PURPOSE: To efficiently and jointly treat night soil and general waste water by inserting an algae fixing body into an aeration chamber, releasing the algae fixed to the body by the circulation of sewage, and exposing the sewage mixed with algae to light in an catalytic oxidation chamber to oxidize the sewage.

CONSTITUTION: Sewage flows into first and second settling and separating tanks 3 and 4, and the suspended matter is separated. The sewage is then introduced into a first contact aeration tank 7, and brought into contact with the algae on the algae fixing body 5 inserted into the tank 7 by the circulation due to aeration to release a part of the algae. The sewage mixed with released algae is transferred to a second contact aeration tank 8, and the algae are released in the same way. Sunlight is introduced into the aeration tanks 7 and 8 from a window 16, and the algae continue to grow. The sewage mixed with the algae is transferred to a catalytic oxidation and sedimentation tank 10, aerated, and circulated by the aeration tank formed therein to release the algae from the algae fixing body 15 in the sedimentation tank 10. Meanwhile, since the mixed sewage is always exposed to sunlight, the algae continue to grow, and the phosphoric acid and nitrogen in the sewage are absorbed. As water is simultaneously purified by the oxygen discharged from the algae, sewage and general waste water are jointly treated.