Abstract: Sequential biological reactor (R), comprising: a tank (1) delimited by a base and peripheral walls (3); an inlet (11) equipped with a selector (S) that determines a baffled route with contacting of the effluent to be treated and a portion (5) of the recirculated sludges; a means (6) for discharging the treated water in the top part of the water surface of the tank, and an outlet (7) for discharging the excess sludges in the bottom of the tank. The selector (S) comprises a vessel (8) placed in the tank (1) at a distance from the peripheral walls (3); the vessel is compartmentalized into several sectors (8.1, 8.2, 8.3, 8.