PURPOSE: To provide a waste water treating apparatus, in which an aeration tower equipped at its lower portion with an aerating tube and a fluidized bed tower filled with filler material having a specific gravity smaller than that of water are communicated together at their upper portions and lower portions, and waste water is circulated in the apparatus by the function of buoyancy of bubbles ejected from said aerating tube, whereby it is enabled to reduce both of initial installation costs and running costs.

CONSTITUTION: Waste water supplied into aeration tower 1 from inlet pipe 9 is made to flow upward in the aeration tower 1 by the function of buoyancy of air ejected from aerating means 3, and passing through communicating tube 6, fluidized bed tower 2 and communicating tube 7, is recirculated to the aeration tower 1. In the fluidized bed tower 2, a straightening vane 4 is provided at its upper portion, and microbe bearing medium consisting of fine particles having a specific gravity samller than that of waste water is filled downwardly of said straightening vane 4. With such an arrangement, waste water is treated biologically by circulating and bringing the same into contact with said microbe bearing medium. After treatment, treated water is discharged to the outside from outlet pipe 10.