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;