An apparatus for at least mixing, advantageously for at least mixing and aerating, a liquid within a liquid body, said apparatus comprising : - A supporting element ; - A top-mounted power drive mounted on said supporting element , said power drive having a rotatable shaft extending downwardly toward the surface of said liquid body; - An impeller mounted on said shaft for rotation therewith; - Possibly, but advantageously, at least one deflecting means selected from a static deflecting plate attached to the supporting element , and a deflecting disc associated to the impeller or to the shaft ; - An at least partly conical housing or volute with an upper portion ending with an upper free edge, and with a lower portion ending with a lower free edge, said housing being attached to the supporting element or to a part attached to the said supporting element , said at least partly conical housing defining an open channel between an upper opening and a lower opening, said housing having substantially a central axis and a substantially circular cross section perpendicular to said central axis adjacent to the lower opening which is greater than a circular cross section adjacent to the upper opening; Whereby the impeller has at least a lower portion extending at least partly within the housing or volute and, advantageously when the impeller is of a screw centrifugal impeller type, an upper portion extending at least partly outside the housing , said impeller being adapted so that the rotation thereof is able to generate a pumping of liquid within the housing through its lower opening, and to expel said pumped liquid through the upper opening of the housing , before being at least partly deflected by contacting the deflecting plate and/or the deflecting disc , and Whereby the upper portion of the housing or volute has a corrugated free upper edge defining peaks and valleys, two successive peaks being separated by a valley, while the distance measured parallel to the axis of the housing or volute between the top of a peak and the bottom of a valley is at least 2 cm or equal to 2 cm, advantageously from 5 to 50cm, preferably from 10 to 30cm. - A supporting element ; - A top-mounted power drive mounted on said supporting element , said power drive having a rotatable shaft extending downwardly toward the surface of said liquid body; - An impeller mounted on said shaft for rotation therewith; - Possibly, but advantageously, at least one deflecting means selected from a static deflecting plate attached to the supporting element , and a deflecting disc associated to the impeller or to the shaft ; - An at least partly conical housing or volute with an upper portion ending with an upper free edge, and with a lower portion ending with a lower free edge, said housing being attached to the supporting element or to a part attached to the said supporting element , said at least partly conical housing defining an open channel between an upper opening and a lower opening, said housing having substantially a central axis and a substantially circular cross section perpendicular to said central axis adjacent to the lower opening which is greater than a circular cross section adjacent to the upper opening; <EOS>