The invention concerns the use of a coagulation-flocculation-decantation device for water treatment to degrease water inside the device itself so as to eliminate therefrom more than 20% of hexane-extractable matter without resorting to separate degreasing means by flotation. The invention also concerns a plant comprising a coagulation-flocculation-decantation device by ballasted flocs having a coagulation zone (1), means supplying (4) a granular material denser than water (4), a flocculation (6), a decantation zone (8) and means for recycling (15) granular material including at least a hydrocyclone (17). The characterised in that it does nut comprise degreasing means provided upstream of the coagulation-flocculation-decantation device, the grease being removed from the water inside the coagulation-flocculation-decantation device itself without means for injecting air or an equivalent gas.