TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The digestion gas containing ammonia, is passed through an absorption process to absorb carbon dioxide and volatile organic substance, using carbon dioxide-volatile organic substance absorption cell (9) by an alkali treatment using soluble alkali. The treatment apparatus further comprises an ammonia absorption cell (11) which absorbs ammonia in the digestion gas (passed after carbon dioxide and volatile organic substance absorption cell) using acid. A path (12) which transfers alkali to the carbon dioxide-volatile organic substance ammonia absorption cell, is provided in the treatment apparatus. EXAMPLE - Sludge was treated with alkali for 30 minutes at 80degreesC in acid fermenter. Consequently, the sludge was solubilized to 70%. The solubilized sludge was introduced to an acid fermenter heated at 55degreesC and souring process was performed for 3 days. Nitrogen gas of 100 L/L reaction vessel was blown into the acid fermenter for 1 hour and ammonia was produced. The sludge after souring was solid-liquid separated with a centrifugation vessel, then the liquid component was introduced to a upflow anaerobic blanket (UASB) tank and the liquid component was heated at 35degreesC and methane fermentation was performed. The methanation amount of the organic material was 510 L/kg total solid. The digestion gas formed during methane fermentation process was returned to souring process and the ammonia was produced with an acid fermenter. The solid component was returned to solubilized process. The digestion gas from the acid fermenter was passed to carbon dioxide and volatile organic substance absorption cell and carbon dioxide and volatile organic substance was removed using alkali and the resulting digestion gas passed through ammonia absorption cell in which the ammonia was removed. The amount of total solid content and volatile solid content in the methane fermentation tank, were 10 wt.% and 8.1 wt.%, respectively. The reaction time was 9 days.