A mechanical aerator for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, the impeller comprising: a shaft rotatable about a substantially vertical axis; and a plurality of backward curved vanes extending outwardly from the shaft, the vanes cuved such that as the impeller rotates about a substantially vertical axis a series of thin film waves emanate spirally outward and upward from the vanes, such that each particle of liquid contained therein moves radially outward from the axis of rotation. The vanes have a portion of increased depth at the area adjacent the shaft so as to gouge a hole of increased depth in the liquid and thereby facilitate rotation of the impeller. The vanes are curved such that as the impeller rotates the liquid is conveyed outward to an area of higher angular velocity.
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