NOVELTY - The bioreactor comprises two flanges (2), a plane zone located between an effluents input (3) and the flange nearest to an effluents output (4), strengthening ribs, an U-shaped chute located in a bottom of a septic water tank (1) on a median longitudinal axis of the tank, a separating semi-partition (14), an air manifold, and a carrying framework for supporting a bacterial bed. The tank is made of high density polyethylene and parallelepipedic. The septic tank comprises a removable pre-filter containing a removable net filter located under the flange nearest to the effluents output.    USE - The submerged fixed culture aerobic bioreactor for domestic waste processing in addition to an anaerobic pretreatment upstream and a secondary decantation downstream on a septic water tank (claimed).    ADVANTAGE - The bioreactor allows easy, simple and economic transformation of septic tank water into the bioreactor.    DETAILED DESCRIPTION - The bioreactor comprises two flanges (2), a plane zone located between an effluents input (3) and the flange nearest to an effluents output (4), strengthening ribs, an U-shaped chute located in a bottom of a septic water tank (1) on a median longitudinal axis of the tank, a separating semi-partition (14), an air manifold, and a carrying framework for supporting a bacterial bed. The tank is made of high density polyethylene and parallelepipedic. The septic tank comprises a removable pre-filter containing a removable net filter located under the flange nearest to the effluents output. The carrying framework is horizontally positioned in a middle of the septic tank and rests on the semi-partition, vertically and horizontally on a side of the strengthening ribs and the horizontal strengthening ribs on a front of the tank. The air manifold is a hollow, tubular structure and L-shaped, and includes an ventilation input and a ventilation path prolonged by a perpendicular section comprising a diffuser of fine air bubbles. The ventilation input forms an external part and fixed to the air manifold, and is inserted and placed in a small circular opening. The ventilation path and the perpendicular section comprise an holding element representing an interior removable part of the air manifold. The ventilation path is dimensioned so that the perpendicular section rests in the U-shaped chute. The perpendicular section is dimensioned so that a length of the perpendicular section is lower or equal to 50-55% of a width of the septic tank. The carrying framework comprises four spacers fixed between two lateral sides of the carrying framework and parallel to two frontal sides of the carrying framework. The spacers are laid out in the plane zone of the tank and between the flanges. An interval between the two successive spacers is greater than or equal to 25 cm. The bacterial bed comprises small blocks of variable sizes superimposed and/or juxtaposed to form a linked block, and is fixed to the spacers on the carrying framework using a iron strip and strapping loops made of rot-resistant plastic to slide the bacterial bed back and forth in a horizontal plane of the carrying framework within limits imposed by the spacers. A clamp made of rot-resistant plastic is inserted between a top of the air manifold and a top of the bacterial bed to prevent the bacterial bed from sliding along the carrying framework. The carrying framework supporting the bacterial bed is assembled from pipes with diameter of 50-32 mm and 90 degrees female/female elbows with a diameter of 50 mm. The blocks are in a form of polyethylene mesh having a large number of cavities and tubes welded together to form cubic blocks. The carrying framework has a flowing region of 70%, a void of 90% and a developing surface of 100 m2 per m3. The air manifold is assembled from pipes with a diameter of 50 mm, a splined end with a diameter of 20 mm, a reducer of type 50 mm/20 mm, two 90 degrees female/female elbows with a diameter of 50 mm, a union with a diameter of 50 mm, a plug with a diameter of 50 mm and the diffuser of air bubbles with a clamp saddle load with a diameter of 50 mm. The diffuser is a standard disc. An INDEPENDENT CLAIM is included for a process for reversible transformation of a septic tank water into an aerobic bioreactor.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic longitudinal sectional view of an aerobic bioreactor.    Septic water tank (1)    Flanges (2)    Effluents input (3)    Effluents output (4)    Separating semi-partition. (14)