Abstract: Disclosed is a process for the treatment of urban or industrial wastewater, in particular a process for the primary treatment of water, said process comprising a first operating mode P1 of the treatment system called a “reagent-free mode”, a second operating mode P2 of transition from the first reagent-free mode P1 to a third mode P3, said third operating mode P3 of the treatment system being called a “mode with reagents”, switching from the first mode P1 to the second mode P2, from the second mode P2 to the third mode P3 and from the third mode P3 to the mode P1 being respectively made after verifying a set of conditions C1, C2 and C3.