble diameter as an object.
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