PURPOSE: To aerate and stir waste liquid and, as required, draw the liquid up and mix the same a single self-priming aerator.

CONSTITUTION: A suction pipe 8 is connected directly to a rotatably movable hollow shaft 2 provided with a screw 3 at its lower end or indirectly thereto via a hollow motor shaft. The suction pipe 8 is provided with a valve 9 for adjusting flow amount, its lower end is immersed in a raw liquid tank 11 and the screw is placed under water in a treating tank 10. By making use of a negative pressure produced in the water area by the rotation of the screw, an air suction valve of the suction pipe provided on the upper end of the hollow shaft is opened to draw air therein and the air is discharged into the water area to aerate the liquid separately. During an aerobic operation, the air suction valve is closed and raw liquid is drawn up by the pump operation due to a negative pressure through the suction pipe, discharged into the water area and mixed and stirred therein. In this way aerobic and anaerobic operations can be conducted selectively and the raw liquid can be supplied as required.