PURPOSE: To efficiently treat waste water containing organic substance, by mixing waste water contg. residual anaerobic sludge having passed through an anaerobic tank with waste water contg. sludge, separating the newly formed sludge from the waste water, and then discharging said sludge to the circulating system of disposal.

CONSTITUTION: Waste water containing organic substance is sent through a conditioner tank 1 and an anaerobic tank 2, the waste water having passed through the anaerobic tank 2, and the first aerobic tank 3 is mixed with waste water from the anaerobic tank 2, and the mixture is sequentially transferred to a separator 4, the second aerobic tank 5 and a precipitation-separating tank 6. Thereafter, a part of aerobic sludge separated in the precipitation-separating tank 6 is mixed with the waste water from the anaerobic tank 2 and returned to the first aerobic tank 3, while the remainder is mixed with a liquid part separated by the separator 4 and returned to the second aerobic tank 5, too. Zoogloea bacteria are essential as aerobes acting in the circulating system of said waste water disposal, while facultative anaerobes such as lactobacilli are essential as said anaerobes.