PROBLEM TO BE SOLVED: To control filter bed clogging and filter clogging caused by the deterioration of water quality by controlling the quantity of sunlight properly in a slow filter basin in a slow filtration method for a water purification plant. SOLUTION: In the filtration process of the slow filter basin of a water purification plant by a slow filtration method for a water supply facility, a float is used to control the quantity of sunlight. A color effective for controlling the transmission of sunlight is applied to the float. The color is not restricted in particular, however, green is preferable to prevent algae generated on the back of the float from standing out. A soft material which floats on water and prevents the elution of harmful substances such as a urethane resin is used, and a cloth which floats on water can be used. In certain cases of raw water quality, the removal of nutritive salts by the generation of algae and the supply of oxygen by photosynthesis to remove organic substances are necessary, and since seasonal changes in water quality, the quantity of sunlight, and the quantity of oxygen in unfiltered water are generated, a function capable of controlling the sunlight at 0-100% is desirably provided.
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