PURPOSE: To utilize town waste and by-product effectively and to convert a sulfate compound with low value into a sulfide with high value, by reducing the sulfate compound contained in activated sludge or town garbage with sulfate reducing microorganism grown in them.

CONSTITUTION: A sulfate compound of industrial waste such as waste liquor, sludge, etc. is dissolved or suspended in water to give a mixed solution, to which activated sludge or garbage is added. When a sulfate reducing microorganism is grown anaerobically at a pH of 5W8 at 15W45°C in the resulted blend, the sulfate compound is converted into sulfide by their growth. When iron sulfate in the waste liquor of acid washing occurring in titanium plant, iron mill, etc. is used as the sulfate compound, iron sulfide is obtained, and it can be separated easily by separation through specific gravity. When gypsum or sodium sulfate is used as the raw materil, H2S is obtained.