PURPOSE: To obtain the titled apparatus capable of inexpensively continuously supplying air to the interior of filthy water without using power, by adopting the constitution that a water wheel provided at a drainage opening in the bottom of a filthy water tank is rotated by the drainage pressure of filthy water to send air into the filthy water tank by the blades of the water wheel.

CONSTITUTION: A water wheel 3 rotatably pivoted to a casing 4 is provided at a drainage opening 2 opened to the bottom of a filthy water tank 1, so that blades 6 are pushed downwards by the drainage pressure of filthy water flowing downwards through a communicating opening 5 leading to the drainage opening 2 to rotate the water wheel 3. The lower surface of the upper part of said casing 4 is formed into a circular arc, the water wheel 3 is rotated while sealing external air in a space defined by the inner surface 7 of said circular arc and the blades 6, and said air is discharged through the drainage opening 2 in the filthy water tank 1 to perform the purification of the filthy water. In addition, the exchange of the filthy water with the air is smoothly performed by a dividing wall 8 provided between a drainage way b and an air-supply passageway c in the drainage opening 2 and the communicating opening 5, to inhibit the energy loss of both of them.