Patent ID: 8480907

Claim:
A method for the clarification of water difficult to treat by flotation, in particular polluted surface water, or municipal or industrial wastewater, rainwater and all types of water difficult to treat by flotation, in particular having a high percentage of mineral matter, or for the clarification of all types of water requiring the injection of a flocculation aid and preceding filters or membrane technologies such as ultrafiltration, comprising: a coagulation step (A 1 ) during which a dose of 10% to 70% by weight of the total dose of a mineral coagulant ( 1 ) is injected into the water to be treated, which has a turbidity of greater than 30 NTU and a concentration of suspended matter SM of greater than 30 mg/l, and the coagulant comprising trivalent cations, especially iron or aluminum salts; a flocculation step (B) during which a flocculant ( 2 ) is injected (B 1 ) into the water after the coagulation step, in order to agglomerate the suspended particles in the form of flocs, wherein the flocculation step comprises: injection of the flocculant into a zone (B 1 ) for mixing the flocculant ( 2 ), followed by a second injection of the same mineral coagulant, having a dose of 30% to 90% by weight of the total dose of the mineral coagulant, said injection being carried out downstream of the flocculant injection in a zone (A 2 ) for mixing, and coarsening of the flocs ( 3 ), suitable for flotation in a single step, in a zone (B 2 ) following the zone (A 2 ) for mixing; and then a flotation step (C) during which, in a flotation apparatus, the flocculated water is mixed with an emulsion of gas microbubbles, generally air microbubbles, which become attached to the flocs and make them rise to the surface, where they are collected and removed, while the clarified water is removed via the bottom of the flotation apparatus.