PURPOSE: To reduce the amt. of a flocculant to be injected, and to improve the dehydrating property of sludge by using an appropriate metallic salt of organic acid as the flocculant in the flocculating separation of organic waste water.

CONSTITUTION: A metallic salt of organic acid is used as a flocculant in the flocculating separation in the treatment of organic waste water using a combination of a biological treatment and the flocculating separation. One or more kinds among Fe2+, Fe3+, Al3+, Ca2+, and Mg2+ is firstly selected, and the salt of said metallic ions combined with biologically decomposable carboxylic acids such as formic acid, acetic acid, oxalic acid, propionic acid, succinic acid, and malic acid is selected as the flocculant. Ferric acetate is used, for example. When the metallic salt of organic acid is added at the stage prior to a biological treatment stage or at the biological treatment stage, metallic ions and colloid are gradually formed in the process of the decomposition and adsorption of the organic acid group along with the substrate in the waste water. And the chromaticity components, COD components, phosphoric acid, etc. are chemically removed by the flocculating sedimentation.