METHOD AND APPARATUS FOR PURIFYING WATER

PROBLEM TO BE SOLVED: To improve the flocculation properties of alga by a method in which in the purification water including a process in which a flocculant is injected into raw water to flocculate suspended substances, before the flocculant being injected, the raw water is pre-treated so the fluorescent color of contained alga is changed from red into orange or yellow. SOLUTION: Raw water 21 from a river, a lake, a marsh, or the like, after coarse impurities being removed in a precipitation sand basin, is distributed into a water purification apparatuses. It is introduced into an inactivation tank 1 equipped with an inactivation apparatus 5 of an ultraviolet type, an ozone injection type, or the like, and the apparatus 5 is operated properly by an inactivation controller 50 so that the destruction of alga is prevented. Next, in a mixing basin 2A, a flocculant in a flocculant tank 7 is injected from flocculant injecting equipment 6 and diffused/mixed in an agitation apparatus 8A, and minute flocks are formed by the flocculant. In an agitation basin 2B, an agitation apparatus is operated slowly, and the minute flocks in a mixed liquid are flocculated further into flocks several mm in size.