To provide a method for microbial degradation of waste water by which a bad smell can be eliminated simply from a sewage line of an anaerobic state to a sewage storage tank and the sewage can be treated in a satisfactorily clean condition without emitting the bad smell and leaving deposits such as waste oil in the sewage line, and to provide an apparatus which is used in this method the structure of which is made comparatively simpler and which is made compact.

In an apparatus for mixing a waste water treating agent, a deposition layer of mineral particles 3 which contain aluminum oxide and have around 10 mm diameter is formed below the standard water surface of a stainless steel-made vessel 2 to which a city water pipeline 1 is connected. An air diffusing member 5 is arranged below the deposition layer via a water-permeating partition table 4 consisting of a punched metal plate between them. A pipe 14 of supplying the compressed air forcibly-sent from an air pump 12 is connected to the member 5. When the mineral particles 3 are vibrated by an air bubble stream emitted from the air diffusing member 5 into the water, each of the particles 3 is broken into pieces by mutual friction to yield a powdery treating agent. A treating agent-mixing pipeline 6 is provided for supplying the powdery treating agent-dispersed water to the water to be treated outside the vessel 2.