NOVELTY - Ventilation unit (1) comprises: a fan (5) equipped with a first duct or suction duct (7a) and a second duct or delivery duct (7b); a first piping arrangement (15); and a switching assembly that is arranged between the fan and the first piping arrangement. The switching assembly comprises at least one switching device movable between two positions in order to allow alternately the passage of a gas flow from the fan to the first piping arrangement and vice versa. The switching assembly includes a casing in which the suction duct and the delivery duct of the fan converge.    USE - The ventilation unit is useful for a plant for the aerobic treatment of organic waste (claimed).    ADVANTAGE - The ventilation unit: is arranged to effect flow reversal, according to two opposite directions, of a gas flow generated by the fan without reversing the rotation direction of the fan; reduces the head losses to a minimum; is reliable and resistant and has very limited manufacturing and operating costs; and withstands chemical aggression by components that may be present in the contaminated air flow coming from the bio-oxidation area.    DETAILED DESCRIPTION - Ventilation unit (1) comprises: a fan (5) equipped with a first duct or suction duct (7a) and a second duct or delivery duct (7b); a first piping arrangement (15); and a switching assembly that is arranged between the fan and the first piping arrangement. The switching assembly comprises at least one switching device movable between two positions in order to allow alternately the passage of a gas flow from the fan to the first piping arrangement and vice versa. The switching assembly includes a casing in which the suction duct and the delivery duct of the fan converge. The casing has four openings of which a first opening connects the casing to the outer environment, a second and a third opening connect the casing to the first piping arrangement and a fourth opening connects the casing to a second piping arrangement and in that the switching assembly includes two switching devices, one of the switching devices is movable between the two positions in order to selectively connect the suction duct of the fan to the first opening or the second opening and the other one of the switching devices is movable between the two positions in order to selectively connect the delivery duct of the fan to the third opening or the fourth opening. An INDEPENDENT CLAIM is also included for a plant for the aerobic treatment of organic waste, comprising a bio-oxidation area in which the waste to be treated is placed, and at least one ventilation unit, where the first piping arrangements of the ventilation units are connected to the bio-oxidation area and the second piping arrangements of the ventilation units are connected to a bio-filter.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the ventilation unit.    Ventilation unit (1)    Fan (5)    Suction duct (7a)    Delivery duct (7b)    First piping arrangement (15)