PURPOSE: To provide an apparatus and a method for treating drainage in which the concentration of solid components is relatively low, a large amount of drainage is treated efficiently with compact equipment, and troubles including blocking in a piping system hardly occur.

CONSTITUTION: Drainage to be treated is received in a drainage vessel 1 for the sedimentation of solid components. The supernatant in the drainage vessel 1 is discharged by a supernatant draw line 5 to concentrate the drainage in the drainage vessel 1. The concentrated drainage in the drainage vessel 1 is agitated by the air discharged from a bubbling pipe 2 and is led into a separation-dewatering machine 8 through a concentration line 7. The filtrate, after the solid component being separated by the separation-dewatering machine, is returned to the drainage vessel 1 through a reflux line 9.