Wastewater treatment method comprising at least one activated sludge treatment step upstream in an aeration tank and at least one settling step downstream in a rapid static decanter without the use of a polymer, the settling being used to pre-thicken a fraction of sludge resulting directly or indirectly from the activated sludge treatment step, before its extraction in the direction of a dewatering treatment unit, the rapid static decanter comprising: - a reactor, - means (14a, 14b) for inlet the sludge fraction at the reactor inlet, - means (15, 16, 30) for regulating the volume flow Q e at the reactor inlet, - means (27a, 27b) for extracting the fraction of sludge at the reactor outlet, - means (28, 29, 30) for regulating the volume flow Q s at the reactor outlet, - the value of the height H and the value of the cross-sectional area S of the reactor are respectively framed as follows: 7 m2 ≤ S ≤ 28 m2 and 2 m ≤ H ≤ 5 m. - the bottom of the decanter being devoid of any means of agitation, - the bottom of the decanter following an inclination with respect to a plane orthogonal to the vertical direction, between 0 and 5%, - the reactor comprising at its upper part a channel allowing the overflow to be evacuated, - the reactor is equipped at its upper part with a bottomless drum called a "tranquilizer" 25 allowing the admission of effluents without disturbing the settling. characterized in that in the decantation stage, in a steady state: - the value of the volume flow Q s of the fraction of sludge leaving the reactor depends on: • a set value CMs0 of the mass load at the reactor outlet, • and the concentration Cs of suspended solids in the sludge measured at the outlet of the reactor, so that: Q s = CMs0 / Cs. - the value of the volume flow rate Q e of the fraction of sludge at the reactor inlet depends on: • a set value CMe0 of the mass load at the reactor inlet, • and the concentration Ce of suspended solids in the sludge measured at the reactor inlet, so that: Q e = CMe0 / Ce, with: - the set value CMs0 of the mass load at the outlet of the reactor being the nominal mass load CM5 specific to the dewatering treatment unit, - the set value CMe0 of the mass charge at the reactor inlet being such that: 1.1 CMs0 ≤ CMe0. - a reactor, - means (14a, 14b) for inlet the sludge fraction at the reactor inlet, - means (15, 16, 30) for regulating the volume flow Q e at the reactor inlet, - means (27a, 27b) for extracting the fraction of sludge at the reactor outlet, - means (28, 29, 30) for regulating the volume flow Q s at the reactor outlet, - the value of the height H and the value of the cross-sectional area S of the reactor are respectively framed as follows: 7 m2 ≤ S ≤ 28 m2 and 2 m ≤ H ≤ 5 m. - the bottom of the decanter being devoid of any means of agitation, - the bottom of the decanter following an inclination with respect to a plane orthogonal to the vertical direction, between 0 and 5%, - the reactor comprising at its upper part a channel allowing the overflow to be evacuated, - the reactor is equipped at its upper part with a bottomless drum called a "tranquilizer" 25 allowing the admission of effluents without disturbing the settling. characterized in that in the decantation stage, in a steady state: - the value of the volume flow Q s of the fraction of sludge leaving the reactor depends on: • a set value CMs0 of the mass load at the reactor outlet, • and the concentration Cs of suspended solids in the sludge measured at the outlet of the reactor, so that: Q s = CMs0 / Cs. - the value of the volume flow rate Q e of the fraction of sludge at the reactor inlet depends on: • a set value CMe0 of the mass load at the reactor inlet, • and the concentration Ce of suspended solids in the sludge measured at the reactor inlet, so that: Q e = CMe0 / Ce, - the value of the volume flow Q s of the fraction of sludge leaving the reactor depends on: • a set value CMs0 of the mass load at the reactor outlet, • and the concentration Cs of suspended solids in the sludge measured at the outlet of the reactor, so that: Q s = CMs0 / Cs. • a set value CMs0 of the mass load at the reactor outlet, • and the concentration Cs of suspended solids in the sludge measured at the outlet of the reactor, so that: Q s = CMs0 / Cs. - the value of the volume flow rate Q e of the fraction of sludge at the reactor inlet depends on: • a set value CMe0 of the mass load at the reactor inlet, • and the concentration Ce of suspended solids in the sludge measured at the reactor inlet, so that: Q e = CMe0 / Ce, • a set value CMe0 of the mass load at the reactor inlet, • and the concentration Ce of suspended solids in the sludge measured at the reactor inlet, so that: Q e = CMe0 / Ce, with: - the set value CMs0 of the mass load at the outlet of the reactor being the nominal mass load CM5 specific to the dewatering treatment unit, - the set value CMe0 of the mass charge at the reactor inlet being such that: 1.1 CMs0 ≤ CMe0. - the set value CMs0 of the mass load at the outlet of the reactor being the nominal mass load CM5 specific to the dewatering treatment unit, - the set value CMe0 of the mass charge at the reactor inlet being such that: 1.1 CMs0 ≤ CMe0.