PURPOSE:To enhance treatment efficiency, by arranging contact materials each of which is formed by drawing out filaments from the spaces of the wound parts of a bundled yarn and forming independent radial loops with a dimension of 11-14mm. to a core wire material from said filaments in a non-bundled state toward the outside in the radial direction. CONSTITUTION:A contact material is constituted by forming radial loops 2 to a core wire material made of an aluminum alloy having a diameter of about 2mm. in the radial direction and the length (a) between leading ends of the loops 2 is set to 20-35mm.. A plurality of contact material units 3 are held to a frame 4 made of a proper corrosion resistant material. Contact material modules are inserted into tanks of an aeration tank 9 and waste water introduced into the aeration tank 9b is contacted with bacteria propagated and grown in the contact materials within a first tank and mixed therewith under aeration and successively transferred to second, third and fourth tanks wile BOD and COD components are reduced and removed. Said BOD and COD components are perfectly removed at last and the supernatant solution is discharged from a line 7.
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