NOVELTY - The apparatus comprises a first circulating pipe for circulating the wastewater contained within an acid fermenter. A first pump forcibly circulates the wastewater in an upper direction of the acid fermenter. A first injection pipe includes a first upper nozzle that is arranged within the acid fermenter. An anaerobic digester has an internal space for accommodating the wastewater, and performs an anaerobic digestion process. A second circulating pipe is installed within the anaerobic digester to forcibly circulate the wastewater stored in the anaerobic digester.    USE - The anaerobic digestion apparatus is useful in a water-treatment apparatus (claimed) to produce the biogas by processing the organic waste.    ADVANTAGE - The apparatus can enhance the anaerobic digestion efficiency, enhance the stirring efficiency, improve the reactivity of the organic compound and microorganism, and has reduced maintenance and management expense. The apparatus can enhance the productivity of the biogas by reducing the sludge amount.    DETAILED DESCRIPTION - The apparatus comprises a first circulating pipe for circulating the wastewater contained within an acid fermenter, where one end of the first circulating pipe is connected with an upper part of the fermenter and the other end is connected with a bottom part of the fermenter. A first pump is installed on the first circulating pipe to forcibly circulate the wastewater in an upper direction in the acid fermenter. A first injection pipe includes a first upper nozzle that is arranged within the acid fermenter and connected with the first circulating pipe. An anaerobic digester has an internal space for accommodating the wastewater, and performs an anaerobic digestion process. A second circulating pipe is installed within the anaerobic digester to forcibly circulate the wastewater stored in the anaerobic digester, where an end of the second circulating pipe is connected with an upper portion of the anaerobic digester and the other end is connected with the lower part of the anaerobic digester. A second pump is installed in the second circulating pipe to forcibly circulate the wastewater contained within the anaerobic digester. The second injection pipe includes a second upper nozzle that is arranged within the anaerobic digester to jet the water in an upper direction. A first heat exchanger heats the wastewater flowing through the first circulating pipe. A second heat exchanger heats the wastewater flowing through the second circulating pipe. The first injection pipe further includes a first lower nozzle that is arranged within the acid fermenter, and a second lower nozzle that is arranged within the anaerobic digester, where the first lower nozzle is diagonally arranged at the lower part of the acid fermenter, and the second lower nozzle is diagonally arranged at the lower part of the anaerobic digester. A first rotary joint comprises a rotating unit that is connected between the first circulating pipe and the first injection pipe. A first motor rotates the rotating unit of the first rotary joint. A second rotary joint includes a fixing unit in which the second circulating pipe is connected to guide the wastewater. A second motor rotates a rotating unit of a second rotary joint. A first water level detector detects the water level within the acid fermenter. A first temperature sensor is installed within the acid fermenter to detect the water temperature within the acid fermenter. A second water level detector is installed within the acid fermenter to detect the water level within the anaerobic digester. A second temperature sensor is installed within the anaerobic digester to detect the water temperature within the anaerobic digester. A control device receives a detection signal from the first water level detector, and controls the operation of the pumps, the heat exchangers, and the temperature sensors. INDEPENDENT CLAIMS are included for:    (1) a water-treatment apparatus; and    (2) an anaerobic digestion processing method.