PURPOSE: To efficiently convert organic substance, etc. in waste water into energy, by treating waste water containing organic substance with an ultrafiltration membrane, methane fermenting the resulting conc. liquid to recover methane, and separating a filtrate into a conc. liquid and pure water using a reverse osmosis membrane.

CONSTITUTION: BOD-contg. waste liquor 13 formed in a factory for processing animal and plant materials is gathered in a reservoir tank 13, bulky garbage is removed from it with a simple filter 32, and then said waste liquor is poured in an ultrafiltration device 34 under pressure. The obtd. filtrate 46 is sent through a filter 47 to a reverse osmosis membrane 49 to separately filter organic substance from it. In a methane fermentation tank 37, its organic oxidation and alcoholic reaction are brought about as such, and fermentation is promoted at an extremely high speed. Formed amino acid, lower aliphatic acids and alcohols are digestively decomposed into methane and carbonic acid gas by the action of methane bacteria in succession. After about 0.4% H2S contained in formed gas 43 formed in the fermentation tank 37 is separated by a desulfurizer 44, said formed gas 43 is reserved in a gas tank 45.