Abstract: An apparatus for degrading the organic fraction of sewage by means of active biomass, in particular active sludge particles, comprising: —at least one tank (1) adapted to contain the sewage and said active biomass; —at least one hollow structure (6, 106, 206), adapted to be at least partially immersed in the sewage, provided with at least one first opening (61) for letting in the sewage and with at least one second opening (62) for letting out the sewage, wherein the ratio between the area of the at least one first opening (61) and the area of the at least one second opening (62) is equal to at least 5:1; —air delivery means (7, 70) adapted to introduce air inside said at least one structure (6, 106, 206); wherein said at least one first opening (61) is proximal to said air delivery means (7, 70) and said at least one second opening (62) is distal from said air delivery means (7, 70), so that the air delivery means (7, 70) are adapted to generate a flow of sewage from said at least one first opening (61) to s