TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The sludge produced by souring and methane fermentation is ejected and removed. The removed sludge is solid-liquid separated, and sodium hydrogen carbonate (10-30 mass%) is added to solid portion. The resulting sludge is heat-processed at 180-300degreesC, and sludge is solubilized. A portion of methane as fuel is generated using internal combustion engine, external combustion engine or fuel cell as mechanism of production of electrical energy. The storage amount of methane gas is detected, and feed rate of organic compound-containing gas is controlled based on the detected value. The digestive supernatant liquor produced by souring and methane fermentation is electrochemically processed, and total nitrogen, total phosphorus, ammoniacal nitrogen and chemical oxygen demand are removed. An iron electrode as an anode is immersed into digestive supernatant liquor, during electrochemical processing. A divalent metal iron ion is produced by flow of electric current. The electrochemically-processed substance is further processed by low frequency pulse wave and high frequency pulse wave, after aggregating suspended particles. A portion of supernatant liquor as culture solution is photosynthesized, by reacting desulfurizing microbe with the produced gas. Hydrogen sulfide is removed from generated gas, and carbon dioxide of gas is converted to carbohydrate. Methanation of digestive supernatant liquor as culture solution is performed using sulfuric acid reduction microbe and carbon dioxide utilizing methane bacterium, and gas is purified.Preferred Apparatus: The methane generator has circular arc-shaped gas chamber (5) provided at upper portion of cylinder-shaped tank (3), a desludging pipe connecting bottom portion of souring portion and methane fermentation portion, and a control system. The desludging pipe is inclined towards the methane fermentation portion from the souring portion. A porous pipe for blowing air for gas stirring, is inserted into the desludging pipe. The control system has a gas sensor for measuring gas volume of gas chamber, and raw material feed rate is adjusted based on the measured value.