NOVELTY - A thickening tank (6) is mounted under the reaction tank for settling sludge in water from reaction tank. An agitator aerates and forms bubbles to increase dissolved oxygen and to decompose the polluting material by aeration microorganism. A sludge separation tank (3) integral to reaction tank receives waste water from it for settling sludge and for returning the waste water to the thickening tank.    USE - For biological treatment of the waste water.    ADVANTAGE - Prevents sludge from accumulating in reaction tank and improves agitation efficiency by installing an inclined blade assembly in the reaction tank, through which fluid can pass in a predetermined direction. Effectively separates water from sludge by making flow direction of the fluid to be identical to direction of the gravity during the process for separating the sludge after biologically treating the waste water by connecting the reaction tank to a sludge separation tank and effectively settles solid material.    DETAILED DESCRIPTION - The reaction tank is divided into multiple sections by partitions and the divisions are provided with a gas intake holes and a gas collecting holes. The agitator has blade assembly (9) having lower and upper co-axial blades (12, 11) to agitate the waste water. A hollow unit is connected to an edge of a circular hole formed on the central portion of the funnel shaped partition (13) of the blade assembly. The upper and lower blades are overlapped on each other circumferentially and the blades are designed to be inclined so that spaces are formed between the blades for water flow. The cross-section of the reaction tank is reduced from upper portion to lower portion and an inclined plate is mounted on the tank portion to direct the waste water downwardly. The plate is provided with a protruding portion through which sludge separated water passes and an exhaust hole is provided on a corner of a tank bottom to return waste water to the intake tube. The guide blade is formed so that the air induced through the air intake tube is moved upward along an inclined surface of the guide plate.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic view of waste water treatment plant.    Sludge separation tank (3)    Thickening tank (6)    Blade assembly (9)    Co-axial blades (11,12)    Funnel shaped partition (13)