Process for treating 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 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 being made after verifying a first set of conditions C1, switching from the second mode P2 to the third mode P3 being made after verifying a second set of conditions C2 and switching from the third mode P3 to the mode P1 being made after verifying a third set of conditions C3, the process being characterised in that the first mode "reagent-free" P1 comprises: a) passing raw water to be treated in a so-called coagulation zone in which no coagulant is present, nor injected and then b) passing water from said coagulation zone into a so-called flocculation zone in which no flocculation agent is present, nor injected and then c) passing water from said flocculation zone into a so-called settling zone, and then d) potentially a step of recirculating the plain settling sludge from said settling zone to said flocculation zone by an external circuit, said flocculation zone being located upstream of said settling zone, and in that the second mode "of transition" P2 that allows at the beginning of the occurrence of high flow rates, the conditioning of recirculated plain settling sludge with addition of reagents, comprises: a) either a step of recirculating the plain settling sludge from said settling zone to said coagulation zone by an external circuit, said coagulation zone being located upstream of said settling zone and said coagulation zone receiving a coagulant b) or a step of recirculating the plain settling sludge from the settling zone to the raw water inlet point located upstream of the measurement of turbidity or suspended matter of said raw water by an external circuit, said raw water inlet being located upstream of the coagulation zone receiving coagulant c) or a step of recirculating the plain settling sludge from said settling zone to said flocculation zone by an external circuit, said circuit being provided with a coagulant injection system into said circuit, said flocculation zone being located upstream of said settling zone, and in that the third mode "with reagent" P3 comprises: a) passing raw water to be treated into a so-called coagulation zone in which a coagulation agent is present; and then b) passing water from said coagulation zone into a so-called flocculation zone in which a flocculation agent is present and then c) passing water from said flocculation zone into a so-called settling zone, and then d) potentially a step of recirculating the settling sludge conditioned during the second mode P2 from said settling zone to said flocculation zone by an external circuit, said flocculation zone being located upstream of said settling zone. a) passing raw water to be treated in a so-called coagulation zone in which no coagulant is present, nor injected and then b) passing water from said coagulation zone into a so-called flocculation zone in which no flocculation agent is present, nor injected and then c) passing water from said flocculation zone into a so-called settling zone, and then d) potentially a step of recirculating the plain settling sludge from said settling zone to said flocculation zone by an external circuit, said flocculation zone being located upstream of said settling zone, a) either a step of recirculating the plain settling sludge from said settling zone to said coagulation zone by an external circuit, said coagulation zone being located upstream of said settling zone and said coagulation zone receiving a coagulant b) or a step of recirculating the plain settling sludge from the settling zone to the raw water inlet point located upstream of the measurement of turbidity or suspended matter of said raw water by an external circuit, said raw water inlet being located upstream of the coagulation zone receiving coagulant c) or a step of recirculating the plain settling sludge from said settling zone to said flocculation zone by an external circuit, said circuit being provided with a coagulant injection system into said circuit, said flocculation zone being located upstream of said settling zone, a) passing raw water to be treated into a so-called coagulation zone in which a coagulation agent is present; and then b) passing water from said coagulation zone into a so-called flocculation zone in which a flocculation agent is present and then c) passing water from said flocculation zone into a so-called settling zone, and then d) potentially a step of recirculating the settling sludge conditioned during the second mode P2 from said settling zone to said flocculation zone by an external circuit, said flocculation zone being located upstream of said settling zone.