PURPOSE:To make it possible to stably treat org. substance even in small scale plottage, by guiding org. waste water to a digestion tank, wherein anaerobic bacteria are held at high concn. to perform stirring treatment. CONSTITUTION:Suspended solid substances in raw waste water are precipitated and separated in a precipitation basin 2 and the treated waste water is guided to a digestion tank 3 under an ambient temp. to perform anaerobic decomposition under stirring by a stirrer 4. The anaerobic decomposition herein is performed, for example, by using a liquid mixture containing anarobic bacteria at high concn. of 15,000ppm or more. When anaerobic decomposition is performed in the disgestion tank 3, methane gas and carbon dioxide are generated and collected by a gas tank 5. Sludge is sent to a final precipitation basin 6 and separated while a part thereof is recirculated to the digestion tank 3 as return sludge. Excessive sludge is discharged from the lower part of the final precipitation basin 6 while treated water 9 isdischarged from the upper part thereof. As a result, excessive sludge 8 is extremely small and a high BOD removal ratio is obtained.
