The invention relates to a method for treating fluids, in particular waste waters involving coagulation/flocculation, clarification by filtration or flotation stages and a stage of filtration through micro-, ultra, and nanomembranes or of hyperfiltration. The inventive method consists in injecting two times several coagulating reagents in a quantity ranging from 75 to 125% of an optimal coagulation dose or a dose cancelling a Zeta potential (pz) in an area (area n.degree. 1) situated upstream of the clarification stage (2) and from 0.1 to 25% of an optimal dose cancelling the pz in a second area (area n.degree. 2) situated upstream of the stage of membrane filtration (4).