| 摘要 |
PROBLEM TO BE SOLVED: To obtain good treated water with a small rate of addition of an inorganic flocculating agent while subjecting the water to stable biological treatment. SOLUTION: A first liquid mixture of a biological treating vessel is separated by a concentrating machine 4 to concentrated sludge and concentrated separation liquid and the concentrated sludge is returned to the biological treating vessel 3 or a fore stage L5 of the biological treating vessel 3 so as to be mixed with the supplied water to be treated. This concentrating machine 4 is so regulated as to equal the amount of the sludge contained by the concentrated separating liquid and the amount of the excess sludge formed by the biological treating vessel 3, by which the sludge concentration of the biological treating vessel 3 is maintained constant. On the other hand, the concentrated separation liquid of the concentrating machine 4 is supplied to an intimate mixing vessel 5 for mixing the inorganic flocculating agent in such a manner that the concentration of the sludge flowing into the intimate mixing vessel 5 and the concentration of the sludge flowing into a solid-liquid separator 6 from the intimate mixing vessel 5 are lowered.
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