NOVELTY - A supply unit (4) supplies iron ion to a methane fermentation tank (1) which performs anaerobic fermentation of organic waste and produces methane gas. A solid-liquid separation device (2) removes solid from digestive liquid obtained from the tank. An electrolytic cell (3) performs electrolytic processing of waste water from which solid has been removed, and which contains nitrogen content and chloride ion.    USE - Organic-waste processing system used to perform anaerobic fermentation and processing of organic waste such as livestock excrement, waste obtained from houses, purification-tank sludge, waste obtained from dairies, farms, so as to generate methane gas used in electric power generation, hot-water supply process, or as hydrogen source of fuel cell.    ADVANTAGE - The iron ion supplied to the fermentation tank reacts with hydrogen sulfide so that hydrogen sulfide is removed from the produced methane gas, without the use of a large-scale iron-oxide packed tower. The iron ion serves as a flocculent and enhances the solid-liquid separation efficiency of the separation device.    DETAILED DESCRIPTION - The supply unit comprises an auxiliary electrolytic cell which elutes iron ion by electrolysis, using an iron electrode under water. A portion of the waste water from which solid has been removed is supplied to the auxiliary electrolytic cell and is used for electrolysis. A biological waste-water treatment unit (5) for complete treatment of waste water, is provided at the later stage of the electrolytic cell. A concentration unit which increases the chloride-ion concentration of waste water, is provided at the later stage of the treatment unit. The concentrated waste water is returned to the electrolytic cell. A mechanical dehydration device dehydrates the concentrate.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the organic-waste processing system. (Drawing includes non-English language text).    tank (1)    separation device (2)    electrolytic cell (3)    supply unit (4)    treatment unit (5)