NOVELTY - A methane fermentation equipment (1) consists of an acid production tank (2) which processes organic wastewater by an acid production microbe and produces and an acid, a wastewater-pH controller (5), a methane fermentation tank (3), and a return line (L2). The pH controller consists of a unit (7) for injecting a compound which reacts with hydrogen sulfide produced in the methane fermentation tank, while neutralizing the alkali in acid production tank, and forms a precipitate of sulfide when wastewater introduction is stopped.    USE - Equipment is used for methane fermentation of organic wastewater (claimed) generated during concentration of black liquor of e.g. kraft pulp manufacturing process.    ADVANTAGE - The methane fermentation equipment enables appropriate adjustment of pH of wastewater when wastewater introduction is stopped. The equipment is economical. Hydrogen-sulfide inhibition is suppressed.    DETAILED DESCRIPTION - A methane fermentation equipment consists of an acid production tank which processes introduced organic wastewater by an acid production microbe and produces and an acid, a wastewater-pH controller, a methane fermentation tank, and a return line. The methane fermentation tank methane-ferments wastewater introduced from the acid production tank. The return line returns some amount of treated water of methane fermentation tank to the acid production tank. The pH controller consists of a unit for injecting a compound which reacts with hydrogen sulfide produced in the methane fermentation tank, while neutralizing the alkali in acid production tank, and forms a precipitate of sulfide when wastewater introduction is stopped. An INDEPENDENT CLAIM is included for methane fermentation method.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the methane fermentation equipment. (Drawing includes non-English language text)    Methane fermentation equipment (1)    Acid production tank (2)    Methane fermentation tank (3)    Wastewater-pH controller (5)    Compound injection unit (7)    Return line (L2)