PURPOSE: To remove an obstacle due to algae and to keep safety in working environment maintaining constant brightness even if flocculating and settling is performed without the use of chlorine by installing a green transparent or translucent window or skylight using glass or plastic in a building where a flocculating and settling tank is installed.

CONSTITUTION: Raw water is introduced from a conduit tube 1 and a flocculant, a pH conditioner, etc., are added from a chemical tank 2. The mixture is agitating in a rapid agitating tank 4 and then agitated in a slow agitated tank 6 to flocculate turbid components in the raw water as floc which is settled and separated in a settling tank 7 as sludge 8. The whole settling tank 7 is installed within a building 10 and sunlight is taken in through a window 11 from the outside. The window 11 consists of green transparent or translucent glass or plastic and shuts out the light of complementary color of green which is necessary for the growth of plankton in the sunlight.