TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Component: The anaerobic, anoxic and aerobic tanks are formed by arranging partition walls with a PE panel in a single tank.The apparatus includes a dissolved oxygen lowering tank for receiving the wastewater returned from the anaerobic tank positioned after the anoxic tank and part of the wastewater from a zone positioned before the anoxic tank to decrease the dissolved oxygen, and then introducing the received wastewater into the anoxic tank.Nozzles are arranged at an inlet through which the wastewater from the lowering tank is introduced into the anoxic tank.The anoxic tank has a wastewater return line for returning part of the wastewater from the anoxic tank to the anaerobic tank.The aerobic tank after the anoxic tank and the clarifier positioned after the anoxic tank has a sludge return line for returning part of the sediment from the clarifier to the aerobic tank.Separate walls are vertically provided in zigzag inside the anaerobic and anoxic tanks.Each separate wall has one side which is in close contact with one side wall of the anaerobic or anoxic tank and the other side is apart from the other side wall of the anaerobic or anoxic tank, thus causing a plug flow of the wastewater in the anaerobic and anoxic tanks.The lowering tank and the separate walls are manufactured by fixing PE panel fixing angles in an inner side of the aerobic, anaerobic or anoxic tank, inserting a PE panel between the PE panel fixing angles, and then caulking a joint.The PE panel is formed by connecting PE panel pieces.    TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The wastewater returned from the aerobic tank to the anoxic tank is introduced in halfway into the dissolved oxygen lowering tank to decrease the dissolved oxygen, and then returned to the anoxic tank.Part of the wastewater prior to entering the anaerobic tank or part of the wastewater from the anaerobic tank is additionally introduced into the dissolved oxygen lowering tank.The wastewater from the zone is the raw wastewater or the wastewater from the anaerobic tank.