NOVELTY - The system comprises an anaerobic tank for storing the raw water including the microorganism and the organic compound, an anoxic chamber in which a biological denitrogen reaction of the nitrate nitrogen is progressed, a first return pipe discharging a portion of the activated sludge from the anoxic tank to the anaerobic tank, a membrane separation aeration tank separating the effluent into the processing water and sludge, a second return pipe returning a part of the sludge, and a mixed liquor suspended solid (MLSS) concentrating unit concentrating the supernatant.    USE - The system is useful for treating wastewater such as agricultural water and river water.    ADVANTAGE - The system treats wastewater with an excellent quality and with minimized usage amount of the coagulant and external carbon source, and enables the reuse of the agricultural water and the river water. The lifetime of the separation membrane is lengthened, and the necessary amount of clean dried air is relatively small, so that the operating cost of the system is reduced and the productivity of the processing water is increased. The amount of dissolved oxygen within the membrane separation aeration tank can be controlled using the baffle.    DETAILED DESCRIPTION - The system comprises an anaerobic tank for storing the raw water including the microorganism and the organic compound, and a biological phosphorus release reaction for the raw water is progressed in the tank, an anoxic chamber in which a biological denitrogen reaction of the nitrate nitrogen is progressed in an anoxic condition, a first return pipe discharging a portion of the activated sludge from the anoxic tank to the anaerobic tank, a membrane separation aeration tank separating the effluent ejected from the anoxic chamber into the processing water and sludge, a second return pipe returning a part of the sludge ejected from the membrane separation aeration tank, and a mixed liquor suspended solid (MLSS) concentrating unit concentrating the supernatant. The microorganism comprises the denitrifying phosphorus accumulating organism. The anoxic chamber comprises a perforated plate that bisects the space within the anoxic chamber, where the perforated plate has a diameter, the thickness and porosity 2-10 mm, 2-30 mm and 30-80%, respectively. A transportable baffle is arranged between the membrane separation aeration tank and the aeration tank. An external carbon source and a feeding dosing device are arranged in an anoxic chamber.