NOVELTY - Providing a wastewater purification apparatus which is organized to maximize the wastewater purification efficiency which has an anaerobic tank, an aerobic tanks and a clarifier.    USE - For purifying wastewater.    ADVANTAGE - The invention maximizes wastewater purification efficiency with a simple arrangement. It allows additional arrangement of the anaerobic and anoxic tanks without changing an active sludge return line in the conventional apparatus comprising the aerobic tank alone, and hardly requires operational interruption of the wastewater purification apparatus. It returns the wastewater from the clarifier to the anaerobic tank to maintain high MLSS level in the aerobic tank and high nitration efficiency at a low water temperature in winter, thus reducing the TN level of the purified wastewater. It lowers the dissolved oxygen in a returned wastewater from the aerobic tank to the anoxic tank, thus enhancing the treatment efficiency in the anoxic tank. It enhances the rate of lowering the dissolved oxygen to minimize the volume of the dissolved oxygen lowering tank. It enables optimized endogenous respiration of the microorganism in the anaerobic and aerobic tanks, thus enhancing the total wastewater purification efficiency. It can be interchangeably applied to various wastewater purification apparatus.    DETAILED DESCRIPTION - A wastewater purification apparatus comprises an anaerobic tank (1a-b) for treating wastewater with anaerobic bacteria; an anoxic tank (2a-b) for treating the wastewater from the anaerobic tank with denitrifying bacteria and then returning part of the wastewater to the anaerobic tank; an aerobic tank (3a-b) for treating the wastewater from the anoxic tank with aerobic bacteria and then returning part of the wastewater to the anoxic tank; and a clarifier (5) for settling the wastewater from the aerobic tank, discharging a purified water, and returning part of a sediment to the aerobic tank through a return line.    An INDEPENDENT CLAIM is also included for a method for purifying wastewater, comprising treating the wastewater introduced into the anaerobic tank with anaerobic bacteria; introducing the wastewater from the anaerobic tank into the anoxic tank for treatment with denitrifying bacteria, and then returning part of the wastewater to the anaerobic tank; introducing the wastewater from the anoxic tank into the aerobic tank for treatment with aerobic bacteria, and then returning part of the wastewater to the anoxic tank; and settling the wastewater from the aerobic tank, and returning part of the sediment to the aerobic tank.    DESCRIPTION OF DRAWING(S) - The figure shows a plan view of a wastewater purification apparatus.    Anaerobic tank (1a-b)    Anoxic tank (2a-b)    Aerobic tank (3a-b)    Clarifier (5)