Abstract: An aerator for the biological purification of waste water comprises an undulating annular disk of sheet material, centered on a vertical axis of rotation, forming interleaved sets of upper and lower sectors interconnected by generally vertical, radially extending webs. Each lower disk sector defines with the adjacent webs an upwardly open channel whereas each upper disk sector defines with its associated webs a downwardly open channel. A central sleeve, descending from the level of the alternately upwardly and downwardly open channels, obstructs the inner ends of the latter but opens into the inner ends of the former whose effective cross-sectional area is adjustable with the aid of a vertically shiftable shutter ring inside the sleeve.